Role of Polymer-Particle Adhesion in the Reinforcement of Hybrid Hydrogels

Model hybrid hydrogels reinforced by silica nanoparticles were carefully designed to selectively tune polymer-particle interactions while maintaining similar dispersion states of nanoparticles. This was achieved by changing the nature of the polymer in the gel matrix from poly(dimethylacrylamide) (PDMA), which adsorbs on silica in aqueous suspension, to red poly(acrylamide) (PAAm), which does not, and by polymerizing and cross-linking the gels in situ. The adsorption on silica of both polymers was first quantified by considering linear chains of PDMA or PAAm or random copolymers of both. Changing the pH may also inhibit PDMA adsorption on silica. The properties of gels of PDMA, PAAm, and copolymers were then characterized. The specific effect of polymer adsorption on mechanical properties was demonstrated, with other parameters, specifically the dispersion state of silica, being kept roughly constant. Young's modulus, or equivalently reinforcement in the linear regime, depends monotonically on the fraction of dimethylacrylamide monomers in the gel. It is enhanced by a factor of 5 due to adsorption only. Adsorption, which is a dynamic process, enhances the dependence of the fracture energy G(c) on the draw rate. While G(c) is slightly increased at a low draw rate, it is increased by 1 order of magnitude at a high draw rate with respect to the value in the same gel built with nonadsorbing monomers.