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

Abstract

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.