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.     

Influence of aspect ratio of carbon nanotubes on crystalline phases and dielectric properties of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF)-multiwalled carbon nanotube (MWNT) composites with different aspect ratios of MWNT were prepared by a coagulation method. Field emission scanning electron and transmission electron microscopic studies reveal that MWNT are well dispersed in the PVDF matrix. The X-ray diffraction and differential scanning calorimeter data indicate that the composites with high aspect ratio of MWNT have the β phase structure at the MWNT loading level of 2.0wt%, and have a mixture of α and β phase below 2wt% MWNT, and that those composites with low aspect ratio of MWNT, however, always have a mixture of α and β phase for MWNT concentrations ?2.0wt%. The dielectric constant values increase with the increase in MWNT loading level and the percent increase in dielectric constant is much greater in the composite filled with high aspect ratio of MWNT than in that loaded with low aspect ratio. And also, it has been found that the dielectric loss of the composites with MWNT loading level ?2.0wt% is still as low as neat PVDF, which is of significance for dielectric application.     

Non-Isothermal Crystallization of Poly(vinylidene Fluoride)/Multiwalled Carbon Nanotube Composites

Poly(vinylidene fluoride)/multiwalled carbon nanotube (PVDF/MWCNT) composites were prepared by the method of solution blending. The non-isothermal crystallization of PVDF and its composites was investigated by differential scanning calorimetry (DSC). The results showed that the crystallization peaks shifted to lower temperature and the exothermic trace became wider when cooling rate increased for PVDF and its composites. The MWCNTs caused an increase in crystallization temperature and initial crystallization temperature. In addition, the Jeziorny and Mo methods were used to analyze the non-isothermal crystallization kinetics. The results showed that the crystallization rate rose with an increase in cooling rate, however, the MWCNTs decreased the crystallization rates of PVDF.     

具有纳米结构的聚偏氟乙烯/1-乙烯基-3-丁基咪唑氯盐离子液体复合材料的结晶行为

用示差扫描量热(DSC)、偏光显微镜(POM)及X射线衍射(XRD)分析考察了具有纳米结构的聚偏氟乙烯(PVDF)/1-乙烯基-3-丁基咪唑氯盐离子液体([VBIM][Cl])复合材料(PVDF/[VBIM][Cl])中经[VBIM][Cl]接枝的PVDF(PVDF-g-[VBIM][Cl])纳米微区对PVDF结晶行为的影响.研究结果表明,[VBIM][Cl]化学接枝在PVDF的分子链上,在PVDF/[VBIM][Cl]复合材料中,PVDF-g-[VBIM][Cl]嵌段形成大量纳米微区,分散在PVDF基体中.PVDF-g-[VBIM][Cl]纳米微区能够显著提高PVDF熔体结晶温度(Tc)并显著降低PVDF晶体的等温结晶时间.与纯PVDF相比,在纳米结构的PVDF/[VBIM][Cl]复合材料中,PVDF-g-[VBIM][Cl]纳米微区大大提高了PVDF晶体的成核速率,PVDF的球晶尺寸明显减小.由于[VBIM][Cl]完全"受限"于PVDF-g-[VBIM][Cl]纳米微区中,无法与PVDF分子链发生相互作用,因此纳米结构的PVDF/[VBIM][Cl]复合材料最终以非极性的α晶体为主.由于PVDF-g-[VBIM][Cl]纳米微区与PVDF基体具有热力学不相容性,因此其界面处的PVDF分子链处于部分有序的状态,有助于PVDF晶体的成核,加速了PVDF晶体的结晶速率.     

Electrospinning of poly(vinylidene fluoride)/dimethylformamide solutions with carbon nanotubes

For conductive carbon nanotube (CN)/polymer composite fibers to be obtained, CNs were incorporated into poly(vinylidene fluoride) (PVDF) in dimethylformamide (DMF) solutions and electrospun to form CN/PVDF fiber mats. The thinnest fiber was 70 nm thick. The percolation threshold for the insulator‐to‐conductor transition was 0.003 wt % CN for CN/PVDF/DMF solutions, 0.015 wt % CN for CN/PVDF spin‐coated films, and 0.04 wt % CN for CN/PVDF electrospun fiber mats. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1572–1577, 2003     

Crystallization Behavior and Mechanical Properties of Poly(vinylidene fluoride)/multi-walled Carbon Nanotube Nanocomposites

