Subphase with four-layer superlattice structure confirmed by resonant soft x-ray scattering in thresholdless antiferroelectric liquid crystal mixtures

Abstract

Binary mixtures developed as thresholdless antiferroelectric (TLAF) materials, MC881-MC815 and MC881-MC452, are studied using optical rotatory power (ORP), resonant soft x-ray scattering (RSoXS), and dynamic light scattering (DLS). A single subphase is confirmed to exist in both mixtures by ORP, stably over a wide temperature range (around 80◦C below RT) but in a narrow concentration range (1–3 wt.%), with curved boundaries; between SmC∗A and the subphase, the ratio of synclinic and anticlinic orderings changes continuously. The antiferroelectric four-layer superlattice structure of the subphase is now firmly established by RSoXS using free-standing films, although a two-layer SmC∗A structure is observed in silicon nitride membrane cells often used in RSoXS, highlighting the influence of surface conditions. The TLAF properties of the subphase are confirmed in homogeneously aligned cells by observing apparent tilt angles induced by an applied electric field. The relaxation times responsible for the TLAF properties are studied by DLS, indicating the smooth flip-flopping of a group of molecules in a single layer, which must also be responsible for the continuous change among SmC∗A, the subphase, and SmC∗. Each smectic layer appears to play an important role as a mesoscopic entity.