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‘Smart’ Janus emulsions
(2021)
Emulsions constitute one of the most prominent and continuously evolving research areas in Colloid Chemistry, which involves the preparation of mixtures or dispersions of immiscible components in a continuous medium. Besides conventional oil-in-water or water-in-oil emulsions, other emulsions of complex droplet morphologies have recently attracted significant research interests. Especially Janus emulsions, in which each droplet is comprised of two distinct sub-regions, have shown versatile potential applications. One of their advantages is the possibility of compartmentalization, which enables to play with two different chemistries in a single droplet. Though microfluidic methods are conventionally used to prepare Janus emulsions, their industrial applications are largely hindered by low throughput and extensive instrumentations. Recently, it has been discovered that simply one-pot moderate/high energy emulsification is also capable of developing Janus morphology, although their preparation and stabilization remain rather substantially challenging. This cumulative doctoral thesis focuses on the preparation and characterization of ‘smart’ Janus emulsions, i.e. Janus emulsions with special stimuli-responsive features. One-step moderate/high energy emulsification of olive and silicone oil in an aqueous medium was carried out. Special consideration was devoted to the interfacial tensions among the components to maintain the criteria of forming characteristic droplet architectures, in addition to avoiding multiple emulsion destabilization phenomena like imminent phase separation or even separated droplet formation. A series of investigations were conducted related to the formation of complexes of charged macromolecules and role of them as stabilizers to achieve stable Janus emulsions for a realistic timeframe (more than 3 months). The correlation between the size of the stabilizer particles and the droplet size of emulsion was established. Furthermore, it was observed that Janus emulsion gels with interesting rheological properties can be fabricated in the presence of suitable polyelectrolyte complexes. Janus emulsions that could be influenced by pH, temperature or magnetic field were successfully produced in presence of characteristic stimuli-responsive stabilizers. Afterwards, the effect of these changes was studied by different characterization techniques. The size and morphology could be tuned easily by changing the pH. The incorporation of iron oxide magnetic nanoparticles (synthesized separately by a co-precipitation method) to one component of the Janus emulsion was carried out so that the movement and orientation of the complex droplets in aqueous media could be controlled by an external magnetic field. Additionally, temperature-triggered instantaneous reversible breakdown of Janus droplets was also accomplished. The responses of the Janus droplets by the stimuli were well-documented and explained. Another goal of the present contribution was to exploit this special morphological feature of emulsions as a template for producing porous materials. This was demonstrated by the preparation of ultralight magnetic responsive aerogels, utilizing Janus emulsion gels. The produced aerogels also showed the capacity to separate toxic dye from water. To the best of our knowledge, this is the first example of investigation towards batch scale production of Janus emulsion with such special stimuli-responsive properties by a simple bulk emulsification method.
Zur Charakterisierung von amphiphilen Blockcopolymeren aus N-Vinylpryrrolidon und Vinylacetat
(2009)
Untersuchungen zur Entwicklung und Synthese neuartiger Gelenkstäbe basierend auf Oligospiroketalen
(2019)
Totalsynthese benzoannellierter Sauerstoffheterocyclen durch Mikrowellen induzierte Tandem-Sequenzen
(2019)
Thermodynamics, kinetics and rheology of surfactant adsorption layers at water/oil interfaces
(2012)
Nanoparticles of magnetite (Fe3O4) are envisioned to find used in diverse applications, ranging from magnetic data storage, inks, ferrofluids as well as in magnetic resonance imaging, drug delivery, and hyperthermia cancer treatment. Their magnetic properties strongly depend on their size and morphology, two properties that can be synthetically controlled. Achieving appropriate control under soft chemical conditions has so far remained a challenging endeavor. One proven way of exerting this desired control has been using a biomimetic approach that emulates the proteome of magnetotactic bacteria by adding poly-L-arginine in the co- precipitation of ferrous and ferric chloride. The objective of the work presented here is to understand the impact of this polycation on the formation mechanism of magnetite and, through rational design, to enhance the control we can exert on magnetite nanoparticle size and morphology. We developed a SAXS setup to