Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, Vol.358, 237-250, 2001
Nematic structures in randomly interconnected porous hosts
Deuteron NMR measurements were performed on the octyl-cyanobiphenyl (8CB) Liquid crystal confined to the randomly interconnected and large cavities of cellulose acetate Millipore filters and to the small pores of silica aerogel. These studies were performed as a function of sample orientation in the field, and by changing the confining size. In almost all cases, 8CB was confined to sizes smaller than the micron size magnetic coherence length, thus the observed line-shapes reflect the director pattern, (n) over cap((r) over right arrow), and the orientational order, Q, as dictated by the porous host. The isotropic nature of aerogel yields DNMR spectral patterns consistent with powder-lineshapes. These patterns indicate a randomized (n) over cap((r) over right arrow), characterized by a single e. No angular dependence is found and nematic domains of finite dimension most likely exceeding the confining size are formed. In contrast, the Millipore anisotropy and larger confining sizes yield DNMR patterns dependent on the sample orientation in the field and show a competition between surface-induced and field induced molecular alignment.