THERMAL-EXPANSION OF POLYMERS - MECHANISMS IN ORTHORHOMBIC POLYETHYLENE

Quasiharmonic lattice dynamics is used to examine the mechanisms under lying 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 exper iment. Tensions caused by vibrations with components away from the bon d directions are responsible for the negative expansion along the poly mer chains, and contribute significantly to the expansion perpendicula r to the chains; associated torques on the bonds have only a small eff ect, 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.