  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. The
evolution of the energy associated with bonded and non-bonded terms was
quantified as a function of strain as well as the evolution of stress in both
the loading and non-loading directions to give insight into how the stress
state is altered within the elastic, yield, strain softening, and strain
hardening regions. 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. 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.
