First-principles investigation of high energy density in PVDF copolymers

PVDF and its copolymers exhibit excellent electromechanical properties and in the case of PVDF-CTFE also a very high energy density [1]. We have investigated the phase diagram of these systems and can quantitatively explain the observed energy density of PVDF-CTFE as due to a para to ferroelectric phase transition in a disordered, multidomain structure [2]. Our results show that pure PVDF prefers the $\alpha$ phase at zero field. Electric field lowers the free energy of the $\beta$ phase, resulting in a structural phase transition at a sufficiently high field. Copolymer admixture lowers the critical field and eventually leads to an energetic preference for the $\beta$ phase even at zero field. For PVDF-CTFE with CTFE content below 17 \%, the $\alpha$ phase is still preferred and the field-induced phase transformation reversibly stores large amounts of energy. For PVDF-TeFE, the total energy difference between the two phases is much smaller, resulting in substantially smaller energy density. [1] B. Chu et al., Science 313, 334 (2006). [2] V. Ranjan et al., PRL 99, 047801 (2007).