Phase separation analysis of Extem/solvent/non-solvent systems and relation with membrane morphology

Abstract

The phase separation behavior of Extem/water/solvent systems for various solvents and Extem/NMP/non-solvent systems for different types of coagulants was investigated. Ternary phase diagrams of studied systems were constructed based on cloud point data obtained by titration at room temperature. In the first part, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were employed as solvent and water was used as coagulant. The cloud point curves from these systems showed that Extem/NMP/water and Extem/DMSO/water has the smallest and largest demixing gap, respectively. Subsequently, water, methanol, and glycerol were employed as coagulants and NMP was selected as solvent. The obtained results indicated that water and methanol had the strongest and weakest coagulant power, respectively. The linearized cloud point and precipitation value were applied to identify the strength of the non-solvent for casting solution (Extem/NMP) with a certain concentration. The solubility curve of Extem was obtained using the Hansen solubility parameter, calculated by a group contribution method. In order to evaluate the influence of different solvents and non-solvents on the final structure of the resulting membranes, scanning electron microscopy (SEM) was used. The morphology of the membranes prepared from Extem/solvent/water systems (except Extem/DMSO/water) showed a finger like sub-layer, while a sponge-like structure was observed for the Extem/DMSO/water system. Using methanol as coagulation medium resulted in a thick dense layer over a macroporous sub layer, while a fully sponge like structure with microporous skin layer was obtained for the Extem/NMP/glycerol system. In order to find the source of these unexpected observations, thermodynamic and kinematic aspects such as binary interaction parameters, heat of mixing, viscosity and diffusion rate were discussed thoroughly.