Membrane surface with antibacterial property by grafting polycation

Biofouling is a major problem for membrane processes in biotechnologies for water treatment. One of the anti-biofouling strategies is to construct an antibacterial membrane surface. In this work, microporous polypropylene membrane (MPPM) was endowed with antibacterial property by grafting polycation and the action mechanism was studied from the viewpoint of the mobility of polycation chains. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was tethered on the membrane surface by a sequential UV-induced graft polymerization, followed by quaternization with benzyl chloride or iodomethane. To explore the antibacterial mechanism, crosslinking structure was introduced by bifunctional alkylating reagents to modulate the chain mobility of quaternized PDMAEMA. Fluorescein (Na salt) staining, water contact angle measurement, FESEM observation and XPS analysis were used to characterize the grafting density, quaternization ratio, positive charge density, and crosslinking degree of PDMAEMA on the membrane surface. The antibacterial efficiencies of the studied membranes against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus were compared with each other and related to the charge density and the chain mobility. These polycation-modified MPPMs exhibit high bactericidal efficiency against both E. coli and S. aureus. We found positive charge density higher than 3.50 μmol cm−2 and 2.50 μmol cm−2, respectively, is enough to obtain 100% antibacterial efficiency in 5 min contact with these two kinds of bacteria. However, once crosslinked the tethered PDMAEMA, the bactericidal activities are almost lost regardless of charge density and contact time. The mobility of grafted polycation is therefore considered to be the prerequisite of antibacterial action.