Orientational ordering in the nematic phase of a thermotropic liquid crystal: A small angle neutron scattering study

I. W. Hamley*, S. Garnett, G. R. Luckhurst, S. J. Roskilly, J. Skov Pedersen, R. M. Richardson, J. M. Seddon

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

22 Citations (Scopus)

Abstract

The long range orientational order in the nematic phase of 4,4′-dimethoxyazoxybenzene (PAA) has been studied using small angle neutron scattering (SANS) from mixtures of protonated and perdeuterated molecules. Unlike spectroscopic methods, the determination of the complete set of orientational order parameters is, in principle, possible using this technique. The orientational order parameters were extracted from the anisotropic single molecule scattering isolated at small scattering angles. These order parameters were compared to those extracted from previous neutron scattering measurements, to second rank order parameters extracted from NMR, and to those predicted from the Maier-Saupe theory of nematic ordering. It was found that only the second and fourth rank terms in the expansion of the single molecule scattering in a basis of Legendre functions are statistically significant, which means that the order parameters P2 and P4 only could be obtained. For the semirigid nematogen PAA, the Maier-Saupe theory is found to underestimate P2 and its dependence on the reduced temperature T/TNI where TNI is the nematic-isotropic (NI) transition temperature. The theoretical values of P4 are, however, in remarkably good agreement with the measured values. Order parameters extracted from NMR are found to be slightly lower than those determined from the SANS experiment, and we also make comparisons with earlier neutron scattering results. Singlet orientational distribution functions are reconstructed from the measured order parameters using the maximum entropy method.

Original languageEnglish
Pages (from-to)10046-10054
Number of pages9
JournalJournal of Chemical Physics
Volume104
Issue number24
DOIs
Publication statusPublished - 22 Jun 1996

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