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A one-step moderate energy vibrational emulsification method was successfully employed to produce thermo-responsive olive/silicone-based Janus emulsions stabilized by poly(N,N-diethylacrylamide) carrying 0.7 mol% oleoyl side chains. Completely engulfed emulsion droplets remained stable at room temperature and could be destabilized on demand upon heating to the transition temperature of the polymeric stabilizer. Time-dependent light micrographs demonstrate the temperature-induced breakdown of the Janus droplets, which opens new aspects of application, for instance in biocatalysis.
The impact of the orientation of zwitterionic groups, with respect to the polymer backbone, on the antifouling performance of thin hydrogel films made of polyzwitterions is explored. In an extension of the recent discussion about differences in the behavior of polymeric phosphatidylcholines and choline phosphates, a quasi-isomeric set of three poly(sulfobetaine methacrylate)s is designed for this purpose. The design is based on the established monomer 3-[N-2-(methacryloyloxy)ethyl-N,N-dimethyl]ammonio-propane-1-sulfonate and two novel sulfobetaine methacrylates, in which the positions of the cationic and the ionic groups relative to the polymerizable group, and thus also to the polymer backbone, are altered. The effect of the varied segmental dipole orientation on their water solubility, wetting behavior by water, and fouling resistance is compared. As model systems, the adsorption of the model proteins bovine serum albumin (BSA), fibrinogen, and lysozyme onto films of the various polyzwitterion surfaces is studied, as well as the settlement of a diatom (Navicula perminuta) and barnacle cyprids (Balanus improvisus) as representatives of typical marine fouling communities. The results demonstrate the important role of the zwitterionic group's orientation on the polymer behavior and fouling resistance
The fabrication of 1D nanostrands composed of stimuli responsive microgels has been shown in this work. Microgels are well known materials able to respond to various stimuli from outer environment. Since these microgels respond via a volume change to an external stimulus, a targeted mechanical response can be achieved. Through carefully choosing the right composition of the polymer matrix, microgels can be designed to react precisely to the targeted stimuli (e.g. drug delivery via pH and temperature changes, or selective contractions through changes in electrical current125).
In this work, it was aimed to create flexible nano-filaments which are capable of fast anisotropic contractions similar to muscle filaments. For the fabrication of such filaments or strands, nanostructured templates (PDMS wrinkles) were chosen due to a facile and low-cost fabrication and versatile tunability of their dimensions. Additionally, wrinkling is a well-known lithography-free method which enables the fabrication of nanostructures in a reproducible manner and with a high long-range periodicity.
In Chapter 2.1, it was shown for the first time that microgels as soft matter particles can be aligned to densely packed microgel arrays of various lateral dimensions. The alignment of microgels with different compositions (e.g. VCL/AAEM, NIPAAm, NIPAAm/VCL and charged microgels) was shown by using different assembly techniques (e.g. spin-coating, template confined molding). It was chosen to set one experimental parameter constant which was the SiOx surface composition of the templates and substrates (e.g. oxidized PDMS wrinkles, Si-wafers and glass slides). It was shown that the fabrication of nanoarrays was feasible with all tested microgel types. Although the microgels exhibited different deformability when aligned on a flat surface, they retained their thermo-responsivity and swelling behavior.
Towards the fabrication of 1D microgel strands interparticle connectivity was aspired. This was achieved via different cross-linking methods (i.e. cross-linking via UV-irradiation and host-guest complexation) discussed in Chapter 2.2. The microgel arrays created by different assembly methods and microgel types were tested for their cross-linking suitability. It was observed that NIPAAm based microgels cannot be cross-linked with UV light. Furthermore, it was found that these microgels exhibit a strong surface-particle-interaction and therefore could not be detached from the given substrates. In contrast to the latter, with VCL/AAEM based microgels it was possible to both UV cross-link them based on the keto-enol tautomerism of the AAEM copolymer, and to detach them from the substrate due to the lower adhesion energy towards SiOx surfaces. With VCL/AAEM microgels long, one-dimensional microgel strands could be re-dispersed in water for further analysis. It has also been shown that at least one lateral dimension of the free dispersed 1D microgel strands is easily controllable by adjusting the wavelength of the wrinkled template. For further work, only VCL/AAEM based microgels were used to focus on the main aim of this work, i.e. the fabrication of 1D microgel nanostrands.
