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) γ-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 µm. 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.

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The swelling behavior and hierarchical structures of poly(acrylic acid) (AA) homopolymer gels and poly(12-acryloyloxydodecanoic acid (ADA)-co-AA) gels carrying ionizable groups as well as hydrophobic groups have been investigated by equilibrium swelling measurement, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The equilibrium swelling degree (Sv) and the structure factors of the copolymer gels were obtained as a function of the ethanol (EtOH)/water (W) solvent composition, x () vol % of EtOH). Sv showed a convex-upward function of x. That is, the gels swelled with increasing x until x ) 50-60 vol % for AA gels and x ) 70-80 vol % for ADA/AA gels and then decreased gradually. This phenomenon is explained by hydrophobic association of long alkyl chains of ADA and a cosolvency effect of poly(AA) in EtOH and W. The DLS results also indicated that the major component of light scattering was frozen inhomogeneities. SANS intensity functions, I(q), for the copolymer gels when x ) 25 vol % showed the presence of an ordered structure in water with a long spacing of ca. 46 Å. These experimental pieces of evidence suggest a strong correlation between the macroscopic properties, such as Sv, and the microscopic structure as well as the dynamics. The diffusion coefficients were obtained by DLS as well as SANS, of which physical meanings are discussed by comparing the x dependence of S v.