Effect of salt additive on the formation of microporous poly(vinylidene fluoride) membranes by phase inversion from LiClO4/water/DMF/PVDF system

The effect of a salt additive, lithium perchlorate, on the morphology and crystal structure of PVDF membranes prepared by wet phase inversion process was studied. The gelation phase boundaries of the quaternary system, LiClO4/water/DMF/PVDF, were determined at 25 degreesC. It was found that the gelation lines shifted up progressively with increasing salt contents in this system. For a salt-free casting dope, the formed membrane exhibited a typical asymmetric structure characterized by the skin, parallel columnar macrovoids, and cellular pores. WAXD analysis indicated that PVDF crystallized into 'alpha' (type II) structure in this membrane. By contrast, when PVDF was precipitated from high salt-content dopes (e.g. greater than or equal to 5 wt%), the macrovoids bent and extended towards the bottom region while the original cellular pores evolved into very large voids. The PVDF crystallites became 'beta' form (type 1) in these membranes. Thermal analysis (DSC) of all membranes showed dual melting peaks at low heating rates (less than or equal to 5 degreesC/min), suggesting that the crystallites formed in the immersion precipitation process were imperfect and they underwent re-crystallization during the heating process. Using low voltage SEM at high magnifications (e.g. 100 KX at 0.55 KV) on uncoated samples, the fine structures (10-20 nm) of the PVDF crystallites were observed. And at very high magnifications (225 KX at 0.59 KV), it was observed that the skin region of the membrane prepared from high salt-content dopes actually contained many nano-pores (e.g. 20 nm). This contributes to the high permeation rate and low solute rejection as revealed from the water-flux measurements. (C) 2002 Elsevier Science Ltd. All rights reserved.