Shaobo Tan

High-field antiferroelectric behaviour and minimized energy loss in poly(vinylidene-co-trifluoroethylene)-graft-poly(ethyl methacrylate) for energy storage application

In this work, we report a novel antiferroelectric-like performance at high poling fields obtained in poly(ethyl methacrylate) (PEMA) grafted poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) series copolymers for application as high energy density and low loss capacitor dielectrics films. Compared with the pristine P(VDF-TrFE) random copolymer, an enhanced discharged energy density but a lowered energy loss has been observed as more PEMA is grafted. This novel antiferroelectric-like behavior at high poling field was explained by the crystalline impediment and polarization confinement effect induced by PEMA side chains. The highest discharged energy density of 14 J cm−3 and a low loss of 30% at 550 MV m−1 are achieved in the sample containing 22 wt% PEMA. This finding represents one of the effective routes to design potential dielectric polymer films for high energy storage applications.

Controlled hydrogenation of P(VDF-co-CTFE) to prepare P(VDF-co-TrFE-co-CTFE) in the presence of CuX (X = Cl, Br) complexes

An environmentally friendly and controllable P(VDF-co-CTFE) hydrogenation route involving the transition-metal complex mediated radical chain transfer reaction is successfully developed to synthesize P(VDF-co-CTFE-co-TrFE). The typical transition metal catalysts of ATRP reaction could be applied in this process.

Significantly improving dielectric and energy storage properties via uniaxially stretching crosslinked P(VDF-co-TrFE) films

Recently, tuning the normal ferroelectric performance of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) to either relaxor ferroelectric or anti-ferroelectric behavior by confining the relaxation of ferroelectric crystal domains physically or chemically has attracted considerable interest to achieve high discharged electric energy density (Ue) and low energy loss (Ul) for energy storage applications in high pulse capacitors. To improve the dielectric and energy storage properties as well as reduce the energy loss induced by the ferroelectric relaxation of P(VDF-co-TrFE), unsaturation containing P(VDF-co-TrFE) films were uniaxially stretched after crosslinking with peroxide in this work. P(VDF-co-TrFE) containing unsaturation was synthesized via controlled hydrogenation and dehydrochlorination of commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)). The properties of the films obtained were characterized with differential scanning calorimetry (DSC), X-ray diffraction (XRD), dielectric constant and electric displacement–electric field (D–E) hysteresis loop measurements. Compared with the as-cast and as-crosslinked films, the stretched films exhibit a significantly enhanced dielectric constant, breakdown field (Eb > 500 MV m−1) and Ue but depressed energy loss. This could be attributed to the enhanced film quality, optimized crystalline properties, improved orientation uniformity of crystal domains as well as accelerated ferroelectric relaxation induced by the crosslinking and mechanical stretching. The best performance was achieved for the stretched film with a dielectric constant of 15 at 1 kHz, a relatively high Ue of 17.5 J cm−3 and a low energy loss of about 30% at 575 MV m−1.

Synthesis of fluoropolymer containing tunable unsaturation by a controlled dehydrochlorination of P(VDF-co-CTFE) and its curing for high performance rubber applications

Fluoropolymer containing unsaturation, an important intermediate for many reactions such as radical addition and Michael addition reaction, could be either utilized to synthesize fluoropolymer with desired functions or cured for rubber applications, which has rarely been investigated because of the absence of a synthetic strategy. A facile method to synthesize fluoropolymer with tunable unsaturation via controlled dehydrochlorination of commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) catalyzed by tertiary monoamines under mild conditions has been reported in this work. The resultant copolymers are carefully characterized with nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), and thermal gravimetric analysis (TGA). It has been shown that the elimination could be well controlled by employing proper solvent, catalyst and reaction conditions. The typical side reactions catalyzed with amines, such as Michael addition reaction and main chain scission during the dehydrofluorination of fluoropolymer, could be avoided in the present reaction system. The kinetics results indicate that the elimination reaction is in a bi-molecular mechanism (E2), which is well recognized in strong base-catalyzed elimination of halogenated hydrocarbon. The concentration, alkalinity and steric bulk of the catalysts, the polarity and capability to absorb HCl acid of solvents, and the reaction time and temperature exhibit dominant influences on the dehydrochlorination of P(VDF-co-CTFE). The fluoropolymer containing unsaturation is readily cured with peroxide, and the crosslinked fluoropolymer exhibits excellent solvent resistance and mechanical properties.

