Ab Initio and molecular dynamics simulations of double-bond defects in dipolar polymers

Polyvinylidene fluoride (PVDF)-based relaxor ferroelectric polymers have attracted significant attention because of their excellent performance in energy storage, electro-actuators, and electrocaloric solid-state cooling. It has been demonstrated that most of the performance gain originates from conformational changes of the polymer chains under applied electric fields. Recent work shows that new "double bond" (DB) defects can tune the magnitude of the electrocaloric effect. However, there is a lack of understanding of how DBs affect the dynamics of structural changes in an electric field. Here, we apply an accurate DFT method to study ferroelectric polymers by adding DBs as defects into the polymer chains. A phase transition path from the non-polar to the polar phase is introduced, in which DBs reduce the energy barrier, and the polar phase forms much easier even at a low field. Based on the simulations, large electrostriction at low field is also predicted even with ~2 mol% DBs. We also examine the DBs' influence on structure in an external electric field by classical molecular dynamics. Different polymer conformations are investigated to reveal the phase transition process.