Yasaku Wada

Piezoelectricity and Pyroelectricity of Polymers

This article surveys theoretical and experimental work on piezo- and pyroelectricity of polymers in the 1970s with special emphasis on the origins of these properties. The origins are classified into three types: (A) internal strain (§2), (B) strain- and temperature-dependences of spontaneous polarization (§3), and (C) elastic and/or dielectric heterogeneity of a system with embedded charges (§4). The origin of piezo- and pyroelectricity of poly(vinylidene fluoride) is discussed as a typical example of electret (§5). Piezoelectric relaxations of polymers are discussed in some detail (§6). Methods of measurements of piezo- and pyroelectric constants of polymer films (§7) and applications of polymer films as new transducer materials (§8) are briefly reviewed.

Nonlinear dielectric constant and ferroelectric‐to‐paraelectric phase transition in copolymers of vinylidene fluoride and trifluoroethylene

Second‐ and third‐order dielectric constant e2 and e3 of three copolymers of vinylidene fluoride and trifluoroethylene were measured over a temperature range from 20 °C to higher than the ferroelectric‐to‐paraelectric phase transition temperature Tc . The thermodynamical theory predicts that e2 is negative below Tc  and vanishes at Tc , whereas, for the second‐order transition, e3 is positive below Tc but turns to negative at Tc and, for the first‐order transition, e3 does not change its sign below and above Tc . According to this criterion, it is concluded that the transition is of the second order for a copolymer with vinylidene fluoride fraction of 52 mole %, but of the first order for 65 and 73 mole % copolymers. This conclusion can explain the difference in slope of e2 versus remanent polarization among the copolymers.

Dielectric Properties and DC Conductivity of Vacuum-Deposited SiO Films

Dielectric properties from 10-2 to 107 c/s and dc conductivity are investigated for vacuum-deposited SiO films. The conductivity decreases with increasing oxidation and the activation energy is estimated to be 8.6 kcal/mole. Two dielectric dispersions are found, the loss peaks of which are at 10-1 c/s (1f dispersion) and at above 107 c/s (hf dispersion), respectively. The strength of 1f dispersion decreases with increasing oxidation and metallic impurities have no appreciable effect on the dispersion. The activation energy of 1f dispersion is almost equal to that to dc conduction. The dc conduction and 1f dispersion are concluded to be attributed to diffusion of free Si ions. The hf dispersion is ascribed to local torsional vibration of SiO chains. A model of the molecular structure of SiO in the glassy state is suggested, which is well consistent with the electrical and optical properties of this material.

A model theory of piezo- and pyroelectricity of poly(vinylidene fluoride) electret

Abstract Calculation of piezoelectric constants e 31 and e 33 and pyroelectric constant p 3, of poly(vinylidene fluoride) electret is made on the basis of a model theory in which spheres with the spontaneous polarization Psc are embedded in a matrix. Results are in good agreement with observed values of these constants, respectively, when the magnitude of P sc is reasonably assumed. It is to be noted that e31, e32;, e33 and p 3 are interpreted in terms of a unified simple composite model.

Temperature Dependence of Velocity and Attenuation of Ultrasonic Waves in High Polymers

Measurements of velocity and attenuation of ultrasonic waves are carried out for polystyrene, polymethyl methacrylate, phenol resin, polyethylene, nylon 6 and other commercial resins in the temperature from -60°C to 90°C and at frequencies of 100 kc, 1.46 Mc, 4.38 Mc and 7.30 Mc. A correlation between sound velocity in the megacycle range and density is pointed out for glassy polymers. In the case of linear polymers, the temperature coefficient of ultrasonic velocity, β, exhibits a relatively abrupt change in the glass trasition region at the temperature T m , which is independent of frequency of sound agrees with the transition point associated with thermal expansion. The ratio of β to volume expansion coefficient α is found to be constant below and above T m for each polymer. These facts indicate that the change in β at T m should be ascribed to the temperature dependence of intermolecular free volume. For some polymers a change in β and a peak of loss factor are observed at lower temperatures.

