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1.
The piezoelectricity of PVDF thermoelect rets formed with vacuum-coated aluminum electrodes has been investigated in detail. The piezoelectricity depends on the β-form crystal structure of PVDF homopolymer and copolymers. However, the piezoelectricity is not attributed to the stress dependence of the spontaneous polarization of β-form crystals, but rather to the persistent polarization arising from trapped charges. The trapping mechanism is discussed.  相似文献   

2.
Persistent polarization in poly(vinylidene fluoride) thermoelectrets prepared under high electric field has been studied by measurements of depolarization and pyroelectricity. Various polarizations are examined in detail; the polarizations related to a characteristic molecular motion near 60°C and the polarizing temperature are not responsible for the major piezoelectric effect in β-form electrets. The piezoelectricity is attributed to a polarization appearing near the melting temperature. The persistent polarization corresponding to d31 of 2 × 10?11 coul/N is about 5 × 10?6 coul/cm2. The pyroelectricity of β-form electrets is linearly correlated with the piezoelectricity.  相似文献   

3.
This report describes isothermal aging of piezoelectricity in poly(vinylidene fluoride) (PVDF) based on long-term heat treatments between 40 and 160°C. The results demonstrate that no piezoelectric decay occurs below about 60°C, that between 60 and 160°C the aging behavior follows logarithmic kinetics, and that aging is linearly dependent on temperature. Both uniaxial and biaxial PVDF show similar trends, but piezoelectric decay is more rapid for uniaxial film. Decay of permanent poling-induced polarization is identified as the likeliest cause of piezoelectric aging, and piezoelectric decay is found to be associated with long-range annealing effects which also produce macroscopic shrinkage of the PVDF film.  相似文献   

4.
To obtain information about microscopic processes involved in the polarization switching in uniaxially oriented poly (vinylidene fluoride) film, a least-squares estimation of nonlinear parameters was developed to yield parameters of an equation which describes the nucleation and domain growth process. Time domain measurements of polarization reversal revealed that switching times decreased as the annealing temperature Ta increased (67.0, 52.4, and 41.3 μs at –20°C under a 200 MV/m pulse field for the as-stretched samples, the samples being annealed at 120 and 160°C, respectively). The analysis showed that the value of the domain growth speed increased as Ta increased. This is consistent with x-ray diffraction data which indicated that the annealing process brought about better chain packing and increased crystallite perfection. The analysis also showed that the nucleation probability significantly increased as Ta increased. This result was interpreted in terms of a morphological transformation, which was indicated by the decrease in elastic modulus with increasing Ta with no corresponding loss of orientation. It is suggested that the annealing process brought about an increase in the number of nucleation sites as a result of a transformation from a fibrous structure to a crystal-amorphous series structure which has increased boundary zone area.  相似文献   

5.
Some of the piezoelectric coefficients of poly(vinylidene fluoride) are distinctly stress dependent. We report on their dependence on the frequency of an oscillatory stress excitation, on the amount of the remanent polarization in the sample materials, and on the magnitude of an offset stress. In order to explain the observed dynamic and nonlinear piezoelectric properties, a model is developed that attributes them to changes in the local polarization in an interphase between the crystalline and amorphous phases in this semicrystalline polymer.  相似文献   

6.
The diffuse-streak x-ray scattering intensity from poly(vinylidene fluoride) form I, which is caused by kink bands with GTn? (n odd) conformation contained in the crystallite, decreases with increasing temperature, while the intensity of the 001 reflection does not change. This is attributed to the disappearance of the kink bands in the crystallite, not to partial melting of crystallites containing kink bands. The disappearance of the kink bands suggests that kink motion takes place in the crystallite. Plots of the intensity of diffuse-streak scattering, estimated from the asymmetric part of the 001 reflection, against 1/T roughly give ΔHv = ?4.6 kcal/mol. This suggests that the kink band is energetically more stable than the regular structure of form I.  相似文献   

