THERMAL EXPANSION OF IDEAL POLYMER CRYSTALS: APPLICATION TO POLYETHYLENE

There are three central force mechanisms underlying thermal expansion: bond stretching due to the asymmetry of pair potentials and two others due to tensions caused by vibrations with components away from the bond direction. Quasi-harmonic methods previously used for skeletal chain models are here applied to a central force model for orthorhombic polyethylene, giving qualitative agreement with experiment. Tension effects neglected in some earlier studies make appreciable negative contributions to alpha(a) and alpha(b); conversely, bond-stretching effects make appreciable positive contributions to the negative alpha(c). The temperature dependence of the setting angle of the skeletal planes has little effect on the macroscopic expansion.