Surface modification of polymers VII.: Photochemical grafting of acrylamide and N-isopropylacrylamide onto polyethylene initiated by anthraquinone-2-sulfonate adsorbed at the surface of the polymer

Photoinitiators anthraquinone-2-sulfonate sodium salt and (4-benzoyl benzyl) trimethylammonium chloride both adsorb from aqueous solutions at the surface of polyethylene and desorb partly in the presence of water. After adsorption of either photoinitiator on polyethylene, surface grafting occurs easily when the polymer, in contact with a thin layer of acrylamide solution, is irradiated at 350 nm. As a consequence of the grafting mechanism, homopolymer is always formed as by-product. Of both photoinitiators anthraquinone-2-sulfonate is the most suitable because it adsorb more strongly and has a higher absorption coefficient in the near UV. Grafting of acrylamide results in a decrease of the contact angle of the polymer surface with a water droplet from 80 to 25°. After grafting of N-isopropyl acrylamide the contact angle measured at room temperature also drops to 25° but remains 80° when measured at 40°C, i.e. above the L.C.S.T. of the grafted polymer.