Achieving Highly Effective Non-biofouling Performance for Polypropylene Membranes Modified by UV-Induced Surface Graft Polymerization of Two Oppositely Charged Monomers

A major problem in membrane technology for applications such as wastewater treatment or desalination is often the loss of membrane permeability due to biofouling initiated from protein adsorption and biofilm formation on the membrane surface. In this study, we developed a relatively simple and yet versatile approach to prepare polypropylene (PP) membrane with highly effective non-biofouling performance. Copolymer brushes were grafted to the surface of PP membrane through UV-induced polymerization of two oppositely charged monomers, i.e., [2-(methacryloyloxy)ethyl]trimethylammonium chloride (TM) and 3-sulfopropyl methacrylate potassium salt (SA), with varying TM:SA molar ratios. Surface analysis with scanning electron microscope (SEM) clearly showed the grafted copolymer brushes on the membrane surfaces and that with X-ray photoelectron spectroscope (XPS) revealed a similar TM:SA ratio of the grafted copolymer brushes to that of the monomer solution used for the polymerization. Water contact angle measurements indicated that the hydrophilicity of the membrane surfaces was remarkably improved by the grafting of the TM/SA copolymer brushes, with the lowest water contact angle of 27° being achieved at the TM:SA ratio of around 1:1. Experiments for antiprotein adsorption with bovine serum album (BSA) and lysozyme (LYZ) and antibiofilm formation with Escherichia coli (E. coli) demonstrated a great dependence of the membrane performance on the TM:SA ratios of the grafted copolymer brushes. It was found that the characteristics of the surface charges of the membrane surfaces played a very important role in the non-biofouling performance, and the membrane surface with balanced positive and negative charges showed the best non-biofouling performance for the proteins and bacteria tested in this study.