THERMAL CONDUCTIVITY OF POLYETHERETHERKETONE AT LOW TEMPERATURES E. Gottardi 1 , G. Bianchini 2 , I. Peroni 3 , A. Peruzzi 4 and G. Ventura 1 1 University of Florence, Florence, Italy 2 CNR-IROE, Florence, Italy 3 CNR-IMGC, Turin, Italy 4 NMi van Swinden Laboratorium, Delft, The Netherlands ABSTRACT We measured the thermal conductivity k between 0.1 K and 6 K of polyetheretherketone, a semicrystalline polymer employed in cryogenics as a thermal insulator or support structure. Our measurements were carried out in a dilution refrigerator, using a steady heat flow method. By fitting the data below 1 K with a k = α T n law, we obtained α = (18.7 ± 0.4) µW/cmK (n+1) and n = 1.47 ± 0.02, which differs appreciably from the theoretical value n = 2 predicted by the tunneling model for polymers. Moreover, at T > 1 K, there is no evidence of the typical plateau expected for amorphous solids, at least up to the maximum temperature reached in our experiment. These anomalies can be ascribed to the presence of crystalline units inside the material, that give rise to fluctuations in the elastic properties of the solid. 1. INTRODUCTION Recent progress in the field of space research, superconducting magnets and cryogenic technology requires polymers that have excellent mechanical properties in severe environments. For this reason, many newly developed engineering plastics have been employed at low temperatures as electrical insulators, thermal insulators and support structures with a low specific weight and high mechanical stiffness. Despite the extensive use of advanced polymers in cryogenics, little information is available on important design parameters (such as low-temperature thermal and mechanical properties) that are essential for estimating the performance of any cryogenic apparatus. At present, a judgement on the utility of a specific polymer at low temperatures usually requires a test demonstration, because of the limited data available: there is still much to be done in order to obtain a systematic knowledge of the cryogenic properties of polymeric materials. In this work we present measurements of the thermal conductivity of polyetheretherketone (PEEK), a semicrystalline polymer which exhibits superior mechanical properties (low coefficient of linear thermal expansion [1], good creep resistance, high tensile strength, etc.) and an excellent chemical resistance. These characteristics enable PEEK to compete with metals and other engineering plastics in many automotive, aerospace and electronic applications. PEEK is also employed as matrix in fibre composites for low temperature use [2]. Our measurements were carried out on a cylindrical sample of PEEK (supplied by Goodfellow Cambridge Limited, Cambridge Science Park, Cambridge, England CB4 4DJ), in the 0.1-6 K temperature range.