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 (NA) and smectic-A---to---reentrant-nematic (AN^') transitions. There is an overall increase in the birefringence at the NA transition and an overall decrease at the AN^' transition. The birefringence change at the transitions decreases with increasing concentration x of 60CB and vanishes at $x=0.28$. The birefringence and the transition enthalpy behave similarly.