Vinylidene Fluoride-trifluoroethylene Copolymer Research Articles

Giant negative electrocaloric effect measured by indirect method in X-ray irradiated P(VDF-TrFE) films

Flexible solid-state cooling devices with high efficiency are attracted to ferroelectric polymers with excellent negative electrocaloric (EC) effects. It is challenging to obtain a large negative EC effect in ferroelectric polymers due to the lack of tunable techniques. A giant negative EC response was obtained in the poly(vinylidene fluoride-trifluoroethylene) copolymers (P(VDF-TrFE), 70/30, in mole ratio) irradiated with high-energy X-ray. The irradiated P(VDF-TrFE) films showed an adiabatic temperature change of −13.5 K at 40 MV/m under a dose of 5 Mrad (1 Mrad=104 J/kg) obtained by the indirect method. This significant negative EC effect is attributed to the enhancement of crystalline due to the entry of polymer molecules into the amorphous to crystalline structure and the reduction of heat capacity due to the increase of crosslinking. In addition, X-ray irradiation improves the dielectric coefficient from 15 to 22. This research indicates that irradiation can modify the negative EC properties of ferroelectric polymers for solid-state cooling.

Structure and ferroelectric properties of P(VDF-TrFE) films prepared under different conditions — Effect of filtration of the copolymer solution

Vinylidene fluoride-trifluoroethylene copolymer films of molar ratio 70/30 with thickness of about 1 [Formula: see text]m have been deposited from solution in ethyl methyl ketone to a glass substrate with an aluminum electrode by spin coating. The solution has been filtrated through a PTFE membrane filter with pore size 0.2 [Formula: see text]m directly before spin coating or it has been used as is (unfiltrated). After deposition of a top electrode, the samples have been polarized by hysteresis loops with an electric field amplitude of about 100 V/[Formula: see text]m. In samples, annealed at temperature 145[Formula: see text]C for 3 h, a high remanent polarization of about 7.5 [Formula: see text]C/cm2has been achieved, without significant differences between samples fabricated of filtrated or unfiltrated solution. Spherulitic lamella are growing in films fabricated of filtrated solution when they are heated above the melting temperature to 159[Formula: see text]C for 3 min before the further annealing process at 145[Formula: see text]C. These films show substantially lower remanent polarization below 4 [Formula: see text]C/cm2. Pyroelectric images recorded with a pyroelectric laser scanning microscope show that the spherulites have very small pyroelectric activity, i.e., the spherulites consist of flat-on lamella. In contrast, no spherulitic lamella are growing in films fabricated of unfiltrated solution heated above the melting temperature, melted and annealed under the same conditions. An explanation for this observation is that filtrating changes the structure of the copolymer in solution from polymer coil to rod. Copolymer rods deposited on a substrate will crystallize in flat-on lamella when heated above the melting temperature, in contrast to copolymer coils which crystallize in edge-on lamella.

Measuring 3D pyroelectric distributions with high resolution in thin films by a laser scanning microscope.

A laser scanning microscope for measuring 3D pyroelectric distributions inside thin vinylidene fluoride-trifluoroethylene copolymer films using the Laser Intensity Modulation Method was developed. The setup consists of a laser unit, a laser driver, an xyz-stepper motor unit, a transimpedance amplifier, and a lock-in amplifier. The focus lens at the laser unit is fixed by magnetic levitation and can correct a defocusing of the system or a tilt of the sample surface. It has been demonstrated in different samples that the system has a lateral resolution of 1μm for measuring the topological surface structure or the pyroelectric distributions. The self-developed laser driver and transimpedance amplifier combined with a fast lock-in amplifier are able to measure small pyroelectric currents and their variation inside a pyroelectric sample in the range of some 1 pA. The maximum measure frequency of 4MHz and the fast lock-in make it possible to measure the 3D pyroelectric distributions with high resolution. A 3D scan of 30 different layers with depths of 100 nm-5μm inside the sample and 100 × 100 points in the xy-direction per layer is performed in 3 days.

