Determination of bend to splay elastic constant ratio (K-3/K-1) of several nematogens and applicability of spherocylindrical model

In the present work we have tried to test the theory of elastic constants of nematogens given by Priest, assuming spherocylindrical shape of the molecules. The bend (K-3) to splay (K-1) elastic constant ratios of four closely related nematic liquid crystals, having cyclohexane carboxylate core, have been determined by us using Freedericksz transition in a magnetic field. The length and width of the molecules and the order parameters <(P)over bar (2)> and <(P)over bar (4)>, required for the theoretical calculations, have been estimated from our earlier X-ray studies on these compounds. The experimental values of the ratio K,IK, for three of these compounds agree quite well with those calculated from the spherocylindrical model. The fourth nematogen, for which the agreement is poor, shows anomalous temperature variation of density and hence the rigid spherocylindrical model is not probably applicable to it. We have also measured K-3/K-1 for n-pentyl cyanobiphenyl (5CB), and have shown that the X-ray experimental data implies that <(P)over bar (4)> is positive even near the nematic-isotropic transition temperature for 5CB, which is at variance with some experimental Raman scattering results. Finally, the K-3/K-1 ratios for pentyloxy cyanobiphenyl (5OCB) at different temperatures obtained experimentally by Bradshaw et al. have been compared with those calculated from spherocylindrical model using our X-ray data.