The effect of water content on the ultimate properties of rubbery nanocomposite gels

An investigation was conducted into the effects of water content (R _H2O) on the ultimate tensile properties of nanocomposite hydrogels (NC gels) based on poly(N-isopropylacrylamide)/clay networks. Rubbery NC gels with low clay contents (>NC10) exhibited unique changes in their stress-strain curves, depending on the R_H2O. At high R_H2O, where PNIPA chains are fully hydrated, NC gels retained their rubbery tensile properties, whereas they changed to exhibit plastic-like deformations with decreasing R_H2O. Consequently, for a series of NC gels with different R_H2O, a failure envelope was obtained by connecting the rupture points in the stress-strain curves. Here, the counterclockwise movement was observed as either the R_H2O decreased or the strain rate increased. This seemed to be analogous to that of a conventional elastomer (e.g., SBR), although the mechanisms are different in the two cases. From the R_H2O and C_clay dependences of the ultimate properties, three critical values of R_H2O were defined, where R_H2O showed a maximum strain at break, a steep increase in initial modulus, and onset of brittle fracture. Compared with NC gels, OR gels (chemically crosslinked hydrogels) showed similar but very small changes in their stress-strain curves on altering R_H2O, whereas LR (viscous PNIPA solution) showed a monotonic decrease (increase) in εb (E _i) with decreasing R_H2O.