Blending modification to porous polyvinylidene fluoride (PVDF) membranes prepared via combined crystallisation and diffusion (CCD) technique

Abstract

PVDF membranes prepared by the combined crystallisation and diffusion (CCD) technique, a variation of ice templating, have shown permeation rates far superior to those made via conventional methods such as nonsolvent induced phase separation (NIPS) or thermally induced phase separation (TIPS). However, so far the smallest pore size achieved for PVDF membranes prepared by CCD is about 30 nm. In this study, the pore size of CCD PVDF membranes is reduced to 15 nm by blending poly(methyl methacrylate) (PMMA) into PVDF-dimethyl sulfoxide (DMSO) dope solutions without altering the operational conditions. Systemic study shows the pore size reduces when increasing PMMA concentration in the membranes; but after an initial drop compared to the pure PVDF membrane, the pure water flux doesn't decline with PMMA concentration. The effect of different molecular weights of PMMA (120 kDa and 35 kDa) was also investigated, where the results showed a smaller pore size for membranes prepared using the higher molecular weight PMMA. Scanning electron microscopy (SEM), cloud-point temperature measurement and Fourier transform infrared spectroscopy (FTIR) was used to characterise the membranes and help understand how PMMA affects membrane formation. From the evidence gathered, we attribute the effects of PMMA to the change in the number of nucleation centres, diffusional rates, as well as the gelation point based on a previously proposed CCD mechanism.