First- and second-order smectic-A to nematic phase transitions in p,p′-dialkylazoxybenzenes studied by birefringence

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Abstract
The birefringence of mixtures of homologues from the terminally non-polar series of p,p′-dialkylazoxybenzenes has been studied. The birefringence is used as a probe of the orientational order parameter, which is coupled to the translational (smectic) order parameter. The tricritical nematic to smectic-A point which results from this coupling is found at a McMillan ratio TAN/TNI= 0.977. This is similar to the values reported for terminally polar systems and is higher than predicated by theory. The critical exponent for the variation of the order parameter across the SA–N phase transition obeys the expected trend.