Specific heat of polytetrafluoroethylene and polychlorotrifluoroethylene within the temperature range 2.5–20 K

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.

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Zusammenfassung Messungen der spezifischen Wärme bei niedrigen Temperaturen wurden an technischen Polymerproben alla rinfusa (Polytetrafluoräthylen und Polychlortrifluoräthylen) zwischen 2.5 und 20 K durchgeführt. Bei diesen experimentellen Untersuchungen wurde eine Wärmepulsierungsmethode und ein adiabatisches Vakuumcalorimeter eingesetzt.

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Résumé On a effectué des mesures de chaleur spécifique à basse température, entre 2.5 et 20 K, sur des échantillons de polymères techniques en vrac (polytétrafluoroéthylène et polychlorotrifluoroé thylène), à l'aide d'une méthode par impulsions de chaleur et d'un calorimètre adiabatique sous vide.

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( ) 2.5 20 . .

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We are indebted to Drs. E. Leyarovski and L. Leyarovska from the Institute of Solid-State Physics, Sofia, Bulgaria for suggesting the experimental method of measurements.

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This work was performed in the international Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw, Poland.     

X-ray and NMR measurements on irradiated polytetrafluoroethylene and polychlorotrifluoroethylene

The effects of ⁶⁰Co γ-radiation on polytetrafluoroethylene (PTFE) and polychlorotri-fluoroethylene (PCTFE) have been studied for radiation doses up to 940 Mrad. The dependence of per cent crystallinity upon irradiation level has been determined from x-ray analysis. An initial increase in crystallinity in PTFE, attributable to chain scission in the amorphous phase of the material, was found, followed (above 300 Mrad) by a gradual decrease associated with a disordering of the crystallites. No initial increase was observed for annealed samples of PCTFE due to the large initial value of the per cent crystallinity. Above 200 Mrad the crystallinity was found to decrease with accumulated dose. Nuclear magnetic resonance measurements on PTFE have indicated a radiation-induced broadening of the amorphous component of the NMR line appearing to maximize above 700 Mrad. Similar measurements of PCTFE have shown a narrowing of the crystalline component of the NMR line and subsequent appearance of the amorphous component at approximately 200 Mrad. The data indicate that the radiation-induced behavior of PTFE and PCTFE is similar above 200 Mrad.     

Thermal conductivity of polytetrafluoroethylene and polytrifluorochloroethylene

Summary Measurements of thermal conductivity were made on polytetrafluoroethylene and polytrifluorochloroethylene which were crystallized by various heat treatments. The variation of thermal conductivity with the degree of crystallinity can be explained qualitatively in terms of the difference of the mean free path. An approximate calculation of the mean free path for the amorphous polytrifluorochloroethylene was made. The calculation gives the value of the mean free path which is very close to the atomic distances in the polymer molecule.

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Zusammenfassung Die thermische Leitf?higkeit von Polytetrafluor?thylen und Polytrifluorchlor?thylen wurde an Proben gemessen, die durch verschiedene W?rmebehandlungen kristallisiert wurden. Die ?nderung der thermischen Leitf?higkeit mit dem Kristallisationsgrad kann qualitativ durch die Unterschiede der mittleren freien Wegl?nge erkl?rt werden. Eine absch?tzende Berechnung der mittleren freien Wegl?nge des Polytrifluorchlor?thylens wurde durchgeführt. Die berechneten Werte liegen nahe an den Atomabst?nden der Polymermoleküle.

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With 4 figures and 1 table

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This article is Part VI of the series “Studies on Thermal Conductivity of High Polymers”. Part I–V of this series were presented in High Polymer Chem., Japan (Kobunshi Kagaku).     

