Fabrication of novel Janus membrane by nonsolvent thermally induced phase separation (NTIPS) for enhanced performance in membrane distillation

© 2018 This study proposed to use the nonsolvent thermally induced phase separation (NTIPS) method to fabricate a novel Janus membrane for MD applications. The as-prepared dual-layer membrane consisted of a thin hydrophobic PVDF top-layer and a relatively thick hydrophilic PVDF-PVA sub-layer. By adopting a facile one-step co-casting technique and water soluble diluent ε-caprolactam (CPL), delamination-free dual-layer membrane was obtained. The SEM morphologies and FTIR crystalline analyses suggested the membrane formation mechanisms, where the hydrophobic top-layer was formed via NTIPS process, resulting in an ultra-thin dense skin with finger-like pores formed beneath; while the hydrophilic sub-layer was induced by TIPS, producing highly porous cellular structure with high degree pore interconnectivity. Combining the structural observation and MD performance results, suitable fabrication parameters were identified as a PVDF concentration of 15 wt% for the hydrophobic layer and coagulation temperature between 20 and 40 °C. The total membrane thickness was optimized as 100–150 µm, given the thickness of hydrophobic layer kept within an optimal range of 30–60 µm to ensure minimal mass transfer resistance. The Janus membrane exhibited stable salt rejection above 99.5% over continuous MD runs and superior permeation flux up to 165.3 kg m-2 h-1 at 80 °C, which was remarkably higher than reported MD membranes.