Ferroelectric relaxation dependence of poly(vinylidene fluoride-co-trifluoroethylene) on frequency and temperature after grafting with poly(methyl methacrylate)

Poly(vinylidene fluoride) (PVDF) based ferroelectric fluoropolymers have attracted considerable attention in high energy density pulse capacitors application for the tunable and tight composition dependence of their ferroelectric performances. Grafting poly(methacrylate esters) (ca. poly(methyl methacrylate) (PMMA)) onto the side chain of P(VDF-TrFE-CTFE) (TrFE refers to trifluoroethyelene and CTFE is chlorotrifluoroethylene) has been shown to be an effective route to tailor the ferroelectric performance of P(VDF-TrFE-CTFE). The resultant grafted copolymers possess advantages including well maintained high energy density and remarkably reduced energy loss. To further disclose the influence of a PMMA side chain on ferroelectric relaxation of a P(VDF-TrFE-CTFE) main chain, a set of P(VDF-TrFE-CTFE)-g-PMMA copolymers bearing PMMA content from 10 wt% to 32 wt% were synthesized and well characterized in this work. The dielectric and energy storage performance of the grafted copolymers were systematically investigated under increased electric field, testing temperature, and frequency. The PMMA side chain was found to exhibit great influence with ferroelectric performance under varied conditions by impeding crystallization and formation of the ferroelectric phase of the P(VDF-TrFE-CTFE) main chain along with increasing the Youngs modulus of amorphous phase. As a result, the grafted copolymer with optimized PMMA content shows excellent stability with testing frequency, temperature, and electric field. That allows them to store and discharge energy consistently, which is rather important for energy storage capacitors working under varied conditions. With respect to promising energy storage performances together with low performance dependence during working conditions, the PMMA grafted P(VDF-TrFE-CTFE) copolymer may offer a great candidate material for high energy density pulse capacitors.

Synthesis and characterization of thermally self-curable fluoropolymer triggered by TEMPO in one pot for high performance rubber applications

The preparation of functional fluorine materials through chemical modification of commercial fluoropolymers has been recognized as an economic and convenient strategy to expand the application field of fluoropolymers. In this work, a thermally self-curable fluoroelastomer triggered by 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) has been successfully synthesized in one pot and carefully characterized. This strategy involves two competitive processes, including the coupling reaction between macroradicals and TEMPO, and the dehydrochlorination of commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) by a route involving a three-molecule process, together with a small amount of elimination by an E2 mechanism and β-H elimination The structure and properties of the target polymer were demonstrated by nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The two competitive reaction processes were carefully investigated under various reaction conditions, including different reaction temperatures, reaction times, ligands, solvents, copper salt, and dosage of TEMPO. The resultant polymer is rather stable at ambient temperature and easily cured at high temperature by ‘pulling the trigger’, namely by breaking C–O or O–N bonds, and the free radicals generated in situ are responsible for initiating the crosslinking of double bonds on the polymer main chain. No other additives are required for the crosslinking of the resultant polymer, which provides a facile chemical route to prepare crosslinked fluoropolymers with high purity and excellent mechanical properties. The curing of the resultant polymer could be accomplished in several minutes at 150–160 °C without the need for a post-cure process.

Synthesis of poly(vinylidene fluoride–trifluoroethylene) via a controlled silyl radical reduction of poly(vinylidene fluoride–chlorotrifluoroethylene)

Poly(vinylidene fluoride–trifluoroethylene) (P(VDF–TrFE)) has long been known for its excellent dielectric, ferroelectric and piezoelectric performances. Besides direct copolymerization of VDF and TrFE, hydrogenation of P(VDF–CTFE) has been extensively investigated to synthesize P(VDF–TrFE) for its advantages of low-cost raw materials and better controlled composition, together with mild reaction conditions. To overcome the negative influence of metal ions from the residue of catalysts on the dielectric performance of the resultant copolymers, in the present contribution, a hydrosilane catalyst system with no metal species has been reported for the hydrogenation of P(VDF–CTFE) in a controlled radical chain transfer reaction process. C–Cl bonds in CTFE are activated by silyl radicals which are in situ formed from the catalysts. The generated P(VDF–CTFE) micro-radicals would undergo a chain transfer reaction to Si–H on hydrosilane and generate new silyl radicals. The repeated chain transfer reactions between C–Cl bonds on CTFE and Si–H bonds on hydrosilane are responsible for the quantitative hydrogenation of P(VDF–CTFE) with respect to the hydrosilane dose. The low sensitivity of hydrosilane and the radical reaction pathway allow the reaction to be conducted under rather mild conditions in most of the good solvents of P(VDF–CTFE). Thanks to the metal-free feature of the catalyst, the resultant copolymer shows significantly improved dielectric performance over that catalyzed with copper complex catalysts. Reducing P(VDF–CTFE) with hydrosilane, such as (Me3Si)3SiH, has been demonstrated to be an environmentally friendly, controllable, metal-free and mild process to synthesize a TrFE containing fluoropolymer, which has glorious prospects for the synthesis of dielectric fluoropolymers.

