Plastic Deformation of Semicrystalline Polyethylene under Extension, Compression, and Shear Using Molecular Dynamics Simulation

Abstract

Plastic deformation of the stack of alternating crystal and amorphous layers typical of semicrystalline polyethylene is studied by molecular dynamics simulation. A previous investigation of the semicrystalline layered stack undergoing isochoric extension1 is extended here to include several new modes of deformation: isostress extension, isostress compression, and isochoric shear, at 350 K and deformation rates of 5 × 107 and 5 × 106 s–1. The observed stress–strain responses are interpreted in terms of the underlying structural evolution of the material for each mode of deformation. Under tensile deformation, crystallographic slip was observed at low strains (0 < e3 < 0.08) regardless of deformation rate. Different yield mechanisms were observed for the different deformation rates. To explain the response at intermediate strains (0.08 < e3 < 0.26), we introduce the concept of “bridging entanglements”, which are temporary, physical bridges between crystal lamellae comprising entanglements involving chain segments belonging to different crystal lamellae. At high strains (e3 > 0.26), melting and recrystallization were observed at the slower deformation rate, while surface melting and cavitation were observed at the faster deformation rate. Under compressive deformation at the slower deformation rate, crystallographic slip was again observed at low strains. For the faster compressive deformation, an initial period of rapid stress growth at low strain was observed. This initial stress growth then transitions to a process of fine crystallographic slip at a strain of e3 = −0.005. At intermediate strains under compressive deformation, the release of bridging entanglements is observed for both strain rates. However, no melting or recrystallization phenomena were observed under compression, even at the highest strains simulated (e3 = −0.33). Under shear deformation, interlamellar slip was observed for both zx and zy shear (strain gradient parallel to stacking direction). Chain segments tend to stretch and align in the shear direction. Interestingly, under shear deformation this semicrystalline polyethylene exhibits transient behavior typical of non-Newtonian fluids.