The Thermal Conductivity of Polymers Below 1K

Abstract

It has been well-established for a number of years that the thermal conductivity, K, of all amorphous materials including amorphous polymers exhibits "universal" behaviour below lK, falling as T2 with magnitudes that are very similar between one material and the next. For semicrystalline polymers, on the other hand, the behaviour is quite different. At lK the values of K are an order of magnitude lower than in amorphous materials, with a temperature dependence in which K is proportional to T. In this report we summarise recent measurements of K below lK, with emphasis on the effects of (i) changes in the degree of crystallinity and (ii) extrusion. Very generally K tends to the universal T2 dependence at very low temperatures (below lOOrnK), and when either the crystallinity is small or the extrusion ratio is large. We shall also report on measurements on a single crystal polymer, polydiacetylene, in which, as expected for a crystalline solid, the temperature dependence of K approaches T3.