Shaping of the structure of vinylidene fluoride-tetrafluoroethylene copolymer membranes

A ¹⁹F NMR study revealed differences in the interaction of hard (phosphoric acid) and soft (acetic acid) precipitants with vinylidene fluoride-tetrafluoroethylene copolymer in dimethylformamide. Phase separation in solutions with additions of mixtures of phosphoric and acetic acids was examined.     

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     

Heterogeneous chemical crosslinking and high-voltage polarization in a ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene

The low-temperature chemical crosslinking of a copolymer of vinylidene fluoride and tetrafluoroethylene was studied through various physicochemical methods. The reaction was conducted in solution in the presence of diethylenetriamine as a crosslinking agent. The penetration of the mixture molecules and the crosslinking agent only in the amorphous phase of the copolymer was provided via selection of the ratio between a good solvent (dimethylformamide) and a poor solvent (ethanol). Owing to this, the crosslinking reaction occurred in the amorphous phase and hardly involved the crystals. This outcome was confirmed by wide-angle X-ray diffraction data. The structural and chemical changes in the amorphous phase during crosslinking were recorded with the use of IR spectroscopy and differential scanning calorimetry. It is shown that crosslinking is accompanied by the formation of C=C bonds in the copolymer chains. The study of high-voltage polarization and conductivity during exposure to bipolar rectangular pulses suggested that crosslinking leads to an increase in the carrier concentration. It was found that the surface potentials in the films increase with an increase in the number of high-voltage pulses applied to the sample. This circumstance is attributed to the fact that the double bonds formed in the copolymer chains can effectively trap negatively charged carriers.     

Molecular mobility and structuring in textured films of the ferroelectric copolymer of vinylidene fluoride with tetrafluoroethylene

The processes of molecular mobility in vinylidene fluoride–tetrafluoroethylene ferroelectric copolymer–based films textured via uniaxial drawing are studied. It is found that the cooperative mobility in the amorphous regions of the copolymer is described by the Vogel–Fulcher equation, whose parameters are sensitive to the drawing conditions. It is shown that a decrease in the drawing temperature causes a reduction in the effective activation energy of the above-mentioned motion. In accordance with the X-ray diffraction data, this circumstance may be explained by an increase in the packing density of chains in interlamellar spaces. This effect is partially provided by deformation-induced processes of formation of small and defective crystals of the paraelectric or antiferroelectric phase in spaces between neighboring lamellar crystals at the primary stage of crystallization. The relationship between the value of the orientational polarization in oriented films and the topology of structure formation in microfibrils is discussed. It is observed that enlargement of the interlamellar space (filled with a liquidlike amorphous phase) between neighboring crystals in microfibrils promotes an increase in the orientational polarization of chains in the amorphous phase.     

Effect of electrode material on contact high-voltage polarization in a vinylidene fluoride-hexafluoropropylene copolymer

High-voltage polarization processes occurring in alternating fields of various frequencies in extruded films of vinylidene fluoride-hexafluoropropylene copolymers are studied. Remanent polarization typical for ferroelectric polymers with a polar cell appears during crystallization in the nonpolar α phase. This effect is associated with the presence of a highly imperfect ferroelectric (or antiferroelectric) phase in the copolymer, where chains assume planar zigzag conformations. The magnitude of remanent polarization depends on the frequency of the applied bipolar sawtooth voltage. This observation may be explained by the fact that the resulting field is created not only by the external source but also by the space charge that is contributed by carriers injected from electrodes. With the use of deposited Au and Al, the role of electrode material in polarization and conduction processes is ascertained. In the case of Al, Al₂O₃ and new Al-C chemical bonds are formed on the polymer surface. Owing to formation of these additional dielectric layers at the copolymer/metal interface, electrodes may be blocked partially. The above new chemical bonds facilitate formation of deeper surface traps for carriers; as a result, the remanent surface potential appears in polarized samples even after storage in the short-circuit mode.     

