X-ray probing of surface anchoring in films of bent-core liquid crystals

Bent-core mesogens (BCMs) are a class of thermotropic liquid crystals (LCs) characterized by a nonlinear shape and exhibiting a wide range of unconventional properties [1]. In particular, their bent shape and transverse molecular dipole result in a marked predisposition to stratification. This tendency strongly favours the formation, throughout the nematic (N) phase, of nanosized clusters featuring smectic (Sm)-like positional order and biaxial (and polar) orientational order (cybotactic order) [2]. Consequently, these materials are among the most promising candidates in the quest for the much sought-after biaxial [3] and ferroelectric [4,5] N phases. Unfortunately, BCMs also exhibit unusual anchoring properties which result in a difficult control of the molecular director by conventional surface treatments, hampering the study and possible applications of these materials. For conventional rod-like LCs, the molecular orientation is efficiently controlled by treating the confining surfaces with specific alignment agents (e.g. surfactants, rubbed polyimide films, oblique SiOx deposition). Despite the nanoscale mechanism is not always fully understood, these treatments allow a very fine control of the surface anchoring. Unfortunately, aligning BCMs with conventional approaches has proved quite difficult and also when a uniform alignment is obtained, its nature is often unclear, making the interpretation of the experimental results ambiguous [6,7]. Here we present a systematic study of the alignment of BCMs (fig. 1a) on different solid substrates under different alignment agents, spanning from ultra-thin films prepared using the Langmuir-Blodgett technique, to spin-coated thicker films (101-102 nm). The challenging investigation of BCMs’ surface anchoring was performed by a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) (fig. 1b) and X-ray reflectometry (XRR) (fig. 1c), a powerful experimental approach to investigate both in-plane and in-depth molecular order in LC films.