EFFECT OF RUBBING ON THE MOLECULAR-ORIENTATION WITHIN POLYIMIDE ORIENTING LAYERS OF LIQUID-CRYSTAL DISPLAYS

The influence of various rubbing parameters on the molecular reorienta tion of thin polyimide orienting layers, used to align liquid-crystal (LC) molecules within liquid-crystal displays, has been studied. For t his purpose the optical phase retardation in the polymer layer, explic itly induced during the rubbing treatment, was determined. The observe d rubbing-induced phase retardation can directly be related to a molec ular orientation within the polymer orienting layer, as could be shown by infrared dichroism studies. Furthermore, it is found that the top of the polymer layer, directly contacting the rubbing cloth during the actual rubbing process, is almost instantaneously oriented to a certa in maximum value as soon as the rubbing is started. Additional or stro nger rubbing has no detectable influence on the orientation within the top layer. Increasing the rubbing density or the rubbing pressure onl y results in an increase of the penetration depth of the rubbing proce ss, i.e., molecular reorientation occurs deeper within the layer. Expe riments show that the penetration depth can be varied from less than 1 0 nm to more than 60 nm by variation in rubbing conditions. These find ings are supported by surface second-harmonic-generation studies of LC monolayers deposited onto rubbed orienting layers and by infrared dic hroism studies.