ATOMIC-FORCE MICROSCOPY STUDIES OF RUBBED POLYIMIDE SURFACES USED FORLIQUID-CRYSTAL ALIGNMENT

Liquid crystal (LC) surface alignment, which controls off-state contra st in LC displays, is commonly achieved by directional rubbing of poly imide-coated substrates. In order to better understand the alignment m echanism we have used tapping-mode atomic force microscopy to study th e dependence of polymer surface morphology on rubbing conditions. Base d on results from three polyimides rubbed at different strengths with two cloths, we have identified five surface feature types induced by t he rubbing: (1) isolated point defects, (2) occasional deep scratches (greater than or equal to 5 nm), (3) numerous very shallow scratches ( mostly less than or equal to 1 nm deep), (4) nanoscale surface islands (3-10 nm high, number density >10(9) cm(-2)), and (5) modified backgr ound texture. The last feature confirms that the entire surface has be en affected by the rubbing process. The finest resolution achieved, ab out 10 nm full width, was insufficient to resolve individual polymer c hains. Of the features observed, only occasional very deep off-axis sc ratches clearly affected azimuthal LC alignment. Quasiperiodic feature s observed under weak rubbing conditions suggest that initial rubbing occurs by a stick-slip mechanism, and the widths of such features give s an estimate of fiber-surface contact size.