Nematic order parameter as determined from dielectric relaxation data and other methods

Abstract

The nematic order parameter S = 〈P₂(cosθ)〉 was calculated from the dielectric relaxation time characterizing the molecular rotation around the short axes using formulae derived by Coffey et al. (W. T. Coffey, Yu. P. Kalmykov and J. P. Waldron, Liq. Cryst., 1995, 18, 677; W. T. Coffey, D. S. F. Crothers, Yu. P. Kalmykov and J. P. Waldron, Physica A, 1995, 213, 551; Yu. P. Kalmykov and W. T. Coffey, Liq. Cryst., 1998, 25, 329). In order to test the approximations on which the theory is based (rod-like shape of molecules, the diffusional model of reorientational motion in the presence of an ordering Maier–Saupe potential) several nematogens were chosen for which the S(T) behaviour is known from other experimental techniques as well. These molecules (1OCB, 5OCB, 7CB, 5PCH, 3CCH, 5CCH and 7CCH) have rigid or flexible cores, strongly polar –CN group at one side and the alkyl or alkoxy tail at the other side. A common analysis of the results coming from different experimental methods show large differences between data. Using the Haller-type formula a re-scaling of the S-values was proposed which reduced the scatter of experimental sets of points to a reasonable range. It was established that the Coffey et al. approach leads to the order parameter being consistent with other data if the molecular cores are sufficiently rigid.