Investigation of dielectric relaxations associated with the glass transition of vinylidene fluoride and trifluoroethylene copolymers by thermally stimulated current

Thermostimulated current (TSC) spectroscopy was applied to the characterization of dielectric relaxations associated with the glass transition of a series of P(VDF/TrFE) copolymers. The maximum temperature of the β-mode increases slightly as the TrFE unit content is increased, in the same manner as the glass transition temperature. By using the technique of fractional polarizations to the resolution of this relaxation mode, we have isolated, for all the polymers investigated, a series of elementary relaxation times that obey a compensation law. This behavior is characteristic of the free amorphous phase of polymers. The mean activation energy of this mode increases as the TrFE unit content is increased, due to a stiffening of molecular chains. Annealing of the copolymers above their ferroelectric-to-paraelectric transition induces a strong crystallinity increase of the materials. As a matter of fact, the amorphous phase is squeezed in to dimensions lower than the characteristic length scale associated with the glass transition. © 1995 John Wiley & Sons, Inc.     

Water adsorption on and desorption from crystalline copolymers of vinylidene fluoride with trifluoroethylene

Water adsorption and absorption on crystalline polyvinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE, 70:30), was examined by thermal desorption spectroscopy. Two distinctly different water adsorption sites are identified: one adsorbed species that resembles ice and another species that interacts more strongly with the polymer thin film. The existence of the latter species is consistent with X-ray diffraction studies of water absorbed into the bulk of copolymers of polyvinylidene fluoride with trifluoroethylene crystalline thin films. There are strong steric effects observed in the angle-resolved thermal desorption that may be a result of the large polymer thin film surface dipoles.     

New copolymers of vinylidene fluoride

After a brief presentation of the standard non-functionalized copolymers of vinylidene fluoride, the preparation of new functionalized copolymers is described, by copolymerization of vinylidene fluoride, with other fluoromonomers and non-fluorinated monomers including at least one hydroxy or other functional group. The copolymers are analysed by the conventional methods. Examples of formulation show that these new copolymers are able to give coatings applicable outdoors and offering the same excellent durability as PVDF.     

Study of ferroelectric transitions of vinylidene fluoride–trifluoroethylene copolymers

Thermally stimulated depolarization (TSD) measurements with superimposed “piezoelectric noise” allow us to identify the ferroelectric-to-paraelectric phase transition in vinylidene fluoride–trifluoroethylene (VDF/TrFE) copolymers with different molar ratios. In addition, a new transition which did not change the ferroelectric features of the material has also been detected. Long relaxation processes always accompany these transitions. By the initial-rise method we calculated the activation energies for these relaxations as 0.73 eV for the ferroelectric-to-paraelectric transition and 0.33 eV for the ferroelectric-to-ferroelectric one.     

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.     

Pyrolysis gas-chromatographic studies of homopolymers and copolymers of chlorotrifluoroethylene with vinylidene fluoride and methyl methacrylate

Homopolymers of chlorotrifluoroethylene (CTFE), vinylidene fluoride (VDF) and methyl methacrylate (MMA) and their copolymers were studied by PGC/IR with regard to the separation and identification of the degraded products and clarification of the degradation mechanism. Besides monomers, HCl CO₂, C₂H₂, HF, C₅H₁₀O₂, C₃F₅Cl and dimers are the observed pyrolysis products. Mechanisms for different degradation reactions have also been studied.     

A simple and convenient route to prepare poly(vinylidene fluoride trifluoroethylene) copolymer nanowires and nanotubes

Poly(vinylidene fluoride trifluoroethylene) copolymer nanowires and nanotubes have been prepared for the first time via a high temperature (相似文献   

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.     

Glass transition temperatures of copolymers of ethyl acrylate and vinylidene chloride

Glass transition temperatures of poly(ethyl acrylate-vinylidene chloride) and reactivity ratios for the copolymerization of the parent monomers are reported. All the copolymers have glass transition temperatures higher than those of polyethylacrylate or polyvinylidene chloride: the copolymer containing equal molar quantities of each monomer has the highest. The data are accounted for in terms of the fractions of AA. AB and BB diads contained in the copolymers.     

