Field induced transitions and interlayer interactions in intermediate smectic phases

Abstract

A series of liquid crystal mixtures displaying wide three- and four-layer intermediate phases are reported. The mixtures are formed from a selenium-containing antiferroelectric material combined with up to 9% (by weight) of a chiral dopant. We describe physical properties including spontaneous polarization, layer spacing and tilt for mixtures including up to 9% concentration of the chiral dopant. Such measurements offer an insight into the factors that affect the stability of the intermediate smectic phases. However, a quantitative measure of the interlayer interaction strength can be obtained from analysis of field-temperature phase diagrams. Therefore, the field-temperature phase diagrams are also determined in the intermediate phase regime of the mixtures containing up to 5% w/w concentration of the chiral dopant and compared with theoretical predictions. Excellent agreement with the theory is observed for the pure material, though for mixtures with increasing concentrations of chiral dopant, deviations from the theory are recorded, in particular in the nature of the transition from the four-layer structure to the three-layer structure. Quantitative measurements of the interlayer interaction constants are deduced from the gradients of the field thresholds, and the interlayer pairing is found to reduce significantly with an increasing concentration of chiral dopant. An interlayer interaction constant of 147 ± 13 N m⁻² K⁻¹ is found in the pure material, reducing to 21 ± 4 N m⁻² K⁻¹ in the mixture with concentration of chiral dopant of 5%. Measurement of the interlayer interaction constants from the field-temperature phase diagrams is shown to give a quantitative understanding of the importance of the interlayer interaction, which is only indicated qualitatively by other measurements. Finally, some evidence is presented for an additional field-induced transition observed in temperature regions close to a triple point on the field-temperature phase diagram.