Analysis of Polyetherimide/N-Methyl-2-Pyrrolidone/nonsolvent phase separation behavior

Nonsolvent Induced Phase Separation (NIPS) is among the most well-known methods for membrane fabrication in which the phase separation behavior of the polymer solution is one factor that governs the structure of the membrane ultimately obtained. In this study, phase separation behavior of the polyetherimide (PEI)-casting dope was investigated for different types of coagulants and nonsolvent additives. Cloud point data were obtained by the titration method on the ternary polyetherimide/solvent/coagulant diagram from a limited number of experiments. The whole cloud point curves were then drawn by calculation using the fitting parameters based on the linearized cloud point relation (LCP). In the first part, water, methanol, ethanol, glycerol, and acetic acid were used as the coagulants for the PEI/NMP solution. The cloud point curves obtained for the above coagulants indicated that water has the strongest coagulation power among them. In the second part, methanol, ethanol, glycerol, and acetic acid were used as nonsolvent additives to NMP in different (nonsolvent additive/NMP) mass ratios. The latter (NMP + nonsolvent additive) mixtures were then used as the solvents to prepare PEI/(NMP + nonsolvent additive) solutions. The cloud point data obtained for the above solutions using water as a coagulant indicated that the cloud point curves shift toward the polymer/solution axis as the (nonsolvent additive/NMP) mass ratio increases.