Effect of the compatibility on dielectric performance and breakdown strength of poly(vinylidene fluoride)/low‐density polyethylene blends

Immiscible polymer blends with high dielectric constant (ε) and improved breakdown strength (E_b) performance were obtained by composing poly(vinylidene fluoride) (PVDF) with low‐density polyethylene (LDPE) or the LDPE grafted with maleic anhydride (LDPE‐g‐MAH) through melt‐blending way. The dielectric properties of these blends were emphasized for considering the compatibility effect on the energy storage application. Interface morphology, co‐continuity behavior, and grafted ratio were simultaneously investigated to detect the compatibility enhancement after introducing MAH. Results showed that the MAH positively improved the dielectric properties. Both the measured E_b of PVDF/LDPE and PVDF/LDPE‐g‐MAH blends showed a minimum value at v^PVDF = 50 vol % because of the worst compatibility; meanwhile, higher E_b of PVDF/LDPE‐g‐MAH than that of PVDF/LDPE blend was observed owing to the better compatibility. For considering the effect interface morphology on the dielectric performance, layer‐structure films composing with pure PVDF and LDPE layers were further constructed and studied. It was revealed that the layered structure could be treated as a helpful way to improve ε and E_b for immiscible polymer blends. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42507.     

Enhanced piezoelectric responses and crystalline arrangement of electroactive polyvinylidene fluoride/magnetite nanocomposites

The well distributed and electroactive polyvinylidene fluoride (PVDF)/magnetite nanocomposites were successfully fabricated using a mixed solvent system (THF/DMF). Dynamic mechanical properties of the fabricated PVDF/magnetite nanocomposites indicate significant enhancements in the storage modulus as compared with that of neat PVDF. By adding 2 wt % magnetite nanoparticles into the PVDF matrix, the thermal stability of nanocomposites could be enhanced about 26°C as compared with that of PVDF. The β‐phase fraction of PVDF is significantly enhanced with increasing the voltage of electric field poling. The piezoelectric responses of PVDF/magnetite films are extensively increased about five times in magnitude with applied strength of electrical field at 35 MV/m. The change of piezoelectric responses during the applied electric field may be due to the relative long arrangement of PVDF units along the direction of electric field poling and thus increases the values of L_p^* and l_c. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40941.     

Characterization of Solution‐Processed Double‐Walled Carbon Nanotube/Poly(vinylidene fluoride) Nanocomposites

Dispersion of CNTs in polymers can yield impressive property enhancements at low volume fractions, thus maintaining the inherent processability of the polymer. In particular, they can improve the electromechanical response of piezoelectric polymers by lowering the actuation voltage and increasing strain and stress response. In this work, piezoelectric PVDF and DWNTs are solution‐cast into films. SEM of fracture surfaces confirms good dispersion, and electrical conductivity measurements reveal a low percolation threshold (0.23 vol.‐%). The effect of CNTs on storage modulus, T_c, T_m and T_g of PVDF is studied. Electromechanical strain is observed at low actuation voltages, possibly due to enhanced local electric field in the presence of DWNTs.

     

Effect of polyvinylidene fluoride on the fracture microstructure characteristics and piezoelectric and mechanical properties of 0-3 barium zirconate titanate ceramic-cement composites

Barium zirconate titanate (40−60 vol.%; BZT), Portland cement (PC) and polyvinylidene fluoride (0−7 vol.%; PVDF) were used as raw materials to produce 0–3 piezoelectric cement-based composites. The highest piezoelectric charge coefficient (d₃₃∼26-27 pC/N) was found at 50−60 vol.% BZT with 5 vol.% PVDF. Moreover, the composite with 50 vol.% BZT and 5 vol.% PVDF had the highest piezoelectric voltage coefficient (g₃₃ = 16.0 × 10⁻³ V·m/N). Scanning electron microscopy was used to investigate the morphology of the fracture surface of the composite. When PVDF was used in the composite, it was observed to fill some pores at the interface zone and within the cement phase. The elastic behaviour of PVDF could also be seen in the fracture surface, where it appeared as a stretched material different from both the BZT ceramic and cement, which are brittle materials. In addition, increasing the PVDF content led to increased fracture toughness.     

