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The field of nanophotonics focuses on the interaction between electromagnetic radiation and matter on the nanometer scale. The elements of nanoscale photonic devices can transfer excitation energy non-radiatively from an excited donor molecule to an acceptor molecule by Förster resonance energy transfer (FRET). The efficiency of this energy transfer is highly dependent on the donor-acceptor distance. Hence, in these nanoscale photonic devices it is of high importance to have a good control over the spatial assembly of used fluorophores. Based on molecular self-assembly processes, various nanostructures can be produced. Here, DNA nanotechnology and especially the DNA origami technique are auspicious self-assembling methods. By using DNA origami nanostructures different fluorophores can be introduced with a high local control to create a variety of nanoscale photonic objects. The applications of such nanostructures range from photonic wires and logic gates for molecular computing to artificial light harvesting systems for artificial photosynthesis.
In the present cumulative doctoral thesis, different FRET systems on DNA origami structures have been designed and thoroughly analyzed. Firstly, the formation of guanine (G) quadruplex structures from G rich DNA sequences has been studied based on a two-color FRET system (Fluorescein (FAM)/Cyanine3 (Cy3)). Here, the influences of different cations (Na+ and K+), of the DNA origami structure and of the DNA sequence on the G-quadruplex formation have been analyzed. In this study, an ion-selective K+ sensing scheme based on the G-quadruplex formation on DNA origami structures has been developed. Subsequently, the reversibility of the G-quadruplex formation on DNA origami structures has been evaluated. This has been done for the simple two-color FRET system which has then been advanced to a switchable photonic wire by introducing additional fluorophores (FAM/Cy3/Cyanine5 (Cy5)/IRDye®700). In the last part, the emission intensity of the acceptor molecule (Cy5) in a three-color FRET cascade has been tuned by arranging multiple donor (FAM) and transmitter (Cy3) molecules around the central acceptor molecule. In such artificial light harvesting systems, the excitation energy is absorbed by several donor and transmitter molecules followed by an energy transfer to the acceptor leading to a brighter Cy5 emission. Furthermore, the range of possible excitation wavelengths is extended by using several different fluorophores (FAM/Cy3/Cy5). In this part of the thesis, the light harvesting efficiency (antenna effect) and the FRET efficiency of different donor/transmitter/acceptor assemblies have been analyzed and the artificial light harvesting complex has been optimized in this respect.
The importance of carbohydrate structures is enormous due to their ubiquitousness in our lives. The development of so-called glycomaterials is the result of this tremendous significance. These are not exclusively used for research into fundamental biological processes, but also, among other things, as inhibitors of pathogens or as drug delivery systems. This work describes the development of glycomaterials involving the synthesis of glycoderivatives, -monomers and -polymers. Glycosylamines were synthesized as precursors in a single synthesis step under microwave irradiation to significantly shorten the usual reaction time. Derivatization at the anomeric position was carried out according to the methods developed by Kochetkov and Likhorshetov, which do not require the introduction of protecting groups. Aminated saccharide structures formed the basis for the synthesis of glycomonomers in β-configuration by methacrylation. In order to obtain α-Man-based monomers for interactions with certain α-Man-binding lectins, a monomer synthesis by Staudinger ligation was developed in this work, which also does not require protective groups. Modification of the primary hydroxyl group of a saccharide was accomplished by enzyme-catalyzed synthesis. Ribose-containing cytidine was transesterified using the lipase Novozym 435 and microwave irradiation. The resulting monomer synthesis was optimized by varying the reaction partners. To create an amide bond instead of an ester bond, protected cytidine was modified by oxidation followed by amide coupling to form the monomer. This synthetic route was also used to isolate the monomer from its counterpart guanosine. After obtaining the nucleoside-based monomers, they were block copolymerized using the RAFT method. Pre-synthesized pHPMA served as macroCTA to yield cytidine- or guanosine-containing block copolymer. These isolated block copolymers were then investigated for their self-assembly behavior using UV-Vis, DLS and SEM to serve as a potential thermoresponsive drug delivery system.
Nanolenses are linear chains of differently-sized metal nanoparticles, which can theoretically provide extremely high field enhancements. The complex structure renders their synthesis challenging and has hampered closer analyses so far. Here, the technique of DNA origami was used to self-assemble DNA-coated 10 nm, 20 nm, and 60 nm gold or silver nanoparticles into gold or silver nanolenses. Three different geometrical arrangements of gold nanolenses were assembled, and for each of the three, sets of single gold nanolenses were investigated in detail by atomic force microscopy, scanning electron microscopy, dark-field scattering and Raman spectroscopy. The surface-enhanced Raman scattering (SERS) capabilities of the single nanolenses were assessed by labelling the 10 nm gold nanoparticle selectively with dye molecules. The experimental data was complemented by finite-difference time-domain simulations. For those gold nanolenses which showed the strongest field enhancement, SERS signals from the two different internal gaps were compared by selectively placing probe dyes on the 20 nm or 60 nm gold particles. The highest enhancement was found for the gap between the 20 nm and 10 nm nanoparticle, which is indicative of a cascaded field enhancement. The protein streptavidin was labelled with alkyne groups and served as a biological model analyte, bound between the 20 nm and 10 nm particle of silver nanolenses. Thereby, a SERS signal from a single streptavidin could be detected. Background peaks observed in SERS measurements on single silver nanolenses could be attributed to amorphous carbon. It was shown that the amorphous carbon is generated in situ.
The incorporation of proteins in artificial materials such as membranes offers great opportunities to avail oneself the miscellaneous qualities of proteins and enzymes perfected by nature over millions of years. One possibility to leverage proteins is the modification with artificial polymers. To obtain such protein-polymer conjugates, either a polymer can be grown from the protein surface (grafting-from) or a pre-synthesized polymer attached to the protein (grafting-to). Both techniques were used to synthesize conjugates of different proteins with thermo-responsive polymers in this thesis.
First, conjugates were analyzed by protein NMR spectroscopy. Typical characterization techniques for conjugates can verify the successful conjugation and give hints on the secondary structure of the protein. However, the 3-dimensional structure, being highly important for the protein function, cannot be probed by standard techniques. NMR spectroscopy is a unique method allowing to follow even small alterations in the protein structure. A mutant of the carbohydrate binding module 3b (CBM3bN126W) was used as model protein and functionalized with poly(N-isopropylacrylamide). Analysis of conjugates prepared by grafting-to or grafting-from revealed a strong impact of conjugation type on protein folding. Whereas conjugates prepared by grafting a pre-formed polymer to the protein resulted in complete preservation of protein folding, grafting the polymer from the protein surface led to (partial) disruption of the protein structure.
Next, conjugates of bovine serum albumin (BSA) as cheap and easily accessible protein were synthesized with PNIPAm and different oligoethylene glycol (meth)acrylates. The obtained protein-polymer conjugates were analyzed by an in-line combination of size exclusion chromatography and multi-angle laser light scattering (SEC-MALS). This technique is particular advantageous to determine molar masses, as no external calibration of the system is needed. Different SEC column materials and operation conditions were tested to evaluate the applicability of this system to determine absolute molar masses and hydrodynamic properties of heterogeneous conjugates prepared by grafting-from and grafting-to. Hydrophobic and non-covalent interactions of conjugates lead to error-prone values not in accordance to expected molar masses based on conversions and extents of modifications.
As alternative to this method, conjugates were analyzed by sedimentation velocity analytical ultracentrifugation (SV-AUC) to gain insights in the hydrodynamic properties and how they change after conjugation. Within a centrifugal field, a sample moves and fractionates according to the mass, density, and shape of its individual components. Conjugates of BSA with PNIPAm were analyzed below and above the cloud point temperature of the thermo-responsive polymer component. It was identified that the polymer characteristics were transferred to the conjugate molecule which than showed a decreased ideality – defined as increased deviation from a perfect sphere model – below and increased ideality above the cloud point temperature. This effect can be attributed to an arrangement of the polymer chain pointing towards the solvent (expanded state) or snuggling around the protein surface depending on the applied temperature.
The last project dealt with the synthesis of ferric hydroxamate uptake protein component A (FhuA)-polymer conjugates as building blocks for novel membrane materials. The shape of FhuA can be described as barrel and removal of a cork domain inside the protein results in a passive channel aimed to be utilized as pores in the membrane system. The polymer matrix surrounding the membrane protein is composed of a thermo-responsive and a UV-crosslinkable part. Therefore, an external trigger for covalent immobilization of these building blocks in the membrane and switchability of the membrane between different states was incorporated. The overall performance of membranes prepared by a drying-mediated self-assembly approach was evaluated by permeability and size exclusion experiments. The obtained membranes displayed an insufficiency in interchain crosslinking and therefore a lack in performance. Furthermore, the aimed switch between a hydrophilic and hydrophobic state of the polymer matrix did not occur. Correspondingly, size exclusion experiments did not result in a retention of analytes larger than the pores defined by the dimension of the used FhuA variant.
Overall, different paths to generate protein-polymer conjugates by either grafting-from or grafting-to the protein surface were presented paving the way to the generation of new hybrid materials. Different analytical methods were utilized to describe the folding and hydrodynamic properties of conjugates providing a deeper insight in the overall characteristics of these seminal building blocks.
Advanced hybrid materials are recognized as one of the most significant enablers for new technologies, which holds true especially on the quest for sustainable energy sources and energy production schemes (e.g., semiconductor based photocatalytic materials). Usually, a single component is far from meeting all the demands needed for these advanced applications. Hybrid materials are composed of at least two components commonly an inorganic and an organic material on the molecular level, which feature novel properties exceeding the sum of the individual parts and might be the milestones of next-generation applications. This dissertation aims to provide novel combinations of the metal-free semiconductor graphitic carbon nitride (g-C3N4) with polymers to obtain materials with advanced properties and applications. Visible light constitutes the core of the present work as it is the only energy source utilized either in synthesis or in the application process. In the area of applications by combination of g-C3N4 and polymers, two different hybrids were thoroughly elucidated, i.e.. their design and construction as well as potential application in photocatalysis. Novel soft 3D liquid objects were formed via charge-interaction driven interfacial jamming between polyelectrolytes in aqueous environment and colloidal dispersions of g-C3N4 in edible sunflower oil. As such, stable liquid objects could be molded into specific shapes and utilized for photodegradation of organic dyes in water. Furthermore, the grafting of polymers onto g-C3N4 was investigated. Allyl-end functionalized polymers were grafted onto g-C3N4 by a photoinitiated process to yield g-C3N4 with versatile and improved properties, e.g. advanced dispersibility enabling processing via spin coating. As g-C3N4 produces radicals under visible light irradiation, which is of significant interest for polymer science, g-C3N4 containing polymer latex and macrogel beads (MGB) were synthesized by emulsion photopolymerization and inverse suspension photopolymerization, respectively. A well-controlled emulsion photopolymerization process via g-C3N4 initiation was designed, which features synthesis of well-defined and cross-linked polymer particles. Furthermore, the polymerization process was investigated thoroughly, indicating an ad-layer polymerization in early stages of the process. The utilization of functionalized g-C3N4 allowed the polymerization of various monomer types. Moreover, g-C3N4 was utilized as photoinitiator in hydrogel MGB formation. The formed MGB properties could be tailored via process design, e.g. stirring rate, cross-linker content and g-C3N4 content. Finally, MGBs were introduced as photocatalyst for waste water remediation, i.e. the degradation of Rhodamine B in aqueous solution was studied. The present thesis therefore builds a bridge between g-C3N4 and polymers and provides strategies for hybrid material formation. Furthermore, several potential applications are revealed with significant implications for photocatalysis, polymerization processes and polymer materials.
The supercapacitor is one of the most important energy storage devices as its construction allows for addressing many of the drawbacks related to batteries, but the low energy density of current systems is a major issue. In this doctoral dissertation, with a view to attaining high energy density supercapacitor systems that can be comparable to those for batteries, new heteroatom-containing carbons in the form of particles and three-dimensional films were investigated. A nitrogen-containing material, acrodam, was chosen as the carbon precursor due to the inexpensiveness, high carbonization yield, oligomerizability, etc. The carbon particles were prepared from acrodam together with caesium acetate as a meltable flux agent, and disclosed excellent properties in hydroquinone-loaded sulphuric acid electrolyte with high energy densities (up to 133.0 Wh kg–1) and sufficient cycle stabilities. These properties are already now comparable to those of batteries. Besides, conductive carbon three-dimensional films were fabricated using acrodam oligomer as the precursor by the inexpensive spin coating method. The films were found to be homogeneous, flat, void- and crack-free, and high conductivities (up to 334 S cm–1) could be obtained at the carbonization temperature of 1000 ºC. Furthermore, a porous carbon three-dimensional film could be formed using an organic template at the first attempt. This finding demonstrates the film’s potentiality for various applications such as supercapacitor electrode; the essential absence of contact resistance within the network should contribute to effective transportation of electron within the electrode. The progress made in this dissertation will open a new way to further enhancement of energy density for supercapacitor as well as other applications that exceeds the current properties.
The valorization of carbohydrates is one of the most promising fields in green chemistry, as it enables to produce bulk chemicals and fuels out of renewable and abundant resources, instead of further exploiting fossil feedstocks. The focus in this thesis is the conversion of fructose, using dehydration and hydrodeoxygenation reactions. The main goal is to find an easy continuous process, including the solubility of the sugar in a green solvent, the conversion over a solid acid as well as over a metal@tungsten carbide catalyst.
At the beginning of this thesis, solid acid catalysts are synthesized by using carbohydrate material like glucose and starch at high temperatures (up to 600 °C). Additionally a third carbon is synthesized, using an activation method based on Ca(OH)2. After carbonization and further sulfonation, using fuming sulfuric acid, the three resulting catalysts are characterized together with sulfonated carbon black and Amberlyst 15 as references. In order to test all solid acid catalysts in reaction, a 250 mm x 4.6 mm stainless steel column is used as a fixed-bed continuous reactor. The temperature (110 °C to 250 °C) and residence time (2 to 30 minutes) is varied, and a direct relationship between contact time and selectivity is determined. The reaction mechanism, as well as the product distribution is showing a dehydration step of fructose towards 5-hydroxymethylfurfural (HMF). These furan-ring molecules are considered as “sleeping giants”, due to the possibility of using them as fuel, but also for upgrading them to chemicals like terephthalic acid or p-xylene. Consecutive reactions are producing levulinic acid, as well as condensation products with ethanol and formic acid. The activated carbon is additionally showing a 2 % yield of 2,5-Dimethylfuran (DMF) production, pointing towards the extraordinary properties of this catalyst. Without a metal catalyst present, what is normally necessary for hydrogenation reactions, a transferhydrogenation (with formic acid) is observed. The active catalyst was therefore carbon itself, what activated the hydrogen on its surface. This phenomenon was just very rarely observed so far. Expensive noble metals are the material of choice, when it comes to hydrogenation reactions nowadays and cheaper alternatives are necessary.
By postulating a similar electronic structure of tungsten carbide (WC) to platinum by Lewy and Boudart, research is focusing on the replacement of Pt. The production of nano-sized tungsten carbide particles (7.5 ± 2.5 nm, 70 m2 g-1) is enabled by the so called “urea glass route” and its catalytic performances are compared to commercial material. It is shown, that the activity is strongly dependent on the size of the particles as well as the surface area. Nano-sized tungsten carbide is showing activity for hydrogenation reactions under mild conditions (maximum 150 °C, 30 bar). This material therefore opens up new possibilities for replacing the rare and expensive platinum with tungsten carbide based catalysts.
Additionally different metal nanoparticles of palladium, copper and nickel are deposited on top of WC to further promote its reactivity. The nickel nanoparticles are strongly connected to WC and showed the best activity as well as selectivity for upgrading HMF with hydrodeoxygenation. The Ni@WC is not leaching and is showing very good hydrodeoxygenation properties with DMF yields up to 90 percent. Copper@WC is not showing good activity and palladium@WC enables undesired consecutive reactions, hydrogenating the furan ring system.
In order to enable the upgrade of fructose to DMF directly in a continuous system, the current H CUBE Pro TM hydrogenation system is customized with a second reaction column. A 250 mm x 4.6 mm stainless steel reactor column is connected ahead of the hydrogen insertion, enabling the dehydration of fructose to HMF derivatives, before pumping these products into the second column for hydrogenation. The overall residence time in the two column reactor system is 14 minutes. The overall results are an almost full conversion with a yield of 38.5 % DMF and 47 % yield of EL. The main disadvantage is the formation of higher mass products, so called humins, which start depositing on top of the catalysts, blocking their active sites.
In general it can be stated, that a two column system goes along with a higher investment as well as more maintenance costs, compared to a one column catalytic approach. To develop a catalyst, which is on the one hand able to dehydrate as well as hydrodeoxygenate the reactants, is aimed for at the last part of the thesis. The activated carbon however shows already activity for hydrodeoxygenation without any metal present and offers itself therefore as an alternative to overcome the temperature instability of Amberlyst 15 (max. 120 °C) for a combined DMF production directly from fructose. The activity for the upgrade to DMF is increased from 2 % to 12 % DMF yield in one mixed continuous column.
In order to scale up the entire one column approach, an 800 mm x 28.5 mm inner diameter column was planned and manufactured. The system is scaled up and assembled, whereas this flow reactor system is able to be run with 50 mL min-1 maximum flow rate, to stand a pressure of maximum 100 bar and be heated to around 500 °C. The tubing and connections, as well as the used devices are planned according to be safe and easy in use. The scaled-up approach offers a reaction column 120 times bigger (510 ml) then the first extension of the commercial system. This further extension offers the possibility of ranging between 1 and 1000 mL min-1, making it possible to use the approach in pilot plant applications.
Hierarchical meso- and macropore architectures by liquid crystalline and polymer colloid templating
(2007)
The needs for sustainable energy generation, but also a sustainable chemistry display the basic motivation of the current thesis. By different single investigated cases, which are all related to the element carbon, the work can be devided into two major topics. At first, the sustainable synthesis of “useful” carbon materials employing the process of hydrothermal carbonisation (HC) is described. In the second part, the synthesis of heteroatom - containing carbon materials for electrochemical and fuel cell applications employing ionic liquid precursors is presented. On base of a thorough review of the literature on hydrothermolysis and hydrothermal carbonisation of sugars in addition to the chemistry of hydroxymethylfurfural, mechanistic considerations of the formation of hydrothermal carbon are proposed. On the base of these reaction schemes, the mineral borax, is introduced as an additive for the hydrothermal carbonisation of glucose. It was found to be a highly active catalyst, resulting in decreased reaction times and increased carbon yields. The chemical impact of borax, in the following is exploited for the modification of the micro- and nanostructure of hydrothermal carbon. From the borax - mediated aggregation of those primary species, widely applicable, low density, pure hydrothermal carbon aerogels with high porosities and specific surface areas are produced. To conclude the first section of the thesis, a short series of experiments is carried out, for the purpose of demonstrating the applicability of the HC model to “real” biowaste i.e. watermelon waste as feedstock for the production of useful materials. In part two cyano - containing ionic liquids are employed as precursors for the synthesis of high - performance, heteroatom - containing carbon materials. By varying the ionic liquid precursor and the carbonisation conditions, it was possible to design highly active non - metal electrocatalyst for the reduction of oxygen. In the direct reduction of oxygen to water (like used in polymer electrolyte fuel cells), compared to commercial platinum catalysts, astonishing activities are observed. In another example the selective and very cost efficient electrochemical synthesis of hydrogen peroxide is presented. In a last example the synthesis of graphitic boron carbon nitrides from the ionic liquid 1 - Ethyl - 3 - methylimidazolium - tetracyanoborate is investigated in detail. Due to the employment of unreactive salts as a new tool to generate high surface area these materials were first time shown to be another class of non - precious metal oxygen reduction electrocatalyst.
Improving Hemocompatibility of poly(ether imide) by surface functionalization with polyethers
(2012)
The formation of colloids by the controlled reduction, nucleation, and growth of inorganic precursor salts in different media has been investigated for more than a century. Recently, the preparation of ultrafine particles has received much attention since they can offer highly promising and novel options for a wide range of technical applications (nanotechnology, electrooptical devices, pharmaceutics, etc). The interest derives from the well-known fact that properties of advanced materials are critically dependent on the microstructure of the sample. Control of size, size distribution and morphology of the individual grains or crystallites is of the utmost importance in order to obtain the material characteristics desired. Several methods can be employed for the synthesis of nanoparticles. On the one hand, the reduction can occur in diluted aqueous or alcoholic solutions. On the other hand, the reduction process can be realized in a template phase, e.g. in well-defined microemulsion droplets. However, the stability of the nanoparticles formed mainly depends on their surface charge and it can be influenced with some added protective components. Quite different types of polymers, including polyelectrolytes and amphiphilic block copolymers, can for instance be used as protecting agents. The reduction and stabilization of metal colloids in aqueous solution by adding self-synthesized hydrophobically modified polyelectrolytes were studied in much more details. The polymers used are hydrophobically modified derivatives of poly(sodium acrylate) and of maleamic acid copolymers as well as the commercially available branched poly(ethyleneimine). The first notable result is that the polyelectrolytes used can act alone as both reducing and stabilizing agent for the preparation of gold nanoparticles. The investigation was then focused on the influence of the hydrophobic substitution of the polymer backbone on the reduction and stabilization processes. First of all, the polymers were added at room temperature and the reduction process was investigated over a longer time period (up to 8 days). In comparison, the reduction process was realized faster at higher temperature, i.e. 100°C. In both cases metal nanoparticles of colloidal dimensions can be produced. However, the size and shape of the individual nanoparticles mainly depends on the polymer added and the temperature procedure used. In a second part, the influence of the prior mentioned polyelectrolytes was investigated on the phase behaviour as well as on the properties of the inverse micellar region (L2 phase) of quaternary systems consisting of a surfactant, toluene-pentanol (1:1) and water. The majority of the present work has been made with the anionic surfactant sodium dodecylsulfate (SDS) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) since they can interact with the oppositely charged polyelectrolytes and the microemulsions formed using these surfactants present a large water-in-oil region. Subsequently, the polymer-modified microemulsions were used as new templates for the synthesis of inorganic particles, ranging from metals to complex crystallites, of very small size. The water droplets can indeed act as nanoreactors for the nucleation and growth of the particles, and the added polymer can influence the droplet size, the droplet-droplet interactions, as well as the stability of the surfactant film by the formation of polymer-surfactant complexes. One further advantage of the polymer-modified microemulsions is the possibility to stabilize the primary formed nanoparticles via a polymer adsorption (steric and/or electrostatic stabilization). Thus, the polyelectrolyte-modified nanoparticles formed can be redispersed without flocculation after solvent evaporation.
