Developing Mode I cohesive traction laws for crystalline Ultra-high molecular weight polyethylene interphases using molecular dynamics simulations

Ultra-high molecular weight polyethylene fiber with a diameter of 17 µm contains over 100,000 fibrils with diameters ranging from 10 to 100 nm. These fibrils can exhibit various relative rotations around the axial direction, forming interphases between distinct crystal planes. Fiber failure can occur due to defibrillation governed by the adhesion between fibrils. In this study, adhesion is quantified through cohesive traction laws that describe the strength, progressive damage, and energy absorption during fibril separation. We predict Mode I cohesive traction laws for polyethylene (PE) interphases between crystals with various orientations using molecular dynamics (MD) simulations. Results were compared with the stress-displacement response of perfect bulk crystals of similar thickness. Surface effects primarily manifested in the outermost layer of PE chains where molecular structure deviates from the bulk crystal structure resulting in a higher surface energy. This resulted in an interphase thickness equivalent to the thickness of two PE chain layers (1.2 nm). The disturbed crystal structure at the interfaces led to a 32% reduction in peak traction and a 46% reduction in energy absorption compared to the perfect bulk crystal. Additionally, results show that strain rate does not have an influence of the traction laws over the range of 10⁸ s⁻¹ to 10¹⁰ s⁻¹. The MD-based traction-separation relations were used to fit parameters for a cohesive zone model. The interphase traction laws predicted in this study can be used as interface properties to bridge length scales in multiscale simulations of defibrillation.