Stress relaxation and its influence on piezoelectric retention characteristics of uniaxially stretched poly(vinylidene fluoride) films Some aspects of piezoelectricity and pyroelectricity in uniaxially stretched poly(vinylidene fluoride) Hysteresis characteristics, which are similar to those observed in ferroelectric crystals, are found in uniaxially stretched poly(vinylidene fluoride) under a high electric field above 300 kV Icm at room temperature. The electrostrictive constant q 13 in poly(vinylidene fluoride) is determined for polarization below about 0.5 J.IC/cm 2 at room temperature to be 0.9 egs esu. The remanent polarization after the poling process can be easily eliminated with a sufficiently high sinusoidal electric field. This effect is confirmed by thermally stimulated currents before and after the depolarization process.

The piezoelectric constant of a uniaxially stretched and polarized poly(vinylidene fluoride) film decreases monotonically with increasing temperature in the range from 65 to 165 °C. After a specimen is cooled under open-circuit conditions, the thermally stimulated current reverses in sign and exhibits a peak at around the glass-transition temperature Tg. Reversible changes in infrared spectra before and after the poling process suggest that the dipoles in the β crystals of poly(vinylidene fluoride) can be aligned along the direction of an applied electric field. The alignment is strongly influenced by the amorphous region; the coercive field in P-E hysteresis curve observed at 50 Hz increases rapidly at around Tg. Under a high electric field, the uniaxially stretched poly(vinylidene fluoride) film behaves above Tg in the same way as ferroelectric ceramics.

Hysteresis characteristics, which are similar to those observed in ferroelectric crystals, are found in uniaxially stretched poly(viny1idene fluoride) under a high electric field above 300 kV/cm a t room temperature. Various transducers such as stereophonic headphones, tweeters, microphones and phonograph cartridges have been developed with polarized poly(viny1idene fluoride) films. These transducers have quite simple structures because the diaphragms are piezoelectric fiims and involve transducing function in themselves.

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Based on the model proposed by Edelman and de Reggi, simple fundamental relations of a sector form electroacoustic transducer with piezoelectric polymer diaphragm are derived. Calculated values of the compliance of the vibrating system are mainly controlled by the bulk modulus of the polyurethane foam backing. As an example, the sensitivity of the piezoelectric polymer headphone with an artificial ear B&K type 4153) is calculated under stiffness control frequency region. Furthermore, a push-pull-type piezoelectric polymer headphone is also studied in order to improve the sensitivity.