Surface anchoring of nematic liquid crystal on swollen polymer brush studied by surface forces measurement

Surface anchoring plays a fundamental role in controlling the molecular alignment of a bulk liquid crystal (LC). It has been previously shown that the pretilt angle of a nematic liquid crystal (NLC) can be controlled by changing the grafting density of a liquid crystalline polymer, poly(6-(4-methoxy-azobenzene-4′-oxy) hexyl methacrylate) (PMMAZO). In this study, the thickness of the swollen PMMAZO brush was measured by employing the surface forces apparatus (SFA). NLC 4-cyano-4′-pentylbiphenyl (5CB) and toluene were used as the solvents. It was shown that both 5CB and toluene were good solvents for PMMAZO. The repulsive force in 5CB appeared at D0 = 98.2 ± 4.6 nm for high grafting density (HD) surfaces, and at 32.1 ± 4.5 nm for the low grafting density (LD) surfaces. These results indicated that the PMMAZO molecules extended nearly perpendicular to the HD surfaces, while they laid on the substrate for the LD surfaces. The interaction between 5CB and the mesogenic group of PMMAZO was stronger than that for toluene. These results could support the expected surface anchoring mechanism of 5CB by the PMMAZO brush. [Display omitted] •Swelling of a side-chain liquid crystalline polymer brush was observed by using SFA.•Configuration of swollen brush was controlled by changing the grafting density.•Results of force measurement showed the stacking between 5CB and mesogenic group.•Surface anchoring was examined by FECO.