H-2-NMR DETECTION OF THE PRESSURE EFFECT ON PACKING MODULATIONS IN A COLUMNAR LIQUID-CRYSTAL

The effect of high pressure on the columnar density modulation of the hexaoctylether of rufigallol, resulting from the incommensurability of the aromatic and aliphatic packing, is studied by 1D and 2D H-2-NMR s pectroscopy. The mobility of the side chains can be used to probe the local packing density along the columns. We show that the numbers of m olecules in regions of high and low packing density are strongly depen dent both on temperature and on pressure. 2D-exchange NMR spectra show that the rate of exchange between these regions decreases considerabl y at increasing pressure, which is attributed to an increase of the le ngth scale of the pinched regions. Our findings are consistent with th e model proposed by de Gennes to resolve the incommensurability proble m. A large decrease in the rate of molecular reorientation about the c olumn axis is observed at the II-III phase transition.