Superhydrophobic Polypropylene Hollow Fiber Membrane Modified By SI-ATRP Method And Its Application In Membrane Distillation

Vacuum Membrane Distillation(VMD),a membrane distillation process,in which the feed contacts with the hot side of membrane while the permeate side is supplemented by vacuum.In light of high flux and low heat loss,it enjoys high favor among people.Currently,among many commercial polymer membranes,polypropylene(PP)hollow fiber membrane is inexpensive and has mature preparation processes so that after modification it will be well valid in membrane distillation.At present,chemical grafting which combines the modified layer with strong chemical bond immobilized on the membrane surface is more useful compared with physical methods.In this paper,polypropylene hollow fiber membrane was used as substrate,explored a controllable-living surface chemical graft modification method in order to obtain modified membrane with excellent performance.First,aqueous solution of ammonium persulfate was used to activate the PP membrane surface to produce a large number of hydroxyl groups.Next,the one-step and multi-step atom transfer radical polymerization(ATRP)methods were employed via using glycidyl monomethacrylate(GMA)or both GMA and 2-hydroxy methacrylate(HEMA)as monomers,respectively to graft functional polymer layer on the surface of the PP membranes.Given that it could increase the grafting degree,repeatedly adding equal doses of the monomer method was both used in two polymerization methods.After that,the graft layers were modified via using low surface energy compound perfluorooctanoyl chloride to prepare superhydrophobic PP membranes.The modified membranes were characterized by Fourier infrared spectrometer(ATR-FTIR),cold field scanning electron microscope(FE-SEM),contact angle measuring instrument(DSA),true color confocal microscope(CSM)and thermogravimetric analyzer(TG),etc.The effects of modification conditions on the performance of modified membranes were investigated.Monomers with mass ratio of10:1,20:1,and 30:1 to the immobilized initiator membranes were used.The results indicated that the water contact angles of membranes prepared by one-step and multi-step modification methods(denoted as L-PP and B-PP)are 167.9° and 179.0°when the amount of monomer was 20 and 30 times,respectively,suggesting that we obtained excellent superhydrophobicity.The obvious rough structure coverage was observed by SEM and CSM.The vacuum membrane distillation performances of L-PP-10/20 and B-PP-10/20 which had the best liquid entry pressure(LEP)were compared to that of the original PP membrane.The results indicated that the modified membranes have excellent flux and acceptable rejection rate at high temperature and low Na Cl solution concentration.When operated 21 h,the average flux of L-PP-20 was 11.01 kg/(m^2h),which was4.53 kg/(m^2h),higher than that of unmodified membrane and the rejection performance scarcely changed.The B-PP-20 was 12.71 kg/(m^2h),but the stability of the performance(the flux and the rejection rate)was unsatisfied,the rejection rate was only 90-96%.Therefore,L-PP-20 had superior performance and can provide a good example for the application of ATRP modified membranes in vacuum membrane distillation.