New clues to the factors governing the perpendicular alignment of liquid crystals on rubbed polystyrene film surfaces

Abstract

We have investigated the surfaces of rubbed polystyrene (PS) films in detail using atomic force microscopy and have discovered a previously unknown surface feature: submicroscale meandering groove-like structures composed of gravel-like grooves in tens of nanometers are present, oriented perpendicular to the rubbing direction. This unusual surface morphology is a significant departure from the surface topographies observed so far for rubbed PS and other polymer films, for which grooves are usually only found parallel to the rubbing direction. We also conclude from retardation analysis and linearly polarized infrared (IR) spectroscopy that the phenyl side groups of the PS chains are reoriented perpendicular to the rubbing direction, with para-directions that are nearly normal to the film plane, whereas the vinyl backbones of the PS chains are reoriented along the rubbing direction. This is the first time IR spectroscopic techniques have been used to determine the three-dimensional reorientation geometry of phenyl rings on the surface of rubbed PS films. Uniform, homogeneous alignment of liquid crystal (LC) molecules is achieved at rubbed PS film surfaces, but the alignment director is usually perpendicular to the rubbing direction. This perpendicular LC alignment was found to have very low anchoring energy (less than 3 x 10(-7) J/cm(2)) and to persist only for a limited time (less than 1 day), indicating that the interaction of LCs with rubbed PS surfaces is very weak. Collectively, the results lead to the conclusion that the unusual well-developed groove topography oriented perpendicular to the rubbing direction plays a critical role in governing the alignment of LC molecules that weakly interact with the perpendicularly reoriented phenyl side groups, overriding the effects of the parallel reoriented vinyl main chains. In addition, the zero degree pretilting behavior of LCs on PS surfaces was discussed, in particular with respect to the rubbing-induced reorientations of the PS polymer segments and their anisotropic interactions with the LC molecules.