Responsive hybrid self-assemblies in aqueous media

Responsive copolymers have been prepared by grafting onto a poly(acrylamide-co-sodium acrylate) backbone [PAM-co-PANa] poly(N-isopropylacrylamide) stickers [PNIPA] characterized by a lower critical solution temperature (LCST) in water. From adsorption isotherms and DSC studies performed on PNIPA/silica mixtures, it was shown that PNIPA chains irreversibly interact with silica particles and that at low coverage they partially lose their responsiveness with temperature. When PNIPA is grafted onto a PAM-co-PANa backbone, which has no specific attraction to silica surfaces (only electrostatic repulsions), their binding process remains very similar to the one analyzed with PNIPA chains alone. Above critical copolymer and silica concentrations (C-p congruent to 1 g/L and C-Si congruent to 30 g/L), hybrid networks can be formed following the rules of percolation theory. The viscoelastic properties of these networks are controlled by the concentration of inorganic cross links and the fraction of PNIPA grafts participating in bridges between particles, the others being involved in inelastic loops or pendant chains. For all of the mixtures investigated, an optimum weight ratio of R-Si/PNIPA = 10-15 was found for the viscoelastic properties, in agreement with the saturation of silica beads by the copolymer. Because of the responsive behavior of PNIPA in aqueous solutions, graft copolymers are able to self-assemble with temperature, giving rise to a sol/gel transition upon heating. In the presence of added silica, hybrid aggregates (silica/PNIPA) coexist at high temperature with organic ones (PNIPA/PNIPA) with synergistic or antagonistic effects on the elastic properties depending on the proportion of PNIPA grafts per silica particle.