Enhanced Electrocaloric Response of Vinylidene Fluoride–Based Polymers via One‐Step Molecular Engineering

Electrocaloric refrigeration is one of the most promising environmentally-friendly technologies to replace current cooling platforms—if a notable electrocaloric effect (ECE) is realized around room temperature where the highest need is. Here, a straight-forward, one-pot chemical modification of P(VDF-ter-TrFE-ter-CTFE) is reported through the controlled introduction of small fractions of double bonds within the backbone that, very uniquely, decreases the lamellar crystalline thickness while, simultaneously, enlarging the crystalline coherence along the a-b plane. This increases the polarizability and polarization without affecting the degree of crystallinity or amending the crystal unit cell—undesirable effects observed with other approaches. Specifically, the permittivity increases by >35%, from 52 to 71 at 1 kHz, and ECE improves by >60% at moderate electric fields. At 40 °C, an adiabatic temperature change >2 K is realized at 60 MV m−1 (>5.5 K at 192 MV m−1), compared to ≈1.3 K for pristine P(VDF-ter-TrFE-ter-CTFE), highlighting the promise of a simple, versatile approach that allows direct film deposition without requiring any post-treatment such as mechanical stretching or high-temperature annealing for achieving the desired performance

Synthesis and Characterization of Novel Fluorinated Electroactive Copolymers

Piezotech, une start-up rachetée en 2010 par le leader français de la chimie, Arkema, développe et commercialise des copolymères fluorés électroactifs (EAPs). Ces copolymères présentent un grand intérêt pour des applications dans l’électronique imprimée, les capteurs, les actionneurs, les muscles artificiels, et le stockage de l’énergie. Les copolymères à base de fluorure de vinylidène (VDF) et de trifluoroéthylène (TrFE) possèdent les meilleures propriétés piézoélectriques et ferroélectriques (FE) parmi les polymères existants. L’introduction d’un termonomère, tel que le 1,1-chlorotrifluoroéthylène (CFE) ou le chlorotrifluoroéthylène (CTFE), modifie la conformation de la chaine polymère et change le comportement du matériau en relaxeur ferroélectrique (RFE). Le travail de thèse s’inscrit dans la recherche de nouveaux EAPs fluorés et la compréhension des relations structure-propriétés de ces matériaux. Après l’étude de la cinétique de copolymérisation avec le VDF et le TrFE de différents termonomères porteurs de groupements CF3, les terpolymérisations ont été examinées. Dans une première partie, le trans-1,3,3,3-tetrafluoropropène (1234ze), un termonomer qui n’homopolymérise pas, a été étudié. Des terpolymères de composition homogène ont été synthétisés. Ensuite, le 3,3,3-trifluoropropène (TFP) et le 2,3,3,3-tetrafluoropropène (1234yf) ont été terpolymérisés avec le VDF et le TrFE. Bien que les terpolymérisations batch amènent à une forte dérive de composition, un procédé de polymérisation en suspension en semi-continu a permis d’obtenir une meilleure homogénéité. Les propriétés thermiques et électroactives de tous les terpolymères ont été caractérisées. Dans une dernière partie, l’introduction d'une faible quantité d’un monomère fonctionnel (2-(trifluorométhyl) acide acrylique, MAF), dans les chaines du copolymère poly(VDF-co-TrFE), a permis une amélioration des propriétés d’adhésion sur le verre et le métal sans altérer les propriétés électroactives. L’étude des terpolymérisations avec le VDF, le TrFE et un termonomère porteur d’un groupement CF3 a contribué l’amélioration de la compréhension des relations structure-propriété dans les EAPs fluorés. En particulier, il a été montré que la réduction de la taille des cristaux n’est pas une condition suffisante pour obtenir un caractère RFE.Piezotech, a start-up acquired in 2010 by the French leader in Chemistry, Arkema, develops and commercializes fluorinated electroactive copolymers (EAPs). These copolymers arouse a significant interest in printed electronics, printed memories, sensors, actuators, artificial muscles, and energy storage devices. Vinylidene fluoride (VDF) and trifluoroethylene (TrFE) -based copolymers are the polymers exhibiting the highest piezoelectric and ferroelectric (FE) properties. The introduction of a third slightly bulkier termonomer such as 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) was shown to modify the polymer chain conformation and change their electroactive behaviors into those of relaxor ferroelectric (RFE) materials. This PhD work aimed to discover new fluorinated EAPs and to better understand their structure-properties relationships. After the study of the kinetics of copolymerization of various CF3-bearing termonomers with VDF and TrFE; the terpolymerizations were investigated. First trans-1,3,3,3-tetrafluoropropene (1234ze), a non-homopolymerizable termonomer, was studied. Terpolymers with homogeneous compositions were synthesized. Then, 3,3,3-trifluoropropene (TFP) and 2,3,3,3-tetrafluoropropene (1234yf) were terpolymerized with VDF and TrFE. Although batch terpolymerizations resulted in terpolymers with strong compositional drift, the semi-continuous suspension process afforded better homogeneity. The thermal and electroactive properties of all the terpolymers were characterized. Finally, the introduction into poly(VDF-co-TrFE) copolymer chains of small amounts of a functional monomer (2-(trifluoromethyl)acrylic acid, MAF) allowed an increase of the adhesion properties onto glass and metal substrates without altering the electroactive properties. The study of the terpolymerizations of VDF, TrFE and a CF3-bearing termonomer contributed to a better understanding of the structure-properties relationships in fluorinated EAPs. Notably, the reduction of the crystal size was shown to be insufficient to afford RFE properties