Thermal expansion of irradiated polytetrafluoroethylene

The thermal expansion coefficient of gamma-irradiated Polytetrafluoroethylene (PTFE) has been measured in the temperature range 80–340 K by using a three-terminal capacitance technique. The samples are irradiated in air at room temperature with gamma rays from a Co⁶⁰ source at a dose rate of 0.26 Mrad/h. The change in crystallinity is measured by an x-ray technique. The expansion coefficient is found to increase with radiation dose below 140 K owing to the predominant effect of degradation. Above 140 K, the expansion coefficient is found to decrease with radiation dose because of the enhanced crystallinity. The temperatures of the two first-order phase transitions are also found to shift to lower temperatures owing to degradation of PTFE.     

Thermal properties at room temperature of polytetrafluoroethylene

Through Differential Scanning Calorimetry (DSC), at least three room temperature transitions are clearly observable for native polytetrafluoroethylene (PTFE). The influence of the thermal history on the room temperature transitions has been investigated. Possible interpretations for the lowest room temperature transition are suggested.

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Zusammenfassung Bei der Untersuchung von nativen Polytetrafluoroethylen (PTFE) lassen sich im Raumtemperaturbereich zumindest drei Umwandlungen eindeutig beobachten. Der Einfluss der termischen Vorgeschichte von PTFE Proben auf die Umwandlungen im Raumtemperaturbereich wurde untersucht.Mögliche ErklÄrungen für den tiefstliegenden übergang im genannten Temperaturbereich werden vorgeschlagen.

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Dr. G. Ajroldi and Prof. G. Guerra are gratefully acknowledged for useful advice and stimulating discussions.     

Thermal conductivities of molecular liquids by reverse nonequilibrium molecular dynamics

The reverse nonequilibrium molecular dynamics method for thermal conductivities is adapted to the investigation of molecular fluids. The method generates a heat flux through the system by suitably exchanging velocities of particles located in different regions. From the resulting temperature gradient, the thermal conductivity is then calculated. Different variants of the algorithm and their combinations with other system parameters are tested: exchange of atomic velocities versus exchange of molecular center-of-mass velocities, different exchange frequencies, molecular models with bond constraints versus models with flexible bonds, united-atom versus all-atom models, and presence versus absence of a thermostat. To help establish the range of applicability, the algorithm is tested on different models of benzene, cyclohexane, water, and n-hexane. We find that the algorithm is robust and that the calculated thermal conductivities are insensitive to variations in its control parameters. The force field, in contrast, has a major influence on the value of the thermal conductivity. While calculated and experimental thermal conductivities fall into the same order of magnitude, in most cases the calculated values are systematically larger. United-atom force fields seem to do better than all-atom force fields, possibly because they remove high-frequency degrees of freedom from the simulation, which, in nature, are quantum-mechanical oscillators in their ground state and do not contribute to heat conduction.     

Density of He adsorbed in micropores at 4.2K

The density of He adsorbed in the cylindrical micropores of zeolites NaY and KL has been determined by He adsorption at 4.2K. He adsorption isotherms were then compared with N₂ adsorption isotherms at 77K. Crystallographic considerations of the micropore volumes gave the density of the He adsorbed layer, which is necessary for assessment of ultramicroporosity of less-crystalline microporous solids, such as activated carbons. The determined density of He adsorbed in the cylindrical micropores of the zeolite was in the range 0.22 to 0.26 gml^–1, greater than that of He adsorbed on a flat surface (0.202 gml^–1). A value for the density of He between 0.20 to 0.22 gml^–1 is recommended for evaluation of ultramicroporosity of a slit-shaped microporous system such as activated carbon.     

Heat capacity of poly(vinylidene fluoride) and polytetrafluoroethylene between 5 and 200°K

Heat capacities of poly(vinylidene fluoride) (PVF₂) and polytetrafluoroethylene (PTFE) have been measured between 5 and 100°K with an accuracy of (1–5)% by adiabatic calorimetry. Calculations based on contributions from known optical lines and the Tarasov continuum model are in good agreement with experimental results down to 30°K for PVF₂ and 10°K for PTFE, and yield characteristic temperatures θ₁ and θ₃ which are consistent with previous values determined from high-temperature (100—350°K) data. At lower temperature the measured heat capacity is significantly higher [(30–100)%] than the model prediction, and can be satisfactorily accounted for by the introduction of localized vibrators at a concentration of about 1% as compared to acoustical oscillators and at a characteristic temperature of about 20°K. Using established data on polyethylene for comparison, the principle of additivity for heat capacities is found to be valid down to at least 20°K, convering the region (<60°K) where interchain vibrations contribute significantly to the heat capacity. Possible reasons for this unexpected behavior are discussed.     

