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The second paper in a set of reviews of radiation effects on thermal properties of polymers is devoted to polytetrafluoroethylene (PTFE). The specificity of the radiation physics and chemistry of this material is relatively well investigated and it is widely used in different applications under radiation impact. Degradation of thermal and structure properties is discussed, and models for the description of these effects are presented. Specific problems such as the action of different kinds of radiation, dose rate effects, especially under vacuum, and deviation of PTFE structure from the two-phase model are also examined.

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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 On the basis of available experimental data, including unpublished results obtained in our laboratory, we show that there are remarkable dose rate effects in physical-chemical and operational (e.g., mechanical, electrical,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