Effects of clay content on the properties of nanocomposite hydrogels composed of poly(N-isopropylacrylamide) and clay

For two different types of poly(N-isopropylacrylamide) (PNIPA) hydrogels, i.e., nano,composite type PNIPA hydrogels (NC gel) and conventional chemically cross-linked PNIPA hydrogels (OR gel), the effects of cross-linker contents on various physical properties were investigated. In NC gels composed of a unique organic (PNIPA)/inorganic (clay) network, the inorganic clay acts as a multifunctional crosslinker in place of an organic cross-linker (BIS) as used in OR gels. In NC gels, which generally exhibit extraordinary mechanical toughness, the tensile moduli and tensile strengths are almost proportional to the clay content (C-clay), while the elongation at break tends to decrease slightly with increasing C-clay. On the other hand, in OR gels, which always exhibit weak and brittle natures, there was no detectable change in properties on altering the concentration of BIS (C-BIS). The deswelling rate was affected markedly by the cross-linker content in both gels though in opposite directions. On increasing cross-linker contents NC gels exhibit decreases and OR gels exhibit increases in rates of deswelling. In NC gels, high deswelling rates and high structural homogeneities (transparencies) were achieved simultaneously. Also, volume changes related to the phase transition of PNIPA at LCST were also inclined to decrease on increasing cross-linker contents in both gels, although swelling ratios at temperatures below LCST were generally larger in NC gels than those in OR gels. As for transparency changes at LCST, in OR gels changes in transparency decrease on increasing C-BIS, because below the LCST the transmittances themselves decrease steeply with increasing C-BIS. On the contrary, NC gels exhibit large transparency changes regardless of C-clay and show a tendency to increase their transmittances above LCST in the high C-clay region. All results obtained were consistent with the proposed model structure for NC gels. On the basis of the theory of rubber elasticity and using tensile mechanical data, the number of effective cross-links and the molecular weight between cross-linking points were evaluated for all NC gels.