Order parameter and the orientational distribution function for 4-cyanophenyl-4′-n-heptyl benzoate in the nematic phase

Abstract

X-ray diffraction studies on oriented samples of 4-cyanophenyl-4′-n-heptylbenzoate in the nematic phase are reported at different temperatures. The orientational distribution function f(β) has been calculated from the angular distribution of the X-ray intensities. The orientational order parameters ₂ and ₄ have been determined from f(β). ₂ is found to be slightly greater than the value predicted by the Maier-Saupe molecular field theory, whereas the ₄ values are much smaller than those given by theory. An approximate expression for the potential of the molecular field is obtained from the temperature variation of the distribution function. The value of l and the intermolecular distance D have been determined from X-ray photograph. l, which is often close to the molecular length is found to be 28 Å. The length as measured from a model in a completely extended form is found to be 21 Å. If we assume antiparallel associations of molecules as in most cyano compounds this difference can be explained.