| Polyimides are used extensively in Liquid Crystal Displays (LCDs) as alignment layers. Thin films are oriented by rubbing, which causes nematic liquid crystals to align parallel to the rub direction with a tilt angle. This tilt angle is referred to as the pretilt angle. The pretilt angle is an important parameter in determining the electrooptical characteristics of an LCD. The more sophisticated devices require high pretilt angles for optimal performance, which are not readily attainable with conventional main-chain polyimides. Recent success in obtaining high pretilt angles has been achieved through the application of side-chain polyimides.; The objective of this research has been the determination of the structure/property relationships for the alignment and pretilt angle of a host of side-chain polyimides that are soluble in conventional organic solvents. One of the key materials that generated a pretilt angle of 60° was synthesized from 2,2 ′-bis(3,4-dicarboxyphenyl)-hexafluoropropane (6FDA) and 4,4 ′ diaminobiphenyl substituted at the 2,2′ positions with the ester linked side-chain 4-hexyloxybiphenyl (C6BP). The pretilt angle of the 6FDA-C6BP side-chain polyimide was investigated extensively using various processing parameters.; The processing parameters that were examined, included, the rubbing strength, heat treatment, and film thickness. The pretilt angle decreased with increasing rubbing strength and the alignment was found to be constant once saturated. The heat treatment temperature had no effect on the pretilt angle provided that most of the solvent was removed from the films. The pretilt angle increased with increasing thickness up to 25 nm, and remained constant for thicker films.; The pretilt angle was stable for long term aging and for electric and magnetic field cycling. The pretilt was not stable with temperature. Heated LC cells demonstrated an irreversible increase in the pretilt angle. The alignment became homeotropic upon heating above the clearing temperature of the nematic liquid crystal.; Aliphatic side-chain polyimides with chain lengths ranging from 4 to 18 Carbons typically resulted in homeotropic alignment. Copolyimides of main-chain and aliphatic side-chain components demonstrated that a maximum stable pretilt angle of approximately 20° could be obtained. Further increases in sidechain content lead to homeotropic alignment. |