Lithium-ion capacitors (LICs) are promising energy storage devices by asymmetrically combining anode with a high energy density close to lithium-ion batteries and cathode with a high power density and long-term stability close to supercapacitors. For the further improvement of LICs, the development of electrode materials with hierarchical porosity, nitrogen-rich lithiophilic sites, and good electrical conductivity is essential. Nitrogen-rich all-carbon composite hybrids are suitable for these conditions along with high stability and tunability, resulting in a breakthrough to achieve the high performance of LICs. In this thesis, two different all-carbon composites are suggested to unveil how the pore structure of lithiophilic composites influences the properties of LICs. Firstly, the composite with 0-dimensional zinc-templated carbon (ZTC) and hexaazatriphenylene-hexacarbonitrile (HAT) is examined how the pore structure is connected to Li-ion storage property as LIC electrode. As the pore structure of HAT/ZTC composite is easily tunable depending on the synthetic factor and ratio of each component, the results will allow deeper insights into Li-ion dynamics in different porosity, and low-cost synthesis by optimization of the HAT:ZTC ratio. Secondly, the composite with 1-dimensional nanoporous carbon fiber (ACF) and cost-effective melamine is proposed as a promising all-carbon hybrid for large-scale application. Since ACF has ultra-micropores, the numerical structure-property relationships will be calculated out not only from total pore volume but more specifically from ultra-micropore volume. From these results above, it would be possible to understand how hybrid all-carbon composites interact with lithium ions in nanoscale as well as how structural properties affect the energy storage performance. Based on this understanding derived from the simple materials modeling, it will provide a clue to design the practical hybrid materials for efficient electrodes in LICs.
The increasing demand for energy in the current technological era and the recent political decisions about giving up on nuclear energy diverted humanity to focus on alternative environmentally friendly energy sources like solar energy. Although silicon solar cells are the product of a matured technology, the search for highly efficient and easily applicable materials is still ongoing. These properties made the efficiency of halide perovskites comparable with silicon solar cells for single junctions within a decade of research. However, the downside of halide perovskites are poor stability and lead toxicity for the most stable ones.
On the other hand, chalcogenide perovskites are one of the most promising absorber materials for the photovoltaic market, due to their elemental abundance and chemical stability against moisture and oxygen. In the search of the ultimate solar absorber material, combining the good optoelectronic properties of halide perovskites with the stability of chalcogenides could be the promising candidate.
Thus, this work investigates new techniques for the synthesis and design of these novel chalcogenide perovskites, that contain transition metals as cations, e.g., BaZrS3, BaHfS3, EuZrS3, EuHfS3 and SrHfS3. There are two stages in the deposition techniques of this study: In the first stage, the binary compounds are deposited via a solution processing method. In the second stage, the deposited materials are annealed in a chalcogenide atmosphere to form the perovskite structure by using solid-state reactions.
The research also focuses on the optimization of a generalized recipe for a molecular ink to deposit precursors of chalcogenide perovskites with different binaries. The implementation of the precursor sulfurization resulted in either binaries without perovskite formation or distorted perovskite structures, whereas some of these materials are reported in the literature as they are more favorable in the needle-like non-perovskite configuration.
Lastly, there are two categories for the evaluation of the produced materials: The first category is about the determination of the physical properties of the deposited layer, e.g., crystal structure, secondary phase formation, impurities, etc. For the second category, optoelectronic properties are measured and compared to an ideal absorber layer, e.g., band gap, conductivity, surface photovoltage, etc.
The negative impact of crude oil on the environment has led to a necessary transition toward alternative, renewable, and sustainable resources. In this regard, lignocellulosic biomass (LCB) is a promising renewable and sustainable alternative to crude oil for the production of fine chemicals and fuels in a so-called biorefinery process. LCB is composed of polysaccharides (cellulose and hemicellulose), as well as aromatics (lignin). The development of a sustainable and economically advantageous biorefinery depends on the complete and efficient valorization of all components. Therefore, in the new generation of biorefinery, the so-called biorefinery of type III, the LCB feedstocks are selectively deconstructed and catalytically transformed into platform chemicals. For this purpose, the development of highly stable and efficient catalysts is crucial for progress toward viability in biorefinery. Furthermore, a modern and integrated biorefinery relies on process and reactor design, toward more efficient and cost-effective methodologies that minimize waste. In this context, the usage of continuous flow systems has the potential to provide safe, sustainable, and innovative transformations with simple process integration and scalability for biorefinery schemes.
This thesis addresses three main challenges for future biorefinery: catalyst synthesis, waste feedstock valorization, and usage of continuous flow technology. Firstly, a cheap, scalable, and sustainable approach is presented for the synthesis of an efficient and stable 35 wt.-% Ni catalyst on highly porous nitrogen-doped carbon support (35Ni/NDC) in pellet shape. Initially, the performance of this catalyst was evaluated for the aqueous phase hydrogenation of LCB-derived compounds such as glucose, xylose, and vanillin in continuous flow systems. The 35Ni/NDC catalyst exhibited high catalytic performances in three tested hydrogenation reactions, i.e., sorbitol, xylitol, and 2-methoxy-4-methylphenol with yields of 82 mol%, 62 mol%, and 100 mol% respectively. In addition, the 35Ni/NDC catalyst exhibited remarkable stability over a long time on stream in continuous flow (40 h). Furthermore, the 35Ni/NDC catalyst was combined with commercially available Beta zeolite in a dual–column integrated process for isosorbide production from glucose (yield 83 mol%).
Finally, 35Ni/NDC was applied for the valorization of industrial waste products, namely sodium lignosulfonate (LS) and beech wood sawdust (BWS) in continuous flow systems. The LS depolymerization was conducted combining solvothermal fragmentation of water/alcohol mixtures (i.e.,methanol/water and ethanol/water) with catalytic hydrogenolysis/hydrogenation (SHF). The depolymerization was found to occur thermally in absence of catalyst with a tunable molecular weight according to temperature. Furthermore, the SHF generated an optimized cumulative yield of lignin-derived phenolic monomers of 42 mg gLS-1. Similarly, a solvothermal and reductive catalytic fragmentation (SF-RCF) of BWS was conducted using MeOH and MeTHF as a solvent. In this case, the optimized total lignin-derived phenolic monomers yield was found of 247 mg gKL-1.
Ultrathin, semi-permeable membranes are not only essential in natural systems (membranes of cells or organelles) but they are also important for applications (separation, filtering) in miniaturized devices. Membranes, integrated as diffusion barriers or filters in micron scale devices need to fulfill equivalent requirements as the natural systems, in particular mechanical stability and functionality (e.g. permeability), while being only tens of nm in thickness to allow fast diffusion times. Promising candidates for such membranes are polyelectrolyte multilayers, which were found to be mechanically stable, and variable in functionality. In this thesis two concepts to integrate such membranes in larger scale structures were developed. The first is based on the directed adhesion of polyelectrolyte hollow microcapsules. As a result, arrays of capsules were created. These can be useful for combinatorial chemistry or sensing. This concept was expanded to couple encapsulated living cells to the surface. The second concept is the transfer of flat freestanding multilayer membranes to structured surfaces. We have developed a method that allows us to couple mm2 areas of defect free film with thicknesses down to 50 nm to structured surfaces and to avoid crumpling of the membrane. We could again use this technique to produce arrays of micron size. The freestanding membrane is a diffusion barrier for high molecular weight molecules, while small molecules can pass through the membrane and thus allows us to sense solution properties. We have shown also that osmotic pressures lead to membrane deflection. That could be described quantitatively.
Interactions involving biological interfaces such as lipid-based membranes are of paramount importance for all life processes. The same also applies to artificial interfaces to which biological matter is exposed, for example the surfaces of drug delivery systems or implants. This thesis deals with the two main types of interface interactions, namely (i) interactions between a single interface and the molecular components of the surrounding aqueous medium and (ii) interactions between two interfaces. Each type is investigated with regard to an important scientific problem in the fields of biotechnology and biology:
1.) The adsorption of proteins to surfaces functionalized with hydrophilic polymer brushes; a process of great biomedical relevance in context with harmful foreign-body-response to implants and drug delivery systems.
2.) The influence of glycolipids on the interaction between lipid membranes; a hitherto largely unexplored phenomenon with potentially great biological relevance.
Both problems are addressed with the help of (quasi-)planar, lipid-based model surfaces in combination with x-ray and neutron scattering techniques which yield detailed structural insights into the interaction processes. Regarding the adsorption of proteins to brush-functionalized surfaces, the first scenario considered is the exposure of the surfaces to human blood serum containing a multitude of protein species. Significant blood protein adsorption was observed despite the functionalization, which is commonly believed to act as a protein repellent. The adsorption consists of two distinct modes, namely strong adsorption to the brush grafting surface and weak adsorption to the brush itself. The second aspect investigated was the fate of the brush-functionalized surfaces when exposed to aqueous media containing immune proteins (antibodies) against the brush polymer, an emerging problem in current biomedical applications. To this end, it was found that antibody binding cannot be prevented by variation of the brush grafting density or the polymer length. This result motivates the search for alternative, strictly non-antigenic brush chemistries. With respect to the influence of glycolipids on the interaction between lipid membranes, this thesis focused on the glycolipids’ ability to crosslink and thereby to tightly attract adjacent membranes. This adherence is due to preferential saccharide-saccharide interactions occurring among the glycolipid headgroups. This phenomenon had previously been described for lipids with special oligo-saccharide motifs. Here, it was investigated how common this phenomenon is among glycolipids with a variety of more abundant saccharide-headgroups. It was found that glycolipid-induced membrane crosslinking is equally observed for some of these abundant glycolipid types, strongly suggesting that this under-explored phenomenon is potentially of great biological relevance.
Interactions of the antimicrobial peptide Arenicin with amphiphiles at planar and curved interfaces
(2010)
We live in an era driven by fossil fuels. The prevailing climate change suggests that we have to significantly reduce greenhouse gas emissions. The only way forward is to use renewable energy sources. Among those, solar energy is a clean, affordable, and sustainable source of energy. It has the potential to satisfy the world’s energy demand in the future. However, there is a need to develop new materials that can make solar energy usable. Photovoltaics (PV) are devices that convert photon energy into electrical energy. The most commonly used solar cells are based on crystalline silicon. However, the fabrication process for silicon solar cells is technologically difficult and costly. Solar cells based on lead halide perovskites (PSCs) have emerged as a new candidate for PV applications since 2009. To date, PSCs have achieved 26% power-conversion-efficiency (PCE) for its single junction, and 33.7% PCE for tandem junction devices. However, there is still room for improvement in overall performance. The main challenge for the commercialization of this technology is the stability of the solar cells under operational conditions. Inorganic perovskite CsPbI3 has attracted researchers’ interest due to its stability at elevated temperatures, however, inorganic perovskites also have associated challenges, e.g. phase stability, larger voltage loss compared to their organic-inorganic hybrid counterparts, and interface energy misalignment. The most efficient inorganic perovskite solar cell is stable for up to a few hundred hours while the most stable device in the field of inorganic PSCs reported so far is at 17% PCE. This suggests the need for improvement of the interfaces for enhanced open circuit voltage (VOC), and optimization of the energy alignment at the interfaces. This dissertation presents the study on interfaces between the perovskite layer and hole transport layer (HTL) for stable CsPbI3 solar cells.
The first part of the thesis presents an investigation of the CsPbI3 film annealing environment and its subsequent effects on the perovskite/HTL interface dynamics. Thin films annealed in dry air were compared with thin films annealed in ambient air. Synchrotron-based hard X-ray spectroscopy (HAXPES) measurements reveal that annealing in ambient air does not have an adverse effect; instead, those samples undergo surface band bending. This surface band modification induces changes in interface charge dynamics and, consequently, an improvement in charge extraction at the interfaces. Further, transient surface photovoltage (tr-SPV) simulations show that air-annealed samples exhibit fewer trap states compared to samples annealed in dry air. Finally, by annealing the CsPbI3 films in ambient air, a PCE of 19.8% and Voc of 1.23 V were achieved for an n-i-p structured device.
Interface engineering has emerged as a strategy to extract the charge and optimize the energy alignment in perovskite solar cells (PSCs). An interface with fewer trap states and energy band levels closer to the selective contact helps to attain improved efficiencies in PSCs. The second part of the thesis presents a design for the CsPbI3/HTM interface. In this work, an interface between CsPbI3 perovskite and its hole selective contact N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine(Spiro-OMeTAD), realized by trioctylphosphine oxide (TOPO), a dipole molecule is introduced. On top of a perovskite film well-passivated by n-octyl ammonium Iodide (OAI), it created an upward surface band-bending at the interface byTOPO that optimizes energy level alignment and enhances the extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. In addition, the work also sheds light on the interfacial charge-selectivity and the long-term stability of CsPbI3 perovskite solar cells.
The third part of the thesis extends the previous studies to polymeric poly(3-hexylthiophene-2,5-diyl) (P3HT) as HTL. The CsPbI3/P3HT interface is critical due to high non-radiative recombination. This work presents a CsPbI3/P3HT interface modified with a long-chain alkyl halide molecule, n-hexyl trimethyl ammonium bromide (HTAB). This molecule largely passivates the CsPbI3 perovskite surface and improves the charge extraction across the interface. Consequently, a Voc of over 1.00 V and 14.2% PCE were achieved for CsPbI3 with P3HT as HTM.
Overall the results presented in this dissertation introduce and discuss methods to design and study the interfaces in CsPbI3-based solar cells. This study can pave the way for novel interface designs between CsPbI3 and HTM for charge extraction, efficiency and stability.
The goal of this work was to study the binding of ions to polymers and lipid bilayer membranes in aqueous solutions. In the first part of this work, the influence of various inorganic salts and polyelectrolytes on the structure of water was studied using Isothermal Titration Calorimetry (ITC). The heat of dilution of the salts was used as a scale of water structure making and breaking of the ions. The heats of dilution could be attributed to the Hofmeister Series. Following this, the binding of Ca2+ to poly(sodium acrylate) (NaPAA) was studied. ITC and a Ca2+ Ion Selective Electrode were used to measure the reaction enthalpy and binding isotherm. Binding of Ca2+ ions to PAA, was found to be highly endothermic and therefore solely driven by entropy. We then compared the binding of ions to the one-dimensional PAA polymer chain to the binding to lipid vesicles with the same functional groups. As for the polymer, Ca2+ binding was found to be endothermic. Binding of calcium to the lipid bilayer was found to be weaker than to the polymer. In the context of these experiments, it was shown that Ca2+ not only binds to charged but also to zwitterionic lipid vesicles. Finally, we studied the interaction of two salts, KCl and NaCl, to a neutral polymer gel, PNIPAAM, and to the ionic polymer PAA. Combining calorimetry and a potassium selective electrode we observed that the ions interact with both polymers, whether containing charges or not.
Health effects, attributed to the environmental pollution resulted from using solvents such as benzene, are relatively unexplored among petroleum workers, personal use, and laboratory researchers. Solvents can cause various health problems, such as neurotoxicity, immunotoxicity, and carcinogenicity. As such it can be absorbed via epidermal or respiratory into the human body resulting in interacting with molecules that are responsible for biochemical and physiological processes of the brain.
Owing to the ever-growing demand for finding a solution, an Ionic liquid can use as an alternative solvent. Ionic liquids are salts in a liquid state at low temperature (below 100 C), or even at room temperature. Ionic liquids impart a unique architectural platform, which has been interesting because of their unusual properties that can be tuned by simple ways such as mixing two ionic liquids.
Ionic liquids not only used as reaction solvents but they became a key developing for novel applications based on their thermal stability, electric conductivity with very low vapor pressure in contrast to the conventional solvents.
In this study, ionic liquids were used as a solvent and reactant at the same time for the novel nanomaterials synthesis for different applications including solar cells, gas sensors, and water splitting.
The field of ionic liquids continues to grow, and become one of the most important branches of science. It appears to be at a point where research and industry can work together in a new way of thinking for green chemistry and sustainable production.
Ionothermal carbon materials
(2016)
Alternative concepts for energy storage and conversion have to be developed, optimized and employed to fulfill the dream of a fossil-independent energy economy. Porous carbon materials play a major role in many energy-related devices. Among different characteristics, distinct porosity features, e.g., specific surface area (SSA), total pore volume (TPV), and the pore size distribution (PSD), are important to maximize the performance in the final device. In order to approach the aim to synthesize carbon materials with tailor-made porosity in a sustainable fashion, the present thesis focused on biomass-derived precursors employing and developing the ionothermal carbonization.
During the ionothermal carbonization, a salt melt simultaneously serves as solvent and porogen. Typically, eutectic mixtures containing zinc chloride are employed as salt phase. The first topic of the present thesis addressed the possibility to precisely tailor the porosity of ionothermal carbon materials by an experimentally simple variation of the molar composition of the binary salt mixture. The developed pore tuning tool allowed the synthesis of glucose derived carbon materials with predictable SSAs in the range of ~ 900 to ~ 2100 m2 g-1. Moreover, the nucleobase adenine was employed as precursor introducing nitrogen functionalities in the final material. Thereby, the chemical properties of the carbon materials are varied leading to new application fields. Nitrogen doped carbons (NDCs) are able to catalyze the oxygen reduction reaction (ORR) which takes place on the cathodic site of a fuel cell. The herein developed porosity tailoring allowed the synthesis of adenine derived NDCs with outstanding SSAs of up to 2900 m2 g-1 and very large TPV of 5.19 cm3 g-1. Furthermore, the influence of the porosity on the ORR could be directly investigated enabling the precise optimization of the porosity characteristics of NDCs for this application. The second topic addressed the development of a new method to investigate the not-yet unraveled mechanism of the oxygen reduction reaction using a rotating disc electrode setup. The focus was put on noble-metal free catalysts. The results showed that the reaction pathway of the investigated catalysts is pH-dependent indicating different active species at different pH-values. The third topic addressed the expansion of the used salts for the ionothermal approach towards hydrated calcium and magnesium chloride. It was shown that hydrated salt phases allowed the introduction of a secondary templating effect which was connected to the coexistence of liquid and solid salt phases. The method enabled the synthesis of fibrous NDCs with SSAs of up to 2780 m2 g-1 and very large TPV of 3.86 cm3 g-1. Moreover, the concept of active site implementation by a facile low-temperature metalation employing the obtained NDCs as solid ligands could be shown for the first time in the context of ORR.
Overall, the thesis may pave the way towards highly porous carbon with tailor-made porosity materials prepared by an inexpensive and sustainable pathway, which can be applied in energy related field thereby supporting the needed expansion of the renewable energy sector.
Förster Resonance Energy Transfer (FRET) plays an important role for biochemical applications such as DNA sequencing, intracellular protein-protein interactions, molecular binding studies, in vitro diagnostics and many others. For qualitative and quantitative analysis, FRET systems are usually assembled through molecular recognition of biomolecules conjugated with donor and acceptor luminophores. Lanthanide (Ln) complexes, as well as semiconductor quantum dot nanocrystals (QD), possess unique photophysical properties that make them especially suitable for applied FRET. In this work the possibility of using QD as very efficient FRET acceptors in combination with Ln complexes as donors in biochemical systems is demonstrated. The necessary theoretical and practical background of FRET, Ln complexes, QD and the applied biochemical models is outlined. In addition, scientific as well as commercial applications are presented. FRET can be used to measure structural changes or dynamics at distances ranging from approximately 1 to 10 nm. The very strong and well characterized binding process between streptavidin (Strep) and biotin (Biot) is used as a biomolecular model system. A FRET system is established by Strep conjugation with the Ln complexes and QD biotinylation. Three Ln complexes (one with Tb3+ and two with Eu3+ as central ion) are used as FRET donors. Besides the QD two further acceptors, the luminescent crosslinked protein allophycocyanin (APC) and a commercial fluorescence dye (DY633), are investigated for direct comparison. FRET is demonstrated for all donor-acceptor pairs by acceptor emission sensitization and a more than 1000-fold increase of the luminescence decay time in the case of QD reaching the hundred microsecond regime. Detailed photophysical characterization of donors and acceptors permits analysis of the bioconjugates and calculation of the FRET parameters. Extremely large Förster radii of more than 100 Å are achieved for QD as acceptors, considerably larger than for APC and DY633 (ca. 80 and 60 Å). Special attention is paid to interactions with different additives in aqueous solutions, namely borate buffer, bovine serum albumin (BSA), sodium azide and potassium fluoride (KF). A more than 10-fold limit of detection (LOD) decrease compared to the extensively characterized and frequently used donor-acceptor pair of Europium tris(bipyridine) (Eu-TBP) and APC is demonstrated for the FRET system, consisting of the Tb complex and QD. A sub-picomolar LOD for QD is achieved with this system in azide free borate buffer (pH 8.3) containing 2 % BSA and 0.5 M KF. In order to transfer the Strep-Biot model system to a real-life in vitro diagnostic application, two kinds of imunnoassays are investigated using human chorionic gonadotropin (HCG) as analyte. HCG itself, as well as two monoclonal anti-HCG mouse-IgG (immunoglobulin G) antibodies are labeled with the Tb complex and QD, respectively. Although no sufficient evidence for FRET can be found for a sandwich assay, FRET becomes obvious in a direct HCG-IgG assay showing the feasibility of using the Ln-QD donor-acceptor pair as highly sensitive analytical tool for in vitro diagnostics.
