Azimuthal anchoring coefficients of nematic liquid crystals on polystyrene brushes formed at the air surface of block copolymer monolayers

ABSTRACT

A nematic liquid crystal (NLC) on a diblock copolymer monolayer shows Néel walls under a polarised optical microscope and the azimuthal anchoring coefficient (A_φ) is estimated from the width of the wall. The block copolymer consists of polystyrene (PS) and poly(3-(triethoxysilyl)propyl methacrylate) (PTESPMA) segments. A monolayer of a pair of PS and PTESPMA layers was prepared by spin coating a copolymer solution on a glass substrate, followed by annealing using methylcyclohexane, exposure to hydrochloric acid vapour and rinsing with acetone. The PS layer is at the air surface and exhibits a thickness five times larger than the radius of gyration of the PS chain, which suggests that the PS chains are more extended towards the air surface, adopting a brush conformation. The A_φ is 6 × 10⁻⁶ J m⁻² at 25°C and slightly decreases to 3 × 10⁻⁶ J m⁻² with increasing temperature up to 90°C. On the other hand, the A_φ on a high-density PS brush, which is prepared via surface-initiated polymerisation method, decreases from 8 × 10⁻⁶ to 3 × 10⁻⁷ J m⁻² with increasing temperature in the same range.

Graphical abstract

KEYWORDS:

• Block copolymers
• monolayers
• polymer
• brushes
• anchoring
• nematic

Acknowledgments

The authors greatly acknowledge Prof. Hidetoshi Matsumoto (Tokyo Institute of Technology) and Dr Osamu Sato (LG Japan Lab Inc.) for their helpful discussions and suggestions. ToF-SIMS studies were performed by Mr Akira Genseki (Technical Department, Tokyo Institute of Technology), who is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by JST Strategic Promotion of Innovative Research and Development [Grant Number JPMJSV0915], Japan.