Toroidal Polar Topology in Strained Ferroelectric Polymer

Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF) and its binary and ternary copolymers, have important applications in reconfigurable electronic devices. In the PVDF polymer, the primary dipolar moments are induced by alternating -CF2 and -CH2 groups along the polymer chains, while the formation of long-range permanent dipoles perpendicular to the polymer chain is hindered, as PVDF has a large portion of amorphous phases with randomly distributed chains. In this work, the researchers focused on the P(VDF-TrFE) polymer, which exhibits a dominant crystalline β phase with an all-trans conformational sequence.

For the first time, with the aid of nano-scale AFM-IR studies, they:

1. Observed toroidal polar topology in ferroelectric polymers, and
2. Were able to identify the driving forces for the topology formation.

This toroidal polar topology was experimentally observed in the face-on P(VDF-TrFE) lamella by high- resolution imaging of in-plane piezo-response force microscopy, and the pattern was also explained by theoretical simulations.

The AFM-IR technique was applied to study the face-on P(VDF-TrFE) lamella, and the AFM-IR spectrum showed characteristic IR absorption bands. IR absorption images were recorded at 880 cm−1 corresponding to the symmetric stretching band of the CF2 group, which has a transition dipole parallel to the in-plane ferroelectric polarizations. When the IR polarization is parallel to the film plane, a spatially periodic absorption pattern was observed, as the local absorptions of face-on lamellae continuously vary because of the alternated toroidal ferroelectric polarizations. The result from AFM-IR studies further verified the toroidal polar topology in the P(VDF-TrFE) polymer.

_{KEY TERMS:}

AFM-IR (PTIR), Ferroelectric materials, Materials science and engineering, Piezoresponse force microscopy (PFM), Polar topology, Polymer-based ferroelectric materials