Porous PVDF/TPU blends asymmetric hollow fiber membranes prepared with the use of hydrophilic additive PVP (K30)

Abstract

Asymmetric blend hollow fiber membranes had been made from a new casting dope containing poly(vinylidene fluoride) (PVDF)/Thermoplastic polyurethane (TPU)/Polyvinylpyrrolidone (PVP) N, N-dimethylacetamide (DMAc). The effect of hydrophilic additive, polyvinylpyrrolidone, on the morphology and crystal structure of PVDF/TPU blends membranes by phase inversion process was studied. The separation property, microstructure and crystalline phase of membranes were characterized by bovine serum albumin (BSA) retention experiments, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) and differential scanning calorimetric (DSC), respectively. The results showed that adding low concentration (less than 3 wt%) of PVP can reduce the hydrophobicity of PVDF and increase its hydrophilicity. With further increment of PVP, the solution demixing is delayed and the kinetic hindrance due to the increase of viscosity. By SEM it was observed that with the increase of PVP the pyriform voids were replaced by the ‘finger-like’ voids, which became longer and widespread. In further increment of PVP (5 wt%), PVP contributes to the suppression rather than the enlargement of macrovoid structure in the membranes. For formed membranes by additive-free casting dope, FTIR–ATR indicated that PVDF crystallized including α and β phase. TPU and low concentration of PVP both can reduce the crystallinity of PVDF membranes. When PVDF/TPU was precipitated from increasing amount of PVP in dopes (e.g. ≥3 wt%), some α phase disappeared and the water flux decreased little by little, the enthalpy of fusion and crystallinity of membranes also declined. When the addition of PVP increased as high as 10 wt%, the heat of fusion and crystallinity of membranes increased slightly compared with membranes adding 3 wt% PVP. In addition the water flux dropped significantly rather than improved.