Control and Modification of Nematic Liquid Crystal Pretilt Angles on Polyimides

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 O2 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 ps 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.