PVDF Membrane Formation via NIPS in Isothermal and Non-Isothermal Conditions: Thermodynamics, Structure, and Properties

Abstract

Previously published experimental data and new data on cloud points were used to plot ternary phase diagram with temperature axis for the poly(vinylidene fluoride) (PVDF)–dimethyl acetamide (DMAc)–water system. The topology of the plotted diagram is different from the published previously. It shows that an increase in temperature leads to a shift of the boundary curves (liquid equilibrium binodal, polymer crystallization curve and swelling curve) to the composition range enriched by the non-solvent. At the same time, a decrease in temperature leads to degeneration of liquid equilibrium binodal. Taking into account the plotted diagram, the expected morphologies of the membranes prepared via non-solvent induced phase separation (NIPS) in both isothermal and non-isothermal conditions were derived. Morphology and properties of the samples prepared from the dope solution of defined composition via NIPS at different temperatures and via thermally assisted non-solvent induced phase separation, T-NIPS (or NIPS in non-isothermal conditions) were studied. It was shown that an increase in temperature leads to formation of cellular structure, resulting from the liquid–liquid phase separation; a decrease in through pore size and improvement of the mechanical properties. Decrease in pore size from 110 to 35 nm accompanied with decrease in permeance from 8.1 to 0.25 L/(m² h bar) and increase in blue dextran rejection from 20 to 94%. A decrease in temperature changes the structure formation process from liquid–liquid phase separation (induced by the mass transfer processes) to solid–liquid phase separation (induced by temperature decrease). In the T-NIPS process (cooling) layers with finger-like pores and sponge-like structure between them, with spherulites surrounded by sponge-like structure and with spherulites connected with each other were formed in the structure of the membrane. T-NIPS process allows to obtain membranes with permeance 6.1 L/(m² h bar) and Blue dextran rejection of 91%.