The crystal structure of the alternating copolymer of hexafluoroisobutylene and vinylidene fluoride

The crystal structure of the alternating copolymer of hexafluoroisobutylene and vinylidene fluoride, P(HFIB/VF₂), was determined using x-ray diffraction techniques. Oriented specimens of the copolymer were produced by drawing melt-pressed film at 300°C. The unit cell of P(HFIB/VF₂) is orthorhombic with dimensions at a = 1.064, b = 1.837, and c = 0.783 nm. The chain conformation is a 2₁ helix with an approximate trans-trans-gauche-gauche backbone dihedral-angle sequence. Severe intrachain steric crowding exists; as a consequence, the chain is quite rigid and some of the backbone bond angles are highly extended. The unit cell contains four such chains packed according to the symmetry of space group Iba2. Neighboring chains in a crystal are mechanically interlocked; in combination with the rigidity of the chains, this is the basis for the superior mechanical properties of the copolymer compared to PTFE. A structure containing up to 20-30% of head-to-head residues was found to be statistically indistinguishable from a completely head-to-tail structure.     

500 MHz 1H NMR studies on the configuration and conformation in the piezoelectric copolymer of vinylidene cyanide and vinyl acetate

The 500-MHz ¹H NMR spectrum of vinylidene cyanide-vinyl acetate copolymer, P(VDCN/VAc), provided detailed information concerning the configuration (ϵ-tacticity) and conformations. The ¹H NMR data also demonstrated that P(VDCN/VAc) is an equimolar and highly alternating copolymer without the regioirregular defects (complete head-to-tail arrangement), which is in good agreement with the previous C-13 NMR result. The calculated ϵ-isotacticity σ(ϵ) is equal to 0.5, suggesting that P(VDCN/VAc) is completely atactic. Due to this nonstereoregular structure, P(VDCN/VAc) in the bulk shows the amorphous phase by X-ray and DSC measurements. Making use of ¹H-homodecoupling, the splittings due to ϵ-tacticity as well as proton-proton spin coupling could be separately assigned. From the vicinal coupling constants, J(AX) and J(BX), obtained in methylene proton resonance, conformer populations around H(A)H(B)C-CH(X)-CH(A ‘)H(B’ ), were calculated. The conformation of P(VDCN/VAc) in DMF solution consists of only TG and TT, forming not a random coil structure but a helical structure with a large radius of curvature.     

Ferroelectric switching characteristics of 73/27 copolymer of vinylidene fluoride and trifluoroethylene

Ferroelectric polarization-reversal switching of vinylidene fluoride—trifluoroethylene copolymer with 73 mol % of vinylidene fluoride is investigated under a variety of conditions in an attempt to derive detailed information on the polarization reversal of the polymer. The electric-field dependence of switching time t_s obeys an exponential law. The temporal change of switching current density J is presented in the form J/(P_s ? P) versus time (P is the polarization, and P_s is its saturation value), which is free of the depolarization-field effect and discriminates in a straightforward manner between acceleration and deceleration of switching. Effects of the annealing of samples upon t_s are observed together with changes in the degree of crystallinity. Effects of space charge distribution upon t_s are investigated by applying two successive pulses, positive and negative, with varied intervals t_w between the two. The dynamics of space charge redistribution is elucidated from the t_w dependence of the t_s of the second pulse. The temperature dependence of t_s indicates that the switching becomes critically fast as the phase-transition temperature is approached.     

