Enhancement of dielectric and energy density properties in the PVDF‐based copolymer/terpolymer blends

We investigated the high dielectric constant and energy storage density for the blends of P(VDF‐TrFE) copolymer and P(VDF‐TrFE‐CFE) terpolymer. The degradation of coercive field (E_c) and remnant polarization (P_r) of the copolymer under an electric field of 125 MV/m was observed and the copolymer changed into a typical relaxor ferroelectric with doping of terpolymer. The dielectric constant of P(VDF‐TrFE) was found to be ～11, but was enhanced to ～55 by blending with P(VDF‐TrFE‐CFE) at 60 wt%. Consequently, a higher energy density of about 4.2 J/cm³ was obtained in these blends in contrast to about 3.6 J/cm³ in the terpolymer at the very low applied electric field of 125 MV/m. These results demonstrate the promise of blend approaches for tailoring and enhancing the dielectric properties of ferroelectric polymers. POLYM. ENG. SCI., 55:1396–1402, 2015. © 2015 Society of Plastics Engineers     

Influence of the processing parameters on the electrocaloric effect of poly(vinylidene fluoride–trifluoroethylene) copolymers

The influence of the processing parameters on the electrocaloric effect of a ferroelectric polymer was investigated. A normal ferroelectric poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] (55:45 mol %) copolymer was selected and fabricated. The crystallinity, microstructure, mechanical properties, and refrigeration capability of this copolymer were characterized with X‐ray diffraction, scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis, respectively. We found that N,N‐dimethylformamide was the most suitable solvent for enhancing the crystallinity of the copolymer film. In combination with the consideration of the effect of the processing parameters on the microstructure of copolymer, a lower spin‐coating temperature and annealing temperature at 140 °C gave the highest crystallinity and uniform microstructure without defects. The highest temperature change was 12.13 °C when the copolymer film was operated at 110 °C under a higher electric field. The direct measurement of the refrigeration was carried out with an IR thermometer. We found that the P(VDF–TrFE) copolymer film had a considerable refrigeration capability, which increased as the applied electric field strength increased before electric breakdown. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44413.     

Crystal phase of poly(vinylidene fluoride‐co‐trifluoroethylene) synthesized via hydrogenation of poly(vinylidene fluoride‐co‐chlorotrifluoroethylene)

Trifluoroethylene addition and thermal treatment induced crystal phase transition in a series of poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐co‐TrFE)] containing varied TrFE molar ratio (6, 9, 12, and 20 mol %) prepared from the hydrogenation of poly(vinylidene fluoride‐co‐chlorotrifluoroethylene have been investigated by means of Fourier transform infrared spectral (FTIR), X‐ray diffraction (XRD), and differential scanning calorimetry (DSC) analyses. The comprehensive applications of the three techniques could distinguish α, β and γ phase of P(VDF‐co‐TrFE) very well. The multipeak fitting technique of DSC is successfully applied to calculate the percentage of different phases in the samples, which allows us to investigate the phase transition process of P(VDF‐co‐TrFE) precisely. It is found that the crystal phase of P(VDF‐co‐TrFE) films is turned from α + γ phase (6 mol % TrFE) to α + γ + β phase (9 and 12 mol % TrFE) to β phase (20 mol % TrFE) at high temperature, and from α + γ phase (6 mol % TrFE) to γ + β phase (9 mol % TrFE) to β phase (>12 mol % TrFE) at low fabricated temperature. Both the fabrication conditions and TrFE addition are responsible for the crystal phase transition of the hydrogenised P(VDF‐co‐TrFE). © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Facile preparation of ferroelectric poly(vinylidene fluoride‐co‐trifluoroethylene) thick films by solution casting

We investigated the effects of annealing temperature and vacuum treatment on the crystallinity and ferroelectric properties of solution‐casted poly(vinylidenefluoride‐co‐trifluoroethylene), P(VDF‐TrFE), thick films. We varied the annealing temperature from 70°C to 150°C and achieved high‐quality ferroelectric thick films annealed at 130°C. Ferroelectric domains and their properties were confirmed using X‐ray diffraction, Fourier transform infrared spectroscopy with attenuated total reflection mode and ferroelectric/piezoelectric measurement systems. Drying and/or annealing in the vacuum allowed for the improvement of crystallinity and ferroelectric/piezoelectric properties. Importantly, the piezoelectric coefficient, d₃₃, of our optimal P(VDF‐TrFE) films after sufficient poling treatment was 36 pC/N and our P(VDF‐TrFE) power generator produced an output voltage of ～6 V under periodic bending and unbending motions. POLYM. ENG. SCI., 54:466–471, 2014. © 2013 Society of Plastics Engineers     