Effect of flow history on poly(vinylidine fluoride) crystalline phase transformation

This study was devoted to the effect of extensional flow during film extrusion on the formation of the β‐crystalline phase and on the piezoelectric properties of the extruded poly(vinylidine fluoride) (PVDF) films after cold drawing. The PVDF films were extruded at different draw ratios with two different dies, a conventional slit die and a two‐channel die, of which the latter was capable of applying high extensional flow to the PVDF melt. The PVDF films prepared with the two‐channel die were drawn at different temperatures, strain rates, and strains. The optimum stretching conditions for the achievement of the maximum β‐phase content were determined as follows: temperature = 90°C, strain = 500%, and strain rate = 0.083 s^?1. The samples prepared from the dies were then drawn under optimum stretching conditions, and their β‐phase content and piezoelectric strain coefficient (d₃₃) values were compared at equal draw ratios. Measured by the Fourier transform infrared technique, a maximum of 82% β‐phase content was obtained for the samples prepared with the two‐channel die, which was 7% higher than that of the samples prepared by the slit die. The d₃₃ value of the two‐channel die was 35 pC/N, which was also 5 pC/N higher than that of the samples prepared with the slit die. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of heat treatment on the electrical properties of lead zirconate titanate/poly (vinylidene fluoride) composites

Ceramic/polymer composites are attracting increasing interest in materials research and practical applications due to the combination of excellent electric properties of piezoelectric ceramics and good flexibility of polymer matrices. In this case, the crystallization of the polymer has a significant effect on the electric properties of ceramic/polymer composites. Based on different heat treatment methods, the crystallization of poly(vinylidene fluoride) (PVDF) in composites of lead zirconate titanate (PZT) and PVDF can be controlled effectively. PZT/PVDF composites with various PVDF crystallizations exhibit distinctive dielectric and piezoelectric properties. When the crystallization of PVDF is 21%, the PZT/PVDF composites show a high dielectric constant (ε) of 165 and a low dielectric loss (tan δ) of 0.03 at 10³ Hz, and when the crystallization of PVDF reaches 34%, the piezoelectric coefficient (d₃₃) of PZT/PVDF composites can be up to ca 100 pC N^?1. By controlling the crystallization of PVDF, PZT/PVDF composites with excellent dielectric and piezoelectric properties were obtained, which can be employed as promising candidates in high‐efficiency capacitors and as novel piezoelectric materials. Copyright © 2010 Society of Chemical Industry     

Effect of titanium dioxide (TiO2) distribution and minute amounts of carbon black on the opacity of PVDF based white composite films

There is a contradiction in making completely opaque and white plastic film with a required high TiO₂ filling fraction, which resulted in inefficient pigment utilization and high cost. Two methods were used here to overcome the contradiction. Firstly, TiO₂ was grafted with poly(methyl methacrylate) (PMMA) by atom transfer radical polymerization to improve the pigment dispersion in poly(vinylidene) fluoride (PVDF). Secondly, minute amounts of carbon black (CB) were added into the PMMA‐g‐TiO₂/PVDF system to enhance opacity and decrease TiO₂ fraction. The structure, morphology, and properties of PMMA‐g‐TiO₂ hybrid particles and composite films were investigated by FTIR, TEM, TGA, SEM, DMA, covering power meter, and UV/VIS spectrophotometer, etc. It was observed that PMMA‐g‐TiO₂ was dispersed uniformly as individual particles in PVDF due to the good compatibility between PMMA and PVDF. Therefore, the opacity of PMMA‐g‐TiO₂/PVDF films was markedly higher than unmodified‐TiO₂/PVDF ones. Adding minute amounts of CB can significantly increase the opacity of the thin film due to its absorption effect on decreasing light transmittance. The contrast ratio (CR) of the PMMA‐g‐TiO₂/PVDF film with 20 μm thickness and 25 vol % TiO₂ was 97.67%, lower than the critical CR 98% for a complete opacity, however, the CR of sample with 10 vol % TiO₂ was 98.1% as the CB concentration was 2 × 10^?4 g/cm³, saved more than 15% TiO₂. We proposed that a critical thickness d₀ existed for the CB/PMMA‐g‐TiO₂/PVDF composite films, under which the light reflectance increased as a function of thickness, otherwise, the reflectance kept constant. Besides, d₀ could be decreased by increasing CB concentration. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43064.     

