Two-step phase separation of a polymer mixture. II. Time evolution of structure factor

Abstract

Nonlinear time evolution of phase-separating structures in the two-step phase separation process was investigated for a deuterated polybutadiene–polyisoprene mixture by using a time-resolved light scattering technique. The mixture studied has a lower critical solution temperature type phase diagram with a spinodal temperature of 36 °C. The first-step phase separation via spinodal decomposition (SD) was conducted by a temperature jump (T-jump) from 23 °C to 42 °C, and to the late stage of the SD for varying time periods, t0, in order to develop phase-separated domains with varying characteristic size Λm,1. This phase separation was followed by the second-step T-jump to a higher temperature of 70 °C so that each phase-separated domain is again quenched into thermodynamically unstable region. Nonlinear time evolution processes of phase-separating structures after the second-step SD were explored as a function of size of the initial structures Λm,1. We found the following intriguing effects of the initial structures on further evolution of phase-separating structure via the second-step SD: (1) When Λm,1≫Λm,0 (characteristic length of composition fluctuations developed in the early stage SD after quenching the system from a single-phase state to 70 °C), small domains were evolved within the initial domains (defined as large domains) developed during the first-step SD process, while (2) when Λm,1⩽Λm,0, the small domains were not developed, but only the large domains grew at a growth rate larger than that at 42 °C. In the former case (1), we succeeded in separating the scattering due to the small domains and that due to large domains from the observed scattering profile. The separation allows us to investigate a coupling of the time evolution of the large and small domains and nonlinear pathways for the system to achieve a new equilibrium structure after the second-step SD process.