A computational study of the Ludwig-Soret effect on the thermal-induced phase separation process in polymer solutions

Abstract

Thermal-induced phase separation (TIPS) is one of the methods used to fabricate functional polymeric materials, i.e. PDLC films for electro-optical devices such flat-panel displays, switchable windows etc., and microporous synthetic membranes from polymer solutions. Since the characteristic thermal, mechanical, and optical properties of these materials are controlled by the morphological features, it it important to understand the phase separation mechanism that forms these materials. In this work, the effect of thermal diffusion, also known as the Ludwig-Soret effect, on the TIPS method of phase separation via the SD mechanism in polymer solutions under non-uniform temperature field has been investigated using the computational technique. The Ludwig-Soret effect occurs when a temperature gradient applied to a fluid mixture induces a net mass flow, which leads to the formation of a concentration gradient. A rigorous mathematical model for TIPS via the spinodal decomposition mechanism based on the nonlinear Cahn-Hilliard and Flory-Huggins theories combined with thermal diffusion phenomenon has been formulated for binary polymer solutions under non-uniform temperature field and solved numerically. Numerical simulation results revealed that the thermal diffusion phenomenon had very little or negligible effect on the phase separation mechanism under a non-uniform temperature field, which was reflected from the studies of the time evolution of structure factor and transition time from the early to the intermediate stages of SD.