Preparation and characterization of poly(vinylidene fluoride) nanocomposites containing multiwalled carbon nanotubes

Poly(vinylidene fluoride) (PVDF) nanocomposites with different loadings of multiwalled carbon nanotubes (MWNT) were prepared by melt‐compounding technique. A homogeneous dispersion of MWNT throughout PVDF matrix was observed on the cryo‐fractured surfaces by scanning electron microscopy. Thermogravimetric analysis results indicated that the thermal stability of neat PVDF was improved with the incorporation of MWNT. Dynamic mechanical analysis showed a significant improvement in the storage modulus over a temperature range from ?125 to 75°C with the addition of MWNT. The melt‐rheological studies illustrated that incorporating MWNT into PVDF matrix resulted in higher complex viscosities (|η*|), storage modulus (G′), loss modulus (G″), and lower loss factor (tan δ) than those of neat PVDF. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermal behaviour and annealing of poly(vinylidene fluoride)

In this study, we report the melting behavior of poly(vinylidene fluoride) (PVF₂) annealed in a differential scanning calorimeter. PVF₂ annealed under isothermal conditions often shows double or triple melting endotherms depending on the annealing temperature (T_a) and the heating rate. The lower melting peak temperature increases as T_a increases. When the annealing time is varied, there is a systematic increase in the size of the lower endotherms. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing PVF₂ not only increases the degree of crystallinity, but also improves the crystal perfection. The ability of an annealing sample to reorganize decreases as the annealing time increases. However, an additional third melting peak appears when PVF₂ is annealed at 140°C for a sufficiently long time. The existence of this peak suggests that more than one kind of distribution of crystal perfection may occur when PVF₂ is quenched from the melt into liquid nitrogen and subsequently annealed.     

Nanocomposites of poly(vinylidene fluoride) with multiwalled carbon nanotubes

The preparation and characterization of nanocomposites of poly(vinylidene fluoride), PVDF, with acid treated multiwalled carbon nanotubes (MWCNT) with a wide composition range, from 0.1 to 5.0% MWCNT by weight, is reported. Effect of uniaxial orientation by zone drawing on these nanocomposites is discussed and compared with unoriented compression molded films. Static room temperature two‐dimensional wide angle X‐ray scattering and Fourier transform infrared spectroscopy were used for phase identification. Differential scanning calorimetry, polarizing optical microscopy, dynamic mechanical analysis (DMA), and thermogravimetic analysis (TGA) were used to study the thermal and mechanical properties. Incorporation of MWCNT into PVDF has no obvious effect in forming beta phase crystal in the PVDF/MWCNT bulk films, while zone drawing cause a significant alpha to beta transition in PVDF/MWCNT. Results indicate that MWCNTs act as nucleation agent during crystallization and slightly increase the degree of crystallinity of PVDF/MWCNT bulk films. TGA indicates the thermal stability is improved when MWCNT concentration increases for unoriented PVDF/MWCNT film. The modulus also increases significantly when MWCNT concentration increases. The glass transition temperature measured by the peak position of tanδ from DMA does not change with MWCNT concentration, but a slightly higher glass transition can be obtained by zone drawing. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Electrical characteristics of fluorinated carbon black‐filled poly(vinylidene fluoride) composites

Dispersion and electrical properties of fluorinated carbon black‐filled poly(vinylidene fluoride) (PVDF) composites were studied as a function of the fluorine content. It was found that with increasing the fluorine content carbon particles tend to stick together to form large aggregates. The percolation concentration increases to a high concentration, whereas the percolation process becomes gradual. The temperature dependence of resistivity measurements show that the fluorinated carbon black‐filled PVDF composites exhibit a high PTC intensity and a low NTC effect. These phenomena were discussed in terms of thermodynamic interactions between fluorinated carbon and the PVDF matrix. The dielectric behavior was also investigated in this study. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1063–1070, 2001     

Electrical and rheological percolation in poly(vinylidene fluoride)/multi‐walled carbon nanotube nanocomposites

Nanocomposites of poly(vinylidene fluoride) (PVDF) and multi‐walled carbon nanotubes (MWCNTs) were prepared through melt blending in a batch mixer (torque rheometer equipped with a mixing chamber). The morphology, rheological behavior and electrical conductivity were investigated through transmission electron microscopy, dynamic oscillatory rheometry and the two‐probe method. The nanocomposite with 0.5 wt% MWCNT content presented a uniform dispersion through the PVDF matrix, whereas that with 1 wt% started to present a percolated network. For the nanocomposites with 2 and 5 wt% MWCNTs the formation of this nanotube network was clearly evident. The electrical percolation threshold at room temperature found for this system was about 1.2 wt% MWCNTs. The rheological percolation threshold fitted from viscosity was about 1 wt%, while the threshold fitted from storage modulus was 0.9 wt%. Thus fewer nanotubes are needed to approach the rheological percolation threshold than the electrical percolation threshold. Copyright © 2010 Society of Chemical Industry     

