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ORaCEL Seminar – Jixiong He
October 15, 2018 | 11:00 am - 12:00 pm
Title: Strain Effects on the Anisotropic Thermal Transport in Crystalline Polyethylene
Abstract: Thermal transport in the axial direction of polymers has been extensively studied while the strain effect on thermal conductivity, especially in the radial direction, remains unknown. In this work, we calculated the thermal conductivity in the radial direction of a crystalline polyethylene model and simulated the uniaxial strain effect on the thermal conductivity tensor by molecular dynamics simulations. We found a strong size effect of the thermal transport in the radial direction and estimated that the phonon means free path can be much larger than the prediction from the classic kinetic theory. We also found that the thermal conductivity in the axial direction increases dramatically with strain while the thermal conductivity in the radial direction decreases with uniaxial strain. We attribute the reduction of thermal conductivity in the radial direction to the decreases of inter-chain van der Waals forces with strains. The chains in the crystalline polyethylene became stiffer and more ordered along the chain direction could be the reasons for the increasing thermal conductivity in the axial direction during stretching. Besides, we observed longer phonon lifetime in acoustic branches and higher group velocity in optical branches after uniaxial stretching. Our work provides a fundamental understanding of the phonon transport in crystalline polymers, a structure-property relationship in crystalline polymers, and the strain effect in highly anisotropic materials.