Fast formation of strong symmetric poly (vinylidene fluoride) membranes via a modified VIPS method

Background: Polymer membranes for microfiltration and membrane distillation should be skinless and symmetric with a bi-continuous structure. To effectively eliminate the inherently formed skin layer and macrovoids in the membrane, proper modifications of preparation parameters are proposed. Methods: Skinless microporous poly(vinylidene fluoride) (PVDF) membranes with a uniform bi-continuous structure were fabricated through phase inversion processes that combine vapor-induced phase separation (40 °C and 70% relative humidity) and non-isothermal immersion precipitation (22 °C) with casting dopes consisting of triethyl phosphate (TEP), PVDF, and a small amount of glycerol, a nonsolvent. Significant findings: Introduction of glycerol to the dope solution induces a rapid polymer crystallization upon exposure to water vapor. Experimental results demonstrate that a skinless microporous membrane can be obtained with a short vapor exposure time of 1.5 min. Additionally, the synthesized membrane possesses ideal filtration properties, including high hydrophobicity (contact angle = 113° for both surfaces), high porosity (83%), adequate strength (2.4 N/mm²), and suitable pore size (0.12 μm). This membrane also exhibits superb rejection capability (99%) and high permeation flux (450 LMH) for filtration of poly(methyl methacrylate) particles with an average size of 0.15 μm.