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.

A novel methodology for non-destructive and real-time determination of the gelation threshold for both chemical and physical systems has been proposed. This method i.e., a time-resolved dynamic light scattering (TRDLS) measurement, allows one not only to determine the gelation threshold but also to investigate critical dynamics near gelation threshold, mechanism of gelation, and architecture of gelling cluster. The gelation threshold was found to be characterized by (1) the appearance of a speckle pattern in the scattering intensity, (2) a power-law in the intensity–time correlation function (ICF), (3) a specific broadening of the distribution function, and (4) a noticeable suppression of the initial amplitude of ICF. All of these features originate from some unique aspects of gels: nonergodicity, frozen inhomogeneities, and divergence of connectivity correlation. As an application of these concepts, we propose four methods for determination of gelation threshold and examine their validity and usefulness for various types of gels; these include chemical gels of N-isopropylacrylamide, a gelling system of silica gel in a reaction batch, thermoreversible physical gels of poly(vinyl alcohol)–Congo Red complex, and biological gels of gelatin and globular protein.

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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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