Structural Regulation And Properties Of Functionalized Polyvinylidene Fluoride Composite Membranes

Polyvinylidene fluoride（PVDF）has been widely used to fabricate the separation membrane due to its high mechanical strength,good thermodynamic and chemical stability,acid and alkali corrosion resistance,excellent aging resistance,good dielectric and biocompatibility,etc.However,PVDF membrane is easily fouled during the filtration process owing to the inherent strong hydrophobicity,which reduces the separation efficiency and service life.Therefore,blending and surface modification are usually used to improve membrane performance and avoid the trade-off phenomenon between permeability and selectivity.In this work,an amphiphilic copolymer and functionalized multi-walled carbon nanotubes（MWCNTs）were simultaneously blended with PVDF to fabricate to the composite membrane by non-solvent induced phase separation（NIPS）.By adjusting the polarity of the amphiphilic copolymer and the functional groups of the multi-walled carbon nanotubes,the separation membrane with uniform pore structures was prepared.Importantly,the as-prepared PVDF composite membrane simultaneously exhibited the high permeability and excellent selective performance due to their synergistic segregation to the membrane surface and the pore wall.Subsequently,a new type of separation layer structure was constructed on the membrane surface by grafting and co-deposition modification,which possessed the high permeability and dye retention,and low salt retention.In general,this study provides a new idea for the functional modification of PVDF separation membranes.The specific research content and main conclusions of this paper are as follows:1.The polarity regulation of styrene-maleic anhydride copolymer（SMA）and its influence on membrane structure and membrane performance.The SMA was modified with alkaline solution,and the effects of treatment time,SMANa addition amount and coagulation bath temperature on the compatibility of the casting solution system,membrane microstructure and properties were studied.It was found that the compatibility between the prepared SMANa and PVDF became better with the extension of the alkali treatment time.The membrane surface morphology was gradually changed from large crescent-shaped pores into a dense small pore structure.When the alkali treatment time was 6 h,the pure water flux and BSA retention of the SMANa/PVDF composite membrane can be increased at the same time,breaking the Trade-off effect.Therefore,the optimal time for alkali treatment was 6 h.In addition,the influence of SMNa addition and coagulation bath temperature on the membrane structure and performance was also studied.The study showed that with the increase of SMANa addition and the decrease of coagulation bath temperature,the phase separation process was changed from liquid-liquid phase dominance into liquid-solid phase dominance,and the pore size distribution of the membrane became narrower and the size became smaller.When the addition amount of SMANa was 3wt%and the coagulation bath temperature was 0℃,the permeation flux of the membrane was1014.29 L/m²·h and the BSA rejection of was 98.86%.2.The effect of functionalized multi-walled carbon nanotubes on composite membrane structure and performance.Functionalized MWCNTs were used to further modify the PVDF membrane by blending,controlled the microstructure and increased the permeation flux by their segregation behavior onto the membrane surface.Moreover,more polar groups on the membrane surface resulted from the segregation not only improved the hydrophilicity and anti-fouling performance,but also provided more reaction sites for subsequent modification.It was found that various functionalized MWCNTs carboxyl（MWCNTs-COOH）,amino（MWCNTs-NH）,and hydroxyl groups（MWCNTs-OH）had differently strong hydrogen bonds with SMANa,and MWCNTs-OH could promote PVDF to form a stableαcrystal structure.Hydrogen bonding and crystallization synergistic affected the microstructure and properties of the composite membrane.Compared with MWCNTs-NH and MWCNTs-OH,the composite membrane with a uniform pore structure was prepared due to the strong hydrogen bond between MWCNTs-COOH and SMANa,and the synergistic segregation effect makes MWCNTs-COOH uniformly dispersed on the membrane surface.However,with increasing the addition of MWCNTs-COOH,it was prone to agglomerate,causing local defects on the membrane surface and reducing the selection performance.When the addition amount of MWCNTs-COOH was 0.08 wt%,the rejection of the composite membrane to Congo red and methyl blue were 79.38%and 35.06%,respectively,and the flux recovery rate was 89.63%showing the good anti-pollution performance.3.The microstructure construction and manipulation of the separation layer on the composite membrane surface.According to the principle that the carboxylic acid groups on the membrane surface can react with polyethyleneimine（PEI）and form hydrogen bonds with triethanolamine（TEA）,PEI and TEA were used to modify the composite membrane by surface grafting and co-deposition techniques.A new type of loose nanofiltration structure is constructed on the surface of the membrane to optimize the selectivity of the membrane.It was found that when PEI or TEA is used alone,the surface structure of the membrane had some defects,leading to a poor selectivity to dyes.The combination of PEI and TEA could make up for the above-mentioned defects.With the increase of PEI/TEA content,the skin layer on the membrane surface became denser and denser,the thickness gradually increased,and the water contact angle of the membrane surface gradually decreased.When the PEI/TEA content was 2/1.6wt%,the as-prepared membrane displayed a high rejection and high permeation flux to Congo red,methyl blue and acid fuchsin.Meanwhile,it exhibited a low rejection to NaCl,MgCl₂,Na₂SO₄ and MgSO₄ salts.In a filtration test for simulated actual textile wastewater,the prepared loose nanofiltration membrane had excellent dye/salt separation performance,high permeation flux,good long-term stability and anti-pollution performance.