Electric Field Induced Phase Transitions in Polymers: A Novel Mechanism for High Speed Energy Storage

Using first-principles simulations, we identify the microscopic origin of the nonlinear dielectric response and high energy density of polyvinylidene-fluoride-based polymers as a cooperative transition path that connects nonpolar and polar phases of the system. This path explores a complex torsional and rotational manifold and is thermodynamically and kinetically accessible at relatively low temperatures. Furthermore, the introduction of suitable copolymers significantly alters the energy barriers between phases providing tunability of both the energy density and the critical fields.

Figure 1

Conformations and transition pathways in PVDF: (a) Trans-gauche configuration (TGTG’), in which the dihedral angles alternate between trans (180°) and gauche ($\pm 57°$); (b) All-trans geometry (TT) where all the dihedral angles are 180°; (c) Starting from the antipolar $\alpha$ phase one can rotate one of the chains about its axis by 180°, to align the dipoles and create the polar $\gamma$ phase; (d) Transformation from the $\gamma$ to the $\beta$ phase is achieved by gradually changing the dihedral angles in each chain from $\pm 57°$ to 180°.

Figure 2

Energetics and polarization along the transformation pathway from the $\alpha$ to the $\beta$ phase, for pure PVDF and P(VDF-CTFE) $90/10\mathrm{mol}\%$. The change in interchain angle from 0° to 180° results in the $\alpha \to \gamma$ transition, while the change in the dihedral angle from $\pm 57°$ to 180° describes the $\gamma \to \beta$ transformation: (a) total energies, and (b) polarizations from Eq. (1). In (b) crosses display polarization values calculated using the first-principles Berry-phase approach. (c) Electric enthalpies for electric fields of 0, 100, 500, and $1000\mathrm{MV}/\mathrm{m}$ as functions of the order parameters.