Atomic force microscopy studies of rubbed polyimide surfaces used for liquid crystal alignment

Liquid crystal (LC) surface alignment, which controls off-state contrast in LC displays, is commonly achieved by directional rubbing of polyimide-coated substrates. In order to better understand the alignment mechanism we have used tapping-mode atomic force microscopy to study the dependence of polymer surface morphology on rubbing conditions. Based on results from three polyimides rubbed at different strengths with two cloths, we have identified five surface feature types induced by the 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 background texture. The last feature confirms that the entire surface has been affected by the rubbing process. The finest resolution achieved, about 10 nm full width, was insufficient to resolve individual polymer chains. Of the features observed, only occasional very deep off-axis scratches clearly affected azimuthal LC alignment. Quasiperiodic features observed under weak rubbing conditions suggest that initial rubbing occurs by a stick-slip mechanism, and the widths of such features gives an estimate of fiber-surface contact size.