The doctoral thesis presented provides a comprehensive view of laser-based ablation techniques promoted to new fields of operation, including, but not limited to, size, composition, and concentration analyses. It covers various applications of laser ablation techniques over a wide range of sizes, from single molecules all the way to aerosol particles. The research for this thesis started with broadening and deepening the field of application and the fundamental understanding of liquid-phase IR-MALDI. Here, the hybridization of ion mobility spectrometry and microfluidics was realized by using IR-MALDI as the coupling technique for the first time. The setup was used for monitoring the photocatalytic performance of the E-Z isomerization of olefins. Using this hybrid, measurement times were so drastically reduced that such photocatalyst screenings became a matter of minutes rather than hours. With this on hand, triple measurements screenings could not only be performed within ten minutes, but also with a minimum amount of resources highlighting its potential as a green chemistry alternative to batch-sized reactions. Along the optimizing process of the IR-MALDI source for microfluidics came its application for another liquid sample supply method, the hanging drop. This demarcated one of the first applications of IR-MALDI for the charging of sub-micron particles directly from suspensions via their gas-phase transfer, followed by their characterization with differential mobility analysis. Given the high spectral quality of the data up to octuply charged particles became experimentally accessible, this laid the foundation for deriving a new charge distribution model for IR-MALDI in that size regime. Moving on to even larger analyte sizes, LIBS and LII were employed as ablation techniques for the solid phase, namely the aerosol particles themselves. Both techniques produce light-emitting events and were used to quantify and classify different aerosols. The unique configuration of stroboscopic imaging, photoacoustics, LII, and LIBS measurements opened new realms for analytical synergies and their potential application in industry. The concept of using low fluences, below 100 J/cm2, and high repetition rates of up to 500 Hz for LIBS makes for an excellent phase-selective LIBS setup. This concept was combined with a new approach to the photoacoustic normalization of LIBS. Also, it was possible to acquire statistically relevant amounts of data in a matter of seconds, showing its potential as a real-time optimization technique. On the same time axis, but at much lower fluences, LII was used with a similar methodology to quickly quantify and classify airborne particles of different compositions. For the first time, aerosol particles were evaluated on their LII susceptibility by using a fluence screening approach.
Hybrid organic-inorganic perovskites have attracted attention in recent years, caused by the incomparable increase in efficiency in energy convergence, which implies the application as an absorber material for solar cells. A disadvantage of these materials is, among others, the instability to moisture and UV-radiation. One possible solution for these problems is the reduction of the size towards the nano world. With that nanosized perovskites are showing superior stability in comparison to e.g. perovskite layers. Additionally to this the nanosize even enables stable perovskite structures, which could not be achieved otherwise at
room temperature.
This thesis is separated into two major parts. The separation is done by the composition and the band gap of the material and at the same time the shape and size of the nanoparticles. Here the division is made by the methylammonium lead tribromide nanoplatelets and the caesium lead triiodide nanocubes.
The first part is focusing on the hybrid organic-inorganic perovskite (methylammonium lead tribromide) nanoplatelets with a band gap of 2.35 eV and their thermal behaviour. Due to the challenging character of this material, several analysis methods are used to investigate the sub nano and nanostructures under the influence of temperature. As a result, a shift of phase-transition temperatures towards higher temperatures is observed. This unusual behaviour can be explained by the ligand, which is incorporated in the perovskite outer structure and adds phase-stability into the system.
The second part of this thesis is focusing on the inorganic caesium lead triiodide nanocubes with a band gap of 1.83 eV. These nanocrystals are first investigated and compared by TEM, XRD and other optical methods. Within these methods, a cuboid and orthorhombic structure are revealed instead of the in literature described cubic shape and structure. Furthermore, these cuboids are investigated towards their self-assembly on a substrate. Here a high degree in self-assembly is shown. As a next step, the ligands of the nanocuboids are exchanged against other ligands to increase the charge carrier mobility. This is further investigated by the above-mentioned methods. The last section is dealing with the enhancement of the CsPbI3 structure, by incorporating potassium in the crystal structure. The results are suggesting here an increase in stability.
The development of novel programmable materials aiming to control friction in real-time holds potential to facilitate innovative lubrication solutions for reducing wear and energy losses. This work describes the integration of light-responsiveness into two lubricating materials, silicon oils and polymer brush surfaces.
The first part focusses on the assessment on 9-anthracene ester-terminated polydimethylsiloxanes (PDMS-A) and, in particular, on the variability of rheological properties and the implications that arise with UV-light as external trigger. The applied rheometer setup contains an UV-transparent quartz-plate, which enables radiation and simultaneous measurement of the dynamic moduli. UV-A radiation (354 nm) triggers the cycloaddition reaction between the terminal functionalities of linear PDMS, resulting in chain extension. The newly-formed anthracene dimers cleave by UV-C radiation (254 nm) or at elevated temperatures (T > 130 °C). The sequential UV-A radiation and thermal reprogramming over three cycles demonstrate high conversions and reproducible programming of rheological properties. In contrast, the photochemical back reaction by UV-C is incomplete and can only partially restore the initial rheological properties. The dynamic moduli increase with each cycle in photochemical programming, presumably resulting from a chain segment re-arrangement as a result of the repeated partial photocleavage and subsequent chain length-dependent dimerization. In addition, long periods of radiation cause photooxidative degradation, which damages photo-responsive functions and consequently reduces the programming range. The absence of oxygen, however, reduces undesired side reactions. Anthracene-functionalized PDMS and native PDMS mix depending on the anthracene ester content and chain length, respectively, and allow fine-tuning of programmable rheological properties. The work shows the influence of mixing conditions during the photoprogramming step on the rheological properties, indicating that material property gradients induced by light attenuation along the beam have to be considered. Accordingly, thin lubricant films are suggested as potential application for light-programmable silicon fluids.
The second part compares strategies for the grafting of spiropyran (SP) containing copolymer brushes from Si wafers and evaluates the light-responsiveness of the surfaces. Pre-experiments on the kinetics of the thermally initiated RAFT copolymerization of 2-hydroxyethyl acrylate (HEA) and spiropyran acrylate (SPA) in solution show, first, a strong retardation by SP and, second, the dependence of SPA polymerization on light. Surprisingly, the copolymerization of SPA is inhibited in the dark. These findings contribute to improve the synthesis of polar, spiropyran-containing copolymers. The comparison between initiator systems for the grafting-from approach indicates PET-RAFT superior to thermally initiated RAFT, suggesting a more efficient initiation of surface-bound CTA by light. Surface-initiated polymerization via PET-RAFT with an initiator system of EosinY (EoY) and ascorbic acid (AscA) facilitates copolymer synthesis from HEA and 5-25 mol% SPA. The resulting polymer film with a thickness of a few nanometers was detected by atomic force microscopy (AFM) and ellipsometry. Water contact angle (CA) measurements demonstrate photo-switchable surface polarity, which is attributed to the photoisomerization between non-polar spiropyran and zwitterionic merocyanine isomer. Furthermore, the obtained spiropyran brushes show potential for further studies on light-programmable properties. In this context, it would be interesting to investigate whether swollen spiropyran-containing polymers change their configuration and thus their film thickness under the influence of light. In addition, further experiments using an AFM or microtribometer should evaluate whether light-programmable solvation enables a change in frictional properties between polymer brush surfaces.
Lignin valorization
(2017)
The topic of this project is the use of lignin as alternative source of aromatic building blocks and oligomers to fossil feedstocks. Lignin is known as the most abundant aromatic polymer in nature and is isolated from the lignocellulosic component of plants by different possible extraction treatments. Both the biomass source and the extraction method affect the structure of the isolated lignin, therefore influencing its further application. Lignin was extracted from beech wood by two different hydrothermal alkaline treatments, which use NaOH and Ba(OH)2 as base and by an acid-catalyzed organosolv process. Moreover, lignin was isolated from bamboo, beech wood and coconut by soda treatment of the biomasses. A comparison of the structural features of such isolated lignins was performed through the use of a wide range of analytical methods. Alkaline lignins resulted in a better candidate as carbon precursor and macromonomers for the synthesis of polymer than organosolv lignin. In fact, alkaline lignins showed higher residual mass after carbonization and higher content of the reactive hydroxy functionalities. In contrast, the lignin source turned out to slightly affect the lignin hydroxyl content.
One of the most common lignin modifications is its deconstruction to obtain aromatic molecules, which can be used as starting materials for the synthesis of fine chemicals. Lignin deconstruction leads to a complex mixture of aromatic molecules. A gas chromatographic analytical method was developed to characterize the mixture of products obtained by lignin deconstruction via heterogeneous catalytic hydrogenolysis. The analytical protocol allowed the quantification of three main groups of molecules by means of calibration curves, internal standard and a preliminary silylation step of the sample. The analytical method was used to study the influence of the hydrogenolysis catalyst, temperature and system (flow and batch reactor) on the yield and selectivity of the aromatic compounds.
Lignin extracted from beech wood by a hydrothermal process using Ba(OH)2 as base, was functionalized by aromatic nitration in order to add nitrogen functionalities. The final goal was the synthesis of a nitrogen doped carbon. Nitrated lignin was reduced to the amino form in order to compare the influence of different nitrogen functionalities on the porosity of the final carbon. The carbons were obtained by ionothermal treatment of the precursors in the presence of the eutectic salt mixture KCl/ZnCl2 Such synthesized carbons showed micro-, macro- and mesoporosity and were tested for their electrocatalytic activity towards the oxygen reduction reaction. Mesoporous carbon derived from nitro lignin displayed the highest electrocatalytic activity.
Lignins isolated from coconut, beech wood and bamboo were used as macromonomers for the synthesis of biobased polyesters. A condensation reaction was performed between lignin and a hyper branched poly(ester-amine), previously obtained by condensation of triethanolamine and adipic acid. The influence of the lignin source and content on the thermochemical and mechanical properties of the final material was investigated. The prepolymer showed adhesive properties towards aluminum and its shear strength was therefore measured. The gluing properties of such synthesized glues turned out to be independent from the lignin source but affected by the amount of lignin in the final material.
This work shows that, although still at a laboratory scale, the valorization of lignin can overcome the critical issues of lignin´s structure variability and complexity.
Lipid monolayers coupled to polyelectrolyte multilayers : stability, dynamics and interactions
(2003)
Many-electron dynamics in molecules by means of time-dependent configuration interaction methods
(2007)
Chitooligosaccharides are composed of glycosamin and N-acetylglycisamin residues. Gel permeations chromatography is employed for the separation of oligomers, cation exchange chromatography is used for the separation of homologes and isomers. Trideuterioacetylation of the chitooligosaccharides followed by MALDI-TOF mass spectrometry allowes for the quantitation of mixtures of homologes. vMALDI LTQ multiple-stage MS is employed for quantitative sequencing of complex mixtures of heterochitooligosaccharides. Pure homologes and isomers are applied to biological assays. Chitooligosaccahrides form high-affinity non-covalent complexes with HC gp-39 (human cartilage glycoprotein of 39 kDa). The affinity of the chitooligosaccharides depends on DP, FA and the sequence of glycosamin and N-acetylglycosamin moieties. (+)nanoESI Q TOF MS/MS is used for identification of a high-affinity binding chitooligosaccharide of a non-covalent chitinase B - chitooligosaccharide complex. DADAA is identified as the heterochitoisomer binding with highest affinity and biostability to HC gp-39. Fluorescence based enzyme assays confirm the results.
Mesoporous organosilica materials with amine functions : surface characteristics and chirality
(2005)
In this work mesoporous organisilica materials are synthesized through the silica sol-gel process. For this a new class of precursors which are also surfactant are synthesized and self-assembled. This leads to a high surface area functionality which is analysized with copper (II) and water adsorption.
Magnetotactic bacteria comprise a heterogeneous group of Gram negative bacteria which share the ability to synthesise intracellular magnetic nanoparticles surrounded by a lipid bilayer, known as magnetosomes, which are arranged in linear chains. The bacteria exert a unique level of control onto the biomineralization of these nanoparticles, which is seen in the controlled size and shape they have. These characteristics have attracted great attention on understanding the process by which the bacteria synthesise the magnetosomes. Moreover, the magnetosome chain impart the bacteria with a net magnetic dipole which makes them susceptible to interact with magnetic fields and thus orient with the Earth’s magnetic field. This feature has attracted as well much interest to understand how the swimming motility of these microorganisms is affected by the presence of magnetic fields. Most of the studies performed in these bacteria so far have been conducted in the traditional manner using large populations of cells. Such studies have the disadvantage of averaging many different individuals with heterogeneous behaviours and fail to consider individual variations. In addition, in large populations each bacterium will be subjected to a different microenvironment that will influence the bacterial behaviour, but which cannot be defined using these traditional methods. In this thesis, different microfluidic platforms are proposed to overcome these limitations and to offer the possibility to study magnetotactic bacteria in defined environments and down to a single-cell resolution. First, a sediment-like microfluidic platform is presented with the purpose of mimicking the porous environment they bacteria naturally dwell in. The platform allows to observe via transmitted light microscopy that bacterial navigation in crowded environments is enhanced by the Earth’s magnetic field strengths (B = 50 μT) rather than by null (B = 0 μT) or higher magnetic fields (B = 500 μT). Second, a microfluidic system to confine single-bacterial cells in physically defined environments is presented. The system allows to study via transmitted light microscopy the interplay between wall curvature, magnetic fields and bacterial speed affect the motion of a confined bacterium, and shows how bacterial trajectories depend on those three parameters. Third, a microfluidic platform to conduct semi in vivo magnetosome nucleation with a single-cell resolution via X-ray fluorescence is fabricated. It is shown that signal arising from magnetosome full chains can be observed individually in each bacterium. Finally, the iron uptake kinetics of a single bacterium are studied via a fluorescent reporter through confocal microscopy. Two different approaches are used for this: one of the previously mentioned platforms, as well as giant lipid vesicles. It is observed how iron uptake rates vary between cells, as well as how these rates are consistent with magnetosome formation taking place within some hours. The present thesis shows therefore how microfluidic technologies can be implemented for the study of magnetotactic bacteria at different degrees, and the level of resolution that can be attained by going into the single- cell scale.
Mixed 1,2-Dümine-1,2-Dithiolate Ligand Complexes : Structure, Proberties and EPR Spectroscopy
(2009)
The interactions between peptides and lipids are of fundamental importance in the functioning of numerous membrane-mediated biochemical processes including antimicrobial peptide action, hormone-receptor interactions, drug bioavailability across the blood-brain barrier and viral fusion processes. Alteration of peptide structure could be a cause of many diseases. Biological membranes are complex systems, therefore simplified models may be introduced in order to understand processes occurring in nature. The lipid monolayers at the air/water interface are suitable model systems to mimic biological membranes since many parameters can be easily controlled. In the present work the lipid monolayers were used as a model membrane and their interactions with two different peptides B18 and Amyloid beta (1-40) peptide were investigated. B18 is a synthetic peptide that binds to lipid membranes that leads to the membrane fusion. It was demonstrated that it adopts different structures in the aqueous solutions and in the membrane interior. It is unstructured in solutions and forms alpha-helix at the air/water interface or in the membrane bound state. The peptide has affinity to the negatively charged lipids and even can fold into beta-sheet structure in the vicinity of charged membranes at high peptide to lipid ratio. It was elucidated that in the absence of electrostatic interactions B18 does not influence on the lipid structure, whereas it provides partial liquidization of the negatively charged lipids. The understanding of mechanism of the peptide action in model system may help to develop the new type of antimicrobial peptides as well as it can shed light on the general mechanisms of peptide/membrane binding. The other studied peptide - Amyloid beta (1-40) peptide, which is the major component of amyloid plaques found in the brain of patients with Alzheimer's disease. Normally the peptide is soluble and is not toxic. During aging or as a result of the disease it aggregates and shows a pronounced neurotoxicity. The peptide aggregation involves the conformational transition from a random coil or alpha-helix to beta-sheets. Recently it was demonstrated that the membrane can play a crucial role for the peptide aggregation and even more the peptide can cause the change in the cell membranes that leads to a neuron death. In the present studies the structure of the membrane bound Amyloid beta peptide was elucidated. It was found that the peptide adopts the beta-sheet structure at the air/water interface or being adsorbed on lipid monolayers, while it can form alpha-helical structure in the presence of the negatively charged vesicles. The difference between the monolayer system and the bulk system with vesicles is the peptide to lipid ratio. The peptide adopts the helical structure at low peptide to lipid ratio and folds into beta-sheet at high ratio. Apparently, Abeta peptide accumulation in the brain is concentration driven. Increasing concentration leads to a change in the lipid to peptide ratio that induces the beta-sheet formation. The negatively charged lipids can act as seeds in the plaque formation, the peptide accumulates on the membrane and when the peptide to lipid ratio increases it the peptide forms toxic beta-sheet containing aggregates.
Abstract. Catalysis is one of the most effective tools for the highly efficient assembly of complex molecular structures. Nevertheless, it is mainly represented by transition metal-based catalysts and typically is an energy consuming process. Therefore, photocatalysis utilizing solar energy is one of the appealing approaches to overcome these problems. A great alternative to classic transition metal-based photocatalysts, carbon nitrides, a group of organic polymeric semiconductors, have already shown their efficiency in water splitting, CO2 reduction, and organic pollutants degradation. However, these materials have also a great potential for the use in functionalization of complex organic molecules for synthetic needs as it was shown in recent years.
This work addresses the challenge to develop efficient system for heterogeneous organic photocatalysis, employing cheap and environmentally benign photocatalysts – carbon nitrides. Herein, fundamental properties of semiconductors are studied from the organic chemistry standpoint; the inherent properties of carbon nitrides, such as ability to accumulate electrons, are deeply investigated and their effect on the reaction outcome is established. Thus, understanding of the electron charging processes allowed for the synthesis of otherwise hardly-achieved diazetidines-1,3 by tetramerization of benzylamines. Furthermore, the high electron capacity of Potassium Poly(heptazine imide)s (K-PHI) made possible a multi-electron reduction of aromatic nitro compounds to bare or formylated anilines. Additionally, two deep eutectic solvents (DES) were designed as a sustainable reaction media and reducing reagent for this reaction. Eventually, the high oxidation ability of carbon nitride K-PHI is employed in a challenging reaction of halide anion oxidation (Cl―, Br―) to accomplish electrophilic substitution in aromatic ring. The possibility to utilize NaCl solution (seawater mimetic) for the chlorination of electron rich arenes was shown. Eventually, light itself is used as a tool in a chromoselective photocatalytic oxidation of aromatic thiols and thioacetatas to three different compounds, using UV, blue, and red LEDs.
All in all, the work enhances understanding the mechanism of heterogeneous photocatalysis in synthetic organic reactions and therefore, is a step forward to the sustainable methods of synthesis in organic chemistry.
Within this work, three physicochemical methods for the hydrophobization of initially hydrophilic solid particles are investigated. The modified particles are then used for the stabilization of oil-in-water (o/w) emulsions. For all introduced methods electrostatic interactions between strongly or weakly charged groups in the system are es-sential. (i) Short chain alkylammonium bromides (C4 – C12) adsorb on oppositely charged solid particles. Macroscopic contact angle measurements of water droplets under air and hexane on flat silica surfaces in dependency of the surface charge density and alkylchain-length allow the calculation of the surface energy and give insights into the emulsification properties of solid particles modified with alkyltrimethylammonium bromides. The measure-ments show an increase of the contact angle with increasing surface charge density, due to the enhanced adsorp-tion of the oppositely charged alkylammonium bromides. Contact angles are higher for longer alkylchain lengths. The surface energy calculations show that in particular the surface-hexane or surface-air interfacial en-ergy is being lowered upon alkylammonium adsorption, while a significant increase of the surface-water interfa-cial energy occurs only at long alkyl chain lengths and high surface charge densities. (ii) The thickness and the charge density of an adsorbed weak polyelectrolyte layer (e.g. PMAA, PAH) influence the wettability of nanoparticles (e.g. alumina, silica, see Scheme 1(b)). Furthermore, the isoelectric point and the pH range of colloidal stability of particle-polyelectrolyte composites depend on the thickness of the weak polye-lectrolyte layer. Silica nanoparticles with adsorbed PAH and alumina nanoparticles with adsorbed PMAA be-come interfacially active and thus able to stabilize o/w emulsions when the degree of dissociation of the polye-lectrolyte layer is below 80 %. The average droplet size after emulsification of dodecane in water depends on the thickness and the degree of dissociation of the adsorbed PE-layer. The visualization of the particle-stabilized o/w emulsions by cryogenic SEM shows that for colloidally stable alumina-PMAA composites the oil-water interface is covered with a closely packed monolayer of particles, while for the colloidally unstable case closely packed aggregated particles deposit on the interface. (iii) By emulsifying a mixture of the corrosion inhibitor 8-hydroxyquinoline (8-HQ) and styrene with silica nanoparticles a highly stable o/w emulsion can be obtained in a narrow pH window. The amphoteric character of 8-HQ enables a pH dependent electrostatic interaction with silica nanoparticles, which can render them interfa-cially active. Depending on the concentration and the degree of dissociation of 8-HQ the adsorption onto silica results from electrostatic or aromatic interactions between 8-HQ and the particle-surface. At intermediate amounts of adsorbed 8-HQ the oil wettability of the particles becomes sufficient for stabilizing o/w emulsions. Cryogenic SEM visualization shows that the particles arrange then in a closely packed shell consisting of partly of aggregated domains on the droplet interface. For further increasing amounts of adsorbed 8-HQ the oil wet-tability is reduced again and the particles ability to stabilize emulsions decreases. By the addition of hexadecane to the oil phase the size of the droplets can be reduced down to 200 nm by in-creasing the silica mass fraction. Subsequent polymerization produces corrosion inhibitor filled (20 wt-%) poly-styrene-silica composite particles. The measurement of the release of 8-hydroxyquinoline shows a rapid increase of 8-hydroxyquinoline in a stirred aqueous solution indicating the release of the total content in less than 5 min-utes. The method is extended for the encapsulation of other organic corrosion inhibitors. The silica-polymer-inhibitor composite particles are then dispersed in a water based alkyd emulsion, and the dispersion is used to coat flat aluminium substrates. After drying and cross-linking the polmer-film Confocal Laser Scanning Micros-copy is employed revealing a homogeneous distribution of the particles in the film. Electrochemical Impedance Spectroscopy in aqueous electrolyte solutions shows that films with aggregated particle domains degrade with time and don’t provide long-term corrosion protection of the substrate. However, films with highly dispersed particles have high barrier properties for corrosive species. The comparison of films containing silica-polystyrene composite particles with and without 8-hydroxyquinoline shows higher electrochemical impedances when the inhibitor is present in the film. By applying the Scanning Vibrating Electrode Technique the localized corrosion rate in the fractured area of scratched polymer films containing the silica-polymer-inhibitor composite particles is studied. Electrochemical corrosion cannot be suppressed but the rate is lowered when inhibitor filled composite particles are present in the film. By depositing six polyelectrolyte layers on particle stabilized emulsion droplets their surface morphology changes significantly as shown by SEM visualization. When the oil wettability of the outer polyelectrolyte layer increases, the polyelectrolyte coated droplets can act as emulsion stabilizers themselves by attaching onto bigger oil droplets in a closely packed arrangement. In the presence of 3 mM LaCl3 8-HQ hydrophobized silica particles aggregate strongly on the oil-water inter-face. The application of an ultrasonic field can remove two dimensional shell-compartments from the droplet surface, which are then found in the aqueous bulk phase. Their size ranges up to 1/4th of the spherical particle shell.
