Mechanism of ac conduction in heat-treated poly (acrylonitrile) and poly (ethylene 2, 6-naphthalate) was studied in the temperature range of -96°C-170°C and in the frequency range of 120 Hz-100 kHz. The temperature and frequency dependences of ac conductivity at low temperatures were compared with Austin-Mott's equation for ac hopping conduction in amorphous materials, which shows fairly reasonable agreements. This fact indicates that at low temperatures electronic ac hopping conduction between localized states takes place in the polymers. At higher temperatures, however, the dc conduction mechanism can be explained in terms of thermally activated process. Dielectric properties due to the molecular motions of polymer chains are also discussed, since they affect ac conductivity.

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The molecular structures of polymers are intrinsically anisotropic. For clarifying the mechanism of electrical conduction in polymers, a new method of the measurement of the anisotropy of electrical conduction in oriented polymers was developed. The limitations of this method as well as the error were discussed. Examples of the conduction anisotropy in uniaxially oriented and unoriented poly (γ-methyl _L-glutamate) films were given. The effect of electric field strength was measured. The method developed here was poorly applicable to the measurement of transient current. An example was shown for the conduction anisotropy along three principal directions of roll-oriented nylon 12.

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The mechanism of the electrical conduction in poly (γ-methyl _L-glutamate) has been studied from the measurements of low- and high-electric-field conductivities, charge-carrier mobility, and the dielectric absorption at the frequencies lower than 100 Hz. The carrier concentration was estimated in the lower or in the higher temperature region than 60°C, by different methods: from the conductivity, high-field-hopping conductivity, and dielectric absorption for the former, or from the conductivity and mobility for the latter. The values of the carrier concentration at 60°C and their temperature independence obtained by both methods were in good agreement (3×10¹⁷ cm^-3). The results confirmed the hopping conduction mechanism in this polymer.

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Thermally stimulated current (TSC) characteristics in poly (ethylene terephthalate) (PET) has been investigated in detail in a high temperature region. The results clearly shows that TSC peak appearing at about 400K is due to ionic space-charge polarization associated with the migration of mobile ions within PET over macroscopic distance. Furthermore a very large TSC peak is observed in the temperature region near 460 K, but no thermally stimulated surface potential (TSSP) corresponding to the TSC peak is measured in the same temperature region. This is related to the neutralization of the mobile ions at PET-electrode interface. Sublinear current versus voltage characteristics are observed in a high temperature region. TSC and TSSP measurements show that the conduction is also related to the neutralization.

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The decay of surface charge and the thermally stimulated current (TSC) for corona charged poly (alkyl methacrylate) s were measured. The decay of surface charge decreased in order of poly (methyl methacrylate) (PMMA), poly (ethyl methacrylate) (PEMA), poly (butyl methacrylate) (PBMA), poly (isopropyl methacrylate) (PiPMA) and poly (tert-butyl methacrylate) (PtBMA). The surface charges on PiPMA and PtBMA were very stable, which appeared due to the low electric conductivity at room temperature. The decay of surface charges on PMMA, PEMA and PBMA might be caused due to the increase of heterocharge which produced by the orientation of dipoles under the innerfield of surface homo-charges. The TSC measured with a contactless electrode showed two peaks above room temperature, except PBMA. The peak at the lower temperature is caused by the polarization current due to the orientation of dipoles. The second peak at the higher temperature is caused by the neutralizing current of homocharge due to increased conductivity. The decay of surface charge on corona-charged PMMA decreased greatly by annealing. The introduction of oriented dipoles by annealing stabilized the surface homocharge.

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The nature of carrier traps, which greatly influence the electrical properties of polymers, was investigated by using the X-ray induced thermally stimulated current (TSC) and thermoluminescence (TL) techniques for various polymers, especially polyethylene (PE). Most polymers except those with high ionic conduction e. g. poly (vinyl chloride), exhibit remarkable TSC and TL peaks arising from trapped carriers. Most of these peaks correspond to the onset of molecular motions, suggesting that carrier detrapping is closely related to molecular motions. There exist many traps in the region related to crystallites. In PE, there are 5-6 kinds of traps which may be some physical defects such as cavities. In an oxidized and a γ-irradiated PE, oxidation products and radiation defects also act as fairly deep traps.

