Anomalous thermal behaviour of polyimide at very low temperatures

The thermal conductivity of an aromatic polyimide sheet (Upilex R) in the 0.1/1 K range was measured. A conductivity k = α · Tn, with α = (1.50±0.06) · 10-5 W/cmKn+1 and n 1.00±0.03, which is substantially different from what predicted by the tunneling model, was found. The linear dependence on temperature, which reveals the presence of crystalline units inside the sample, has been explained within the framework of the structure scattering theory, together with the standard tunneling model, and the size of the crystallites has been estimated. Our results are compared with the thermal conductivity of other semicrystalline materials