In this study, natural rubber vulcanizates which were deformed to variable strains were exposed to r-ray, and their chemorheological properties were examined by the stress reiaxation measurements in nitrogen atmosphere. The rate of relaxation decrcased with increasing extension ratlos.
The rate of the degradation which was caused by peroxyradicals produced in the presence of oxygen was too fast to observe the effects on extension ratios. In nitrogen atmosphere the rate of stress relaxation varied in proportion to the extention ratios, and the slope ef the proportional constant changed with crosslinking density. Further, the rate of stress relaxation of such vuleanizates seemed to be affected by the crosslinking structures, i. e., peroxide or sulfur crosslinking.
Nielsen has reported that the force of contraction was directly proportional to the degree of the orientation of polymer chains. The degree of orientation was inverse proportion to the relaxat ion rate. Thus, it was suggested that the decrease in rate of degradation in high extension ratios was not cagsed by the crystallization, but mainly by the orientation of polymer chains.
The oriented molecular chains were easily cut by r-ray irradiation, so-called by mechanochemical process. The produced macroradicals activated adjacent chains or recombined with other macroradicals. As a result, induced network increased and retarded the stress relaxation in large deformation.