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.     

Molecular mobility of poly(vinylidene fluoride) in the anisotropic state

The effect of uniaxial orientational drawing and subsequent isometric annealing on the molecular mobility of poly(vinylidene fluoride) was studied by dielectric spectroscopy and dynamic mechanical analysis. The influence of orientation, the polymorphic composition of the crystalline phase, and the structure of disordered regions on dielectric and mechanical relaxation in isotropic and oriented samples was investigated.     

Molecular mechanism for elongation: poly(vinylidene fluoride) form II

Elongation of an unoriented sample of poly(vinylidene fluoride) form II at high temperature (above 140 °C) results in a uniaxially oriented sample of form II during so-called necking. The short-range and long-range order parameters of poly(vinylidene fluoride) form II and measures of the structural order on thec-projection are characterized by the intensity and half-width of the 120 reflection. The elongation under the same condition gives almost the same order parameters, independent of the order parameters before the elongation. Furthermore, the dependence of the order parameters on the elongation temperature behaves in the same way as the dependence of the quenched unoriented sample on the annealing temperature. This suggests that the elongation of the sample during necking corresponds to the crystallization or recrystallization after a much disordered state, approximately molten state; this supports the mechanism proposed by Yoon and Flory.     

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.     

Brillouin scattering of oriented films of poly(vinylidene fluoride) and poly(vinylidene fluoride) -poly( methyl methacrylate) blends

Temperature dependent Brillouin scattering studies of PVF₂ films stretched to various stretch ratios have been carried out. Elastic constants for unstretched and stretched films have been obtained as functions of temperature. The elastic constant C₃₃ of the stretched films has a greater temperature dependence than that of unstretched films. To elucidate the effect of the surrounding amorphous matrix on the Brillouin spectrum of semicrystalline PVF₂ film, we carried out Brillouin scattering studies of films made from blends of PVF₂ and PMMA.     

Melting studies of poly(vinylidene fluoride) and its blends with poly(methyl methacrylate)

Blends of poly(vinylidene fluoride) (PVF₂) and poly(methyl methacrylate) exhibit complex melting behavior when crystallized at low undercoolings. Three crystals comprised of two different PVF₂ forms grow. Hoffman-Weeks plots of the observed melting points T_m of these crystals versus crystallization temperatures are constructed. The lowest-melting-point species, the α form, shows a change in slope which is attributed to fewer head-to-head PVF₂ units trapped in the crystal at higher temperatures. Defect energies in the crystal due to these units are calculated to be from 6.3 to 10.3 kJ/mol. Estimating lamellar thicknesses from the slopes of the two regions gives much more reasonable values when the high-temperature data are used. Removal of kinetic effects that lower the observed T_m by extrapolating the data to obtain T permits the thermodynamic interaction energy density B between the two polymers to be obtained. The low-temperature α-form data give B = ?8.83 × 10⁶ J/m³. The high-temperature α-form data and the T of the γ-form crystals both show B to vary from ?5.40 × 10⁶ to ?2.96 × 10⁷ J/m³ as the blend composition goes from 40.1 vol % to pure PVF₂.     

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.     

Morphological studies of poly(vinylidene fluoride) and its blends with poly(methyl methacrylate)

Both pure poly(vinylidene fluoride) (PVF₂) and its blends with poly(methyl methacrylate) (PMMA) develop a variety of morphologies when they are crystallized above the 420–424 K range. Two populations of spherulites as well as axialitelike growths are observed. Addition of the PMMA lowers the temperature where these new morphologies develop, makes the spherulites more open, causes the banding periodicity to decrease, and increases the number of small, coarse spherulites. These structures melt in three regimes. The highest-melting-point crystals arise only from a solid-solid transformation of the lowest-melting-point ones. This solid-state transition sometimes causes mixed spherulites to be formed in the blends. Electron and wide-angle x-ray diffraction show the lowest-melting-point species to be α crystals, while the other two are γ crystals. The highest-melting-point species, labeled γ′, and the α crystals seem to be more ordered than the other γ crystals.     

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.     

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     

Vibrational analysis of poly(vinylidene fluoride)

A vibrational analysis has been carried out for the two crystalline forms of poly(vinylidene fluoride) (PVF₂). The Raman spectrum of the planar form of PVF₂ is also reported. The band assignments are made on the basis of the spectral properties including the infrared dichroism and Raman intensities. A force field is derived based on a force constant refinement procedure utilizing the frequency data for both crystal forms.     

Carbon nanotubes induced poly(vinylidene fluoride) crystallization from a miscible poly(vinylidene fluoride)/poly(methyl methacrylate) blend

Different contents of carbon nanotubes (CNTs) were introduced into a miscible poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend. The interfacial affinity between CNTs and components of the blend was evaluated by calculating the interfacial tension. The dispersion and microstructure of CNTs in the nanocomposites were investigated through scanning electron microscope and rheological measurement. The effect of CNTs on the crystallization of PVDF was comparatively investigated through nonisothermal and isothermal crystallization processes. The results showed that CNTs exhibited stronger interfacial affinity to PMMA. Homogeneous dispersion of CNTs in the nanocomposites was achieved. Largely enhanced crystallization temperature and increased crystallinity of PVDF were obtained by adding CNTs during the nonisothermal crystallization process. The results obtained from the isothermal crystallization process proved that CNTs induced the concentration fluctuation in the sample, which resulted in the formation of spherulites with different types, i.e., the banded spherulites and compact spherulites. Furthermore, both the crystallization temperature and the content of CNTs exhibited great influence on the crystalline morphology of PVDF.     

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.     

Persistent polarization in poly(vinylidene fluoride). III. Depolarization and pyroelectricity of poly(vinylidene fluoride) thermoelectrets

Persistent polarization in poly(vinylidene fluoride) thermoelectrets prepared under high electric field has been studied by measurements of depolarization and pyroelectricity. Various polarizations are examined in detail; the polarizations related to a characteristic molecular motion near 60°C and the polarizing temperature are not responsible for the major piezoelectric effect in β-form electrets. The piezoelectricity is attributed to a polarization appearing near the melting temperature. The persistent polarization corresponding to d₃₁ of 2 × 10^?11 coul/N is about 5 × 10^?6 coul/cm². The pyroelectricity of β-form electrets is linearly correlated with the piezoelectricity.     

Dielectric relaxations in poly(vinylidene fluoride)

Molecular motions in poly(vinylidene fluoride) were studied by the dielectric technique. Three distinct absorption peaks (α_c, α_a, and β) were observed in the frequency range from 0.1 cps to 300 kcps and in the temperature range from ?66 to 100°C. The molecular mechanisms for these absorptions and their temperature dependence are discussed, and results are compared with x-ray diffraction and the NMR measurements. It is concluded that the α_c absorption located at 97°C (1 kcps) is related to molecular motion in the crystalline region. The α_a absorption located at ?27°C (1 kcps) can be interpreted as due to the micro-Brownian motion of the amorphous main chains. The β absorption located at ?47°C (1 kcps) is attributed to local oscillation of the frozen main chains.