Miscibility and morphology of poly(vinylidene fluoride)/poly[(vinylidene fluoride)‐ran‐trifluorethylene] blends

This study presents an investigation of the effect of the different crystalline phases of each blend component on miscibility when blending poly(vinylidene fluoride) (PVDF) and its copolymer poly[(vinylidene fluoride)‐ran‐trifluorethylene] [P(VDF–TrFE)] containing 72 mol % of VDF. It was found that, when both components crystallized in their ferroelectric phase, the PVDF showed a strong effect on the crystallinity and phase‐transition temperature of the copolymer, indicating partial miscibility in the crystalline state. On the other hand, immiscibility was observed when both components, after melting, were crystallized in their paraelectric phase. In this case, however, a decrease in crystallization temperatures suggested a strong interaction between monomers in the liquid state. Blend morphologies indicated that, in spite of the lack of miscibility in the crystalline state, there is at least miscibility between PVDF and P(VDF–TrFE) in the liquid state, and that a very intimate mixture of the two phases on the lamellar level can be maintained upon crystallization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1362–1369, 2002     

Structure of P(VDF‐TrFE) (80/20) copolymers under electron irradiation and recrystallization

The structural changes of irradiated and recrystallized copolymers of vinylidene fluoride and trifluoroethylene [P(VDF‐TrFE)] (80/20) copolymers have been investigated through IR spectra, X‐ray, and DSC. It is found that recrystallization has a reverse effect on irradiated P(VDF‐TrFE) copolymers, by which the irradiated samples are turned back to original nonirradiated state partially. During recrystallization, trans‐gauche sequences are replaced by all‐trans conformation gradually and the irradiated P(VDF‐TrFE) (80/20) copolymers change from nonpolar phase to polar one. The average crystallite size of polar phase is larger than that in irradiated samples, and the difference between recrystallized and irradiated samples reduces gradually with irradiation dosage. The C?C and conjugated C?C bonds are also found in the recrystallized‐irradiated P(VDF‐TrFE) (80/20) due to rearrangement of broken bonds. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4258–4263, 2006     

Electrical energy discharging performance of poly(vinylidene fluoride‐co‐trifluoroethylene) by tuning its ferroelectric relaxation with polymethyl methacrylate

Poly(methyl methacrylate) (PMMA) was introduced into ferroelectric Poly(vinylidene fluoride‐co‐trifluoroethylene) P(VDF‐co‐TrFE) via a simple solution blending process and a series of P(VDF‐co‐TrFE)/PMMA blends with varied PMMA content was obtained in an effort to investigate the confinement effect of PMMA on the crystalline, dielectric, and electric energy storage properties of P(VDF‐co‐TrFE). PMMA addition could reduce the crystallinity dramatically as well as the crystal size due to its dilution effect and impediment effect on the crystallization of P(VDF‐co‐TrFE). PMMA introduction is also responsible for the phase transition of P(VDF‐co‐TrFE) from α phase into γ phase. As expected, both the dielectric constant and loss of the blends are reduced as PMMA addition increases for the dilute, decoupling, and confinement effect of PMMA on the relaxation behavior of crystal phases of P(VDF‐co‐TrFE) under external electric field. As a result, both the maximum and remnant polarization of the blends are significantly depressed. The irreversible polarization of P(VDF‐co‐TrFE) is effectively restricted by the addition of PMMA due to its impeding effect on the crystallization of P(VDF‐co‐TrFE) and restricting effect on the switch of the polar crystal domains. Therefore, the energy loss induced by the ferroelectric relaxation of P(VDF‐co‐TrFE) is significantly reduced to less than 25% at an electric field of 450 MV/m while the energy storage density is well maintained at about 10 J/cm^?3 in the blend with 30 wt % PMMA. The results may help to understand how the ferroelectric relaxation affects the energy loss of ferroelectrics fundamentally and design more desirable materials for high energy storage capacitors. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40114.     