temporally and structurally resolve the formation of magnetite in the presence of poly-L-arginine in situ. Using analytical scattering models, we were able to separate the scattering contribution of a low-density 5 nm iron structure from the contribution of the growing nanoparticles. We identified that the low-density iron structure is a metastable precursor to the magnetite particles and that it is electrostatically stabilized by poly-L-arginine. In a process analogous to biomineralization, the presence of the charged macromolecule thus shifts the reaction mechanism from a thermodynamically controlled one to a kinetically controlled one. We identify this shift in reactions mechanism as the cornerstone of the proposed mechanism and as the crucial step in the paradigm of this extraordinary nanoparticle morphology and size control. Based on SAXS data, theoretical considerations suggest that an observed morphological transition between spherical, solid, and sub-structured mesocrystalline magnetite nanoparticles is induced through a pH-driven change in the wettability of the nanoparticle surface. With these results, we further demonstrate that SAXS can be an invaluable tool for investigating nanoparticle formation. We were able to change particle morphology from spherically solid particles to sub-structured mesocrystals merely by changing the precipitation pH. Improving the synthesis sustainability by substituting poly-L-arginine with renewable, polysaccharide-based polycations produced at the metric ton scale, we demonstrated that the ability to alter the reaction mechanism of magnetite can be generically attributed to the presence of polycations. Through meticulous analysis and the understanding of the formation mechanism, we were able to exert precise control over particle size and morphology, by adapting crucial synthesis parameters. We were thus able to grow mesocrystals up to 200 nm and solid nanocrystals of 100 nm by adding virtually any strong polycation. We further found a way to produce stable single domain magnetite at only slightly increased alkalinity, as magnetotactic bacteria do it. Thus through the understanding of the biological system, the consecutive biomimetic synthesis of magnetite and the following understanding of the mechanism involved in the in vitro synthesis, we managed to improve the synthetic control over the co-precipitation of magnetite, coming close biomineralization of magnetite in magnetotactic bacteria. Polyanions, in both natural as well as in synthetic systems, have been in the spotlight of recent research, yet our work shows the pivotal influence polycations have on the nucleation of magnetite. This work will contribute significantly to our ability to tailor magnetite nanoparticle size and morphology; in addition, we presume it will provide us with a model system for studying biomineralization of magnetite in vitro, putting the spotlight on the important influence of polycations, which have not had the scientific attention they deserve.
Tetrahalidocuprat(II)-komplexe : Untersuchungen zur Relation von Struktur- und EPR-Parametern
(2013)
Synthesis of stimuli-responsive and switchable inorganic nanoparticles for biomedical applications
(2010)
This thesis focuses on the synthesis of novel functional materials based on plasmonic nanoparticles. Three systems with targeted surface modification and functionalization have been designed and synthesized, involving modified perylenediimide doped silica-coated silver nanowires, polydopamine or TiO2 coated gold-palladium nanorods and thiolated poly(ethylene glycol) (PEG-SH)/dodecanethiol (DDT) modified silver nanospheres. Their possible applications as plasmonic resonators, chiral sensors as well as photo-catalysts have been studied. In addition, the interaction between silver nanospheres and 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules has also been investigated in detail.
In the first part of the thesis, surface modification on Ag nanowires (NWs) with optimized silica coating through a modified Stöber method has been firstly conducted, employing sodium hydroxide (NaOH) to replace ammonia solution (NH4OH). The coated silver nanowires with a smooth silica shell have been investigated by single-particle dark-field scattering spectroscopy, transmission electron microscopy and electron-energy loss spectroscopy to characterize the morphologies and structural components. The silica-coated silver nanowires can be further functionalized with fluorescent molecules in the silica shell via a facile one-step coating method. The as-synthesized nanowire is further coupled with a gold nanosphere by spin-coating for the application of the sub-diffractional chiral sensor for the first time. The exciton-plasmon-photon interconversion in the system eases the signal detection in the perfectly matched 1D nanostructure and contributes to the high contrast of the subwavelength chiral sensing for the polarized light.
