Fabrication and characterization of novel asymmetric polyvinylidene fluoride (PVDF) membranes by the nonsolvent thermally induced phase separation (NTIPS) method for membrane distillation applications

Abstract

Nonsolvent-induced phase separation (NIPS) and thermally induced phase separation (TIPS) are widely adopted membrane fabrication methods with both advantages and drawbacks. In an attempt to combine the benefits of both processes, in this work, a novel hybrid method, i.e., nonsolvent thermally induced phase separation (NTIPS), was proposed to fabricate polyvinylidene difluoride (PVDF) membranes by careful choice of a water-soluble diluent, ε-caprolactam (CPL), for membrane distillation (MD) applications. Combined with polymer/diluent compatibility analysis, a series of PVDF/CPL binary solutions was studied by differential scanning calorimetry (DSC) and polarized light microscopy (PLM) to identify an appropriate PVDF concentration range (<30wt% PVDF) for the fabrication of membranes with desirable pore structures in the NTIPS process. The SEM images showed an asymmetric structure of NTIPS membranes with an ultrathin top skin layer of 0.5μm and a highly porous support layer with a bicontinuous network of pores. Based on the X-ray diffractometry (XRD) examination and attenuated total reflection-Fourier transform infrared spectroscopic (ATR-FTIR) analysis for the NTIPS membranes, combined NIPS and TIPS mechanisms were identified through the formation of different PVDF crystalline phases at different locations (surface and bulk layers). Further characterization confirmed that, compared to the currently reported MD membranes, the novel NTIPS membrane exhibited a much higher overall porosity of 86% and a higher liquid entry pressure (LEP) above 3.5bar, as well as exceptional mechanical strength, which are desirable characteristics for MD applications to avoid membrane pore wetting and to ensure high salt rejection. Through the performance evaluation of NTIPS membranes using synthetic seawater (3.5wt% sodium chloride solutions) in direct contact membrane distillation (DCMD), superior permeation flux of 85.6kgm−2h−1 at a feed temperature of 80°C and a salt rejection rate greater than 99.99% were achieved. This study is expected to have profound implications in the development of PVDF membrane fabrication methods not only for MD applications but also for other membrane processes.