Nature and ESR anisotropy of trapped radicals in UV-irradiated isotactic polypropylene at 4.2 K and 77 K

A comparative ESR study of low-temperature photolysis at 77 and 4.2 K of isotactic polypropylene, preoxidized under controlled conditions, and of 2.6.8 trimethylnonane-4-one as a model compound of the inchain PP ketones ? CH₂COCH₂? is presented. The results show conclusively that the ESR spectra of low-temperature PP photolysis reported in the literature arise from the decomposition of tertiary hydroperoxides, whereas the contribution by the Norrish I photolytic cleavage of ketones is of minor importance; the relative efficiency of radical production at 77 K (λ ≥ 300 nm) by both types of photoactive compound has been measured by ESR and found to be about 15 times greater for the hydroperoxides. The mechanism of solid-state photolysis of PP hydroperoxides is dominated by the nearly complete decay of excited alkoxyls during the photolytic events that yield the chain end radicals ? CH(CH₃)CH₂· by β scission (87.8%) and the inchain radicals ? CH₂C(CH₃)CH₂? by hydrogen abstraction (10.4%). The low yield of methyl radicals detected (1.8%) and their delayed appearance strongly suggest that these species are of secondary origin and that the decomposition of alkoxyls via C? CH₃ scission is of negligible importance. From the ESR anisotropy of the deliberately prepared peroxyradicals ? CH(CH₃)CH₂O₂· and ? CH₂C(CH₃)(O₂)CH₂? evidence of the misalignment of terminal chain segments following scission of alkoxyls at 77 K was obtained. An unexpected result of the measurements at 4.2 K is represented by the identification of a clear signal by H atoms: the H atoms apparently are not trapped in perprotiated matrices, even at 4.2 K, because of their unusually high reactivity toward the hydrogen abstraction assisted by tunneling effects.     

The heat capacity of the heusler alloy Pd2Dy0.2Y0.8Sn between 0.25 and 4.2 K

The heat capacity of the Heusler alloy Pd₂Dy_0.2Y_0.8Sn has been measured between 0.25 and 4.2 K. The magnitude is very large and can be accounted for over most of the temperature range by the sum of lattice and Schottky type contributions. Evidence is found for two transitions at about T = 0.4 and T = 1.1 K. The upper one is reasonably consistent with the superconducting temperature reported for an alloy of the same nominal composition.     

Fluorescence spectrum of the benzyl radical in methylcyclohexane at 4.2 K

The fluorescence spectrum of the benzyl radical (BR) is characterized by participation of many totally symmetric vibrational modes with less intensity activeness. To make this point more clear, the fluorescence spectra of the benzyl radical and its deuterated compounds in methylcyclohexane (MCH) have been observed at 4.2 K and detailed vibrational analysis has been carried out. Two non-totally symmetric modes such as nu6b and nu8b play a role of Herzberg-Teller type vibronic coupling together with one totally symmetric mode nu6a. There are only a few progression bands with very weak intensity. These are nu12, nu1 and nu(phi-CH2) vibrational modes.     

Dielectric loss of high-density polyethylene below 4.2°K

Drawing of oxidized high-density polyethylene (HDPE) and subsequent annealing greatly reduce the low-temperature dielectric loss when the electric field is applied perpendicular to the draw direction. This supports our model (J. Polym. Sci. Polym. Phys. Ed., 15 , 43 (1977)) of the proton moving parallel to the c axis of the PE crystal. A particular antioxidant (Ionox 330) in unoxidized HDPE induces a dielectric loss with a frequency and temperature dependence which differs from that for the loss in oxidized HDPE. The antioxidant loss seems to be an overlapping of tunneling and a thermal activation process. The possibility that the hydroperoxide group in the PE crystal is the origin of the loss in oxidized HDPE was theoretically examined in a manner similar to that used for the hydroxyl group in the previous paper. Results suggest that the hydroperoxide group is less probable than hydroxyl as the origin of the low-frequency loss in HDPE.     

