This work dealt with the improvement of the pretilt angle of the photo-induced liquid crystal alignment on the alignment layer prepared from the polyimide containing fluorine group. The incorporation of a fluorine group into the polyimide is the strategy for the improvement of the pretilt angle. The incorporation of fluorine group into a polyimide was approached in two ways. The first approach is a physical blending of a polyimide containing fluorine group, and the second approach is a chemical one via copolymerization of polyimide containing fluorine group into polyimide.
It was difficult to prepare a miscible blend of the PMDA-ODA and PMDA-BDAF polyimide with sufficient content of PMDA-BDAF. The phase separation of polyimide alignment layer causes a poor liquid crystal alignment on the polyimide blend alignment layer. The miscible blend of the PMDA-ODA and PMDA-BDAF blend was possible when PMDA-BDAF content was 10 mol%, but in this case, the pretilt angle of the polyimide blend alignment layer was below 1°.
To overcome the phase separation problem, we prepared the copolymer of polyimides. The PMDA-BDAF-ODA polyimides were successfully synthesized and characterized. The phase separation of alignment layer was not observed. The pretilt angle of the polyimide alignment layer containing BDAF unit was larger than that of PMDA-ODA. As the BDAF content in polyimide increases, the pretilt angle of alignment layer increases. This may be attributed to the change of the relative content of fluorine atoms, which have a low surface energy, in the polyimide. The polar surface energy decreases with increasing BDAF content in polyimide and the interaction between liquid crystal and alignment layer is weaken, so the pretilt angle increases with BDAF content.
For the alignment layer of polyimide containing fluorine group, we found that the UV irradiation increases the pretilt angle of nematic liquid crystal on alignment layer. From the results of surface energy analysis, th...