History-dependent orientational order of rubbed polyimide for liquid-crystal alignment

A polyimide film that was spin coated onto a glass substrate was multiply rubbed along different directions and studied using ellipsometry and atomic force microscopy. The data show a minimum required rubbing strength for the onset of orientational order in the polyimide. When over rubbed along an axis perpendicular to the first rubbing direction, a smaller rubbing strength was required for the onset of order along this direction. This behavior indicates that the polyimide had been partially disentangled by the initial rubbing, needing only weaker rubbing to be reoriented by the second rubbing.