As an alternative to the unspecific and harsh UV cross-linking, the host-guest complexation via diazobenzene cross-linkers and cyclodextrin hosts was explored. The idea behind this approach was to give means to a future construction kit-like approach by incorporation of cyclodextrin comonomers in a broad variety of particle systems (e.g. microgels, nanoparticles). For this purpose, VCL/AAEM microgels were copolymerized with different amounts of mono-acrylate functionalized β-cyclodextrin (CD). After successfully testing the cross-linking capability in solution, the cross-linking of aligned VCL/AAEM/CD microgels was tried. Although the cross-linking worked well, once the single arrays came into contact to each other, they agglomerated. As a reason for this behavior residual amounts of mono-complexed diazobenzene linkers were suspected. Thus, end-capping strategies were tried out (e.g. excess amounts of β-cyclodextrin and coverage with azobenzene functionalized AuNPs) but were unsuccessful. With deeper thought, entropy effects were taken into consideration which favor the release of complexed diazobenzene linker leading to agglomerations. To circumvent this entropy driven effect, a multifunctional polymer with 50% azobenzene groups (Harada polymer) was used. First experiments with this polymer showed promising results regarding a less pronounced agglomeration (Figure 77). Thus, this approach could be pursued in the future. In this chapter it was found out that in contrast to pearl necklace and ribbon like formations, particle alignment in zigzag formation provided the best compromise in terms of stability in dispersion (see Figure 44a and Figure 51) while maintaining sufficient flexibility.
For this reason, microgel strands in zigzag formation were used for the motion analysis described in Chapter 2.3. The aim was to observe the properties of unrestrained microgel strands in solution (e.g. diffusion behavior, rotational properties and ideally, anisotropic contraction after temperature increase). Initially, 1D microgel strands were manipulated via AFM in a liquid cell setup. It could be observed that the strands required a higher load force compared to single microgels to be detached from the surface. However, with the AFM it was not possible to detach the strands in a controllable manner but resulted in a complete removal of single microgel particles and a tearing off the strands from the surface, respectively. For this reason, to observe the motion behavior of unrestrained microgel strands in solution, confocal microscopy was used. Furthermore, to hinder an adsorption of the strands, it was found out that coating the surface of the substrates with a repulsive polymer film was beneficial. Confocal and wide-field microscopy videos showed that the microgel strands exhibit translational and rotational diffusive motion in solution without perceptible bending. Unfortunately, with these methods the detection of the anisotropic stimuli responsive contraction of the free moving microgel strands was not possible. To summarize, the flexibility of microgel strands is more comparable to the mechanical behavior of a semi flexible cable than to a yarn. The strands studied here consist of dozens or even hundreds of discrete submicron units strung together by cross-linking, having few parallels in nanotechnology.
With the insights gained in this work on microgel-surface interactions, in the future, a targeted functionalization of the template and substrate surfaces can be conducted to actively prevent unwanted microgel adsorption for a given microgel system (e.g. PVCL and polystyrene coating235). This measure would make the discussed alignment methods more diverse. As shown herein, the assembly methods enable a versatile microgel alignment (e.g. microgel meshes, double and triple strands). To go further, one could use more complex templates (e.g. ceramic rhombs and star shaped wrinkles (Figure 14) to expand the possibilities of microgel alignment and to precisely control their aspect ratios (e.g. microgel rods with homogeneous size distributions).
