Designstrategien für photoschaltbare Polymer-Nanokomposite

Abstract

Silica and gold nanoparticles were functionalized with a developed photo responsive polymer in order to construct tailored self-assembled architectures from polymer nanocopmosites. Silica surfaces were functionalized with transfer agents for a surface initiated Reversible-Addition–Fragmentation-chain-Transfer- (RAFT-) polymerization and studied systematically. Hence, mono-, di, and trialkoxysilylether were incorporated in the chemical structure of RAFT agents. These RAFT agents were immobilized on planar surfaces and analyzed via atomic force microscopy. It could be found that di- and trifunctional anchor groups were bound to the surface as interconnected cluster, when toluene was used as an immobilization solvent. Dynamic light scattering (DLS) revealed that the reason is an accelerated aggregation of the anchor groups compared to the reaction with the surface. By using 1,2-dimethoxyethane as a solvent, this aggregation could be prevented and better defined surface structures could be obtained. The latter could also be achieved by using monoalkoxysilylether as anchor groups, which do not provide any possibilities for aggregation. These results could be confirmed by analyzing functionalized spherical silica nanoparticles by transmission electron microscopy (TEM). It could be shown, that aggregated anchor groups lead to interconnected silica particles. These particles were used in RAFT polymerizations and the obtained products were analyzed by size exclusion chromatography and thermo gravimetric analysis. The analyses showed that the polymer grafting density does not only increase with the RAFT agent’s grafting density but rather with its accessibility for macroradicals. Additionally, it could be found that the content of low molar mass byproducts is independent of the anchor groups’ degree of aggregation. Based on these principles silica and gold particles were tailored and selectively incorporated in certain domains of microphase separated block copolymers and analyzed via TEM. For the first time RAFT polymer functionalized silica particles could be integrated in a specific polymer phase. Furthermore, it was observed that silica particles with a smaller diameter from the applied size distribution were selectively incorporated into the block copolymer matrix. In order to form photo responsive nanocomposites out of these inorganic nanoparticles, a new photo responsive polymer (PAzoPMA) was developed. The reversible light-induced trans-cis isomerization of the switchable azobenzene units in the polymer causes a decrease of the molecular size and an increase in the dipole moment. These changes were characterized with water contact angle analyses, DLS and ion-mobility-mass-spectrometry. Photo responsive nanocomposites were obtained by functionalizing silica and gold nanoparticles by binding PAzoPMA via RAFT agents to the surface. The irradiation of a dispersion of these particles with ultraviolet light induced the trans-cis isomerization, which was followed by self-assembly of the primary particles into superstructures. Especially the functionalized gold nanoparticles aggregated into well-defined spherical superstructures, which could be proven by DLS and optical absorption spectroscopy. The latter also revealed that the switched state is stable for longer times compared to literature known systems, based on small molecules as photoswitches. Another advantage of the developed system could be illustrated by TEM. Tunable molar masses of PAzoPMA in the particle shell allow the inter-particle distances within the superstructures to be tailored – demonstrating the high potential of this system.