Dependence of mechanical properties on crystal orientation of semi-crystalline polyethylene structures

Abstract

Molecular dynamics simulations based on a united atom method are employed to study the molecular response of semi-crystalline polyethylene structures with unidirectional and bidirectional molecular orientation. The initial structures are generated from isothermal crystallization at 375 K of pre-oriented melts. Deformation simulations are conducted at a constant strain rate of 1.24 nm/ns at both constant volume (NVT) and constant lateral stress (NLzσxσyT) conditions. In order to connect characterization results at various length scales two-point statistics are introduced to capture the morphological changes in semi-crystalline structures during the deformation, based on the density and orientational differences of the amorphous and crystalline regions. In this new approach, molecular conformational changes are well captured in the two-point auto- and cross-correlation spectra, to understand the morphological evolution of different domains consisting of macromolecular chains on a statistical level. The pseudo-laminar semi-crystalline structures are preserved for both structures up to a strain level of 0.5, with the rotation and orientation of chains in the crystalline region facing towards the uniaxial deformation direction. A laminar to fibrillar transition is observed for the bidirectional oriented semi-crystalline structure at higher strain levels. Recovery tests confirm the stability of the laminar and fibrillar structures at certain stages of the deformation.