Asymmetric forward osmosis membranes from p-aramid nanofibers

p-Aramid is an ideal building block for forward osmosis (FO) membranes due to its extraordinary thermal resistance, chemical stability, and mechanical properties. However, existing aramid membranes have certain limitations such as large pore diameters and low salt rejection rates. In this work, we describe a facile solvent exchange-delay phase inversion strategy to prepare p-aramid nanofibrous membranes that would be suitable for FO applications. In this strategy, p-aramid nanofibers with an average diameter of 16 ± 4 nm and an average length of 382 ± 89 nm were employed as membrane matrices. Prior to the immersion of the cast film into a coagulation bath, a pre-evaporation protocol was carefully designed and introduced to provide a slower exchange rate between the good solvent and the non-solvent, which delayed the demixing process between p-aramid nanofibers and thus yielded an asymmetric membrane with a denser active layer as well as a loose substrate layer. The resultant membrane showed excellent FO water flux, NaCl rejection ratios, tensile strength, thermal properties, and solvent resistance. The membrane reported in this work may provide a promising candidate for separation applications and the results reported herein will facilitate the development of high-performance nanofibrous membranes.