Large-area PVDF membranes with through-thickness porosity prepared by uni-directional freezing

Abstract

An ideal microporous membrane has uniformity through-thickness pores with low tortuosity. In this study, a novel uni-directional freezing method to manufacture poly(vinylidene fluoride) (PVDF) membranes with ideal pore structures is proposed. Crack-free large-area membranes can be produced by inducing the directional growth of cylindrical solvent crystals at a controlled freezing speed and removing them by freeze drying or solvent etching. These membranes are uniform and are regularly arrayed through-thickness pores and relatively strong mechanical properties (modulus=75 MPa and tensile strain to break=0.1). Filterability tests where membranes are produced via freeze drying reveal a fast average filtration rate compared to commercial membranes. In addition, Xray diffraction and infrared spectroscopy results confirmed that uni-directional freezing increased the beta phase content, possibly by inducing polymer chain alignment through directional solvent crystal growth. The obtained findings show that the uni-directional freezing method is a simple and effective process to control through-thickness pore structures.