Preparation And Characterization Of Polyimide Liquid Crystal Vertical Alignment Layer

Liquid crystal displays (LCDs) have been widely applied in mobile, notebook andmany other electronic appliances. Traditional LCDs, which adopt the rubbedparallel alignment layer to achieve uniform alignment of liquid crystals (LCs), haveshown some defects, such as narrow viewing angle, low contrast ratio and slowresponse time. Because the vertical alignment LCDs can improve these defectsgreatly, in recent years, the research of the vertical alignment layer has become achief topic in the field of the LC alignment layer. Many workers have reported onvertical alignment of LCs caused by polyimide (PI) films. Lee et al. reported on aPI containing a dianhydride with an aromatic side chain, which induced a pretiltangle of 70°～80°after a rubbing process. However, the pretilt angle did not meetthe requirement for the multi-domain vertical alignment mode, which requested apretilt angle above 89°after a rubbing process. Lee and Yi et al. reported that a PIwith side chain containing rigid alicyclic rings and long alkyl end group gave apretilt angle of 86°after the rubbing process and discussed the effect of the sidechain structure on the pretilt angle. But the thermal stability and uniformity of thevertical alignment were not mentioned in their paper. The induction of the verticalalignment is important, but maintenance and uniformity of the alignment may bemore important for application. It is necessary and interesting to conduct furtherstudies because of the importance of the vertical alignment method in LCDs.Therefore, we will explore to prepare a PI film to induce a uniform and thermallystable vertical alignment in our research. In the present study, a series of diamines with a side chain containing rigidbiphenyl unit and nonpolar alkoxy side end group[4-alkoxy-biphenol-3',5'-diaminobenzoate] (Cm-BBDA, m=4, 6, 12) weresynthesized and their chemical structures were confirmed by Fourier TransformInfrared Spectroscopy (FT-IR) and Nuclear Magnetic Resonance Spectroscopy(~1H-NMR). Then three PIs were prepared by copolymerization of pyromelliticdianhydride (PMDA), 4,4'-methylenedianiline (MDA) and Cm-BBDA inN-Methyl-2-pyrrolodone (NMP), and an aromatic PI without any side chain wasalso prepared by polymerization of PMDA and MDA in NMP as a reference.Chemical structures of all PIs were confirmed by FT-IR. LC cells were fabricatedusing these PIs as the alignment layer for characterization of the alignmentproperties of LCs.The conclusions were summarized as follows:1) Long alkyl chain must be introduced into the side chain of PI to obtain a highpretilt angle. It was found that the vertical alignment of 5CB was induced by PIscontaining C6—BBDA and C12—BBDA, and a pretilt angle above 89°wasobtained. However, the vertical alignment was not produced by the PI containingC4—BBDA.2) When the rigid biphenyl was used as the connection unit between the main chainand side chain of PIs, the side chain did not fall down after a rubbing process. So apretilt angle closely to 90°still was induced by PIs containing C6—BBDA and C12—BBDA after a rubbing process.3) The pretilt angle of LCs increased rapidly with a decrease of the polarity part ofsurface tension of PI films.4) Due to the similar structure of the side chain of PI and LCs, the intermolecularforce between side chains and LCs is strong enough to produce a uniform alignment.5) The aromatic main chain and side chain should be incorporated into the PI tomaintain the thermal stability of LC vertical alignment.