Equilibrium swelling behavior and elastic properties of polymer–clay nanocomposite hydrogels

Nanocomposite hydrogels were prepared by free-radical polymerization of the monomers acrylam- ide (AAm), N,N-dimethylacrylamide (DMA), and N-iso- propylacrylamide (NIPA) in aqueous clay dispersions at 218C. Laponite XLS was used as clay nanoparticles in the hydrogel preparation. The hydrogels based on DMA or NIPA monomers exhibit much larger moduli of elasticity compared with the hydrogels based on AAm monomer. Calculations using the theory of rubber elasticity reveal that, in DMA-clay or NIPA-clay nanocomposites, both the effective crosslink density of the hydrogels and the func- tionality of the clay particles rapidly increase with increas- ing amount of Laponite up to 10% (w/v). The results sug- gest that DMA-clay and NIPA-clay attractive interactions are stronger than AAm-clay interactions due to the forma- tion of multiple layers on the nanoparticles through hydro- phobic associations. It was also shown that, although the nanocomposite hydrogels do not dissolve in good solvents such as water, they dissolve in dilute aqueous solutions of acetone or poly(ethylene oxide) of molecular weight 10,000 g/mol, demonstrating the physical nature of the crosslink points. 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 3714-3724, 2008