Cross-link density dependence of spatial inhomogeneities and dynamic fluctuations of poly(N-isopropylacrylamide) gels

Dynamic light scattering studies have been carried out on poly(N-isopropylacrylamide) (NIPA) gels having different cross-link densities. Intensity-intensity time correlation functions obtained at 25 degrees C were successfully analyzed by taking account of the nonergodic nature of gels. By taking the ensemble average ([I](E)) of the time average scattered intensity ([I](T))), the dynamic component of the concentration fluctuations ([I-F](T)) and the spatial inhomogeneities ([I](E); the ensemble average) were evaluated as a function of cross-link density. While [I-F](T) remained more or less constant, [I](E) increased with the increasing ratio of cross-linker concentration (C-BIS) to the monomer concentration (C-NIPA), r = C-BIS/C-NIPA, where BIS is the abbreviation of the cross-linker, N,N'-methylenebis(acrylamide). This indicates domination of the static inhomogeneity for large r. The correlation length, xi, a measure of the range of dynamic fluctuations, was evaluated from the time correlation function. A linear relationship was obtained for xi(-1), i.e., 1/xi = (1/xi(soln) + (1/xi(gel))r, with xi(soln) = 144 Angstrom and xi(gel) = 8.12 Angstrom. The value of xi(gel) was found to be in good agreement with the literature value for the segment length of NIPA polymers, 8.12 Angstrom.