A computational study of long range surface-directed phase separation in polymer blends under a temperature gradient

Abstract

The surface-directed phase separation phenomena of a model binary polymer blend quenched into the unstable region of its binary symmetric phase diagram was studied numerically using the nonlinear Cahn–Hilliard theory coupled with the Flory–Huggins–de-Gennes theory. Long range surface potential within a square geometry, where one side of the domain is exposed to a surface with preferential attraction to one component of a binary polymer blend under a temperature gradient in the x-direction, was incorporated in the model. No transition from complete wetting to partial wetting for all quench depths and/or long range surface potentials was observed. The structure factor analysis for the bulk presented an exponential growth rate at the early stage of phase separation, which slowed down at the intermediate stage with a slope of 0.33, which is in agreement with the Lifshitz–Slyozov power law. As the diffusion coefficient increased, the rate of phase separation increased accordingly in the bulk. This led to faster transition time from the early to intermediate stage within the bulk. In deeper quenches, a higher rate of phase separation was observed along with lower surface enrichment. Deeper quenches also led to the faster rupture of spinodal waves in the bulk. The process of surface enrichment, however, was continuous for all quench depths studied. The effect of different temperature gradient values on the surface enrichment rate was studied for the first time within a long range surface potential setting. No noticeable change in surface enrichment was observed for different temperature gradients.