Anomalous thermal behaviour of polyimide at very low temperatures

The thermal conductivity of an aromatic polyimide sheet (Upilex R) in the 0 .1 divided by 1K range was measured. A conductivity k = alpha.T-n, with alpha = (1.50 +/- 0.06).10(-5) W/cmK(n+1 ) and n = 1.00 +/- 0.03, which is substantially different from what predict ed by the tunneling model, was found. The linear dependence on temperature, which reveals the presence of crystal line units inside the sample, has been explained within the framework of th e 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 semicrystal line materials.