Modification of hollow fibers by UV surface grafting

Hollow-fiber membranes, made of polysulfone and originally having ultrafiltration properties, were modified in their outer surface layer by UV photografting. The monomer that was grafted onto the membrane surface was diallyl dimethyl ammonium chloride (DADMAC): it makes the surface hydrophilic, closes membrane pores and adds positively charged sites to the surface. The different UV doses were obtained by changing the light intensity. The membrane permeability was reduced with respect to the untreated membrane and high retentions of the calcium ion were obtained even though the MWCO remained high. The retention of ions is controlled by the electrostatic repulsion betweens the ions and the membrane charges. Increasing the UV dose at a constant line speed reduces membrane permeability and increases selectivity. This is the first step towards production of hollow-fiber membranes for nanofiltration.