Influence of ensemble boundary conditions (thermostat and barostat) on the deformation of amorphous polyethylene by molecular dynamics

Abstract

Molecular dynamics simulations are increasingly being used to investigate the structural evolution of polymers during mechanical deformation, but relatively few studies focus on the influence of boundary conditions on this evolution, in particular the dissipation of both heat and pressure through the periodic boundaries during deformation. The research herein explores how the tensile deformation of amorphous polyethylene, modelled with a united atom method potential, is influenced by heat and pressure dissipation. The stress-strain curves for the pressure dissipation cases (uniaxial tension) are in qualitative agreement with experiments and show that heat dissipation has a large effect on the strain hardening modulus calculated by molecular dynamics simulations. Moreover, in addition to quantifying the evolution of the energy associated with bonded and non-bonded terms as a function of strain, the evolution of stress associated with these different components in both the loading and non-loading directions was also calculated as a function of strain to give insight into how the stress state is altered within the elastic, yield, strain softening, and strain hardening regions. The energy partitioning shows that the majority of energy increase during deformation is associated with the non-bonded Van der Waal's interactions, similar to previous studies. The stress partitioning shows a competition between 'tensile' Van der Waal's interactions and 'compressive' bond stretching forces, with the characteristic yield stress peak clearly associated with the non-bonded stress. Subsequent analyses concentrates on the evolution of several internal structure metrics with strain: bond length, bond angle, dihedral conformation, chain orientation, and chain entanglement. The lack of heat dissipation had the largest effect on the strain hardening regime, where an increase in the calculated temperature correlated with faster chain alignment in the loading direction and more rapid conformation changes. In part, these observations demonstrate the role that heat and pressure dissipation play on deformation characteristics of amorphous polymers, particularly for the strain hardening regime.

Stress–Strain Relationships in Hydroxyl Substituted Polyethylene

“…All simulations were performed in the isothermal−isobaric (NPT) ensemble with orthogonal periodic boundary conditions. 34,36 Simulations were carried out using the GPU accelerated LAMMPS package. 37−39 Equations of motion were integrated using the velocity-Verlet algorithm with a time step of 1 fs.…”