Effects of transverse compression on the structure and axial tensile properties of polyethylene: A molecular simulation study

This paper investigates the effects of transverse pressure on the structure and axial tensile properties of polyethylene (PE) crystals and fibril (i.e., defective crystal with chain ends) using all-atom molecular dynamics (MD) simulations. The Hugoniot equation of state of the PE crystal is developed using both isotropic and anisotropic compression. To investigate the transverse pressure effects, both the crystal and fibrils models of length 0.02 to 0.2 micro-meter are subjected to axial tensile loading at different levels of transverse pressure from 0.0 to 20 GPa. The tensile stress-strain response and properties are predicted as functions of transverse pressure and fibril length. The MD predicted equation of state agrees well with theoretical Pastine's model and simulation captures orthorhombic to monoclinic phase transformation in anisotropic compression near 22 GPa hydrostatic pressure. Simulation results show that the transverse pressure significantly affects the tensile properties of PE especially for the fibril models by intensifying the inter-molecular interactions and changing the damage modes from chain end slippage to chain scission. In the presence of transverse pressure, the improvement in modulus is predicted to increase by 49% for crystal and 50%-73% for fibril, whereas the tensile strength is increased by 42% for crystal and 263%-600% for fibrils. These predictions provide insight into the response of PE subjected to high velocity impact where the PE structures experience multiaxial loadings at transverse pressure several orders of magnitude greater than the ambient pressure conditions.