PRESSURE AND TEMPERATURE DEPENDENCE OF THE ORIENTATIONAL ORDER IN THE NEMATIC PHASE OF 4-N-PENTYL-D11-4 prime -CYANOBIPHENYL. A DEUTERIUM NMR STUDY.

Deuterium NMR spectroscopy has been used to measure the temperature and pressure dependence of quadrupolar splittings in alkyl chain deuteriated 4-n-pentyl-4 prime -cyanobiphenyl. The quandrupolar splittings are used to obtain order parameters S**i//C//D for each position in the chain, and these are found to be independent of pressure at the nematic-isotropic transition temperatures. The data are used together with an equation of state for the nematic phase to derive the variation of S**i//C//D with temperature at constant volumes of 0. 243 dm**3 mol** minus **1 and 0. 248 dm**3 mol** minus **1. These two sets of S**i//C//D are compared with the predictions of a mean field theory of flexible molecules. The density dependences are obtained of the averaged interaction coefficients which appear in the theory, and the results suggest that relatively short range forces dominate the potential of mean torque.