Facile formation of symmetric microporous PVDF membranes via vapor-induced phase separation of metastable dopes

Symmetric microporous PVDF membranes were prepared from the ternary system water/triethyl phosphate/PVDF, by a vapor induced phase separation scheme (VIPS) that employed a non-solvent additive (i.e. water) in the dope to facilitate phase inversion. Incorporation of water into the dope generates a metastable system and induces precipitation even upon limited vapor-exposure at ambient conditions. Additionally, the resulted membranes exhibit a special bi-continuous bulk structure comprising interlinked crystallites with three-dimensionally porous networks. Moreover, the structure of top surface can be controlled by the duration of vapor exposure and ranges from a dense (nonporous) skin to an open top over the period of 0-8 min. Optimally, a symmetric microporous membrane with high hydrophobicity (CA = 1191, large porosity (84.5%) and submicron pore size (0.36 mu m) appropriate for membrane distillation in desalination applications is obtained. In actual practice, this membrane could yield nearly pure water at the rate of 17.2 LMH from a 3.5% NaCl(aq) feed by direct contact membrane distillation operated at a temperature difference (Delta Thot-cold) of 30 degrees C.