Determination of Dose Rate Effects in Polymers Irradiated in Vacuum

Abstract

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 On the basis of available experimental data, including unpublished results obtained in our laboratory, we show that there are remarkable dose rate effects in physical-chemical and operational (e.g., mechanical, electrical,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. NomenclatureC * = nonequilibrium state concentration c = concentration of the crosslinks D = absorbed dose, MGy E = resulting radiolysis product concentration F = surface area, m 2 G = radiation-chemical yield, 1/100 eV I = dose rate, Gy/s k = rate constant of chemical reaction, kJ/mol k D = diffusivity, m 2 /s l = sample characteristic size, m M n = number-averaged molecular weight P = radiolysis product concentration q = gel fraction R = intermediate active species concentration S = optical density T ir = temperature of irradiation t = exposure time in seconds W = product accumulation rate, s −1 δ = dielectric losses ε = elongation at rupture λ = heat conductivity, W/m · K σ = relative tensile strength, MPa τ = radiolysis species lifetime, s Subscripts a = under accelerated conditions g = gaseous products

Investigation on the effects of beta and gamma irradiation on conducting polymers for sensor applications

“…It is clear that the lower dose rate caused much more severe chain scission (reduction in conductivity) than the higher dose rate. This dose rate effect has been confirmed by others (Briskman et al, 2004).…”

Effects of Tritium Gas Exposure on Electrically Conducting Polymers

“…It is clear that the lower dose rate caused much more severe chain scission (reduction in conductivity) than the higher dose rate. This dose rate effect seen even in vacuum has been confirmed by others [Biskman, 2004]…”

In-situ strain analysis of potential habitat composites exposed to a simulated long-term lunar radiation exposure

“…In addition, it was found that oxidative degradation was a possible explanation for the scission dominance at decreased dose rates. Other studies (Briskman, et al 2004, Gillen and Clough, 1981a, Gillen and Clough, 1989, Sasuga, et al, 1985, and Seguchi, et al, 1981 have also discussed similar findings of scission related effects due to oxidation at low dose rates. Given that this study is performed in air at two extreme dose rates, it is possible that the strain effects are also due to oxidative degradation occurring more heavily in the slow dose rate samples, giving rise to the increased strain behavior.…”