Solidification dynamics of polymer membrane by solvent extraction: Spontaneous stratification

Nonsolvent-induced phase separation is widely used to create polymer membranes, but its demixing process is generally understood by liquid-liquid separation. In this work, the solidification dynamics of polymer solutions for membrane formation are explored through dissipative particle dynamics simulations. The shrinkage of the symmetric film of polymer solution is monitored, and the evolutions of the concentration profiles of polymer, solvent, and nonsolvent are analyzed. Additionally, the evolution of the degree of crystallinity (local alignment) and the morphology of the developing membrane are also studied. Three regions can be identified: (i) interfacial region, (ii) dense layer, and (iii) middle region. The demixing process associated with liquid-solid separation is found to be caused by the extraction of solvent and concentration of polymers rather than nonsolvent-induced separation. Our results provide valuable insights into directional solidification and spontaneous stratification of the membrane, driven by solvent loss and oversaturation (beyond the maximum solubility of polymer).