Dielectric relaxations in poly(vinylidene fluoride)

Molecular motions in poly(vinylidene fluoride) were studied by the dielectric technique. Three distinct absorption peaks (α_c, α_a, 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 α_c absorption located at 97°C (1 kcps) is related to molecular motion in the crystalline region. The α_a 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.     

Dielectric relaxation and molecular motion in poly(vinylidene fluoride)

The dielectric properties of poly(vinylidene fluoride) have been studied in the frequency range 10 Hz to 100 kHz at temperatures between ?196 and 150°C. Three dielectric relaxations were observed: the α relaxation occurred near 130°C, the β near 0°C, and the γ near ?30°C at 100 kHz. In the α relaxation the magnitude of loss peak and the relaxation times increased not only with increasing lamellar thickness, but also with decrease of crystal defects in the crystalline regions. In the light of the above results, the α relaxation was attributed to the molecular motion in the crystalline regions which was related to the lamellar thickness and crystal defects in the crystalline phase. In the β relaxation, the magnitude of the loss peak increased with the amount of amorphous material. The relaxation times were independent of the crystal structure and the degree of crystallinity, but increased slightly with orientation of the molecular chains by drawing. The β relaxation was ascribed to the micro-Brownian motions of main chains in the amorphous regions. The Arrhenius plots were of the so-called WLF type, and the “freezing point” of the molecular motion was about ?80°C. The Cole-Cole distribution parameter of the relaxation time α increased almost linearly with decreasing temperature in the temperature range of the experiment. The γ relaxation was attributed to local molecular motions in the amorphous regions.     

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δε is analyzed on the basis of the two-site model. A linear relation between Δε/χξ and cos²θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.     

Dielectric absorption in oriented poly(vinylidene fluoride)

The dielectric behavior of poly(vinylidene fluoride) is affected by orientation and crystal modification. The loss peak caused by molecular motion of the molecules in crystalline regions appears at about 70°C (110 Hz) (α₁ absorption) for the α form, and at about 110°C (110 Hz) (α₂ absorption) for the β form. Orientation significantly affects the magnitude of the β absorption which appears at about ?40°C. The very high value of the dielectric constant for stretched film is believed to be due to the orientation effect. The γ absorption, which is assumed to be local-mode absorption, is not so much affected by orientation. An additional loss peak has been found at around 0°C in dynamic mechanical measurements, but the molecular mechanism is unknown.     

Molecular mechanism for elongation: poly(vinylidene fluoride) form II

Elongation of an unoriented sample of poly(vinylidene fluoride) form II at high temperature (above 140 °C) results in a uniaxially oriented sample of form II during so-called necking. The short-range and long-range order parameters of poly(vinylidene fluoride) form II and measures of the structural order on thec-projection are characterized by the intensity and half-width of the 120 reflection. The elongation under the same condition gives almost the same order parameters, independent of the order parameters before the elongation. Furthermore, the dependence of the order parameters on the elongation temperature behaves in the same way as the dependence of the quenched unoriented sample on the annealing temperature. This suggests that the elongation of the sample during necking corresponds to the crystallization or recrystallization after a much disordered state, approximately molten state; this supports the mechanism proposed by Yoon and Flory.     

Kink motion in poly(vinylidene fluoride) form I

The diffuse-streak x-ray scattering intensity from poly(vinylidene fluoride) form I, which is caused by kink bands with GT_n? (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 ΔH_v = ?4.6 kcal/mol. This suggests that the kink band is energetically more stable than the regular structure of form I.     

Persistent polarization in poly(vinylidene fluoride). II. Piezoelectricity of poly(vinylidene fluoride) thermoelectrets

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.     

Dielectric relaxations in poly(thiocarbonates)

The dielectric absorptions observed on six poly(thiocarbonates) (PTC) containing different substituents in the interphenylic carbon atom are subjected to a study. The α relaxation process associated with the glass transition temperature, which appears at the highest temperature studied, is overlapped with a strong conductive component. Between 80 and 100°C dielectric activity (β) is observed, which is related to structural relaxation phenomena. Polymers with double substitution on the interphenylic carbon atom show a γ relaxation with an activation energy of ca. 11 kcal mol^?1 while those where the carbon atom is linked to a cyclohexyl group show higher activation energies and their respective characteristics. A cryogenic absorption around ?120°C, for at least three polymers with an activation energy of about 5 kcal mol^?1, is detected. © 1994 John Wiley & Sons, Inc.     

Effect of ionic liquid on crystallization kinetics and crystal form transition of poly(vinylidene fluoride) blends

Journal of Thermal Analysis and Calorimetry - Semicrystalline poly(vinylidene fluoride) (PVDF) incorporated with ionic liquids(IL) exhibits applicability as electrolyte. In this paper,...     

The morphology of poly(vinylidene fluoride) crystallized from blends of poly(vinylidene fluoride) and poly(ethyl acrylate)

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.     

Dielectric phenomena and electrical energy storage of poly(vinylidene fluoride) based high-k polymers

Since the discovery of relaxor ferroelectric behavior was firstly reported in irradiated poly(vinylidene fluoridetrifluoroethylene) (P(VDF-TrFE)) copolymer, many strategies have been developed to enhance the electrical energy storage capability, including copolymerization, grafting, blending and fabricating of multilayer. This review article mainly summarizes the recent progresses on these strategies and aims to motivate the development of novel PVDF-based polymers for electrical energy storage and dielectric applications.     

Quantitative confirmation of the crystal-amorphous interphase in semicrystalline poly(vinylidene fluoride) and poly (vinylidene fluoride)/poly (ethyl methacrylate) blends

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.     

Persistent polarization in poly(vinylidene fluoride). III. Depolarization and pyroelectricity of poly(vinylidene fluoride) thermoelectrets

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.     

Carbon nanotubes induced poly(vinylidene fluoride) crystallization from a miscible poly(vinylidene fluoride)/poly(methyl methacrylate) blend

Different contents of carbon nanotubes (CNTs) were introduced into a miscible poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend. The interfacial affinity between CNTs and components of the blend was evaluated by calculating the interfacial tension. The dispersion and microstructure of CNTs in the nanocomposites were investigated through scanning electron microscope and rheological measurement. The effect of CNTs on the crystallization of PVDF was comparatively investigated through nonisothermal and isothermal crystallization processes. The results showed that CNTs exhibited stronger interfacial affinity to PMMA. Homogeneous dispersion of CNTs in the nanocomposites was achieved. Largely enhanced crystallization temperature and increased crystallinity of PVDF were obtained by adding CNTs during the nonisothermal crystallization process. The results obtained from the isothermal crystallization process proved that CNTs induced the concentration fluctuation in the sample, which resulted in the formation of spherulites with different types, i.e., the banded spherulites and compact spherulites. Furthermore, both the crystallization temperature and the content of CNTs exhibited great influence on the crystalline morphology of PVDF.     

Brillouin scattering of oriented films of poly(vinylidene fluoride) and poly(vinylidene fluoride) -poly( methyl methacrylate) blends

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.     

Solubility parameters of poly(vinylidene fluoride)

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 δ_t,P were evaluated. The latter was also estimated from limiting viscosity number data in the eight solvents found. Both experimental methods gave δ_t,P values in very good agreement. Comparisons among our findings, the literature, and calculated results are discussed.     

Infrared spectrum of poly(vinylidene fluoride)

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.