DEUTERIUM NMR OF A PENTYLCYANOBIPHENYL LIQUID-CRYSTAL CONFINED IN A SILICA AEROGEL MATRIX

The temperature dependence of the deuterium NMR line shape was studied in a pentylcyanobiphenyl (5CB) nematic liquid crystal embedded into l ecithin-treated and nontreated continuous-pore silica aerogel matrices . In the lecithin-treated matrix the temperature dependence of the NMR absorption spectra of 5CB exhibits a pronounced first-order nematic-i sotropic transition, while in the nontreated matrix only a weak discon tinuity was found. The observed almost continuous onset of nematic ord ering in the latter case appears to be due to the spread of nematic-is otropic-transition temperatures T(NI) in different aerogel cavities. I n the isotropic phase of 5CB a surface ordering effect of the silica a erogel matrix was observed and the surface order parameter was estimat ed to be S0=0.02+/-0.01. The surface anchoring of the 5CB molecules is much stronger in the case of a nontreated matrix than in the lecithin -treated one.