Electroactive phases of poly(vinylidene fluoride) : determination, processing and applications

Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylide...

Ferroelectric properties of vinylidene fluoride copolymers

The crystal structures of three forms of poly(vinylidene fluoride) were studied by X-ray diffraction method. Although the structure of form I has been determined to be a fully extended planar zigzag by Lando, et al. [orthorhombic; a=8.58 A, b=4.91 A, and c(fiber axis)=2.56 A; space group Cm2m(C2v14)], an alternately-deflected molecular structure was postulated in order to release the steric hindrance between the fluorine atoms along the chain. A satistically disordered packing of such deflected chains satisfies the observed fiber period and improves appreciably the structure factor agreement. Form II is monoclinic [pseudo-orthorhombic; a=4.96 A, b=9.64 A, c(fiber axis)=4.62 A, and β=90°; space group P21/c(C2h5)], and its cell contains two molecular chains. The molecular conformation is essentially the TGTG type (internal rotation angles, 179° and 45°), and the glide plane of the molecular chain coincides with the c glide plane of the lattice. It is suggested that form III is monoclinic [a=8.66 A, b=4.93 A, c(fiber axis)=2.58 A, and β=97°; space group C121(C23)], and the structural features similar to that of form I.

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Crystal Structures of Three Crystalline Forms of Poly(vinylidene fluoride)

After an introduction reporting the properties and the applications of fluoropolymers, a first part deals with i) the main routes to produce vinylidene fluoride (VDF) monomer, ii) its homopolymerization, and iii) the advantages and uses of polyvinylidene fluoride (PVDF). In a second section, this review, illustrated by numerous examples, extensively reports the synthesis, properties and applications of the copolymers based on VDF with non-halogenated, fluorinated, commercially available or synthesized comonomers. These comonomers exhibit XYC=CZ-Sp-R structures where X, Y, and Z represent H, F, and CF3 groups, Sp a spacer and R a function such as OH, OAc, SAc, CO2R' (R' being a H atom or an alkyl group), CN, P(O)(OR')2 and SO3H. According to the nature and to the amount of the comonomer, the copolymers can be thermoplastic, elastomeric or thermoplastic elastomers. Introducing reactive R side groups brings complementary properties such as hydrophily, ionic exchange or surface properties, or further crosslinking of the resulting copolymers. Then, the kinetics of radical copolymerization of VDF with M comonomers led to the assessment of the reactivity ratios which are compared. Hence, a reactivity series of these M comonomers with respect to a macroradical terminated by VDF is proposed. Usually, these copolymers exhibit random structures but only three comonomers produced alternating copolymers with VDF: hexafluoroisobutylene, F2C=CFCO2CH3, and H2C=C(CF3)CO2R. The controlled radical copolymerizations of VDF with other comonomers (such as chlorotrifluoroethylene, 3,3,3-trifluoropropene, hexafluoropropylene, perfluoromethyl vinyl ether or-trifluoromethacrylic acid) either in the presence of xanthates, borinates or iodo-compounds are also reported. In addition, new VDF-containing copolymers exhibit well-defined architectures, such as block and graft copolymers. They can be synthesized either by conventional techniques or by controlled radical copolymerization. Chemical modifications of PVDF and poly(VDF-co-monomer) copolymers are also presented. Several properties and applications (such as surfactants, dielectrical polymers, thermoplastic elastomers, fuel cell and ultrafiltration membranes, or polycondensates, the fluorinated segments of which bringing softness and thermal stability) of these VDF-containing copolymers will illustrate this review.

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From vinylidene fluoride (VDF) to the applications of VDF-containing polymers and copolymers: recent developments and future trends.

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.