Influence of extrusion,stretching and poling on the structural and piezoelectric properties of poly (vinylidene fluoride‐hexafluoropropylene) copolymer films

Three types of poly (vinylidene fluoride‐hexafluoropropylene) (PVDF–HFP) copolymer films were prepared by extrusion, stretching as well as simultaneously stretching and static electric field poling (SSSEP), respectively, and measured by the differential scanning calorimetric, wide angle X‐ray diffraction, fourier transformation infrared‐attenuated total reflection, and Dynamic mechanical analysis. The experimental results showed that the films prepared by stretching and SSSEP have higher crystallinity and β phase than by extrusion. SSSEP improved the chain orientation enormously both in crystalline and amorphous regions, resulting in the highest storage modulus. Because of the lower β phase content, the extruded films exhibited the lowest piezoelectric coefficient d₃₃. For the stretched and SSSEP films, although the β phase content was similar, the d₃₃ was distinct because of the different potential energy for the rotation of the dipoles. In addition, the SSSEP films gave the maximum d₃₃ (24 pC/N), higher than the other PVDF–HFP copolymer films that have been reported. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 858–862, 2007     

Piezoelectric Properties of a Pioneering 3‐1 Type PZT/Epoxy Composites Based on Freeze‐Casting Processing

Lead zirconate titanate (PbZr_1 ? xTi_xO₃, PZT)/epoxy composites with one‐ dimensional epoxy in PZT matrix (called 3‐1 type piezocomposites) have been fabricated by tert‐butyl alcohol (TBA)‐based directional freeze casting of PZT matrix and afterward infiltration of epoxy. The composites with PZT volume fraction ranging from 0.36 to 0.69 were obtained by adjusting initial solid loading in freeze‐casting slurry. The effect of poling voltage on piezoelectric properties of the composites was studied for various volume fraction of PZT phase. With the increasing of PZT volume fraction, relative permittivity (ε_r) increased linearly and piezoelectric coefficient (d₃₃ and d₃₁) increased step by step. The resultant composites with 0.57 PZT volume fraction possessed the highest hydrostatic piezoelectric strain coefficient (d_h) value (184 pC/N), voltage coefficient (g_h) value (13.6 × 10^?3 V/m Pa), and hydrostatic figure of merit (HFOM) value (2168 × 10^?15 Pa^?1).     

Experimental investigation of electrostrictive polarization biased direct apparent piezoelectric properties in polyurethane elastomer under quasistatic conditions

Polyurethane elastomer was recently discovered to demonstrate a very high field induced electrostrictive response. In this work an experimental setup, consisting of an electric circuit and a mechanical system, was designed and constructed for the measurement of the electrostrictive polarization biased apparent piezoelectric response of polyurethane elastomers in a direct piezoelectric effect under quasistatic conditions. The electric circuit design allows the application of a direct current (dc) bias electric field to the sample and the possibility of picking up the generated quasistatic electrical signal separately. The mechanical system provides the function of a vibration source from which the stress and strain of the sample can be measured. Therefore, such effective piezoelectric properties as d₃₁ and k₃₁ can be measured. The electromechanical coupling coefficient was derived by two different methods. One was from the deduction based on the piezoelectric equations. The other was from the calculation based on the basic definition of the electromechanical coupling coefficient (i.e., through the exact measurement of input mechanical energy and output electric energy). In the latter case, the internal resistance of the sample and the dc bias blocking capacitor were found to be the critical factors for precision determination of the total electrical energy output. The different approaches led to close agreement. The effective d₃₁ can be 184 pC/N under a 25 MV/m bias electric field in a 30-µm thick sample, which is much higher than that of typical piezoelectric polymers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2603–2609, 1999     

