Influence of initial composition of casting solution on morphology of porous thin polymer films produced via phase separation

In this study, variation in the thermodynamic and kinetic properties of the casting solution achieved by tailoring initial composition (IC) of the polymer-solvent-nonsolvent ternary mixture is investigated to engineer pores in a polymer film prepared by liquid-liquid phase separation (LLPS). The driving force for liquid-liquid phase separation, identified as thermodynamic enhancement factor (T-E), is observed to influence the LLPS rate. At an IC closer to the phase boundary, the LLPS rate is higher. The kinetic restraint (K-R), which is dictated by the viscosity of the casting solution, also alters the LLPS rate. The interplay of these two opposing factors determines the final film morphology. The higher LLPS rate obtained from a larger T-E value leads to finger-like pore structure, while lowering LLPS rate by considering a casting solution with lower T-E results in polymer film with spherical sponge-like pores irrespective of casting solution viscosity. On the other hand, when the concentration of both the polymer and nonsolvent is high, i.e., for high K-R value, polymer film with interconnected pores and higher pore number density are obtained.

Automated summary

This study investigates the variation in thermodynamic and kinetic properties of a casting solution to engineer pores in a polymer film. The driving force for liquid-liquid phase separation and the viscosity of the casting solution are found to play key roles in determining the final film morphology and pore density.