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

Low molecular mass liquid-crystal materials can be aligned homogeneous ly by polymer layers that are rubbed. Therefore, the rubbing process i s widely used in the mass production of liquid-crystal displays. The r ubbing treatment orients the polymer molecules in the layer. The exten t of orientation of rubbed polyimide layers was investigated, using un iaxially drawn polymer tapes as a reference. For this, the experimenta l results from optical phase retardation, infrared dichroism, and X-ra y diffraction studies were combined with a molecular modeling study. T he results indicate that the Hermans' orientation factor of the top of the rubbed polyimide layer, i.e., the part directly contacting the ru bbing cloth during the rubbing process, clearly exceeds 0.5. In view o f the fact that the studied polyimide is an amorphous polymer, this fa ctor is rather high, indicating that the rubbing process is an effecti ve way to induce molecular orientation of a polymer layer. The rubbing process is found to be more effective to orient a solvent-free thin p olyimide layer than uniaxial drawing.