Non-Ohmic Conduction in Vacuum-Deposited SiO Films

The current through vacuum-deposited SiO films in the thickness range 1–4 µ is measured as a function of voltage and temperature between 10° and 80°C. Three regions are distinguished in the current-voltage relationship; (1) voltage below 0.1 V, (2) from 0.1 to 10 V and (3) above 10 V. The current-voltage characteristic is linear in the region (1) but becomes nonlinear in the regions (2) and (3). In the region (3), the current depends on voltage and thickness only through the electric field E, the activation energy decreases linearly with increasing E1/2 and the logarithmic current increases linearly with increasing E1/2. These results are quantitatively explained in terms of field enhanced thermal excitation of trapped electrons into the conduction band (Poole-Frenkel effect). The energy gap between trap level and conduction band is estimated to be 0.54 eV.

Dielectric Relaxations in Various Crystal Modifications of Cyclohexanol and 2,3-Dimethylbutane

Abstract The dielectric constant and loss of crystalline cyclohexanol (CHOL) and 2,3-dimethylbutane (DMB) were measured from 4°K to the melting point, 299°K for CHOL and 145°K for DMB. The supercooled plastic crystal of CHOL above the glass temperature Tg exhibits a prominent relaxation (α) due to molecular rotation. The temperature dependence of therelaxation time was found to obey the Fogel-Tamman equation. A secondary relaxation (β) was observed below Tg , 149°K for CHOL and 76°K for DMB. The mechanism proposed for β is internal rotation; rotationof OH bond for CHOL and trans-gauche transformation for DMB. The β relaxation of DMB is well correlated with acoustic relaxation in the liquid state in the relaxation time versus timperature plot. Relaxations in the intermediate state, crystal III, of these two substances and the most stable state, crystal II of DMB were studied.

Mechanical Dispersions and Transition Phenomena in Semicrystalline Polymers

The existing data of mechanical dispersions in seven semicrystalline polymers (polyethylene, polypropylene, polyethylene terephthalate, polytrifluoromonochioro ethylene, polyamide, polyurethane and polytetra-fluoroethylene) are compiled and the frequency dependence of loss peak temperature (“transition map”) is illustrated. The primary and secondary dispersions occurring in the amorphous regions are identified on the basis of these results. The correspondence of the mechanical dispersions to dielectric ones, transition temperatures found in dilatometric measurements and change in NMR line width with temperature is discussed for these polymers.

Effect of space charge on polarization reversal in a copolymer of vinylidene fluoride and trifluoroethylene

Polarization‐reversal phenomena in ferroelectric copolymers of vinylidene fluoride and trifluoroethylene are affected by the time interval elapsed after poling of samples. Measurements of switching current demonstrated that this waiting effect is enhanced with the increase of space charges and switching curves with different waiting intervals satisfy the scaling rule when each switching curve is scaled by the switching time. These facts suggest that the electric field that reverses the spontaneous polarization decreases due to the electric field coming from space charges in the amorphous phase. A model with a prolate ellipsoid crystal and space charges in the amorphous phase explains such experimental facts. The difference of the waiting effect among samples crystallized under different conditions is also explained through the difference in the depolarization field coefficient of crystals.

New Apparatus for Measuring the Complex Dielectric Constant of a Highly Conductive Material

Difficulties in measuring the contplex dielectric constant of a highly conductive material such as aqueous polyelectrolytes including biopolymers arise from (l) electrode polarization effect and (2) d.c. conductance of the specimen which is very much higher than the conductance due to dielectric loss and drifts with time. A new method has been developed in which the first difficulty is avoided by employing a four-electrode cell and the second orue is overcome by direct measurement of the frequency difference spectrum of specimen impedance, ΔZ(ω)=Z(ω)-Z(ω0), between the measuring frequency ω and the reference ω0. By this procedure, the effect of frequency-independent d.c. conductance is eliminated. With the new method, the frequency range of dielectric measurement is extended from 0.2 Hz to 30 kHz.