Towards new strategies for the synthesis of functional vinylidene fluoride-based copolymers with tunable wettability

International audienceSynthesis of poly(vinylidene fluoride)-based functional polymers with tunable wettability was achieved viaradical copolymerizations of vinylidene fluoride (VDF) with tert-butyl 2-trifluoromethacrylate (MAF-TBE)followed by hydrolysis of the tert-butyl ester groups. Radical copolymerizations of VDF with 2-trifluoromethylacrylic acid (MAF) and MAF-TBE were investigated under various experimental conditions:initiators, temperatures, and solvents. The polymerizations using peroxide initiators led to good yield(≥57%), while azo initiators did not lead to any polymerization. The compositions and microstructures ofall the obtained copolymers were determined by 1H and 19F NMR spectroscopies. The very low yield(23%) achieved in radical copolymerization of VDF with MAF in water was attributed to the regioselectivenucleophilic addition of water onto MAF, yielding a potential fluorosurfactant, 3-hydroxy-2-(trifluoromethyl)propanoic acid in good yield (85%). In contrast, no reaction was observed between MAF-TBE andwater, even in the presence of ammonium perfluorooctanoate as a (fluorinated) surfactant. Thepoly(VDF-co-MAF-TBE) copolymer was shown to be easily hydrolyzed under mild conditions to prepare–COOH-functionalized PVDF copolymers, without any dehydrofluorination of VDF units. The wettabilitiesof these copolymers were studied by water contact angle (WCA) measurement. Compared to thepoly(VDF-co-MAFTBE) copolymers (WCA = 102°), the –COOH functionalized PVDF copolymer exhibits alower WCA (57°). This work paves the way for the synthesis of PVDF-based functional copolymers withtunable wettability for potential applications in the preparation of membranes for water purification, coatingsor oil recovery systems

Vinylidene fluoride- and trifluoroethylene-containing fluorinated electroactive copolymers. How does chemistry impact properties?

