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

We have studied, via x-ray scattering, the nematic to smectic-[ital 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 [ital T]=[ital T][sub [ital N][ital A]]=33.7 [degree]C in bulk samples and replacing it by a short range smectic ordering having a correlation length which at [ital T][sub [ital N][ital A]] is [xi][similar to]40 A, much smaller than the aerogel pore and nematic domain size [xi][sub [ital p]][similar to]200 A. As [ital T] is lowered this smectic disorder is eliminated from the pores, [xi] increasing continuously over a 20 [degree]C range to saturate at [xi][sub [ital p]] for [ital T][lt][ital T][sub [ital N][ital A]][minus]20 [degree]C.