Multi-scale characterisation of a ferroelectric polymer reveals the emergence of a morphological phase transition driven by temperature

[EN] Ferroelectric materials exhibit a phase transition to a paraelectric state driven by temperature - called the Curie transition. In conventional ferroelectrics, the Curie transition is caused by a change in crystal symmetry, while the material itself remains a continuous three-dimensional solid crystal. However, ferroelectric polymers behave differently. Polymeric materials are typically of semi-crystalline nature, meaning that they are an intermixture of crystalline and amorphous regions. Here, we demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) strongly affects its Curie transition, as not only a change in crystal symmetry but also in morphology occurs. We demonstrate, by high-resolution nanomechanical measurements, that the semicrystalline microstructure in the paraelectric state is formed by crystalline domains embedded into a softer amorphous phase. Using in situ X-ray diffraction measurements, we show that the local electromechanical response of the crystalline domains is counterbalanced by the amorphous phase, effectively masking its macroscopic effect. Our quantitative multiscale characterisations unite the nano- and macroscopic material properties of the ferroelectric polymer P(VDF-TrFE) through its semi-crystalline nature.
European Union’s Horizon 2020 research and Innovation programme under the Marie Skłodowska-Curie grant agreement number 721874 (SPM2.0). RG acknowledges funding from the European Research Council ERC–AdG–340177 (3DNanoMech). This work was supported by the UK government’s Department for Business, Energy and Industrial Strategy. The dynamic mechanical analysis was supported by T. Koch from the Institute of Materials Science and Technology, TU Wien. We gratefully thank A. Muhamedagić for the contribution of artworks to the figures (armindesign.li).