Evidence for the Presence of a Twist-Grain-Boundary Structure in Confined Hybrid Non-Chiral Smectic Liquid Crystal Films
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ABSTRACT We have used Grazing Incidence X-ray Scattering to study the depth dependence of the interfacial structure of smectic liquid crystal (8CB) hybrid films. These films are bounded by a grated glass substrate and air, and have been studied as a function of both temperature and thickness. Our measurements indicate the presence of both a chevron structure and a structure similar to the helical twist-grain-boundary (TGB) phase. Our measurements constitute the first to show that this structure can also be observed in hybrid smectic liquid crystal films. The twisted structure is the result of anchoring at two dissimilar competing confining surfaces.
INTRODUCTION Widely used liquid crystal applications, such as displays, projector and optic modulators, 1 make use of confined LC materials inside low dimensional geometries, which makes it important to understand the effect of different substrate materials and the different methods used to prepare these substrates. The twist-Grain-Boundary structure usually exists in chiral smectic liquid crystal materials. 2 However, Patel 3 found that nonchiral smectic LC may also exhibit a TGB structure under twisted boundary condition. In addition, a previous theoretical study 4 on possible structures under confinement shows a chevron structure can be stable in weak homeotropic anchoring The Grazing Incidence X-ray Scattering (GIXS) 5 is an ideal technique for obtaining direct information on surface or interface of materials, especially for liquid crystal thin films. This technique exploits the fact that the index of refraction for X-rays is slightly less than 1; this means that there exists a critical angle, otc, below which the X-rays are totally externally reflected. For ct
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