Thermal expansion and Grüneisen parameters of isotropic and oriented polyethylene

The linear thermal expansion coefficients α of three isotropic samples (crystallinity v = 0.42‐0.8) and one oriented sample (v = 0.8, draw ratio 11) of polyethylene have been measured between 2 and 100 K. Together with our previous measurements at high temperatures, these data provide general patterns for the crystallinity and orientation dependence over a wide temperature range. α varies only slightly with crystallinity from 50 to 120 K, but at higher and lower temperatures, it increases significantly with decreasing crystallinity. The increase at high temperature arises from segmental motion in the amorphous regions, while the large increase near 5 K is similar to that observed in many glasses. The tranverse expansion coefficient of the oriented sample is 40‐100% higher than the values for isotropic polyethylene, but the axial expansion coefficient is much smaller and is negative over the entire temperature range. The values of these coefficients are quite close to those for the polyethylene crystal, a feature readily understood in terms of existing models. However, as a result of the much larger elastic anisotropy of the crystal, the Grüneisen parameters (especially the axial Grüneisen parameters) of these two types of polyethylene have rather different values.