Under the in vivo condition, a cell is continually interacting with its surrounding microenvironment, which is composed of its neighboring cells and the extracellular matrix (ECM). These components generate and transmit the microenvironmental signals to regulate the fate and function of the target cells. Except the signals from the microenvironment, stimuli from the ambient environment, such as temperature changes, also play an important in modulating the cell behaviors, which are considered as regulators from the macroenvironment. In this regard, recapitulation of these environmental factors to steer cell function will be of crucial importance for therapeutic purposes and tissue regeneration. Although the role of a variety of environmental factors has been evaluated, it is still challenging to identify and provide the appropriate factors, which are required for optimizing the survival of cells and for ensuring effective cell functions.
Thus, in vitro recreating the environmental factors that are present in the extracellular environment would help to understand the mechanism of how cells sense and process those environmental signals. In this context, this thesis is aimed to harness these environmental parameters to guide cell responses. Here, human induced pluripotent stem cells (hiPSCs) and human keratinocytes (KTCs), HaCaT cells, were used to investigate the impact of signals from the microenvironment or stimuli from the macroenvironment.
Firstly, polydopamine (PDA) or chitosan (CS) modifications were applied to generate different substrate surfaces for hiPSCs and KTCs (Chapter 4 to Chapter 6). Our results showed that the PDA modification was efficient to increase the cell-substrate adhesion and consequently promoted cell spreading. While CS modification was able to decrease the cell-substrate adhesion and enhance the cell-cell interaction, which enabled the morphology shift from monolayered cells to multicellular spheroids. The quantitative result was acquired using the atomic force microscopy (AFM)-based single-cell force spectroscopy. The balance between the cell-substrate and cell-cell adhesion yielded a net force, which determined the preference of the cell to adhere to its neighboring cells or to the substrate. The difference in the adhesive behaviors further affected the cellular function, such as the proliferation and differentiation potential of both hiPSCs and HaCaT cells.
Next, the cyclic temperature changes (ΔT) were selected here to study the influence of macroenvironmental stimuli on hiPSCs and KTCs (Chapter 7 and Chapter 8). The macroenvironmental temperature ranging from 10.0 ± 0.1 °C to 37.0 ± 0.1 °C was achieved using a thermal chamber equipped with a temperature controller. This temperature range was selected to explore the responses of hiPSCs to the extreme environments, while a temperature variation between 25.0 ± 0.1 °C and 37.0 ± 0.1 °C was applied to mimic the ambient temperature variations experienced by the skin epithelial KTCs. The ΔT led to cell stiffening in both hiPSCs and HaCaT cells in a cytoskeleton-dependent manner, which was measured by AFM. Specifically, in hiPSCs, the cell stiffening was resulted from the rearrangement of the actin skeleton; in HaCaT cells, was due to the difference of the Keratin (KRT) filaments. Except for inducing cell hardening, ΔT also caused differences in the protein expression profiles in hiPSCs or HaCaT cells, compared to those without ΔT treatment, which might be attributed to the alterations in their cytoskeleton structures.
To sum up, the results of the thesis demonstrated how individual factors from the micro-/macro-environment can be harnessed to modulate the behaviors of hiPSCs and HaCaT cells. Engineering the microenvironmental cues using surface modification and exploiting the macroenvironmental stimuli through temperature control were identified as precise and potent approaches to steer hiPSC and HaCaT cell behaviors. The application of AFM served as a non-invasive and real-time monitoring platform to trace the change in cell topography and mechanics induced by the environmental signals, which provide novel insights into the cell-environment interactions.
Polymeric films and coatings derived from semi-crystalline oligomers are of relevance for medical and pharmaceutical applications. In this context, the material surface is of particular importance, as it mediates the interaction with the biological system. Two dimensional (2D) systems and ultrathin films are used to model this interface. However, conventional techniques for their preparation, such as spin coating or dip coating, have disadvantages, since the morphology and chain packing of the generated films can only be controlled to a limited extent and adsorption on the substrate used affects the behavior of the films. Detaching and transferring the films prepared by such techniques requires additional sacrificial or supporting layers, and free-standing or self supporting domains are usually of very limited lateral extension. The aim of this thesis is to study and modulate crystallization, melting, degradation and chemical reactions in ultrathin films of oligo(ε-caprolactone)s (OCL)s with different end-groups under ambient conditions. Here, oligomeric ultrathin films are assembled at the air-water interface using the Langmuir technique. The water surface allows lateral movement and aggregation of the oligomers, which, unlike solid substrates, enables dynamic physical and chemical interaction of the molecules. Parameters like surface pressure (π), temperature and mean molecular area (MMA) allow controlled assembly and manipulation of oligomer molecules when using the Langmuir technique. The π-MMA isotherms, Brewster angle microscopy (BAM), and interfacial infrared spectroscopy assist in detecting morphological and physicochemical changes in the film. Ultrathin films can be easily transferred to the solid silicon surface via Langmuir Schaefer (LS) method (horizontal substrate dipping). Here, the films transferred on silicon are investigated using atomic force microscopy (AFM) and optical microscopy and are compared to the films on the water surface.
The semi-crystalline morphology (lamellar thicknesses, crystal number densities, and lateral crystal dimensions) is tuned by the chemical structure of the OCL end-groups (hydroxy or methacrylate) and by the crystallization temperature (Tc; 12 or 21 °C) or MMAs. Compression to lower MMA of ~2 Å2, results in the formation of a highly crystalline film, which consists of tightly packed single crystals. Preparation of tightly packed single crystals on a cm2 scale is not possible by conventional techniques. Upon transfer to a solid surface, these films retain their crystalline morphology whereas amorphous films undergo dewetting.
The melting temperature (Tm) of OCL single crystals at the water and the solid surface is found proportional to the inverse crystal thickness and is generally lower than the Tm of bulk PCL. The impact of OCL end-groups on melting behavior is most noticeable at the air-solid interface, where the methacrylate end-capped OCL (OCDME) melted at lower temperatures than the hydroxy end-capped OCL (OCDOL). When comparing the underlying substrate, melting/recrystallization of OCL ultrathin films is possible at lower temperatures at the air water interface than at the air-solid interface, where recrystallization is not visible. Recrystallization at the air-water interface usually occurs at a higher temperature than the initial Tc.
Controlled degradation is crucial for the predictable performance of degradable polymeric biomaterials. Degradation of ultrathin films is carried out under acidic (pH ~ 1) or enzymatic catalysis (lipase from Pseudomonas cepcia) on the water surface or on a silicon surface as transferred films. A high crystallinity strongly reduces the hydrolytic but not the enzymatic degradation rate. As an influence of end-groups, the methacrylate end-capped linear oligomer, OCDME (~85 ± 2 % end-group functionalization) hydrolytically degrades faster than the hydroxy end capped linear oligomer, OCDOL (~95 ± 3 % end-group functionalization) at different temperatures. Differences in the acceleration of hydrolytic degradation of semi-crystalline films were observed upon complete melting, partial melting of the crystals, or by heating to temperatures close to Tm. Therefore, films of densely packed single crystals are suitable as barrier layers with thermally switchable degradation rates.
Chemical modification in ultrathin films is an intricate process applicable to connect functionalized molecules, impart stability or create stimuli-sensitive cross-links. The reaction of end-groups is explored for transferred single crystals on a solid surface or amorphous monolayer at the air-water interface. Bulky methacrylate end-groups are expelled to the crystal surface during chain-folded crystallization. The density of end-groups is inversely proportional to molecular weight and hence very pronounced for oligomers. The methacrylate end-groups at the crystal surface, which are present at high concentration, can be used for further chemical functionalization. This is demonstrated by fluorescence microscopy after reaction with fluorescein dimethacrylate. The thermoswitching behavior (melting and recrystallization) of fluorescein functionalized single crystals shows the temperature-dependent distribution of the chemically linked fluorescein moieties, which are accumulated on the surfaces of crystals, and homogeneously dispersed when the crystals are molten. In amorphous monolayers at the air-water interface, reversible cross-linking of hydroxy-terminated oligo(ε-caprolactone) monolayers using dialdehyde (glyoxal) lead to the formation of 2D networks. Pronounced contraction in the area occurred for 2D OCL films in dependence of surface pressure and time indicating the reaction progress. Cross linking inhibited crystallization and retarded enzymatic degradation of the OCL film. Altering the subphase pH to ~2 led to cleavage of the covalent acetal cross-links. Besides as model systems, these reversibly cross-linked films are applicable for drug delivery systems or cell substrates modulating adhesion at biointerfaces.
The world energy consumption has constantly increased every year due to economic development and population growth. This inevitably caused vast amount of CO2 emission, and the CO2 concentration in the atmosphere keeps increasing with economic growth. To reduce CO2 emission, various methods have been developed but there are still many bottlenecks to be solved. Solvents easily absorbing CO2 such as monoethanol-amine (MEA) and diethanolamine, for example, have limitations of solvent loss, amine degradation, vulnerability to heat and toxicity, and the high cost of regeneration which is especially caused due to chemisorption process. Though some of these drawbacks can be compensated through physisorption with zeolites and metal-organic frameworks (MOFs) by displaying significant adsorption selectivity and capacity even in ambient conditions, limitations for these materials still exist. Zeolites demand relatively high regeneration energy and have limited adsorption kinetics due to the exceptionally narrow pore structure. MOFs have low stability against heat and moisture and high manufacturing cost.
Nanoporous carbons have recently received attention as an attractive functional porous material due to their unique properties. These materials are crucial in many applications of modern science and industry such as water and air purification, catalysis, gas separation, and energy storage/conversion due to their high chemical and thermal stability, and in particular electronic conductivity in combination with high specific surface areas. Nanoporous carbons can be used to adsorb environmental pollutants or small gas molecules such as CO2 and to power electrochemical energy storage devices such as batteries and fuel cells. In all fields, their pore structure or electrical properties can be modified depending on their purposes.
This thesis provides an in-depth look at novel nanoporous carbons from the synthetic and the application point of view. The interplay between pore structure, atomic construction, and the adsorption properties of nanoporous carbon materials are investigated. Novel nanoporous carbon materials are synthesized by using simple precursor molecules containing heteroatoms through a facile
templating method. The affinity, and in turn the adsorption capacity, of carbon materials toward polar gas molecules (CO2 and H2O) is enhanced by the modification of their chemical construction. It is also shown that these properties are important in electrochemical energy storage, here especially for supercapacitors with aqueous electrolytes which are basically based on the physisorption of ions on carbon surfaces. This shows that nanoporous carbons can be a “functional” material with specific physical or chemical interactions with guest species just like zeolites and MOFs.
The synthesis of sp2-conjugated materials with high heteroatom content from a mixture of citrazinic acid and melamine in which heteroatoms are already bonded in specific motives is illustrated. By controlling the removal procedure of the salt-template and the condensation temperature, the role of salts in the formation of porosity and as coordination sites for the stabilization of heteroatoms is proven. A high amount of nitrogen of up to 20 wt. %, oxygen contents of up to 19 wt.%, and a high CO2/N2 selectivity with maximum CO2 uptake at 273 K of 5.31 mmol g–1 are achieved. Besides, the further controlled thermal condensation of precursor molecules and advanced functional properties on applications of the synthesized porous carbons are described. The materials have different porosity and atomic construction exhibiting a high nitrogen content up to 25 wt. % as well as a high porosity with a specific surface area of more than 1800 m2 g−1, and a high performance in selective CO2 gas adsorption of 62.7. These pore structure as well as properties of surface affect to water adsorption with a remarkably high Qst of over 100 kJ mol−1 even higher than that of zeolites or CaCl2 well known as adsorbents. In addition to that, the pore structure of HAT-CN-derived carbon materials during condensation in vacuum is fundamentally understood which is essential to maximize the utilization of porous system in materials showing significant difference in their pore volume of 0.5 cm3 g−1 and 0.25 cm3 g−1 without and with vacuum, respectively.
The molecular designs of heteroatom containing porous carbon derived from abundant and simple molecules are introduced in the presented thesis. Abundant precursors that already containing high amount of nitrogen or oxygen are beneficial to achieve enhanced interaction with adsorptives. The physical and chemical properties of these heteroatom-doped porous carbons are affected by mainly two parameters, that is, the porosity from the pore structure and the polarity from the atomic composition on the surface. In other words, controlling the porosity as well as the polarity of the carbon materials is studied to understand interactions with different guest species which is a fundamental knowledge for the utilization on various applications.
The growing energy demand of the modern economies leads to the increased consumption of fossil fuels in form of coal, oil, and natural gases, as the mains sources. The combustion of these carbon-based fossil fuels is inevitably producing greenhouse gases, especially CO2. Approaches to tackle the CO2 problem are to capture it from the combustion sources or directly from air, as well as to avoid CO2 production in energy consuming sources (e.g., in the refrigeration sector). In the former, relatively low CO2 concentrations and competitive adsorption of other gases is often leading to low CO2 capacities and selectivities. In both approaches, the interaction of gas molecules with porous materials plays a key role. Porous carbon materials possess unique properties including electric conductivity, tunable porosity, as well as thermal and chemical stability. Nevertheless, pristine carbon materials offer weak polarity and thus low CO2 affinity. This can be overcome by nitrogen doping, which enhances the affinity of carbon materials towards acidic or polar guest molecules (e.g., CO2, H2O, or NH3). In contrast to heteroatom-free materials, such carbon materials are in most cases “noble”, that is, they oxidize other matter rather than being oxidized due to the very positive working potential of their electrons. The challenging task here is to achieve homogenous distribution of significant nitrogen content with similar bonding motives throughout the carbon framework and a uniform pore size/distribution to maximize host-guest interactions. The aim of this thesis is the development of novel synthesis pathways towards nitrogen-doped nanoporous noble carbon materials with precise design on a molecular level and understanding of their structure-related performance in energy and environmental applications, namely gas adsorption and electrochemical energy storage.
A template-free synthesis approach towards nitrogen-doped noble microporous carbon materials with high pyrazinic nitrogen content and C2N-type stoichiometry was established via thermal condensation of a hexaazatriphenylene derivative. The materials exhibited high uptake of guest molecules, such as H2O and CO2 at low concentrations, as well as moderate CO2/N2 selectivities. In the following step, the CO2/N2 selectivity was enhanced towards molecular sieving of CO2 via kinetic size exclusion of N2. The precise control over the condensation degree, and thus, atomic construction and porosity of the resulting materials led to remarkable CO2/N2 selectivities, CO2 capacities, and heat of CO2 adsorption. The ultrahydrophilic nature of the pore walls and the narrow microporosity of these carbon materials served as ideal basis for the investigation of interface effects with more polar guest molecules than CO2, namely H2O and NH3.
H2O vapor physisorption measurements, as well as NH3-temperature programmed desorption and thermal response measurements showed exceptionally high affinity towards H2O vapor and NH3 gas. Another series of nitrogen-doped carbon materials was synthesized by direct condensation of a pyrazine-fused conjugated microporous polymer and their structure-related performance in electrochemical energy storage, namely as anode materials for sodium-ion battery, was investigated.
All in all, the findings in this thesis exemplify the value of molecularly designed nitrogen-doped carbon materials with remarkable heteroatom content implemented as well-defined structure motives. The simultaneous adjustment of the porosity renders these materials suitable candidates for fundamental studies about the interactions between nitrogen-doped carbon materials and different guest species.
Foam fractionation of surfactant and protein solutions is a process dedicated to separate surface active molecules from each other due to their differences in surface activities. The process is based on forming bubbles in a certain mixed solution followed by detachment and rising of bubbles through a certain volume of this solution, and consequently on the formation of a foam layer on top of the solution column. Therefore, systematic analysis of this whole process comprises of at first investigations dedicated to the formation and growth of single bubbles in solutions, which is equivalent to the main principles of the well-known bubble pressure tensiometry. The second stage of the fractionation process includes the detachment of a single bubble from a pore or capillary tip and its rising in a respective aqueous solution. The third and final stage of the process is the formation and stabilization of the foam created by these bubbles, which contains the adsorption layers formed at the growing bubble surface, carried up and gets modified during the bubble rising and finally ends up as part of the foam layer.
Bubble pressure tensiometry and bubble profile analysis tensiometry experiments were performed with protein solutions at different bulk concentrations, solution pH and ionic strength in order to describe the process of accumulation of protein and surfactant molecules at the bubble surface. The results obtained from the two complementary methods allow understanding the mechanism of adsorption, which is mainly governed by the diffusional transport of the adsorbing protein molecules to the bubble surface. This mechanism is the same as generally discussed for surfactant molecules. However, interesting peculiarities have been observed for protein adsorption kinetics at sufficiently short adsorption times. First of all, at short adsorption times the surface tension remains constant for a while before it decreases as expected due to the adsorption of proteins at the surface. This time interval is called induction time and it becomes shorter with increasing protein bulk concentration. Moreover, under special conditions, the surface tension does not stay constant but even increases over a certain period of time. This so-called negative surface pressure was observed for BCS and BLG and discussed for the first time in terms of changes in the surface conformation of the adsorbing protein molecules. Usually, a negative surface pressure would correspond to a negative adsorption, which is of course impossible for the studied protein solutions. The phenomenon, which amounts to some mN/m, was rather explained by simultaneous changes in the molar area required by the adsorbed proteins and the non-ideality of entropy of the interfacial layer. It is a transient phenomenon and exists only under dynamic conditions.
The experiments dedicated to the local velocity of rising air bubbles in solutions were performed in a broad range of BLG concentration, pH and ionic strength. Additionally, rising bubble experiments were done for surfactant solutions in order to validate the functionality of the instrument. It turns out that the velocity of a rising bubble is much more sensitive to adsorbing molecules than classical dynamic surface tension measurements. At very low BLG or surfactant concentrations, for example, the measured local velocity profile of an air bubble is changing dramatically in time scales of seconds while dynamic surface tensions still do not show any measurable changes at this time scale. The solution’s pH and ionic strength are important parameters that govern the measured rising velocity for protein solutions. A general theoretical description of rising bubbles in surfactant and protein solutions is not available at present due to the complex situation of the adsorption process at a bubble surface in a liquid flow field with simultaneous Marangoni effects. However, instead of modelling the complete velocity profile, new theoretical work has been started to evaluate the maximum values in the profile as characteristic parameter for dynamic adsorption layers at the bubble surface more quantitatively.
The studies with protein-surfactant mixtures demonstrate in an impressive way that the complexes formed by the two compounds change the surface activity as compared to the original native protein molecules and therefore lead to a completely different retardation behavior of rising bubbles. Changes in the velocity profile can be interpreted qualitatively in terms of increased or decreased surface activity of the formed protein-surfactant complexes. It was also observed that the pH and ionic strength of a protein solution have strong effects on the surface activity of the protein molecules, which however, could be different on the rising bubble velocity and the equilibrium adsorption isotherms. These differences are not fully understood yet but give rise to discussions about the structure of protein adsorption layer under dynamic conditions or in the equilibrium state.
The third main stage of the discussed process of fractionation is the formation and characterization of protein foams from BLG solutions at different pH and ionic strength. Of course a minimum BLG concentration is required to form foams. This minimum protein concentration is a function again of solution pH and ionic strength, i.e. of the surface activity of the protein molecules. Although at the isoelectric point, at about pH 5 for BLG, the hydrophobicity and hence the surface activity should be the highest, the concentration and ionic strength effects on the rising velocity profile as well as on the foamability and foam stability do not show a maximum. This is another remarkable argument for the fact that the interfacial structure and behavior of BLG layers under dynamic conditions and at equilibrium are rather different. These differences are probably caused by the time required for BLG molecules to adapt respective conformations once they are adsorbed at the surface.
