Responsive macroscopic materials from self-assembled cross-linked SiO ₂-PNIPAAm core/shell structures

A way to obtain macroscopic responsive materials from silicon-oxide polymer core/shell microstructures is presented. The microparticles are composed of a 60 nm SiO ₂-core with a random copolymer corona of the temperature responsive poly-N-isopropylacrylamide (PNIPAAm) and the UV-cross-linkable 2-(dimethyl maleinimido)-N-ethyl-acrylamide. The particles shrink upon heating and form a stable gel in both water and tetrahydrofuran (THF) at 3-5 wt% particle content. Cross-linking the aqueous gel results in shrinkage when the temperature is increased above the lower critical solution temperature and it regains its original size upon cooling. By freeze drying with subsequent UV irradiation, thin stable layers are prepared. Stable fibers are produced by extruding a THF gel into water and subsequent UV irradiation, harnessing the cononsolvency effect of PNIPAAm in water/THF mixtures. The temperature responsiveness translates to the macroscopic materials as both films and fibers show the same collapsing behavior as the microcore/shell particle. The collapse and re-swelling of the materials is related to the expelling and re-uptake of water, which is used to incorporate gold nanoparticles into the materials by a simple heating/cooling cycle. This allows for future applications, as various functional particles (antibacterial, fluorescence, catalysis, etc.) can easily be incorporated in these systems.