Super-porous nanocomposite PNIPAm hydrogels reinforced with titania nanoparticles, displaying a very fast temperature response as well as pH-sensitivity

Super-porous nanocomposite hydrogels, based on PNIPAm matrix and reinforced by acid- and base-resistant TiO2 nanoparticles were prepared via freezing polymerization. The hydrogels display a fast swelling response to temperature (potential application as mechanical actuators), which is enabled by the nano-reinforcement. The reinforcing effect of TiO2 was strongly enhanced (by nearly one order) through the incorporation of small amounts (2.5 mol%) of sodium methacrylate (SMA) as comonomer, which strongly adsorbs on the TiO2 particles' surface, in analogy to ionic double layer adsorption of salts on free TiO2 colloid particles. The SMA adsorption is so strong, that the pH-responsivity of the swelling of the PNIPAm-SMA copolymer is practically suppressed. A significant pH-responsivity was found only in such poly(NIPAm-co-SMA)/TiO2 gels, which contained high amounts of chemical crosslinker. In these rather rigid gels, some of the SMA units were topologically prevented from adsorption to TiO2 surface. (C) 2014 Elsevier Ltd. All rights reserved.