Microscopic molecular re-orientation of polymer surfaces induced by rubbing and factors determining LC pretilt angles

It is proposed that liquid crystal (LC) pretilt angles on a polymer alignment layer (AL) surface with a certain rubbing strength are determined by three major factors: an anchoring effect and electronic and steric interactions between the LC molecules and the polymer AL surface. As the rubbing strength increases, the pretilt angle reaches a maximum plateau and then slowly decreases. The anchoring effect includes how LC mesogens interact with the AL surface as well as the effects of a locally tilted orientation of an AL surface on the pretilt angle. A polar functionality at the AL surface lowers the pretilt angle due to an electronic interaction between LC molecules and the AL surface, while a long alkyl side-chain in a repeat unit of the AL polymer increases the pretilt angle because of steric repulsion between LC molecules and the AL surface. It has been previously reported that rubbing a polyimide film (AL) gives a macroscopic orientation. In this work, we have discovered that rubbing also induces microscopic molecular re-orientation such that polar or dipole functional groups at the surface are oriented out of the plane of the surface and non-polar aliphatic chains are partially oriented towards the bulk of the film. Heating the oriented (rubbed) film at a temperature higher than the glass transition temperature (T_g) of the polymer prior to LC introduction does not give uniform LC alignment. Also, the LC alignment obtained on the oriented (rubbed) film is destroyed by heating the LC cell at a temperature higher than the T_g. The pretilt angle tends to decrease with annealing the LC cell, even at a temperature lower than the T_g.