Low surface energy nanofibrous membrane for enhanced wetting resistance in membrane distillation process

Abstract

Membranes with hierarchical structure have higher water contact angle due to affording more air pockets in its nano-micro regions, and then have been known as more hydrophobic. However, the hydrophobicity could be vanished after air in pockets has been exhausted such as in the vacuum membrane distillation (VMD) process. Here, an intrinsically hydrophobic electrospun nanofiber membrane (ENM), whose hydrophobicity is entirely depended on these materials surface energy, was proposed to promote the wetting resistance without the aid of any hierarchical structure. The novel composite ENMs were fabricated using polyvinylidene fluoride (PVDF) as matrix material, and then covered it with polydimethylsiloxane (PDMS) of much lower surface energy via a facile dip-coating. Scanning electron microscope (SEM) images of nanofibers in composite ENMs showed almost no hierarchical structure was existed, while the hierarchical structure was supposed to lower water contact angle in theory. However, the composite ENMs exhibited a higher water-repellency as the surface energy of nanofibers became lower. Therefore, the composite ENMs cannot be wetted by condensed vapor throughout the MD process, and a thinner membrane exhibited an excellent durability in the continuous vacuum MD tests. The result indicated that for the ENMs material of intrinsic hydrophobicity, the low surface energy is the essential quality to exhibit the strong and durable wetting resistance in VMD process, what is difficult to achieve by conventionally hierarchical architecture constructing on membrane surface.