Inserting –CHCH– into P(VDF-TrFE) by C–F activation mediated with Cu(0) in a controlled atom transfer radical elimination process

Chemical functionalization of poly(vinylidene fluoride) (PVDF) based fluoropolymers is an economic and convenient strategy to extend the application field of fluoropolymers. To enrich the functionalization tool box, in this contribution a controlled single electron transfer radical elimination (SET-RE) reaction has been successfully developed to introduce CHCH bonds into poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), which was synthesized through the full hydrogenation of poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE)). In this SET-RE process, C–F bonds in TrFEs are directly activated by Cu(0) and 2,2′-bipyridine (Bpy) to generate CF2–CH*, followed by a controlled β-H elimination to complete the dehydrofluorination. The high selectivity of the elimination and the reaction mechanism could be well confirmed by the chemical composition of the resultant copolymers. The kinetics results show that the reaction rate is first-order with respect to the concentration of TrFE units and SET-RE is a finely controlled process. The reaction conditions, including the temperature, reaction time, dosage of Cu(0) and Bpy, concentration and even solvents, exhibit an influence on the reaction and the resultant copolymers. The introduction of CHCH leads to reduced crystallinity and an elevated ferro- to para-electric transition temperature of P(VDF-TrFE). This work offers a robust and controlled chemical tool to introduce reactive CHCH into the fluoropolymer for subsequent functionalization by directly activating C–F bonds.

A light-mediated metal-free atom transfer radical chain transfer reaction for the controlled hydrogenation of poly(vinylidene fluoride-chlorotrifluoroethylene)

A light-mediated metal-free atom transfer radical chain transfer reaction (ATRCT) strategy is proposed for the controlled hydrogenation of poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE)) in present work. In this process, C–Cl bonds on CTFE units in P(VDF-CTFE) are firstly activated by the photoexcited catalyst under light. The subsequent chain transfer reaction of the generated macro-radicals to either polar solvents or chain transfer reagents could finely convert part or all the CTFE units into trifluoroethylenes (TrFEs) depending on the loading concentration of the chain transfer reagents and the reaction time. The hydrogenation reaction could be conducted under mild conditions and shows excellent controllable characteristics, which offers a facile, efficient, economic, and metal-free approach to synthesize P(VDF-TrFE) or P(VDF-TrFE-CTFE) from P(VDF-CTFE). In particular, this process could completely avoid issues associated with the residue of the metal ions in the target product, which have been recognized as the major resource of dielectric loss in the polymers utilized under high electric field.

Synthesis of poly(vinylidene fluoride-co-chlorotrifluoroethylene)-g-poly(methyl methacrylate) with low dielectric loss by photo-induced metal-free ATRP

The grafting of poly(methacrylate ester) or polystyrene onto the side chain of poly(vinylidene fluoride) (PVDF)-based fluoropolymers maintains their high energy density and remarkably reduces the energy loss due to the confinement (or insulation) effect, which has potential application in high-pulse capacitors. The graft copolymers were previously synthesized from C–Cl bonds via the transition metal-catalyzed atom transfer radical polymerization (ATRP) process. To overcome the negative influence of the residual metal ions from the catalyst on the dielectric properties of the resultant copolymers, in the present contribution, a facile strategy is reported for photo-mediated ATRP using organic-based photoredox catalysts to directly synthesize a poly(methyl methacrylate) (PMMA)-grafted copolymer from the commercial poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF–CTFE)). The graft copolymerization is efficiently activated and deactivated with light and exhibits first-order kinetics. The detailed structural information of the graft copolymer, including average grafting density and side chain length, are also determined by converting the uninitiated Cl atoms into H atoms. When compared with the traditional Cu-catalyzed ATRP process, the current photo-induced ATRP method used for preparing the graft copolymer results in improved dielectric performances such as reduced dielectric loss at low frequency and high temperature, decreased conduction loss, and enhanced breakdown strength.