Slow relaxation of the elastic N-isopropylacrylamide gel

The tensile moduli of N-isopropylacrylamide (NIPA) gels having various cross-linking degrees, q, are measured at temperatures crossing over the volume phase transition temperature. The moduli increase with rising temperature to exhibit a transition at the volume phase transition temperature. The relaxation of the tensile modulus, Y(t), of the shrunken gel can be fitted to Y(t) = (Y-i - Y-e)(1 + t/(tau1))(-beta) + Y,,, which can be theoretically explained by a gel model consisting of dangling chain ends in cross-linking networks with topological constraints. The experiments show that the value is proportional to q(0.4) and that the tau(1), and (Y-i - Y-e) values are independent of q.