Effect Of Polyimide Alignment Film Manufacturing Conditions On The Pretilt Angle Of Liquid Crystal

Liquid crystal (LC) alignment layer influences the alignment behavior of LC molecules on glass substrate, and then affects the electro-optic properties of liquid crystal display (LCD), so it is one of the key materials of manufacturing LCD. In the current LCD industry, polyimide is used as LC alignment layer owing to their outstanding and excellent properties such as resistance to elevated temperatures, resistance to chemical attack, stabilized orientation, forming film easily, cost of manufacturing cheaply. Pretilt angle (θp) is one of the important parameter used to evaluate LC alignment layer performances and it prevents the creation of reverse tilt disclinations of LC molecules. The domestic research and technology on LC alignment layer is very simple. Polyimide LC alignment layer used to produce super twist nematic liquid crystal display (STN-LCD) in current domestic LCD industry are almost imported. It has very important to synthesize a kind of polyimide with cheap price, big pretilt angle, and the good thermal stability of pretilt angle.4-(4-(trifluoromethoxy)benzoyl)cyclohexyl-3,5-diaminobenzoate (TBCA), 4-(4-aminobenzyl)benzenamine (MDA) and pyromellitic dianhydride (PMDA) were firstly used to synthesize main chain polyimide (PMDA/MDA) and side chain polyimide (PMDA/MDA/TBCA) which were all used as LC alignment layer, and the LC cells were manufactured with them. By means of polarized optical microscope (POM) and pretilt angle test (PAT), the alignment of LC molecules was investigated, and pretilt angle induced by rubbed polyimide alignment layerwas measured. The effect of monomer mole ratio, monomer charge order, reactiontime, polyamic acid (PAA) concentration, prebake temperature, curing conditions,and rubbing strength on LC pretilt angle was studied. The effect of diamine moleratio, monomer charge order and rubbing strength on LC pretilt angle was alsoinvestigated.The main chain polyimide (PMDA/MDA) was used to LC alignment layer.The research results were concluded as follows: LC pretilt angle increased firstlyand then decreased with reaction time. Pretilt angle increased with an increase ofprebake temperature, curing temperature, curing time and rubbing strength.Based on the research results of main chain polyimide above, side chainpolyimide (PMDA/MDA/TBCA) were used as LC alignment layer. The effect ofdiamine mole ratio, dianhydride/diamine mole ratio, monomer charge order, andrubbing strength on LC pretilt angle was studied. The results were concluded asfollows: LC pretilt angle increased with TBCA content in polyimide. The pretiltangle increased obviously under this condition that PMDA was firstly added toN-Methyl pyrrolidone (NMP) solution of TBCA, and then MDA was added to themixture when PMDA was dissolved completely. The pretilt angle increased firstlyand then decreased with rubbing strength, and it reached the biggest value whenrubbing strength was 360 mm. At the same time, the effect of diamine mole ratio,monomer charge order and rubbing strength on thermal stability of LC pretiltangle was also studied. Decreasing TBCA content in polyimide was in favour ofimproving thermal stability of LC pretilt angle. The thermal stability of pretiltangle also increased under this condition that TBCA was firstly dissolved to NMPwhile polyimides used as LC alignment layer were synthesized.For main chain and side chain polyimides, LC pretilt angle all increasedfirstly and then decreased with increasing dianhydride content of polyimide whilePAA was synthesized, and it reached the biggest value when dianhydride/diaminemole ratio was 1.04:1. Based on study and analysis on influencing factors of LC pretilt angle, a kind of polyimide alignment layer was synthesized. A pretilt angle as high as 4.1°was induced, and the thermal stability of the pretilt angle was good. The LC alignment layer is expected to be applied to twist nematic liquid crystal display (TN-LCD) and STN-LCD.