A description is given of the molecular and morphological structure of polyvinylidene fluoride and from this description a classical model is proposed for calculating the piezoelectric and pyroelectric properties. The model consists of an array of crystal lamellae with a net moment from aligned dipoles in the crystals and compensating space charge on the crystal surfaces. The results for no compensation and complete compensation essentially bracket experimentally observed results and indicate that the largest contribution to the activity of this polymer arises from bulk dimensional changes rather than from changes in molecular dipole moments.

Piezoelectricity and pyroelectricity in polyvinylidene fluoride—A model

This chapter presents an extensive review of the important ferroelectric polymer, poly(vinylidene fluoride). Covered topics include: polymerization, molecular structure, and defects; dilute-solution properties; crystalline structure of the various polymorphic phases, crystallization, and morphology; polymorphic transitions induced through a variety of means (e.g. thermal, mechanical, electrical); relaxational characteristics; melting behaviour and thermal degradation; ferroelectric properties, with emphasis on piezoelectricity and pyroelectricity; copolymers and compatible blends with other polymers and general engineering properties and uses.

Poly(Vinylidene Fluoride)

Dielectric and piezoelectric responses caused by a high electric field and a 10‐Hz small strain have been simultaneously measured for stretched polyvinylidene fluoride using a minicomputer system. The electric field was applied up to 240 MV/m in a sinusoidal or triangular wave form in the frequency range 10−4–10−2 Hz at temperatures between −100 and 100 °C. Ferroelectric hysteresis loops were observed even below the glass‐transition temperature of −60 °C. At 20 °C the 120‐MV/m electric field gave the remanent polarization Pr of 50 mC/m2 which resulted in the piezoelectric activities e31 and e32 of 70 and 7.5 mC/m2, respectively. The origin of Pr was confirmed to be the molecular dipole orientation because the value of Pr was independent of the frequency of applied field. The coercive field Ec significantly decreased with increasing temperature from 180 MV/m (−100 °C) to 25 MV/m (100 °C), while Pr showed only a slight temperature dependence.

Hysteresis phenomena in polyvinylidene fluoride under high electric field

Apatitic cements have shown excellent biocompatibility and adequate mechanical properties but have slow resorption in the human body. To assure that new bone tissue grows into the bone defect, a certain porosity is necessary although hard to achieve in injectable cements with suitable mechanical properties. An attempt was made by mixing alpha-tricalcium phosphate (alpha-TCP), calcium sulphate hemihydrate (CSH) and an aqueous solution containing 2.5 wt% of Na(2)HPO(4). The aim was to obtain a material containing two phases: a) one apatitic phase (calcium-deficient hydroxyapatite; CDHA) and b) one resorbable phase (calcium sulphate dihydrate; CSD). alpha-TCP and CSH mixtures were produced at relative intervals of 20 wt%. The liquid-to-powder (L/P) ratio to obtain a paste was 0.32 mLg(-1). The highest compressive strength (34 MPa) was obtained for the pure alpha-TCP sample. The strength was, in a first approximation, directly correlated to the weight proportions of the powders. X-ray diffraction analysis showed that the relative intensity for CDHA increased linearly, and the one for CSD decreased exponentially, when the amount of alpha-TCP increased. Thus, CSH ceased to transform to CSD when the amount of alpha-TCP increased. Observations in environmental scanning electron microscopy confirmed the X-ray diffraction results. CSH-crystals (100 microm) were embedded in the HA-matrix permitting gradual porosity in the material when resorbed.

Characterization of a novel calcium phosphate/sulphate bone cement.

The ferroelectric switching characteristics of polyvinylidene fluoride have been investigated by the time domain measurement of electric displacement under a step‐function field up to 200 MV/m at temperatures between −100 and 20 °C. Almost complete reversal of polarization is found to occur in a fairly short time if the applied field is sufficiently high. The switching time is 4 μsec at 20 °C under a field of 200 MV/m and increases significantly as either field or temperature decreases. The rate of change of polarization is greater than the rate associated with a single relaxation process. This is an indication of cooperative effects in the switching process of this ferroelectric polymer.

Measurements of ferroelectric switching characteristics in polyvinylidene fluoride

The piezoelectric stress coefficient and electrostriction coefficient for unpoled and poled films of polyvinylidene fluoride have been determined by applying a sinusoidal electric field and detecting stresses with the same and double frequency as the applied field, respectively. The piezoelectric coefficient shows a hysteresis with the cyclic change of the d.c. bias field. Under the poling field, the dipoles in polymers are preferentially oriented in the direction of the field, thus producing a residual polarization associated with a residual stress. Both coefficients increase with increasing temperature. A phenomenological interpretation for the piezoelectricity in poled polymers is given in terms of electrostriction and residual stress.

Eiichi Fukada

Papers

Inverse piezoelectric effect and electrostrictive effect in polarized poly(vinylidene fluoride) films