Crystal transformation and thermomechanical properties of poly(vinylidene fluoride)/clay nanocomposites

The crystal transformation and thermomechanical properties of melt‐intercalated poly(vinylidene fluoride) (PVDF)/clay nanocomposites are reported in this study. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal properties of PVDF and its nanocomposites with various clay concentrations. The incorporation of clay in PVDF results in the formation of β‐form crystals of PVDF. DSC study of melting behavior suggested the presence of only α‐phase crystals in neat PVDF and both α‐ and β‐phase crystals in the nanocomposite. This conclusion was corroborated by findings from Fourier‐transform infrared (FTIR) spectroscopy and X‐ray diffraction (XRD). Dynamic mechanical analysis (DMA) indicated significant improvements in storage modulus over a temperature range of 20–150 °C. The coefficient of thermal expansion (CTE) decreases with increasing clay loading. Copyright © 2004 Society of Chemical Industry     

Structure and morphology of solution blended poly(vinylidene fluoride)/montmorillonite nanocomposites

Nanocomposites based on poly(vinylidene fluoride) were prepared with montmorillonite by solution blending. The samples were characterized by small angle X‐ray scattering, wide angle X‐ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry. Different crystallization conditions, that is, evaporation of the solvent and coprecipitation with two different antisolvents, H₂O or supercritical CO₂ (scCO₂), were tested and their influence on the resulting structure and morphology of the samples were studied. Coprecipitation with scCO₂ induced an ordinate crystalline framework and an intercalated morphology of clay, with a consequent large improvement in modulus. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Conductive network formation and electrical properties of poly(vinylidene fluoride)/multiwalled carbon nanotube composites: Percolation and dynamic percolation

Conductive network formation and its dynamic process for multiwalled carbon nanotubes (MWNTs) and carboxyl‐tethered MWNT (MWNT‐COOH) filled poly(vinylidene fluoride)(PVDF) systems were investigated. Based on real‐time tracing the variation of electrical resistivity of systems with isothermal treatment time, the conductive network formation was evaluated. It was found that the conductive network formation was temperature and time dependent. The percolation time, characterized at a certain annealing time where the electrical resistivity started to decrease drastically, decreased with the increase of the filler concentration or the annealing temperature. However, the values of the percolation time and the activation energy of conductive network formation for the PVDF/MWNT‐COOH system were higher than those of the PVDF/MWNT system, indicating that the interaction between MWNTs and PVDF molecules played an important role in the conductive network formation of the composites. Furthermore, a modified thermodynamic percolation model was proposed to predict the percolation time of PVDF/MWNT composites. It was found that the calculated results fit the experimental data very well. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Morphology and melting behaviour of poly(vinylidene fluoride) crystallized from the melt

The influence of crystallization temperature on the melting behaviour and the morphology of poly(vinylidene fluoride) (PVF₂) has been investigated. The DSC endotherms of PVF₂ crystallized from the melt show at least two peaks. The peak areas depend on the thermal history of the samples and the heating scan rate. The area of the first peak was found to increase as the crystallization temperature or the scan rate increased. The double peak configuration was attributed to a melting–recrystallization process. Electron microscopy supports these results, for which only one type of lamella was found in the spherulitic structure.     

Preparation,crystallization behavior,and dynamic mechanical property of nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplate

Nanocomposites based on poly(vinylidene fluoride) (PVDF) and exfoliated graphite nanoplate (xGnP) were prepared by solution precipitation method. The resulting nanocomposites were investigated with respect to their structure and properties by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, and dynamic mechanical analysis. Both SEM and TEM examinations confirmed the good dispersion of xGnP in the PVDF matrix. The nonisothermal crystallization behavior of the PVDF/xGnP nanocomposites was studied using DSC technique at various cooling rates. The results indicated that the xGnPs in nanometer size might act as nucleating agents and accelerated the overall nonisothermal crystallization process. Meanwhile, the incorporation of xGnP significantly improved the storage modulus of the PVDF/xGnP nanocomposites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structurally Induced Dielectric Constant Promotion and Loss Suppression for Poly(vinylidene fluoride) Nanocomposites

