Crystallite Rotation Drives Strain Softening in Semicrystalline Polyethylene

This work examines the response of semicrystalline polyethylene under triaxial strain. The Shinoda-DeVane-Klein (SDK) model of polyethylene (PE) was used in coarse-grained (CG) molecular dynamics (MD) simulations of more than 2 × 106 particles, representing more than 2 × 107 atoms and a volume of greater than 105 nm3. For strains in the range of ϵ = 0.25–0.5, the stress was related to the overall orientation of the polymer backbone with respect to the extension axis. However, at intermediate strains (ϵ = 0.05–0.3), where strain softening typically occurs, there were large variations in mechanical response as a result of the initial distribution of crystallite orientations. At intermediate strains, both the compressive and tensile stress also depended strongly on the initial distribution of crystallite orientations. The orientation distribution also determined the onset of the strain-hardening regime. We found that, due to the inextensible nature of the crystallites, compressive stresses drove rotation for crystallites oriented away from the extension axis, which in turn strongly decreased the tensile stress. Results from MD simulations suggested that, while spatial averaging smooths the overall macroscopic response of semicrystalline samples, intermediate scale or local stress imbalances due to crystallite rotation have a key role in driving strong local strain and potentially leading to plastic deformation like banding or cavitation.