Study of interfacial adhesion in piezoelectric composites having a fluoropolymer matrix

Les matériaux électroactifs sont des matériaux dont les propriétés physiques (déformation, polarisation…) évoluent sous l’application d'un champ électrique. Parmi les différentes propriétés électroactives, la piézoélectricité caractérise la capacité d’un matériau à accumuler des charges électriques en réponse à une contrainte mécanique. Des propriétés piézoélectriques ont été attribuées à de nombreux types de matériaux comme les monocristaux, les céramiques ou les polymères. Ces matériaux sont utilisés dans un large domaine d’applications comme actionneurs comme capteurs ou transducteurs. Les céramiques, même si elles présentent les meilleurs coefficients piézoélectriques, présentent tout de même certains inconvénients. En effet, leur dureté les rend particulièrement fragiles. Les polymères piézoélectriques, même s’ils possèdent un coefficient piézoélectrique moindre par rapport aux céramiques piézoélectriques, sont faciles à mettre en œuvre du fait de leur souplesse et sont capables de déformations plus importantes.Cette thèse concerne essentiellement l’étude de composites de fluoropolymères piézoélectriques et de céramiques piézoélectriques préparés par voie solvant et s’intéresse plus particulièrement à l’amélioration de l’adhésion interfaciale dans ce type de composite. L’objectif de cette étude est d’utiliser les techniques de greffage de polymère « Grafting To » et « Grafting From » pour fixer des agents de couplage polymériques capables d’interactions aussi bien avec la céramique qu’avec le polymère piézoélectrique. Cette étude vise à mieux comprendre l’impact de l’amélioration de l’adhésion interfaciale sur les propriétés morphologiques, structurales et piézoélectriques des composites.Electroactive materials are materials whose physical properties (deformation, polarization, etc.) change under the application of an electric field. Among the various electroactive properties, piezoelectricity characterizes the ability of a material to accumulate electrical charges in response to mechanical stress. Piezoelectric properties have been attributed to many types of materials such as single crystals, ceramics or polymers. These materials are used in a wide range of applications as actuators, sensors or transducers. Ceramics, even if they have the best piezoelectric coefficients, still have certain drawbacks. Indeed, their hardness makes them particularly brittle. Piezoelectric polymers, although they have a lower piezoelectric coefficient compared to piezoelectric ceramics, are easy to process due to their flexibility and are capable of greater deformation.This thesis mainly concerns the study of composites of piezoelectric fluoropolymers and piezoelectric ceramics prepared by a solvent route and is more particularly interested in improving the interfacial adhesion in this type of composite. The objective of this study is to use the "Grafting To" and "Grafting From" polymer grafting techniques to fix polymeric coupling agents capable of interactions with both the ceramic and the piezoelectric polymer. This study aims to better understand the impact of improved interfacial adhesion on the morphological, structural and piezoelectric properties of composites

NMR investigations of polytrifluoroethylene (PTrFE) synthesized by RAFT

International audienceTrifluoroethylene (TrFE) is a relatively rare fluorinated monomer mainly used in copolymerisation with vinylidene fluoride (VDF) to prepare ferroelectric materials. While VDF homopolymerisation has been relatively well studied, that of TrFE is still poorly understood and the reversible deactivation radical polymerisation of this monomer has never been studied in depth. To better understand the RAFT polymerisation of TrFE, accurate assignments of PTrFE synthesized by RAFT polymerisation are necessary. Thus, this article reports detailed 19F, 1H and 13C 1D and 2D experiments carried out to determine and assign the different NMR chemical shifts and splitting patterns of the α- and ω-chain ends of PTrFE synthesized by RAFT polymerisation

RAFT Polymerisation of Trifluoroethylene: The importance of understanding reverse additions

International audienceThis article is the first report of the RAFT polymerisation of trifluoroethylene (TrFE). Trifluoroethylene is a rare but very important fluoromonomer, as it allows the preparation of materials endowed with unique electroactivity via copolymerisation with vinylidene fluoride (VDF) and other fluoromonomers. RAFT polymerisations carried out using O-ethyl-S-(1-methoxycarbonyl) ethyldithiocarbonate as a chain transfer agent and a thermal initiator were carefully examined. The polymerisation, its kinetics and the chain-end evolution were investigated by GPC, 1H{19F} and 19F{1H} NMR spectroscopy as well as MALDI-TOF mass spectrometry. Similar to the RAFT polymerisation of VDF, irreversible transfer reactions and reverse additions significantly affect the control of the polymerisation as well as the chain-end functionality. However, in contrast to VDF, unusual reverse propagation of TrFE, although limited to a few monomer units, was evidenced thanks to a combined NMR spectroscopy and DFT calculation approach. RAFT polymerisation afforded relatively well-defined PTrFE with a crystalline structure consistent with previous reports

Relations entre la structure et les propriétés de composites piézoélectriques de BTO/P(VDF-co-TrFE)

National audienc

New Insight into Nanoscale Identification of the Polar Axis Direction in Organic Ferroelectric Films

International audienceFerroelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-co-TrFE)] thin films have been deposited by spin-coating onto the Bi0.5Na0.5TiO3(BNT)/LNO/SiO2/Si heterostructure. The copolymer microstructure investigated by using grazing-incidence wide-angle X-ray diffraction (GIWAXD) and deduced from the (200)/(110) reflections demonstrates that the b-axis in the P(VDF-co-TrFE) orthorhombic unit cell is either in the plane or out of the plane, depending on the face-on or on the two types of edge-on (called I and II) lamellar structures locally identified by atomic force microscopy (AFM). For edge-on I lamellae regions, the electroactivity (dzzeff ∼ −50.3 pm/V) is found to be twice as high as that measured for both edge-on II or face-on crystalline domains, as probed by piezoresponse force microscopy (PFM). This result is directly correlated to the direction of the ferroelectric polarization vector in the P(VDF-co-TrFE) orthorhombic cell: larger nanoscale piezoactivity is related to the b-axis which lies along the normal to the substrate plane in the case of the edge-on I domains. Here, the ability to thoroughly gain access to the as-grown polar axis direction within the edge-on crystal lamellae of the ferroelectric organic layers is evidenced by combining the nanometric resolution of the PFM technique with a statistical approach based on its spectroscopic tool. By the gathering of information at the nanoscale, two orientations for the polar b-axis are identified in edge-on lamellar structures. These findings contribute to a better understanding of the structure–property relationships in P(VDF-co-TrFE) films, which is a key issue for the design of future advanced organic electronic devices

Utilization of Catechol End-Functionalized PMMA as a Macromolecular Coupling Agent for Ceramic/Fluoropolymer Piezoelectric Composites

International audienceAn approach based on the use of a macromolecular coupling agent and the aim to improve the interfacial adhesion between piezoelectric ceramics and piezoelectric polymer matrix in piezoelectric composites is presented. Poly(methyl methacrylate) (PMMA) bearing a catechol moiety was used as a macromolecular coupling agent, as it is known to be miscible to piezoelectric fluoropolymers and catechol groups can strongly bind to a large variety of surfaces. Thus, entanglement between the PMMA chains and the amorphous segments of the fluoropolymer would ensure the desired interfacial adhesion. Well-defined PMMA was synthesized via RAFT polymerization using 2-cyano-2-propyl dodecyl trithiocarbonate as a chain-transfer agent. The PMMA ω-chain end was then functionalized with a catechol group via a one-pot aminolysis/thia-Michael addition procedure using a dopamine acrylamide (DA) derivative as a Michael acceptor. The presence of the catechol moiety at the chain end of PMMA was controlled by 1H NMR and cyclic voltammetry measurements. The resulting PMMA-DA was then grafted onto the surface of a lead-free piezoelectric ceramic film (i.e., a thin film of H2O2-activated (Bi0.5Na0.5)TiO3 (BNT) with a large contact area). The increase of the water contact angle confirmed the efficiency of the grafting. A commercial piezoelectric copolymer P(VDF-co-TrFE) was then spin-coated onto the modified BNT surface to form a bilayer composite. The composite cross section prepared by cryofracture was examined by scanning electron microscopy and revealed that the ceramic/polymer interface of the BNT-PMMA/P(VDF-co-TrFE) bilayer composite exhibits a much better cohesion than its counterpart composite prepared from nonmodified BNT. Moreover, the grazing incidence wide-angle X-ray scattering confirmed that the copolymer crystal structure was not impacted by the presence of the PMMA-DA coupling agent. A strong piezoelectric response was locally detected by piezoresponse force microscopy. This study highlights the potential of PMMA-DA as a macromolecular coupling agent to improve the ceramic/polymer interface in piezoelectric composite materials

Atomic force microscopy as a suitable tool for probing the polar axis direction in electroactive P(VDF-co-TrFE) films at the nanoscale

National audienc

Atomic force microscopy study of local polarization direction in ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) organic films

International audienc

Revitalizing Inert Materials: Grafting Self‐Oscillating, Stimuli‐Responsive Organometallic Polymers for Pulsating Systems

Abstract A major challenge in materials science is dynamically adjusting material properties using sensors and control systems. This contribution develops a new approach using a self‐oscillating copolymer to autonomously change material surface properties in response to environmental changes. A redox‐sensitive terpolymer of N‐isopropylacrylamide (NIPAM), dimethylacrylamide (DMAc), and an iron‐based comonomer ([(phen)2(phen‐5‐yl‐acrylamide)FeII](PF6)2) is synthesized via Reversible Addition‐Fragmentation Chain Transfer (RAFT) polymerization, catalyzing an oscillating redox reaction (Belousov‐Zhabotinsky, BZ). The terpolymer oscillates from soluble to insoluble around 35 °C based on the iron's oxidation state. A catechol unit is incorporated to enhance versatility, enabling grafting onto different surfaces. Optimal BZ reagent concentrations are explored for maximum oscillation amplitude and frequency. By selecting a working temperature between redox transition points, the copolymer's oscillation from coil to globular conformation is observed due to redox oscillations. The self‐oscillating copolymer is grafted onto an ultrafiltration membrane, where conformational changes cause variations in pore size, leading to rapid negative flux peaks that disrupt the flux and reduce membrane fouling during protein filtration. This study highlights self‐oscillating polymers' ability to impart dynamic properties to inert materials, paving the way for smart materials with self‐regulating properties to adapt to changing conditions