Effect of processing parameters of the continuous wet spinning system on the crystal phase of PVDF fibers

Poly(vinylidene difluoride) (PVDF) has been widely used in piezoelectric applications as films and nanofiber mats, but there are limited publications on piezoelectric wet‐spun fibers. In this work, PVDF fibers were prepared using the wet spinning method, and the processing parameters, including the drawing ratio and heat setting temperature, were controlled in the continuous wet spinning system to increase the β‐phase crystallinity of the fibers. In addition, the wet‐spun PVDF fibers were compressed by a rolling press to eliminate voids in the fibers. Then, the compressed PVDF fibers were poled to align the molecular dipoles. The crystal structures of the PVDF fibers were investigated using X‐ray diffraction and Fourier‐transform infrared spectroscopy. Single filament tensile tests were performed to measure the tensile strength of the fibers. The morphologies of the PVDF fibers with respect to the processing parameters were observed by scanning electron microscope (SEM) and polarization optical microscopy. The piezoelectric constant of the prepared PVDF fibers was then measured using a d₃₃ meter. The wet‐spun PVDF fibers showed the highest β‐phase and piezoelectric constants when the drawing ratio and heat setting temperature were 6 and 150 °C, respectively. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45712.     

New piezoelectric damping composites of poly(vinylidene fluoride) blended with clay and multi‐walled carbon nanotubes

Damping materials are used to control mechanical vibrations, and piezoelectric damping composite is a very promising material due to its unique mechanism. In this study, a potential piezoelectric damping composite was developed by simply melt mixing poly(vinylidene fluoride) (PVDF) with small amounts of organic modified montmorillonite (OMMT) and multi‐walled carbon nanotubes (MWCNTs). The piezoelectric, mechanical and electrical properties were investigated using a dynamic mechanical analyser, direct current electrical resistivity measurements, X‐ray diffraction, Fourier transform infrared spectroscopy and the direct quasi‐static d₃₃ piezoelectric coefficient method. It was found that the damping property of PVDF can be greatly improved by adding both MWCNTs and OMMT, and the composite containing 1.9 wt% of MWCNTs and 3 wt% of OMMT showed the best damping property. A model and an approximate calculation were applied to explain the improved damping property. Moreover, similar mechanical properties of PVDF composites were observed in the tensile testing and dynamic mechanical analyser measurements. Copyright © 2012 Society of Chemical Industry     

An experimental model for predicting the piezo and dielectric constant of PVDF-PZT nanocomposite fibers with 0–3 and 1–3 connectivity

Energy harvesting from mechanical energy around ambient by flexible nanogenerators is one of the most efficient ways to generate green and renewable energy. Lead zirconate titanate (PZT) particles were embedded into a polyvinylidene fluoride (PVDF) polymer matrix to prepare mixed 0–3 and 1–3 connectivity nanocomposite fibers by electrospinning method. Various theoretical models of Maxwell-Garnett, Rayleigh, and Tinga etc were presented at two different Classes to predict the dielectric constant of PVDF-PZT nanocomposite fibers and compared the predicted results with the experimental results. Also, the piezoelectric properties like the piezoelectric coefficient (d₃₃) and piezoelectric voltage coefficient (g₃₃) were predicted by the Furukawa model and the predicted values were compared with the experimental values. Finally, the experimental model was derived to predict the dielectric constant of binary composites with mixed 0–3 and 1–3 connectivity. Compared to well-known models, the proposed experimental model accurately predicted the dielectric constant of PVDF-PZT nanocomposite fibers. The highest and lowest difference between the theoretical and the experimental results were obtained 12.24% and 0.12% for PZT volume fractions 1.1 and 17, respectively. Also, due to the linear relationship between the dielectric constant and piezoelectric coefficients, this model was generalized to predict the piezoelectric coefficients.     