Poly(vinylidene fluoride)(PVDF)/multi-walled carbon nanotube(MWCNT) nanocomposites were prepared by means of ultrasonic dispersion method. X-ray diffraction(XRD) results indicate that incorporating MWCNTs into PVDF caused the formation of β phase. A thermal annealing at 130 °C confirmed that the β phase was stable in the nanocomposites. Differential scanning calorimetry(DSC) results indicate that the melting temperature slightly increased while the heat of fusion markedly decreased with increasing MWCNT content. The tensile strength and modulus of PVDF were improved by loading the MWCNTs. The scanning electron microscopy(SEM) observations showed that MWCNTs were uniformly dispersed in the PVDF matrix and an interfacial adhesion between MWCNT and PVDF was achieved, which was responsible for the enhancement in the tensile strength and modulus of PVDF.     

Crystallization behavior and hydrophilicity of poly (vinylidene fluoride) (PVDF)/poly (styrene-<Emphasis Type="Italic">co</Emphasis>-acrylonitrile) (SAN) blends

Poly (styrene-co-acrylonitrile) (SAN) is a hydrophilic non-crystalline copolymer, which is initially used in this paper to improve the hydrophilicity of poly (vinylidene fluoride) (PVDF). Investigation of the crystallization behavior of PVDF/SAN blends showed that the samples presented only α phase regardless of SAN content as cooling from the melt. A double-melting phenomenon was related to the perfection or crystal size of PVDF crystals. As the SAN content is increasing, crystallization of PVDF was limited, leading to a decreased crystallinity and lamellar growth. Besides, the hydrophilicity of PVDF was improved by blending with SAN. The sample containing 70 wt.% SAN performed a similar surface property of the neat SAN owing to the besieging of the PVDF phase by SAN. Observed from the cross section of the blends, PVDF/SAN blends were partially miscible with less than 50 wt.% SAN addition. As the SAN content was more than 50 wt.%, the crystalline PVDF particles clearly dispersed in the amorphous matrix.     

Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (T_m ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.     

Effect of poly(butylene succinate) on the morphology evolution of poly(vinylidene fluoride) in their blends

The effect of PBS on the morphological features of PVDF has been investigated by optical and atomic force microscopies under various conditions.It was found that neat PVDF forms largeγform spherulites with extraordinarily weak birefringence at 170℃.Adding 30%PBS makes PVDF exhibit intrigued flower-like spherulitic morphology.The growth mechanism was explained by the decrease of the supercooling and the materials dissipation.Increasing the PBS content to 70%favors the formation of ring banded spherulites.Temperature dependent experiments verify theα→γphase transition occurs from the junction sites of theαandγcrystals,while starts from the centers ofαspherulites in the blends.Ring banded structures could be observed in neat PVDF,70/30 blend and 30/70 blend when crystallized at 155℃,withoutγcrystals.The band period of PVDFαspherulites increases with crystallization temperature as well as the amount of PBS content.At 140℃,spherulites in neat PVDF lose their ring banded feature,while coarse spherulites consisting of evident lamellar bundles could be found in 30/70 blend.     

Polymerisation of vinylidene fluoride in supercritical carbon dioxide: Formation of PVDF macroporous beads

Poly(vinylidene fluoride) (PVDF) macroporous beads with diameter in the range of hundreds of micrometers were produced by batch polymerisation of vinylidene fluoride (VDF) in supercritical carbon dioxide (scCO₂) using diethyl peroxydicarbonate (DEPDC) as a free radical initiator. The rate and type of stirring were found to influence strongly the morphology of polymers, and the results indicated that the shear force was the key factor. A low shear force and a suitable monomer concentration range at 6-8 mol/l were needed for the formation of PVDF macroporous beads. Scanning electron microscopy (SEM) was employed to characterize the polymer morphology, and Brunauer-Emmett-Teller (BET) method was used to analyze the surface area of the polymer macroporous beads. In addition, polydimethylsiloxane monomethylacrylate (PDMS-ma) and poly(1H,1H,2H,2H-perfluorooctyl methacrylate) (PFOMA) were found to be able to control the size of PVDF macroporous beads. We propose that the formation of PVDF macroporous beads results from the aggregation of semi-crystalline PVDF primary particles.     

