Synthesis and Nano-Patterning of Novel Stimuli-Responsive Polymers

Abstract

In this thesis reactive ester polymers were utilized for both the preparation of novel multi-stimuli responsive polymers and for the fabrication of polymeric nano-objects. The reactive ester polymers were synthesized via the controlled radical RAFT polymerization technique, yielding in well-defined polymers with narrow molecular weight distribution and could be further modified with different amines in a subsequent post-polymerization modification. As a result highly-functionalized polymers could be obtained. This reactive ester approach was successfully applied for different individual projects: Dual-responsive poly(N-isopropyl acrylamide) (PNIPAM) polymers were synthesized, equipped with carbon dioxide (CO2) sensitive moieties based on. The tertiary amine side groups were able to form bicarbonate/ammonium complexes, when CO2 was passed through the polymeric solution. The formed complex rendered the polymeric system more hydrophilic, which in turn lead to an increase of the lower critical solution temperature (LCST). The full reversibility of the CO2 binding process was investigated by turbidity measurement and consecutive purging with either CO2 or argon. The principle of triggering the LCST by introducing stimuli-sensitive moieties was further enhanced and triple-responsive polymers were synthesized. Based on PNIPAM again, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (Amino-TEMPO) and an amine-modified azobenzene were introduced to the polymer. Induced by the independent addressing of the amino-TEMPO (redox-sensitive) and azobenzene (light-sensitive) moieties, the extensive investigation of the LCST behavior revealed an even more complex LCST characteristic in contrast to the dual-responsive system. The quantity of the LCST was dependent on the stimulus, which was applied first to the polymeric system. Above that, the LCST could be fine-tuned by the degree of the oxidized TEMPO derivative present in the polymer. In contrast to the investigation of stimuli-responsive polymers in solution, the reactive ester strategy was transferred to polymer brushes, which could be obtained by a surface initiated RAFT polymerization, utilizing siloxane containing chain transfer agent (CTA) immobilized on a solid substrate. The fabricated reactive brushes of poly(pentafluorophenyl acrylate) (PPFPA) were further modified with a spiropyrane derivative. The resulting light-responsive wetting behavior could be monitored by determination of the contact angle. Furthermore, a photo-lithography technique was applied, caused by the photo-degradation of the CTA. The resulting micro-pattern of polymer brushes was modified with a fluorescent dye and visualized by confocal laser scanning microscopy. The translation of stimuli-responsive polymers to nano-confined matter was accomplished by a template-assisted approach, utilizing anodic aluminum oxide (AAO). AAO substrates provide hexagonal ordered, mesoscopic pores, which were precisely adjusted by the anodization conditions. Multilayered covalently-bonded polymeric nanotubes were fabricated by an AAO-assisted layer-by-layer (LbL) approach, by the alternating deposition of PPFPA and poly(allylamine) within the nano-confined geometry. The nano-tubes revealed remaining amine functionalities, which were further modified with the fluorescent dye rhodamine B in a subsequent post-modification and visualized by confocal laser scanning microscopy. The polymeric nanotubes, exhibited excellent resistance against any organic solvent and allow further modification by a post-modification. Finally, the funnel-shape of the AAO pores was exploited to fabricate hetero-structured Janus Particles. The particles, comprising of an isotropic silica sphere and an anisotropic polymer rod, were fabricated by wetting the AAO substrate with both a siloxane- and a reactive ester containing polymer, followed by the deposition of the silica particles on top of the AAO substrate. The subsequent thermal annealing ensured the covalent linkage of the inorganic to the organic part. The results suggest the potential for the fabrication of highly functionalized stimuli-responsive softmatter and hybrid nano-objects, which can be utilized in both research and diverse applications.