Thermoresponsive nanocomposite double network hydrogels.

The utility and efficacy of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels as smart materials is limited by their physical properties. In this study, we sought to design PNIPAAm nanocomposite hydrogels which displayed enhanced mechanical properties as well as deswelling-reswelling kinetics but without reducing equilibrium swelling or altering the convenient volume phase transition temperature (VPTT) of PNIPAAm. PNIPAAm hydrogels were formed as double networks (DN) comprised of a tightly crosslinked 1st network and a loosely crosslinked 2nd network. In addition, polysiloxane nanoparticles of two different average diameters (~50 nm and ~200 nm) were incorporated during formation of the 1st or 2nd network. The influence of the hydrogel composition on VPTT, morphology, equilibrium swelling, deswelling-reswelling kinetics and mechanical properties was evaluated. We observed that DN hydrogels formed with ~200 nm polysiloxane nanoparticles introduced during formation of the 1st network achieved the best combination of the desired properties.