Chemistry in spinning solutions: Surface modification of PVDF membranes during phase inversion

This article discusses the chemistry in spinning solution of surface-modified poly(vinylidene fluoride) (PVDF) hollow fibre membranes, during phase inversion process. Hydrophilic hollow fibre PVDF membranes have been prepared through the in-situ grafting of spinning solution. The resultant hollow fibre membranes demonstrated improvements on the hydrophilic properties, where the contact angle was substantially reduced. The highest water flux of 406.2 L/m2 h (at 1 bar) was observed on a membrane prepared with ethanol addition and 30 wt% 1-methyl-2-pyrrolidinone (NMP) in water as an internal coagulant. Hollow fibre membranes spun using a wet spinning method (zero air gap) provide higher water flux compared to the ones spun at a higher air gap. By increasing the copolymer contents from 1 wt% to 2 wt%, the water flux increases for both membranes irrespective of their air gaps, while the water flux for hollow fibres with 3 wt% of the copolymer decreases significantly. Morphology of the hollow fibre membranes spun from different air gap changes significantly due to the presence of the hydrophilic graft copolymer, PVDF-g-PEGMA and excess monomer, poly(ethylene glycol) methyl ether methacrylate (PEGMA). The modified hollow fibre PVDF membranes have been also characterised by the FTIR-ATR, SEM, FESEM, bovine serum albumin (BSA) filtration, and tensile strength measurements. The mechanical properties of the modified hollow fibres show relatively smaller tensile strength and higher elongation at break, demonstrating a ductile behaviour. With increasing dope extrusion rate, the tensile strength of the hollow fibre membrane is enhanced due to greater molecular orientation and closer package of the molecules.