International audienceFluoropolymers are attractive niche polymers used in high added value materials for high-tech appli-cations in aerospace, electronics, coatings, membranes, cables, and the automotive industries. Amongthem, VDF- and TrFE-based copolymers exhibit remarkable electroactive properties allowing their incor-poration into a wide range of devices such as printed memories, sensors, actuators, artificial muscles,and energy storage devices. In a first section, a detailed overview of semi-crystalline poly(VDF-co-TrFE)copolymers and of their ferroelectric (FE) properties from the point of view of polymer chemists is sup-plied. In addition to the polymer microstructure that may sometimes be controlled or influenced by thesynthesis strategies, physical properties such as the phase transitions, and electroactivity are also affectedby processing, such as annealing for example, and film thickness for example. Building on the conclusionsand understanding obtained from the first section, the effect of the introduction of a termonomer (leadingto poly(VDF-ter-TrFE-ter-M) terpolymers) is detailed in a second section of this review. Modifying theterpolymer chain microstructure has a major impact on the crystalline phase of the terpolymers that mayresult in a relaxor-ferroelectric behavior (RFE). The distribution of the termonomer along the polymerchain, the capacity of the termonomer units to enter the crystal lattice, as well as its dipole momentgovern in large part the terpolymer electroactive properties. Poly(VDF-ter-TrFE-ter-CFE) and poly(VDF-ter-TrFE-ter-CTFE) terpolymers appeared to be the best candidates for RFE properties and were thus themost studied. In two following sections, the block or graft architectures of VDF- and TrFE- based copoly-mers, and the various crosslinking strategies used so far for such copolymers are described. Chemicalmodification is indeed a very powerful tool to tune electroactive properties of copolymers or to impartadditional properties. Finally, in the last section, a few examples of emerging applications for these fluo-rinated electroactive polymers (EAPs) are briefly discussed. This review aims to provide a comprehensivereport on the use of polymer chemistry as a tool to produce better electroactive fluorinated polymers, andhighlights possible opportunities and perspectives for future progress in this field. Research in this inter-disciplinary field requires different kinds of expertise, ranging from organic and polymer chemistries,polymer films engineering, physics of semi-crystalline polymers and electroactivity, to the design andfabrication of electronic devices

Differences in electroactive terpolymers based on VDF, TrFE and 2,3,3,3-tetrafluoropropene preparedby batch solution and semi-continuous aqueous suspension polymerizations.

International audienceIn the search for new fluorinated electroactive copolymers, 0–8 mol% of 2,3,3,3-tetrafluoropropene(1234yf) was terpolymerized with vinylidene fluoride (VDF) and trifluoroethylene (TrFE). The reactivity inbatch solution polymerization of 1234yf was evaluated. First, reactivity ratio measurements showed thathomopropagation of 1234yf is favored compared to cross propagation with VDF or TrFE (rVDF-1234yf =0.079, r1234yf-VDF = 2.6; and rTrFE-1234yf = 0.089, r1234yf-TrFE = 4.9, at 48 °C). Then, the study of a typicalbatch solution polymerization revealed that the polymerization time was not affected by the initialamount of 1234yf and that polymer chains with strong compositional heterogeneity were obtained. Based onthese results, a semi-continuous aqueous suspension polymerization process was used for the synthesis ofterpolymers with a more homogeneous composition. Calculation of addition probabilities exemplified thechanges in the drift of composition. The monomer sequences in the poly(VDF-ter-TrFE-ter-1234yf) terpolymers,carefully studied by 19F NMR spectroscopy, were not affected by the polymerization process and weredominated by VDF-TrFE and VDF-1234yf dyads. Finally, the thermal and ferroelectric (FE) properties of thehomogeneous and heterogeneous terpolymers were characterized and compared. In the case of heterogeneousterpolymers, increasing the 1234yf content from 0 to 6 mol% slightly decreased the crystal size andthe overall crystallinity (Tm = 60 °C and ΔHm = 144 J g−1 for a poly(VDF66-ter-TrFE26-ter-1234yf8) terpolymer).On the contrary, for homogeneous terpolymers, 5 mol% of 1234yf strongly affected both the Curie andmelting transitions at TCurie = 37 °C and Tm = 107 °C. The reduced coercive field (Ec = 40 MV m−1) andremnant polarization (Pr = 22 mC m−2) for the homogeneous terpolymer compared to a referencepoly(VDF65-co-TrFE35) (Ec = 62 MVm−1 and Pr = 45 mCm−2) transcribed modifications of the FE domains