In the second part of the thesis, dumbbell-shaped Au-Pd nanorods coated with a layer of polydopamine (PDA) or titanium dioxide (TiO2) have been constructed. The PDA- and TiO2- coated Au-Pd nanorods show a strong photothermal conversion performance under NIR illumination. Moreover, the catalytic performance of the particles has been investigated using the reduction of 4-nitrophenol (4-NP) as the model reaction. Under light irradiation, the PDA-coated Au-Pd nanorods exhibit a superior catalytic activity by increasing the reaction rate constant of 3 times. The Arrhenius-like behavior of the reaction with similar activation energies in the presence and absence of light irradiation indicates the photoheating effect to be the dominant mechanism of the reaction acceleration. Thus, we attribute the enhanced performance of the catalysis to the strong photothermal effect that is driven by the optical excitation of the gold surface plasmon as well as the synergy with the PDA layer.
In the third part, the kinetic study on the adsorption of 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4TCNQ) on the surface of Ag nanoparticles (Ag NPs) in chloroform has been reported in detail. Based on the results obtained from the UV-vis-NIR absorption spectroscopy, cryogenic transmission electron microscopy (cryo-TEM), scanning nano-beam electron diffraction (NBED) and electron energy loss spectroscopy (EELS), a two-step interaction kinetics has been proposed for the Ag NPs and F4TCNQ molecules. It includes the first step of electron transfer from Ag NPs to F4TCNQ indicated by the ionization of F4TCNQ, and the second step of the formation of Ag-F4TCNQ complex. The whole process has been followed via UV-vis-NIR absorption spectroscopy, which reveals distinct kinetics at two stages: the instantaneous ionization and the long-term complex formation. The kinetics and the influence of the molar ratio of Ag NPs/F4TCNQ molecules on the interaction between Ag NPs and F4TCNQ molecules in the organic solution are reported herein for the first time. Furthermore, the control experiment with silica-coated Ag NPs indicates that the charge transfer at the surface between Ag NPs and F4TCNQ molecules has been prohibited by a silica layer of 18 nm.
This thesis covers the synthesis of conjugates of 2-Deoxy-D-ribose-5-phosphate aldolase (DERA) with suitable polymers and the subsequent immobilization of these conjugates in thin films via two different approaches.
2-Deoxy-D-ribose-5-phosphate aldolase (DERA) is a biocatalyst that is capable of converting acetaldehyde and a second aldehyde as acceptor into enantiomerically pure mono- and diyhydroxyaldehydes, which are important structural motifs in a number of pharmaceutically active compounds. Conjugation and immobilization renders the enzyme applicable for utilization in a continuously run biocatalytic process which avoids the common problem of product inhibition. Within this thesis, conjugates of DERA and poly(N-isopropylacrylamide) (PNIPAm) for immobilization via a self-assembly approach were synthesized and isolated, as well as conjugates with poly(N,N-dimethylacrylamide) (PDMAA) for a simplified and scalable spray-coating approach. For the DERA/PNIPAm-conjugates different synthesis routes were tested, including grafting-from and grafting-to, both being common methods for the conjugation. Furthermore, both lysines and cysteines were addressed for the conjugation in order to find optimum conjugation conditions. It turned out that conjugation via lysine causes severe activity loss as one lysine plays a key role in the catalyzing mechanism. The conjugation via the cysteines by a grafting-to approach using pyridyl disulfide (PDS) end-group functionalized polymers led to high conjugation efficiencies in the presence of polymer solubilizing NaSCN. The resulting conjugates maintained enzymatic activity and also gained high acetaldehyde tolerance which is necessary for their use later on in an industrial relevant process after their immobilization.
The resulting DERA/PNIPAm conjugates exhibited enhanced interfacial activity at the air/water interface compared to the single components, which is an important pre-requisite for the immobilization via the self-assembly approach. Conjugates with longer polymer chains formed homogeneous films on silicon wafers and glass slides while the ones with short chains could only form isolated aggregates. On top of that, long chain conjugates showed better activity maintenance upon the immobilization.
