Switching of banana liquid crystal mesophases under field

Abstract

Taking advantage of the great number of bent-core or "banana" compounds synthesized and studied in the laboratory, we describe their behaviour under the application of an external electric field. If the field were a static one, we would work within the frame of an equilibrium phase diagram in a (field E, temperature T) space where some phases would be simple dielectrics and others ferroelectric ones with a macroscopic polarization, either spontaneous or induced by the field. In this paper, we deal with the basic responses of "banana" liquid crystals under the application of a low frequency (1 to 100 Hz) AC field. Firstly square-wave voltages allow us to locate the phase boundary between dielectric (at lower field) and ferroelectric phases (higher field) at a given temperature and field threshold. Then we apply slowly varying AC voltages with shapes like triangle or "triple-plateau" to check out the stability of the induced ferroelectric phase versus field removal. Three behaviours are encountered, the unstable one (short lifetime of the high-field ferroelectric phase) where the macroscopic polarization is destroyed and then rebuilt in the opposite direction during each half period and usually called "antiferroelectric"; the stable one (long lifetime) with a polarization that rotates at constant modulus which is labeled as "ferroelectric" and a new one where the macroscopic polarization is proportional to the applied field, we named this behaviour as "superparaelectric". Let us stress that these observations apply to the ferroelectric phases of the (E, T) phase diagram not to the zero field (0,T) phases observed in the usual phase characterization experiments except for an eventual spontaneous ferroelectric phase.

Ferroelectric Response and Induced Biaxiality in the Nematic Phase of Bent‐Core Mesogens

“…Consistently with the X-ray experiment with an applied electric field, above a threshold amplitude of the input wave voltage (V pp ¼ 50 V at n ¼ 1.1 Hz), the switching current arising from the reversal of the polarization direction in the LC sample gives a nonlinear contribution to the total current superimposed to the linear background generated by the ionic flow, [36,37] as shown in Figure 3A. The observed ferroelectric response indicates, differently from normal nematics, the existence of a switchable macroscopic bulk polarization in the direction of the applied electric field, and hence of polar ordering.…”

“…To verify the presence of polar ordering we measured the electric response and switching behavior of 9BPO in the temperature range between T ¼ 120 8C and T ¼ 270 8C by means of the repolarization current technique, [36] where an applied triangular voltage waveform drives a current through a thin film of material, measured by the voltage drop across a high precision resistor connected in series with the sample (capacitor) and the voltage generator.…”

“…without vanishing, thus implying a ferroelectric ordering [36] both in the Sm and N mesophases. We observed a similar behavior over the entire nematic range up to the clearing temperature, while in the isotropic phase the peak completely disappears (inset of Fig.…”

“…This electric response is different from that expected for paraelectric phases (even what could be called super or nonlinear paraelectric, given the high P s values involved) that do not show any switching threshold and whose polarization switching should exhibit a single peak per half-period centered at the zero-crossing point of the applied voltage. [36] The unique delayed peak per half-period observed in the repolarization measurements under triangular wavevoltage shows [36,38] that the polarization vector P s (whose characteristic lifetime is greater than the half-period of the input voltage) rotates at constant modulus. The electric response is therefore the same observed in true ferroelectrics, i.e., ferroelectric materials having an ideally infinite lifetime of the polarization, even if in our case the field-on ferroelectric nematic phase eventually goes back to a non polar ground phase as the field is definitively switched off.…”

Low electric-field-induced switching in the B1 bent-core liquid crystal phase

“…There are eight known major bent-core mesophases, of which the smecticlike ͑or layered͒ B2 phase is the most heavily investigated due to its ferroelectric and antiferroelectric switching properties. 4,5 The B1 phase has a somewhat controversial twodimensional columnar structure and has generated significant interest recently as it has been shown that, under the influence of relatively high electric fields, in certain materials a transition to the B2 phase may occur and ferroelectric molecular switching in this state has been observed. The main disadvantage of this field-induced transition for applications and study involves the high electric field strengths required to induce it ͑Ͼ25 V / m reported by Ortega et al 6 ͒.…”