Mixing Effects on the Cotton-Mouton Constant of 3OCB and 5OCB in Isotropic Phase

Abstract

To find a material with large magnetic-field-induced birefringence Δn/H2, we have studied the mixing effects on KC and (Δn/H2)T=Tc of 4'-n-propyloxy-4-cyanobiphenyl (3OCB), which has the largest value of KC, and 4'-n-pentyloxy-4-cyanobiphenyl (5OCB), which has the largest value of (Δn/H2)T=Tc among the cyanobiphenyls so far investigated. Here KC is the proportionality constant in the relation Δn/H2=KC/(T-Tc*); (Δn/H2)T=Tc is necessary because the relation may not hold as the temperature approaches the clearing point Tc. The mixing is effective and both KC and (Δn/H2)T=Tc attain the maxima at x=0.2 and 0.6, respectively, where x=[5 OCB]/([3 OCB]+[5 OCB]). The phase field in which the isotropic and nematic phases coexist appears to be less than 0.07°C. The KC(x) and Tc(x) curves have mirror symmetry with respect to a line parallel to the abscissa; larger KC corresponds to lower Tc. To understand their dependence on x, we have also discussed what phenomenological parameters in the Landau-de Gennes theory are responsible for KC and (Δn/H2)T=Tc.