An increase in the dielectric constant (800 ± 15; 10⁰ ≤ f ≤ 10⁴ Hz) of PVDF without a coresponding increase in the dielectric loss is reported. This is realized by preparing sandwich structures composed of CNF/PVDF composite layers and a pure PVDF interlayer. The influence of the interlayer thickness on the dielectric properties is investigated. It is shown that a 30 µm interlayer is sufficient to prevent formation of the conductive network in the sandwich structures that would result in lower loss. It is demonstrated that with better CNF dispersion, a further increase of the dielectric constant and a lower loss can be obtained simultaneously. The sandwich approach thus leads to nanocomposites with enhanced dielectric constants while maintaining low loss.

     

Multiple melting in blends of poly(vinylidene fluoride) with isotactic poly(ethyl methacrylate)

Multiple melting phenomena have been studied in blends of poly(vinylidene fluoride) (PVF₂) with low molar mass isotactic poly(ethyl methacrylate) (it-PEMA). In all blends, as well as in pure PVF₂, a transition (T₁) was observed prior to the main melting point (T₂). T₁ is probably connected with the melting of secondarily-crystallized material. In addition to this, a high temperature melting endotherm (T₃) was observed, which could be ascribed completely to recrystallization of PVF₂. The highest transition (T₄) was caused by melting of the σ form of PVF₂. From Hoffman-Weeks plots—T₂ vs. crystallization temperature, T_c — it could be concluded that no thermody amic depression of the melting point of PVF₂ occurred in the blends. The stabilities of PVF₂ crystallites in the various blends were derived from the slopes of Hoffman-Weeks plots and were in good agreement with lamellar thicknesses found from SAXS measurements.     

COOHMWCNTs‐induced BiFeO3‐poly(vinylidene fluoride) (PVDF) composites with enhanced dielectric and ferroelectric properties

In this work, flexible three phase composite films were prepared with surface functionalized multi‐walled carbon nanotubes (f‐MWCNTs) and bismuth ferrite (BiFeO₃;BFO) particles embedded into the poly(vinylidene fluoride) (PVDF) matrix via solution casting technique. The properties and the microstructure of prepared composites were investigated using an impedance analyzer and field emission scanning electron microscope. The micro‐structural study showed that the f‐MWCNTs and BFO particles were dispersed homogeneously within the PVDF matrix, nicely seated on the floor of the f‐MWCNTs separately. The dielectric measurement result shows that the resultant composites with excellent dielectric constant (≈96) and relatively lower dielectric loss (<0.23 at 100 Hz). Furthermore, the percolation theory is explored to explain the dielectric properties of the resultant composites. It says that the percolation threshold of f_MWCNTs = 0.9 wt % and the enhancement of the dielectric constant of the composite was also discussed. In addition, the remnant polarization of the un‐poled PVDF‐BFO‐f‐MWCNTs composites (2P_r ～1.34 µC/cm² for 1.1 wt % of f‐MWCNTs) is also improved. These three phase composites provide a new insight to fabricate flexible and enhanced dielectric properties as a promising application in modern electrical and electronic devices. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46002.     

Thermal transitions,microstructure and miscibility in ternary polyblends based on poly(vinylidene fluoride)

The morphology, microstructure and compatibility of binary and ternary polyblends based on poly(vinylidene fluoride) have been studied by means of glass transition temperature, isothermal crystallization kinetics analysis and scanning electron microscopy in order to determine the possibilities offered by poly(vinyl acetate) for compatibilizing poly(vinylidene fluoride) and polystyrene. Calorimetric and dynamic mechanical analyses demonstrate that poly(vinyl acetate) tends to ‘compatibilize’ blends of PVF₂/PS, although the results of the two analyses do not agree with each other; we presume that this is due to differences in the techniques employed.     