The crosslinking of conjugates, as well as their fixation on the support materials, are important for the mechanical stability of the films obtained from the self-assembly process. Therefore, in a second step, we introduced the UV-crosslinkable monomer DMMIBA to the PNIPAm polymers to be used for conjugation. The introduction of DMMIBA reduced the lower critical solution temperature (LCST) of the polymer and thus the water solubility at ambient conditions, resulting in lower conjugation efficiencies and in turn slightly poorer acetaldehyde tolerance of the resulting conjugates. Unlike the DERA/PNIPAm, the conjugates from the copolymer P(NIPAM-co-DMMIBA) formed continuous, homogenous films only after the crosslinking step via UV-treatment. For a firm binding of the crosslinked films, a functionalization protocol for the model support material cyclic olefin copolymer (COC) and the final target support, PAN based membranes, was developed that introduces analogue UV-reactive groups to the support surface. The conjugates immobilized on the modified COC films maintained enzymatic activity and showed good mechanical stability after several cycles of activity assessment. Conjugates with longer polymer chains, however, showed a higher degree of crosslinking after the UV-treatment leading to a pronounced loss of activity. A porous PAN membrane onto which the conjugates were immobilized as well, was finally transferred to a dead end filtration membrane module to catalyze the aldol reaction of the industrially relevant mixture of acetaldehyde and hexanal in a continuous mode. Mono aldol product was detectable, but yields were comparably low and the operational stability needs to be further improved
Another approach towards immobilization of DERA conjugates that was followed, was to generate the conjugates in situ by simply mixing enzyme and polymer and spray coat the mixture onto the membrane support. Compared to the previous approach, the focus was more put on simplicity and a possible scalability of the immobilization. Conjugates were thus only generated in-situ and not further isolated and characterized. For the conjugation, PDMAA equipped with N-2-thiolactone acrylamide (TlaAm) side chains was used, an amine-reactive comonomer that can react with the lysine residues of DERA, as well as with amino groups introduced to a desired support surface. Furthermore disulfide formation after hydrolysis of the Tla groups causes a crosslinking effect. The synthesized copolymer poly(N,N-Dimethylacrylamide-co-N-2-thiolactone acrylamide) (P(DMAA-co-TlaAm)) thus serves a multiple purpose including protein binding, crosslinking and binding to support materials. The mixture of DERA and polymer could be immobilized on the PAN support by spray-coating under partial maintenance of enzymatic activity. To improve the acetaldehyde tolerance, the polymer in used was further equipped with cysteine reactive PDS end-groups that had been used for the conjugation as described in the first part of the thesis. The generated conjugates indeed showed good acetaldehyde tolerance and were thus used to be coated onto PAN membrane supports. Post treatment with a basic aqueous solution of H2O2 was supposed to further crosslink the spray-coated film hydrolysis and oxidation of the thiolactone groups. However, a washing off of the material was observed. Optimization is thus still necessary.
Synthesis and Characterization of Upconversion Nanaparticles for Applications in Life Sciences
(2021)
Die vorliegende Arbeit beschäftigt sich mit der Synthese von Disulfiden, der Thiol-Disulfid Metathesereaktion als Möglichkeit, Polymere zu funktionalisieren, und der Synthese von Polydisulfiden. Im ersten Teil der Arbeit wird die Aminolyse von RAFT-Polymeren und die Abhängigkeit der Polymer-Polymer Disulfidbildung von der Molmasse untersucht. Dabei wurde durch die Aufnahme von Reaktionskinetiken mittels Gel-Permeations-Chromatographie (GPC) festgestellt, dass je länger die Polymerketten sind, desto weniger Disulfid Polymerkopplung tritt auf. RAFT-Polymere werden oft genutzt, um die RAFT-Polymer Endgruppe nach der Polymerisation zu modifizieren