Orientational order parameter of a binary liquid crystal mixture showing induced smectic phase

Binary mixture of 4 - pentyl 4' - cyanobiphenyl (5CB) and 4-n-pentyl phenyl 4 - n'- pentyloxy benzoate (ME50.5) shows the presence of an induced smect ic A phase. In the present work we report the density and refractive indice s on this system in the entire composition range. The density and refractiv e indices are used to calculate molecular polarizability values parallel an d perpendicular to the molecular long axes using Neugebauer's method. Orien tational order parameters (OOP's) as a function of temperature and concentr ation are determined. The relative change in density at the smectic A-nemat ic phase transition shows a maximum near equimolar concentration. The orien tational order parameter values and calculated packing fraction show a mini mum around x approximate to 0.4 at a temperature T=35 degreesC. Similar beh aviour is also observed at a temperature T = 0.98T(C), in the nematic phase where the induced smectic A phase is the precursor of the nematic phase. A verage lateral distance (D) between the molecules is measured from the anal ysis of x-ray diffraction data. The variation of D with molar concentration in smectic A phase shows a maximum near equimolar concentration.