The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride)

Ilias Katsouras, Kamal Asadi, Mengyuan Li, Tim B. van Driel, Kasper S. Kjær, Dong Zhao, Thomas Lenz, Yun Gu, Paul W. M. Blom, Dragan Damjanovic, Martin M. Nielsen & Dago M. de Leeuw.

Nature Materials (2015) doi:10.1038/nmat4423

Piezoelectricity, which describes interconversion between electrical charge and mechanical strain, is presently attracting huge attention due to its wide application range. There are however open fundamental issues,ii specifically understanding the physical processes that govern the coupling between stress and polarization.

Typically, the electro-mechanical coupling constants are positive in the sense that piezoelectric materials expand when an applied electric field is aligned with the poling direction. The only exceptions are the ferroelectric polymer poly(vinylidene-fluoride) (PVDF) and its copolymers with trifluoroethylene P(VDF-TrFE), which show an unusual “negative piezoelectric effect”. Counterintuitively, these polymers contract in the direction of the applied electric field. Reported explanations consider exclusively contraction of either the crystallineiii  or the amorphous partiv of these semi-crystalline polymers. Curiously, these approaches account for the total experimentally measured piezoelectric strain, suggesting that they are inadequate. To resolve the controversy, we have performed in-situ X-ray diffraction measurements on P(VDF-TrFE) capacitors, a severe experimental challenge.


FIG 1. Schematic illustration of strain on application of an electric field in a poled ferroelectric material. a, The initial state. b, When an electric field is applied in the direction of the polarization in a material with a positive piezoelectric coecient, such as lead zirconate titanate (PZT), the material expands. c, The counterintuitive behaviour of PVDF and its copolymers. These materials contract, exhibiting a negative piezoelectric coecient. Schematic crystal structures are shown in b and c.

The minute changes can only be determined dynamically, by measuring the diffracted X-ray intensity as a function of a time-varying electric field. Only then can the strain, defined as the relative change in lattice constant, be obtained by statistical averaging over thousands of acquired X-ray diffractograms. Furthermore, to arrive at a quantitative description of the piezoelectric effect in PVDF, the changes in lattice constant have to be measured simultaneously with the ferroelectric polarization, yielding constrains on data acquisition and synchronization.

We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the response of the crystalline part alone. We developed a phenomenological model to explain the data and our quantitative analysis shows that the crystalline contribution only amounts to a third of the total piezoelectric response. The major contribution is a previously overseen 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.

The work is opening a new page in the understanding of the piezoelectricity in organic materials, and has technological implications for organic transducers and actuators.

 S. H. Baek et al., Science, 334, 958 (2011); Z. L. Wang and J. Song, Science, 312, 242 (2006); J. X. Zhang et al., Nat. Nanotech. 6, 98 (2011).
ii R. J. Zeches et al., Science, 326, 977 (2009); T. Sluka et al., Nat Comm. 3, 748 (2012).
iii V. S. Bystrov et al., J. Mol. Model. 19, 3591 (2013).
iv T. Furukawa and N. Seo, Jpn. J. Appl. Phys. 29, 675 (1990).






Martin Meedom Nielsen
DTU Physics
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22 JULY 2018