Synthesis and ferroelectric investigations of poly(vinylidene fluoride‐co‐hexafluoropropylene)‐Mg(NO3)2 films

The free‐standing, flexible, and ferroelectric films of poly(vinylidenefluoride‐co‐hexafluoropropylene) [P(VDF‐HFP)] were prepared by spin coating method. The ferroelectric phase of the films was enhanced by adding magnesium nitrate Mg(NO₃)₂ in different wt % as the additive during the film fabrication. The effects on the structural, compositional, morphological, ferroelectric, dielectric, and leakage current behaviors of the films due to the addition of salt were analyzed. Based on the X‐ray diffraction (XRD) patterns and Fourier Transform Infrared (FTIR) spectra, it is confirmed that the addition of Mg(NO₃)₂ promotes the electroactive β phase that induces the ferroelectric property. The fiber‐like topography of the films exhibits a nodule‐like structure, and the roughness of the films increases by the addition of Mg(NO₃)₂. The ferroelectric studies show the higher polarization values for the composite films than that of the plain P(VDF‐HFP) film. The Piezo‐response force microscope images also confirm the domain switching behavior of the samples. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44008.     

Evolution of morphology,ferroelectric, and mechanical properties in poly(vinylidene fluoride)–poly(vinylidene fluoride‐trifluoroethylene) blends

Considering the complementary properties of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride‐trifluoroethylene) [P(VDF‐TrFE)], it appears that their blends have the potential to be promising candidates for device applications. We report the evolution of morphology, ferroelectric, and mechanical properties (modulus and hardness) and their dependence on preparation temperature for PVDF–P(VDF‐TrFE) blends. From ferroelectric hysteresis measurements it was found that P(VDF‐TrFE) rich blends treated at higher temperature show significant values of remanent polarization. Remanent polarization values show a fourfold increase in these P(VDF‐TrFE) rich blends treated at higher temperature. Interestingly, blends prepared from high temperature showed greater value of remanent polarization even though they were found to consist of smaller amount of electroactive phase as compared to their low temperature treated counterpart. Nanoindentation experiments revealed that high temperature treatment improves the modulus of blends by at least 100%. This report attempts to tie these findings to the morphology and crystallinity of these blends. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45955.     

Curie transition and piezoelectricity of the blends of a ferroelectric VDF/TrFE copolymer and PMMA

The Curie transition, even though the conformational change at the Curie transition primarily arises from intermolecular interaction, is highly dependent on the crystallization conditions. A slower cooling rate from the melt during paraelectric crystallization lowers T_c, increases the portion of F_β at the expense of F_α reduction, and produces a more unstable ferroelectric phase. T is rarely dependent upon the amount of PMMA, but T is increased with the PMMA content. PMMA has a favorable action in forming a more stable ferroelectric phase in the P(VDF/TrFE)/PMMA blend and elevating the Curie transition point because of the all-trans sequence conformation of PMMA and a specific intermolecular interaction with P(VDF/TrFE) in the melt state. However, PMMA reduces the total amount of the crystalline phase, the electric response, and the piezoelectricity. © 1993 John Wiley & Sons, Inc.     

Structural analysis of the P(VDF/TrFE) copolymer film

We have examined the morphology of an ultra-thin film P(VDF/TrFE) copolymer by the atomic force microscopy and the transmission electron microscopy, which show the same images with “rod-like” structures in the size of in width and few hundreds nanometer in length. In the electron diffraction pattern, several rings are observed which indicate that the crystallites have no preferred orientation. According to the sharpest and strongest observed Bragg spacing from inner to outer ring, and to the strongest diffraction which is related to the d(h00) spacing, we can conclude that the crystals tend to form a fibrous texture keeping the a-axis aligned parallel to the substrate surface. These results enable us to improve our knowledge on the mechanism of the ferroelectric and piezoelectric activities in P(VDF/TrFE) copolymer and thus constitute a new study on the crystallite axis orientation.     

