Structure Control And Property Of Functionalized Polypropylene Hollow Fiber Membrane Prepared By In Situ Ultrasonic Wave-assisted Polymerization

Polypropylene membrane is easily contaminated during the waste water treatment due to the intrinsic hydrophobic properties,decreasing the separation efficiency.Lots of methods have been used to improve the antifouling property of hydrophobic polymer membrane,such as physical coating,blending,surface grafting polymerization,and so on.However,Most of the studies reported the poor stability and hydrophilicity,and the reduction in the water flux with the increase of rejection,indicating the trade-off between the permeability and selectivity in the polymer-based membranes.How to controll the microstructure of polymer membrane for improving properties is a key in the surface modification.Ultrasonic wave can not only promote the polymer synthesis by generating free radicals and activating free radical initiators,but also control the structure of polymers via the cavitation effect.Therefore,functionalized polypropylene hollow fiber membrane with stable structures and excellent performances may be fabricated by in situ ultrasonic wave-assisted polymerization in order to address the above mentioned issue.Polypropylene hollow fiber membranes with an acrylic hydrogel surface was prepared by in situ ultrasonic wave-assisted polymerization.The effect of formula and technological conditions on the surface composition and morphology of modified membranes was studied.The structure-property relationship for modified membranes was established.The results show that the ultrasonic irradiation can significantly improve the grafting efficiency of acrylic acid on the membrane surface.The polyacrylic acid mainly existed on the outer surface of modified membranes,and only a few appeared on the inner.Interestingly,the optimized membrane PPM1.49 exhibited excellent retentions of 99.8%,99.5%and 98.7%to BSA,Congo red and methylthionine chloride,respectively.These promising results indicate that modified membranes developed in this study are potentially applicable for dye removal from wastewater.To overcome the permeability-selectivity trade-off in the polymer-based membranes,our key strategy is to construct the microstructures of polypropylene hollow fiber membranes by in situ ultrasonic wave-assisted graft polymerization,and incorporate hydrophilic groups into the inner of membranes.Functional groups were incorporated on the surface and inner of membranes through altering grafted monomers and the amino acids induced epoxy ring opening reaction.The effect of formula and technological conditions on the surface composition and morphology of glycidyl methacrylate grafted membranes was studied.The formation mechanism of microstructures of modified membranes was revealed according to the ultrasonic capillary effect.The effect of amino acids on the extent of reaction and microstructure was investigated based on the nucleophilic substitution(S_N2).The structure-property relationship for modified membranes was established.The results show that the functional epoxy groups can be introduced into the inner of the polypropylene hollow fiber membrane in the study,providing the reaction activity point for the introduction of the subsequent hydrophilic groups.The electron-donating ability of nitrogen atom in the threonine was very strong,so it has powerful nucleophilic ability and high reactivity in the epoxy ring opening reaction.Thus the corresponding PGMA/threonine modified membrane(PPM3)obviously broke through the selectivity-permeability trade-off.Here,a novel method was presented to fabricate porous styrene-maleic anhydride(SMA)copolymers on the surface of polypropylene hollow fiber membranes through the ultrasonic wave-assisted interfacial suspension copolymerization of styrene and maleic anhydride.The effect of formula and technological conditions on the surface composition and morphology of SMA grafted membranes was studied.The structure and property of pores in the SMA layer were controlled by adjusting the polymerization and phase separation,and the formation mechanism of inter-connected pores was elucidated in view of the selective swelling.The structure-property relationship for modified membranes was established.The results show that when polypropylene hollow fiber membranes are modified via the free radical copolymerization of styrene and maleic anhydride,the reaction might mainly occur on the outer surface because of their high copolymerization activity and different wetting.In the deionized water/acetone system with the weight ratio of 3:7,a SMA layer with many inter-connected pores was fabricated on the membrane outer surface.Compared with the pristine membrane,the water flux,rejections and antifouling performance of the as-prepared Janus membrane PPM2 was simultaneously enhanced.One hand,the surface charges induced the deemulsification of emulsions,increasing the the size of oil droplets getting to the membrane surface.On the other hand,the hydrophobic inner surface of the Janus membrane prolonged the retention time of emulsion in the membrane,thus facilitated the demulsification of entrapped emulsions,making small oil droplets merge into bigger droplets.Therefore,the separation efficiency of oil/water emulsion was up to 99.9%.