Surface hydroxylation of poly(vinylidene fluoride) (PVDF) film

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     

Thermal stability of poly(vinylidene fluoride) films pre-annealed at various temperatures

This paper investigates the relationship between the pre-annealing conditions and the thermal stability of uniaxially-drawn poly(vinylidene fluoride) (PVDF) films in order to clarify their technical limits in terms of temperatures that can be used for assembly processes and for practical applications. Specimens that are pre-annealed below their melting temperature apparently shrink in the stretch-direction when they are exposed to elevated temperatures above the pre-annealing temperature. Since the content of β-PVDF in the films decreases simultaneously with the shrinkage, their piezoelectric properties also deteriorate. In addition, there is a suggestion that the level of polarization in the remaining β-phase decreases significantly during annealing above 90-100 °C. However, the dimensions and the piezoelectric coefficients of the films remain stable during annealing below the pre-annealing temperature. Therefore, the thermal stability of PVDF films can be controlled practically by using the appropriate pre-annealing temperature. By contrast, the films were softened at 90-100 °C when the pre-annealing treatment was conducted above the melting temperature. The softening of films that are pre-annealed above the melting temperature is a different phenomenon from that observed in specimens that are pre-annealed below the melting temperature.     

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.     

Crystallization kinetics of poly(ethylene oxide) confined in semicrystalline poly(vinylidene) fluoride

Polymer blends based on poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO) have been prepared to analyze the crystallization kinetics of poly(ethylene oxide) confined in semicrystalline PVDF with different ratios of both polymers. Both blend components were dissolved in a common solvent, dimethyl formamide. Blend films were obtained by casting from the solution at 70 °C. Thus, PVDF crystals are formed by crystallization from the solution while PEO (which is in the liquid state during the whole process) is confined between PVDF crystallites. The kinetics of crystallization of the confined PEO phase was studied by isothermal and nonisothermal experiments. Fitting of Avrami model to the experimental DSC traces allows a quantitative comparison of the influence of the PVDF/PEO ratio in the blend on the crystallization behavior. The effect of melting and further recrystallization of the PVDF matrix on PEO confinement is also studied. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 588–597     

Far-infrared and x-ray studies on poled semicrystalline poly(vinylidene fluoride)

Far-infrared (FIR) Fourier-transform transmission spectra and x-ray diagrams of poly(vinylidene fluoride) films are examined in terms of poling-field action on the crystalline phase (α and β forms) and on the amorphous phase. We show that the poling field induces an increase of crystallinity in the α phase as well as in the β phase, and for the latter an orientation of dipoles in the field direction. For samples containing a mixture of phases α and β we note an α to β phase transition, a rotation of the dipoles in the β phase, and an increase of the crystallinity of the sample. Part of the β phase is generated from the amorphous phase.     

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.     

Fracture behavior of amorphous and semicrystalline blends of poly(vinylidene fluoride) and poly(methyl methacrylate)

The fracture behavior of blends of poly(vinylidene fluoride) and poly(methyl methacrylate) was investigated all over the composition range. A detailed analysis of the net stress versus crack opening displacement curves was performed. Fracture surface observations allowed statements on the process zone characteristics ahead of the crack tip. For the amorphous blends, the crack initiation energy is well related to the glass transition temperature. For the semicrystalline blends, the fracture energy is correlated with the degree of crystallinity. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Single-step synthesis of proton conducting poly(vinylidene fluoride) (PVDF) graft copolymer electrolytes

The single-step synthesis of proton conducting poly(vinylidene fluoride) (PVDF) graft copolymer electrolytes is demonstrated. The graft copolymers of PVDF backbone with poly(sulfopropyl methacrylate) (PVDF-g-PSPMA) and poly(styrene sulfonic acid) (PVDF-g-PSSA) were synthesized using PVDF as a macroinitiator for atom transfer radical polymerization (ATRP). ¹H NMR and FT-IR spectroscopy show that the “grafting from” method using ATRP was successful and the maximum grafting degrees were 35 and 25 wt% for PVDF-g-PSPMA and PVDF-g-PSSA, respectively. The IEC values were 0.63 and 0.45 meq/g, the water uptakes were 46.8 and 33.4 wt% and the proton conductivities were 0.015 and 0.007 S/cm at room temperature, for PVDF-g-PSPMA and PVDF-g-PSSA, respectively. Both membranes exhibited excellent thermal stability up to around 350 °C, verified by thermal gravimetric analysis (TGA).     

