The pressure and temperature dependence of the orientational order in the nematic phase of 4-n-pentyl-d11-4'-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'-cyanobiphenyl. The quadrupolar splittings are used to obtain order parameters SiCD 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 Si CD with temperature at constant volumes of 0.243 dm3 mol -1 and 0.248 dm3 mol-1. These two sets of SiCD 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.