Photo-bromination and photo-induced graft polymerization as a two-step approach for surface modification of polyacrylonitrile ultrafiltration membranes

A two-step heterogeneous polymer surface modification was developed, involving photo-bromination as activation and subsequent UV-induced graft polymerization with acrylic monomers. With polyacrylonitrile (PAN) ultrafiltration (UF) membranes, extent and effects of photo-bromination compared with the thermal reaction were studied with gravimetry, diffuse reflectance UV and ATR-IR spectroscopy, and scanning electron microscopy (SEM) as well as the UF properties. The yields of thermal and photo-bromination reached constant values of about 60 and 150 mu mol/g, after 24 and 3 hr, respectively. The former Br content was assigned to Br-2 addition to double bonds, the latter to Pr-radical CH substitution plus addition products. Excess bromine was bound as a charge-transfer complex to PAN nitrile groups, causing partially irreversible changes of the membrane matrix indicated by UF permeability reductions. The formation of polymer radicals after UV irradiation of photo-brominated PAN was verified with ESR spectroscopy. UV irradiation-induced graft polymerization of acrylic acid and methyl acrylate from the gas phase on to PAN membranes was accomplished. It was found that activation by thermal bromination is sufficient to initiate heterogeneous graft polymerization. Sorbed bromine promoted homopolymerization, increasing concentrations of hydroquinone improved the grafting efficiency. The graft-polymer modified surfaces were characterized by ATR-IR, SEM and contact angles. A pronounced selectivity towards the UV exposed membrane surface was achieved. For relatively low degrees of modification (up to 150 mu g/cm(2)), thin and smooth graft polymer layers were created which specifically altered the membrane surface hydrophilicity. UF membrane permeability and selectivity were a direct function of the degree of modification, suggesting adjustability of average separation layer pore size. Copyright (C) 1996 Elsevier Science Ltd