7.
The surface of PVDF film was selectively modified by wet chemistry. Treatment with aqueous LiOH produced HF-elimination and the emergence of an oxygen-containing functionality. The XPS analysis clearly indicated the presence of ketone-, ether(epoxide)-, and alcohol motifs. The percentage of alcohols could be significantly increased by reduction of the ketones with NaBH4 in 2-propanol, followed by reduction of the epoxides with DIBAL-H in hexane. Thus, the full treatment led to a PVDF surface displaying 7 to 16% of oxygen-containing units, of which about 60% consisted in alcohol motifs. The reactvity of the surface-displayed hydroxyl functions was assayed by radiolabeling with [3H]-Ac2O. © 1997 John Wiley & Sons, Inc. J. Polym Sci A: Polym Chem 35: 1227–1235, 1997  相似文献   

8.
The effects of annealing temperatures on stretched poly(vinylidene fluoride) film were systematically studied from the “as-received” condition up to the melting range (180°C). X-ray diffraction studies indicate that the annealing process brings about better chain packing and increased crystallite perfection. The elastic modulus and piezoelectric strain and stress constants, d3, and e31, decrease as the annealing temperature Ta increases up to 160°C, while the remanent polarization Pr remains almost constant. Some of these characteritics may be interpreted in terms of a morphological transformation of microfibrils. The values of Pr, d31, and e31 increase dramatically as Ta increases from 160 to 180°C; Pr increases from 56 to 85 mC/m2, d31 from 20.2 to 27.7 pC/N, and e31 from 51.4 to 65.2 mC/m2. As a result, the values of Pr and d31 were the largest recorded from any of the samples used in the present study. e31 showed a value close to the largest one; this usually occurs in unannealed samples. Samples annealed in the melting range also exhibit significantly improved ageing characteristics. The large value of Pr and the small relaxation strength of both the dielectric constant and elastic modulus indicates that the largest crystallinity obtained is approximately 70%.  相似文献   

9.
A combined optical and electron microscopical study has been carried out of the crystallization habits of poly(vinylidene fluoride) (PVF2) when it is crystallized from blends with noncrystallizable poly(ethyl acrylate) (PEA). The PVF2/PEA weight ratios were 0.5/99.5,5/95, and 15/85. Isothermal crystallization upon cooling the blends from the single-phase liquid region was carried out in the range 135–155°C, in which the polymer crystallizes in the α-orthorhombic unit cell form. The 0.5/99.5 blend yielded multilayered and planar lamellar crystals. The lamellae formed at low undercoolings were lozenge shaped and bounded laterally by {110} faces. This habit is prototypical of the dendritic lateral habits exhibited by the crystals grown from the same blend at high undercoolings as well as by the constituent lamellae in the incipient spherulitic aggregates and banded spherulites that formed from the 5/95 and the 15/85 blends, respectively. In contrast with the planar crystals grown from the 0.5/99.5 blend, the formation of the aggregates grown from the 5/95 blend is governed by a conformationally complex motif of dendritic lamellar growth and proliferation. The development of these aggregates is characterized by the twisting of the orientation of lamellae about their preferential b-axis direction of growth, coupled with a fan-like splaying or spreading of lamellae about that axis. The radial growth in the banded spherulites formed from the 15/85 blend is governed by a radially periodic repetition of a similar lamellar twisting/fan-like spreading growth motif whose recurrence corresponds to the extinction band spacing. This motif differs in its fan-like splaying component from banding due to just a helicoidal twisting of lamellae about the radial direction. © 1993 John Wiley & Sons, Inc.  相似文献   

10.
The oriented superstructure of poly(vinylidene fluoride) is controlled by using a forced-quenching type of zone drawing apparatus. Systematic variation of the weight fraction χ(I) of form-I crystals and the orientation function fa of amorphous chains shows that the piezoelectricity increases with increasing χ(I) and fa. A change in the state of molecular aggregation during poling is also effective in increasing the piezoelectricity and the orientation of the crystal b axis along the poling direction. Equations relating piezoelectricity to the form-I crystallinity, the orientation of amorphous chains, and the orientation of the crystal b axis along the poling direction are derived. These are based on a mechanical model having regions of taut tie molecules in parallel with composite regions consisting of crystalline and amorphous blocks in series.  相似文献   

11.
Molecular motions in poly(vinylidene fluoride) were studied by the dielectric technique. Three distinct absorption peaks (αc, αa, and β) were observed in the frequency range from 0.1 cps to 300 kcps and in the temperature range from ?66 to 100°C. The molecular mechanisms for these absorptions and their temperature dependence are discussed, and results are compared with x-ray diffraction and the NMR measurements. It is concluded that the αc absorption located at 97°C (1 kcps) is related to molecular motion in the crystalline region. The αa absorption located at ?27°C (1 kcps) can be interpreted as due to the micro-Brownian motion of the amorphous main chains. The β absorption located at ?47°C (1 kcps) is attributed to local oscillation of the frozen main chains.  相似文献   