Small Reduction of the Piezoelectric d33 Response in Potassium Sodium Niobate Thick Films

We have investigated the electromechanical response of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. The high‐field strain hysteresis loops and weak‐field converse piezoelectric d₃₃ coefficient of the films were measured and compared with those of KNN bulk ceramics under the same electric field conditions. The converse d₃₃ values of the thick films and bulk ceramics were equal to 82.5 and 138 pm/V, respectively, at 0.4 kV/mm. The fundamental difference between the piezoelectric response of the KNN films and the ceramics was studied in terms of the effective (“clamped”) piezoelectric d₃₃ coefficient. The reduction in the piezoelectric d₃₃ coefficient of the KNN films, resulting from the clamping by the substrate, was compared to lead‐based ferroelectric thick films, including Pb(Zr,Ti)O₃ (PZT) and (1 ? x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ (PMN‐PT). We propose a possible explanation, based on the particular elastic properties of KNN, for the small relative difference observed between the “clamped” and “unclamped” (“bulk”) d₃₃ of KNN, in comparison with lead‐based systems.     

Mechanical and piezo‐resistive properties of styrene–butadiene–styrene copolymer covalently modified with graphene/styrene–butadiene–styrene composites

As novel piezoelectric materials, carbon‐reinforced polymer composites exhibit excellent piezoelectric properties and flexibility. In this study, we used a styrene–butadiene–styrene triblock copolymer covalently grafted with graphene (SBS‐g‐RGO) to prepare SBS‐g‐RGO/styrene–butadiene–styrene (SBS) composites to enhance the organic solubility of graphene sheets and its dispersion in composites. Once exfoliated from natural graphite, graphene oxide was chemically modified with 1,6‐hexanediamine to functionalize with amino groups (GO–NH₂), and this was followed by reduction with hydrazine [amine‐functionalized graphene oxide (RGO–NH₂)]. SBS‐g‐RGO was finally obtained by the reaction of RGO–NH₂ and maleic anhydride grafted SBS. After that, X‐ray diffraction, X‐ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and other methods were applied to characterize SBS‐g‐RGO. The results indicate that the SBS molecules were grafted onto the graphene sheets by covalent bonds, and SBS‐g‐RGO was dispersed well. In addition, the mechanical and electrical conductivity properties of the SBS‐g‐RGO/SBS composites showed significant improvements because of the excellent interfacial interactions and homogeneous dispersion of SBS‐g‐RGO in SBS. Moreover, the composites exhibited remarkable piezo resistivity under vertical compression and great repeatability after 10 compression cycles; thus, the composites have the potential to be applied in sensor production. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46568.     

Melt electrowriting of electroactive poly(vinylidene difluoride) fibers

Poly(vinylidene difluoride) (PVDF) has piezoelectric properties suitable for numerous applications such as flexible electronics, sensing and biomedical materials. In this study, individual fibers with diameters ranging from 17 to 55 µm were processed using melt electrowriting (MEW). Electroactive PVDF fibers can be fabricated via MEW, while the polymer can remain molten for up to 10 h without noticeable changes in the resulting fiber diameter. MEW processing parameters for PVDF were investigated, including applied voltage, pressure and temperature. A rapid fiber characterization methodology for MEW that automatically determines the fiber diameters from camera images taken of microscope slides was developed and validated. The outputs from this approach followed previous MEW processing trends already identified with different polymers, although overestimation of fiber diameters <25 µm was observed. The transformation of the PVDF crystalline phase to the electroactive β phase was confirmed using piezo‐force microscopy and revealed that the PVDF fibers possess piezoelectric responses showing d₃₃ ≈ 19 pm V^–1. © 2018 Society of Chemical Industry     

Photonic Sintering of Aerosol Jet Printed Lead Zirconate Titanate (PZT) Thick Films

