Controllable alignment of nematics by nanostructured polymeric layers

Abstract

A method for a continuous control of the pretilt angle of the easy axis in the range 0–90° degrees and of the anchoring strength by using nanostructured polymers as alignment layers is described. The nanostructured polymers are blends of two different side-chain polymers each of them promoting planar and homeotropic alignment, respectively. A model to interpret the alignment of a nematic liquid crystal induced by such polymer layers is proposed. We show that in this case the anisotropic part of the surface tension can be approximated by a simple extension of the Rapini–Papoular expression. The predicted trend of the pretilt of the easy axis versus the concentration of the side-chain polymer promoting the planar alignment, for instance, is in good agreement with the experimental data. We also show that the effective anchoring strength of the system depends on the concentration of the side-chain polymer promoting planar alignment, and exhibits a minimum for a well-defined value of this quantity. The results obtained in this work seems to be of importance for liquid crystal displays technology since the control of the pretilt and the anchoring strength strongly affect the performance of liquid crystal displays.

Keywords:

• titled alignment
• anchoring strength
• nanostructured polymers
• reverse pretilt

Acknowledgement

GB is grateful to the University of Gothenburg for awarding him the position of Marks Guest-Professorship.

Notes

^†Permanent address: Dipartimento di Fisica, Politecnico di Torino, corso Duca degli Abruzzi 24, I-10129 Torino, Italia. Currently a holder of the Marks Guest-Professorship at Gothenburg University.