Thermal Conductivity and Specific Heat of Some Polymers between 4.5° and 1°K

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Abstract
Measurements of the thermal conductivity, using a steady‐state technique, and of the specific heat, using a transient technique, have been performed between 4.5° and 1°K on commercial samples of polytetrafluoroethylene (Teflon), polychlorotrifluoroethylene (Kel‐F), nylon, and three samples of polyethylene of varying densities. In all cases the specific heat is proportional to T3. For the polyethylene samples, the constant of proportionality was found to depend strongly on the density of the sample, indicating that amorphous regions have a higher specific heat in this temperature range than do crystalline regions. Extrapolation to completely crystalline material gives the heat capacity as C = 100 T3 erg/°K·cm3, while extrapolation to completely amorphous material gives the heat capacity as C = 297 T3 erg/°K·cm3. The thermal conductivity varies more rapidly than linearly with temperature except in the case of polychlorotrifluoroethylene. Analysis of these thermal‐conductivity measurements and some similar results which have been published previously on the basis of a combination of structure scattering and internal boundary scattering yield sizes of the internal boundaries which show good correlation with the average size of crystalline regions determined optically for the same samples.

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