The diffuse behavior of the ferroelectric transition in poly(vinylidene fluoride-trifluoroethylene) copolymers

The ferroelectric-to-paraelectric phase transition in poly(vinylidene fluoride-trifluoroethylene) copolymers has been investigated using calorimetric and dielectric technics. In these materials, as it is well known, the ferro- and paraelectric phases coexist at large temperature intervals, which produces a smearing of the physical anomalies within the transition region. Thus, in order to treat our data, we have used an extended Landau-Devonshire treatment, which has provided a quantitative analysis of the macroscopic behavior of the systems. In particular, the Landau expansion parameters, the sample crystallinities, the spontaneous polarizations, and the coercive electric fields have been estimated for the copolymers with 25, 30, and 40 mol% of trifluoroethylene. © 1994 John Wiley & Sons, Inc.     

Ferroelectric and piezoelectric properties of a quenched poly(vinylidene fluoride-trifluoroethylene) copolymer

The ferroelectric and piezoelectric properties of melt-quenched unoriented poly(vinylidene fluoride-trifluoroethylene) (73 : 27) copolymer films as a function of the number of poling cycles have been studied. The investigation revealed that quenched films exhibit a decrease in D-E hysteresis behavior as the number of poling cycles increases when the samples are poled at room temperature. Corresponding decreases in remanent polarization, Pr, as well as small increases in the coercive field, Ec, were observed as the material was subjected to successive poling cycles. The piezoelectric coefficients, d₃₁ and e₃₁, also decreased as the number of poling cycles increased. In addition, a clear reduction in the “apparent” Curie transition temperature between unpoled and poled material was observed. Preliminary evidence indicates that films quenched from the melt to below T_c do not form a stable ferroelectric crystal phase as previously believed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2671–2679, 1997     

Thermoreversible gelation of blend of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) in γ-butyrolactone solution

Thermoreversible gelation behavior of blend of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) in γ-butyrolactone solution was studied. Sol-gel transition temperature increased with the increase of polymer concentration, but was independent of the blend ratio of two polymers. An equation for gelation rate was derived, assuming that the gelation is a first-order reaction and that the gelation rate obeys an Arrhenius type. According to the equation, the growth index of gelation and supercooling temperature had a dominant effect on gelation rate. The growth index of gelation, which was calculated from the dependence of activation energy on the supercooling temperature in the isothermal gelation, varied with the blend ratio of two polymers. Growth index of gelation larger than 2 was obtained for the blend gels studied in this experiment. It may suggest that the multidimensional growth of gels occurs in such polymer blend solutions. X-ray diffraction and differential scanning calorimetry measurements showed existence of separate crystals due to each component of polymer in the blend gels. © 1996 John Wiley & Sons, Inc.     

Low-frequency dielectric behavior of poly(vinylidene fluoride)

An explicit mechanism is described for the anomalous increase in dielectric constant and dielectric loss at low frequencies and high temperatures for poly(vinylidene fluoride) containing ionic impurities. Relations are proposed for the ionic contributions, ε_i″ and ε_i″, to the dielectric constant and dielectric loss: where v₀ and D₀ are the concentration and the diffusion coefficient of the mobile ions at infinite temperature, q is the charge of an ion (in cgs electrostatic units), l is the distance between electrodes, k is the Boltzmann constant, T is the absolute temperature, E_d is the apparent activation energy for diffusion of the ions, and W is the dissociation energy of the ionic impurities. From the slopes of curves of log εT′ versus 1/T and log ε″T versus 1/T for poly(vinylidene fluoride), energies E_d = 34 kcal/mole and W = 342 kcal/mole were obtained.     

