Kinetics of Thermally Induced Phase separation in Ternary Polymer Solutions. I. Modeling of Phase Separation Dynamics

摘要

Simulations based on Cahn-Hilliard spinodal decomposition theory for phase separation in thermally quenched polymer/solventonsolvent systems are presented. Two common membrane-forming systems are studied, cellulose acetate [CA]/acetone/water, and poly(ethersulfone) [PES]/dimethylsulfoxide {DMSO]/water. The effects of initial polymer and nonsolvent composition on the structure-formation dynamics are elucidated, and growth rates at specific points within the ternary phase diagram are quantified. Predicted pore growth rate curves exhibit a relative maximum with nonsolvent composition. For shallow quenches (lower nonsolvent content) near a phase boundary, the pore growth rate increases with increasing quench depth, whereas for deep quenches, where the compositon of the polymer-rich phase approaches that of a glass, teh pore growth rate decreases with increasing quench depth. With increasing initial polymer concentration, the overall rate of structure growth is lowered and the growth rate maximum shifts to higher nonsolvent compositions. This behavior appears to be a universal phenomenon in quenched polymer solutions which can undergo a glass transition, and is a result of an interplay between thermodynamic and kinetic driving forces. These results suggest a mechanismk for the locking-in of the two-phase structure that occurs during nonsolvent-induced phase inversion.