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Molecular dynamics simulations are increasingly being used to investigate thestructural evolution of polymers during mechanical deformation, but relativelyfew studies focus on the influence of boundary conditions on this evolution, inparticular the dissipation of both heat and pressure through the periodicboundaries during deformation. The research herein explores how the tensiledeformation of amorphous polyethylene, modelled with a united atom methodpotential, is influenced by heat and pressure dissipation. The stress-straincurves for the pressure dissipation cases (uniaxial tension) are in qualitativeagreement with experiments and show that heat dissipation has a large effect onthe strain hardening modulus calculated by molecular dynamics simulations. Theevolution of the energy associated with bonded and non-bonded terms wasquantified as a function of strain as well as the evolution of stress in boththe loading and non-loading directions to give insight into how the stressstate is altered within the elastic, yield, strain softening, and strainhardening 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 thenon-bonded stress. The lack of heat dissipation had the largest effect on thestrain hardening regime, where an increase in the calculated temperaturecorrelated with faster chain alignment in the loading direction and more rapidconformation changes. In part, these observations demonstrate the role thatheat and pressure dissipation play on deformation characteristics of amorphouspolymers, particularly for the strain hardening regime.

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