Synthesis of new aromatic polyimides with various side chains containing a biphenyl mesogen unit and their abilities to control liquid-crystal alignments on the rubbed surface

Abstract

New poly(m-phenylene 4,4'-oxydiphthalimide)s containing various side chains, such as 6-(4-biphenylmethoxy)hexyloxy group and 6-(phenylphenoxy)hexyloxy isomers, were synthesized, giving thin films of a high quality. All the polyimides apparently were almost amorphous, but exhibited short-range ordering in some extent, depending on the side chains. By incorporating side chains, the thermal properties, including stability, thermal expansivity, and glass transition temperature, were generally degraded, whereas the optical and dielectric properties were improved. All the polyimides exhibited a good rubbing processability and excellent performance in the controlling of both the alignment and the pretilt of LC molecules in the LC cell. The pretilt angle of LC molecules was easily achieved in a wide-angle range of 8-27 degrees, depending upon the rubbing density as well as the incorporated side chains. The pretilting of LC molecules was very sensitive to all the molecular parameters (namely, the flexibility of polymer chain backbone as well as the isomeric structure of biphenyl mesogen end group, spacer length, and spacer conformation in the side chain) in the polyimide, in addition to the rubbing process. In particular, the side chains, which are much shorter in length than the long alkyl side chains in the polyimides being used widely as LC alignment layers, were evident to involve effectively in the alignment of and the pretilt of LC molecules, which are highly desired in the LC display industry. This might mainly be attributed to a strong interaction between the biphenyl mesogen end group of the side chain and the LC molecule.