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

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.

Thermodynamic and kinetic variation is a key to understand the phase inversion process with addition of PVP into PVDF/TPU casting dope. Thermodynamically, PVP works as a demixing enhancer that accelerates the phase inversion reduces α crystal structure resulting in reduction of the crystallinity of PVDF in membranes, contributing to the enlargement macrovoid structure in the membranes, which improves the membrane flux. However, in further increment of PVP, kinetic impact is dominant. The demixing of the casting dope delayed and the crystallinity of PVDF increases, resulting in macrovoids being stretched down like a finger from the underneath of membrane skin to the bottom of a sublayer. PVP in the PVDF/TPU dope suppress macrovoid formation at high concentration. The flux is decreased rather than increased. It reveals that kinetic effect takes over thermodynamic enhancement.