Studies on two types of built-in inhomogeneities for polymer gels: Frozen segmental concentration fluctuations and spatial distribution of cross-links

The shrinking kinetics of poly(N-isopropylacrylamide) (PNIPA) gels has been studied for two types of PNIPA gels prepared by (i) copolymerization of constituent monomer and cross-linker (monomer cross-linked gels) and (ii) gamma-ray irradiation in the PNIPA solutions(polymer cross-linked gels) in order to investigate the role of cross-linking on shrinking kinetics. The shrinking kinetics of the monomer cross-linked gels is quite similar to that of the polymer cross-linked gels. For example, the rapid shrinking is attained by simply lowering the cross-linking density for both types of gels with a skin formation with skin thickness of ca. 3 mum. On the other hand, a significant difference was found when the microscopic structure and the dynamics were investigated by small-angle neutron scattering (SANS) and static/dynamic light scattering (SLS/DLS). The degree of built-in inhomogeneities and dynamic fluctuations were evaluated as a function of the cross-linking degree and the gel preparation temperature by intensity decomposition methods for both types of gels. It is concluded from the SANS and SLS/DLS results that the monomer cross-linked gels have extra built-in inhomogeneities due to the spatial distribution of crosslinks in addition to the frozen concentration fluctuations inherent in polymer gels.