Improving ferro- and piezo- electric properties of hydrogenized poly(vinylidene fluoride-trifluoroethylene) films by annealing at elevated temperatures

After annealing the solution cast P(VDF-TrFE) films at elevated temperatures, which were synthesized via a full hydrogenation process from P(VDF-CTFE) with a composition of VDF/TrFE = 80/20(mol%), a series of P(VDF-TrFE) films were fabricated in present work. The crystalline and ferroelectric phases of the films were carefully characterized and their dielectric, ferroelectric and piezoelectric properties were systematically investigated. The improved crystalline and ferroelectric phases in the films induced by annealing at elevated temperatures are responsible for the significant improved electric properties of the films. The optimized annealing temperature is found to be 130 °C and the best performance including the highest dielectric constant of 12.5 at 1 kHz, the largest maximum polarization of 11.21 μC/cm~2 and remnant polarization of 7.22 μC/cm~2, the lowest coercive electric field of 56 MV/m, and the highest piezoelectric coefficient of -25 pC/N is observed.     

Thermal dehydrochlorination of poly(vinylidene chloride)

The kinetics of the early stages of thermal degradation below 1% dehydrochlorination of emulsion-polymerized poly(vinylidene chloride) (PVDC) is studied by the variation of the pH value of potassium hydroxide aqueous solution between 160 and 190°C in the presence of air and other gas streams. The results turned out that the thermal degradation of PVDC can be divided into three stages, which correspond to an induction period, a period with conversion below 0.1% dehydrochlorination, and that with conversion ranging from 0.1 to 1%. For the induction stage, the induction time depends upon the types of environment gas and degradation temperature. Both of the second and the third stages are zero-order reactions, which also result in the discoloration and crosslinking of the neat polymer. The average apparent activational energy of the zero-order degradation reaction was about 21 kcal/mol, which is independent of the types of environment gas. The whole degrading kinetics data can be well explained by the mechanism of a free-radical-induced dehydrochlorination. The viscosity of the degraded sample increases rapidly with degradation and becomes insoluble in regular solvents. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2035–2044, 1999     

Structural dependence of phase transition and dielectric relaxation in ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s

A series of ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s, P(VDF-CTFE-TrFE), with systematically varied chemical compositions have been synthesized via a two-step approach consisting of copolymerization and dechlorination. The effect of polymer structure on polarization responses and dielectric properties has been investigated over a broad frequency and temperature range. As shown in the X-ray diffraction patterns, multiple phases coexist within the terpolymers as a result of the gauche conformation induced by the CTFE unit. The polarization hysteresis loops reveal the variation of remanent polarization and coercive electric field with the CTFE content due to the changes of crystallinity and crystalline phase. The observed broad dielectric constant peak with Vogel-Fulcher dielectric dispersion behavior suggests a transformation from a normal ferroelectric to a ferroelectric relaxor of the polymers. The relationship between the local relaxation process and relaxor ferroelectric behavior has been examined on the basis of the dielectric and mechanical loss tangents as a function of temperature.     

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.     

Alternating-current ionic conduction and dielectric relaxation of poly(vinylidene fluoride) at high temperatures

Dielectric properties of poly(vinylidene fluoride) have been studied in the frequency range 20 Hz to 1 MHz and between 100 and 220°C, during heating and cooling. The dielectric constant and loss change abruptly at the temperature T_m corresponding to the melting point. At lower frequencies, two types of ionic conductin are observed. One appears below T_m and is attributed to interfacial polarization. The other occurs above T_m and is related to electrode polarization. These results suggest that a crystalline polymer is a heterogeneous medium for ionic transport, while the melt is a homogeneous medium. From these results, the nature of ac ionic conduction in crystalline polymers is discussed. At high frequency, the α relaxation is observed below T_m. It is due to the molecular motion in the crystalline region and disappears at T_m.     

