Synchrotron x-ray study of dimensional crossover in solid-phase smectic liquid-crystal films

Abstract

The crystalline phases of the liquid crystal 8OSI [racemic 4-(2'-methylbutyl)phenyl 4'-(octyloxy)-(1,1')-biphenyl-4-carboxylate] have been studied using high-resolution synchrotron x-ray-scattering techniques. A careful analysis has been performed on the x-ray-scattering profiles of samples with thicknesses varying from 1000 molecular layers to five layers. Three-dimensional long-range positional order with large-amplitude thermal fluctuations is observed for a very thick film. As the thickness is decreased, two-dimensional characteristics become apparent. The scattering profiles of thin films are described by the power-law line shape, 1/\ensuremath{\Vert}Q-G${\mathrm{\ensuremath{\Vert}}}_{\mathrm{eff}}^{2\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\eta}}}$, with an effective exponent ${\mathrm{\ensuremath{\eta}}}_{\mathrm{eff}}$, which evolves with thickness.