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The effects of molecular weight, molecular architecture, reaction mechanism, and sample thickness on the radiation-induced response of high-polymer materials are analyzed. A statistical evaluation of the main-chain degradation process shows that, up to high doses, the molecular weight of the degraded system is a function of the initial molecular weight. Radiation-induced changes in physical properties also exhibit a sensitive dependence on the initial molecular weight distribution in the important early stages of degradation. Different degradative characteristics can arise from variations in mer sequence, mer orientation, branching, and crystallization. In addition, these elements of molecular architecture affect the diffusion coefficient and thus the residence time of free radicals in the sample. Considerable induction periods may precede steady-state conditions as a result of diffusion processes and chemical mechanism effects. Thus, if measurements are made during the induction period, they may lead to incorrect conclusions about the sample's ultimate behavior under steady-state conditions. It is concluded that test results are of little comparative value unless the polymer systems are defined in terms of their determinant molecular parameters. Sample thickness, test environment, and testing time should be chosen carefully to avoid vitiation by diffusion and mechanism effects.
Fritz, E. G.
Senior Nuclear Engineer, Convair, A Division of General Dynamics Corp., Fort Worth, Tex.