Formation mechanism of ferroelectric poly (vinylidene fluoride-trifluoroethylene) copolymers with in-plane dipole alignment under low electric field from melt and its SPR based pyroelectric sensor

P(VDF/TrFE) thin films were annealed under a low electric field using microgapped comb-like electrodes (IDT structure), and their structures and pyroelectric properties were investigated. In-plane dipole alignment of P(VDF/TrFE) was achieved by applying a low electric field (7.7 MV m−1), and the I phase crystal structure with in-plane dipole alignment was transformed following nucleation of the III phase structure. Despite the absence of a Fabry–Perot-type infrared absorption structure, the P(VDF/TrFE) thin films, which were continuously subjected to a low electric field from the melting point to para-ferroelectric phase transition temperature, exhibited a large pyroelectric signal. The voltage sensitivity was 242 V W−1, which is much higher than that of the sandwich type sensor equipped with a Fabry–Perot-type infrared absorption structure on Si substrates (72 V W−1). The improved sensitivity could be attributed to the suppression of the injected charges under the application of a low electric field and the enhancement of the infrared absorption and infrared-to-thermal conversion owing to the surface plasmon resonance on the microgapped IDT structure.

Simultaneous enhancement of tunability and reduction of permittivity in P(VDF-TrFE)-MgO ferroelectric-dielectric composite films

Ferroelectric-dielectric composite has the potential to combine low dielectric constant of the dielectrics with the high tunability of ferroelectrics. However, one significant drawback of the composites is that the decreased dielectric constant is at the expense of tunability. Here, we eliminate the doping effect through organic-inorganic compositing and redistribute electric field to let the tunability increases while dielectric constant decreases. Poly(vinylidene fluoride-trifluoroethylene) copolymers based ferroelectric-dielectric composite films were prepared through spinning-coating method and the effect of MgO content on the dielectric tunable properties was studied. With increasing MgO content up to 30 wt%, the tunability of composite films increased while the dielectric constant decreased. The tunability of P(VDF-TrFE)-MgO composite films with 30 wt% MgO at 10 kHz under 32.5 kV/mm can reach to 25.6%, 1.22 times that of pure P(VDF-TrFE) (20.9%). This research paves a new pathway to increase the tunability and decrease the dielectric constant of ferroelectric materials simultaneously.

Elastic and anelastic properties of vinylidene fluoride-trifluoroethylene (VDF-Tr) copolymers in the vicinity of phase transition

Temperature dependences of internal friction Q−1 and elastic modulus E of the VDF60Tr40 copolymer were studied in the temperature range of 20–100 °C at a frequencies f ≈ 3–17 Hz. A distinct softening of the modulus E was found near the Curie temperature. In the vicinity of 60 °C, a wide maximum of Q−1 is revealed, which is a superposition of two Q−1 peaks. The “low-temperature” peak of internal friction is due to the process of α-relaxation in the non-crystalline polymer fraction. “High-temperature” one is associated with a first-order ferroelectric phase transition in the crystalline fraction.

The Influence of γ Radiation on the Dielectric Properties of Ferroelectric Langmuir–Blodgett Films

Dielectric relaxation of Debye type has been found for thin vinylidene fluoride‒trifluoroethylene copolymer films of different composition, fabricated by the Langmuir–Blodgett (LB) technique. The influence of γ radiation on the dielectric response of LB films is studied. It is established that the relaxation time increases after the γ irradiation, the phase transition is observed at a lower temperature, and the temperature hysteresis is reduced.

Investigation of the electrocaloric effect in ferroelectric polymer film through direct measurement under alternating electric field

The electrocaloric (EC) effect of ferroelectric polymer film was investigated at a high frequency. The analysis used the finite element method and revealed that the extractive heat density per unit of time increases and the thermal diffusion length decreases with an increase in the frequency of the driving electric field. Given the small thermal diffusion coefficient of ferroelectric polymers, the high-frequency drive of the EC effect for ferroelectric polymer film is promising for cooling devices. To discuss the EC effect under an alternating current (AC) electric field, the EC temperature change of the vinylidene fluoride-trifluoroethylene copolymer 75/25 P(VDF-TrFE) film with a thickness of 2 μm was characterized through direct measurement using a thin-film thermocouple. The EC temperature change increases with an increase in the electric field and frequency and reached up to 0.23 °C at 1.5 MV cm−1 and 10 kHz. The extractive heat density was estimated to be as high as 0.69 W cm−2.

