LIGHT SCATTERING STUDY OF SPINODAL DECOMPOSITION AND ELASTIC INSTABILITY OF POLYMER GELS

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1. Dynamical mechanism of spinodal decomposition (SD) in NIPA gel. By combining multi-channel photodetectors and a personal computer, we made a time- and position-sensitive light-scattering photometer. With this apparatus, evolution of the unstable fluctuation in NIPA gel brought into an unstable region was investigated. It was found that at least two kinds of SD coexists in this gel, which were tentatively assigned as the surface SD and the bulk SD. On the basis of the Flory-type phenomenological theory, it was pointed out that three types of SD, i.e. those between swollen phase-shrunken phase, swollen phase-pure solvent, and shrunken phase-pure solvent, exist in NIPA gel. To make correspondence between the theoretical prediction and the experimental results, further detailed studies will have to be made.2. High-frequency elastic measurements by Brillouin scattering. Velocity and attenuation of hyper sound near the volume phase transition in NIPA gel were measured by means of Brillouin scattering. Large step-like anomalies in both quantities were found at the transition point.3. Evolution of patterns on the surface of gels during swelling. It was found on AA and NIPA gels that the evolution of honey-comb like pattern on gel surface follows a power law, the index of which is nearly 1.0.4. Development of a new method to impose a homogeneous uniaxial tension on gels and some measurements utilizing this technique. (1) Dependence of the volume phase transition on external stresses. (2) Measurements of softening of the bulk modulus and the negative Poisson's ratio. It is remarkable that this measurement is the first report to confirm the existence in nature of a material with negative Poisson's ratio. (3) Light-scattering measurement of the dependence of concentration fluctuation of gel network upon external stresses.