Fine-tuning surface porosity of polyimide-based membranes for membrane distillation

Despite growing research interest in developing membrane materials for membrane distillation (MD), efforts remain focused on modifying existing membranes, which increases costs and raises concerns about the toxicity of modifying agents, particularly long-chain fluoroalkyl substances and nanoparticles. Surface engineering offers significant potential for enhancing the properties and performance of polymeric membranes. In this study, we developed polyimide membranes from 4,4 '-(hexafluoroisopropylidene) diphthalic anhydride and m-phenylenediamine (6FDA-mPDA) for desalinating highly saline water (70,000 ppm), achieving enhanced properties and performance for MD applications. Additionally, we introduced a straightforward surface engineering process employing vapor-induced phase separation (VIPS). Notably, membranes prepared using ethanol vapor exhibited exceptional characteristics, with the highest water contact angle (WCA) reaching 150 degrees with a flux of 26.26 kg m- 2h- 1 and a salt rejection of 99.98 % in air gap membrane distillation (AGMD). The membranes displayed good fouling resistance with stable performance in long-term experiment. The recycled membranes were tested and displayed similar performance to the pristine ones with marginal changes in the hydrophobicity. The resulting membrane morphologies were characterized using various surface analysis techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and atomic force microscopy (AFM). The combination of VIPS with advanced polyimides holds promise for developing high-performance membranes for desalination applications.