Specific heat dependence on orientational order at cylindrically confined liquid crystal phase transitions.

Abstract

Results from a high resolution calorimetric study for the alkylcyanobiphenyl liquid crystal series nCB, with n the number of carbons in the alkyl chain, confined to the 0.2 \ensuremath{\mu}m diam cylindrical pores of Anopore membranes are reported. The orientational order at the pore wall is controlled and varied from tangential to homeotropic by surface treatment. Effects due to confinement and director configuration on the specific heat were studied at the weakly first-order nematic to isotropic, the continuous smectic-A to nematic, and the first-order smectic-A to isotropic phase transitions. Deviations from bulk are considerable and the confined specific heat, strongly dependent on the orientational order, is dominated by elastic distortions and influenced by surface order and disorder contributions. A surface induced nematiclike ordered shell near the pore wall is retained deep in the isotropic phase. Over a limited reduced temperature range bulklike criticality appears to be preserved under confinement. The average scalar orientational order parameter temperature dependence can be extracted from the specific heat results using a simplified Landau--de Gennes theory and is found to be consistent with nuclear magnetic resonance measurements. In spite of the large pore size, the smectic translational order within the pores is inhibited for liquid crystals that also exhibit a nematic phase.