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 °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, ξ, a measure of the range of dynamic fluctuations, was evaluated from the time correlation function. A linear relationship was obtained for ξ-1, i.e., 1/ξ = (1/ξsoln) + (1/ ξgel)r, with ξsoln = 144 Å and ξgel = 8.12 Å. The value of ξgel was found to be in good agreement with the literature value for the segment length of NIPA polymers, 8.12 Å.