MOLECULAR-ORIENTATION IN RUBBED POLYIMIDE ALIGNMENT LAYERS USED FOR LIQUID-CRYSTAL DISPLAYS

Low molecular mass liquid-crystal materials can be aligned homogeneously by polymer layers that are rubbed. Therefore, the rubbing process is widely used in the mass production of liquid-crystal displays. The rubbing treatment orients the polymer molecules in the layer. The extent of orientation of rubbed polyimide layers was investigated, using uniaxially drawn polymer tapes as a reference. For this, the experimental results from optical phase retardation, infrared dichroism, and X-ray diffraction studies were combined with a molecular modeling study. The results indicate that the Hermans' orientation factor of the top of the rubbed polyimide layer, i.e., the part directly contacting the rubbing cloth during the rubbing process, clearly exceeds 0.5. In view of the fact that the studied polyimide is an amorphous polymer, this factor is rather high, indicating that the rubbing process is an effective way to induce molecular orientation of a polymer layer. The rubbing process is found to be more effective to orient a solvent-free thin polyimide layer than uniaxial drawing.