Deformation and fracture behavior of poly(vinylidene fluoride‐trifluorethylene) ferroelectric copolymer films under uniaxial tension

The deformation and fracture behavior under uniaxial tension was characterized for P(VDF‐TrFE) 68/32 mol % copolymer films prepared under two different processing conditions. It was found that the copolymer films prepared by solution casting and then annealing show a typical polymeric brittle fracture feature. For the copolymer films prepared by stretching the solution‐cast films and then annealing process, a typical linearly strengthening stage occurs in the stress–strain curve after yielding, and the polymer film samples fracture at a much larger maximum strain and a higher tensile strength than those prepared by the former process. SEM observation and XRD analysis were carried out to examine the morphology and microstructure change during uniaxial tension. The results show that for the stretched film samples, the polymer chains undergo slipping or further reorientation during uniaxial tension, causing the increase of the peak intensity in the X‐ray diffraction pattern. For the directly annealed ones, no yielding phenomenon is observed and there is no apparent X‐ray diffraction intensity change. It was suggested that the highly‐oriented fibril structure of the stretched film samples contributes to the linearly strengthening stage after yielding in the stress–strain curve. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3255–3260, 2005     

Poling-time dependence of the field-induced phase transition and piezoelectric response of poly(vinylidene fluoride) films

We have examined the poling time dependence of the field-induced phase transition (from phase II to polar phase II) of biaxially oriented poly(vinylidene fluoride) films by x-ray methods. These results were compared with the poling-time dependence of the piezoelectric response (d₃₁ and e₃₁) determined using a piezotron Model U (Toyo Seiki, Tokyo). The piezoelectric response shows an initial rapid increase with poling time followed by a slow increase as the poling time increases. The x-ray results show that the field-induced phase transition is time dependent, and occurs first for those crystallites with their a axes perpendicular to the film surface. Crystallites with a in the plane of the film transform at a much slower rate. The data indicate that the poling time dependence of d₃₁ and e₃₁ (and, presumably, film polarization) are dependent on the transition rates.     

Curie transition,ferroelectric crystal structure,and ferroelectricity of a VDF/TrFE(75/25) copolymer 1. The effect of the consecutive annealing in the ferroelectric state on curie transition and ferroelectric crystal structure

We have completely deconvoluted multiple ferroelectric-to-paraelectric phase transitions of a VDF/TrFE copolymer often observed on DSC by consecutive annealing below the Curie transition temperature and obtained three clear endothermic peaks resolved well from each other. We could also increase the Curie transition temperature by using the consecutive annealing performed in the ferroelectric state. Annealing time, as well as annealing temperature, was found to affect the Curie transition behavior significantly. Each ferroelectric crystalline phase was characterized by IR, Raman scattering, x-ray diffraction, and DSC measurements. The ferroelectric phase, having higher Curie transition temperature, has been found to have more trans sequences and less gauche defects and to require larger thermal energy for the Curie transition. © 1994 John Wiley & Sons, Inc.     

Brillouinspectroscopic investigations of the ferroelectric phase transition in a Polyvinylidenefluoride — Trifluoretylene copolymer

The hypersonic and thermal behaviour of mechanically oriented and electrically poled random copolymers of vinylidenefluoride and trifluoroethylene (70 mol % / 30 mol %) has been investigated by Brillouin spectroscopy and Differential Scanning Calorimetry (DSC) around the glass transition temperatureT _g and the ferroelectric transition temperatureT _c . From the DSC measurements it appears that a further structural transition may exist in the vicinity ofT _c . The influence of poling and annealing the samples on the elastic stiffness constants at room temperature has been studied. Appropriate sound velocity polar plots are compared. Both glass and ferroelectric transtitions are clearly seen in acoustic behaviour and are discussed from the view point of the degree of crystallinity and disorder, respectively.On leave of absence from Institute of Physics, Czech. Acad. Sci., Praha, Czechoslovakia.     

Electro-optic characteristics of surface-stabilized ferroelectric liquid crystal devices with the stripe-shaped domain structure

The stripe-shaped domain (SSD) structure has been obtained in the initial ferroelectric liquid crystal (FLC) alignment after the doping of a naphthalocyanine compound into rubbed polyimide films. Pre-tilt angles on the aligning films have been measured. The memory capability of aligned SSFLC cells has been enhanced nearly to completion, and contrast ratios reached 86:1 with the appearance of the SSD structure. Thus fabricated, 64 64 electrically controlled FLC spatial light modulator has better electro-optic characteristics and stability than a non-doped device.     