Homogeneous liquid crystal alignment on poly(vinylidene fluoride-trifluoroethylene) films subjected to ion-beam irradiation

We demonstrate homogeneous and uniform liquid crystal (LC) alignment on poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE] films using ion-beam (IB) irradiation and a performance improvement of twisted nematic (TN) cells using IB-irradiated PVDF-TrFE films. Spontaneous ferroelectricity of the PVDF-TrFE films was modified by IB irradiation, which affected the LC alignment properties. The variation in the pre-tilt angles of the LC molecules on the IB-irradiated PVDF films is attributed to surface reformation, including defluorination and oxidation because the pre-tilt angles of LC molecules can be controlled by adjusting the fluorine content. The results of contact angle measurements supported this phenomenon. A 58% reduction in the switching voltage was observed for TN cells, indicating that the IB-irradiated PVDF-TrFE films are a promising candidate for use as an alignment layer.     

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.     

Microwave frequency dielectric properties of poly(vinylidene fluoride) films

The frequency dependence of dielectric constant ?′, dielectric loss ?″, and dielectric anisotropy were determined for poly(vinylidene fluoride) (PVDF) in microwave frequencies from 4 to 13 GHz. The ?′ and ?″ for PVDF films decreased with increasing frequency. Both ?′ and ?″ were larger in the transverse direction than in the machine direction or draw direction, but the values at 12 GHz were smaller than those observed at 4.0 GHz. The angular dependence of ?″ at microwave frequency reflects the orientational distribution of molecules in the amorphous region. The orientation function was determined to be about 0.04 and 0.01 for uniaxially and biaxially stretched PVDF, respectively. © 1995 John Wiley & Sons, Inc.     

Multi‐functional poly(vinylidene fluoride) graft copolymers

Poly(vinylidene fluoride) (PVDF) is known for its biocompatibility, piezo and pyro‐electricity, and membrane forming capability. In order to tune its properties, modification through grafting from approach by atom transfer radical polymerization (ATRP) is preferred. Hydrophilic polymers like poly(ethylene glycol) methacrylate, poly(methacrylic acid), poly(dimethylaminoethyl methacrylate) (PDMAEMA), and so forth have been anchored from PVDF backbone in order to make permeation of water molecules through the PVDF based membranes. The successful solution grafting of PDMAEMA chains from PVDF backbone by ATRP resulted appreciable graft conversion and hence its bulk properties showed a significant change. This water soluble graft copolymer shows incredible mechanical and adhesive properties. PVDF‐g‐poly(n‐butyl methacrylate) generates honey‐comb porous film using “breath figure” technique. Recently, they have used further improvement of grafting where model ATRP initiators are anchored using atom transfer radical coupling and used them as macroinitiators for grafting. This approach simplified the grafting reactions even more and enabled successful grafting of a large number of monomers under relatively less drastic conditions with appreciable conversion compared with the previous conditions. This technique has resulted interesting solution properties, ion and electron conducting PVDF, antifouling membrane, super glue and super tough materials, capable of generating metal nanoparticles tunable with pH and temperature. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2569–2584     

Thermal degradation of poly(siloxane-urethane) copolymers

The aim of this study was to investigate the characteristics and mechanism of the degradation of poly(siloxane-urethane) (PSiU) copolymers by thermogravimetric analysis (TGA) and TGA coupled with Fourier-transform infra-red spectroscopy (TG-FTIR). The PSiU copolymers consisted of 4,4′-diphenylmethane diisocyanate (MDI), 1,4-butanediol (1,4-BD), and OH-terminated polydimethylsiloxane (PDMS). In TGA they exhibited a two-stage degradation at 250-650 °C. The two stages of degradation have been found to comprise eight degradation steps and two interchange reactions, as revealed by TG-FTIR analysis. The main decomposition products have been identified as CO₂, tetrahydrofuran, cyclosiloxane, and macrocyclic species. In addition, the effects of hard segment content (HSC) on the degradation and thermal stability of PSiU copolymers have been investigated by means of TG and DTG curves; notably, a stability region at 410-470 °C is caused by the cyclosiloxane, as verified by TG-FTIR.     

