Thermal Conductivities of Polyethylene and Nylon from 1.2° to 20°K

Abstract

The thermal conductivities of four samples of polyethylene whose densities lie between 0.914 g/cc and 0.971 g/cc have been determined from 1.2° to 20°K. An empirical fit to the data between 1.2° and 4.0°K is effected for longitudinal phonons in terms of ``structure scattering'' and a frequency-independent scattering mechanism. However, the average spherulite sizes as observed with the optical microscope do not agree with the size parameters extracted from the frequency-independent scattering term which gives a best fit to the data. An excellent empirical fit to the data has been obtained up to 20°K by assuming that transverse phonons make an additional contribution to the thermal conductivity. The mechanisms which scatter these waves are chosen to be the same as those scattering the longitudinal phonons at the lower temperatures. The temperature-independent size parameter extracted from this portion of the fit to the data increases with increasing lamella thickness determined from small-angle x-ray scattering measurements, and is in order-of-magnitude agreement with this periodicity. The thermal conductivity of a sample of nylon has been measured and found to be in agreement with previously published data. An analysis similar to that effected for polyethylene failed to reveal consistency between spherulite or lamella dimensions with size parameters extracted from the data.