Dynamics of Hybrid Poly(acrylamide-co-N,N-dimethylacrylamide) Hydrogels Containing Silica Nanoparticles Studied by Dynamic Light Scattering

Nanohybrid gels based on poly(acrylamide-co-N,N-dimethylacrylamide), P(AAm-co-DMA), and colloidal silica nanoparticles were synthesized by radical polymerization, and the influence of hydrogen bonding between DMA and silica nanoparticles on the dynamics of the network and the nanoparticles were studied by dynamic light scattering. As previously reported, in the case of polyacrylamide homopolymer, PAAm, which have no hydrogen bonding with silica, we observed two decay modes in the hybrid gels, a gel mode and a Brownian diffusion mode of silica nanoparticles. When we increased the DMA weight fraction (or physical cross-link density), we observed (1) a higher scattered light intensity than the silica suspension and the gel without silica, (2) an increased plateau value of the autocorrelation function, (3) a slowing down of the silica diffusion mode, and (4) no influence on the gel mode. These results, compared with those of the mixtures of the linear PDMA and silica nanoparticles in solution (showing an increase in the scattered light intensity and in the hydrodynamic radius), indicate that the P(AAm-co-DMA) hybrid gels have a more heterogeneous structure due to the adsorption of the polymer on the silica nanoparticles, and that the silica nanoparticles, trapped in the network due to the adsorption of the polymer, show a cage dynamics in the network. The local viscoelastic properties probed by the bound nanoparticles are discussed.