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Thermally stimulated current (TSC) and thermoluminescence (TL) of poly (vinylidene fluoride) with the variety of crystalline form are discussed in terms of molecular motions. The TSC peak at around 70°C, which corresponds to the crystalline dispersion temperature, gives a current reversal with suitable charging conditions. This fact suggests that the crystals play an impotant role for homo-charge trapping. The TL measurements give two peaks, one is located at around 70°C and another at around 140°C. The former is likely assigned to the crystalline dispersion and the latter may be a reflection of a new molecular motion due to the crystals formed under high pressure.

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The 100-μm thick films of poly (vinylidene fluoride) (PVDF) were stretched four times their original lengths at 80°C and heat treated at 140°C for 30min under the condition of fixed length. Arrays of needle electrodes were placed 5-mm above and below the film. The d. c. voltage 12 kV applied between the electrodes produced corona discharge in the air gap and polarized the film. The piezoelectric constant d₃₁ was about 12 pC/N, independent of poling temperature. When the process of heat-treatment at 140°C was omitted, the piezoelectric constant attained a maximum at the poling temperature 80°C and the stabilized value of d₃₁ was about 20 pC/N. When the corona poling and stretching were performed simultaneously, the piezoelectric constant became the highest at the poling temperature 50°C and the stabilized value of d₃₁ reached 40pC/N.

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The electrostriction and the piezoelectric constants were measured for uniaxially stretched and rolled poly (vinylidene fluoride) films. In order to control amorphous orientations, the techniques of thermal shrinkage were employed for the uniaxially stretched films. The electrostriction constant of thermally shrunk samples is in proportion to the degree of amorphous orientation. Analysis of infrared dichroism revealed that the dichroic ratio, which is ascribed to amorphous region at 600cm^-1, changed with the applied strain; it increased with the increase of draw ratio. The draw ratio dependences of the dichroism change are very similar to those of the electrostriction constant. Further, the theoretical expression of the electrostriction effect, derived from the orientation model of the amorphous chains, is in good agreement with experiment. Consequently, the positive electrostriction effect stems mainly from the change of the amorphous orientation with the applied strain. It is clarified that the contributions of the electrostriction effect to the piezoelectricity of highly oriented PVDF films are up to 50%.

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Dielectric properties of poly (trifluoroethylene) were studied over a frequency range of 1 Hz to 1 MHz at temperatures from -180 to 140°C. The dynamic tensile modulus was measured over temperatures ranging from -70 to 130°C at 110 Hz. Two relaxation processes, a and β, were observed above and below the glass transition at 31°C, respectively. With increasing the degree of crystallinity the mechanical loss tangent decreased in the a relaxation and the loss peak for the β relaxation shifted to lower terriperature. The dielectric loss peak of the β process was much greater in size than that for the α process. The activation energy in the dielectric relaxation was 280 and 71 kJ mol_-1 for the a and β process, respectively. The higher the temperature, the larger was the relaxation intensity for the β process. It was concluded that the α process is related to the micro-Brownian motions of molecular-chain backbone in amorphous regions and the β process is attributable to the local molecular motions of short segments. It was noted that the β relaxation may consist of multiple processes as found for the γ relaxation in poly (chlorotrifluoroethylene).

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The dielectric constant (ε′) and the loss factor (ε″) of wood for various moisture content (M.C.) were measured in temperature range from -196°C to 0°C. At M.C. of 0.7%, ε″-peak arising from the rotational motion of CH₂OH groups shifted to the higher temperature with an apparent activation energy (ΔE) of 10.7 kcal/mol. Another ε″-peak appeared at ca. -40°C in frequency range 30 Hz-1 kHz. The latter peak was enhanced and shifted to lower temperatures with increasing M.C. ΔE of that peak decreased from 28 to 12.4 kcal/mol and levelled off above M.C. of 5% corresponding to the lower temperature shift of the peak. A similar situation was observed for the variation of dynamic loss modulus at 110 Hz. It was suggested that the ε″-peak at ca. -40°C came from the local mode motion due to the cooperative motion between adsorbed moisture and cellulose, hemicellulose and lignin with the tight hydrogen bonding.

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