Synthesis and characterization of soluble ester-containing polyimide alignment layers with high voltage holding ratio features and potential applications in TFT-LCDs

Novel ester-containing polyimide (PI) resins have been synthesized from an alicyclic dianhydride monomer, 4,4'dihydroxybiphenyldicyclohexanecarboxylate-3,3',4,4'-tetracarboxylic acid dianhydride (HTABP) and various aromatic diamines.The derived PI resins (PI-1~PI-5) were easily soluble in N-methyl-2-pyrrolidinone (NMP) and gamma-butyrolactone (GBL).The PI alignment agents prepared from the HTABP-PI resins, NMP solvent, and the butyl cellulose (BC) leveling agent with a solid content of 6 wt% showed good stability during the storage at 25 °C for half a year.Smooth and uniform PI alignment layers were formed by spin-coating the PI varnishes onto indium tin oxide (ITO) substrates, followed by thermally baking at the temperature up to 230 °C.Liquid crystal (LC) cells were successfully fabricated by using the rubbing treated PI layers as the alignment components for LC molecules.The LC cells fabricated with the newly-developed PI alignment layers exhibited good electro-optical properties with the pretilt angles around 1.26-2.82°and voltage holding ratio (VHR) as high as 98.36% at room temperature.The residual direct current voltage (RDC) values of the HTABP-PI alignment layers were in the range of 915-2146 mV due to the high volume resistivity values of the PI layers.In addition, the HTABP-PI layers showed good thermal stability with the 5% weight loss temperatures (T 5% ) higher than 415 °C and glass transition temperatures (T g ) in the range of 150.5-201.4°C.