A Journey into the Microstructure of PVDF Made by RAFT

International audiencePoly(vinylidene fluoride) (PVDF) is a very important fluoropolymer. It possesses high resistances to weathering or ageing and to chemical and thermal aggressions, as well as unique electroactive properties. The reversible-deactivation radical polymerization (RDRP) of VDFcan, so far, only be achieved via degenerative transfer using ITP (iodine transfer polymerization) or RAFT (reversible addition–fragmentation chain transfer). However, due to chain defects, and transfer to solvents, the RAFT polymerization of VDF produces PVDF chains with different chain ends. This article presents the results obtained from advanced 1H, 13C, and 19F NMR spectroscopy experiments using decoupling strategies, to ascertain unequivocally the microstructure of the PVDF chains synthesized using RAFT polymerization. This article provides a very detailed description of the different α- and ω-chain ends of PVDF 51-XA and reveals an uncommon NMR heteronuclear coupling between the proton of the stereocenterof the CTA R-group and the fluorine atoms of the CF2 moiety of the first added VDF unit

Photopatternable High-k Fluoropolymer Dielectrics Bearing Pendent Azido Groups

International audiencePhotopatternable fluoropolymers with high dielec. const. were prepd. by direct azidation of com. available poly(vinylidene fluoride-​trifluoroethylene-​chlorotrifluoroethylene)​. The produced fluoropolymers exhibit very high dielec. const., while being photopatternable without the use of any additives. These cross-​linked polymers appear to have reduced crystallinity compared to the pristine ones, as crosslinking is known to introduce defects in the polymer conformation, leading to a redn. in crystallinity. Crosslinking can occur both thermally and photochem. because of the presence of the pendent azido groups, which upon heating or irradn. with UV light release nitrogen and act as crosslinking sites. The cross-​linked films exhibit excellent dielec. properties which greatly depend on the reaction conditions. Appropriate photolithog. protocols have been developed to obtain excellent quality patterned films using the azide-​contg. fluoropolymer as neg. photoresist

Semicrystalline Organization of VDF- and TrFE-Based ElectroactiveTerpolymers: Impact of the trans-1,3,3,3-TetrafluoropropeneTermonomer

International audienceIn the search for fluorinated polymers with newelectroactive properties, the radical polymerization of vinylidenefluoride (VDF), trifluoroethylene (TrFE), and trans-1,3,3,3-tetrafluoropropene (1234ze) was achieved. The crystallineorganization and the electroactive properties of semicrystallinepoly(VDF-ter-TrFE-ter-1234ze) terpolymer films with 1234zemolar contents ranging from 0 to 6%, obtained by solventcasting,were investigated using a combination of structural,dielectric, and electromechanical techniques. For 0 mol %1234ze, poly(VDF-co-TrFE) copolymers exhibited a Curietransition from the ferroelectric (FE) phase to the paraelectric (PE) phase at the Curie temperature (TC ∼ 95 °C). DSC,dielectric spectroscopy, and FTIR experiments revealed the preservation of this Curie transition (TC ∼ 75 °C) while increasingthe termonomer content. WAXS measurements confirmed the persistence of the FE phase as the main phase at roomtemperature but also showed the appearance of a secondary ZFE phase (expanded FE phase incorporating 1234ze units) in lowerproportion. The WAXS crystallinity was halved (42 to 23%) from 0 to 6 mol % 1234ze terpolymer annealed films. Furthermore,SAXS and WAXS experiments highlighted a decrease of the crystalline lamellae thickness and a significant modification of theelectronic density distribution upon increase of 1234ze units. A new model for this particular arrangement of crystalline lamellaewas thus proposed with the location of the ZFE phase at the interface between the FE crystalline lamellae and the amorphousphase. Finally, reduced remnant polarization (Pr ∼ 15 mC/m2) and transverse piezoelectric coefficient (d33 ∼ −5 pC/N),compared to those of the reference poly(VDF-co-TrFE) copolymer (Pr ∼ 50 mC/m2 and d33 ∼ −20 pC/N), were mainlyassigned to the decrease of the FE phase content in terpolymer poled films