Preparation And Performance Characterization Of Polyvinylidene Fluoride (PVDF) Homogeneous Blend Ultrafiltration Membrane

In order to obtain a porous surface and improve the pure water flux of the polyvinylidene fluoride(PVDF)membrane,two PVDF polymers with different molecular weights were blended to prepare PVDF homogeneous blend membranes by the non-solvent induced phase separation(NIPS)method.The influence of the homogeneous blending ratio,solvent type,coagulation bath temperature and non-solvent vapor treatment time on the structure and performance of PVDF homogeneous blend membrane was studied.First,the influence of homogeneous blending ratio on the membrane structure and performance was studied.The rheological behavior and dynamic thermodynamic analysis showed that the two PVDF polymers with different molecular weights were not completely compatible,which will promote the formation of phase inversion pores and interface pores on the membrane surface.With the increase of mass fraction of low molecular weight PVDF(PVDF_L),the number of surface pores,roughness and porosity increased.When the mass fraction of PVDF_L was 50 wt%,the pure water flux and rejection rate of the blending membrane were higher,which was about 8 times that of the pure PVDF membrane,and the bovine serum albumin(BSA)rejection rate was92.83%.The optimal blending ratio of PVDF_L was selected to be 50 wt%,the effects of DMSO and DMF on the structure and properties of PVDF homogeneous blending membrane were discussed.The results showed that PVDF homogeneous blend membrane prepared by DMSO had porous surface and larger finger-like pores,and the pure water flux increased to 27.95 L·m^-2·h^-1·bar^-1,which was 1.5 times of DMF solvent system membrane.Then,using DMSO as the solvent,the effects of the coagulation bath temperature and non-solvent vapor treatment time on the membrane structure and performance were studied.The results showed that when the temperature of the coagulation bath increased from 25 ? to 85 ?,the density of the surface layer increased,the surface roughness decreased,the finger-like pores became wider,the mechanical properties and rejection rate of the membrane increased,and the water flux decreased.Prolonged non-solvent steam treatment time was beneficial to the formation of porous surface and loose membrane pore structure.When the treatment time was increased to 60 s,the PVDF spherulitic structure appeared on the membrane surface.When the treatment time was150 s,a porous membrane surface with PVDF spherulites was formed,the surface roughness and contact angle increased sharply,and the cross-section changed from an asymmetric structure to a symmetric structure with spherulites stacked.When the non-solvent steam treatment time exceeded 60 s,the tensile mechanical properties of the membrane were significantly reduced,which was not conducive to practical applications.According to the above analysis,the best preparation conditions:the mass fraction of PVDF_L was 50 wt%,DMSO was the solvent,the coagulation bath temperature was 25 ?,and the non-solvent steam treatment time was 60 s.At this time,the pure water flux of the PVDF homogeneous blend membrane was 82.17 L·m^-2·h^-1·bar^-1,which was about 27 times that of the pure PVDF membrane.The BSA rejection rate was 89.91%,the tensile strength was 1.52 MPa,and the elongation at break was17.76%.This study showed that blending two PVDF polymers with different molecular weights was an effective method to control the pore structure of the membrane and obtained porous membrane surface,and by changing the solidification conditions,the porous PVDF homogeneous blend membrane can be given higher pure water flux.