Crystallization kinetics and morphology of poly(vinylidene fluoride)/poly(ethylene adipate) blends

The miscibility, isothermal crystallization kinetics and morphology of the poly(vinylidene fluoride)(PVDF)/poly(ethylene adipate)(PEA) blends have been studied by differential scanning calorimetry(DSC), optical microscopy(OM) and scanning electron microscopy(SEM). A depression of the equilibrium melting point of PVDF was observed. From the melting point data of PVDF, a negative but quite small value of the interaction parameter ?PVDF-PEA is derived using the Flory-Huggins equation, implying that PVDF shows miscibility with PEA to some extent. Nonisothermal and isothermal crystallization kinetics suggest that the crystallization rate of PVDF decreases with increasing the amount of PEA, and a contrary trend was found when PEA crystallizes with the increase of the amount of PVDF. It was further disclosed that the blend ratio and crystallization temperature affect the texture of PVDF spherulites greatly, which determines the subsequent crystallization of PEA. At high temperatures, e.g. 150 ℃, the band spacing of PVDF spherulites increases with the addition of PEA content and the spherulitic structure becomes more open. In this case, spherulitic crystallization of PEA is not observed for all blend compositions. At low temperatures, e.g. 130 ℃, for the PEA-rich blends, the interpenetrated structures are eventually formed by the penetration of the spherulites of PEA growing within the pre-existing PVDF spherulites.     

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.     

Crystallization kinetics of poly(ethylene oxide) confined in semicrystalline poly(vinylidene) fluoride

Polymer blends based on poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO) have been prepared to analyze the crystallization kinetics of poly(ethylene oxide) confined in semicrystalline PVDF with different ratios of both polymers. Both blend components were dissolved in a common solvent, dimethyl formamide. Blend films were obtained by casting from the solution at 70 °C. Thus, PVDF crystals are formed by crystallization from the solution while PEO (which is in the liquid state during the whole process) is confined between PVDF crystallites. The kinetics of crystallization of the confined PEO phase was studied by isothermal and nonisothermal experiments. Fitting of Avrami model to the experimental DSC traces allows a quantitative comparison of the influence of the PVDF/PEO ratio in the blend on the crystallization behavior. The effect of melting and further recrystallization of the PVDF matrix on PEO confinement is also studied. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 588–597     

Crystallization and morphology of poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) blends. I. Spherulitic morphology and growth by polarized microscopy

The development of the poly(3‐hydroxybutyrate) (PHB) morphology in the presence of already existent poly(vinylidene fluoride) (PVDF) spherulites was studied by two‐stage solidification with two separate crystallization temperatures. PVDF formed irregular dendrites at lower temperatures and regular, banded spherulites at elevated temperatures. The transition temperature of the spherulitic morphology from dendrites to regular, banded spherulites increased with increasing PVDF content. A remarkable amount of PHB was included in the PVDF dendrites, whereas PHB was rejected into the remaining melt from the banded spherulites. When PVDF crystallized as banded spherulites, PHB could consequently crystallize only around them, if at all. In contrast, PHB crystallized with a common growth front, starting from a defined site in the interfibrillar regions of volume‐filling PVDF dendrites. It formed by itself dendritic spherulites that included a large number of PVDF spherulites. For blends with a PHB content of more than 80 wt %, for which the PVDF dendrites were not volume‐filling, PHB first formed regular spherulites. Their growth started from outside the PVDF dendrites but could later interpenetrate them, and this made their own morphology dendritic. These PHB spherulites melted stepwise because the lamellae inside the PVDF dendrites melted at a lower temperature than those from outside. This reflected the regularity of the two fractions of the lamellae because that of those inside the dendrites of PVDF was controlled by the intraspherulitic order of PVDF, whereas that from outside was only controlled by the temperature and the melt composition. The described morphologies developed without mutual nucleating efficiency of the components. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 873–882, 2003     

Crystallization and morphology of poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) blends. III. Crystallization and phase diagram by differential scanning calorimetry

The crystallization of poly(vinylidene fluoride) (PVDF)/poly(3‐hydroxybutyrate) (PHB) blends was studied with differential scanning calorimetry, from which the phase diagram was derived. Strong miscibility was underlined by the large negative Flory–Huggins interaction parameter (?0.25). The crystallization of the blend components differed remarkably. Whereas PVDF always crystallized in the surroundings of a homogeneous melt, PHB crystallized in a volume that was confined by the already existing PVDF spherulites, partly in their surroundings and partly inside. Under isothermal conditions, PVDF usually crystallized regularly in three dimensions with predominant quench‐induced athermal nucleation. The Avrami exponent for PVDF dendritic spherulitic growth was, however, distinctly smaller than that for compact growth, and this revealed the two‐dimensional lamellar growth inside. This deviation from ideal Avrami behavior was caused by the development of compositional inhomogeneities as PVDF crystallization proceeded, and this decelerated the kinetics. PHB crystallized three‐dimensionally with mixed thermal and athermal nucleation outside the PVDF spherulites. Inside the PVDF spherulites, PHB crystallization proceeded in a fibrillar fashion with thermal nucleation; the growth front followed the amorphous paths inside the dendritic PVDF spherulites. The crystallization was faster than that in the melt of uncrystallized PVDF. Solid PVDF acts possibly heterogeneously nucleating, accelerating PHB crystallization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 287–295, 2005     

添加剂对PVDF膜结构和性能的影响

以亲水性的聚乙二醇(PEG)和聚乙烯吡咯烷酮(PVP)为添加剂,利用浸没沉淀相转化法制备聚偏氟乙烯(PVDF)多孔膜。利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、傅立叶-红外光谱仪(FT-IR)对所制得膜进行表征测试,分析了添加剂对膜孔结构、结晶度和膜性能的影响。结果表明:添加剂能有效改善膜的孔径结构,使得膜的微孔增多、孔间连通性增强、膜结晶度降低,达到提高膜的纯水通量和孔隙率的效果。     

Crystallization behavior,tensile behavior and hydrophilicity of poly(vinylidene fluoride)/poly(vinyl pyrrolidone) blends

Binary polymer blends of hydrophobic poly(vinylidene fluoride) (PVDF) and hydrophilic poly(vinylpyrrolidone) (PVP) were prepared by melt blending. The crystallization behavior, mechanical properties and hydrophilicity of the binary blends were investigated using Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffractometry (WAXD), differential scanning calorimeter (DSC) scanning, non-isothermal crystallization kinetics, successive self-nucleation and annealing (SSA) fractionation, tensile tests and contact angle tests. The analysis of FTIR, WAXD, DSC scanning, non-isothermal crystallization kinetics and SSA fractionation showed that the addition of PVP greatly influenced the crystallization behavior of the sample. As the PVP content increased, the crystallization temperature, crystallization rate, degree of crystallinity, and the amount of thick lamellaes decreased gradually. Meanwhile, PVP favored the formation of β-phase of PVDF. The results of tensile test revealed that the addition of PVP increased the elongation at break of the sample, and lowered the yield stress. Besides, the result of contact angle test indicated that the hydrophilicity of PVDF was remarkably improved in the presence PVP. The relationship between crystallization behavior and the tensile behavior, hydrophilicity were discussed.     

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.     

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.     

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.     

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.     

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.     

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.     

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.