Nonreentrant and reentrant nematic liquid crystals under high pressure: A volumetric study

The results of volumetric measurements under high pressure near three different phase transitions in heptyloxyazoxybenzene (70AB), a nonreentrant nematic liquid crystal, and octyloxycyanobiphenol (80CB), a reentrant nematic liquid crystal, are reported. Volume changes at the isotropic–nematic transition in these two liquid crystals are similar at low pressure, but behave differently at high pressure. Volume changes at the nonreentrant nematic–smectic C transition are an order of magnitude larger than at the reentrant nematic–smectic A transition. The volume dependence along these two nematic–smectic coexistence curves also reveals the stronger role played by attractive forces in these transitions as compared to the isotropic–nematic transition. Finally, these data do not support a previously reported nematic–smectic C-solid triple point in 70AB.