Volumetric study of the nematic-isotropic pretransition region

Specific-volume measurements on methoxybenzylidene butylaniline (MBBA) and $p$-azoxyanisole (PAA) using the differential-dilatometer method are reported. The nematic-isotropic pretransition region over a range ± 15°K about the transition point is examined. The data, interpreted in terms of the de Gennes-Landau theory, are consistent with an $\alpha =0.5\mathrm{}$ critical exponent. The pseudocritical temperature derived from the volume data is significantly different from that reported for optical experiments.