Precision agriculture (PA) strongly relies on spatially differentiated sensor information. Handheld instruments based on laser-induced breakdown spectroscopy (LIBS) are a promising sensor technique for the in-field determination of various soil parameters. In this work, the potential of handheld LIBS for the determination of the total mass fractions of the major nutrients Ca, K, Mg, N, P and the trace nutrients Mn, Fe was evaluated. Additionally, other soil parameters, such as humus content, soil pH value and plant available P content, were determined. Since the quantification of nutrients by LIBS depends strongly on the soil matrix, various multivariate regression methods were used for calibration and prediction. These include partial least squares regression (PLSR), least absolute shrinkage and selection operator regression (Lasso), and Gaussian process regression (GPR). The best prediction results were obtained for Ca, K, Mg and Fe. The coefficients of determination obtained for other nutrients were smaller. This is due to much lower concentrations in the case of Mn, while the low number of lines and very weak intensities are the reason for the deviation of N and P. Soil parameters that are not directly related to one element, such as pH, could also be predicted. Lasso and GPR yielded slightly better results than PLSR. Additionally, several methods of data pretreatment were investigated.
Excellent conversion efficiencies of over 20% and facile cell production have placed hybrid perovskites at the forefront of novel solar cell materials, with CH3NH3PbI3 being an archetypal compound. The question why CH3NH3PbI3 has such extraordinary characteristics, particularly a very efficient power conversion from absorbed light to electrical power, is hotly debated, with ferroelectricity being a promising candidate. This does, however, require the crystal structure to be non-centrosymmetric and we herein present crystallographic evidence as to how the symmetry breaking occurs on a crystallographic and, therefore, long-range level. Although the molecular cation CH3NH3+ is intrinsically polar, it is heavily disordered and this cannot be the sole reason for the ferroelectricity. We show that it, nonetheless, plays an important role, as it distorts the neighboring iodide positions from their centrosymmetric positions.
Temperature-memory technology was utilized to generate flat substrates with a programmable stiffness pattern from cross-linked poly(ethylene-co-vinyl acetate) substrates with cylindrical microstructures. Programmed substrates were obtained by vertical compression at temperatures in the range from 60 to 100 degrees C and subsequent cooling, whereby a flat substrate was achieved by compression at 72 degrees C, as documented by scanning electron microscopy and atomic force microscopy (AFM). AFM nanoindentation experiments revealed that all programmed substrates exhibited the targeted stiffness pattern. The presented technology for generating polymeric substrates with programmable stiffness pattern should be attractive for applications such as touchpads. optical storage, or cell instructive substrates.
Due to its bioavailability and (bio)degradability, poly(lactide) (PLA) is an interesting polymer that is already being used as packaging material, surgical seam, and drug delivery system. Dependent on various parameters such as polymer composition, amphiphilicity, sample preparation, and the enantiomeric purity of lactide, PLA in an amphiphilic block copolymer can affect the self-assembly behavior dramatically. However, sizes and shapes of aggregates have a critical effect on the interactions between biological and drug delivery systems, where the general understanding of these polymers and their ability to influence self-assembly is of significant interest in science.
The first part of this thesis describes the synthesis and study of a series of linear poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA)-based amphiphilic block copolymers with varying PLA (hydrophobic), and poly(ethylene glycol) (PEG) (hydrophilic) chain lengths and different block copolymer sequences (PEG-PLA and PLA-PEG). The PEG-PLA block copolymers were synthesized by ring-opening polymerization of lactide initiated by a PEG-OH macroinitiator. In contrast, the PLA-PEG block copolymers were produced by a Steglich-esterification of modified PLA with PEG-OH.
