Hybrid Hydrogels: Macromolecular Assemblies through Inorganic Cross-Linkers

The adsorption process onto a silica surface of oligomers from three different monomers, N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPA) and ethylene oxide (EO), was studied. From the adsorption isotherms and calorimetric experiments, it was shown that N-alkylacrylamide derivatives strongly interact with silica nanoparticles. At low coverage (Gamma < 0.5 mg m(-2)), the polymers were adsorbed in a flat conformation with a high proportion of trains, while for higher coverages (Gamma = 0.5 - 1 mg m(-2)), the adsorption of new chains proceeded by chain displacement, forming an increasing proportion of loops and tails. Similarly, PEO chains readily interacted with silica nanoparticles but showed a lower affinity with an adsorbed amount (Gamma congruent to 0.6 mg m(-2)) that was approximately half of the value obtained for both PDMA and PNIPA. When these oligomers were grafted onto a non-adsorbing poly(acrylamide-co-sodium acrylate) backbone (PAMH), the binding process of these adsorbing side-chains with silica nanoparticles proceeded very similarly and gave rise to the formation of hybrid hydrogels above critical copolymer and silica concentrations. The viscoelastic properties of these networks were controlled by the concentration of inorganic cross-links and the fraction of adsorbing grafts that participated in the formation of bridges between particles, the others being involved in inelastic loops or pendant chains. For all the hybrid mixtures investigated, an optimum weight ratio between silica and grafts was found for the viscoelastic properties, which was in agreement with the saturation of silica beads by the graft precursors. Due to the temperature-dependence of the solubility of the PNIPA side-chains in aqueous solutions, the PAMH-g-PNIPA copolymer was also able to self-assemble with temperature, giving rise to a hybrid conetwork in the presence of added silica.