Patterning flat-sheet Poly(vinylidene fluoride) membrane using templated thermally induced phase separation

The presence of surface patterns on separation membranes, when properly designed, can enhance the membrane performance by increasing the active surface area, tuning the surface wettability, and promoting mass transfer in the boundary layer. This work reports a new method of patterning microporous membranes, namely, lithographically templated thermally induced phase separation (lt-TIPS). Templated by surface-patterned elastomeric molds, the TIPS of poly (vinylidene fluoride) (PVDF) and tributyl O-acetylcitrate (ATBC) lead to successful pattern replication onto the PVDF membranes. We systematically examined the TIPS processing conditions, including PVDF/ATBC concentration (20 wt%, 25 wt% and 30 wt%) and quenching temperature (25 degrees C and 100 degrees C), on the fidelity of pattern replication, as well as structure and properties of the PVDF membranes. Experimental results show that lower solution concentration and direct quenching to 25 degrees C lead to higher fidelity of pattern replication. Lower concentration and higher quenching temperature result in membranes with larger pores and higher porosity. In comparison with non-patterned PVDF membranes, the patterned PVDF membranes display similar pore structures, mechanical characteristics and crystallinity. In contrast, the presence of the surface pattern significantly increases the water contact angle and pure water flux, as expected from the increase in active surface area. The work shows that lt-TIPS is an effective method for patterning membranes, complementary to existing imprinting and phase-inversion based methods.