聚偏氟乙烯/鳞片石墨/碳纤维高导热复合材料的制备

以聚偏氟乙烯（PVDF）树脂为基体,天然鳞片石墨（FG）、碳纤维（CF）为填料,采用熔融共混法制备了PVDF/FG/CF复合导热材料,并研究了FG、CF含量及其改性对复合材料导热性能和力学性能的影响。结果表明,复合材料的热导率随FG含量的增加而增大,力学性能随着FG含量的增加而降低;CF的加入提高了复合材料的力学性能,但热导率略有降低;对CF进行表面氧化处理将使得复合材料的热导率以及力学性能有所提高,当CF含量为5 %、FG含量为50 %时,复合材料的热导率为11.4 W/(m·K),拉伸强度为48 MPa,断裂伸长率为11 %。     

Structure,properties, and phase transitions of melt‐spun poly(vinylidene fluoride) fibers

Three different experimental techniques were used to study structural phase transitions in melt‐spun poly(vinylidene fluoride) fibers, which were produced with different process parameters and processed in the draw‐winding process at different temperatures and draw ratios. The fibers are examined with the help of wide‐angle X‐ray diffraction at elevated temperatures, differential scanning calorimetry with stochastic temperature modulation, and dynamic mechanical analysis. An oriented mesophase and deformed crystal structures can be observed in all fibers and assigned to the mechanical stress occurring in the processes. Furthermore, several phase transitions during melting and two mechanical relaxation processes could be detected. The observed transitions affect the crystal geometry, the orientation distribution, anisotropic thermal expansion, and the mechanic response of the fiber samples. The relaxation processes can be related with an increasing amount of crystalline β‐phase in fibers drawn at different temperatures. The detailed information about phase transitions and the related temperatures are used to produce fibers with an extended amount of β‐phase crystallites, which are responsible for piezoelectric properties of the material. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

FTIR-microspectroscopy and DSC studies of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) samples, obtained by casting from tetrahydrofuran solutions and submitted to various thermal treatments, have been examined by Fourier transform–infrared microspectroscopy (FTIR-M) and differential scanning calorimetry (DSC). This kind of analysis allowed us to examine microdomains of samples with different morphological characteristics and to obtain an indication of the polymorphism of PVDF. In some cases the simultaneous presence of two or three forms has been evidenced thanks to the comparison of FTIR-M spectra and DSC traces. Vibrational spectra of single crystalline forms can be recorded by FTIR-M on phase homogeneous microdomains.     

Formation of nanostructured composites with environmentally-dependent electrical properties based on poly(vinylidene fluoride)-polyaniline core-shell latex system

Submicron poly(vinylidene fluoride) (PVDF)/polyaniline (PANI) core-shell latex particles are synthesized and examined as an active component in a simple conductometric chemical sensor. The structure and physical properties of these particles and nanostructured composite PVDF-PANI polymer films built of them are characterized with transmission electron, atomic force, and helium ion microscopy techniques, differential scanning calorimetry, and conductivity measurements. The nanostructured composite films with conductivity of about 4 × 10⁻⁴ S/cm suitable for sensor applications are prepared by casting from the core-shell particles dispersions on glass substrates patterned with silver electrodes followed by annealing at 180 °C, i.e. above T_m of the PVDF component. Sensor properties of these films are tested by measuring current-voltage (I-V) characteristics in response to varying concentration of NH₃ or HCl vapors. The developed thin film sensor heterostructures with electrically conductive percolation network of PANI as an active component and employing the conductometric detection scheme show high sensitivity to both analytes. However, the polymer material is especially efficient for application to NH₃ sensing with the detection limit as low as 100 ppb, and good reproducible recovery behavior upon repeated exposure to NH₃ at ambient conditions.     

Spatially‐confined crystallization of poly(vinylidene fluoride)

Nanometre‐sized poly(vinylidene fluoride) (PVDF) particle domains in a confined space were obtained by blending PVDF with excess poly(methyl methacrylate) (PMMA). When these particles were small enough they showed β‐form structure, which was different from the structure of bigger particles or PVDF bulk. However, the β‐form was thermodynamically metastable because it could eventually be transformed to a more stable phase by annealing at a certain temperature. Larger particle domains were of identical phase to the bulk, indicating that small size favours the formation of the β‐form. © 2000 Society of Chemical Industry     

Thermal and electrical conductivity of carbon–filled liquid crystal polymer composites

The thermal and electrical conductivity of resins can be increased by adding conductive carbon fillers. One emerging market for thermally and electrically conductive resins is for bipolar plates for use in fuel cells. In this study, varying amounts of five different types of carbon, one carbon black, two synthetic graphites, one natural flake graphite, and one calcined needle coke, were added to Vectra A950RX Liquid Crystal Polymer. The resulting composites containing only one type of filler were then tested for thermal and electrical conductivity. The objective of this work was to determine which carbon filler produced a composite with the highest thermal and electrical conductivity. The results showed that composites containing Thermocarb TC‐300 synthetic graphite particles had the highest thermal and electrical conductivity. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99; 1552–1558, 2006