Radiation-initiated graft copolymerization of 4-vinylpyridine onto polyethylene and polytetrafluoroethylene films and anion-exchange membranes therefrom

Radiation-initiated graft copolymerization of 4-vinylpyridine (4VP) onto low-density polyethylene (LDPE) and polytetrafluoroethylene (PTFE) films was carried out by gamma-rays from Co-60 source in nitrogen atmosphere. The direct method of multiple grafting was used, connected with posteffect taking place at irradiation doses from 1 to 35 kGy. Thus the monomer diffused into the polymer matrix together with the stepwise generation of free radicals to reduce the thickness of the nongrafted layer. The grafting proceeds in the amorphous phase of the polymers which is further amorphized by the irradiation. Copolymers with grafting degree (P) from 17 to 75% for LDPE and from 0.7 to 13% for PTFE were obtained. The dependence of the degree of grafting of 4VP on irradiation dose was established. The spectral and thermodynamic properties of the synthesized copolymers were studied. The degree of crystallinity was found to decrease and a two-stage mechanism of thermooxidative destruction was observed. The synthesized copolymers contain pyridine groups determining their anion-exchange properties. The initial pyridine groups in the grafted copolymers were quaternized. Some of the basic characteristics, such as water content and specific electric resistance of anion-exchange membranes (AEM) were studied depending on the grafting degree of 4VP. The ionomers studied possess high transport number ((t) over bar(-) similar to 0.8-0.9), good diffusion characteristics (9.2 X 10(-8)-1.7 x 10(-7) cm(2)/s), and suitable tensile properties. The AEM obtained, combined with the cation exchange membranes we have synthesized earlier on the same polymeric basis, can be used successfully in various electrodialysis processes. (C) 1997 John Wiley & Sons, Inc.