Compositional effects on mechanical properties of nanocomposite hydrogels composed of poly(N,N-dimethylacrylamide) and clay

Nanocomposite type hydrogels (DMAA-NC gels) consisting of organic (polymer)/inorganic (clay) networks were prepared by in-situ free-radical polymerization of N,N-dimethylacrylamide (DMAA) in the presence of inorganic clay in aqueous solution. The composition of the NC gels could be controlled directly by altering the composition of the initial reaction mixture. The resulting DMAA-NC gels were mostly uniform and transparent, irrespective of their clay and polymer contents. From DSC, X-ray, TEM, and tensile mechanical measurements, the network structure was established. Contrary to conventional chemically cross-linked hydrogels (DMAA-OR gels) prepared by chemical cross-linking with a difunctional monomer, DMAA-NC gels exhibit superb mechanical properties with astonishingly large elongations at break, near to or greater than 1500%. The effects of the composition, such as the amounts of clay, polymer, and water content in DMAA-NC gels, on the tensile mechanical properties were investigated in detail. With increasing clay content from C-clay = 1 to 7 (C-clay is proportional to the weight of clay per unit volume of water, and a value of 1 corresponds to 0.762 g clay per 100 mL of water), the modulus and the ultimate tensile strength increased almost proportionally to the clay content, but the elongation at break decreased slightly. By altering the polymer content over 2 orders of magnitude, it was observed that the nature of the mechanical properties of DMAA-NC gels changed markedly. Above a lower critical polymer content (C-p approximate to 0.13), the elongation at break increased rapidly from near zero to more than 1000%. Thereafter, the hydrogels became very tough NC gels. With further increases in polymer content, the strength and the elongation at break showed maxima at certain polymer contents, whereas the modulus increased monotonically. The mechanical properties were also changed by altering the water content. The cross-link density was estimated for DMAA-NC gels with different clay contents. The effects of clay and polymer contents on the mechanical properties, as described above, were discussed on the basis of the network structure model for NC gel.