oder in einer chemischen Reaktion zu funktionalisieren. Hier kann die Aminolyse in Anwesenheit von kurzkettigen Disulfiden, wie zum Beispiel Cystin, durchgeführt werden, um die Bildung von Polymer-Polymer Disulfiden vollständig zu unterdrücken und ein endgruppenfunktionalisiertes Polymer zu erhalten. Bei dieser Reaktion greift das bei der Aminolyse entstehende Polymerthiolat die kurzkettigen Disulfide an, und es kommt zur Bildung von funktionalisierten Polymeren. Es wurde ein Polyethylenglykoldisulfid eingesetzt, um ein amphiphiles Blockcopolymer zu erhalten. Als RAFT-Polymer wurde Polystyrol (PS) verwendet, und es konnte die Bildung von Polystyrol-Polyethylenglykol Copolymeren nachgewiesen werden. Das amphiphile Polymer bildet im wässrigen Medium Vesikel. Die Oberfläche der Vesikel konnte mittels der Thiol-Disulfid Metathese umfunktionalisiert werden. Die Aminolyse von PS RAFT-Polymeren mit einem Polylaktiddisulfid oder einem Polybenzylglutamatdisulfid ergab Polystyrol-block-Polyester und Polystyrol-block-Polyaminosäuren Copolymere. Im zweiten Teil der Arbeit liegt der Fokus auf der Synthese von Polydisulfiden und ihren thermischen Eigenschaften. Es wurden verschiedene Alkyldithiole synthetisiert und mittels Wasserstoffperoxid und Triethylamin polymerisiert. Dabei konnte gezeigt werden, dass die Polymere teilkristallin sind und dass der Schmelzpunkt und die Kristallinität der Polymere mit steigender Alkylkettenlänge zwischen den Disulfidbindungen zunehmen. Die Möglichkeit einer Polymerkettenerweiterung nach der Polymerisation ist mit diesem System gegeben. Die Abbaubarkeit der Polydisulfide konnte durch den Einsatz von Thiolen im basischen Milieu gezeigt werden.
Synthese und Charatkerisierung von Rhenium(IV/V)-Komplexen mit dreizähnigen diaciden Liganden
(1998)
Synthese und Charakterisierung monomolekularer dicker Polyisobutenfilme mit ionischen Ankergruppen
(1999)
Stereoselektive Synthese von asymmetrisch Benzyl-H-deuterierten Dibenzylbutyrolacton-Lignanzen
(2004)
In recent years the development of renewable energy sources attracted much attention due to the increasing environmental pollution induced by burning fossil fuels. The growing public interest in reducing greenhouse gases and the use of pollution-free energies (bio-mass-, geothermal-, solar-, water- or wind energy) paved the way for scientific research in renewable energies. [1] Solar energy provides unlimited access and offers high applicational flexibility, which is needed for energy consumption in a modern society. The scientific interest in photovoltaics (PV) nowadays focuses on discovering new materials and improving materials properties, aiming for the production of highly efficient solar cells. Lately, a new type of absorber material based on the perovskite type structure reached power conversion efficiencies of more than 24%. [2] By varying the chemical composition the electronic properties as e.g. the band gap energy can be tuned to increase the absorption range of this absorber material. This makes them in particular attractive for use in tandem solar cells, where silicon and perovskite absorber layers are combined to absorb a large range of the vible light (28.0% efficiency). [2] However, perovskite based solar cells not only suffer from fast degradation when exposed to humidity, but also from the use of toxic elements (e.g. lead), which can result in long-term environmental damage. Therefore, the aim of this study was to determine the fundamental structural and optoelectronical properties of highly interesting hybrid perovskite materials, the MAPbX3 solid solution (MA=CH3NH3; X=I,Br,Cl) and the triple cation (FA1-xMAx)1-yCsyPbI3 solid solution (FA=HC(NH2)2). The study was performed on powder samples by using X-ray diffraction, revealing the crystal structure and solubility behavior of all solid solutions. Moreover the temperature-dependent behavior was studied using in-situ high resolution synchrotron X-ray diffraction and combinatorial thermal analysis methods. The influence of compositional changes on the band gap energy variation were observed using spectroscopic methods as photoluminescence and