All bubble studies described in this work refer to stages of the foam fractionation process. Experiments with different systems, mainly surfactant and protein solutions, were performed in order to form foams and finally recover a solution representing the foamed material. As foam consists to a large extent of foam lamella – two adsorption layers with a liquid core – the concentration in a foamate taken from foaming experiments should be enriched in the stabilizing molecules. For determining the concentration of the foamate, again the very sensitive bubble rising velocity profile method was applied, which works for any type of surface active materials. This also includes technical surfactants or protein isolates for which an accurate composition is unknown.
The present work focuses on minimising the usage of toxic chemicals by integration of the biobased monomers, derived from fatty acid esters, to photopolymerization processes, which are known to be nature friendly. Internal double bond present in the oleic acid was converted to more reactive (meth)acrylate or epoxy group. Biobased starting materials, functionalized by different pendant groups, were used for photopolymerizing formulations to design of new polymeric structures by using ultraviolet light emitting diode (UV-LED) (395 nm) via free radical polymerization or cationic polymerization.
New (meth)acrylates (2,3 and 4) consisting of two isomers, methyl 9-((meth)acryloyloxy)-10-hydroxyoctadecanoate / methyl 9-hydroxy-10-((meth)acryloyloxy)octadecanoate (2 and 3) and methyl 9-(1H-imidazol-1-yl)-10-(methacryloyloxy)octadecanoate / methyl 9-(methacryloyloxy)-10-(1H-imidazol-1-yl)octadecanoate (4), modified from oleic acid mix, and ionic liquid monomers (1a and 1b) bearing long alkyl chain were polymerized photochemically. New (meth)acrylates are based on vegetable oil, and ionic liquids (ILs) have nonvolatile behaviour. Therefore, both monomer types have green approach. Photoinitiated polymerization of new (meth)acrylates and ionic liquids was investigated in the presence of ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (Irgacure® TPO−L) or di(4-methoxybenzoyl)diethylgermane (Ivocerin®) as photoinitiator (PI). Additionally, the results were discussed in comparison with those obtained from commercial 1,6-hexanediol di(meth)acrylate (5 and 6) for deeper investigation of biobased monomer’s potential to substitute petroleum derived materials with renewable resources for possible coating applications. Kinetic study shows that methyl 9-(1H-imidazol-1-yl)-10-(methacryloyloxy)octadecanoate / methyl 9-(methacryloyloxy)-10-(1H-imidazol-1-yl)octadecanoate (4) and ionic liquids (1a and 1b) have quantitative conversion after irradiation process which is important for practical applications. On the other hand, heat generation occurs in a longer time during the polymerization of biobased systems or ILs.
The poly(meth)acrylates modified from (meth)acrylated fatty acid methyl ester monomers generally show a low glass transition temperature because of the presence of long aliphatic chain in the polymer structure. However, poly(meth)acrylates containing aromatic group have higher glass transition temperature. Therefore, new 4-(4-methacryloyloxyphenyl)-butan-2-one (7) was synthesized which can be a promising candidate for the green techniques, such as light induced polymerization. Photokinetic investigation of the new monomer, 4-(4-methacryloyloxyphenyl)-butan-2-one (7), was discussed using Irgacure® TPO−L or Ivocerin® as photoinitiator. The reactivity of that monomer was compared to commercial 2-phenoxyethyl methacrylate (8) and phenyl methacrylate (9) basis of the differences on monomer structures. The photopolymer of 4-(4-methacryloyloxyphenyl)-butan-2-one (7) might be an interesting candidate for the coating application with the properties of quantitative conversion and high molecular weight. It also shows higher glass transition temperature.
In addition to the linear systems based on renewable materials, new crosslinked polymers were also designed in this thesis. Therefore, isomer mixture consisting of ethane-1,2-diyl bis(9-methacryloyloxy-10-hydroxy octadecanoate), ethane-1,2-diyl 9-hydroxy-10-methacryloyloxy-9’-methacryloyloxy10’-hydroxy octadecanoate and ethane-1,2-diyl bis(9-hydroxy-10-methacryloyloxy octadecanoate) (10) was synthesized by derivation of the oleic acid which has not been previously described in the literature. Crosslinked material based on this biobased monomer was produced by photoinitiated free radical polymerization using Irgacure® TPO−L or Ivocerin® as photoinitiator. Furthermore, material properties were diversified by copolymerization of 10 with 4-(4-methacryloyloxyphenyl)-butan-2-one (7) or methyl 9-(1H-imidazol-1-yl)-10-(methacryloyloxy)octadecanoate / methyl 9-(methacryloyloxy)-10-(1H-imidazol-1-yl)octadecanoate (4). In addition to this, influence of comonomer with different chemical structure on the network system was investigated by analysis of thermo-mechanical properties, crosslink density and molecular weight between two crosslink junctions. An increase in the glass transition temperature caused by copolymerization of biobased monomer 10 with the excess amount of 4-(4-methacryloyloxyphenyl)-butan-2-one (7) was confirmed by both techniques, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). On the other hand, crosslink density decreased as a result of copolymerization reactions due to the reduction in the mean functionality of the system. Furthermore, surface characterization has been tested by contact angle measurements using solvents with different polarity.
This work also contributes to the limited data reported about cationic photopolymerization of the epoxidized vegetable oils in the literature in contrast to the widely investigation of thermal curing of the biorenewable epoxy monomers. In addition to the 9,10-epoxystearic acid methyl ester (11), a new monomer of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) has been synthesized from oleic acid. These two biobased epoxies have been polymerized via cationic photoinitiated polymerization in the presence of bis(t-butyl)-iodonium-tetrakis(perfluoro-t-butoxy)aluminate ([Al(O-t-C4F9)4]-) and isopropylthioxanthone (ITX) as photinitiating system. Polymerization kinetic of 9,10-epoxystearic acid methyl ester (11) and bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) was investigated and compared with the kinetic of commercial monomers being 3,4-epoxycyclohexylmethyl-3’,4’-epoxycyclohexane carboxylate (13), 1,4-butanediol diglycidyl ether (14), and diglycidylether of bisphenol-A (15). Both biobased epoxies (11 and 12) showed higher conversion than cycloaliphatic epoxy (13), and lower reactivity than 1,4-butanediol diglycidyl ether (14). Additional network systems were designed by copolymerization of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) and diglycidylether of bisphenol-A (15) in different molar ratios (1:1; 1:5; 1:9). It addresses that, final conversion is dependent on polymerization rate as well as physical processes such as vitrification during polymerization. Moreover, low glass transition temperature of homopolymer derived from bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) was successfully increased by copolymerization with diglycidylether bisphenol-A (15). On the other hand, the surface produced from bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) shows hydrophobic character. Higher concentration of biobased diepoxy (12) in the copolymerizing mixture decreases surface free energy. Network systems were also investigated according to the rubber elasticity theory. Crosslinked polymer derived from the mixture of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) and diglycidylether of bisphenol-A (15) (molar ratio=1:5) exhibits almost ideal polymer network.
Functional materials, also called "Smart Materials", are described by their ability to fulfill a desired task through targeted interaction with its environment. Due to this functional integration, such materials are of increased interest, especially in areas where the increasing micronization of components is required. Modern manufacturing processes (e.g. microfluidics) and the availability of a wide variety of functional materials (e.g. shape memory materials) now enable the production of particle-based switching components. This category includes micropumps and microvalves, whose basic function is the active control of liquid flows. One approach in realizing those microcomponents as pursued by this work, enables variable size-switching of water-filled microballoons by implementing a stimulus-sensitive switching motif in the capsule's membrane shell, while being under the influence of a constant driving force. The switching motif with its gatekeeper function has a critical influence on one or more material parameters, which modulate the capsule's resistance against the driving force in microballoon expansion process. The advantage of this concept is that even non-variable analyte conditions, such as concentration levels of ions, can be capitalized to generate external force fields that, under the control of the membrane, cause an inflation of the microballoon by an osmotically driven water influx. In case of osmotic pressure gradients as the driving force for the capsule expansion, material parameters associated with the gatekeeper function are specifically the permeability and the mechanical stiffness of the shell material. While a modulation of the shell permeability could be utilized to kinetically impede the water influx on large time scales, a modulation of the shell's mechanical stiffness even might be utilized to completely prevent the capsule inflation due to a possible non-deformability beneath a certain threshold pressure. In polymer networks, which are a suitable material class for the demanded capsule shell because of their excellent elasticity, both the permeability and the mechanical properties are strongly influenced by the crystallinity of the material. Since the permeability is effectively reduced with increasing crystallinity, while the mechanical stiffness is simultaneously greatly increased, both effects point in the same direction in terms of their functional relationship. For this reason and due to a reversible and contactless modulation of the membrane crystallinity by heat input, crystallites may be suitable switching motifs for controlling the capsule expansion. As second design element of reversible expandable microballoons, the capsule geometry, defined by an aqueous core enveloped by the temperature-sensitive polymer network membrane, should allow an osmotic pressure gradient across the membrane layer. The strength of the inflation pressure and the associated inflation velocity upon membrane melting should be controlled by the salt concentration within the aqueous core, while a turn in the osmotic gradient should furthermore allow the reversible process of capsule deflation. Therefore, it should be possible to build either microvalves and micropumps, while their intended action of either pumping or valving is determined by their state of expansion and the direction of the osmotic pressure gradient.. Microballoons of approximately 300 µm in diameter were formed via droplet-based microfluidics from double-emulsion templates (w/o/w). The elastomeric capsule membrane was formed by photo-crosslinking of methacrylate (MA) functionalized oligo(ε-caprolactone) precursors (≈ 3.8 MA-arms, Mn ≈ 12000 g mol-1) within the organic medium layer (o) via UV-exposure after droplet-formation. After removal of the toluene/chloroform mixture by slow extraction via the continuous aqueous phase, the capsules solidified under the development of a characteristic "mushroom"-like shape at specific experimental conditions (e.g. λ = 308 nm, 57 mJ·s-1·cm-2, 16 min). It could be furthermore shown that in dependency to the process parameters: oligomer concentration and curing-time also spherical capsules were accessible. Long curing-times and high oligomer concentrations at a fixed light-intensity favored the formation of "mushroom"-like capsules, whereas the contrary led to spherical shaped capsules. A comparative study on thin polymer network films of same composition and equal treatment proved a correlation between the film's crosslink density and their contraction capability, while stronger crosslinked polymer networks showed a stronger contraction after solvent removal. In combination with observations during capsule solidification via light-microscopy, where a continuous shaping from almost spherical crosslinked templates to "mushroom"-shaped and solidified capsules was stated, the following mechanism was proposed. In case of low oligomer contents and short curing-times, the contraction of the capsule shell during solvent removal is strongly diminished due to a low degree of crosslinking. Therefore, the solidifying shell could freely collapse onto the aqueous core. In the other case, high oligomer concentrations and long curing-times will favor the formation of highly crosslinked capsule membranes with a strong contraction capability. Due to an observed decentered location of the aqueous core within the swollen polymer network, an uneven radial stress along the capsule's circumference is exerted to the incompressible core. This lead to an uneven contraction during solvent removal and a directed flow of the core fluid into the direction of the minimal stress vector. In consequence, the initially thicker spherical cap contracts, whereas the opposing thinner spherical cap get stretched. The "mushroom"-shape over some advantages over their spherical shaped counterparts, why they were selected for the further experiments. Besides the necessity of a high density of crosslinking for the purpose of extraordinary elasticity and toughness, the form-anisotropy promotes a faster microballoon expandability due to a partial reduction of the membrane thickness. Additionally, pre-stretched regions of thin thickness might provide a better resistance against inflation pressure than spherical but non-stretched capsules of equal membrane thickness. The resulting "mushroom"-shaped microcapsules exhibited a melting point of Tm ≈ 50 - 60 °C and a degree of crystallinity of Xc ≈ 29 - 38 % depending on the membrane thickness and internal salt content, which is slightly lower than for the non-crosslinked oligomer and reasoned by a limited chain mobility upon crosslinking. Nonetheless, the melting transition of the polymer network was associated with a strong drop in its mechanical stiffness, which was shown to have a strong influence on the osmotic driven expansion of the microcapsules. Capsules that were subjected to osmotic pressures between 1.5 and 4.7 MPa did not expand if the temperature was well below the melting point of the capsule's membrane, i.e. at room temperature. In contrast, a continuous expansion, while approaching asymptotically to a final capsule size, was observed if the temperature exceeded the melting point, i.e. 60 °C. Microballoons, which were kept for 56 days at ∆Π = 1.5 MPa and room temperature, did not change significantly in diameter, why the impact of the mechanical stiffness on the expansion behavior is considered to be the greater than the influence of the shell permeability. The time-resolved expansion behavior of the microballoons above their Tm was subsequently modeled, using difusion equations that were corrected for shape anisotropy and elastic restoring forces. A shape-related and expansion dependent pre-factor was used to dynamically address the influence of the shell thickness differences along the circumference on the inflation velocity, whereas the microballoon's elastic contraction upon inflation was rendered by the inclusion of a hyperelastic constitutive model. An important finding resulting from this model was the pronounced increase in inflation velocity compared to hypothetical capsules with a homogeneous shell thickness, which stresses the benefit of employing shape anisotropic balloon-like capsules in this study. Furthermore, the model was able to predict the finite expandability on basis of entropy-elastic recovery forces and strain-hardening effects. A comparison of six different microballoons with different shell thicknesses and internal salt contents showed the linear relationship between the volumetric expansion, the shell thickness and the applied osmotic pressure, as represented by the model. As the proposed model facilitates the prediction of the expansion kinetics depending on the membranes mechanical and diffusional characteristics, it might be a screening tool for future material selections. In course of the microballoon expansion process, capsules of intermediate diameters could be isolated by recrystallization of the membrane, which is mainly caused by a restoration of the membrane's mechanical stiffness and is otherwise difficult to achieve with other stimuli-sensitive systems. The capsule's crystallinity of intermediate expansion states was nearly unchanged, whereas the lamellar crystal size tends to decreased with the expansion ratio. Therefore, it was assumed that the elastic modulus was only minimally altered and might increased due to the networks segment-chain extension. In addition to the volume increase achieved by inflation, a turn in the osmotic gradient also facilitated the reversible deflation, which was shown in inflation/deflation cycles. These both characteristics of the introduced microballoons are important parameter regarding the realization of micropumps and microvalves. The fixation of expanded microcapsules via recrystallization enabled the storage of entropy-elastic strain-energy, which could be utilized for pumping actions in non-aqueous media. Here, the pumping velocity depended on both, the type of surrounding medium and the applied temperature. Surrounding media that supported the fast transport of pumped liquid showed an accelerated deflation, while high temperatures further accelerate the pumping velocity. Very fast rejection of the incorporated payload was furthermore realized with pierced expanded microballoons, which were subjected to temperatures above their Tm. The possible fixation of intermediate particle sizes provide opportunities for vent constructions that allowed the precise adjustment of specific flow-rates and multiple valve openings and closings. A valve construction was realized by the insertion of a single or multiple microballoons in a microfluidic channel. A complete and a partial closing of the microballoon-valves was demonstrated as a function of the heating period. In this context, a difference between the inflation and deflation velocity was stated, summarizing slower expansion kinetics. Overall, microballoons, which presented both on-demand pumping and reversible valving by a temperature-triggered change in the capsule's volume, might be suitable components that help to design fully integrated LOC devices, due to the implementation of the control switch and controllable inflation/deflation kinetics. In comparison to other state of the art stimuli-sensitive materials, one has to highlight the microballoons capability of stabilizing almost continuously intermediate capsule sizes by simple recrystallization of the microballoon's membrane.
This research addressed the question, if it is possible to simplify current microcontact printing systems for the production of anisotropic building blocks or patchy particles, by using common chemicals while still maintaining reproducibility, high precision and tunability of the Janus-balance
Chapter 2 introduced the microcontact printing materials as well as their defined electrostatic interactions. In particular polydimethylsiloxane stamps, silica particles and high molecular weight polyethylenimine ink were mainly used in this research. All of these components are commercially available in large quantities and affordable, which gives this approach a huge potential for further up-scaling developments. The benefits of polymeric over molecular inks was described including its flexible influence on the printing pressure. With this alteration of the µCP concept, a new method of solvent assisted particle release mechanism enabled the switch from two-dimensional surface modification to three-dimensional structure printing on colloidal silica particles, without changing printing parameters or starting materials. This effect opened the way to use the internal volume of the achieved patches for incorporation of nano additives, introducing additional physical properties into the patches without alteration of the surface chemistry.
The success of this system and its achievable range was further investigated in chapter 3 by giving detailed information about patch geometry parameters including diameter, thickness and yield. For this purpose, silica particles in a size range between 1µm and 5µm were printed with different ink concentrations to change the Janus-balance of these single patched particles. A necessary intermediate step, consisting of air-plasma treatment, for the production of trivalent particles using "sandwich" printing was discovered and comparative studies concerning the patch geometry of single and double patched particles were conducted. Additionally, the usage of structured PDMS stamps during printing was described. These results demonstrate the excellent precision of this approach and opens the pathway for even greater accuracy as further parameters can be finely tuned and investigated, e.g. humidity and temperature during stamp loading.
The performance of these synthesized anisotropic colloids was further investigated in chapter 4, starting with behaviour studies in alcoholic and aqueous dispersions. Here, the stability of the applied patches was studied in a broad pH range, discovering a release mechanism by disabling the electrostatic bonding between particle surface and polyelectrolyte ink. Furthermore, the absence of strong attractive forces between divalent particles in water was investigated using XPS measurements. These results lead to the conclusion that the transfer of small PDMS oligomers onto the patch surface is shielding charges, preventing colloidal agglomeration. However, based on this knowledge, further patch modifications for particle self-assembly were introduced including physical approaches using magnetic nano additives, chemical patch functionalization with avidin-biotin or the light responsive cyclodextrin-arylazopyrazoles coupling as well as particle surface modification for the synthesis of highly amphiphilic colloids. The successful coupling, its efficiency, stability and behaviour in different solvents were evaluated to find a suitable coupling system for future assembly experiments. Based on these results the possibility of more sophisticated structures by colloidal self-assembly is given.
Certain findings needed further analysis to understand their underlying mechanics, including the relatively broad patch diameter distribution and the decreasing patch thickness for smaller silica particles. Mathematical assumptions for both effects are introduced in chapter 5. First, they demonstrate the connection between the naturally occurring particle size distribution and the broadening of the patch diameter, indicating an even higher precision for this µCP approach. Second, explaining the increase of contact area between particle and ink surface due to higher particle packaging, leading to a decrease in printing pressure for smaller particles.
These calculations ultimately lead to the development of a new mechanical microcontact printing approach, using centrifugal forces for high pressure control and excellent parallel alignment of printing substrates. First results with this device and the comparison with previously conducted by-hand experiments conclude this research. It furthermore displays the advantages of such a device for future applications using a mechanical printing approach, especially for accessing even smaller nano particles with great precision and excellent yield.
In conclusion, this work demonstrates the successful adjustment of the µCP approach using commercially available and affordable silica particles and polyelectrolytes for high flexibility, reduced costs and higher scale-up value. Furthermore, its was possible to increase the modification potential by introducing three-dimensional patches for additional functionalization volume. While keeping a high colloidal stability, different coupling systems showed the self-assembly capabilities of this toolbox for anisotropic particles.
Polyelectrolyte microcapsules containing stimuli-responsive polymers have potential applications in the fields of sensors or actuators, stimulable microcontainers and controlled drug delivery. Such capsules were prepared, with the focus on pH-sensitivity and carbohydrate-sensing. First, pH-responsive polyelectrolyte capsules were produced by means of electrostatic layer-by-layer assembly of oppositely charged weak polyelectrolytes onto colloidal templates that were subsequently removed. The capsules were composed of poly(allylamine hydrochloride) (PAH) and poly(methacrylic acid) (PMA) or poly(4-vinylpyridine) (P4VP) and PMA and varied considerably in their hydrophobicity and the influence of secondary interactions. These polymers were assembled onto CaCO3 and SiO2 particles with diameters of ~ 5 µm, and a new method for the removal of the silica template under mild conditions was proposed. The pH-dependent stability of PAH/PMA and P4VP/PMA capsules was studied by confocal laser scanning microscopy (CLSM). They were stable over a wide pH-range and exhibited a pronounced swelling at the edges of stability, which was attributed to uncompensated positive or negative charges within the multilayers. The swollen state could be stabilized when the electrostatic repulsion was counteracted by hydrogen-bonding, hydrophobic interactions or polymeric entanglement. This stabilization made it possible to reversibly swell and shrink the capsules by tuning the pH of the solution. The pH-dependent ionization degree of PMA was used to modulate the binding of calcium ions. In addition to the pH-sensitivity, the stability and the swelling degree of these capsules at a given pH could be modified, when the ionic strength of the medium was altered. The reversible swelling was accompanied by reversible permeability changes for low and high molecular weight substances. The permeability for glucose was evaluated by studying the time-dependence of the buckling of the capsule walls in glucose solutions and the reversible permeability modulation was used for the encapsulation of polymeric material. A theoretical model was proposed to explain the pH-dependent size variations that took into account an osmotic expanding force and an elastic restoring force to evaluate the pH-dependent size changes of weak polyelectrolyte capsules. Second, sugar-sensitive multilayers were assembled using the reversible covalent ester formation between the polysaccharide mannan and phenylboronic acid moieties that were grafted onto poly(acrylic acid) (PAA). The resulting multilayer films were sensitive to several carbohydrates, showing the highest sensitivity to fructose. The response to carbohydrates resulted from the competitive binding of small molecular weight sugars and mannan to the boronic acid groups within the film, and was observed as a fast dissolution of the multilayers, when they were brought into contact with the sugar-containing solution above a critical concentration. It was also possible to prepare carbohydrate-sensitive multilayer capsules, and their sugar-dependent stability was investigated by following the release of encapsulated rhodamine-labeled bovine serum albumin (TRITC-BSA).
