Effects of quenched disorder on the orientational order of the octylcyanobiphenyl liquid crystal

Deuteron NMR (DNMR) measurements were performed with high-temperature and spectral resolution on the octylcyanobiphenyl (8CB) liquid crystal confined to the randomly interconnected pores of silica aerogel as a function of temperature and silica density. The aerogel density was varied by one order of magnitude and the temperature spanned the isotropic (I), nematic (N), and smectic-ii (SmA) phases of 8CB. For all samples, the liquid crystal was confined to pores smaller than the micron-sized 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 DNMR spectral patterns are consistent with powder line shapes representative of a randomized (n) over cap((r) over right arrow) characterized by a single Q. The nematic domains formed are of finite dimension that likely exceeds the confining size. The weakly first-order nematic-to-isotropic phase transition becomes less discontinuous with decreasing pore size, eventually becoming continuous at an aerogel density between 0.36 and 0.5 g/cm(3). In the most severe confinement, no phase transitions are observed with only a continuous evolution of Q present. The orientational order is suppressed from the bulk's with no enhancement upon the onset of the SmA phase. This indicates a decoupling of nematic and smectic order parameters and a severe suppression of the SmA phase by this porous media. [S1063-651X(49)08711-5].