Control and modification of nematic liquid crystal pretilt angles on polyimides

• Published in: Japanese Journal of Applied Physics Vol. 36; no. 6A; pp. 3591 - 3597
• Main Authors: LEE, K.-W, LIEN, A, STATHIS, J. H, PAEK, S.-H
• Format: Journal Article
• Language: English
• Published: Tokyo Japanese journal of applied physics 01.06.1997
• Subjects: Applied sciences; Display; Electronics; Exact sciences and technology; Tilt angle; Polyimide; Alignment; Nematic state; Experimental study; Liquid crystals; Display
• ISSN: 0021-4922; 1347-4065
• DOI: 10.1143/JJAP.36.3591

The pretilt angle (Θ p ) of nematic liquid crystals (LCs) could be controlled with alignment layer (AL) polyimides (PIs) and/or LCD processes in the case that a cell gap and a rubbing condition are fixed. Θ p was increased by introducing long, linear alkyl side chains and/or other nonpolar groups to the AL polyimide. On the other hand, Θ p was decreased by modifying the polyimide surfaces (with UV exposure or O 2 plasma) and LCD processes such as heating or cleaning the rubbed polyimide and annealing the LC-filled cell. Thus, the AL polyimide and the LCD process can be designed so as to obtain a desired pretilt angle, which is important in applications. In the UV-type two-domain twisted nematic approach for wide-viewing angle LCDs, two different Θ p s were obtained in one pixel and a mechanism for modifying the Θ p was discovered. Upon UV-exposing the PI films, aromatic or conjugated radicals with long lifetimes (greater than 2 weeks) were generated and some side chains were removed, and these PI surfaces subsequently became more polar since polar functional groups such as OH were introduced. The Θ p of ZLI-5080 on the UV-exposed/rubbed PI film decreased by 4.5°–8.8° in comparison with that on the rubbed film probably because the lower steric repulsion and greater electronic attraction would decrease the Θ p .