X-ray scattering study of smectic ordering in a silica aerogel.

Abstract

We have studied, via x-ray scattering, the nematic to smectic-A (NA) translational ordering of the liquid-crystal--forming compound octylcyanobiphenyl (8CB) incorporated into the pores of a silica aerogel. We find that the aerogel dramatically alters 8CB phase behavior, eliminating the NA transition found at T=${\mathit{T}}_{\mathit{N}\mathit{A}}$=33.7 \ifmmode^\circ\else\textdegree\fi{}C in bulk samples and replacing it by a short range smectic ordering having a correlation length which at ${\mathit{T}}_{\mathit{N}\mathit{A}}$ is \ensuremath{\xi}\ensuremath{\sim}40 \AA{}, much smaller than the aerogel pore and nematic domain size ${\ensuremath{\xi}}_{\mathit{p}}$\ensuremath{\sim}200 \AA{}. As T is lowered this smectic disorder is eliminated from the pores, \ensuremath{\xi} increasing continuously over a 20 \ifmmode^\circ\else\textdegree\fi{}C range to saturate at ${\ensuremath{\xi}}_{\mathit{p}}$ for T${\mathit{T}}_{\mathit{N}\mathit{A}}$-20 \ifmmode^\circ\else\textdegree\fi{}C.