High‐resolution solid‐state NMR study of the domain structure and molecular motion in drawn films of a vinylidene fluoride and trifluoroethylene copolymer with 73 mol % vinylidene fluoride

The heterogeneous higher order structure and molecular motion in a single crystalline film of a vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer with 73 mol % VDF was investigated with the ¹H–¹³C cross‐polarization/magic‐angle spinning NMR technique. A transient oscillation was observed in plots of the ¹³C peak intensity versus the contact time for the CH₂, CHF, and CF₂ groups. On the basis of the extended cross‐relaxation theory of spin diffusion, we determined that the oscillation behavior was caused by the TrFE‐rich segments in the chain and that the crystal consisted of VDF‐rich and TrFE‐rich domains. The former had TrFE‐rich segments in VDF and TrFE fractions of 0.24 and 0.27, respectively, and the latter had VDF‐rich segments in a VDF fraction of 0.49. The spin–lattice relaxation time T_1ρH in the rotating frame for each group was minimal in the three temperature regions of β, α_b, and α_c (↑) on heating and in the two temperature regions of α_1D and α_c (↓) on cooling. The α_c (↑) and α_c (↓) processes depended on the first‐order ferroelectric phase‐transition regions on heating and cooling, respectively. The motional modes for the other processes were confirmed by the T_1ρH minimum behavior of the VDF and TrFE groups in the TrFE‐rich domain and the VDF‐rich segments in the VDF‐rich domain. The β and α_b processes were attributed to the flip–flop motion of the TrFE‐rich segments and the competitive motion of the TrFE‐ and VDF‐rich segments in the ferroelectric phase, respectively. The α_1D process was due to the one‐dimensional diffusion motion of the conformational defects along the chain in the paraelectric phase, accompanied by the trans and gauche transformation of the VDF conformers of ttg⁺tg^? and g⁺tg^?tt. The effect of the competitive motion of the TrFE‐rich segment on the thermal stability of the VDF‐rich segment in the chain near the Curie temperature was examined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1026–1037, 2002     

Miscibility and ferroelectric behavior in blends of poly(vinylidene fluoride/trifluoroethylene) and poly(vinyl acetate)

The blend system containing a poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] copolymer (68/32 mol %) and poly(vinyl acetate) (PVAc) was miscible from the results of differential scanning calorimetry (DSC) studies that exhibit the presence of a single, composition‐dependent glass transition temperature (T_g) and a strong melting point depression for the semicrystalline P(VDF/TrFE) component. However, differences between the DSC and dielectric measurements, which showed a separate P(VDF/TrFE) T_g peak, suggests that the P(VDF/TrFE)/PVAc blends are actually partially miscible. Because of the lower dielectric constant of PVAc and the reduced sample crystallinity caused by the addition of PVAc, both the dielectric constant and the remanent polarization of the copolymer blends decrease with increasing PVAc content. The presence of a small amount of PVAc stabilized the anomalous ferroelectric behavior of ice–water‐quenched P(VDF/TrFE), and the blend portrayed normal polarization reversal behavior after adding only 1 wt % PVAc. The piezoelectric response suggests small changes with an increasing number of poling cycles. It is believed that PVAc affects the D‐E hysteresis behavior at the interface between crystalline and amorphous phases, although much work remains to be done to confirm this hypothesis. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 927–935, 2003     

Mild-temperature Mn2(CO)10-photomediated controlled radical polymerization of vinylidene fluoride and synthesis of well-defined poly(vinylidene fluoride) block copolymers

By contrast to typical high-temperature (100-250 °C) telo-/polymerizations of gaseous fluorinated monomers, carried out in high-pressure metal reactors, the visible light, Mn(2)(CO)(10)-photomediated initiation of vinylidene fluoride (bp = -83 °C) polymerization occurs readily from a variety of alkyl, semifluorinated, and perfluorinated halides at 40 °C, in low-pressure glass tubes and in a variety of solvents, including water and alkyl carbonates. Perfluorinated alkyl iodide initiators also induce a controlled radical polymerization via iodine degenerative transfer (IDT). While IDT proceeds with accumulation of the less reactive P(m)-CF(2)-CH(2)-I vs the P(n)-CH(2)-CF(2)-I chain ends, Mn(2)(CO)(10) enables their subsequent quantitative activation toward the synthesis of well-defined poly(vinylidene fluoride) block copolymers with a variety of other monomers.     

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.     

Crystallite damage studies on irradiated poly(vinylidene fluoride)

The effect of γ-radiation on crystallite of poly(vinylidene fluoride) was studied with X-ray diffraction, differential scanning calorimetry and polarizing macroscopy techniques. It has been shown that the degree of crystallite damage relates to the imperfection of crystallites. The lower the imperfection of crystallite, the stronger the radiation-induced crystallite damage. The long period, lamellae thickness, contour and optical anisotropy of sheprulites in irradiated samples remains unchanged. “Crystallite core damage mechanism” gives a more satisfactory explanation of the results.     

Thermal and dielectric behaviors of poly(vinylidene fluoride-trifluoroethylene) copolymers at the curie transition

The thermal and dielectric behaviors of poly(vinylidene fluoride-trifluoroethylene) copolymers near the ferroelectric-to-paraelectric phase transition are investigated for samples with 20, 25, 30, and 40 mol% trifluoroethylene (TrFE). The data suggest that the transition becomes continuous for a particular composition near 50 mol% TrFE. Experimental data are sensitive to thermal history (kinetics of crystallization, and kinetics and cycling over the structural transition). It is found that several anomalies are present at the structural change, and in particular the 30 mol% TrFE sample shows the most marked anomalies. These phenomena can be attributed to defects, but another possibility would be the existence of an intemediate supplementary phase. Both hypotheses are discussed.     

