Fabrication of hydrophobic/hydrophilic composite hollow fibers for DCMD: Influence of dope formulation and external coagulant

Hydrophobic/hydrophilic dual-layer composite membranes were prepared for DCMD in this research. The polymers for constructing the main framework in both layers were PVDF. PVA was blended with PVDF to enhance hydrophilicity of inner layer. The effect of outer layer dope formulation and external coagulant on membrane structures and separation properties was investigated. It was found that fibers with the less PVA in the inner layer generally exhibited higher and more stable salt rejections, regardless of the conditions of the outer layer formation. For fibers with the higher PVA content in the inner layer, however, the variation of both flux and rejection was significant with changing outer layer fabrication conditions; i.e. the fibers prepared with no additive in its outer layer and using water as external coagulant led to negative flux and null rejection. Adding ethanol into the outer layer dope or replacing water with water/ethanol in external coagulant resulted in remarkably better membrane performance. The difference was attributed to the larger macrovoids developed in the membrane of poorer performance. In addition, the membrane with more hydrophilic inner layer could possess higher flux than those with the same thickness but a more hydrophobic inner structure.