Poly(vinylidene fluoride)/polyaniline/carbon nanotubes nanocomposites: Influence of preparation method and oscillatory shear on morphology and electrical conductivity

Nanocomposites of poly(vinylidene fluoride) (PVDF)/polyaniline (PAni)/carbon nanotubes (CNT) were prepared through melt blending using three different methods. The PVDF, CNT and PAni were added into the internal mixer at the same time in method I. In method II, PAni was polymerized in the presence of different amounts of CNT, and then added to PVDF. In method III, PAni was obtained with half the CNT content, and then added to PVDF with the other half of CNT. The morphology, rheological behavior and electrical conductivity of these systems were investigated through transmission electron microscopy (TEM) and combined electro-rheological measurements. As expected, the preparation method strongly influenced the final morphology of the nanocomposites, as shown by TEM analysis. The deformation and destruction of PAni-PAni and CNT-CNT bonds during the oscillatory shear experiments strongly affected the electrical conductivity, probably in two different ways: it breaks the PAni into smaller domains and also disrupts the CNT percolated network. Concluding, a good correlation between the electrical, flow behavior and preparation method could be achieved for PVDF/PAni/CNT nanocomposites, mainly by means of the combined electro-rheological measurements.     

Morphology Evolution of Poly（vinylidene fluoride） Membranes during Supercritical CO2 Assisted Phase Inversion

A supercritical carbon dioxide（Sc CO2） assisted phase inversion was developed to produce microporous poly（vinylidene fluoride）（PVDF） membranes whose morphology characteristics arise from both liquid-liquid demixing and solid-liquid demixing（crystallization）. This result was confirmed by Fourier transform infrared spectroscopy（FTIR）, from which both α and β crystals were found. As revealed by contact angle experiment, the PVDF membranes prepared via Sc CO2 assisted phase inversion were more hydrophobic compared with the control membrane produced via conventional immersionprecipitation technique. In particular, the sample with 15 wt% PVDF prepared at 45 °C and 13 MPa exhibited a contact angle of 142°, which was mainly caused by the multilevel micro- and nano- structure. The effects of polyethylene glycol（PEG）, polyvinyl pyrrolidone（PVP） and lithium chloride（Li Cl） on the structures and crystal form were investigated. PVP promoted the formation of β phase crystal form, while PEG boosts the evolution of α phase. Li Cl restrained the crystallization degree of PVDF membrane under Sc CO2.     

Crystallization behavior and electroactive properties of PVDF,P(VDF-TrFE) and their blend films

PVDF, poly(vinylidene fluoride), as a semi-crystalline polymer, has interesting electroactive properties but usual melt and solution processing techniques result in its thermodynamically favored non-polar α-phase. By comparison, poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE), PT for short, directly crystallizes in the polar β-phase under the same conditions as PVDF. In this study, blend thin films comprising PVDF and P(VDF-TrFE) were prepared by solvent casting method. The difference in the crystallization behavior is comprehensively investigated between the polymers: PVDF, P(VDF-TrFE), and the resulting blend films. It is found that replacement of the fluoride atom in TrFE monomer induces a strong steric hindrance that may alter the crystallization process to become more favorable for nucleation of the PVDF β-phase. To figure out the effect of TrFE content on the crystallization behavior and electroactive properties, films with different blend ratios of PVDF and P(VDF-TrFE) were prepared. We found that the PVDF films exhibit higher crystallization activation energy (ΔE) as PT content increases. The atomic force microscopy (AFM) in the piezoresponse force microscopy (PFM) mode illustrated that P5T5 films with equal contents of PVDF and P(VDF-TrFE) induced the highest d₃₃ values.     

Viscometric behaviour of polymer blends based on poly (vinylidene fluoride)

The viscosity behaviour of dilute dimethylformamide solutions of poly(vinylidene fluoride)-poly (methyl methacrylate) and poly(vinylidene fluoride)-polystyrene has been studied at 25°C. The polymer concentration ranges are such that neither phase separation nor microgel formation occurs, although we are very close to theta conditions. The intrinsic viscosity and viscosity interaction parameter of the ternary mixtures have been calculated. The estimation of the compatibility of the above polymer pairs has been studied based on: a) specific viscosities; b) viscosity interaction parameters, according to Krigbaum and Wall formalism, and c) viscosity interaction parameters of a system formed by a dilute probe polymer in the presence of a matrix polymer and a small molecule solvent.     

