Thermomechanical properties dependence on chain length in bulk polyethylene: Coarse-grained molecular dynamics simulatio
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Mechanical and thermodynamical properties of bulk polyethylene have been scrutinized using coarse-grained (CG) molecular dynamics simulations. Entangled but cross-link-free polymer clusters are generated by the semicrystalline lattice method for a wide range chain length of alkane modeled by CG beads, and tested under compressive and tensile stress with various temperature and strain rates. It has been found that the specific volume and volumetric thermal expansion coefficient decrease with the increase of chain length, where the specific volume is a linear function of the bond number to all bead number ratios, while the thermal expansion coefficient is a linear rational function of the ratio. Glass-transition temperature, however, does not seem to be sensitive to chain length. Yield stress under tension and compression increases with the increase of the bond number to all bead number ratio and strain rate as well as with decreasing temperature. The correlation found between chain length and these physical parameters suggests that the ratio dominates the mechanical properties of the present CG-modeled linear polymer material. I. INTRODUCTION
Glassy polyethylene (PE) is one of the most fundamental polymer materials that have been widely applied to various fields in modern industry. The outstanding combination of physical and mechanical properties of the material attracts many materials engineers, chemists, and physicists.1–3 Great efforts have been made for decades to clarify these properties in both theoretical and experimental studies. Hanscomb and Kaahwa4,5 reported the high electrical conductivity of PE terephthalate at elevated temperature. Donald et al.6,7 investigated strain hysteresis under tensile stress, and found that cracklike defects (crazes) appear within the apparent elastic strain; crazes bridged by fibrils are initiated with increasing load, and macroscopic fracture occurs immediately after the failure of these fibrils.6,7 G’Sell et al.,8 Arruda and Boyce,9 and Boyce et al.10 independently provided video-recorded tension and compression tests for various amorphous polymers, showing that typical stress–strain response exhibits normal viscoelasticity under the initial loading process, followed by strain softening after yielding, and then followed by strain hardening. Recently, Melick et al.11 revealed that cross-linking network density in polymer materials plays an important role in the mechanisms of the strain hardening. Our previous work12 discovered that the constitutive properties of spherical polymer microsized a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0061 J. Mater. Res., Vol. 25, No. 3, Mar 2010
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particles vary according to their size, confirmed by the high-resolution depth-sensing nanoindentation technique equipped with a flat-punch diamond indenter.12 Phenomenological explanations by continuum mechanics are not sufficient for the above experimental findings since the mechanisms o
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