Low-temperature specific heat measurements on bulk technical polymer samples (polytetrafluoroethylene and polychlorotrifluoroethylene) were carried out between 2,5 and 20 K. In these experimental investigations a heat pulse method and an adiabatic vacuum calorimeter were used.The low-temperature specific heats of polytetrafluoroethylene (T4) and polychlorotrifluoroethylene (T3) have not been studied in sufficient detail [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The extrapolation of the experimental values at 20 K down to 4 K by means of the Debye-model is inadmissible [5]. Thus, investigations of the thermal properties giving the necessary physical quantities and constants for cryogenic design of these highly crystalline polymers are quite important.In the present work, calorimetric measurements on the bulk polymer samples were made in the range 2.5-20 K. The length and diameter of the samples did not surpass 50 mm and 12 ram, respectively. By using a shield in a vacuum calorimeter [16], entirely adiabatic conditions of operation were ensured. The cooling of the calorimeter, as well as that of the sample and the shield, was effected with a mechanical heat switch [17]. The heat capacity was derived from a well-known relation between the increase of the sample temperature (A T ~ 10-2 T) and the amount of heat supplied to the sample by a 650 f2 heater, consisting of 0.050 mm diameter constantan wire. This heater gives heat impulses lasting 40 seconds. The temperatures of the samples and the shield were measured by 300~ Allen-Bradley resistors. The calibration of these resistors was performed using a germanium (NIG) substandard. The time for establishment of the heat equilibrium was a few seconds at 2.5 K, and about 15 minutes at 20 K. ExperimentalThe temperature-dependence of the specific heat is presented in Fig. 1 for polychlorotrifluoroethylene, and in Fig. 2

The specific heat and superconducting critical temperature of VN0.74 and VN0.89 are measured i n the temperature range 1.8 < T < 20 K. The density of states a t the Fermi level is determined on the basis of these results. The dependence of the density of states on the parameter of the electronphonon interaction is discussed as well. It may be expected that increasing the N content of the mentioned compounds would lead to the intensification of the electron-phonon interaction and the increase of the density of states at the Fermi level and the critical temperature, respectively.