Thermal stability of poly(vinylidene fluoride) films pre-annealed at various temperatures

This paper investigates the relationship between the pre-annealing conditions and the thermal stability of uniaxially-drawn poly(vinylidene fluoride) (PVDF) films in order to clarify their technical limits in terms of temperatures that can be used for assembly processes and for practical applications. Specimens that are pre-annealed below their melting temperature apparently shrink in the stretch-direction when they are exposed to elevated temperatures above the pre-annealing temperature. Since the content of β-PVDF in the films decreases simultaneously with the shrinkage, their piezoelectric properties also deteriorate. In addition, there is a suggestion that the level of polarization in the remaining β-phase decreases significantly during annealing above 90-100 °C. However, the dimensions and the piezoelectric coefficients of the films remain stable during annealing below the pre-annealing temperature. Therefore, the thermal stability of PVDF films can be controlled practically by using the appropriate pre-annealing temperature. By contrast, the films were softened at 90-100 °C when the pre-annealing treatment was conducted above the melting temperature. The softening of films that are pre-annealed above the melting temperature is a different phenomenon from that observed in specimens that are pre-annealed below the melting temperature.     

Features and origin of the dynamic and the nonlinear piezoelectricity in poly(vinylidene fluoride)

Some of the piezoelectric coefficients of poly(vinylidene fluoride) are distinctly stress dependent. We report on their dependence on the frequency of an oscillatory stress excitation, on the amount of the remanent polarization in the sample materials, and on the magnitude of an offset stress. In order to explain the observed dynamic and nonlinear piezoelectric properties, a model is developed that attributes them to changes in the local polarization in an interphase between the crystalline and amorphous phases in this semicrystalline polymer.     

Intercalated poly(vinylidene fluoride)/clay nanocomposites: Structure and properties

The preparation and characterization of melt‐intercalated poly(vinylidene fluoride) (PVDF)/clay nanocomposites are reported. Organophilic clay (clay treated with dimethyl dihydrogenated tallow quaternary ammonium chloride) was used for the nanocomposite preparation. The composites were characterized with X‐ray diffraction (XRD), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). XRD results indicated the intercalation of the polymer in the interlayer spacing. The incorporation of clay in PVDF resulted in the β form of PVDF. DSC nonisothermal curves showed an increase in the melting and crystallization temperatures along with a decrease in crystallinity. Isothermal crystallization studies show an enhanced rate of crystallization with the addition of clay. DMA indicated significant improvements in the storage modulus over a temperature range of ?100 to 150 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 31–38, 2003     

Non-Isothermal Crystallization of Poly (vinylidene fluoride)/Poly (methyl methacrylate)/Cellulose Nanocrystal Nanocomposites

Poly (vinylidiene fluoride) (PVDF)/poly (methyl methacrylate) (PMMA)/cellulose nanocrystal (CNC) nanocomposites were prepared by solution blending. Non-isothermal crystallization of PVDF/PMMA (70/30) blend and its composites was investigated using differential scanning calorimetry. It was found that the addition of CNCs played a positive role in both the crystallization rate and crystallization percentage. The addition of CNCs increased the initial crystallization temperature, peak crystallization temperature, and crystalline enthalpy. The Avrami index indicated that CNCs did not change the crystallization mechanism; while other parameters derived from Jeziorny theory and Mo's method, including Z _c , F(t), and α, further verified the positive role played by CNCs.     

Ferroelectric properties of nylon 11 and poly(vinylidene fluoride) blends

Both poly(vinylidene fluoride) (PVF₂) and nylon 11 are ferroelectric polymers, and have been extensively studied over the past two decades. Blend films were made from mixed powders of these two polymers, which were then melt pressed and cold drawn. The ferroelectric properties of these blend films were investigated. The remnant polarization, P_r, was found to vary with composition, and to be 60% larger than that of either component at a 50/50 (by weight) composition where P_r exhibited a maximum of about 90 mC/m². The magnitude of the coercive field, E_c, also exhibited a maximum at this composition. Both P_r and E_c are also observed to change significantly with the draw ratio. The results are discussed based on a two-phase dielectric composite model. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3217–3225, 1999     

Poly（vinylidene fluoride）/Clay Nanocomposites by Melt Compounding

The crystalline structures, morphologies, and mechanical properties of poly（vinylidene fluoride）/clay nanocomposites were studied using X-ray diffraction（XRD）, transmission electron microscopy（TEM）, Fourier transform infrared spectroscopy（FTIR）, polarized optical microscopy（POM）, and tensile tests. The results of XRD and TEM show that organoclays are dispersed in the poly（vinylidene fluoride）（PVDF） matrix. A clay-induced crystal transformation from α-phase to β-phase of PVFD was confirmed by XRD and FTIR. Clay layers restricted the growth of spherulite. The tensile tests indicate that the tensile modulus and yield strength as well as the elongation at break decrease when clay is loaded.