Effects of Co-diluent on the pore structure, patterning fidelity, and properties of membranes fabricated by lithographically templated thermally induced phase separation

Surface patterning as a physical surface modification method is promising for improving performance of liquid-based membrane processes. Lithographically templated, thermally induced phase separation (lt-TIPS) process is recently demonstrated for fabricating surface patterned microporous polymer membranes. However, the mechanical properties of the poly(vinylidene fluoride) (PVDF) membranes casted from tributyl O-acetylcitrate (ATBC) were poor, due to the predominant spherulitic morphology. In this study, we investigate the influences of diphenyl carbonate (DPC) as co-diluent to ATBC on the pore structure and patterning fidelity of lt-TIPS fabricated PVDF membranes. With the increase of DPC content in the mixed diluents, the phase separation path of the PVDF solution transitions from solid-liquid (S-L) to liquid-liquid (L-L), and correspondingly, the membrane pore structures evolve from dispersed spherulitic to bi-continuous cellular. The interconnected cellular morphology resulted from ATBC/DPC ratio of 20/80 shows superior mechanical properties of membranes compared with all other membranes, improving the toughness by nine times compared with pure ATBC system. The patterning fidelity improves from 59% to 79% as DPC content increases. The presence of the surface patterns on the PVDF membranes improves both the water contact angle WCA (by 12–18%) and the liquid entry pressure LEP (by 5–20%) which are desirable for membrane distillation (MD) applications. Concomitantly, patterned membranes also showed improved pure water permeance and water vapor flux in MD measurement.