Spinodal decomposition of a symmetric critical mixture of deuterated and protonated polymer

Abstract

A nearly symmetric critical mixture (φ_c=0.486) of perdeuterated and protonated 1,4‐polybutadiene exhibiting an upper critical solution temperature T_c=61.5±1.5 °C has been quenched from the homogeneous state (≂75 °C) to various temperatures between 25 and 57.5 °C. Light scattering measurements document the subsequent spinodal decomposition process which we describe based on a four‐stage model: early, intermediate, transition, and final. The early stage is accounted for by the Cahn theory, yielding initial correlation lengths and effective diffusion coefficients in quantitative agreement with mean‐field predictions. Nonlinear effects mark the beginning of the intermediate stage, which exhibits a simple power‐law growth of heterogeneity length L_m(t)∼t^n_eff, but with a temperature dependent exponent n_eff. As the composition fluctuation amplitude approaches the equilibrium values, the spinodal decomposition process enters the transition stage, characterized by a decreasing interfacial thickness and an increasing L_m(t). Once the interfacial profile equilibrates, a crossover to the final stage occurs. Subsequent growth of L(t) leaves the morphology unaffected as evidenced by a universal structure factor. These findings are discussed in the context of the current theory and are compared with prior studies involving polymer–polymer and simple liquid mixtures.