Control and modification of nematic liquid crystal pretilt angles on polyimides

The pretilt angle (Theta(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. Theta(p) was increased by introducing long, linear alkyl side chains and/or other nonpolar groups to the AL polyimide. On the other hand, Theta(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, tyro different Theta(p)s were obtained in one pixel and a mechanism for modifying the Theta(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 Theta(p) of ZLI-5080 on the UV-exposed/rubbed PI film decreased by 4.5 degrees-8.8 degrees in comparison with that on the rubbed film probably because the lower steric repulsion and greater electronic attraction would decrease the Theta(p).