Dielectric,piezoelectric and ferroelectric properties of a poly (vinylidene fluoride-co-trifluoroethylene) synthesized via a hydrogenation process

The dielectric, piezoelectric and ferroelectric properties of a poly (vinylidene fluoride-co-trifluoroethylene) (P (VDF-co-TrFE)) copolymer synthesized via a novel hydrogenation process are presented in this work. Comparing with the direct copolymerized P(VDF-co-TrFE), the head-to-head (H–H) connection of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) units in hydrogenized P(VDF-co-TrFE) results in the increasing crystal domains and the improvement of overall crystallinity and. Therefore, excellent electric properties, including dielectric constant of 11 at a frequency of 100 Hz, a piezoelectric value (d₃₃) of ?23 pC/N, a remnant polarization (P_r) of 7 μC/cm² and a maximum polarization (P_s) of 10 μC/cm² were observed in hydrogenized P(VDF-co-TrFE), which are rather close to that obtained in copolymerized copolymer. The results indicate that the improved condensed structures offer the low cost and convenient hydrogenized P(VDF-co-TrFE) broader application scenery in piezoelectric devices than the direct copolymer.     

Pyroelectric and dielectric properties of spin‐coated thin films of vinylidene fluoride–trifluoroethylene copolymers

This work emphasizes the use of vinylidene fluoride and trifluoroethylene copolymer P(VDF‐TrFE) as a pyroelectric sensor. The pyroelectric and dielectric properties of the copolymer have been investigated in the temperature interval 150–350 K. The samples were prepared by using a spin‐coating technique with 70/30 mol% VDF/TrFE copolymer. The final film thickness of the samples, which is mainly determined by the concentration of the copolymer, spinning rate and spin time, was measured with a surface profiler. The samples were annealed at 150 °C for 10 min to improve the crystallinity of the copolymer. The crystallinity of the annealed and non‐annealed samples was compared by IR spectroscopy. The most effective process by which to improve the pyroelectric response of the material is to pole the sample with huge poling field‐strengths at elevated temperatures. Both pyroelectric and dielectric activities of the samples were measured after each successful poling process. It was observed that while the pyroelectric activity of the material increases, the dielectric activity decreases, so the figure‐of‐merit of the material, which shows the sensor capability of the material, was increased by a significant amount. It was found that the pyroelectric coefficient of VDF/TrFE (70/30 mol%) copolymer is 68.7 µC m^?2 K^?1 at 300 K. © 2001 Society of Chemical Industry     

Mesophase Structure‐Enabled Electrostrictive Property in Nylon‐12‐Based Poly(ether‐block‐amide) Copolymers

To search for alternative electrostrictive polymers and to understand the underlying mechanism, the structure‐ferroelectric/electrostrictive property relationship for nylon‐12‐based poly(ether‐b‐amide) multiblock copolymers (PEBAX) is investigated. Two PEBAX samples are studied, namely, P6333 and P7033 with 37 and 25 mol.% of soft poly(tetramethylene oxide) (PTMO) blocks, respectively. In both samples, poorly hydrogen‐bonded mesophase facilitates electric field‐induced ferroelectric switching. Meanwhile, the longitudinal electrostrictive strain (S₁)–electric field (E) loops are obtained at 2 Hz. Different from conventional poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)]‐based terpolymers, uniaxially stretched nylon‐12‐based PEBAX samples exhibit negative S₁, that is, shrinking rather than elongation in the longitudinal direction. This is attributed to the unique conformation transformation of nylon‐12 crystals during ferroelectric switching. Namely, at a zero electric field, crystalline nylon‐12 chains adopt a more or less antiparallel arrangement of amide groups. Upon high‐field poling, ferroelectric domains are enforced with more twisted chains adopting a parallel arrangement of amide groups. Meanwhile, extensional S₁ is observed for P6333 at electric fields above 150 MV m^?1. This is attributed to the elongation of the amorphous phases (i.e., amorphous nylon‐12 and PTMO). Therefore, competition between shrinking S₁ from mesomorphic nylon‐12 crystals (i.e., nanoactuation) and elongational S₁ from amorphous phases determines the ultimate electrostriction behavior in stretched PEBAX films.     

High polarization levels in poly(vinylidene fluoride–trifluoroethylene) ferroelectric thin films doped with diethyl phthalate

With a spin‐coating technique, ferroelectric thin films of poly(vinylidene fluoride–trifluoroethylene) copolymer [P(VDF–TrFE; with a content (mol %) ratio of 68/32 vinylidene fluoride/trifluoroethylene] were fabricated on silicon wafers covered with platinum and were doped with a nucleation agent, diethyl phthalate (DEP). The remnant polarization of copolymer thin films increased 70% after doping with DEP, and the coercive field was reduced, which is highly desirable in bistable memory devices. The dielectric constant of thin films also increased after doping. However, the effect of doping on the ferroelectric response was not remarkable in freestanding copolymer films. The results demonstrated that the ferroelectric dipole orientation in P(VDF–TrFE) thin films was significantly enhanced by the presence of DEP because the crystallinity of thin films decreased after doping, as revealed by X‐ray results. The dopant DEP acted as both a nucleation agent during the crystallization process and a plasticizer in the noncrystalline regions, which greatly enhanced the dipole orientation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1416–1419, 2003     

