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Dive into the research topics where G. T. Davis is active.

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Featured researches published by G. T. Davis.


Journal of Applied Physics | 1978

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

G. T. Davis; John E. McKinney; M. G. Broadhurst; Steven C. Roth

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.


Journal of Applied Physics | 1978

Piezoelectricity and Pyroelectricity in Polyvinylidene Fluoride -- A Model.

M. G. Broadhurst; G. T. Davis; John E. McKinney; R. E. Collins

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.


Polymer | 1983

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

Andrew J. Lovinger; T. Furukawa; G. T. Davis; M. G. Broadhurst

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.


Journal of Applied Physics | 1970

Thermal Expansion of Polyethylene Unit Cell: Effect of Lamella Thickness

G. T. Davis; R. K. Eby; J. P. Colson

The unit cell dimensions have been measured at temperatures between 93 and 333 K for linear polyethylene samples with long periods of 385, 220, and 99 A. The angular positions of 6 x‐ray diffraction lines were obtained at 5–10 K intervals with a powder diffractometer and the positions corrected for beam penetration so as to agree with powder camera results obtained with more lines at 296 and 155 K. At lower temperatures, the cell dimensions are nearly independent of long period, but at higher temperatures, the basal area of the cell appears to vary linearly with the reciprocal of the long period. The value of the slope increases with temperature and at 293 K is nearly the same for sets of data obtained with a number of different molecular weight distributions, crystallization and annealing conditions as well as for n‐paraffins. The specific volume data for all three polymer samples can be represented between 133 and 333 K with a standard deviation of 2.6×10−4 cm3 g−1 by the equation V=0.8341(1055.5−T)/(93...


Journal of Applied Physics | 1973

Glass transition of polyethylene: Volume relaxation

G. T. Davis; R. K. Eby

Data are presented to show that when linear polyethylene is quenched from room temperature to temperatures below 273 K, it exhibits a volume decrease for times long compared with that required to establish temperature equilibrium. The time, temperature, and density dependence of this decrease is shown to be consistent with a relaxation occurring in the amorphous portion (lamella boundary layers) of the samples. The data can be superposed and the shift factors follow the WLF formalism. Analysis by this method yields a Tg of 231 ± 9 K but the uncertainties preclude any correlation with specific volume over the range 1.01–1.05 cm3 g−1. The data indicate the absence of any comparably strong time dependence of the volume near 150 K. This method of detecting a glass transition in partially crystalline polymers is relatively freer of subjective judgment than most.


Ferroelectrics | 1984

Hysteresis in copolymers of vinylidene fluoride and trifluoroethylene

G. T. Davis; M. G. Broadhurst; Andrew J. Lovinger; T. Furukawa

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...


Ferroelectrics | 1983

Curie transitions in copolymers of vinylidene fluoride

Andrew J. Lovinger; T. Furukawa; G. T. Davis; M. G. Broadhurst

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...


Journal of Applied Physics | 1980

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

John E. McKinney; G. T. Davis; M. G. Broadhurst

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.


Journal of Applied Physics | 1974

Cell dimensions of hydrocarbon crystals: Surface effects

G. T. Davis; J. J. Weeks; G. M. Martin; R. K. Eby

The unit‐cell dimensions of a given polyethylene have previously been shown to vary nearly linearly with the reciprocal of lamella thickness. Data obtained at 153.2 and 296.2°K are presented to show that the slope of this dependence is different for crystals of orthorhombic n ‐paraffins, melt‐crystallized polyethylene, and solution‐crystallized polyethylene. Within the limits of error, all extrapolate to the same basal area at infinite lamella thickness, and this agrees with the measured value for a sample crystallized from the melt under high pressure to yield a long period of about 3500 A. Since the effect is a surface one, it is proposed that these differences result from the differences between methyl interactions, fold interactions, different fold planes, domains, etc. It is shown that the variation of cell dimension with lamella thickness leads to a quadratic term in the variation of macroscopic density. This term permits the separation of the thickness and density of a lower‐density surface layer i...


Polymer | 1982

Pyroelectricity and charge transport in a copolymer of vinylidene fluoride and tetrafluoroethylene

M. G. Broadhurst; G. T. Davis; Aime S. DeReggi; Steven C. Roth; R.E. Collins

Previous investigations have shown that a copolymer of vinylidene fluoride and tetrafluoroethylene can be made piezoelectric and pyroelectric by the temporary application of an electric field.∗1, 2 For applied fields in excess of about 250 kV/cm, the electrical response became independent of applied field, independent of poling temperature between 0° and 60°C, and independent of poling time between 5 minutes and 7 hours.1, 2 Such behavior implied saturation of polarization, and values of polarization required to account for the piezoelectric response were well within the maximum to be expected from alignment of dipoles within the crystalline phase of the copolymer. Measurements on similar films now indicate that the polarization distribution within the films is non-uniform and varies with poling temperature. This paper presents these results and will propose a model of charge transfer within the film to reconcile the evidence for saturation polarization and nonuniform charge distribution.

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M. G. Broadhurst

National Institute of Standards and Technology

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John E. McKinney

National Institute of Standards and Technology

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R. K. Eby

National Institute of Standards and Technology

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Aime S. DeReggi

National Institute of Standards and Technology

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G. M. Martin

National Institute of Standards and Technology

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Steven C. Roth

National Institute of Standards and Technology

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Brian Dickens

National Institute of Standards and Technology

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Frederick I. Mopsik

National Institute of Standards and Technology

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