In-situ SAXS study and modeling of the cavitation/crystal-shear competition in semi-crystalline polymers: Influence of temperature and microstructure in polyethylene

This study focuses on the first occurrence of either cavitation or crystal shear in relation to temperature and microstructure during the tensile drawing of polyethylene. Four high density polyethylenes covering a range of crystallinity have been thermally treated to generate different microstructures displaying a large range of crystal thickness from 8 to 29 nm. The testing temperature spanned the domain 25–100 °C. In-situ SAXS measurements on synchrotron have been performed to capture the initiation of cavitation in parallel with stress-strain measurements. Depending on microstructure and temperature the strain onset of cavitation proved to be either before or after yielding associated with homogeneous or localized cavitation regimes respectively. The transition between the two regimes can be defined by a critical value of lamella thickness at each temperature. A physical modeling based on a thermally activated nucleation process has been developed for predicting the macroscopic stress for generation of cavities as well as the one for initiating crystal shearing. This modeling accounts for both temperature and microstructure effects on yielding. It allows describing successfully the delayed apparition of cavitation with increasing temperature and decreasing crystal thickness. The observation of complete disappearance of cavitation at high temperature is also predicted by the model in relation to crystal thickness. The more relevant aspects as well as the shortcomings of the model are discussed in the conclusion.