High cyclic stability of electrocaloric effect in relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymers in the absence of ferroelectric phase transition

The incorporation of bulky defects into ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymers can result in a relaxor behavior, accompanied by outstanding electrocaloric properties near room temperature. However, it remains elusive whether ferroelectric transition occurs in relaxor terpolymers, which is critical for the design of electrocaloric cooling devices. In this work, we study the electrocaloric fatigue in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (61.8/30.4/7.8 mol. %) terpolymer, in which we revisit its controversial issue of the phase transition. We observe that the electrocaloric response of the terpolymers remains nearly constant (ΔS = 25.5 J K−1 kg−1, ΔT = 5.1 K at 100 MV m−1) within 50 cycles at different temperatures around room temperature. Moreover, we conduct temperature-dependent X-ray diffraction and Fourier-transform infrared spectroscopy measurements on the terpolymer to provide insights into the evolution in intermolecular lattice spacing and intramolecular conformation. We find no clear evidence of the ferroelectric-to-paraelectric phase transition near the temperature range, where differential scanning calorimeter measurement displays an endothermic peak.

Phenomenological analysis of elastocaloric effect in ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymers

In this work, based on thermodynamic theory, we theoretically derived the elastocaloric coefficient at the zero electric field, including the first kind of elastocaloric coefficient at constant polarization and the second kind of elastocaloric coefficient arising from the piezoelectric effect. In addition, using the Landau-Ginzburg-Devonshire approach, we predicted a giant elastocaloric effect in poly(vinylidene fluoride-trifluoroethylene) [P(VEF-TrFE)] copolymer films with a large elastocaloric strength, which is one or two orders of magnitude larger than typical elastocaloric materials reported in the literature. The adiabatic elastocaloric temperature change is found to be huge [∼32.2 K for P(VEF-TrFE) 65/35 and 65 K for P(VEF-TrFE) 70/30] under a uniaxial compressive stress (−100 MPa) due to negative piezoelectricity. Moreover, even a low tensile stress (∼15 MPa) can induce a modest elastocaloric effect (∼−1.3 K), which is in good agreement with the previous literature [Patel et al., Appl. Phys. Lett. 108, 072903 (2016)]. Our study not only stimulates further investigations on elastocaloric effects in ferroelectric polymers but also offers opportunities for elastocaloric solid-state refrigeration.

Localized Fretting-Vibrotactile Sensations for Large-Area Displays.

Tactile perception in large-area displays is currently attracting substantial research attention since, in conjunction with visible and auditory sensations, it provides more immersive and realistic interactions with displayed contents. Here, a new vibrotactile display based on the fretting phenomenon is developed for the first time to provide localized tactile feedback on a large-area display. Normal pressure by a human fingertip activates a locally concentrated electric field in a relaxor ferroelectric polymer (RFP) film under the contact area, which produces a localized electrostrictive strain. The synergistic interplay among the localized electric field, electrostrictive deformation of the RFP film, and contact area dramatically amplifies acoustic vibrations near the contact edge of a human fingertip. A blend of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and poly(vinylidene fluoride-trifluoroethylene) (55:45) copolymer is proposed for the RFP to provide an enhanced actuation performance even at elevated temperatures. The fretting-vibrotactile mechanism has several interesting properties, such as tactile feedback on a stationary fingertip, pressure-responsive simple on-off mechanism, multitouch interaction, excellent transparency, and easy integration with capacitive or resistive touch sensors and friction-based haptic-feedback mechanisms. An array of RFP film vibrators can provide addressable content-related multiple tactile feedback on large-area displays by modulating the frequency, amplitude, and profile of the driving voltage signals.