12.
Dielectric and thermal characterizations were performed for poly (vinylidene fluoride) (PVDF)/poly (ethyl methacrylate) (PEMA) blends of different composition. The characteristics of PVDF β relaxation were shown to be little affected in the semicrystalline blends with PEMA. The relaxation strength, however, depends strongly on the PEMA content and a linear relation was found between the intensity of the β relaxation and the weight fraction of the PVDF crystal-amorphous interphase. Phase structures of the PVDF/PEMA blends are also proposed. © 1994 John Wiley & Sons, Inc.  相似文献   

13.
Temperature dependent Brillouin scattering studies of PVF2 films stretched to various stretch ratios have been carried out. Elastic constants for unstretched and stretched films have been obtained as functions of temperature. The elastic constant C33 of the stretched films has a greater temperature dependence than that of unstretched films. To elucidate the effect of the surrounding amorphous matrix on the Brillouin spectrum of semicrystalline PVF2 film, we carried out Brillouin scattering studies of films made from blends of PVF2 and PMMA.  相似文献   

14.
A vibrational analysis has been carried out for the two crystalline forms of poly(vinylidene fluoride) (PVF2). The Raman spectrum of the planar form of PVF2 is also reported. The band assignments are made on the basis of the spectral properties including the infrared dichroism and Raman intensities. A force field is derived based on a force constant refinement procedure utilizing the frequency data for both crystal forms.  相似文献   

15.
16.
The thermal expansion behavior of oriented poly(vinylidene fluoride) films has been studied over the temperature range ?75 to +20°C. Representative high draw, low draw, and voided samples have been examined. For all samples at low temperatures the transverse thermal expansion coefficients, both in the plane of the sheet and perpendicular to it, are similar and have positive values of about 10?4 K?1. In the draw direction the thermal expansion coefficients are much smaller in magnitude and can be either positive or negative, the room temperature values varying in the range +4 × 10?6 K?1 for low draw samples to ?14 × 10?6 K?;1 for high draw samples. As the temperature is raised the coefficients also increase but, above the glass transition temperature, the value in the draw direction, α1, shows a rapid fall in value. It is shown that this effect can be related quantitatively to the presence of an internal shrinkage stress. Differences between samples can then be primarily related to differences in the magnitude of this internal stress and to differences in the temperature dependence of the modulus of the sample.  相似文献   

17.
We report the transcrystallinity of poly(vinylidene fluoride) on several different types of substrate materials. The supermolecular structure and its development were characterized with polarization microscopy, whereas differential scanning calorimetry was used for monitoring the isothermal and nonisothermal crystallization kinetics. Although only approximately applicable, an Avrami–Ozawa analysis of the latter yielded reliable exponents, which characterized the transcrystalline nucleation conditions, the related dimensionality of growth, and the resulting texture. The results complemented and agreed quantitatively with those of light microscopy. Several polymers, including poly(ethylene terephthalate), polytetrafluoroethylene, and polyimide, induced distinct transcrystallinity, but only a spherulitic supermolecular structure developed on glass and metallic substrates. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2130–2139, 2001  相似文献   

18.
19.
The solubility behavior of poly(vinylidene fluoride) (PVDF) in about 50 liquids was investigated. The results were input to a computer program to obtain a three-dimensional representation of the polymer solubility region in the Hansen space; the values of dispersion, hydrogen bonding, and polar components of the total solubility parameter δt,P were evaluated. The latter was also estimated from limiting viscosity number data in the eight solvents found. Both experimental methods gave δt,P values in very good agreement. Comparisons among our findings, the literature, and calculated results are discussed.  相似文献   

20.
Two crystalline forms (α and β) of poly(vinylidene fluoride) were studied by infrared spectroscopy. The spectral differences permitted the study of the transformation and the ratio of the two forms. The ordinary \documentclass{article}\pagestyle{empty}\begin{document}$ \vec G,\vec F $\end{document} matrix method was used to calculate the fundamental mode with a Urey-Bradley type potential field, and a preferred set of the force constants was obtained.  相似文献   

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