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Orientation of molecular solutes in nematic liquid crystals : size and shape effects Kok, Mei Yeng


An understanding of the mechanisms of orientation of solutes in anisotropic solvents is of fundamental importance in the theory of liquid crystalline systems. A systematic study of these mechanisms is presently being conducted in this and other laboratories. As part of this investigation, the orientational order of a series of C[sub 2v] solutes in the liquid crystal mixture 55 wt% 1132 (Merck ZLI 1132) and 45 wt% partially deuterated EBBA (N-(4-ethoxybenzylidene)-2,6-dideutero-4-n-butylaniline) was studied using NMR spectroscopy. This technique provides a description of the average orientation of a solute in terms of the order parameters. The results obtained indicate a good correlation between order parameters and the size and shape of the solutes. A model based on short range hard body interactions, which depend on the dimensions of the solute, was used to predict orientational ordering. Excellent agreement was obtained between observed and calculated order parameters. These results provide supportive evidence that the mechanism responsible for orientation in the 55 wt% 1132 system is short range hard body interactions. A side study was conducted on the ordering of furan and thiophene in the liquid crystalline solvents 1132 and EBBA. Temperature dependence of order parameters in 55 wt% 1132 were examined. Orientation in 1132 and EBBA and temperature dependence in 55 wt% 1132 can be explained in terms of: (i) the short range interactions dependent on the dimensions of the solute, and (ii) the interaction between the molecular quadrupole moment of the solute and the average electric field gradient which is due to the solvent.

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