Ultrastiff, Thermoresponsive Nanocomposite Hydrogels Composed of Ternary Polymer-Clay-Silica Networks

Anomalous increases in mechanical stiffness and tensile strength were achieved in a poly(N-isopropylacrylamide) nanocomposite gel (NC gel) by incorporating a small amount of silica through the sol-gel reaction of tetraethyl orthosilicate in the polymer-clay network. The resulting NC-Si gels, with ternary polymer-clay-silica structures, exhibited very high tensile moduli (∼3500 kPa) and strengths (∼1700 kPa), as well as well-defined thermoresponsive swelling/deswelling behavior, through the incorporation of 0.2-2.5 wt % silica (relative to the gel weight). The reinforcing efficiency of in situ formed silica in the NC gel is more than 50 times that of clay, while the incorporation of preformed silica nanoparticles led to no observable changes in mechanical properties. The controllable range of hydrogel tensile mechanical properties was significantly extended by the preparation of NC-Si gels. The ternary polymer-clay-silica network structure was proposed on the basis of optical transmittance, TEM, EDS, and ²⁹Si NMR analyses, in addition to its mechanical and swelling/deswelling properties.