Tunable quasi-homeotropic liquid crystal pretilt angle based on competing alignment layers

Abstract

The liquid crystal alignment properties of a spincoated mixture of two commercial homeotropic and planar aligning polyamic acids are investigated. Baking the coated substrates induces cleaving of the long alkyl side chains of the first polyamic acid which weakens the tendency for homeotropic alignment while the imidisation ratio of both polyamic acids changes in favour of planar alignment. By detailed measurement of the retardation induced by the liquid crystal, we show that the combination of these two mechanisms in the proposed mixture yields a spatially uniform liquid crystal pretilt angle that is tunable with the baking temperature in the quasi-homeotropic range 0–10°, which is important for the Vertically Aligned Nematic technology.

Keywords:

• liquid crystal alignment
• quasi-homeotropic pretilt
• competing alignment layers
• Vertically Aligned Nematic (VAN) technology

Acknowledgements

Pieter Vanbrabant is a PhD Fellow of the Research Foundation Flanders (FWO Vlaanderen) and Jeroen Beeckman is a Postdoctoral Fellow of the same institution. The work has been carried out in the framework of the Interuniversity Attraction Pole (IAP) project Photonics@be of the Belgian Science Foundation. Pietes Vanbrabant would like to thank Casper van Oooster far discussions.