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 determined the average inclination angle of the backbone structure of a series of polyimides with alkyl side chains induced by rubbing. We examined four polyimides. They have an identical backbone structure, but the number (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 also measured the contact angle of water on the film surface. For the rubbed films of polyimide with short alkyl side chains $(n=1,$ 3, and 6), almost all the alkyl side chains at the film surface orient inward, toward the bulk of the film. On the other hand, the alkyl side chains that orient outward exist at the rubbed film surface for the polyimide with a long alkyl side chain $(n=11).$ The pretilt angle of liquid crystals (LC) is proportional to the inclination angle of the polyimide backbone structure independent of the orientation of the alkyl side chain. From these results we conclude that the rubbing-induced inclination of the polyimide backbone structure has the dominant effect in determining the pretilt angle of LC.