Molecular dynamics and morphology of confined 4-heptyl-4′-isothiocyanatobiphenyl liquid crystals

Abstract

Molecular dynamics and orientational order of 4-heptyl-4′-isothiocyanatobiphenyl (7BT) in non-intersecting nano-pores of mean diameters from 4 nm to 10.5 nm are studied by a combination of Broadband Dielectric and Fourier-Transform Infrared Spectroscopy. The smectic E phase observed in bulk 7BT is replaced by short-range molecular order imposed by the surface potential within the pores. In contrast to bulk dielectric properties of 7BT, geometrical confinement leads to modification of the molecular dynamics. Two dielectric relaxation processes exhibiting Arrhenius-like thermal activation are detected for molecules in nanopores of mean diameters from 6 nm to 10.5 nm. The slower process is assigned to molecular reorientation around the short axis (δ-relaxation) whereas the faster dipolar relaxation process (β-process) is attributed to librational motion of the molecules close to the pore-walls. Infrared Transition Moment Orientational Analysis reveals different molecular arrangement in pores of diameters 10.5 nm compared to the molecules in 4 nm and 6 nm pores.