Ionic liquids induce crystalline β phase and ferroelectric polarization in sub-micrometer films of poly(vinylidene fluoride) (PVDF)

In this paper, ferroelectric poly(vinylidene fluoride) (PVDF) films with a thickness of about 430 nm were obtained by spin coating from a solution that contained a small portion of the ionic liquid 1-ethyl-3-methylimidazolium nitrate ([EMIM][NO₃]). Before the experiments, the PVDF films were dried and thermally annealed. The sub-micrometer films on substrates show clear ferroelectric hysteresis behavior. A remanent polarization of about 60 mC/m² was detected by means of two independent methods: Non-linear current-voltage characteristics and time-dependent current response after application of voltage steps. The coercive field decreases from 190 to 165 MV/m when the width of triangular bipolar/unipolar electric-field cycles increases from 3 to 300 ms. Quasi-static pyroelectricity increases from 14 to 18 μC/(m² K) between -20 and +40 °C. X-ray diffraction prove that the ionic liquid enhances the crystallinity. The dipolar interaction between anions of the ionic liquid and CH₂ groups in the PVDF chain during spin coating is considered to facilitate the formation of the β phase.