Nematic liquid crystal anchoring strengths of high density polymer brush surfaces

Abstract

The azimuthal anchoring coefficient A₂ of a nematic liquid crystal (NLC) on high-density polymer brushes was investigated using the threshold voltage for the in-plane switching of LC cells consisting of polymer-grafted substrates. A₂ was affected by the bulk glass transition temperature (T_g) of the grafted polymers and decreased with increasing temperature. The NLCs on poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(styrene) (PS) brushes had an A₂ of 10⁻⁵ J m⁻² at 25°C. In contrast, the NLC on a poly(hexyl methacrylate) (PHMA) brush exhibited a small A₂ (1.1 × 10⁻⁶ J m⁻²) at 25°C and was oriented along the electrodes owing to the brush surface unevenness. With increasing temperature to 90°C, A₂ of the NLCs on the PEMA and PS brushes decreased to the values of 10⁻⁷ J m⁻², and the NLCs were oriented similar to the PHMA brush cell. Although the T_gs of PMMA and PS are similar, at 90°C the A₂ of the NLC on the PMMA brush was 4.5 × 10⁻⁶ J m⁻² and required a magnetic field for LC orientation. Such a difference in A₂ is explained by the fact that PS has better compatibility with the NLC than PMMA.

Keywords:

• nematic liquid crystal
• anchoring
• polymer brush
• in-plane switching
• glass transition temperature