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Chirality-induced relaxor properties in ferroelectric polymers

Abstract

Relaxor ferroelectrics exhibit outstanding dielectric, electromechanical and electrocaloric properties, and are the materials of choice for acoustic sensors, solid-state coolers, transducers and actuators1,2,3,4. Despite more than five decades of intensive study, relaxor ferroelectrics remain one of the least understood material families in ferroelectric materials and condensed matter physics5,6,7,8,9,10,11,12,13,14. Here, by combining X-ray diffraction, atomic force microscope infrared spectroscopy and first-principles calculations, we reveal that the relaxor behaviour of ferroelectric polymers originates from conformational disorder, completely different from classic perovskite relaxors, which are typically characterized by chemical disorder. We show that chain chirality is essential to the formation of the disordered helix conformation arising from local distortions of gauche torsional angles, which consequently give rise to relaxor properties in polymers. This study not only sheds light on the fundamental mechanisms of relaxor ferroelectrics, but also offers guidance for the discovery of new ferroelectric relaxor organic materials for flexible, scalable and biocompatible sensor and energy applications.

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Fig. 1: Disordered helical chain structure driven by chirality in relaxor polymers.
Fig. 2: AFM-IR characterization of conformational disorder in relaxor ferroelectric polymers.
Fig. 3: Emergence of relaxor behaviour induced by chirality.
Fig. 4: Structural characterization by X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR).
Fig. 5: Electrostrictive properties of PTrFE and PCTFE at room temperature.

Data availability

The data that support the findings of this study are publicly available at https://figshare.com/projects/Nature_Materials_datasource/81800. Source data are provided with this paper.

Code availability

Quantum Espresso for DFT calculations can be downloaded from https://www.quantum-espresso.org/. LAMMPS and ReaxFF for molecular dynamics or MM calculations can be found at https://lammps.sandia.gov/. Source data are provided with this paper.

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Acknowledgements

This research was funded by the US Air Force Office of Scientific Research through MURI FA9550-19-1-0008 (Q.W.) and the US Office of Naval Research (grant N000141912033, J.B.). The supercomputer time at the National Center for Supercomputing Applications (NSF OCI-0725070 and ACI-1238993) was provided by NSF grant ACI-1615114 (J.B.). Y.L. thanks T. Williams for technical assistance.

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Authors

Contributions

Y.L. and Q.W. designed the research; W.X. and Z.H. were responsible for the polymer synthesis and NMR measurements; Y.L. prepared the polymer films and collected X-ray diffraction data; Y.L. and A.H. performed FTIR, dielectric and electromechanical property measurements; and Y.L. collected the electrical conduction and AFM-IR data. B.Z., W.L. and J.B. performed the simulations and analysis. Y.L. and Q.W. wrote the manuscript with input from all authors. Q.W. supervised the research.

Corresponding author

Correspondence to Qing Wang.

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Extended data

Extended Data Fig. 1 XRD patterns of P(VDF-TrFE) copolymers.

θ-2θ scans of P(VDF-TrFE). The black dashed line indicates the (001) plane characteristic of long-range ferroelectricity while the light blue dash line indicates characteristic peaks originating from relaxor phases.

Extended Data Fig. 2 XRD patterns of ferroelectric in the paraelectric phase.

θ-2θ scans were obtained at 100 °C except for P(VDF-TrFE) 65/35 mol% at 130 °C. The dashed lines indicate characteristic peaks originating from paraelectric phases.

Extended Data Fig. 3

Intermolecular lattice spacing versus temperature in PTrFE.

Extended Data Fig. 4 Super cells of six different conformations used in DFT calculations to determine the stable crystalline PTrFE.

a, Left: The legend of elements. The chirality center carbon is colored orange in this figure. Right: schematic views of syndiotactic all-trans PTrFE chain from two directions, where the boxed -CHF-’s exhibit opposite chirality. b, Six-chain conformations in primitive unit cells in views from three directions as indicated in the rows. Column 1: axes; Column 2: iso/b; Column 3: iso/g; Column 4: iso/h; Column 5: syn/b; Column 6: syn/g; Column 7: syn/h. Iso and syn are abbreviations for isotactic and syndiotactic while h, b and g denote the (TG)3 (3/1-helical phase), the all trans (the β phase) and the T3GT3Ḡ (the γ phase) conformations, respectively. c, Supercells of six different conformations used in DFT calculations to determine the stable crystalline PTrFE in views from two different directions as indicated in the rows. Column 1: axes; Column 2: syn/b; Column 3: syn/g; Column 4: iso/h; Column 5: syn/b; Column 6: syn/g; Column 7: syn/h.

Extended Data Fig. 5 Supercells used in ReaxFF calculations for gauche distortion.

a, Top: the PTrFE supercell where all the 24 gauche torsional angles are constrained; Bottom left: the two finite chains used in DFT calculations where the two torsional angles in the black boxes are constrained. Bottom right pictures demonstrate how the gauche torsional angles (formed by the four carbon atoms in yellow) are changed in views from two directions. b, A summary of differences between the DFT calculations and the MM/ReaxFF calculations.

Extended Data Fig. 6 AFM-IR characterization of conformational disorder.

a and d, b and c, Simultaneously measured topographies (a,d) and AFM-IR chemical maps (b,e) exposed to a 1275 cm−1 and 1190 cm−1 laser light. The size of ae is 1 × 1 µm2. c and f, Local spectra of the positions marked in (a,d) and (b,e). The upper panels (a,b,c) P(VDF-CTFE) 90/10 mol%. The bottom panels (d,e,f) PCTFE.

Extended Data Fig. 7 Fit of dielectric data by the Vogel-Fulcher law.

a, PTrFE. b, PCTFE. c, P(VDF-CTFE) 90/10 mol%. d, P(VDF-CTFE) 80/20 mol%. The red line is a fit of the dielectric constant (solid black symbols) by the Vogel-Fulcher law. The fitting parameters can be found in Supplementary Table 2. Error bars represent standard deviations obtained from at least three measurements using different samples.

Extended Data Fig. 8 Ionic conductivity in PTrFE at different temperatures and frequencies.

a, Dielectric constant. b, dielectric loss. c, Conductivity. The gray region indicates the dominant region of ionic conductivity.

Extended Data Fig. 9 FTIR spectra.

a, From top to down: P(VDF-TrFE) 65/35 mol%, P(VDF-TrFE) 55/45 mol%, P(VDF-TrFE) 45/55 mol%, PTrFE, P(VDF-TrFE-CTFE) 61.8/30.4/7.8 mol%, P(VDF-TrFE-CFE) 61.5/30.3/8.2 mol%. b, From top to down: PVDF, P(VDF-CTFE) 90/10 mol%, and P(VDF- CTFE) 80/20 mol%.

Supplementary information

Supplementary Information

Supplementary Figs. 1–6, Tables 1–2 and refs. 39–47.

Source data

Source Data Fig. 1

Source data for Fig. 1b–d,f,g.

Source Data Fig. 2

Source data for Fig. 2g–i.

Source Data Fig. 3

Source data for Fig. 3a–d.

Source Data Fig. 4

Source data for Fig. 4a,b.

Source Data Fig. 5

Source data for Fig. 5a–f.

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Liu, Y., Zhang, B., Xu, W. et al. Chirality-induced relaxor properties in ferroelectric polymers. Nat. Mater. 19, 1169–1174 (2020). https://doi.org/10.1038/s41563-020-0724-6

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