J. Phys. II France
Volume 6, Number 12, December 1996
Page(s) 1727 - 1741
DOI: 10.1051/jp2:1996158
J. Phys. II France 6 (1996) 1727-1741

2H-NMR Detection of the Pressure Effect on Packing Modulations in a Columnar Liquid Crystal

A. Hollander1, J. Hommels1, K.O. Prins1, H.W. Spiess2 and M. Werth2

1  Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
2  Max-Planck-Institut für Polymerforschung, Postfach 3148, 55021 Mainz, Germany

(Received 4 June 1996, revised 20 August 1996, accepted 6 September 1996)

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 2H-NMR spectroscopy. The mobility of the side chains can be used to probe the local packing density along the columns. We show that the numbers of molecules in regions of high and low packing density are strongly dependent both on temperature and on pressure. 2D-exchange NMR spectra show that the rate of exchange between these regions decreases considerably at increasing pressure, which is attributed to an increase of the length scale of the pinched regions. Our findings are consistent with the model proposed by de Gennes to resolve the incommensurability problem. A large decrease in the rate of molecular reorientation about the column axis is observed at the II-III phase transition.

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