Low-temperature thermal properties of nylon and polyethylene

Abstract

The thermal conductivity and heat capacity of polyethylene and nylon samples have been measured in the temperature range 0.15–4°K. A good theoretical fit is obtained for the thermal conductivity above 1°K by using a Debye ω2 phonon frequency distribution with constant mean free path internal boundary scattering and amorphous structure scattering. Below 1°K the experimental results show a lower power dependence on temperature than would be predicted by the above theory. The theory can be modified to fit the low-temperature data by the addition of a term containing a frequency distribution proportional to ω, corresponding to two-dimensional vibrations due to the platelike nature of the individual crystallites. At temperatures below 1°K the heat-capacity data show a lower power dependence on temperature than that predicted by the Debye model. The proposed phonon frequency distribution also provides a satisfactory explanation for this behavior.