Thermal expansion of polymers: Mechanisms in orthorhombic polyethylene

Quasiharmonic lattice dynamics is used to examine the mechanisms underlying the anisotropic thermal expansion of orthorhombic polyethylene, with particular attention to low temperature behavior. Several sets of interatomic potentials all give good qualitative agreement with experiment. Tensions caused by vibrations with components away from the bond directions are responsible for the negative expansion along the polymer chains, and contribute significantly to the expansion perpendicular to the chains; associated torques on the bonds have only a small effect, except at very low temperatures. The anisotropy of the expansion perpendicular to the chains results from a subtle interplay of thermal stress and elasticity. The results suggest that this anisotropy will be greatly reduced or even reversed at low temperatures.