Preparation and properties of poly (vinylidene fluoride) membranes via the low temperature thermally induced phase separation method

Tensile strength is of paramount importance to poly (vinylidene fluoride) (PVDF) membranes in expanding their industrial application. In this paper, porous PVDF membranes with higher tensile strength were prepared by the low temperature thermally induced phase separation (LT-TIPS) method. The effects of mixed diluents (MD) composition on the morphology, polymorphism, and tensile strength of such prepared flat sheet membranes were investigated. The competition of membrane formation mechanisms between the nonsolvent induced phase separation (NIPS) and TIPS was demonstrated by observing the membrane morphology in the LT-TIPS process. It was found that the tensile strength was improved by suppressing the formation of finger-like macrovoids and spherulitic morphologies through adjusting the composition of MD. PVDF crystallized into α phase for all the investigated cases, and as the MD became poorer, the total crystallinity increased slightly. Based on these experimental results, PVDF hollow fiber membranes were fabricated via LT-TIPS. The influences of MD composition and polymer concentration on the morphology, water permeability and tensile strength of the formed hollow fiber membranes were studied. The properties of optimized hollow fiber membranes associated with the surface and cross-section morphologies were promising and the performance can be further enhanced in future work.