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

• Published in: Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics Vol. 60; no. 5 Pt A; p. 5607
• Main Authors: Zeng, H, Zalar, B, Iannacchione, G S, Finotello, D
• Format: Journal Article
• Language: English
• Published: United States 01.11.1999
• ISSN: 1063-651X; 1095-3787
• DOI: 10.1103/PhysRevE.60.5607

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-A (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(r-->), 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(r-->) 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.