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

A nematic liquid crystal (NLC) on a diblock copolymer monolayer shows Néel walls under a polarised optical microscope and the azimuthal anchoring coefficient (Aφ) 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 Aφ 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 Aφ 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.