F.C. Chen

Thermal conductivity of highly oriented polyethylene

Abstract We have measured the thermal conductivity of oriented polyethylene both along and perpendicular to the draw direction for draw ratio λ between 1–25 and in the temperature range of 120 to 320K. The results for λ ⩽ 5 have been analysed in terms of the modified Maxwell model while the further increase of thermal conductivity along the draw direction at higher λ has been explained by the Takayanagi model. The Youngs modulus and thermal conductivity along the draw direction for the sample with λ = 25 were found to be 64 GN/m2 (at 220K) and 140 mW/cm K (at 300K), respectively, which are extremely high values for polymers. This material, which is a good electrical insulator and yet has a high thermal conductivity, may be useful in electrical applications requiring large dissipation of heat.

Anistropic thermal expansion of oriented crystalline polymers

Abstract The linear thermal expansitivity of oriented high-density polyethylene (HDPE) and polypropylene (PP) with draw ratio between 1 and 18 has been measured between 120 and 300 K. The expansivity perpendicular to the draw direction z (α ⊥ ) increases with λ, because of the alignment of the crystallite chain axes along ẑ; the expansivity parallel to z (α  ) decreases very sharply with λ, becoming negative at about λ = 3 for HDPE and λ = 7 for PP — a consequence of the negative expansivity of the crystalline phase along its chain axis and the constraining effect of the stiff intercrystalline bridges. A model treating the drawn crystalline polymer as a composite consisting of partly aligned crystallites embedded in an isotropic amorphous matrix is adequate for explaining the behaviour of α⊥, whereas a parallel-series model can give reasonable estimates of α  for ultra-oriented samples, which is largely independent of λ and decreases with increasing temperature. However, neither model can account for the behaviour of α  at low draw ratio.

Thermal diffusivity of polymers by the flash method

Abstract We have adopted the flash method to the measurement of thermal diffusivity α of polymers in the temperature range 100–400K. The pulsed radiant energy from a flash tube is applied to the ‘front’ side of a suspended sample disc, and α is deduced from the exponential decay time constant of the subsequent transient temperature difference between the ‘front’ and the ‘back’ side, while correction against radiation loss is made by measuring the much longer decay time of the back-side temperature. Calibration runs on polycarbonate (PC) samples of several thicknesses show that the method is quick, precise and fairly accurate, and the results obtained are in reasonable agreement with previous determinations. We have also carried out measurements on polyoxymethylene (POM), poly (vinylidene fluoride) (PVF2) and poly (ethylene terephthalate) (PET) and computed their thermal conductivities. Results on POM and PVF2, which are semicrystalline, are analysed in the framework of several two-phase models, and the effect of crystallization (produced by annealing) on the glass transition behaviour of PET has also been studied.

Mechanical relaxations and moduli of oriented semicrystalline polymers

Abstract A systematic investigation was carried out on the mechanical relaxations and moduli of four drawn semicrystalline polymers: polyoxymethylene, polypropylene, polyvinylidene fluoride and polychlorotrifluoroethylene. Low-frequency tensile and torsional measuremnts were made between-140 and 140°C, and ultrasonic measurements of all five moduli were made by the water-tank method between 0 and 60°C. The patterns of relaxations remain essentially unchanged upon orientation, but there is a marked reduction of the height of relaxation peaks associated with the amorphous phase and, correspondingly, a smaller drop of moduli in the relaxation region. This reflects a lowering of molecular mobility in the amorphous phase due to the constraining effect of taut tie-molecules. The modulus C 33 increases sharply with draw ratio λ while the other moduli show little variation, which result from the alignment of molecular chain axes and the production of taut tie-molecules. The λ-dependence of the moduli is consistent with the aggregate model only when the polymer is glassy, that is, when its amorphous phase is comparable in stiffness to the crystalline phase and the polymer can reasonably be regarded as a one-phase material for which the aggregate model is valid.

