Integrated RAFT Polymerization With Click Chemistry For Polypropylene Membrane Surface Modification And Its Properties

Membranes, such as polypropylene porous membranes, were widelyused in many fields such as ultrafiltration and microfiltration because oflow cost and easy processing. However, some disadvantages(hydrophobicity, poor anti-fouling characteristics and so on) limit itswider application. Surface modification of membranes becomes more andmore important in the membrane science to endow membranes withdesired properties. Traditionally there are two main routes for surfacemodification: grafting to and grafting from. The grafting to approach isexperimentally simple and can provide better control of the graftedpolymer, but it usually suffers from a lower grafting density. The"grafting from" technique is a heterogeneous, surface-initiatedpolymerization process whereby polymer chains grow from initiator siteson the membrane surface by monomer addition from solution.Oneimportant distinction in consideration of membrane modification bygrafted polymers is that the polymer chain density achievable by the"grafting to" method depends on the chain molecular weight; whereas,grafting density and polymermolecular weight are independent designparameters in "grafting from" strategies.In the present work, a novel three-step method for polyacrylamidegrafting to the polypropylene macroporous membrane was carried out bymarrying click chemistry with reversible addition-fragmentation chaintransfer radical polymerization. First, the membrane was brominated via agas phase free radical photochemical pathway, followed by SN2nucleophilic exchange of bromine atoms in the brominated membranewith azide groups in NaN3; second, alkyne-terminated polyacrylamide with determined structure was synthesized by using reversibleaddition-fragmentation transfer radical polymerization method; third,alkyne-terminated polyacrylamide was coupled onto theazide-functionalized membrane surface by the CuI-catalyzed azide-alkynecycloaddition click reaction. The permeation performances of themodified membranes were tested by the filtration of a protein dispersion.The protein filtration experiments show that, in comparison with theunmodified membrane, the modified membrane can effectively rejectproteins due to the densely grafted polymer chains.In order to further improve the experiment steps, a novel two-stepmethod for N-vinyl-2-pyrrolidone grafting to the polypropylenemacroporous membrane was carried out based on the experimentalmethod above. Firstly, the membrane was brominated and azidefunctionalized as mentioned above except shortened UV irradiation timeto decrease the grafting density of Br. Secondly, combining step2andstep3above, a tandem process that simultaneously employs RAFTpolymerization and click chemistry was provided to modify the surface ofpolypropylene membrane. Similarly, the permeation performances of themodified membranes were tested by the filtration of a protein dispersion.The experiment results showed that the contact angle decreased lessbecause of the low grafting degree of N-vinyl-2-pyrrolidone. The proteinfiltration experiments show that, in comparison with the unmodifiedmembrane, the modified membrane can effectively reject proteins due tothe densely grafted polymer chains.