Effect of Side Chain Density of Polyimides on Properties of Liquid Crystal Alignment Film

4-dodecyloxy-phenyl-4′,4″-diaminotriphenylamine(DPDTA) containing triphenylamine were successfully synthesized.Polyimides(PIs) containing triphenylamine units in the main chain were synthesized with the co-condensation of DMMDA,DPDTA and aromatic dianhydride 4,4′-oxydiphthalic anhydride(ODPA) by thermal imidization.The structures of the PIs were confirmed by FT-IR.The alignment behavior of PIs as liquid crystal alignment layers was also examined with polarized optical micrograph(POM).The properties of the PIs were characterized by TGA,UV-Vis and DSC etc.It was found that side chain density of PIs had important influence on properties of liquid crystal alignment film.Before rubbing,PI containing 25% DPDTA could induce uniform vertical alignment and a pretilt angle of 89.7°.The liquid crystal was not aligned on the surface of PI film which contained 10% DPDTA without the rubbing treatment.After the rubbing process,the pretilt angle of liquid crystal was 2.4,when PI film contained 25% DPDTA and the pretilt angle of liquid crystal was 1.8°,when PI contained 10% DPDTA.The transmittances of the PIs in 500～800 nm wavelengths are more than 80%,and their glass transition temperatures are above 230 ℃.