H/D isotopic exchange in water interactions with the ferroelectric copolymer: Poly(vinylidene fluoride-trifluoroethylene) (70%:30%)

For both water and heavy water adsorption and absorption on crystalline poly(vinylidene fluoride with trifluoroethylene (30%)), P(VDF-TrFE 70:30), two distinctly different adsorption sites have been identified by thermal desorption spectroscopy. One adsorbed water species resembles ice and there is also an absorbed water species that interacts more strongly with the polymer thin film, and in addition, there is a polymer surface (polymer to ice interface) water species. We find that there is H/D exchange between the water or heavy water molecules and the ferroelectric polymer (largely -(CH2-CF2)-), particularly at the polymer surface.     

Effect of multiple extrusion on molecular structure of polypropylene impact copolymer

Neat and multiple processed polypropylene impact-copolymer (ICPP) were fractionated using series of hydrocarbon solvents with increasing solvent power. The analyses of the fractions obtained in successive extractions showed significant decrease in weight-average molecular weight (Mw) and narrowing the molecular weight distribution (MWD) of investigated samples after extrusions. Although the changes due to thermooxidation were observed in all phases of the system, the most intensive degradation was found in the prevailing PP homopolymer phase.     

Vibronic coupling in the valence band photoemission of the ferroelectric copolymer: poly(vinylidene fluoride) (70%) and trifluoroethylene (30%)

Two different vibrational contributions to the photoemission fine structure of the ferroelectric copolymer poly(vinlylidene fluoride) with trifluoroethylene (CH2-CF2:CHF-CF2, 70%:30%) are identified. The vibrational contributions at the higher photoemission binding energies are associated with two closely placed upsilon(a,s) (CH2) stretching modes while at the smaller photoemission binding energies, the fine structure is due to a delta (CH2) bending mode. The contribution of the delta (CH2) mode to the photoemission fine structure decreases with decreasing temperature. We associate this temperature dependence to the importance of symmetry in vibronic coupling to the photoemission process and increased dipole ordering with decreasing temperature in this organic ferroelectric system.     

Non-linear dielectric response of a ferroelectric liquid crystal

Dielectric spectroscopy is a very useful method for investigation of the structure and dynamics of liquid crystals. However, with few exceptions, most investigations have been only in the linear regime. In this note we present a simple method for extraction of the non-linear contributions to the dielectric constant and as an example give the results obtained for a ferroelectric liquid crystal.     

Process influences on the structure,piezoelectric, and gas‐barrier properties of PVDF‐TrFE copolymer

The influence of annealing between the Curie transition and the melting point of solvent cast polyvinylidene fluoride trifluoroethylene copolymer films on the crystalline structure, mechanical and electrical properties, and oxygen permeability is investigated. Annealing leads to remarkable changes in the structure and properties of the copolymer, within the first four hours of treatment, and with kinetics depending on the temperature. The crystallinity increases by 19% (relative), resulting in a 10 K increase in the Curie transition, a 4 K increase of the melting temperature and a 2 K decrease in the glass transition temperature. A crystalline phase transition from the paraelectric α‐phase to the ferroelectric β‐phase is also evidenced using in‐situ X‐ray diffraction. The elastic modulus is found to increase by more than three‐fold at room temperature and the loss peak at the glass transition is considerably reduced. The piezoelectric coefficient is found to increase by 40% and the dielectric properties are significantly changed. The most remarkable influence is the ten‐fold reduction of the oxygen permeability, with a drastic reduction of the activation energy for oxygen transport. The improvement in oxygen barrier properties of the annealed copolymer is attributed to the restricted mobility of oxygen molecules in the semicrystalline polymer with nanometer sized crystallites. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 496–506