Thermal expansion and Grüneisen parameter of polyethylene between 5 and 320 K

The linear thermal expansion coefficient, α, of polyethylene with crystallinity X=0.44, 0.62, 0.77 and 0.98 has been measured between 5 and 320 K with an experimental accuracy of about 5% above 15 K and 10% below 15 K using an interferometric dilatometer system. In the temperature range 40 to 100 K the thermal expansion is found to be independent of crystallinity, whereas at lower as well as higher temperatures, α increases with increasing noncrystallinity. By linear extrapolation the thermal expansion coefficient of the noncrystalline phase can be estimated. The influence of the phonon spectrum on the thermal expansion is discussed in terms of the Grüneisen parameter. For completely crystalline polyethylene, a bulk-Grüneisen parameter of $\text{γ}{}_{\mathrm{<mtext>b</mtext>}}\text{?2}$ is found below 20 K. This value is smaller than expected from theories for polymer crystals. For noncrystalline polyethylene, γ_b is constant ($\text{γ}{}_{\mathrm{<mtext>b</mtext>}}\text{?1.6}$) between 30 and 100 K, showing a rapid increase at lower temperatures with $\text{γ}{}_{\mathrm{<mtext>b</mtext>}}\text{?12.5}$ at 5 K. This high value for γ_b can be related to excess modes in the vibration spectrum of the noncrystalline phase, which have previously been found to contribute also to the heat capacity at these temperatures.