Effect of additives in the casting solution on the formation of PVDF membranes

Porous asymmetric hydrophobic membranes were prepared from poly(vinylidenefluoride-co-hexafluoropropylene) and polyvinylidenefluoride (PVDF) homopolymer by the phase inversion process induced by a nonsolvent. The effect of pore-forming hydrophilic additives on the membrane morphology and transport properties was investigated. Mean pores size and effective porosity were calculated by the gas permeation method. It was found that membranes made of PVDF copolymer offer a higher resistance to mass transport compared to membranes prepared from PVDF homopolymer under the same experimental conditions. This result is due to lower membrane porosity and pore size and to the presence of a double skin layer (on the upper and bottom surface) for these membranes, reducing the permeate flux. However, membrane morphology and transport properties can be modified by additives in the casting solution. In particular, it was found that polyvinylpyrrolidone improves membrane permeability; LiCl can be used in order to reduce macrovoid formation and increase the mechanical stability of the membranes.