IMPORTANCE OF RUBBING-INDUCED INCLINATION OF POLYIMIDE BACKBONE STRUCTURES FOR DETERMINATION OF THE PRETILT ANGLE OF LIQUID-CRYSTALS

By using polarized infrared absorption spectroscopy, we have determine d the average inclination angle of the backbone structure of a series of polyimides with alkyl side chains induced by rubbing. We examined f our polyimides. They have an identical backbone structure, but the num ber (n) of carbon atoms in the alkyl side chain is different: n = 1, 3 , 6, and 11. The inclination angle of the polyimide backbone structure increases with the number of carbon atoms in the alkyl side chain. To learn the behavior of the alkyl side chain under rubbing, we have als o measured the contact angle of water on the film surface. For the rub bed films of polyimide with short alkyl side chains (n=1, 3, and 6), a lmost all the alkyl side chains at the film surface orient inward, tow ard the bulk of the him. On the other hand, the alkyl side chains that orient outward exist at the rubbed film surface for the polyimide wit h a long alkyl side chain (n=11). The pretilt angle of liquid crystals (LC) is proportional to the inclination angle of the polyimide backbo ne structure independent of the orientation of the alkyl side chain. F rom these results we conclude that the rubbing-induced inclination of the polyimide backbone structure has the dominant effect in determinin g the pretilt angle of LC. [S1063-651X(98)04811-9].