diffuse reflectance spectroscopy. The obtained results have shown that for the MAPb(I1-xBrx)3 solid solution a large miscibility gap in the range of 0.29 ( ± 0.02) ≤ x ≤ 0.92 ( ± 0.02) is present. This miscibility gap limits the suitable compositional range for use in thin film solar cells of mixed halide compounds. From the temperature-dependent in-situ synchrotron X-ray diffraction studies the complete T-X-phase diagram was established. Studies on the MAPb(Cl1-xBrx)3 solid solution revealed that MAPb(Cl1-xBrx)3 forms a complete solid solution series. For the triple cation (FA1-xMAx)1-yCsyPbI3 solid solution the aim was to study the formation of the d-modification in FAPbI3, which is undesired for solar cell application. This can be overcome by stabilizing the favored high temperature cubic a-modification at ambient conditions. By partial substituting the formamidinium molecule by methylammonium and cesium the stabilization of the cubic modification was successful. The solubility limit of FA1-xCsxPbI3 solid solution was determined to be x=0.1, while a full miscibility was observed for the FA1-xMAxPbI3 solid solution. For the triple cation (FA1-xMAx)1-yCsyPbI3 solid solution a solubility limit of cesium was observed to be y=0.1. The optoelectronic properties were investigated, revealing a linear change of band gap energy with chemical composition. It is demonstrated that the stabilized triple cation compound with cubic perovskite-type crystal structure shows enhanced stability of approximately six months. Furthermore, a short insight into lead-free perovskite-type materials is given, using germanium as non-toxic alternative to lead. For germanium based perovskites a fast decomposition in air was observed, due to the preferred formation of GeI4 in oxygen atmosphere. In-situ low temperature synchrotron X-ray diffraction measurements revealed a yet unknown low temperature modification of MAGeI3. [1] WESSELAK, Viktor; SCHABBACH, Thomas; LINK, Thomas; FISCHER, Joachim: Handbuch Regenerative Energietechnik. Springer, 2017 [2] NREL: Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies-190416.pdf. – 25.04.2019
The ongoing trend of miniaturizing multifunctional devices, especially for minimally-invasive medical or sensor applications demands new strategies for designing the required functional polymeric micro-components or micro-devices. Here, polymers, which are capable of active movement, when an external stimulus is applied (e.g. shape-memory polymers), are intensively discussed as promising material candidates for realization of multifunctional micro-components. In this context further research activities are needed to gain a better knowledge about the underlying working principles for functionalization of polymeric micro-scale objects with a shape-memory effect. First reports about electrospun solid microfiber scaffolds, demonstrated a much more pronounced shape-memory effect than their bulk counterparts, indicating the high potential of electrospun micro-objects.
Based on these initial findings this thesis was aimed at exploring whether the alteration of the geometry of micro-scale electrospun polymeric objects can serve as suitable parameter to tailor their shape-memory properties. The central hypothesis was that different geometries should result in different degrees of macromolecular chain orientation in the polymeric micro-scale objects, which will influence their mechanical properties as well as thermally-induced shape-memory function. As electrospun micro-scale objects, microfiber scaffolds composed of hollow microfibers with different wall thickness and electrosprayed microparticles as well as their magneto-sensitive nanocomposites all prepared from the same polymer exhibiting pronounced bulk shape-memory properties were investigated. For this work a thermoplastic multiblock copolymer, named PDC, with excellent bulk shape-memory properties, associated with crystallizable oligo(ε-caprolactone) (OCL) switching domains, was chosen for the preparation of electrospun micro-scale objects, while crystallizable oligo(p-dioxanone) (OPDO) segments serve as hard domains in PDC.
