Abstract:

Poly(vinylidene chloride-co-vinyl chloride)[P(VDC-co-VC)] porous membrane, which had a homogeneous structure, was prepared by thermally induced phase separation(TIPS) method with dimethyl phthalate(DMP) as the diluent. The microstructures of P(VDC-co-VC) porous membranes were observed by field emission scanning electron microscopy(FESEM), differential scanning calorimetry(DSC), X-ray diffraction(XRD), atomic force microscopy(AFM). The effects of polymer content on the morphology and properties of P(VDC-co-VC) porous membranes were investigated by polymer-diluent binary system phase diagram, terms of porosity, pure water flux, contact angle and mechanical strength test etc. The results showed that the membrane formation process of P(VDC-co-VC)-DMP binary system was dominated by liquid-liquid(L-L) phase separation. With the increase of the polymer content, the cross-sectional structure of the membrane came from petal-like structure to cavity structure. In the structural transformation, the pores connectivity of the membrane cross-section were reduced and the structure became denser. At the same time, the porosity on the top surface of the membrane decreased and the roughness increased. The changes of L-L phase separation time and the polymer content caused the crystallinity of the membrane decreased and the partial crystallization peak disappeared or decreased. The membrane crystallinity increased as the polymer content reached to 40%, which indicated the greater correlativity of the phase separation time and the polymer content to the membrane crystallinity. In the flux testing stage, the pure water flux of membranes prepared with low polymer content appeared a large attenuation with the prolonging of testing time, which indicated poor compaction resistant ability for the membrane fabricated by low polymer content, while the increase of the polymer content could improve the membrane compaction resistant ability strongly. During the protein retention stage, the concentration polarization formed a filter cake layer that increased the protein rejection, while the permeate flux was tended to be stable. The increase of the polymer content brought about the increase of the contact angle of top surface, tensile strength and protein rejection, but the mean pore size, porosity and pure water flux increased first and then decreased. When the polymer content was 30%, the prepared membrane had excellent permeability and the tensile strength was 7.5 MPa.

Key words: Thermally induced phase separation, Poly(vinylidene chloride-co-vinyl chloride) porous membrane, Polymer content, Liquid-liquid phase separation