Tailored ferroelectric responses and enhanced energy density in PVDF‐based homopolymer/terpolymer blends

Blend of polymers is an effective way to tailor the ferroelectric responses and improve the energy storage properties of polymers. In this work, the microstructure and dielectric responses of the blends of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) [P(VDF‐TrFE‐CFE)] have been studied. It is found that the addition of PVDF disturbs the crystallization process of P(VDF‐TrFE‐CFE), leading to lower crystallinity and smaller crystalline size. The aforementioned microstructure changes result in tailored ferroelectric responses. Dielectric responses show that the blend with 10 wt % PVDF achieves larger polarization response under high electric field (above 300 MV/m) due to the interfacial polarization. Because of the tailoring effect and the interfacial polarization, the blend with 10 wt % PVDF exhibits higher energy density and efficiency. Moreover, the breakdown strength (E_b) is also improved by adding a small amount of PVDF into the terpolymer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40994.     

Effect of water absorption on the mechanical properties of poly(vinylidene fluoride‐trifluoroethylene) copolymer films

Electroactive polymers are smart materials that respond to electrical stimulation by changing their shape and size. The objective of this study is to investigate the applicability of the poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) copolymer in sensors or actuators in an in vivo environment. The basic mechanical properties of P(VDF‐TrFE) thin films were estimated by carrying out tensile and creep tests using specimens submerged in water at 30°C for a few weeks. Furthermore, X‐ray diffraction analysis was employed to observe the micro‐structural changes in the P(VDF‐TrFE) films due to water absorption wherein water intercalated in the interstitial sites between the chains of the P(VDF‐TrFE) films. Consequently, the mechanical properties of the P(VDF‐TrFE) films were enhanced due to water absorption. POLYM. ENG. SCI., 54:2654–2659, 2014. © 2013 Society of Plastics Engineers     

Ferroelectric nanodot formation from spin‐coated poly(vinylidene fluoride‐co‐trifluoroethylene) films and their application to organic solar cells

We demonstrated a facile route to the preparation of self‐assembled poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)] nanodots from spin‐coated thin films. We found that the initial film thickness would play an important role in the formation of such P(VDF‐TrFE) nanodots. Interestingly, the electric dipoles of such nanodots were self‐aligned toward the bottom electrode and their ferroelectric properties were determined by using piezoresponse force microscopy. In addition, the self‐polarized ferroelectric nanostructures were introduced to small molecular organic photovoltaic devices and allowed for enhancing the short circuit current density (J_sc) from 9.4 mA/cm² to 10.2 mA/cm² and the power conversion efficiency from 2.37% to 2.65%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41230.     

Strengthening of electromechanical properties for poly(vinylidene fluoride‐trifluoroethylene) films under tailored electric cycling

Copolymers of polyvinylidene fluoride and trifluorethylene [P(VDF‐TrFE)] have potential applications in wearable and implantable electromechanical devices since they are mechanically flexible, and biocompatible. A tailored electric cyclic process is employed to enhance both electrical and mechanical properties in P(VDF‐TrFE) 65/35 mol % copolymer films. The films are subjected to lower and higher field magnitude electric cycling successively. For electrical properties, enhancement in remnant polarization, dielectric and piezoelectric constants occurs. From mechanical point of view, strengthening in the fracture strength happens. Wide‐angle X‐ray diffraction techniques examine changes of the orientation of the molecular chains, grain size, and crystallinity after electric cycling for the copolymer films. Scanning electron microscopy reveals evolutions of the microstructure, including rod‐like textures and fractography of the films. The results indicate that electric cycling causes the molecular chains to orient gradually along the direction perpendicular to the applied electric field. Consequently, an enhancement of 12.2% and 45% is realized for the remnant polarization and fracture strength, respectively for P(VDF‐TrFE) 65/35 copolymer film. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45926.     

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

The molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10^?1–10⁷ Hz), differential scanning calorimetry DSC (?100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (α_a‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. α_c‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (α_p‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007