Birefringence study of reentrant-nematic liquid-crystal mixtures

The birefringence in mixtures of 4-cyano-4′-octyloxybiphenyl (80CB) and 4-cyano-4′-hexyloxybiphenyl (60CB) has been studied near the nematic—to—smectic-$A$ ($\mathrm{NA}$) and smectic-$A$—to—reentrant-nematic ($A{N}^{\prime }$) transitions. There is an overall increase in the birefringence at the $\mathrm{NA}$ transition and an overall decrease at the $A{N}^{\prime }$ transition. The birefringence change at the transitions decreases with increasing concentration $x$ of 60CB and vanishes at $x=0.28\mathrm{}$. The birefringence and the transition enthalpy behave similarly.