SLOW SPINODAL DECOMPOSITION IN BINARY-LIQUID MIXTURES OF POLYMERS .5. MOLECULAR-WEIGHT DEPENDENCE IN THE LATER STAGE OF UNMIXING

The unmixing process via spinodal decomposition of poly(styrene-ran-butadiene) (SBR)/polyisoprene (PI) mixtures was investigated by the time-resolved light-scattering method to see how it is affected by the temperature of phase separation T and by the molecular weight of PI with that of SBR fixed. Time changes in the wave number q(m)(t;T) and intensity I(m)(t;T) at the peak of the scattering function were followed to explore the coarsening process in the later stage of spinodal decomposition. For each mixture the observed changes at different T were reduced to a master curve when plotted against a reduced time tau. This finding is consistent with the scaling postulate by Langer et al. and Chou and Goldburg. The master curves for mixtures with different PI molecular weights were divergent, showing the phenomenon called the N branch. However, it was possible to bring them to a master curve when tau was multiplied by a shift factor depending on the average number of entanglements per chain. The dependence was weaker than that predicted theoretically by Onuki.