Complex emulsions are dispersions of kinetically stabilized multiphasic emulsion droplets comprised of two or more immiscible liquids that provide a novel material platform for the generation of active and dynamic soft materials. In recent years, the intrinsic reconfigurable morphological behavior of complex emulsions, which can be attributed to the unique force equilibrium between the interfacial tensions acting at the various interfaces, has become of fundamental and applied interest. As such, particularly biphasic Janus droplets have been investigated as structural templates for the generation of anisotropic precision objects, dynamic optical elements or as transducers and signal amplifiers in chemo- and bio-sensing applications. In the present thesis, switchable internal morphological responses of complex droplets triggered by stimuli-induced alterations of the balance of interfacial tensions have been explored as a universal building block for the design of multiresponsive, active, and adaptive liquid colloidal systems. A series of underlying principles and mechanisms that influence the equilibrium of interfacial tensions have been uncovered, which allowed the targeted design of emulsion bodies that can alter their shape, bind and roll on surfaces, or change their geometrical shape in response to chemical stimuli. Consequently, combinations of the unique triggerable behavior of Janus droplets with designer surfactants, such as a stimuli-responsive photosurfactant (AzoTAB) resulted for instance in shape-changing soft colloids that exhibited a jellyfish inspired buoyant motion behavior, holding great promise for the design of biological inspired active material architectures and transformable soft robotics.
In situ observations of spherical Janus emulsion droplets using a customized side-view microscopic imaging setup with accompanying pendant dropt measurements disclosed the sensitivity regime of the unique chemical-morphological coupling inside complex emulsions and enabled the recording of calibration curves for the extraction of critical parameters of surfactant effectiveness. The deduced new "responsive drop" method permitted a convenient and cost-efficient quantification and comparison of the critical micelle concentrations (CMCs) and effectiveness of various cationic, anionic, and nonionic surfactants. Moreover, the method allowed insightful characterization of stimuli-responsive surfactants and monitoring of the impact of inorganic salts on the CMC and surfactant effectiveness of ionic and nonionic surfactants. Droplet functionalization with synthetic crown ether surfactants yielded a synthetically minimal material platform capable of autonomous and reversible adaptation to its chemical environment through different supramolecular host-guest recognition events. Addition of metal or ammonium salts resulted in the uptake of the resulting hydrophobic complexes to the hydrocarbon hemisphere, whereas addition of hydrophilic ammonium compounds such as amino acids or polypeptides resulted in supramolecular assemblies at the hydrocarbon-water interface of the droplets. The multiresponsive material platform enabled interfacial complexation and
thus triggered responses of the droplets to a variety of chemical triggers including metal ions, ammonium compounds, amino acids, antibodies, carbohydrates as well as amino-functionalized solid surfaces.
In the final chapter, the first documented optical logic gates and combinatorial logic circuits based on complex emulsions are presented. More specifically, the unique reconfigurable and multiresponsive properties of complex emulsions were exploited to realize droplet-based logic gates of varying complexity using different stimuli-responsive surfactants in combination with diverse readout methods. In summary, different designs for multiresponsive, active, and adaptive liquid colloidal systems were presented and investigated, enabling the design of novel transformative chemo-intelligent soft material platforms.
Heterophase polymerization is a technique widely used for the synthesis of high performance polymeric materials with applications including paints, inks, adhesives, synthetic rubber, biomedical applications and many others. Due to the heterogeneous nature of the process, many different relevant length and time scales can be identified. Each of these scales has a direct influence on the kinetics of polymerization and on the physicochemical and performance properties of the final product. Therefore, from the point of view of product and process design and optimization, the understanding of each of these relevant scales and their integration into one single model is a very promising route for reducing the time-to-market in the development of new products, for increasing the productivity and profitability of existing processes, and for designing products with improved performance or cost/performance ratio. The process considered is the synthesis of structured or composite polymer particles by multi-stage seeded emulsion polymerization. This type of process is used for the preparation of high performance materials where a synergistic behavior of two or more different types of polymers is obtained. Some examples include the synthesis of core-shell or multilayered particles for improved impact strength materials and for high resistance coatings and adhesives. The kinetics of the most relevant events taking place in an emulsion polymerization process has been investigated using suitable numerical simulation techniques at their corresponding time and length scales. These methods, which include Molecular Dynamics (MD) simulation, Brownian Dynamics (BD) simulation and kinetic Monte Carlo (kMC) simulation, have been found to be very powerful and highly useful for gaining a deeper insight and achieving a better understanding and a more accurate description of all phenomena involved in emulsion polymerization processes, and can be potentially extended to investigate any type of heterogeneous process. The novel approach of using these kinetic-based numerical simulation methods can be regarded as a complement to the traditional thermodynamic-based macroscopic description of emulsion polymerization. The particular events investigated include molecular diffusion, diffusion-controlled polymerization reactions, particle formation, absorption/desorption of radicals and monomer, and the colloidal aggregation of polymer particles. Using BD simulation it was possible to precisely determine the kinetics of absorption/desorption of molecular species by polymer particles, and to simulate the colloidal aggregation of polymer particles. For diluted systems, a very good agreement between BD simulation and the classical theory developed by Smoluchowski was obtained. However, for concentrated systems, significant deviations from the ideal behavior predicted by Smoluchowski were evidenced. BD simulation was found to be a very valuable tool for the investigation of emulsion polymerization processes especially when the spatial and geometrical complexity of the system cannot be neglected, as is the case of concentrated dispersions, non-spherical particles, structured polymer particles, particles with non-uniform monomer concentration, and so on. In addition, BD simulation was used to describe non-equilibrium monomer swelling kinetics, which is not possible using the traditional thermodynamic approach because it is only valid for systems at equilibrium. The description of diffusion-controlled polymerization reactions was successfully achieved using a new stochastic algorithm for the kMC simulation of imperfectly mixed systems (SSA-IM). In contrast to the traditional stochastic simulation algorithm (SSA) and the deterministic rate of reaction equations, instead of assuming perfect mixing in the whole reactor, the new SSA-IM determines the volume perfectly mixed between two consecutive reactions as a function of the diffusion coefficient of the reacting species. Using this approach it was possible to describe, using a single set of kinetic parameters, typical mass transfer limitations effects during a free radical batch polymerization such as the cage effect, the gel effect and the glass effect. Using multiscale integration it was possible to investigate the formation of secondary particles during the seeded emulsion polymerization of vinyl acetate over a polystyrene seed. Three different cases of radical generation were considered: generation of radicals by thermal decomposition of water-soluble initiating compounds, generation of radicals by a redox reaction at the surface of the particles, and generation of radicals by thermal decomposition of surface-active initiators "inisurfs" attached to the surface of the particles. The simulation results demonstrated the satisfactory reduction in secondary particles formation achieved when the locus of radical generation is controlled close to the particles surface.
Nanoparticles (NPs) are particles between 1 and 100 nanometers in size. They have attracted enormous research interests owing to their remarkable physicochemical properties and potential applications in the optics, catalysis, sensing, electronics, or optical devices. The thesis investigates systems of NPs attached to planar substrates.
In the first part of the results section of the thesis a new method is presented to immobilize NPs. In many NP applications a strong, persistent adhesion to substrates is a key requirement. Up to now this has been achieved with various methods, which are not always the optimum regarding adhesion strength or applicability. We propose a new method which uses capillarity to enhance the binding agents in the contact area between NP and substrate. The adhesion strength resulting from the new approach is investigated in detail and it is shown that the new approach is superior to older methods in several ways.
The following section presents the optical visualization of nano-sized objects through a combination of thin film surface distortion and interference enhanced optical reflection microscopy. It is a new, fast and non-destructive technique. It not only reveals the location of NPs as small as 20nm attached to planar surfaces and embedded in a molecularly thin liquid film. It also allows the measurement of the geometry of the surface distortion of the liquid film. Even for small NPs the meniscus reaches out for micrometers, which is the reason why the NPs produce such a pronounced optical footprint.
The nucleation and growth of individual bubbles is presented in chapter 5. Nucleation is a ubiquitous natural phenomenon and of great importance in numerous industrial processes. Typically it occurs on very small scales (nanometers) and it is of a random nature (thermodynamics of small systems). Up to now most experimental nucleation studies deal with a large number of individual nucleation processes to cope with its inherently statistical, spatio-temporal character. In contrast, in this thesis the individual O2-bubble formation from single localized platinum NP active site is studied experimentally. The bubble formation is initiated by the catalytic reaction of H2O2 on the Pt surface. It is studied how the bubble nucleation and growth depends on the NP size, the H2O2 concentration and the substrate surface properties. It is observed that in some cases the bubbles move laterally over the substrate surface, driven by the O2-production and the film ablation.
Nanoporous carbon materials are widely used in industry as adsorbents or catalyst supports, whilst becoming increasingly critical to the developing fields of energy storage / generation or separation technologies. In this thesis, the combined use of carbohydrate hydrothermal carbonisation (HTC) and templating strategies is demonstrated as an efficient route to nanostructured carbonaceous materials. HTC is an aqueous-phase, low-temperature (e.g. 130 – 200 °C) carbonisation, which proceeds via dehydration / poly-condensation of carbon precursors (e.g. carbohydrates and their derivatives), allowing facile access to highly functional carbonaceous materials. Whilst possessing utile, modifiable surface functional groups (e.g. -OH and -C=O-containing moieties), materials synthesised via HTC typically present limited accessible surface area or pore volume. Therefore, this thesis focuses on the development of fabrication routes to HTC materials which present enhanced textural properties and well-defined porosity. In the first discussed synthesis, a combined hard templating / HTC route was investigated using a range of sacrificial inorganic templates (e.g. mesoporous silica beads and macroporous alumina membranes (AAO)). Via pore impregnation of mesoporous silica beads with a biomass-derived carbon source (e.g. 2-furaldehyde) and subsequent HTC at 180 oC, an inorganic / carbonaceous hybrid material was produced. Removal of the template component by acid etching revealed the replication of the silica into mesoporous carbonaceous spheres (particle size ~ 5 μm), representing the inverse morphological structure of the original inorganic body. Surface analysis (e.g. FTIR) indicated a material decorated with hydrophilic (oxygenated) functional groups. Further thermal treatment at increasingly elevated temperatures (e.g. at 350, 550, 750 oC) under inert atmosphere allowed manipulation of functionalities from polar hydrophilic to increasingly non-polar / hydrophobic structural motifs (e.g. extension of the aromatic / pseudo-graphitic nature), thus demonstrating a process capable of simultaneous control of nanostructure and surface / bulk chemistry. As an extension of this approach, carbonaceous tubular nanostructures with controlled surface functionality were synthesised by the nanocasting of uniform, linear macropores of an AAO template (~ 200 nm). In this example, material porosity could be controlled, showing increasingly microporous tube wall features as post carbonisation temperature increased. Additionally, by taking advantage of modifiable surface groups, the introduction of useful polymeric moieties (i.e. grafting of thermoresponsive poly(N-isopropylacrylamide)) was also demonstrated, potentially enabling application of these interesting tubular structures in the fields of biotechnology (e.g. enzyme immobilization) and medicine (e.g. as drug micro-containers). Complimentary to these hard templating routes, a combined HTC / soft templating route for the direct synthesis of ordered porous carbonaceous materials was also developed. After selection of structural directing agents and optimisation of synthesis composition, the F127 triblock copolymer (i.e. ethylene oxide (EO)106 propylene oxide (PO)70 ethylene oxide (EO)106) / D-Fructose system was extensively studied. D-Fructose was found to be a useful carbon precursor as the HTC process could be performed at 130 oC, thus allowing access to stable micellular phase. Thermolytic template removal from the synthesised ordered copolymer / carbon composite yielded functional cuboctahedron single crystalline-like particles (~ 5 μm) with well ordered pore structure of a near perfect cubic Im3m symmetry. N2 sorption analysis revealed a predominantly microporous carbonaceous material (i.e. Type I isotherm, SBET = 257 m2g-1, 79 % microporosity) possessing a pore size of ca. 0.9 nm. The addition of a simple pore swelling additive (e.g. trimethylbenzene (TMB)) to this system was found to direct pore size into the mesopore size domain (i.e. Type IV isotherm, SBET = 116 m2g-1, 60 % mesoporosity) generating pore size of ca. 4 nm. It is proposed that in both cases as HTC proceeds to generate a polyfuran-like network, the organised block copolymer micellular phase is essentially “templated”, either via hydrogen bonding between hydrophilic poly(EO) moiety and the carbohydrate or via hydrophobic interaction between hydrophobic poly(PO) moiety and forming polyfuran-like network, whilst the additive TMB presumably interact with poly(PO) moieties, thus swelling the hydrophobic region expanding the micelle template size further into the mesopore range.
The present thesis focuses on the synthesis of nanostructured iron-based compounds by using β-FeOOH nanospindles and poly(ionic liquid)s (PILs) vesicles as hard and soft templates, respectively, to suppress the shuttle effect of lithium polysulfides (LiPSs) in Li-S batteries. Three types of composites with different nanostructures (mesoporous nanospindle, yolk-shell nanospindle, and nanocapsule) have been synthesized and applied as sulfur host material for Li-S batteries. Their interactions with LiPSs and effects on the electrochemical performance of Li-S batteries have been systematically studied.
In the first part of the thesis, carbon-coated mesoporous Fe3O4 (C@M-Fe3O4) nanospindles have been synthesized to suppress the shuttle effect of LiPSs. First, β-FeOOH nanospindles have been synthesized via the hydrolysis of iron (III) chloride in aqueous solution and after silica coating and subsequent calcination, mesoporous Fe2O3 (M-Fe2O3) have been obtained inside the confined silica layer through pyrolysis of β-FeOOH. After the removal of the silica layer, electron tomography (ET) has been applied to rebuild the 3D structure of the M-Fe2O3 nanospindles. After coating a thin layer of polydopamine (PDA) as carbon source, the PDA-coated M-Fe2O3 particles have been calcinated to synthesize C@M-Fe3O4 nanospindles. With the chemisorption of Fe3O4 and confinement of mesoporous structure to anchor LiPSs, the composite C@M-Fe3O4/S electrode delivers a remaining capacity of 507.7 mAh g-1 at 1 C after 600 cycles.
In the second part of the thesis, a series of iron-based compounds (Fe3O4, FeS2, and FeS) with the same yolk-shell nanospindle morphology have been synthesized, which allows for the direct comparison of the effects of compositions on the electrochemical performance of Li-S batteries. The Fe3O4-carbon yolk-shell nanospindles have been synthesized by using the β-FeOOH nanospindles as hard template. Afterwards, Fe3O4-carbon yolk-shell nanospindles have been used as precursors to obtain iron sulfides (FeS and FeS2)-carbon yolk-shell nanospindles through sulfidation at different temperatures. Using the three types of yolk-shell nanospindles as sulfur host, the effects of compositions on interactions with LiPSs and electrochemical performance in Li-S batteries have been systematically investigated and compared. Benefiting from the chemisorption and catalytic effect of FeS2 particles and the physical confinement of the carbon shell, the FeS2-C/S electrode exhibits the best electrochemical performance with an initial specific discharge capacity of 877.6 mAh g-1 at 0.5 C and a retention ratio of 86.7% after 350 cycles.
In the third part, PILs vesicles have been used as soft template to synthesize carbon nanocapsules embedded with iron nitride particles to immobilize and catalyze LiPSs in Li-S batteries. First, 3-n-decyl-1-vinylimidazolium bromide has been used as monomer to synthesize PILs nanovesicles by free radical polymerization. Assisted by PDA coating route and ion exchange, PIL nanovesicles have been successfully applied as soft template in morphology-maintaining carbonization to prepare carbon nanocapsules embedded with iron nitride nanoparticles (FexN@C). The well-dispersed iron nitride nanoparticles effectively catalyze the conversion of LiPSs to Li2S, owing to their high electrical conductivity and strong chemical binding to LiPSs. The constructed FexN@C/S cathode demonstrates a high initial discharge capacity of 1085.0 mAh g-1 at 0.5 C with a remaining value of 930.0 mAh g-1 after 200 cycles.
The results in the present thesis demonstrate the facile synthetic routes of nanostructured iron-based compounds with controllable morphologies and compositions using soft and hard colloidal templates, which can be applied as sulfur host to suppress the shuttle behavior of LiPSs. The synthesis approaches developed in this thesis are also applicable to fabricating other transition metal-based compounds with porous nanostructures for other applications.
New bio-based polymers
(2018)
Redox-responsive polymers, such as poly(disulfide)s, are a versatile class of polymers with potential applications including gene- and drug-carrier systems. Their degradability under reductive conditions allows for a controlled response to the different redox states that are present throughout the body. Poly(disulfide)s are typically synthesized by step growth polymerizations. Step growth polymerizations, however, may suffer from low conversions and therefore low molar masses, limiting potential applications. The purpose of this thesis was therefore to find and investigate new synthetic routes towards the synthesis of amino acid-based poly(disulfide)s.
The different routes in this thesis include entropy-driven ring opening polymerizations of novel macrocyclic monomers, derived from cystine derivatives. These monomers were obtained with overall yields of up to 77% and were analyzed by mass spectrometry as well as by 1D and 2D NMR spectroscopy. The kinetics of the entropy-driven ring-opening metathesis polymerization (ED-ROMP) were thoroughly investigated in dependence of temperature, monomer concentration, and catalyst concentration. The polymerization was optimized to yield poly(disulfide)s with weight average molar masses of up to 80 kDa and conversions of ~80%, at the thermodynamic equilibrium. Additionally, an alternative metal free polymerization, namely the entropy-driven ring-opening disulfide metathesis polymerization (ED-RODiMP) was established for the polymerization of the macrocyclic monomers. The effect of different solvents, concentrations and catalyst loadings on the polymerization process and its kinetics were studied. Polymers with very high weight average molar masses of up to 177 kDa were obtained. Moreover, various post-polymerization reactions were successfully performed.
This work provides the first example of the homopolymerization of endo-cyclic disulfides by ED-ROMP and the first substantial study into the kinetics of the ED-RODiMP process.
This work describes the synthesis and characterization of stimuli-responsive polymers made by reversible addition-fragmentation chain transfer (RAFT) polymerization and the investigation of their self-assembly into “smart” hydrogels. In particular the hydrogels were designed to swell at low temperature and could be reversibly switched to a collapsed hydrophobic state by rising the temperature. Starting from two constituents, a short permanently hydrophobic polystyrene (PS) block and a thermo-responsive poly(methoxy diethylene glycol acrylate) (PMDEGA) block, various gelation behaviors and switching temperatures were achieved. New RAFT agents bearing tert-butyl benzoate or benzoic acid groups, were developed for the synthesis of diblock, symmetrical triblock and 3-arm star block copolymers. Thus, specific end groups were attached to the polymers that facilitate efficient macromolecular characterization, e.g by routine 1H-NMR spectroscopy. Further, the carboxyl end-groups allowed functionalizing the various polymers by a fluorophore. Because reports on PMDEGA have been extremely rare, at first, the thermo-responsive behavior of the polymer was investigated and the influence of factors such as molar mass, nature of the end-groups, and architecture, was studied. The use of special RAFT agents enabled the design of polymer with specific hydrophobic and hydrophilic end-groups. Cloud points (CP) of the polymers proved to be sensitive to all molecular variables studied, namely molar mass, nature and number of the end-groups, up to relatively high molar masses. Thus, by changing molecular parameters, CPs of the PMDEGA could be easily adjusted within the physiological interesting range of 20 to 40°C. A second responsivity, namely to light, was added to the PMDEGA system via random copolymerization of MDEGA with a specifically designed photo-switchable azobenzene acrylate. The composition of the copolymers was varied in order to determine the optimal conditions for an isothermal cloud point variation triggered by light. Though reversible light-induced solubility changes were achieved, the differences between the cloud points before and after the irradiation were small. Remarkably, the response to light differed from common observations for azobenzene-based systems, as CPs decreased after UV-irradiation, i.e with increasing content of cis-azobenzene units. The viscosifying and gelling abilities of the various block copolymers made from PS and PMDEGA blocks were studied by rheology. Important differences were observed between diblock copolymers, containing one hydrophobic PS block only, the telechelic symmetrical triblock copolymers made of two associating PS termini, and the star block copolymers having three associating end blocks. Regardless of their hydrophilic block length, diblock copolymers PS11 PMDEGAn were freely flowing even at concentrations as high as 40 wt. %. In contrast, all studied symmetrical triblock copolymers PS8-PMDEGAn-PS8 formed gels at low temperatures and at concentrations as low as 3.5 wt. % at best. When heated, these gels underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurs. The gel-sol transition shifted to markedly higher transition temperatures with increasing length of the hydrophilic inner block. This effect increased also with the number of arms, and with the length of the hydrophobic end blocks. The mechanical properties of the gels were significantly altered at the cloud point and liquid-like dispersions were formed. These could be reversibly transformed into hydrogels by cooling. This thesis demonstrates that high molar mass PMDEGA is an easily accessible, presumably also biocompatible and at ambient temperature well water-soluble, non-ionic thermo-responsive polymer. PMDEGA can be easily molecularly engineered via the RAFT method, implementing defined end-groups, and producing different, also complex, architectures, such as amphiphilic triblock and star block copolymers, having an analogous structure to associative telechelics. With appropriate design, such amphiphilic copolymers give way to efficient, “smart” viscosifiers and gelators displaying tunable gelling and mechanical properties.
