Recent advances in ferroelectric polymers

In the Polymer Electroprocessing Laboratory at Rutgers University, we have discovered that the odd-numbered nylons constitute the second known class of ferroelectric polymers^1–3, and that polarized films exhibit piezoelectric properties similar to poly(vinylidene fluoride) (PVF₂) and copolymers of PVF₂ and trifluoroethylene (PVF₂/PVF₃). A study of a series of these polymers including nylon 11, nylon 9, nylon 7 and nylon 5 showed that the remanent polarization produced in quenched, cold-drawn films was a linear function of polymer dipole density⁴. The highest remanent polarization produced was that of nylon 5, the value attained (P₁=125mC/m²) being approximately 2.5 times that of PVF₂. We also discovered that (unlike PVF₂ or PVF₂/PVF₃) the remanent polarization could be stabilized to elevated temperatures (close to the polymer melting point). For nylon 5, the remanent polarization and piezoelectric response was stable to over 250°C⁵. We showed that the hydrogen-bonded sheet structure in nylon 11 for quenched cold-drawn films was parallel to the plane of the film, and that after application of high electric fields the hydrogen-bonded sheet structure was rotated 90° to an orientation perpendicular to the plane of the film³. A detailed X-ray diffraction study of the effects of humidity and electroprocessing on the switching behavior of nylon 5, nylon 7 and nylon 11 films was carried out⁵. The piezoelectric and pyroelectric response⁶ of these films was also determined. The different switching mechanisms observed and the measured piezoelectric and pyroelectric properties will be presented and discussed.