Determination of dose rate effects in polymers irradiated in vacuum

The most reliable information on polymeric material radiation resistance and lifetime may be obtained in tests conducted under operational conditions. An important problem of the environment simulation is the adequacy of the accelerated test results, because the dose rates used in the test and under operational conditions can differ by up to several orders of magnitude. Accelerated testing to determine operational engineering performance is valid only when the change in a material property does not depend on the dose rate. It is well known that under irradiation in an air environment, the dose rate effects in polymers are very significant and are connected with oxidizing radiation destruction. For example, they are taken into account for polymer materials used in nuclear energy facilities. We examine an opposite situation, the irradiation of materials in vacuum that is typical for operation in space environments. It is often assumed that irreversible radiation changes in polymer properties do not depend on the dose rate. On the basis of available experimental data, including unpublished results obtained in our laboratory, we show that there are remarkable dose rate effects in physical-chemical and operational (e.g., mechanical, electrical, thermal, and optical) properties for a wide variety of polymers irradiated in vacuum. The material property change attributed to dose rate is often significant, varying by as much as an order of magnitude when compared to material response in an operational environment. In a number of cases, dose rate effects produce a nonlinear response in the material property.