Phase equilibria in P(TrFE-VDF): conformation and chirality

Ferro- and piezo-electric materials near a phase boundary usually possess enhanced dielectric or piezoelectric properties. We have discovered a morphotropic phase boundary (MPB) in P(VDF-TrFE) polymers [1], the first for any organic material. We show that the MPB forms due to intrachain rather than interchain conformation competition between the planar all-trans and 3/1-helical conformations in single P(VDF-TrFE) chains [2]. Our ab-initio calculations reveal low energy barrier electric dipole moment rotations between the nearly energetically degenerate chain conformations, which result in the piezoelectric enhancement near MPB. In particular, chirality plays a crucial role in the formation of a helical conformation and the relative stability of chain conformations strongly depends on the spatial arrangement of chirality centers. Our results explain the experimental data and the near degeneracy of multiple phases in the P(VDF-TrFE) material. They also provide a guideline for designing new polymers with MPB from a molecular point of view.
[1] Y. Liu, A. Haibibu, B. Zhang, W. Xu, W. Lu, J. Bernholc, Q. Wang, Nature 562, 96 (2018).
[2] Y. Liu, B. Zhang, A. Haibibu, W. Xu, Z. Han, W. Lu, J. Bernholc, Q. Wang, J. Phys. Chem. C 123, 8727 (2019).