Effects of Non-solvent Additives on PVDF Solution Viscosity and Membrane Performance

Fabrication of polyvinylidene fluoride (PVDF) membranes from non-solvent induced phase separation process (NIPS) was studied. Particular focus was given to the effects of triglycol and Tween80 as non-solvent additive (NSA) on the polymer solution viscosity and membrane performance. The results showed that the viscosity of polymer solution increased after NSA addition, while it decreased with further incremental content of NSA. When VNSA/VS ratio approached its cloud point value, the viscosity did increase again. Under the same concentration level, unlike Tween80, triglycol could lead to higher incremental value in viscosity. All the membranes obtained in this research work had dense outer surface and the pores in subsurface were formed by the gaps between spherulites. Similarly, the cross-section images showed that, asymmetric structure consisted of a dense top layer and a porous sub-layer. The addition of triglycol promoted the formation of sponge-like structure and suppressed the formation of finger-like structure. While, Tween80 promoted the formation of macrovoid structure and induced the disappearance of the finger-like pores. In addition, Tween80 improved the membrane surface hydrophilicity due to the exposure of the residual hydrophilic groups on the membrane surface. The addition of NSA improved the water flux, especially for additive such as Tween80 (from 3.73 L. (m2. h . 0.1 MPa)-1 to 440.25 L . ( m2. h . 0.1 MPa)-1. After adding NSA into the polymer solution, the breaking tenacity of the obtained membrane decreased, while the elongation-at-break increased.