Submillimeter infrared absorption and chain conformations of poly(vinylidene fluoride) and copolymers. II. Phase transition analysis

Absorption spectra, and hence the dielectric properties, of poly(vinylidene fluoride-tri-fluoroethylene) copolymers have been obtained by far infrared spectroscopy in the frequency range 15–55 cm^?1. We have studied the evolutions of the absorption coefficient and imaginary part of the dielectric constant as a function of poling field and copolymer composition. During the phase transition from form II to form I, the whole absorption presents a strong increase. This behavior is explained by polarizability changes at phase transition.     

Tailoring porous structure of ferroelectric poly(vinylidene fluoride-trifluoroethylene) by controlling solvent/polymer ratio and solvent evaporation rate

Ferroelectric macroporous poly(vinylidene fluoride-trifluoroethylene) membranes have been produced by isothermal crystallization from the solution at different temperatures starting from different diluted solutions of the co-polymer in dimethylformamide. In this way pore architecture, consisting in interconnected spherical pores can be obtained. The mechanism and kinetics of solvent evaporation was investigated and related to the evolution of the polymer microstructure. The mechanism underlying the pattern formation has been discussed on the light of the Flory–Huggins (FH) lattice theory, grain boundary effects and the Cahn–Hilliard equation for mass conservation systems. It was also observed that the temperature or initial concentration of the crystallization process does not affect the phase, ferroelectric transition temperature or the melting temperature of the polymer.     

Thermal stability of poly(mono-n-alkylitaconates) and mono-n-alkylitaconate-co-vinylpyrrolidone copolymers I

Poly(monoitaconates) containing octyl, decyl and dodecyl groups and random monoalkylitaconate-co-vinylpyrrolidone copolymers were studied by thermogravimetric analysis. Copolymers of mono-n-octylitaconate (MOI), mono-n-decylitaconate (MDI), and mono-n-dodecylitaconate (MDoI), respectively, with N-vinyl-2-pyrrolidone (VP) of different compositions were studied by dynamic thermogravimetric analysis. The thermal stability of the copolymers depends on the structure of the monoitaconate comonomer and on the composition of the copolymer The kinetic analysis of the degradation data shows that the thermal decomposition of these copolymers can be described by several kinetic orders depending on the copolymer and on the composition. The relative thermal stability of the copolymers increases as the VP content increases and as the length of the side chain of the itaconate increases, following the same trend as the flexibility of the copolymers in solution.     

Blends of glycidyl methacrylate/methyl methacrylate copolymers with poly(vinylidene fluoride)

Blends of glycidyl methacrylate (GMA)/methyl methacrylate (MMA) copolymers with poly (vinylidene fluoride) (PVDF) were found to be miscible when the GMA content of the copolymer is 35.7 wt % or less. The miscible blends did not phase separate upon heating prior to thermal decomposition. The melting point depression method, based on both the Flory-Huggins theory and the equation of state theory of Sanchez-Lacombe, was used to evaluate interaction parameters for each pair. The magnitude of these parameters appears to be much larger than interaction energies evaluated by other methods. Possible reasons for this are discussed. © 1995 John Wiley & Sons, Inc.     

Evolution of nanopolar phases, interfaces, and increased dielectric energy storage capacity in photoinitiated cross-linked poly(vinylidene fluoride)-based copolymers

Photoinitiated cross-linking of poly(vinylidene fluoride-co-chlorotrifluoroethylene) can offer a significant increase in electric energy storage capacity. This improvement is related to the structural changes in the copolymer crystals brought by cross-linking. Cross-linking favors formation of polar crystalline phase, drastic reduction of spherulite sizes, and increase in copolymer inner interface area. This copolymer case demonstrates the greatly enhanced energy storage behavior, including increased discharge energy density at reduced field strength, and improved capacitor efficiency at relatively high degree of cross-linking, which may facilitate a better design for polymer dielectric materials in their application of high energy density capacitors.