Relation between diffusion controlled decay of radicals and α-relaxation in polyethylene and polyoxymethylene

Abstract

Diffusion controlled process theory was applied to the analysis of the data concerning decay reactions of free radicals in irradiated polyethylene and polyoxymethylene in detail. Diffusion constants of free radical sites were estimated from the data of the decay reactions and their time constants were calculated by using Stokes-Einstein relation. These time constants were plotted against the inverse temperature in the relaxation maps, which were arranged by Wada. In the case of polyethylene, the time constants of decay reactions of alkyl radicals trapped in the lamellar surface and the inner part of crystallites were in good agreement with the relaxation times of α′- and α-processes, respectively. However, it was found that the time constants of main chain radicals trapped in polyoxymethylene were closely related to α-relaxation processes. The activation energies of the decay reactions of free radicals were in good agreement with those of the corresponding relaxation processes in both cases. From these results, it was concluded that the decay reactions of the free radicals reflect the molecular motions associated with the respective relaxation processes. Concerning the decay reaction of free radicals in the inner crystallite of polyethylene, effects of lamellar thickness and crystal surface were also studied by using solution grown crystals, which were crystallized at various temperatures, and the fuming nitric acid treated materials. It was found that the diffusion constant of free radical sites and its activation energy were strongly dependent on the lamellar thickness and half-life of free radicals was a function of the amount of the fold surface; i.e., the reaction rate was much depressed as the lamellar thickness was increased and the amount of the fold surface was decreased.