Abstract
We have investigated the role of the backbone structures of polyimide with alkyl side-chains in the determination of the pretilt angle of liquid crystal (LC) in contact with a rubbed polyimide film. We have determined the inclination angles of the polyimide backbone structures in rubbed films as well as the pretilt angles of LCs in contact with the rubbed films. We examined four different polyimdes with the same backbone structure but different length of alkyl side-chains. The inclination angle of the polyimide backbone structure increases with the number of carbon atoms in the alkyl side-chains. We found a linear relation between the inclination angle of the backbone structure and the pretilt angle of LC. Furthermore,the linear relation holds for a polyimide without side-chains [Arafune et al., Surf. Sci. 368, 208 (1996)] as well. Thus we conclude that the inclination of the polyimide backbone structure determines the pretilt angle of LC. The alkyl side-chains affect the alignment of the polyimide backbone structure in the rubbing process, but they do not affect directly the pretilt angle of LC. The effect due to the alkyl side-chains is only indirect.