Wide angle X-ray diffraction studies of polytetrafluoroethylene and ethylen-tetrafluoroethylen-bipolymers in the range 9.5 K–270 K

PTFE shows in the whole temperature range investigated a triclinic crystal structure with cell parametersa=0.952 nm,b=0.559 nm,c=1.706 nm,=87.9°, = 90.0° and=91.8°. The temperature dependence ofa andb-axis is linear, thec-axis shows a change in thermal expansion at 150 K. The inherent strain is small. The crystallite sizes are measured, paracrystalline distortions are absent.The bipolymers show only an ordered arrangement in lateral direction under maintenance of the helical structure. The lateral arrangement is very distorted and the range of order is aboutD _L 10 nm.     

The two-photon excitation spectrum of a dibenzofuran single crystal at 4.2 K

The polarised two-photon excitation spectrum of crystalline dibenzofuran at 4.2 K has been measured. Only totally-symmetric vibrations are active, and their relative intensities are similar to those in the one-photon spectrum already reported. No factor-group splittings are found. The origin in the two-photon spectrum has a width of ? 10 cm^-1 in contrast to a width of ? 250 cm^-1 in the one-photon spectrum, now attributed to extreme inhomogeneous broadening.     

The two-photon excitation spectrum of a carbazole single crystal at 4.2 K

The low-energy system in the polarized two-photon excitation spectrum of carbazole at 4.2 K has been measured by detecting fluorescence pulses from deliberately added anthracene. Herzberg—Teller activity was detected for Some A₁ vibrations. The two-photon operators contribute to the two-photon cross section in the order yy ≈ zz ? xx.     

Kinetics of the 20°C phase transformation in polytetrafluoroethylene

The rate at which PTFE transforms from the triclinic to the hexagonal phase has been measured under isothermal conditions. Samples in a series of decreasing density and crystallinity show increased isothermal rates of transformation. Observed kinetic data are interpreted on the basis of a modified Avrami-Johnson-Mehl treatment. A model for the transformation in which planes of helix-hand reversal propagate through the lattice is shown to fit the experimental results. The transformation rate is observed to be proportional to the total (001) surface area in the polytetrafluoroethylene specimens, suggesting that nucleation of the transformation takes place at grain boundaries.     

Fluorescence of solid tetracene in the temperature range 4.2 to 300 K

The fluorescence spectrum of polycrystalline tetracene has been measured in the temperature range 4.2 to 300 K. The 0,0 transition occurs at 18700 ± 20 cm^?1 independent of temperature and crystal structure. The unusually large Stokes shift between absorption and fluorescence is traced back to structural relaxation.     

Helium induced pressure broadening and shifting of HCN hyperfine transitions between 1.3 and 20 K

We have measured the helium induced pressure broadening and shifting of the distinct hyperfine components of the j = 1 <-- 0 and j = 2 <-- 1 transitions of HC14N at temperatures between 1.3 and 20 K. The HCN molecules were cooled to these temperatures using the collisional cooling technique. As a test of this cooling technique we measured the Doppler contribution to the spectral lines, and these measurements confirm that the molecules are at the same temperature as the walls of the spectroscopic cell. We observed that the hyperfine components of the 2 <-- 1 transition have distinct broadening coefficients that differ from one another by as much as 5%. The measured differences are in reasonable agreement with theoretical predictions. We have also performed molecular scattering calculations on three He-HCN potential energy surfaces in order to compare our results with theoretical expectations. At the lowest temperatures these calculations predict broadening coefficients that are considerably larger than the measured coefficients. We have previously found a similar discrepancy for two other molecules at these low temperatures, and we discuss possible experimental and theoretical origins for this persistent discrepancy.