Director Elasticity above the Nematic-Smectic a Transition.

Director elasticity above the nematic-smectic A (NA) transition for 4-n-nonylphenyl-4'-hexyloxybenzoate ((')609) and for oxtyloxycyanobiphenyl (80CB) was investigated using Freedericksz transition and light-scattering techniques. The first technique yields information about the bend curvature coefficient, and the second the twist elastic constant. The measurements in this study show that the bend elastic coefficient diverges with an exponent (nu)(,(PARLL)) = 0.66 (+OR-) 0.01 for (')609 and (nu)(,(PARLL)) = 0.62 (+OR-) 0.03 for 80CB. The light-scattering results in the twist mode show that the twist elastic constant exponent is (zeta)(,2) = 0.56 (+OR-) 0.01 for (')609 and 0.34 (+OR -) 0.03 for 80CB. Measurements on (nu)(,(PARLL)) do not agree with X-ray measurements for t(, )<(, )10('-3) but they do agree for t(, )>(, )10('-3) where the (chi)('2) is poor. This shows a discrepancy between the exponents measured with X-ray scattering and those measured with optical techniques. Also, the study showed that values for (zeta)(,2) increase as the nematic range increases. The determination of the values of (zeta)(,2) establishes the fact that (nu)(,(PERP)) can be determined by light-scattering techniques, where (nu)(,(PERP)) = ((zeta)(,2 )+(, )(nu)(,(PARLL)))/2.