Poly(vinylidene fluoride) (PVDF) membranes have been intensively investigated and commercialised with broad applications in water purification and wastewater treatment for decades due to its outstanding properties. Currently, PVDF membranes are mainly produced by the phase inversion technique, which is predominant in both laboratory research and industrial manufacturing. Various modification methods based on the phase inversion technique have also been developed to improve the membrane performances, but these improvements are incremental and there have been no important breakthroughs during the past decade.
This thesis first explores the preparation of reinforced PVDF flat sheet membranes by blending nanoclay followed by the phase inversion process. Although the membranes showed improved water permeation flux and enhanced abrasion resistance, further improvements are limited by the phase inversion technique itself. Consequently, a new concept of membrane manufacturing procedure has been proposed by combining unidirectional crystallisation of green solvent and polymer diffusion. The new method uses crystallites of a solvent dimethyl sulfoxide with controlled sizes as pore templates to create enormous nanosized flow passages. It follows a completely different pore formation mechanism and therefore overcomes the drawbacks of the phase inversion technique. The resultant PVDF membranes have an asymmetric structure composed of a highly porous separation layer and gradually opened micro-channels. Due to the unique structure, the prepared membranes showed excellent permeation performances and mechanical properties overwhelming commercial PVDF membranes prepared by the phase inversion technique. The filtration performance of the PVDF membranes can be further improved by modification of the membrane material, for example, by blending polyethylene glycol in the dope solution. The obtained membrane with pore size of 36 nm showed extraordinary high flux of 1711 L/m2h and could withstand 35 bar in the test. Moreover, the new manufacturing process is of much fewer influencing factors compared to the phase inversion approach and thus highly reliable and repeatable. In principle, it is also applicable to other common polymeric membrane materials such as polyethersulfone and cellulose acetate.Open Acces
