LIGHT-SCATTERING STUDY OF KINETICS OF SPINODAL DECOMPOSITION IN A POLYMER-SOLUTION

Small-angle light scattering was used to measure the time evolution of the structure factor S(k,t) at various times t after quenching polystyrene-cyclohexane (PS-CH) solutions to a temperature below the spinodal temperature. Qualitatively, we observe the usual features of spinodal decomposition kinetics. The initial part of the phase-separation process shows an exponential increase in intensity. An analysis in terms of the linear Cahn-Hilliard theory was made to determine the extrapolated growth rate at zero time and hence the initial diffusion constant. By comparing the initial growth rates for two different molecular weights of PS, we find evidence of Zimm dynamics in these dilute solutions. The characteristic domain size shows power law growth with an exponent ranging between 0.4 and 0.5. However, the growth of intensity at the maximum of the structure factor does not exhibit any extensive region of power law growth; i.e., the exponent varies with time. The later stages of the structure factor data appear to collapse when rescaled with appropriate units of length and time. From a detailed analysis of the k-dependence of the scaled structure factor, we present some possible explanations of the interesting morphology occurring in the intermediate stages. Our results are discussed in terms of current theory and compared with the results seen in polymer blends and simple liquid mixtures.