Spontaneous pinning of domain growth during spinodal decomposition of off-critical polymer mixtures

Spinodal decomposition of off-critical mixtures of poly(styrene–random–butadiene) copolymer with polyisoprene or polybutadiene was investigated as a function of the composition w of the mixtures and of the phase separation temperature T by time-resolved light scattering. The coarsening behavior of the mixtures was characterized in terms of the time change of the maximum intensity Im and that of the magnitude of the scattering vector qm at which the intensity becomes maximum. It was found that the time changes of Im and qm for the off-critical mixtures studied here are effectively pinned at a certain time defined here as the ‘‘pinning’’ time. This spontaneous pinning of the growth of the unmixing structure was found to occur earlier when w is biased toward 0 or 1 at a given T and when the phase separation temperature is raised at a given w, for these systems which seemingly have upper critical solution temperature behavior.