Radiation induced embrittlement of PTFE

The radiochemical degradation of polytetrafluoroethylene (PTFE) samples has been studied in air at dose rate 100 Gy/h for doses up to 5000 Gy, at ambient temperature. The polymer degradation has been monitored by DSC, tensile testing and Essential Work of Fracture (EWF) testing. Some fractured samples have been observed by scanning electron microscopy. The polymer undergoes a fast chain scission, its number average molar mass is divided by about 20 for a dose of 1000 Gy and tends towards a pseudo asymptotic value of ∼20 kg mol⁻¹ (against 6200 kg mol⁻¹ initial value). The modulus and yield characteristics seem to be almost unaffected whereas ultimate properties undergo strong variations. The ultimate elongation ε_R and the EWF plastic work characteristic βw_p first increase and then decrease. The ultimate stress decreases and tends towards a pseudo asymptotic value. The mechanisms of radiation induced ultimate property changes are discussed. The first stage could be due to the destruction of non-extended tie molecules (due to the presence of very long chains) responsible for interfibrillar bridging during fracture. The (more classical) second stage is a progressive embrittlement due to the destruction of the entanglement network. The critical molar mass M′_c for embrittlement is such as M′_c∼50M_e, M_e being the molar mass between entanglements in the melt. This relationship could be a general characteristic of high crystallinity non-polar polymers.