In this work, the development of temperature- and protein-responsive sensor materials based on biocompatible, inverse hydrogel opals (IHOs) is presented. With these materials, large biomolecules can be specifically recognised and the binding event visualised. The preparation of the IHOs was performed with a template process, for which monodisperse silica particles were vertically deposited onto glass slides as the first step. The obtained colloidal crystals with a thickness of 5 μm displayed opalescent reflections because of the uniform alignment of the colloids. As a second step, the template was embedded in a matrix consisting of biocompatible, thermoresponsive hydrogels. The comonomers were selected from the family of oligo(ethylene glycol)methacrylates. The monomer solution was injected into a polymerisation mould, which contained the colloidal crystals as a template. The space in-between the template particles was filled with the monomer solution and the hydrogel was cured via UV-polymerisation. The particles were chemically etched, which resulted in a porous inner structure. The uniform alignment of the pores and therefore the opalescent reflection were maintained, so these system were denoted as inverse hydrogel opals. A pore diameter of several hundred nanometres as well as interconnections between the pores should facilitate a diffusion of bigger (bio)molecules, which was always a challenge in the presented systems until now. The copolymer composition was chosen to result in a hydrogel collapse over 35 °C. All hydrogels showed pronounced swelling in water below the critical temperature. The incorporation of a reactive monomer with hydroxyl groups ensured a potential coupling group for the introduction of recognition units for analytes, e.g. proteins. As a test system, biotin as a recognition unit for avidin was coupled to the IHO via polymer-analogous Steglich esterification. The amount of accessible biotin was quantified with a colorimetric binding assay. When avidin was added to the biotinylated IHO, the wavelength of the opalescent reflection was significantly shifted and therefore the binding event was visualised. This effect is based on the change in swelling behaviour of the hydrogel after binding of the hydrophilic avidin, which is amplified by the thermoresponsive nature of the hydrogel. A swelling or shrinking of the pores induces a change in distance of the crystal planes, which are responsible for the colour of the reflection. With these findings, the possibility of creating sensor materials or additional biomolecules in the size range of avidin is given.
In this thesis, a route to temperature-, pH-, solvent-, 1,2-diol-, and protein-responsive sensors made of biocompatible and low-fouling materials is established. These sensor devices are based on the sensitivemodulation of the visual band gap of a photonic crystal (PhC), which is induced by the selective binding of analytes, triggering a volume phase transition.
The PhCs introduced by this work show a high sensitivity not only for small biomolecules, but also for large analytes, such as glycopolymers or proteins. This enables the PhC to act as a sensor that detects analytes without the need of complex equipment.
Due to their periodical dielectric structure, PhCs prevent the propagation of specific wavelengths. A change of the periodicity parameters is thus indicated by a change in the reflected wavelengths. In the case explored, the PhC sensors are implemented as periodically structured responsive hydrogels in formof an inverse opal.
The stimuli-sensitive inverse opal hydrogels (IOHs) were prepared using a sacrificial opal template of monodispersed silica particles. First, monodisperse silica particles were assembled with a hexagonally packed structure via vertical deposition onto glass slides. The obtained silica crystals, also named colloidal crystals (CCs), exhibit structural color. Subsequently, the CCs templates were embedded in polymer matrix with low-fouling properties. The polymer matrices were composed of oligo(ethylene glycol) methacrylate derivatives (OEGMAs) that render the hydrogels thermoresponsive. Finally, the silica particles were etched, to produce highly porous hydrogel replicas of the CC. Importantly, the inner structure and thus the ability for light diffraction of the IOHs formed was maintained.
The IOH membrane was shown to have interconnected pores with a diameter as well as interconnections between the pores of several hundred nanometers. This enables not only the detection of small analytes, but also, the detection of even large analytes that can diffuse into the nanostructured IOH membrane. Various recognition unit – analyte model systems, such as benzoboroxole – 1,2-diols, biotin – avidin and mannose – concanavalin A, were studied by incorporating functional
comonomers of benzoboroxole, biotin and mannose into the copolymers. The incorporated recognition units specifically bind to certain low and highmolar mass biomolecules, namely to certain saccharides, catechols, glycopolymers or proteins.
Their specific binding strongly changes the overall hydrophilicity, thus modulating the swelling of the IOH matrices, and in consequence, drastically changes their internal periodicity. This swelling is amplified by the thermoresponsive properties of the polymer matrix. The shift of the interference band gap due to the specific molecular recognition is easily visible by the naked eye (up to 150 nm shifts). Moreover, preliminary trial were attempted to detect even larger entities. Therefore anti-bodies were immobilized on hydrogel platforms via polymer-analogous esterification. These platforms incorporate comonomers made of tri(ethylene glycol) methacrylate end-functionalized with a carboxylic acid. In these model systems, the bacteria analytes are too big to penetrate into the IOH membranes, but can only interact with their surfaces. The selected model bacteria, as Escherichia coli, show a specific affinity to anti-body-functionalized hydrogels. Surprisingly in the case functionalized IOHs, this study produced weak color shifts, possibly opening a path to detect directly living organism, which will need further investigations.
Neolignans, dehydrodimers of phenylpropenes, are natural products that exhibit different biological activities. 8,5’-Neolignans containing a trans- dihydrobenzofuran skeleton are the most abundant neolignans in nature. The published syntheses of trans-dihydrobenzofurans are multistep procedures that are time consuming and provide the product in low yield. Furthermore, all dimerisation reactions either in the presence of enzymes or mediated by metal salts are yielding dimers consisting of two units of the same phenylpropene compound, narrowing substantially the substitution pattern. Two different general synthetic approaches were examined. The first strategy was the enantioselective deprotonation at the α-carbon of the ο-alkyl phenols in the presence of a chiral diamine and sBuLi. Synthesis of several new phosphorous-based directed ortho-metalation groups was studied. The examined compounds having these new groups decomposed even under very mild reaction conditions and are not suitable for the application in the synthesis. The second strategy was to examine one [3+2] cycloaddition reaction, transition metal catalysed Heck oxyarylation reaction, in the synthetic approach to compounds having trans-dihydrobenzofuran skeleton. Palladium catalysed Heck oxyarylation reaction with halogenophenols or ortho-diazonium phenols as the starting material allowed the trans-dihydrobenzofuran compounds as the major products in acceptable yield and in one step. The products were formed under ligand free condition, as well as in the presence of some strong coordinating ligands (Ph3P). The experiments with several chiral ligands, showed that the obtained trans-dihydrobenzofurans were racemic mixtures. This result suggests formation of an achiral intermediate along the reaction pathway, which causes the lack of stereoselectivity in the products. Initially formed trans-dihydrobenzofuran compounds are the key precursors of many naturally occurring neolignans, and can be easily converted to 8,5’-neolignan derivatives.
Proteins are natural polypeptides produced by cells; they can be found in both animals and plants, and possess a variety of functions. One of these functions is to provide structural support to the surrounding cells and tissues. For example, collagen (which is found in skin, cartilage, tendons and bones) and keratin (which is found in hair and nails) are structural proteins. When a tissue is damaged, however, the supporting matrix formed by structural proteins cannot always spontaneously regenerate. Tailor-made synthetic polypeptides can be used to help heal and restore tissue formation.
Synthetic polypeptides are typically synthesized by the so-called ring opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCA). Such synthetic polypeptides are generally non-sequence-controlled and thus less complex than proteins. As such, synthetic polypeptides are rarely as efficient as proteins in their ability to self-assemble and form hierarchical or structural supramolecular assemblies in water, and thus, often require rational designing. In this doctoral work, two types of amino acids, γ-benzyl-L/D-glutamate (BLG / BDG) and allylglycine (AG), were selected to synthesize a series of (co)polypeptides of different compositions and molar masses.
A new and versatile synthetic route to prepare polypeptides was developed, and its mechanism and kinetics were investigated. The polypeptide properties were thoroughly studied and new materials were developed from them. In particular, these polypeptides were able to aggregate (or self-assemble) in solution into microscopic fibres, very similar to those formed by collagen. By doing so, they formed robust physical networks and organogels which could be processed into high water-content, pH-responsive hydrogels. Particles with highly regular and chiral spiral morphologies were also obtained by emulsifying these polypeptides. Such polypeptides and the materials derived from them are, therefore, promising candidates for biomedical applications.
The self-assembly of amphiphilic polymers in aqueous systems is important for a plethora of applications, in particular in the field of cosmetics and detergents. When introducing thermoresponsive blocks, the aggregation behavior of these polymers can be controlled by changing the temperature. While confined to simple diblock copolymer systems for long, the complexity - and thus the versatility - of such smart systems can be strongly enlarged, once designed monomers, specific block sizes, different architectures, or additional functional groups such as hydrophobic stickers are implemented. In this work, the structure-property relationship of such thermoresponsive amphiphilic block copolymers was investigated by varying their structure systematically. The block copolymers were generally composed of a permanently hydrophobic sticker group, a permanently hydrophilic block, and a thermoresponsive block exhibiting a Lower Critical Solution Temperature (LCST) behavior. While the hydrophilic block consisted of N,N dimethylacrylamide (DMAm), different monomers were used for the thermoresponsive block, such as N n propylacrylamide (NPAm), N iso propylacrylamide (NiPAm), N,N diethylacrylamide (DEAm), N,N bis(2 methoxyethyl)acrylamide (bMOEAm), or N acryloylpyrrolidine (NAP) with different reported LCSTs of 25, 32, 33, 42 and 56 °C, respectively. The block copolymers were synthesized by successive reversible addition fragmentation chain transfer (RAFT) polymerization. For the polymers with the basic linear, the twinned hydrophobic and the symmetrical quasi miktoarm architectures, the results were well defined block sizes and end groups as well as narrow molar mass distributions (Ɖ ≤ 1.3). More complex architectures, such as the twinned thermoresponsive and the non-symmetrical quasi miktoarm one, were achieved by combining RAFT polymerization with a second technique, namely atom transfer radical polymerization (ATRP) or single unit monomer insertion (SUMI), respectively. The obtained block copolymers showed well defined block sizes, but due to the complexity of these reaction paths, the dispersities were generally higher (Ɖ ≤ 1.8) and some end groups were lost.
The thermoresponsive behavior of the block copolymers was investigated by turbidimetry and dynamic light scattering (DLS). Below the phase transition temperature, the polymers were soluble in water and small micellar structures were visible. However, above the phase transition temperature, the aggregation behavior was strongly dependent on the architecture and the chemical structure of the thermoresponsive block. Thermoresponsive blocks comprising PNAP and PbMOEAm with DPn = 40 showed no cloud point (CP), since their already high LCSTs were further increased by the attached hydrophilic block. Depending on the architecture as well as on the block size, block copolymers with PNiPAm, PDEAm and PNPAm showed different CP’s. Large aggregates were visible for block copolymers with PNiPAm and PDEAm above their CP. For PNPAm containing block copolymers, the phase transition was very sensitive towards the architecture resulting in either small or large aggregates.
In addition, fluorescence studies were performed using PDMAm and PNiPAm homo and block copolymers with linear architecture, functionalized with complementary fluorescence dyes introduced at the opposite chain ends. The thermoresponsive behavior was studied in pure aqueous solution as well as in an oil in water (o/w) microemulsion. The findings indicate that the block copolymer behaves as polymeric surfactant at low temperatures, with one relatively small hydrophobic end group and an extended hydrophilic chain forming ‘hairy micelles’ similar as the other synthesized architectures. Above the phase transition temperature of the PNiPAm block, however, the copolymer behaves as associative telechelic polymer with two non-symmetrical hydrophobic end groups, which do not mix. Thus, instead of a network of bridged ‘flower micelles’, large dynamic aggregates are formed. These are connected alternatingly by the original micellar cores as well as by clusters of the collapsed PNiPAm blocks. This type of bridged micelles is even more favored in the o/w microemulsion than in pure aqueous solution.
The remarkable antifouling properties of zwitterionic polymers in controlled environments are often counteracted by their delicate mechanical stability. In order to improve the mechanical stabilities of zwitterionic hydrogels, the effect of increased crosslinker densities was thus explored. In a first approach, terpolymers of zwitterionic monomer 3-[N -2(methacryloyloxy)ethyl-N,N-dimethyl]ammonio propane-1-sulfonate (SPE), hydrophobic monomer butyl methacrylate (BMA), and photo-crosslinker 2-(4-benzoylphenoxy)ethyl methacrylate (BPEMA) were synthesized. Thin hydrogel coatings of the copolymers were then produced and photo-crosslinked. Studies of the swollen hydrogel films showed that not only the mechanical stability but also, unexpectedly, the antifouling properties were improved by the presence of hydrophobic BMA units in the terpolymers.
Based on the positive results shown by the amphiphilic terpolymers and in order to further test the impact that hydrophobicity has on both the antifouling properties of zwitterionic hydrogels and on their mechanical stability, a new amphiphilic zwitterionic methacrylic monomer, 3-((2-(methacryloyloxy)hexyl)dimethylammonio)propane-1-sulfonate (M1), was synthesized in good yields in a multistep synthesis. Homopolymers of M1 were obtained by free-radical polymerization. Similarly, terpolymers of M1, zwitterionic monomer SPE, and photo-crosslinker BPEMA were synthesized by free-radical copolymerization and thoroughly characterized, including its solubilities in selected solvents.
Also, a new family of vinyl amide zwitterionic monomomers, namely 3-(dimethyl(2-(N -vinylacetamido)ethyl)ammonio)propane-1-sulfonate (M2), 4-(dimethyl(2-(N-vinylacetamido)ethyl)ammonio)butane-1-sulfonate (M3), and 3-(dimethyl(2-(N-vinylacetamido)ethyl)ammonio)propyl sulfate (M4), together with the new photo-crosslinker 4-benzoyl-N-vinylbenzamide (M5) that is well-suited for copolymerization with vinylamides, are introduced within the scope of the present work. The monomers are synthesized with good yields developing a multistep synthesis. Homopolymers of the new vinyl amide zwitterionic monomers are obtained by free-radical polymerization and thoroughly characterized. From the solubility tests, it is remarkable that the homopolymers produced are fully soluble in water, evidence of their high hydrophilicity. Copolymerization of the vinyl amide zwitterionic monomers, M2, M3, and M4 with the vinyl amide photo-crosslinker M5 proved to require very specific polymerization conditions. Nevertheless, copolymers were successfully obtained by free-radical copolymerization under appropriate conditions.
Moreover, in an attempt to mitigate the intrinsic hydrophobicity introduced in the copolymers by the photo-crosslinkers, and based on the proven affinity of quaternized diallylamines to copolymerize with vinyl amides, a new quaternized diallylamine sulfobetaine photo-crosslinker 3-(diallyl(2-(4-benzoylphenoxy)ethyl)ammonio)propane-1-sulfonate (M6) is synthesized. However, despite a priori promising copolymerization suitability, copolymerization with the vinyl amide zwitterionic monomers could not be achieved.
In this work, the nonaqueous synthesis of binary and ternary metal oxide nanoparticles is investigated for a number of technologically important materials. A strong focus was put on studying the reaction mechanisms leading to particle formation upon solvothermal treatment of the precursors, as an understanding of the formation processes is expected to be crucial for a better control of the systems, offering the potential to tailor particle size and morphology. The synthesis of BaTiO3 was achieved by solvothermal reaction of metallic barium and titanium isopropoxide in organic solvents. Phase-pure, highly crystalline particles about 6 nm in size resulted in benzyl alcohol, whereas larger particles could be obtained in ketones such as acetone or acetophenone. In benzyl alcohol, a novel mechanism was found to lead to BaTiO3, involving a C–C coupling step between the isopropoxide ligand and the benzylic carbon of the solvent. The resulting coupling product, 4-phenyl-2-butanol, is found in almost stoichiometric yield. The particle formation in ketones proceeds via a Ti-mediated aldol condensation of the solvent, involving formal elimination of water which induces formation of the oxide. These processes also occurred when reacting solely the titanium alkoxide with ketones or aldehydes, leading to highly crystalline anatase nanoparticles for all tested solvents. In ketones, also the synthesis of nanopowders of lead zirconate titanate (PZT) was achieved, which were initially amorphous but could be crystallized by calcination at moderate temperatures. Additionally, PZT films were prepared by simply casting a suspension of the powder onto Si substrates followed by calcination.Solvothermal synthesis however is not restricted to alkoxides as precursors but is also achieved from metal acetylacetonates. The use of benzylamine as solvent proved particularly versatile, making possible the synthesis of nanocrystalline In2O3, Ga2O3, ZnO and iron oxide from the respective acetylacetonates. During the synthesis, the acetylacetonate ligand undergoes a solvolysis under C–C cleavage, resulting in metal-bound enolate ligands which, in analogy to the synthesis in ketones, induce ketimine and aldol condensation reactions. In the last section of this work, surface functionalization of anatase nanoparticles is explored. The particles were first capped with various organic ligands via a facile in situ route, which resulted in altered properties such as enhanced dispersibility in various solvents. In a second step, short functional oligopeptide segments were attached to the particles by means of a catechol linker to achieve advanced self-assembly properties.
Supercapacitors are electrochemical energy storage devices with rapid charge/discharge rate and long cycle life. Their biggest challenge is the inferior energy density compared to other electrochemical energy storage devices such as batteries. Being the most widely spread type of supercapacitors, electrochemical double-layer capacitors (EDLCs) store energy by electrosorption of electrolyte ions on the surface of charged electrodes. As a more recent development, Na-ion capacitors (NICs) are expected to be a more promising tactic to tackle the inferior energy density due to their higher-capacity electrodes and larger operating voltage. The charges are simultaneously stored by ion adsorption on the capacitive-type cathode surface and via faradic process in the battery-type anode, respectively. Porous carbon electrodes are of great importance in these devices, but the paramount problems are the facile synthetic routes for high-performance carbons and the lack of fundamental understanding of the energy storage mechanisms. Therefore, the aim of the present dissertation is to develop novel synthetic methods for (nitrogen-doped) porous carbon materials with superior performance, and to reveal a deeper understanding energy storage mechanisms of EDLCs and NICs.
The first part introduces a novel synthetic method towards hierarchical ordered meso-microporous carbon electrode materials for EDLCs. The large amount of micropores and highly ordered mesopores endow abundant sites for charge storage and efficient electrolyte transport, respectively, giving rise to superior EDLC performance in different electrolytes. More importantly, the controversial energy storage mechanism of EDLCs employing ionic liquid (IL) electrolytes is investigated by employing a series of porous model carbons as electrodes. The results not only allow to conclude on the relations between the porosity and ion transport dynamics, but also deliver deeper insights into the energy storage mechanism of IL-based EDLCs which is different from the one usually dominating in solvent-based electrolytes leading to compression double-layers.
The other part focuses on anodes of NICs, where novel synthesis of nitrogen-rich porous carbon electrodes and their sodium storage mechanism are investigated. Free-standing fibrous nitrogen-doped carbon materials are synthesized by electrospinning using the nitrogen-rich monomer (hexaazatriphenylene-hexacarbonitrile, C18N12) as the precursor followed by condensation at high temperature. These fibers provide superior capacity and desirable charge/discharge rate for sodium storage. This work also allows insights into the sodium storage mechanism in nitrogen-doped carbons. Based on this mechanism, further optimization is done by designing a composite material composed of nitrogen-rich carbon nanoparticles embedded in conductive carbon matrix for a better charge/discharge rate. The energy density of the assembled NICs significantly prevails that of common EDLCs while maintaining the high power density and long cycle life.
Facing the environmental crisis, new technologies are needed to sustain our society. In this context, this thesis aims to describe the properties and applications of carbon-based sustainable materials. In particular, it reports the synthesis and characterization of a wide set of porous carbonaceous materials with high nitrogen content obtained from nucleobases. These materials are used as cathodes for Li-ion capacitors, and a major focus is put on the cathode preparation, highlighting the oxidation resistance of nucleobase-derived materials. Furthermore, their catalytic properties for acid/base and redox reactions are described, pointing to the role of nitrogen speciation on their surfaces. Finally, these materials are used as supports for highly dispersed nickel loading, activating the materials for carbon dioxide electroreduction.
Nanostructured materials are materials consisting of nanoparticulate building blocks on the scale of nanometers (i.e. 10-9 m). Composition, crystallinity and morphology can enhance or even induce new properties of the materials, which are desirable for todays and future technological applications. In this work, we have shown new strategies to synthesise metal oxide and metal nitride nanomaterials. The first part of the work deals with the study of nonaqueous synthesis of metal oxide nanoparticles. We succeeded in the synthesis of In2O3 nanopartcles where we could clearly influence the morphology by varying the type of the precursors and the solvents; of ZnO mesocrystals by using acetonitrile as a solvent; of transition metal oxides (Nb2O5, Ta2O5 and HfO2) that are particularly hard to obtain on the nanoscale and other technologically important materials. Solvothermal synthesis however is not restricted to formation of oxide materials only. In the second part we show examples of nonaqueous, solvothermal reactions of metal nitrides, but the main focus lies on the investigation of the influence of different morphologies of metal oxide precursors on the formation of the metal nitride nanoparticles. In spite of various reports, the number and variety of nanocrystalline metal nitrides is marginally small by comparison to metal oxides; hence preformed metal oxides as precursors for the preparation of metal nitrides are a logical choice. By reacting oxide nanoparticles with cyanamide, urea or melamine, at temperatures of 800 to 900 °C under nitrogen flow metal nitrides could be obtained. We studied in detail the influence of the starting material and realized that size, crystallinity, type of nitrogen source and temperature play the most important role. We have managed to propose and verify a dissolution-recrystallisation model as the formation mechanism. Furthermore we could show that the initial morphology of the oxides could be retained when ammonia flow was used instead.