Dielectric and Piezoelectric Properties of Composite Poly(vinylidene fluoride-trifluoroethylene) Copolymer with Carbon Nanotubes

Film samples of poly(vinylidene fluoride-trifluoroethylene) copolymer containing 2% of carbon nanotubes were synthesized. The temperatures of the ferroelectric phase transition and the value of permittivity at the frequency of 1 MHz were determined. The surface was visualized using scanning probe microscopy; the values of the effective piezoelectric coefficient were obtained. It was shown that incorporation of carbon nanotubes into the polymer matrix improves the values of the permittivity and piezoelectric coefficient of the polymer.

Direct piezoelectric response in vinylidene fluoride–trifluoroethylene copolymer films

The effective transverse piezoelectric coefficient e31,f in vinylidene fluoride–trifluoroethylene copolymer [P(VDF–TrFE), 75/25 mol %] films was investigated. 2-µm-thick P(VDF–TrFE) films were deposited by spin coating on a Pt/Si substrate at a valid annealing temperature from 100 to 160 °C. The phase development and surface morphology suggest that the films were melted by annealing at 150 °C and above, and that gauche defects increase with the annealing temperature. Although all the films have similar dielectric constants between 8 and 10, the remanent polarization Pr and coefficient e31,f show maximum values of 9.1 µC/cm2 and 140 mC/m2, respectively, at 140 °C. These results indicate that the ferroelectric and piezoelectric properties of P(VDF–TrFE) films are enhanced by annealing just below the melting point.

Fabrication and Characterization of Aligned Flexible Lead-Free Piezoelectric Nanofibers for Wearable Device Applications.

Flexible lead-free piezoelectric nanofibers, based on BNT-ST (0.78Bi0.5Na0.5TiO3-0.22SrTiO3) ceramic and poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) copolymers, were fabricated by an electrospinning method and the effects of the degree of alignment in the nanofibers on the piezoelectric characteristics were investigated. The microstructure of the lead-free piezoelectric nanofibers was observed by field emission scanning electron microscope (FE-SEM) and the orientation was analyzed by fast Fourier transform (FFT) images. X-ray diffraction (XRD) analysis confirmed that the phase was not changed by the electrospinning process and maintained a perovskite phase. Polarization-electric field (P-E) loops and piezoresponse force microscopy (PFM) were used to investigate the piezoelectric properties of the piezoelectric nanofibers, according to the degree of alignment—the well aligned piezoelectric nanofibers had higher piezoelectric properties. Furthermore, the output voltage of the aligned lead-free piezoelectric nanofibers was measured according to the vibration frequency and the bending motion and the aligned piezoelectric nanofibers with a collector rotation speed of 1500 rpm performed the best.

Coalescence and split of high-entropy polymer lamellar cocrystals

The solvent evaporation has been identified able to induce the cocrystallization of dissolved polyvinylidene fluoride (PVDF) homopolymer and poly(vinylidene fluoride trifluoroethylene) (PVDF-TrFE) copolymer solutes in this research. Upon the random incorporation of either trifluoroethylene (TrFE) or vinylidene fluoride (VDF) motif during this selected route of cocrystallization, several ferroelectric crystalline phases grow concurrently. For the development of each crystalline form, there is an appropriate composition range of TrFE motif, instead of a specific composition. When the inclusion of TrFE is beyond a threshold level, the non-polar HT-form crystals were found to solely develop upon slow heating above Curie temperature, and therefore inherits various compositions of TrFE motif. During the stay above Curie temperature, stick-like HT-form lamellar cocrystals preferably associate together and coalesce into coarser crystalline lamellae. This coalescence behavior is deduced to be driven by the secondary crystallization above Curie temperature, presumably enabled by the acceptable level of lattice-packing entropy of HT-form cocrystals. During the subsequent cooling below Curie temperature, the coalesced HT-form lamellar cocrystals reversely split into thinner lamellar cocrystals with various types of ferroelectric lattice packing comprising narrower ranges of TrFE composition. The Curie transition of cocrystals is thus accompanied with the alteration of the stacking dispersion of crystalline lamellae within materials, which associates with the variation of motif composition and therefore the lattice-packing entropy.

