EPR application to polymers: Radiation induced crosslinking and graft polymerization

EPR application to polymers has been closely related to the research and development of polymer modification by radiation processing. The free radicals induced in polymer chains by irradiation can initiate chemical reactions at rather low temperature ranges, therefore, the radical reactions can be followed by EPR measurement. EPR studies of polymers were applied mainly for the analysis of graft polymerization by the pre-irradiation method and the analysis of radiation crosslinking, which contributed to the development of polymer materials by radiation processing. For graft polymerization, the radicals trapped in the crystalline regions of polymers migrate to the surface and initiate the graft reaction. The concentration of the trapped radicals and their rate of decay are closely related to the graft yield and rate of the grafting reaction. As the radical migration rate in the crystalline part, namely the decay rate of the trapped radicals, is determined by the temperature, the graft polymerization is much affected by the temperature. And also the trapped radicals in any polymers can be conserved for a long time by cooling the irradiated polymers even in air. Therefore, the irradiation and the graft-polymerization can be separated in place and in time. For the crosslinking of polymers, polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN) fiber and polycarbosilane (PCS) fiber were selected for a specific application. The PAN fiber is a precursor of carbon fiber formation and PCS fiber is the precursor of SiC ceramic fiber. Especially the EPR studies for PCS fiber and the pyrolysis were useful for the development of radiation processing. The process of pyrolysis of PCS fiber at a high temperature involves radical reactions and could be followed by EPR measurements. A knowledge of the radical reaction mechanism contributed much to the development of a new method for SiC fiber synthesis.