M. G. Broadhurst

Electric‐field‐induced phase changes in poly(vinylidene fluoride)

The antipolar crystal form of poly(vinylidene fluoride) can be made piezoelectric and pyroelecric by the temporary application of electric fields in excess of 1 MV/cm at room temperature. Infrared and x‐ray diffraction data reveal that the polarization occurs in two stages. At fields near 1 MV/cm, a phase transition to a polar form II occurs with presumably no change in chain conformation. Fields near 5 MV/cm cause a change in conformation to produce form I. Our results indicate that at least a portion of the residual polarization occurs within the crystal phase of the polymer.

Piezoelectricity and Pyroelectricity in Polyvinylidene Fluoride -- A Model.

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.

Crystallographic changes characterizing the Curie transition in three ferroelectric copolymers of vinylidene fluoride and trifluoroethylene: 1. As-crystallized samples

Abstract Copolymers of vinylidene fluoride/trifluoroethylene of molar composition 65 35 , 73 27 and 78 22% respectively, are ferroelectric and undergo a Curie transition to the paraelectric state at high temperatures. In contrast to the irregular structure found earlier for the 52 48 mol % copolymer, the structures of these three compositions in the low-temperature state are all well ordered and analogous to that of β-poly(vinylidene fluoride): they consist of molecular chains in a polar trans conformation whose order is improved with increasing vinylidene fluoride content, packed pseudo-hexagonally in unit cells whose dimensions decrease with increasing vinylidene fluoride content. In their paraelectric phase, the chains assume a partly disordered conformation consisting of irregular TG, TḠ and TT sequences and are packed on an expanded pseudo-hexagonal lattice. The Curie transitions were found to occur over a broad temperature range, encompassing ∼30°C, and in the case of the 78 22 mol % copolymer to extend into the melting region; they were also found to exhibit hysteresis by occurring at much lower temperatures upon cooling than upon heating.

Extrapolation of the Orthorhombic n‐Paraffin Melting Properties to Very Long Chain Lengths

An analysis of the equation TM=T0 (n+a) / (n+b) is made in order to determine the convergence temperature T0 of the melting temperatures of the n‐paraffins from C44 to C100. A good fit is obtained using T0=414.3°K, a=—1.5, and b=5.0. This value of T0 (414.3°K=141.1°C) has an estimated total uncertainty of ±2.4°K and is proposed as the correct value of the equilibrium melting temperature of polyethylene. It is shown that presently available data do not permit accurate extrapolation of the n‐paraffin heats and entropies of fusion to the polymer limit.

Molecular dipole electrets

The total polarization due to molecular dipoles in a glassy electret is computed using an Onsager cavity approach. From this result, all the possible contributions to the piezoelectric and pyroelectric coefficients are considered. It is shown that there are major contributions from the variation in dielectric constant and, for pyroelectricity, from thermal motion. These results account well for experimental data for polyvinyl chloride.

Physical basis for piezoelectricity in PVDF

Abstract Consideration of the molecular and bulk structures of PVDF and related semicrystalline polymers, and the effects of processing to make transducer films, leads to a novel way of analysing the elastic and piezoelectric constant data. The results of this analysis support the conclusion that thickness changes contribute about 2/3 of the piezoelectric activity. An increase in thickness decreases the charge on the electrodes. The probable mechanism is simply the displacement of the electrodes in the fields of the constant dipole moments of the crystals. The remaining 1/3 of the activity is attributed to changes in the films dipole moment at constant thickness. Strains in the orientation direction of the film increase the charge on the surface while strains in the plane of the film normal to the orientation direction decrease it. Eight separate possible contributions to the change in moment are discussed, and qualitative evaluations of their importance are given.

Hysteresis in copolymers of vinylidene fluoride and trifluoroethylene

Abstract Copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE), with more than 50 mole percent VDF exhibit D-E hysteresis loops at room temperature which are much sharper than those exhibited by various crystal phases of the PVDF homopolymer. For the copolymer samples investigated here, appreciable conductivity develops at elevated temperatures which in the presence of electric fields leads to trapped charges in the polymer film. These charges then prevent the switching of dipoles at values of electric fields that were previously applied, the room temperature hysteresis is greatly reduced and polarization through the thickness of the film is highly non-uniform. Upon heating the copolymers above the ferroelectric to paraelectric transition temperature, the polarization is destroyed, the space charges are apparently released and room temperature hysteresis is restored. Experiments with aluminum and gold electrodes and with mica blocking electrodes lead to the conclusion that the charges are g...

Bulk Modulus and Grüneisen Parameters for Linear Polymers

Expressions are derived for the isothermal bulk modulus BT and Anderson–Gruneisen parameter δ for a simple two‐dimensional bundle‐of‐chains model where parallel nearest‐neighbor chains are assumed to interact with a simple separation‐dependent potential. Bulk modulus data for general linear polymers are examined and shown to be essentially temperature independent at constant volume in accord with the model. The theory is shown to predict quite accurately for reasonable assumed pair potentials both BT and δ of high‐density polyethylene and n‐paraffins with no adjustable parameters. The values of δ obtained for polyethylene from BT data are in excellent agreement with those reported from ultrasonic data. The analysis is also shown to provide a sensitive measure of the form of the effective chain pair potential near its minimum. Also, a close relationship between δ and the Gruneisen constant γ is derived for this model.

Curie transitions in copolymers of vinylidene fluoride

Abstract A series of random copolymers of vinylidene fluoride and trifluoroethylene containing 52, 65, 73, and 78 mol % VF2 has been shown by X-ray and dielectric techniques to undergo Curie transi...

Plasma Poling of Poly(Vinylidene Fluoride): Piezo- and Pyroelectric Response

A plasma poling technique and its use with polyvinylidene fluoride (PVDF) films is described. Specimens of biaxially drawn (blow extruded) PVDF containing both Form I ( β) and Form II (α) crystals were poled under various conditions in a plasma field while the charging current was monitored to determine the polarization. Subsequently, both piezoelectric and pyroelectric activity were measured in order to evaluate their magnitudes with respect to the remnant polarization (that is, the polarization remaining after returning the applied field to zero). The results are shown to be in reasonable quantitative agreement with the predictions of a model of PVDF consisting of a mixture of preferentially aligned crystals in randomized amorphous material.