Evolution of the precipitation kinetics, morphologies, permeation performances, and crystallization behaviors of polyvinylidenefluoride (PVDF) hollow fiber membrane by adding different molecular weight polyvinylpyrrolidone (PVP)

Polyvinylidenefluoride ( PVDF) hollow fiber membranes were fabricated by wet spinning (wet/wet) and dry-jet wet spinning (dry/wet; 3 cm air gap) processes with four types of polyvinylpyrrolidone (PVP) of different molecular weight as additives. Evolution of the precipitation kinetics, morphologies, permeation performances, and crystallization behaviors of the as-spun PVDF membranes were investigated. The PVDF membranes were well characterized by numerous state-of-the-art analytical techniques: scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and attenuated total reflectance fourier Transform Infrared (FTIR-ATR) and elucidated accompanying with its precipitation kinetics obtained by light transmittance measurements. The precipitation kinetics results confirm that four PVDF/PVP/NMP dopes experience instantaneous demixing mechanism and the precipitation rate decreases as PVP molecular weight increases. Little peaks are found in the precipitation curves of the PVDF dopes containing PVP of low molecular weight. The SEM images indicate that the middle sponge-like layer sandwiched by double finger-like layers becomes thinner for the special precipitation behaviors. Visible large pores exist in the internal surfaces of the PVDF membranes spun by both wet/wet and dry/wet spinning processes. The increase in PVP molecular weight restricts the formation of large pores in the internal surfaces of the PVDF membranes for the increase in dope viscosity. The pure water permeability (PWP) of the as-spun PVDF membranes increases initially and then decreases as PVP molecular weight increases. The largest PWP flux of 316.7 Lm(-2) h(-1) bar(-1) is obtained for the PVDF membrane containing PVP K25 by wet/wet spinning process. The rejections for bovine serum albumin (BSA) by the as-spun PVDF membranes range from 35.4 to 82.9%. It illustrates that typical PVDF ultrafiltration membranes were obtained in this research. The melting temperature(T-m) of the PVDF hollow fiber membranes decreases with the increase in the PVP molecular weight as a whole. IR spectra and XRD patterns verify the exclusive formation of beta crystalline phase structure in the as-spun PVDF membranes. Copyright (C) 2010 John Wiley & Sons, Ltd.