Surface modification of polymers .4. Grafting of acrylamide via an unexpected mechanism using a water soluble photo-initiator

A water soluble derivative of benzophenone has been used to initiate the grafting of acrylamide onto a copolymer styrene-(ethylene-co-butene)-styrene (SEBS). Irradiation at 350 nm is carried out through the polymer film in contact with aqueous solution of monomer and photo-initiator in such a way that the light is absorbed very close to the surface of the film. In the absence of hydrogen donor in solution, no surface modification can tie detected but addition of 2 vol% isopropanol results in simultaneous surface grafting and homopolymerization. Adsorption of the photo-initiator is a necessary condition for hydrogen abstraction to occur from the surface of the polymer but this is not sufficient to initiate the grafting of acrylamide. The results show that an induction period, during which homopolymerization proceeds, is required before surface modification can be detected. Grafting results from the recombination of SEBS radicals formed at the surface of the him with growing polyacrylamide chains initiated in solution, very close ro the surface. That mechanism is possible because of the inhomogeneous absorption of light and the existence of a thin viscous layer of homopolymer produced during the induction period. Surface grafting of acrylamide by the same mechanism onto other polymers is also possible.