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

• Main Authors: X. X. Zhi, H. S. Bi, J. G. Liu, Y. S. Gao, Y. L. Zhang, Y. C. Zhou, Y. Zhang, X. Wu, X. M. Zhang
• Format: Article
• Language: English
• Published: Budapest University of Technology 2019-10-01
• Series: eXPRESS Polymer Letters
• Subjects: Industrial applications; Polyimide; Alignment layer; TFT-LCD; Voltage holding ratio
• Online Access: http://www.expresspolymlett.com/letolt.php?file=EPL-0010039&mi=cd

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 HTABPPI layers showed good thermal stability with the 5% weight loss temperatures (T5%) higher than 415 °C and glass transition temperatures (Tg) in the range of 150.5–201.4 °C.