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 /τ 1 ) -β + Y e, 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 β value is proportional to q 0.4 and that the τ 1 and ( Y i − Y e ) values are independent of q .