(576j) Deformation of Linear and Short Chain Branched Semicrystalline Polyethylene

High Density Polyethylene (HDPE) and Linear Low Density Polyethylene (LLDPE) are the two polyethylenes (PE) produced in largest volume with global production exceeding 80 million metric tons. Their solid-state performance and mechanical properties are fundamentally controlled by their semicrystalline structures, specifically, their crystalline morphology and noncrystalline topology. Herein, we examine the deformation behavior of HDPE and LLDPE using atomistic Monte Carlo and molecular dynamics simulations. Detailed atomistic semicrystalline structures of unbranched (HDPE) and Short Chain Branched (SCB) PE (containing methyl, ethyl or butyl branches, LLDPE) were generated via a thermodynamic equilibrium distribution of topological features of tails, loops and bridges using Monte Carlo simulations. These lamellar stack structures were subsequently deformed under uniaxial tension, compression, and shear modes by molecular dynamics to establish the relationships between noncrystalline topology and mechanical response. Topological features that are responsible for cavitation and melting-recrystallization in tensile deformation, are found to be less important in compressive and shear deformations. Qualitatively similar effects of noncrystalline topology on deformation behavior are observed in HDPE and LLDPE. It is found that entanglements between topological features, both in the initial configurations and as a result of deformation, are key factors controlling deformation in HDPE and LLDPE.