Ferroelectric poly(vinylidene fluoride) PVDF films derived from the solutions with retainable water and controlled water loss

To obtain β‐phase dominant ferroelectric poly(vinylidene fluoride) (PVDF) homopolymer thin films on aluminum‐coated silicon substrates, the retaining and loss of water were manipulated by introducing several hydrated and hygroscopic chemicals in the precursor solutions, including aluminum nitrate nonahydrate, aluminum chloride hexahydrate, chromium nitrate nonahydrate, tetra‐n‐butylammonium chloride, and one hygroscopic but nonhydrated chemical, ammonium acetate. Their ability of retaining water during the thermal annealing of the films and the relationship between water retaining and the effects on promoting the β phase were investigated. The results showed an ideal scenario was that the added hydrated salts should be able to retain substantial amount of water during the PVDF crystallization to effectively promote the β phase but completely dehydrate or decompose at the further elevated annealing temperature in order to obtain β‐phase dominant PVDF film without substantially incorporating water and deteriorating the electrical properties. As one of the hydrated chemicals well satisfying the above requirements, Al(NO₃)₃·9H₂O, of different amounts was introduced to the PVDF precursor solutions and the optimal resulting β‐phase dominant ferroelectric PVDF thin films exhibited smooth morphology, low dielectric loss, high remnant polarization of 89 mC/m², and large effective piezoelectric coefficient d₃₃ of ?14.5 pm/V (under the clamping of the substrate). © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2410–2418, 2009     

Viscometric behaviour of polymer blends based on poly (vinylidene fluoride)

The viscosity behaviour of dilute dimethylformamide solutions of poly(vinylidene fluoride)-poly (methyl methacrylate) and poly(vinylidene fluoride)-polystyrene has been studied at 25°C. The polymer concentration ranges are such that neither phase separation nor microgel formation occurs, although we are very close to theta conditions. The intrinsic viscosity and viscosity interaction parameter of the ternary mixtures have been calculated. The estimation of the compatibility of the above polymer pairs has been studied based on: a) specific viscosities; b) viscosity interaction parameters, according to Krigbaum and Wall formalism, and c) viscosity interaction parameters of a system formed by a dilute probe polymer in the presence of a matrix polymer and a small molecule solvent.     

Effect of molecular weight distribution on the rheological behaviour of poly(vinylidene fluoride) (PVDF)

In this study, six samples of commercial poly(vinylidene fluoride) are investigated. Dynamic and steady-state shear flow experiments are carried out in the melt state in order to relate their rheological behaviour to their molecular weight distributions. This rheological investigation is performed through the use of some simple parameters related to the viscosity, elasticity and relaxation times. Plots of flow curves in reduced coordinates are used to confirm the above results.     

Ferroelectric properties of nylon 11 and poly(vinylidene fluoride) blends

Both poly(vinylidene fluoride) (PVF₂) and nylon 11 are ferroelectric polymers, and have been extensively studied over the past two decades. Blend films were made from mixed powders of these two polymers, which were then melt pressed and cold drawn. The ferroelectric properties of these blend films were investigated. The remnant polarization, P_r, was found to vary with composition, and to be 60% larger than that of either component at a 50/50 (by weight) composition where P_r exhibited a maximum of about 90 mC/m². The magnitude of the coercive field, E_c, also exhibited a maximum at this composition. Both P_r and E_c are also observed to change significantly with the draw ratio. The results are discussed based on a two-phase dielectric composite model. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3217–3225, 1999     

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.     

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.     

Transcrystallization kinetics of poly(vinylidene fluoride)

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     

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.     

Thermal expansion of poly(vinylidene fluoride)

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.