Nonaqueous synthesis of metal oxide nanoparticles and their assembly into mesoporous materials
(2006)
This thesis mainly consist of two parts, the synthesis of several kinds of technologically interesting crystalline metal oxide nanoparticles via nonaqueous sol-gel process and the formation of mesoporous metal oxides using some of these nanoparticles as building blocks via evaporation induced self-assembly (EISA) technique. In the first part, the experimental procedures and characterization results of successful syntheses of crystalline tin oxide and tin doped indium oxide (ITO) nanoparticles are reported. SnO2 nanoparticles exhibit monodisperse particle size (3.5 nm in average), high crystallinity and particularly high dispersibility in THF, which enable them to become the ideal particulate precursor for the formation of mesoporous SnO2. ITO nanoparticles possess uniform particle morphology, narrow particle size distribution (5-10 nm), high crystallinity as well as high electrical conductivity. The synthesis approaches and characterization of various mesoporous metal oxides, including TiO2, SnO2, mixture of CeO2 and TiO2, mixture of BaTiO3 and SnO2, are reported in the second part of this thesis. Mesoporous TiO2 and SnO2 are presented as highlights of this part. Mesoporous TiO2 was produced in the forms of both films and bulk material. In the case of mesoporous SnO2, the study was focused on the high order of the porous structure. All these mesoporous metal oxides show high crystallinity, high surface area and rather monodisperse pore sizes, which demonstrate the validity of EISA process and the usage of preformed crystalline nanoparticles as nanobuilding blocks (NBBs) to produce mesoporous metal oxides.
The Reversible Addition Fragmentation Chain Transfer (RAFT) process using the new RAFT agent benzyldithiophenyl acetate is shown to be a powerful polymerization tool to synthesize novel well-defined amphiphilic diblock copolymers composed of the constant hydrophobic block poly(butyl acrylate) and of 6 different hydrophilic blocks with various polarities, namely a series of non-ionic, non-ionic comb-like, anionic and cationic hydrophilic blocks. The controlled character of the polymerizations was supported by the linear increase of the molar masses with conversion, monomodal molar mass distributions with low polydispersities and high degrees of end-group functionalization. The new macro-surfactants form micelles in water, whose size and geometry strongly depend on their composition, according to dynamic and static light scattering measurements. The micellization is shown to be thermodynamically favored, due to the high incompatibility of the blocks as indicated by thermal analysis of the block copolymers in bulk. The thermodynamic state in solution is found to be in the strong or super strong segregation limit. Nevertheless, due to the low glass transition temperature of the core-forming block, unimer exchange occurs between the micelles. Despite the dynamic character of the polymeric micellar systems, the aggregation behavior is strongly dependent on the history of the sample, i.e., on the preparation conditions. The aqueous micelles exhibit high stability upon temperature cycles, except for an irreversibly precipitating block copolymer containing a hydrophilic block exhibiting a lower critical solution temperature (LCST). Their exceptional stability upon dilution indicates very low critical micelle concentrations (CMC) (below 4∙10<sup>-4 g∙L<sup>-1). All non-ionic copolymers with sufficiently long solvophobic blocks aggregated into direct micelles in DMSO, too. Additionally, a new low-toxic highly hydrophilic sulfoxide block enables the formation of inverse micelles in organic solvents. The high potential of the new polymeric surfactants for many applications is demonstrated, in comparison to reference surfactants. The diblock copolymers are weakly surface-active, as indicated by the graduate decrease of the surface tension of their aqueous solutions with increasing concentration. No CMC could be detected. Their surface properties at the air/water interface confer anti-foaming properties. The macro-surfactants synthesized are surface-active at the interface between two liquid phases, too, since they are able to stabilize emulsions. The polymeric micelles are shown to exhibit a high ability to solubilize hydrophobic substances in water.
The aim of this work was the generation of carbon materials with high surface area, exhibiting a hierarchical pore system in the macro- and mesorange. Such a pore system facilitates the transport through the material and enhances the interaction with the carbon matrix (macropores are pores with diameters > 50 nm, mesopores between 2 – 50 nm). Thereto, new strategies for the synthesis of novel carbon materials with designed porosity were developed that are in particular useful for the storage of energy. Besides the porosity, it is the graphene structure itself that determines the properties of a carbon material. Non-graphitic carbon materials usually exhibit a quite large degree of disorder with many defects in the graphene structure, and thus exhibit inherent microporosity (d < 2nm). These pores are traps and oppose reversible interaction with the carbon matrix. Furthermore they reduce the stability and conductivity of the carbon material, which was undesired for the proposed applications. As one part of this work, the graphene structures of different non-graphitic carbon materials were studied in detail using a novel wide-angle x-ray scattering model that allowed precise information about the nature of the carbon building units (graphene stacks). Different carbon precursors were evaluated regarding their potential use for the synthesis shown in this work, whereas mesophase pitch proved to be advantageous when a less disordered carbon microstructure is desired. By using mesophase pitch as carbon precursor, two templating strategies were developed using the nanocasting approach. The synthesized (monolithic) materials combined for the first time the advantages of a hierarchical interconnected pore system in the macro- and mesorange with the advantages of mesophase pitch as carbon precursor. In the first case, hierarchical macro- / mesoporous carbon monoliths were synthesized by replication of hard (silica) templates. Thus, a suitable synthesis procedure was developed that allowed the infiltration of the template with the hardly soluble carbon precursor. In the second case, hierarchical macro- / mesoporous carbon materials were synthesized by a novel soft-templating technique, taking advantage of the phase separation (spinodal decomposition) between mesophase pitch and polystyrene. The synthesis also allowed the generation of monolithic samples and incorporation of functional nanoparticles into the material. The synthesized materials showed excellent properties as an anode material in lithium batteries and support material for supercapacitors.
Conventional energy sources are diminishing and non-renewable, take million years to form and cause environmental degradation. In the 21st century, we have to aim at achieving sustainable, environmentally friendly and cheap energy supply by employing renewable energy technologies associated with portable energy storage devices. Lithium-ion batteries can repeatedly generate clean energy from stored materials and convert reversely electric into chemical energy. The performance of lithium-ion batteries depends intimately on the properties of their materials. Presently used battery electrodes are expensive to be produced; they offer limited energy storage possibility and are unsafe to be used in larger dimensions restraining the diversity of application, especially in hybrid electric vehicles (HEVs) and electric vehicles (EVs). This thesis presents a major progress in the development of LiFePO4 as a cathode material for lithium-ion batteries. Using simple procedure, a completely novel morphology has been synthesized (mesocrystals of LiFePO4) and excellent electrochemical behavior was recorded (nanostructured LiFePO4). The newly developed reactions for synthesis of LiFePO4 are single-step processes and are taking place in an autoclave at significantly lower temperature (200 deg. C) compared to the conventional solid-state method (multi-step and up to 800 deg. C). The use of inexpensive environmentally benign precursors offers a green manufacturing approach for a large scale production. These newly developed experimental procedures can also be extended to other phospho-olivine materials, such as LiCoPO4 and LiMnPO4. The material with the best electrochemical behavior (nanostructured LiFePO4 with carbon coating) was able to delive a stable 94% of the theoretically known capacity.
The key to reduce the energy required for specific transformations in a selective manner is the employment of a catalyst, a very small molecular platform that decides which type of energy to use. The field of photocatalysis exploits light energy to shape one type of molecules into others, more valuable and useful.
However, many challenges arise in this field, for example, catalysts employed usually are based on metal derivatives, which abundance is limited, they cannot be recycled and are expensive. Therefore, carbon nitrides materials are used in this work to expand horizons in the field of photocatalysis.
Carbon nitrides are organic materials, which can act as recyclable, cheap, non-toxic, heterogeneous photocatalysts. In this thesis, they have been exploited for the development of new catalytic methods, and shaped to develop new types of processes.
Indeed, they enabled the creation of a new photocatalytic synthetic strategy, the dichloromethylation of enones by dichloromethyl radical generated in situ from chloroform, a novel route for the making of building blocks to be used for the productions of active pharmaceutical compounds.
Then, the ductility of these materials allowed to shape carbon nitride into coating for lab vials, EPR capillaries, and a cell of a flow reactor showing the great potential of such flexible technology in photocatalysis.
Afterwards, their ability to store charges has been exploited in the reduction of organic substrates under dark conditions, gaining new insights regarding multisite proton coupled electron transfer processes.
Furthermore, the combination of carbon nitrides with flavins allowed the development of composite materials with improved photocatalytic activity in the CO2 photoreduction.
Concluding, carbon nitrides are a versatile class of photoactive materials, which may help to unveil further scientific discoveries and to develop a more sustainable future.
Various synthetic approaches were explored towards the preparation of poly(N-substituted glycine) homo/co-polymers (a.k.a. polypeptoids). In particular, monomers that would facilitate in the preparation of bio-relevant polymers via either chain- or step-growth polymerization were targeted. A 3-step synthetic approach towards N-substituted glycine N-carboxyanhydrides (NNCA) was implemented, or developed, and optimized allowing for an efficient gram scale preparation of the aforementioned monomer (chain-growth). After exploring several solvents and various conditions, a reproducible and efficient ring-opening polymerization (ROP) of NNCAs was developed in benzonitrile (PhCN). However, achieving molecular weights greater than 7 kDa required longer reaction times (>4 weeks) and sub-sequentially allowed for undesirable competing side reactions to occur (eg. zwitterion monomer mechanisms). A bulk-polymerization strategy provided molecular weights up to 11 kDa within 24 hours but suffered from low monomer conversions (ca. 25%). Likewise, a preliminary study towards alcohol promoted ROP of NNCAs suffered from impurities and a suspected alternative activated monomer mechanism (AAMM) providing poor inclusion of the initiator and leading to multi-modal dispersed polymeric systems. The post-modification of poly(N-allyl glycine) via thiol-ene photo-addition was observed to be quantitative, with the utilization of photo-initiators, and facilitated in the first glyco-peptoid prepared under environmentally benign conditions. Furthermore, poly(N-allyl glycine) demonstrated thermo-responsive behavior and could be prepared as a semi-crystalline bio-relevant polymer from solution (ie. annealing). Initial efforts in preparing these polymers via standard poly-condensation protocols were insufficient (step-growth). However, a thermally induced side-product, diallyl diketopiperazine (DKP), afforded the opportunity to explore photo-induced thiol-ene and acyclic diene metathesis (ADMET) polymerizations. Thiol-ene polymerization readily led to low molecular weight polymers (<2.5 kDa), that were insoluble in most solvents except heated amide solvents (ie. DMF), whereas ADMET polymerization, with diallyl DKP, was unsuccessful due to a suspected 6 member complexation/deactivation state of the catalyst. This understanding prompted the preparation of elongated DKPs most notably dibutenyl DKP. SEC data supports the aforementioned understanding but requires further optimization studies in both the preparation of the DKP monomers and following ADMET polymerization. This work was supported by NMR, GC-MS, FT-IR, SEC-IR, and MALDI-Tof MS characterization. Polymer properties were measured by UV-Vis, TGA, and DSC.
In recent years, the aim of supramolecular syntheses is not only the creation of particular structures but also the introduction of specific functions in these supramolecules. The present work describes the use of the ionic self-assembly (ISA) route to generate nanostructured materials with integrated functionality. Since the ISA strategy has proved to be a facile method for the production of liquid-crystalline materials, we investigated the phase behaviour, physical properties and function of a variety of ISA materials comprising a perylene derivative as the employed oligoelectrolyte. Functionality was introduced into the materials through the use of functional surfactants. In order to meet the requirements to produce functional ISA materials through the use of functional surfactants, we designed and synthesized pyrrole-derived monomers as surfactant building blocks. Owing to the presence of the pyrrole moiety, these surfactants are not only polymerizable but are also potentially conductive when polymerized. We adopted single-tailed and double-tailed N-substituted pyrrole monomers as target molecules. Since routine characterization analysis of the double-tailed pyrrole-containing surfactant indicated very interesting, complex phase behaviour, a comprehensive investigation of its interfacial properties and mesophase behavior was conducted. The synthesized pyrrole-derived surfactants were then employed in the synthesis of ISA complexes. The self-assembled materials were characterized and subsequently polymerized by both chemical and electrochemical methods. The changes in the structure and properties of the materials caused by the in-situ polymerization were addressed. In the second part of this work, the motif investigated was a property rather than a function. Since chiral superstructures have obtained much attention during the last few years, we investigated the possibility of chiral ISA materials through the use of chiral surfactants. Thus, the work involved synthesis of novel chiral surfactants and their incorporation in ISA materials with the aim of obtaining ionically self-assembled chiral superstructures. The results and insights presented here suggest that the presented synthesis strategy can be easily extended to incorporate any kind of charged tectonic unit with desired optical, electrical, or magnetic properties into supramolecular assemblies for practical applications.
For more than 70 years, understanding of the mechanism of particle nucleation in emulsion polymerization has been one of the most challenging issues in heterophase polymerization research. Within this work a comprehensive experimental study of particle nucleation in emulsion polymerization of styrene at 70 °C and variety of conditions has been performed. To follow the onset of nucleation, on-line conductivity measurements were applied. This technique is highly sensitive to the mobility of conducting species and hence, it can be employed to follow aggregation processes leading to particle formation. On the other hand, by recording the optical transmission (turbidity) of the reaction mixture particle growth was followed. Complementary to the on-line investigations, off-line characterizations of the particle morphology and the molecular weight have been performed. The aim was to achieve a better insight in the processes taking place after starting the reaction via particle nucleation until formation of colloidally stable latex particles. With this experimental protocol the initial period of styrene emulsion polymerization in the absence as well as in the presence of various surfactants (concentrations above and below the critical micellization concentration) and also in the presence of seed particles has been investigated. Ionic and non-ionic initiators (hydrophilic and hydrophobic types) have been applied to start the polymerizations. Following the above algorithm, experimental evidence has been obtained showing the possibility of performing surfactant-free emulsion polymerization of styrene with oil-soluble initiators. The duration of the pre-nucleation period (that is the time between starting the polymerization and nucleation) can be precisely adjusted with the initiator hydrophobicity, the equilibration time of styrene in water, and the surfactant concentration. Spontaneous emulsification of monomer in water, as soon as both phases are brought into contact, is a key factor to explain the experimental results. The equilibration time of monomer in water as well as the type and concentration of other materials in water (surfactants, seed particles, etc.) control the formation rate and the size of the emulsified droplets and thus, have a strong influence on the particle nucleation and the particle morphology. One of the main tasks was to investigate the effect of surfactant molecules and especially micelles on the nucleation mechanism. Experimental results revealed that in the presence of emulsifier micelles the conductivity pattern does not change essentially. This means that the presence of emulsifiers does not change the mechanism of particle formation qualitatively. However, surfactants assist in the nucleation process as they lower the activation free energy of particle formation. Contrary, seed particles influence particle nucleation, substantially. In the presence of seed particles above a critical volume fraction the formation of new particles can be suppressed. However, micelles and seed particles as absorbers exhibit a common behavior under conditions where monomer equilibration is not allowed. Results prove that the nucleation mechanism comprises the initiation of water soluble oligomers in the aqueous phase followed by their aggregation. The process is heterogeneous in nature due to the presence of monomer droplets.
The properties of a series of well-defined new surfactant oligomers (dimers to tetramers)were examined. From a molecular point of view, these oligomeric surfactants consist of simple monomeric cationic surfactant fragments coupled via the hydrophilic ammonium chloride head groups by spacer groups (different in nature and length). Properties of these cationic surfactant oligomers in aqueous solution such as solubility, micellization and surface activity, micellar size and aggregation number were discussed with respect to the two new molecular variables introduced, i.e. degree of oligomerization and spacer group, in order to establish structure – property relationships. Thus, increasing the degree of oligomerization results in a pronounced decrease of the critical micellization concentration (CMC). Both reduced spacer length and increased spacer hydrophobicity lead to a decrease of the CMC, but to a lesser extent. For these particular compounds, the formed micelles are relatively small and their aggregation number decreases with increasing the degree of oligomerization, increasing spacer length and sterical hindrance. In addition, pseudo-phase diagrams were established for the dimeric surfactants in more complex systems, namely inverse microemulsions, demonstrating again the important influence of the spacer group on the surfactant behaviour. Furthermore, the influence of additives on the property profile of the dimeric compounds was examined, in order to see if the solution properties can be improved while using less material. Strong synergistic effects were observed by adding special organic salts (e.g. sodium salicylate, sodium vinyl benzoate, etc.) to the surfactant dimers in stoichiometric amounts. For such mixtures, the critical aggregation concentration is strongly shifted to lower concentration, the effect being more pronounced for dimers than for analogous monomers. A sharp decrease of the surface tension can also be attained. Many of the organic anions produce viscoelastic solutions when added to the relatively short-chain dimers in aqueous solution, as evidenced by rheological measurements. This behaviour reflects the formation of entangled wormlike micelles due to strong interactions of the anions with the cationic surfactants, decreasing the curvature of the micellar aggregates. It is found that the associative behaviour is enhanced by dimerization. For a given counterion, the spacer group may also induce a stronger viscosifying effect depending on its length and hydrophobicity. Oppositely charged surfactants were combined with the cationic dimers, too. First, some mixtures with the conventional anionic surfactant SDS revealed vesicular aggregates in solution. Also, in view of these catanionic mixtures, a novel anionic dimeric surfactant based on EDTA was synthesized and studied. The synthesis route is relatively simple and the compound exhibits particularly appealing properties such as low CMC and σCMC values, good solubilization capacity of hydrophobic probes and high tolerance to hard water. Noteworthy, mixtures with particular cationic dimers gave rise to viscous solutions, reflecting the micelle growth.
In the first section of the thesis graphitic carbon nitride was for the first time synthesised using the high-temperature condensation of dicyandiamide (DCDA) – a simple molecular precursor – in a eutectic salt melt of lithium chloride and potassium chloride. The extent of condensation, namely next to complete conversion of all reactive end groups, was verified by elemental microanalysis and vibrational spectroscopy. TEM- and SEM-measurements gave detailed insight into the well-defined morphology of these organic crystals, which are not based on 0D or 1D constituents like known molecular or short-chain polymeric crystals but on the packing motif of extended 2D frameworks. The proposed crystal structure of this g-C3N4 species was derived in analogy to graphite by means of extensive powder XRD studies, indexing and refinement. It is based on sheets of hexagonally arranged s-heptazine (C6N7) units that are held together by covalent bonds between C and N atoms. These sheets stack in a graphitic, staggered fashion adopting an AB-motif, as corroborated by powder X-ray diffractometry and high-resolution transmission electron microscopy. This study was contrasted with one of many popular – yet unsuccessful – approaches in the last 30 years of scientific literature to perform the condensation of an extended carbon nitride species through synthesis in the bulk. The second section expands the repertoire of available salt melts introducing the lithium bromide and potassium bromide eutectic as an excellent medium to obtain a new phase of graphitic carbon nitride. The combination of SEM, TEM, PXRD and electron diffraction reveals that the new graphitic carbon nitride phase stacks in an ABA’ motif forming unprecedentedly large crystals. This section seizes the notion of the preceding chapter, that condensation in a eutectic salt melt is the key to obtain a high degree of conversion mainly through a solvatory effect. At the close of this chapter ionothermal synthesis is seen established as a powerful tool to overcome the inherent kinetic problems of solid state reactions such as incomplete polymerisation and condensation in the bulk especially when the temperature requirement of the reaction in question falls into the proverbial “no man’s land” of classical solvents, i.e. above 250 to 300 °C. The following section puts the claim to the test, that the crystalline carbon nitrides obtained from a salt melt are indeed graphitic. A typical property of graphite – namely the accessibility of its interplanar space for guest molecules – is transferred to the graphitic carbon nitride system. Metallic potassium and graphitic carbon nitride are converted to give the potassium intercalation compound, K(C6N8)3 designated according to its stoichiometry and proposed crystal structure. Reaction of the intercalate with aqueous solvents triggers the exfoliation of the graphitic carbon nitride material and – for the first time – enables the access of singular (or multiple) carbon nitride sheets analogous to graphene as seen in the formation of sheets, bundles and scrolls of carbon nitride in TEM imaging. The thus exfoliated sheets form a stable, strongly fluorescent solution in aqueous media, which shows no sign in UV/Vis spectroscopy that the aromaticity of individual sheets was subject to degradation. The final section expands on the mechanism underlying the formation of graphitic carbon nitride by literally expanding the distance between the covalently linked heptazine units which constitute these materials. A close examination of all proposed reaction mechanisms to-date in the light of exhaustive DSC/MS experiments highlights the possibility that the heptazine unit can be formed from smaller molecules, even if some of the designated leaving groups (such as ammonia) are substituted by an element, R, which later on remains linked to the nascent heptazine. Furthermore, it is suggested that the key functional groups in the process are the triazine- (Tz) and the carbonitrile- (CN) group. On the basis of these assumptions, molecular precursors are tailored which encompass all necessary functional groups to form a central heptazine unit of threefold, planar symmetry and then still retain outward functionalities for self-propagated condensation in all three directions. Two model systems based on a para-aryl (ArCNTz) and para-biphenyl (BiPhCNTz) precursors are devised via a facile synthetic procedure and then condensed in an ionothermal process to yield the heptazine based frameworks, HBF-1 and HBF-2. Due to the structural motifs of their molecular precursors, individual sheets of HBF-1 and HBF-2 span cavities of 14.2 Å and 23.0 Å respectively which makes both materials attractive as potential organic zeolites. Crystallographic analysis confirms the formation of ABA’ layered, graphitic systems, and the extent of condensation is confirmed as next-to-perfect by elemental analysis and vibrational spectroscopy.