Soluble polyimide films as alignment layers for bistable chiral-tilted homeotropic nematic liquid crystal display applications

• Increasing the side-chain content improves the long-term alignment stability.
• Useful in bistable chiral-tilted homeotropic nematic liquid crystal display.
• The pretilt angle of the liquid crystal cell is almost the same after 6 months.
• Vertical alignment properties are stable at a temperature lower than 180 °C.

Soluble polyimides (PI) with and without aliphatic side chains were synthesized by a one-pot synthesis method and acted as homogeneous or homeotropic liquid crystal alignment layers. Homeotropic PI was synthesized by grafting an alkyl side chain on the backbone of homogeneous PI. The vertical alignment properties can be tuned by adjusting the grafting density and the length of the side chains. In addition, various homogeneous and homeotropic PIs were blended by a different ratio to adjust the pretilt angle of the alignment film. The long-term alignment properties and thermal resistance of the alignment films were investigated. Combining the dual frequency liquid crystal with the alignment layers which have an intermediate pretilt angle of 75°, a bistable chiral-tilted homeotropic nematic liquid crystal display device is demonstrated.