The aqueous self-assembly at room temperature of the enantiomerically pure PLLA-based block copolymers and their stereocomplexed mixtures was investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). Spherical micelles and worm-like structures were produced, whereby the obtained self-assembled morphologies were affected by the lactide weight fraction in the block copolymer and self-assembly time. The formation of worm-like structures increases with decreasing PLA-chain length and arises from spherical micelles, which become colloidally unstable and undergo an epitaxial fusion with other micelles. As shown by DSC experiments, the crystallinity of the corresponding PLA blocks increases within the self-assembly time. However, the stereocomplexed self-assembled structures behave differently from the parent polymers and result in irregular-shaped clusters of spherical micelles. Additionally, time-dependent self-assembly experiments showed a transformation, from already self-assembled morphologies of different shapes to more compact micelles upon stereocomplexation.
In the second part of this thesis, with the objective to influence the self-assembly of PLA-based block copolymers and its stereocomplexes, poly(methyl phosphonate) (PMeP) and poly(isopropyl phosphonate) (PiPrP) were produced by ring-opening polymerization to implement an alternative to the hydrophilic block PEG. Although, the 1,8 diazabicyclo[5.4.0]unde 7 ene (DBU) or 1,5,7 triazabicyclo[4.4.0]dec-5-ene (TBD) mediated synthesis of the corresponding poly(alkyl phosphonate)s was successful, however, not so the polymerization of copolymers with PLA-based precursors (PLA-homo polymers, and PEG-PLA block copolymers). Transesterification, obtained by 1H-NMR spectroscopy, between the poly(phosphonate)- and PLA block caused a high-field shifted peak split of the methine proton in the PLA polymer chain, with split intensities depending on the used catalyst (DBU for PMeP, and TBD for PiPrP polymerization). An additional prepared block copolymer PiPrP-PLLA that wasn’t affected in its polymer sequence was finally used for self-assembly experiments with PLA-PEG and PEG-PLA mixing.
This work provides a comprehensive study of the self-assembly behavior of PLA-based block copolymers influenced by various parameters such as polymer block lengths, self-assembly time, and stereocomplexation of block copolymer mixtures.
Plasmonic metal nanostructures can be tuned to efficiently interact with light, converting the photons into energetic charge carriers and heat. Therefore, the plasmonic nanoparticles such as gold and silver nanoparticles act as nano-reactors, where the molecules attached to their surfaces benefit from the enhanced electromagnetic field along with the generated energetic charge carriers and heat for possible chemical transformations. Hence, plasmonic chemistry presents metal nanoparticles as a unique playground for chemical reactions on the nanoscale remotely controlled by light. However, defining the elementary concepts behind these reactions represents the main challenge for understanding their mechanism in the context of the plasmonically assisted chemistry.
Surface-enhanced Raman scattering (SERS) is a powerful technique employing the plasmon-enhanced electromagnetic field, which can be used for probing the vibrational modes of molecules adsorbed on plasmonic nanoparticles. In this cumulative dissertation, I use SERS to probe the dimerization reaction of 4-nitrothiophenol (4-NTP) as a model example of plasmonic chemistry. I first demonstrate that plasmonic nanostructures such as gold nanotriangles and nanoflowers have a high SERS efficiency, as evidenced by probing the vibrations of the rhodamine dye R6G and the 4-nitrothiophenol 4-NTP. The high signal enhancement enabled the measurements of SERS spectra with a short acquisition time, which allows monitoring the kinetics of chemical reactions in real time.
To get insight into the reaction mechanism, several time-dependent SERS measurements of the 4-NTP have been performed under different laser and temperature conditions. Analysis of the results within a mechanistic framework has shown that the plasmonic heating significantly enhances the reaction rate, while the reaction is probably initiated by the energetic electrons. The reaction was shown to be intensity-dependent, where a certain light intensity is required to drive the reaction. Finally, first attempts to scale up the plasmonic catalysis have been performed showing the necessity to achieve the reaction threshold intensity. Meanwhile, the induced heat needs to quickly dissipate from the reaction substrate, since otherwise the reactants and the reaction platform melt. This study might open the way for further work seeking the possibilities to quickly dissipate the plasmonic heat generated during the reaction and therefore, scaling up the plasmonic catalysis.
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.