In the first part of the thesis microfiber scaffolds with different microfiber geometries (solid or hollow with different wall thickness) were discussed. Hollow microfiber based PDC scaffolds were prepared by coaxial electrospinning from a 1, 1, 1, 3, 3, 3 hexafluoro-2-propanol (HFP) solution with a polymer concentration of 13% w·v-1. Here as a first step core-shell fiber scaffolds consisting of microfibers with a PDC shell and sacrificial poly(ethylene glycol) (PEG) core are generated. The hollow PDC microfibers were achieved after dissolving the PEG core with water. The utilization of a fixed electrospinning setup and the same polymer concentration of the PDC spinning solution could ensure the fabrication of microfibers with almost identical outer diameters of 1.4 ± 0.3 µm as determined by scanning electron microscopy (SEM). Different hollow microfiber wall thicknesses of 0.5 ± 0.2 and 0.3 ± 0.2 µm (analyzed by SEM) have been realized by variation of the mass flow rate, while solid microfibers were obtained by coaxial electrospinning without supplying any core solution. Differential scanning calorimetry experiments and tensile tests at ambient temperature revealed an increase in degree of OCL crystallinity form χc,OCL = 34 ± 1% to 43 ± 1% and a decrease in elongation of break from 800 ± 40% to 200 ± 50% associated with an increase in Young´s modulus and failture stress for PDC hollow microfiber scaffolds when compared with soild fibers. The observed effects were enhanced with decreasing wall thickness of the single hollow fibers. The shape-memory properties of the electrospun PDC scaffolds were quantified by cyclic, thermomechanical tensile tests. Here, scaffolds comprising hollow microfibers exhibited lower shape fixity ratios around Rf = 82 ± 1% and higher shape recovery ratios of Rr = 67 ± 1% associated to more pronounced relaxation at constant strain during the first test cycle and a lower switching temperature of Tsw = 33 ± 1 °C than the fibrous meshes consisting of solid microfibers. These findings strongly support the central hypothesis that different fiber geometries (solid or hollow with different wall thickness) in electrospun scaffolds result in different degrees of macromolecular chain orientation in the polymeric micro-scale objects, which can be applied as design parameter for tailoring their mechanical and shape-memory properties.
The second part of the thesis deals with electrosprayed particulate PDC micro-scale objects. Almost spherical PDC microparticles with diameters of 3.9 ± 0.9 μm (as determined by SEM) were achieved by electrospraying of HFP solution with a polymer concentration of 2% w·v-1. In contrast, smaller particles with sizes of 400 ± 100 nm or 1.2 ± 0.3 μm were obtained for the magneto-sensitive composite PDC microparticles containing 23 ± 0.5 wt% superparamagnetic magnetite nanoparticles (mNPs). All prepared PDC microparticles exhibited a similar overall crystallinity like the PDC bulk material as analyzed by DSC. AFM nanoindentation results revealed no influence of the nanofiller incorporation on the local mechanical properties represented by the reduced modulus determined for pure PDC microparticles and magneto-sensitive composite PDC microparticles with similar diameters around 1.3 µm. It was found that the reduced modulus of the nanocomposite microparticles increased substantially with decreasing particles size from 2.4 ± 0.9 GPa (1.2 µm) to 11.9 ± 3.1 GPa (0.4 µm), which can be related to a higher orientation of the macromolecules at the surface of smaller sized microparticles. The magneto-sensitivity of such nanocomposite microparticles could be demonstrated in two aspects. One was by attracting/collecting the composite micro-objects with an external permanent magnet. The other one was by a inductive heating to 44 ± 1 °C, which is well above the melting transition of the OCL switching domains, when compacted to a 10 x 10 mm2 film with a thickness of 10 µm and exposed to an alternating magnet field with an magnetic field strength of 30 kA·m-1. Both functions are of great relevance for designing next generation drug delivery systems combining targeting and on demand release.
By a compression approach shape-memory functionalization of individual microparticles could be realized. Here different programming pressures and compression temperatures were applied. The shape-recovery capability of the programmed PDC microparticles was quantified by online and off-line heating experiments analyzed via microscopy measurement. The obtained shape-memory properties were found to be strongly depending on the applied programming pressure and temperature. The best shape-memory performance with a high shape recovery rate of about Rr = 80±1% was obtained when a low pressure of 0.2 MPa was applied at 55 °C. Finally, it was demonstrated that PDC microparticles can be utilized as micro building parts for preparation of a macroscopic film with temporary stability by compression of a densely packed array of PDC microparticles at 60 °C followed by subsequent cooling to ambient temperature. This film disintegrates into individual microparticles upon heating to 60 °C. Based on this technology the design of stable macroscopic release systems can be envisioned, which can be easily fixed at the site of treatment (i.e. by suturing) and disintegrate on demand to microparticles facilitating the drug release.
In summary, the results of this thesis could confirm the central hypothesis that the variation of the geometry of polymeric micro-objects is a suitable parameter to adjust their shape-memory performance by changing the degree of macromolecular chain orientation in the specimens or by enabling new functions like on demand disintegration. These fundamental findings might be relevant for designing novel miniaturized multifunctional polymer-based devices.