Lead Zirconate Titanate (PZT) is a commonly used piezoelectric material due to its high piezoelectric response. We demonstrate a new method of printing and sintering micro‐scale PZT films with low substrate temperature increase. Self‐prepared PZT ink was Aerosol‐Jet printed on stainless steel substrates. After drying for 2 h in vacuum at 200°C, the printed PZT films were divided into two groups. The first group was traditionally sintered, using a thermal process at 1000°C for 1 h in an Argon environment. The second group was photonically sintered using repetitive sub‐msec pulses of high intensity broad spectrum light in an atmospheric environment. The highest measured substrate temperature during photonic sintering was 170.7°C, enabling processing on low melting point substrates. Ferroelectric measurements were performed with a low‐frequency sinusoidal signal. The remanent polarization (P_r) and coercive field (E_c) for thermally sintered PZT film were 17.1 μC/cm² and 6.3 kV/cm, respectively. The photonically sintered film had 32.4 μC/cm² P_r and 6.7 kV/cm E_c. After poling the samples with 20 kV/cm electric field for 2 h at 150°C, the piezoelectric voltage constant (g₃₃) was measured for the two film groups yielding ?16.9 × 10^?3 (V·m)·N^?1 (thermally sintered) and ?17.9 × 10^?3 (V·m)·N^?1 (photonically sintered). Both factors indicate the PZT films were successfully sintered using both methods, with the photonically sintered material exhibiting superior electrical properties. To further validate photonic sintering of PZT on low melting point substrates, the process and measurements were repeated using a polyethylene terephthalate (PET) substrate. The measured P_r and E_c were 23.1 μC/cm² and 5.1 kV/cm, respectively. The g₃₃ was ?17.3 × 10^?3 (V·m)·N^?1. Photonic sintering of thick film PZT directly on low melting point substrates eliminates the need for complex layer transfer processes often associated with flexible PZT transducers.     

Polymer multilayer films for high temperature capacitor application

Advanced film capacitors require polymers with high thermal stability, high breakdown strength, and low loss for high temperature dielectric applications. To fulfill such requirements, two polymer multilayer film systems were coextruded via the forced assembly technique. High glass transition temperature (T _g) polycarbonate (HTPC, T_g = 165 °C) and polysulfone (PSF, T_g = 185 °C) were multilayered with a high dielectric constant polymer, poly(vinylidene fluoride) (PVDF), respectively. The PSF/PVDF system was more thermally stable than the HTPC/PVDF system because of the higher T_g for PSF. At temperatures lower than 170 °C, the HTPC/PVDF system exhibited comparable breakdown strength and hysteresis loss as the PSF/PVDF system. While at temperatures above 170 °C, the PSF/PVDF system exhibited a higher breakdown strength because of the higher T_g of PSF. The electric displacement-electric field (D-E) loop behavior of the PSF/PVDF system was studied as a function of temperature. Moreover, a melt-recrystallization process could further decrease the hysteresis loss for the PSF/PVDF system due to better edge-on crystal orientation. These results demonstrate that PSF/PVDF and HTPC/PVDF systems are applicable for high temperature film capacitors. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47535.     

Properties of CNTs-modified lead-free BCTS ceramic–Portland fly ash cement composites

Fly ash (FA) is widely used as a supplementary cementitious material in the production of Portland cement concrete. The effect of addition of carbon nanotubes (CNTs) and FA on the properties of barium calcium stannate titanate (BCTS) ceramic–Portland FA cement composites was investigated. These composites have potential for use as sensors and transducers in the monitoring of structural health in concrete structures containing FA. CNTs were found to have filled the pores of the composites. All composites showed good compatibility with the concrete mix. The dielectric constant and electrical conductivity of composites were in the range 200–257 and 1.04 × 10^–6 to 1.66 × 10⁻⁶ S/m, respectively. The presence of FA in composites increased the piezoelectric voltage coefficient (g₃₃). Adding CNTs increased the piezoelectric charge coefficient (d₃₃), thickness electromechanical coupling coefficient (K_t), and also g₃₃ but decreased mechanical quality factor (Q_m), which is related to good for the receiving sensor and transducer application. CNTs can improve the properties of these composites and composite with FA content at 10 vol.%, and CNTs at 1 vol.% exhibited the highest compressive strength and piezoelectric values (d₃₃ = 44 pC/N, g₃₃ = 20.21×10^–3 V m/N, and K_t = 18.9%), along with higher g₃₃ values, than pure BCTS ceramic.