PHASE-DIAGRAMS FROM SPIN-PROBE STUDIES FOR BINARY-MIXTURES OF NON-MESOMORPHIC SOLUTES IN 5CB

The effect of low concentrations of a non-mesomorphic solute impurity on the nematicto-isotropic transition of 4-cyano-4'-n-pentylbiphenyl ( 5CB), has been investigated by ESR spectroscopy, using euterated-2,2,6 ,6-tetramethyl-4-piperidone-N-oxide (PD-Tempone), at mole fractions of the order of 10(-5), as a probe. It has been found that at such low m ole fractions PD-Tempone simply probes the nematic-to-isotropic transi tion without observably perturbing it. This fast approach relies on th e fact that a smaller coupling constant is obtained for the spin probe in the orientationally ordered nematic phase relative to the isotropi c phase. The method has been further refined in this study by the use of an especially designed sample holder which, in conjunction with a t emperature control and monitoring system, allowed control of the tempe rature of the sample to within +/-0.02 degrees C. Additionally measure ments were made, for each solute, at several solute mole fractions wit hin the range from 0.01 to 0.07. The results were used to obtain beta( N) and beta(I), the moduli of the slopes of the nematic and isotropic boundary lines respectively in the reduced nematic-to-isotropic transi tion temperature (T) versus solute mole fraction (x(2)) diagrams. The non-mesomorphic solutes used in this study are n-nonane, 3,3-diethylp entane, tetrapropyltin and tetrabutyltin.