Small angle neutron scattering studies on structural inhomogeneities in polymer gels: irradiation cross-linked gels vs chemically cross-linked gels

A comparison of network structure in a solvent was made for two types of poly(N-isopropylacrylamide) gels cross-linked by chemical reaction with N,N-I-methylenebisacrylamide (BIS) (chemical gels) and by gamma-ray irradiation (gamma-ray gels). The cross-linking density dependence for these gels was examined by small angle neutron scattering (SANS). The SANS results indicated an increase of frozen inhomogeneities with an introduction of cross-links for both chemical and gamma-ray gels. However, it was found that the effect of cross-linking is much stronger in the chemical gels than in the gamma-ray gels. The differences in the structure were successfully interpreted by a statistical-mechanical theory of gels proposed by Panyukov-Rabin (Phys. Rep. 269 (1996) 1). The degree of polymerization between cross-links, N, was a decreasing function of cross-linking content for both types of gels, while that for the gamma-ray gels was a weak function of irradiation dose. Quantitative analyses on BIS concentration and gamma-ray dose dependence led to an experimental evidence of the existence of cross-linking saturation threshold. (C) 2002 Elsevier Science Ltd. All rights reserved.