Design of super-hydrophobic microporous polytetrafluoroethylene membranes

Abstract

Polytetrafluoroethylene (PTFE) membranes with unique structures were fabricated from a mixture of a PTFE emulsion and a poly(vinyl alcohol) (PVA) aqueous solution. The formation of the micro–nano structures in the PTFE membrane was ascribed to PTFE crystallization. By controlling the cooling rate during membrane formation, various PTFE membranes with different structures and properties prepared, which showed a super-hydrophobicity with a water static contact angle (155°) and a sliding angle (8.3°) owing to their micro–nano structures. The results of DSC thermal analysis, XRD and polarized light microscope examination revealed that the micro–nano structures in the membrane were derived from improved PTFE crystallization during the cold-stretching process, resulting in a crystalline phase comprising of the micro aggregates and an amorphous phase comprising of the nano structures. The resulting membrane was shown to be highly porous while still maintaining a super-hydrophobicity. A critical transmembrane pressure of as high as 0.65 MPa was achieved because of the super-hydrophobicity of the membrane. The membranes were further investigated for use in vacuum membrane distillation (VMD), and the effects of membrane structures on the water vapour flux were studied. The VMD flux increased obviously with the increase in membrane pore size when the membrane suffered stretching.