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

The phenomenon of polymorphism in poly(vinylidene fluoride) has been observed recently by several authors. It has also been reported that high-resolution NMR measurements demonstrate the presence in this polymer of head-to-head linkages, resulting from the “backward” addition of from 5-6% of the monomer units. Since the van der Waals radii of fluorine (1.35 A) and hydrogen (1.1-1.2 A) are similar, the cocrystallization in a polymer chain of units that differ only by the substitution of fluorine atoms for hydrogen atoms is not unexpected. The two polymorphic forms of poly(vinylidene fluoride), examined in this investigation, have different chain conformations. Chains in phase I have a planar zigzag conformation, while chains in phase II are assumed to exhibit a 21 helical conformation. The incorporation into the polymer chain of small amounts of tetrafluoroethylene or trifluoroethylene comonomer favored the crystallization of phase I. This is in accord with the relative abilities, deduced from con...

The polymorphism of poly(vinylidene fluoride). I. The effect of head-to-head structure

In this study, X-ray diffraction has been used to determine the crystal structure of poly(vinylidene fluoride) phase II. Three principal chain conformations were considered, which included two 21 helical forms [degenerate (planar) and nondegenerate] and the trans-gauche-trans-gauche' form. An X-ray crystal structure determination has shown that the trans-gauche-trans-gauche' (TGTG′) conformation is the correct one for phase II. Experimental X-ray intensity data were taken at both −145° and 20° C in order to study the effect of temperature on the resulting structure. From the comparison between observed and calculated intensities, there are two possible crystal symmetries, P21 and P1, depending upon whether the two chains are taken to be antiparallel or parallel to one another. For both cases, the reliability indices for the chosen structure are almost identical. For either symmetry the unit cell is metrically orthorhombic (all angles equal to 90° within experimental error) with the following latt...

Polymorphism of poly(vinylidene fluoride). III. The crystal structure of phase II

The crystallization and melting of poly(vinylidene fluoride) as a function of hydrostatic pressure was investigated. Poly(vinylidene fluoride) can be crystallized in at least two crystalline modifications at pressures of 1 atm (phase I and phase II). The crystallization of poly(vinylidene fluoride) under hydrostatic pressure over a range of conditions yields a new crystalline phase, designated phase III. The new phase melts at 185°C, approximately 25C° higher than phase I or phase II. A noticeable similarity in the melting behavior of phase II and phase III as a function of pressure was observed. Over a range of pressures (up to 3000 atm) the rate of change of the melting temperature with pressure for phases II and III has a single constant value within experimental error. The specific volume of melting (ΔVm) and the entropy of melting (ΔSm) of the two phases varied with pressure in a similar manner. The higher values of ΔVm and ΔSm observed for phase III apparently reflect the difference in dens...

The polymorphism of poly(vinylidene fluoride). II. The effect of hydrostatic pressure

A detailed study of high-pressure-crystallized poly(vinylidene fluoride) has indicated that a mixture of low-melting phase II and high-melting phase I is present, rather than a new crystalline phase (phase III) as originally suggested. The relative amounts of phase I and phase II resulting from crystallization under pressure are a function of pressure and the degree of supercooling. Pressure crystallization at 285°C and 5500 atm results in samples which were pure phase I with an increased melting point of 187°C.

The polymorphism of poly(vinylidene fluoride) IV. The structure of high-pressure-crystallized poly(vinylidene fluoride)

The high-pressure melting behavior of samples of vinylidene fluoride copolymerized with vinyl fluoride, trifluoroethylene, or tetrafluoroethylene indicate that the copolymers have a lower entropy of melting than the poly(vinylidene fluoride) homopolymers in the same phase. As the comonomer size increases, the entropy of melting decreases. High-pressure crystallization of copolymers of 91-9 mole% vinylidene fluoride-trifluoroethylene and 93-7 mole% vinylidene fluoride-tetrafluoroethylene results in a high-melting form of phase I (planar zig-zag).

W. W. Doll

Papers

The polymorphism of poly(vinylidene fluoride). I. The effect of head-to-head structure

Polymorphism of poly(vinylidene fluoride). III. The crystal structure of phase II

The polymorphism of poly(vinylidene fluoride). II. The effect of hydrostatic pressure

The polymorphism of poly(vinylidene fluoride) IV. The structure of high-pressure-crystallized poly(vinylidene fluoride)

The polymorphism of poly(vinylidene fluoride) V. The effect of hydrostatic pressure on the melting behavior of copolymers of vinylidene fluoride