Dielectric properties of composite based on ferroelectric copolymer of poly(vinylidene fluoride-trifluoroethylene) and ferroelectric ceramics of barium lead zirconate titanate

A study of dielectric properties of composite films on the base of poly(vinylidene fluoride-trifluoroethylene) copolymer P(VDF-TrFE) and ferroelectric ceramics of barium lead zirconate titanate (BPZT) solid solution is presented in this work. The composite films containing up to 50 vol.% of BPZT grains with size [Formula: see text]1[Formula: see text][Formula: see text]m were prepared by the solvent cast method. Frequency dependences of real and imaginary components of the complex permittivity were determined. The concentration dependence of the dielectric constant was discussed.

Analysis of vinylidene fluoride–trifluoroethylene copolymer film for nonpolarized energy harvester

In this paper, an investigation of a vinylidene fluoride–trifluoroethylene (VDF/TrFE) copolymer film for use in a nonpolarized energy harvester is presented. The Fourier transform infrared spectroscopy and X-ray diffraction analysis of a nonpolarized VDF/TrFE copolymer film indicate that polymer crystallinity depends on film thickness. Furthermore, the growth of the polar structure (β phase) is observed upon the reduction in the thickness. However, power generation does not exhibit a linear behavior because the generation capacity increases with the thickness. These results indicate that the film thickness for a nonpolarized harvester must be optimized to obtain the maximum output.

Facile coating of conductive poly(vinylidene fluoride-trifluoroethylene) copolymer on Li1.2Mn0.54Ni0.13Co0.13O2 as a high electrochemical performance cathode for Li-ion battery

Coating a protection layer on the surface of Li- and Mn-rich oxide cathode is an effective approach to improve its cycling stability. In this paper, a conductive copolymer P(VDF-TrFE) coating on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) was synthesized via wet-chemical method. The P(VDF-TrFE) coating layer with uniform thickness around 5 nm is confirmed by X-ray diffraction, scanning electronic microscopy, high-resolution transmission electron microscopy, infrared and X-ray photoelectron spectroscopy. Results reveal that β-phase P(VDF-TrFE) coated LMNCO electrode exhibits remarkably improved Coulombic efficiency, cycleability and rate capability, even at elevated temperature. Such improved electrochemical performance is attributed to the fact that the uniform and continuous P(VDF-TrFE) coating can not only suppress the undesirable side reactions between the electrode and electrolyte, but also enhance the structural stability of LMNCO cathode. Most importantly, the conductive P(VDF-TrFE) layer is in favour of Li+ diffusion and electron transport of the coated cathode.

The third-order nonlinear optical properties in polymer ferroelectrics

The third-order susceptibility of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer are analyzed by using the z-scan technique with a nanosecond laser as an excitation source. It is established that optical transmittance of the VDF-TrFE samples decreases after their polarization in an electric field.

A flexible, wave-shaped P(VDF-TrFE)/metglas piezoelectric composite for wearable applications

In this work, a wave-shaped piezoelectric composite (WSPC) made of fine β-phase vinylidene fluoride trifluoroethylene copolymer (P(VDF-TrFE)) polymer and high-elastic FeSiB amorphous alloy (metglas) ribbon has been successfully fabricated for wearable device applications. X-ray diffraction and the Fourier Transform Infrared Spectrum studies reveal P(VDF-TrFE) exhibiting the fine β-phase. Both theoretical analysis and experimental results show that unique wave-shaped structure enhances the electromechanical coupling significantly, because of the combination piezoelectric effects of d33 and d31 modes in P(VDF-TrFE) polymer, as well as the enhanced effective piezoelectric coefficient caused by the pre-stretch in P(VDF-TrFE) film. Two application examples of WSPC, (i) mechanical force sensor or energy harvester, and (ii) the medical blood-pressure pulse sensor, have been investigated, which show that the WSPC is a promising candidate for future wearable device applications.