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.     

Structural studies of vinylidene fluoride-tetrafluoroethylene copolymers by nuclear magnetic resonance spectorscopy

The application of high-resolution nuclear magnetic resonance (n.m.r.) spectroscopy to structural studies of copolymers is illustrated by an investigation of the vinylidene fluoride (VF₂)-tetrafluoroethylene (F₄E) system. Copolymers of VF₂ with F₄E are of topical interest owing to their ferroelectric behavior, and n.m.r. provides the only method for accurate determination of their composition and chemical microstructure. Many copolymer properties are highly sensitive to these structural variables, which must be characterized prior to the evaluation of physical behavior. A series of copolymers with compositions ranging from 100 to 50 mol% VF₂ is studied by proton, carbon-13, and fluorine-19 n.m.r. Proton and carbon-13 n.m.r. experiments distinguish monomer sequence triads. Fluorine-19 n.m.r. is the method of choice for the most detailed study because this probe resolves all monomer sequence pentads, as well as some longer-range sequences. Assignment of microstructure at this detailed level is aided by an extrapolation of known assignments for PVF₂ homopolymer, and computer simulation of peak probabilities based on a terpolymerization scheme involving both forward and reverse VF₂ addition, as well as F₄E addition. Necessary statistical relationships between observed carbon sequences and monomer sequences are derived to facilitate the study. The VF₂-F₄E system provides a particularly good example of the potential of high-resolution n.m.r. as an analytical tool for copolymer studies.     

Functional polymers and sequential copolymers by phase transfer catalysis. XXII. Vinylidene fluoride–trifluoroethylene copolymers by surface modification of polyvinylidene fluoride

Polyvinylidene fluoride films and powders have been used to synthesize vinylidene fluoride-trifluoroethylene copolymers. The synthetic procedure involves a phase transfer catalyzed heterogenous dehydrofluorination, followed by electrophilic chlorination or bromination. A phase transfer catalyzed displacement of bromine or chlorine by fluoride ion completes the synthesis. Dehydrofluorinations were up to 40% complete at room temperature and up to 100% complete at 90°C. Electrophilic chlorination of the resulting carbon–carbon double bonds were complete on both powder or films. The bromination of dehydrohalogenated polymers does not go to completion even when the reaction is carried out in solution. Displacement reactions were incomplete and resulted in further dehydrohalogenation when tetrabutylammonium hydrogen sulfate (TBAH) or 18-crown-6 and KF were used in organic solvents. Using TBAH, KHF₂, and water, high degrees of displacement were achieved only on the brominated compounds. The chlorinated products gave both displacement and elimination.     

Fluorinated copolymers and terpolymers based on vinylidene fluoride and bearing sulfonic acid side‐group

The radical co‐ and terpolymerization of perfluoro(4‐methyl‐3,6‐dioxaoct‐7‐ene) sulfonyl fluoride (PFSVE) with 1,1‐difluoroethylene (or vinylidene fluoride, VDF or VF₂), hexafluoropropene (HFP), chlorotrifluoroethylene (CTFE), and bromotrifluoroethylene (BrTFE) is presented. Although PFSVE could not homopolymerize under radical initiation, it could be copolymerized in solution under a radical initiator with VDF, while its copolymerizations with HFP or CTFE led to oligomers in low yields. The terpolymerizations of PFSVE with VDF and HFP, with VDF and CTFE, or with VDF and BrTFE also led to original fluorinated terpolymers bearing sulfonyl fluoride side‐groups. The conditions of co‐ and terpolymerization were optimized in terms of the nature and the amount of the radical initiators, of the nature of solvents (fluorinated or nonhalogenated), and of the initial amounts of fluorinated comonomers. The different mol % contents of comonomers in the co‐ and terpolymers were assessed by ¹⁹F NMR spectroscopy. A wide range of co‐ and terpolymers containing mol % of PFSVE functional monomer ranging from 10 to 70% was produced. The kinetics of copolymerization of VDF with PFSVE enabled to assess the reactivity ratios of both comonomers: r_VDF = 0.57 ± 0.15 and r_PFSVE = 0.07 ± 0.04 at 120 °C. The thermal and physicochemical properties were also studied. Moreover, the glass transition temperatures (T_gs) of poly(VDF‐co‐PFSVE) copolymers containing different amounts of VDF and PFSVE were determined and the theoretical T_g of poly(PFSVE) homopolymer was deduced. Then, the hydrolysis of the ? SO₂F into ? SO₃H function was investigated and enabled the synthesis of fluorinated copolymers bearing sulfonic acid functions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1814–1834, 2007