Two-step phase separation of a polymer mixture. I. New scaling analysis for the main scattering peak

Time-resolved light scattering experiments have been performed on the two-step phase separation process for a binary mixture of deuterated polybutadiene and protonated polyisoprene with the critical composition. The first-step phase separation was induced by quenching the system from a temperature T₀ in a single-phase state to T₁. At various times in the late stage spinodal decomposition (SD) at T₁, the mixture was further subjected to the second-step phase separation by temperature jump (T-jump) from T₁ to T₂ in such a way that the driving force for the phase separation increases and hence phase separation process is accelerated. The comparison between the single-step phase separation behavior from T₀ to T₁, or to T₂ and the two-step phase separation behavior was made in terms of the time evolution of the main scattering peak arising from a dominant mode of the composition fluctuations developed in the phase separation process. To do so, we proposed and applied a new scaling method for the scattering peak position, q_m,1 and intensity, I_m,1, to explore the time evolution of the fluctuations for the two-step phase separation process. The new scaling method, which takes into account an abrupt change in the spatio-temporal scale of phase-separating system involved by the second-step T-jump clearly elucidates a nonlinear pathway according to which the initial structures developed in the first-step process is relaxed and transformed toward an equilibrium structure at T₂ after the second-step process.