title: The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride).
abstract: Piezoelectricity describes interconversion between electrical charge and mechanical strain. As expected for lattice ions displaced in an electric field, the proportionality constant is positive for all piezoelectric materials. The exceptions are poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)), which exhibit a negative longitudinal piezoelectric coefficient. Reported explanations exclusively consider contraction with applied electric field of either the crystalline or the amorphous part of these semi-crystalline polymers. To distinguish between these conflicting interpretations, we have performed in situ dynamic X-ray diffraction measurements on P(VDF-TrFE) capacitors. We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the polarization-biased electrostrictive contribution of the crystalline part alone. Our quantitative analysis shows that an additional contribution is operative, which we argue is due to an electromechanical coupling between the intermixed crystalline lamellae and amorphous regions. Our findings tie the counterintuitive negative piezoelectric response of PVDF and its copolymers to the dynamics of their composite microstructure.
authors.affiliation: Max-Planck Institute for Polymer Research, Ackermannweg 10 55128 Mainz, Germany.|Holst Centre, High Tech Campus 31, 5656 AE, Eindhoven, The Netherlands.
authors.forename: Ilias
authors.initials: I
authors.lastname: Katsouras
authors.affiliation: Max-Planck Institute for Polymer Research, Ackermannweg 10 55128 Mainz, Germany.
authors.forename: Kamal
authors.identifier: http://orcid.org/0000-0003-0447-4337
authors.initials: K
authors.lastname: Asadi
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4 9747 AG, Groningen, The Netherlands.|Electronic Information Industry, CCID Thinktank, China Center for Information Industry Development No. 27, Wanshou Road 100846 Beijing, China.
authors.forename: Mengyuan
authors.initials: M
authors.lastname: Li
authors.affiliation: Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Tim B
authors.initials: TB
authors.lastname: van Driel
authors.affiliation: Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Kasper S
authors.initials: KS
authors.lastname: Kjær
authors.affiliation: Max-Planck Institute for Polymer Research, Ackermannweg 10 55128 Mainz, Germany.
authors.forename: Dong
authors.initials: D
authors.lastname: Zhao
authors.affiliation: Max-Planck Institute for Polymer Research, Ackermannweg 10 55128 Mainz, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Lenz
authors.affiliation: Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Yun
authors.initials: Y
authors.lastname: Gu
authors.affiliation: Max-Planck Institute for Polymer Research, Ackermannweg 10 55128 Mainz, Germany.
authors.forename: Paul W M
authors.initials: PW
authors.lastname: Blom
authors.affiliation: Ceramics Laboratory, Swiss Federal Institute of Technology-EPFL, 1015 Lausanne, Switzerland.
authors.forename: Dragan
authors.identifier: http://orcid.org/0000-0002-9596-7438
authors.initials: D
authors.lastname: Damjanovic
authors.affiliation: Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Martin M
authors.identifier: http://orcid.org/0000-0002-8135-434X
authors.initials: MM
authors.lastname: Nielsen
authors.affiliation: Max-Planck Institute for Polymer Research, Ackermannweg 10 55128 Mainz, Germany.|King Abdulaziz University, Abdullah Sulayman, 22254 Jeddah, Saudi Arabia.
authors.forename: Dago M
authors.initials: DM
authors.lastname: de Leeuw