Yu. I. Polikarpov

Energy of the elastic loading of anharmonic solids

This paper reports on the results of measurements of changes in the temperature of a solid under the adiabatic elastic loading (thermoelastic effect), the coefficient of thermal expansion, and the Young’s modulus of a rigid-chain vitrified polymer, namely, polyimide. It has been found that there are differences in the sign and magnitude of the changes in the energy of thermal origin in samples and the work of the external force. The energy of the thermoelastic effect has been explained in terms of the influence exerted by the anharmonic expansion of a solid, with the separation of the quasi-static potential and dynamic components of the thermal energy of the solid. The loading with an external force causes a redistribution of the thermal energy. A change in the temperature of the solid corresponds to a change in the dynamic component. An energy analysis of the adiabatically loaded anharmonic oscillator has confirmed the conclusion regarding the mechanism of energy transfer and revealed that, under loading, there is a redistribution of the kinetic and potential components of the internal energy of the oscillator.

Determination of the activation energy for complicated relaxation processes

Different methods are considered for analyzing the temperature dependence of the relaxation rate in the Arrhenius form. It is emphasized that the possible changes in the barrier to elementary acts with variations in temperature should be taken into account in order to determine the relaxation activation energy correctly. Changes in the barrier to elementary acts are illustrated using experimental data on the temperature-frequency dependence of the dielectric relaxation in polymers. A theoretical approach offering realistic activation energies is proposed.

On the determination of the energy of activation of relaxation transitions in polymers by differential scanning calorimetry

The determination of the energy of activation (barrier height) of elementary events involved in relaxation (fluctuation) transitions in polymers from the temperature dependence of the specific heat is discussed. The dependence is derived by the method of differential scanning calorimetry. It is emphasized that the correct determination of the energy of activation must include the temperature variation of the barrier. The deviation of the preexponential in the Arrhenius temperature dependence from the value predicted theoretically demonstrates that the barrier does depend on temperature. Experimental data from which the realistic energy of activation of the α relaxation in polymers can be found are given.

Detailing of deformation processes in polymeric crystals

Structural changes in polymer crystals (polyethylene, polyimide, and others) have been studied using the X-ray diffraction and Raman spectroscopy methods under different influences: tensile loading along the chain molecule axis and heating from 90 to 350 K. An increase in the molecule axial length under loading and a decrease in the molecule axial length upon heating have been identified and measured using X-ray diffraction. A decrease in the skeletal vibration frequency during loading and heating has been identified and measured using Raman spectroscopy, which indicates an increase in the molecule contour length in both cases. A technique for determining the change in the polyethylene molecule contour length in the crystal from the measured change in the skeletal vibration frequency has been justified. The contributions of two components, namely, skeletal (carbon–carbon) bond stretching and the change (an increase during stretching and a decrease during heating) in the angle between skeletal bonds, to the longitudinal deformation of polyethylene crystals, have been quantitatively estimated. It has been shown that the negative thermal expansion (contraction) of the polymer crystal is caused by the dominant contribution of the decrease in the bond angle.

Mechanisms of elementary events in the kinetics of electrical breakdown of polymer and ceramic dielectrics

The kinetics of electrical breakdown of thin (15–70 μm) layers of polymers and ceramics in a constant-sign field at 77–480 K has been investigated. The temperature dependences of the longevity (breakdown waiting time) of both dielectrics have been found to be similar to each other. At elevated temperatures, the longevity of the dielectrics varies exponentially with increasing temperature, and at reduced temperatures, it is temperature-independent (there is an athermal plateau). The mechanisms of elementary events controlling the process of preparation of the dielectrics for breakdown at elevated and reduced temperatures are the thermal-fluctuation over-barrier electron transition from trap to trap and the tunneling (under-barrier) transition, respectively. The hopping electron transport in the field direction gives rise to critical space charges causing breakdown of the dielectrics. The transition barrier heights (trap depths) have been determined. The low-temperature longevities of the polymer and the ceramic have been found to be similar, whereas the transition barrier for the ceramic is much higher than that for the polymer and the applied field in the former case is significantly (by a factor of tens) lower than that in the latter case. Electron traps in the polymer are adequately described by the Coulomb center model, whereas this is not the case for the ceramic.

Effect of mechanical loading on the kinetics of electrical breakdown in polymers

The decrease in the electrical lifetime (time of expectation of a breakdown in an electric field of constant polarity) under tensile loading of disoriented polyethylene, Teflon, and lavsan films in the range of 100–300 K is studied. It is found that tensile loading decreases the depth of intermolecular electron traps due to an increase in the spacing of chain molecules in amorphous regions of polymers. This accelerates the hopping (from one trap to another) transport of electrons, leading to the formation of critical (breakdown) volume charges.

Characteristics of elementary acts in the mechanical fracture kinetics of polymers

The main characteristics of elementary acts in the mechanical fracture kinetics of oriented polymers, namely, the initial potential barrier U0 and the activation volume VA, are determined. The values of U0 And VA are obtained from analyzing the temperature and force dependences of the life of polymers with allowance for the actual stresses (overstresses) applied to chain molecules, which are found by IR spectroscopy. The values obtained (U0 ≈ 4 eV, VA ≈ 0.03 nm3) agree satisfactorily with the dissociation energy and activation volume of the fluctuation breaking of covalent skeleton bonds in polymer molecules.

Negative longitudinal expansion and the amplitude of longitudinal vibrations in poly(ethylene) crystals

The experimental temperature dependences of the thermal expansion coefficient and the mean-square amplitude of atomic vibrations in the longitudinal direction (along the axes of chain molecules) in poly(ethylene) crystals of different sizes are measured using x-ray diffraction in the temperature range 5–350 K. Theoretical calculations of the temperature dependences of the thermal expansion coefficient and the mean-square amplitude of atomic vibrations in poly(ethylene) crystals are carried out. It is shown that the results of calculations are in good agreement with experimental data. The temperature ranges in which the quantum character of lattice dynamics affects the thermal expansion coefficient and the mean-square amplitude of vibrations are determined. It is revealed that the shear longitudinal modes play an important role in the lattice dynamics of crystals with a chain structure.

Application of the temperature wave technique for the study of relaxation transitions in polymers

Abstract The paper describes the principles, theoretical calculation procedure, and apparatus used for the realization of the temperature wave technique. The efficiency of the technique for the studies of relaxation transitions in polymers when the thermophysical characteristics are in the complex form has been demonstrated. The temperature dependences of the moduli and arguments of thermophysical characteristics for different frequencies of temperature oscillations have been obtained for polyvinylacetate. The vibrational and configurational components of the heat capacity have been found. The effect of polymer plasticization and also of changes in the chemical composition of molecules on complex thermophysical characteristics has been shown using styrene-butylacrylate as an example.

Features of the kinetics of electrical damage of polymers

The kinetics of electrical damage (breakdown) of polymer films 20–50 μm thick in a constant-sign field of 0.5–0.6 GV/m at 77–300 K has been studied. At elevated temperatures (250–300 K), the exponential temperature dependence of the durability and the above-barrier thermal-fluctuation mechanism of electron emission from traps, i.e., space charge accumulation leading to breakdown, take place. At low temperatures (77–200 K), there are separate local decreases in the durability (minima) at the athermal durability level. The identity of the temperatures of durability minima and measured thermoluminescence maxima of polymers was found. A conclusion is made about the mechanism of thermally stimulated tunneling (subbarrier emission) of electrons from traps.