Effect of electric field on the infrared spectrum of a ferroelectric poly(vinylidene fluoride-co-hexafluoropropylene) film

• We find butterfly-shaped IR intensity loops characteristic of ferroelectricity for P(VDF-HFP).
• We find electric field induced inversion of a polymer chain around the P(VDF-HFP) polymer long axis.
• We find electric field induced motions of polar crystallites.

We have used infrared spectroscopy to study electric-field induced motions of polymer chains in a ferroelectric uniaxially drawn film of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] (molar ratio, 1:0.18) consisting mainly of the polar crystalline β phase. We analyzed the observed spectra under the C2v symmetry of an infinite poly(vinylidene fluoride) (PVDF) chain. The changes in intensity of the a1 bands (2979, 1432, 1280, and 842 cm−1), b1 bands (1073 cm−1), and b2 bands (3017, 1168, and 880 cm−1) showed butterfly-shaped hysteresis loops characteristic of ferroelectricity when the electric field was varied from 1.4 to −1.4 MV/cm. The changes in intensity of the a1 and b2 bands with transition dipole moments perpendicular to the polymer chain were large, whereas those of the b1 bands with transition dipole moments parallel to the polymer chain were small. The changes in intensity of the a1 and b2 bands were mainly attributed to the inversion of the polymer chain around the polymer axis. On the other hand, the changes in intensity of the b1 band were attributed to the motion of a polar crystallite.