Development of PVDF membranes for membrane distillation via vapour induced crystallisation

This study explores vapour induced crystallisation technique for developing polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD). It works based on the observation that a sufficiently long exposure of PVDF film under humid air allows the occurrence of crystallisation of semi-crystalline PVDF. Three PVDF polymer grades with different molecular weights (MWs) were used to investigate crystallisation behaviour during vapour induced phase separation (VIPS). Later, the performances of four membranes, prepared at exposure times of 30, 60, 90 and 120 min and using PVDF-370 kDa, were evaluated in a lab-scale direct contact MD (DCMD) set-up. Vapour induced crystallisation was proven to be an effective way to adjust PVDF crystallinity, being higher at longer exposure time. There exists a transition time in humidity exposure (between 60 and 120 min) at which a shift of membrane morphology occurs from highly amorphous, flat and smooth to rough and grainy morphology. This finding is persistent for the three PVDF MWs. This shift in morphology has clear consequences on the membrane properties and its MD performance. The increase in degree of crystallinity led to higher contact angle, larger pore size and reduced membrane thickness. The combination of these properties led to an overall improvement in membrane performance manifested in an increase in its flux and salt rejection.