X-Ray Study of Liquid Crystal Alignment: Evidence for an Alternate Mesophase at the Free Surface?

Abstract

Abstract We studied the alignment of the free surface of a liquid crystal film as a function of depth using grazing incidence x-ray diffraction. We have produced well defined gratings on soda lime glass using conventional photolithography. After treatment with MAP, the substrates induced homogeneous alignment with a preferred orientation along the direction of the grooves in the bulk of an 8CB film in the SmA phase at room temperature. At the air/liquid crystal interface, these molecules align homeotropically. Using conventional Bragg diffraction, we observed a layer spacing of 31.5 A for the perpendicularly aligned molecules near the surface (2q = 2.8[ddot]). However, using a collimated beam in the grazing incidence geometry at a nominal incident angle of 0.00[ddot], a diffraction peak with a large angular spread at 2q = 2.2[ddot] showed a layer spacing of 40.1 A at the surface. With a slight increase in angle, a small peak at 2q = 4.4[ddot] (d-spacing of 20 A) emerged. As the incident angle was further ...