Ultrasonic studies of three fluoropolymers

Abstract The ultrasonic velocities and attenuations of poly(vinylidene fluoride), poly trifluoroethylene and polychlorotrifluoroethylene at 10 MHz from 170 to 300 K have been measured by the pulse echo-overlap and the pulse-height comparison methods, respectively. Discontinuities in the slopes of the velocity — temperature curves and concurrent sharp rises of attenuations were observed, and were identified with previously reported transitions. An analysis of the fractional changes in the corresponding temperature coefficients was made. The velocity data extrapolated to 0 K by Raos rule were combined with presently available thermal data to yield the effective three-dimensional intermolecular force constants in the Tarasov model, and to confirm the existence of low-frequency vibrational modes contributing to the specific heat of polychlorotrifluoroethylene. The magnitude and temperature dependence of the elastic moduli and the Poisson ratio in relation to those of the velocities are also discussed.

Ultrasonic measurements of the mechanical relaxations and complex stiffnesses in oriented linear polyethylene

Abstract The five complex stiffnesses of highly oriented linear polyethylene produced by hydrostatic extrusion and die drawing have been determined by ultrasonic measurements. The results are compared with the elastic stiffnesses for crystalline polyethylene calculated theoretically. The development of anisotropic mechanical behaviour with draw ratio is discussed in terms of present structural understanding of highly oriented polyethylene. Although the very high stiffnesses obtained at the highest draw ratios are attributed to increasing crystal continuity, it is noted that the development of anisotropy in terms of low-temperature ultrasonic behaviour can be predicted to a good approximation by the reorienting unit aggregate model. This surprising result suggests that the overall orientation may still be the key parameter at low temperatures and high frequencies where there is no molecular mobility in the structure.

Mechanical relaxations in polybutene-1 and poly-4-methylpentene-1

Abstract Dynamic mechanical measurements between — 180°C and 180°C were made on both isotropic and drawn samples of polybutene-1 (PB-1) and poly-4-methylpentene-1 (P4MB1) over a wide frequency range by the use of a torsional pendulum (0.3–3 Hz), a viscoelastic spectrometer (5–90 Hz) and ultrasonic technique (3 MHz). The relaxation peaks were identified and the associated activation energies determined from Arrhenius plots. For PB-1 it was observed that orientation reduces the height and shifts up the temperature of the α a -peak associated with large scale main-chain motion in the amorphous regions, but has little effect on the β-peak associated with side-group motion. In addition to the α a and β relaxations a high-temperature crystalline relaxation ( α c ) is also observed in P4MP1. For both the α c and β relaxations the mechanical loss at 45° to the draw direction is much larger than that at 90°, which indicates that shear processes are involved in these relaxations.

Heat capacity of linear high polymers

Abstract The molar heat capacity of a 45:55% copolymer of ethylene and tetrafluoroethylene has been measured from 80 to 340 K by the use of an adiabatic calorimeter, to an accuracy of 0.3%. The results are found to be in close agreement with values calculated from the known optical lines of related polymers and the Tarasov model; they are also analysed together with the available data on polyethylene and four other fluoropolymers, showing that the principle of additivity for heat capacities is generally valid to within 2%. The logical relations between these two phenomenological frameworks and their resultant implication on the effective one-dimensional force constant of the carbon backbone are discussed at some length.

Heat capacity of copolymers of chlorotrifluoroethylene and vinylidene fluoride

Abstract The heat capacities of two copolymers of chlorotrifluorethylene and vinylidene fluoride, one 30:70 mol % and the other 44:56 mol % in composition, were measured in an adiabatic calorimeter from 80 to 340 K. The glass-transition points T g observed at 256 and 269 K for the 30:70 and 44:56 samples, respectively, closely agree with the prediction of the theory of Flory and Fox; the accompanying jump of heat capacities at these points was found to be 2.2 cal/K ‘per bead’ for both samples, in fair agreement with the ‘constant ΔC p rule’. A small peak was observed immediately above T g , and interpreted as a time-dependent effect arising from the lack of equilibrium in the region. The data of the heat capacities were analysed in detail in the framework of the Tarasov model and the principle of additivity, and were found to agree well (≲ 1–3%) with theory up to the respective glass-transition points in all cases, thus giving support to the validity of the previously published segmental values of heat capacity and of characteristic temperature θ 1 .