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.