Study On Pretilt Angle Induced By Polyimide Alignment Films With Side Chain Containing Biphenyl Unit

Polyimide films are widely used as liquid crystal alignment layers for liquid crystal molecules in advanced liquid crystal display (LCD) devices owing to their outstanding and excellent properties. Three types of main chain polyimides(PIs), PMDA/ODA,ODPA/ODA,BTDA/ODA and four types of side chain PIs with biphenyl unit, ODPA/DABBE,ODPA/DABBBE,ODPA/C2-DABBE, ODPA/C6-DABBE were prepared for alignment films. These PI films were rubbed and assembled to form liquid crystal cells. Based on the study of three types of main chain PIs, four types of side chain PIs with biphenyl unit were investigated. By means of polarized optical microscope (POM), atomic force microscope (AFM), XPS , ATR-FTIR , polarized infrared and contact angle test, the alignment of the liquid crystal molecules induced by rubbed PI alignment films was investigated, and the micro-morphology of the PI alignment films surface before and after rubbing was characterized. At the same time, pretilt angle was compared before and after annealing, and the effect of rubbing strength, curing temperature, solid content of polyamide acids and types of liquid crystal on the pretilt angle was studied. It was found that that the pretilt angle decreased with lower rubbing strength for main chain PIs, while increased for side chain PIs. Meanwhile, the pretilt angle increased with curing temperature improvement for both main chain and side chain PIs. C6-DABBE/0DPA alignment film, which containing flexible alkyl side chain with six methene members, shows good ability to align liquid crystal molecules, and it can induce a pretilt angle as high as 10° . Because of the rigid ester bond connection, the stability of liquid crystal cells pretilt angle is good stocking at room temperature. ODPA/DABBE alignment film was processed at room temperature by soaking...