Heat capacity of the discotic mesogen, 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene, between 17 and 397 K and phase transitions in the solid and liquid states

Abstract

Abstract The heat capacity of the discotic mesogen, 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene whose purity was better than 99.3 mole per cent has been measured with an adiabatic calorimeter between 17 and 397 K. Five solid-solid phase transitions were observed in the crystalline state at 105.90, 220.86, 233.91, 330.81, and 337.10 K. The melting and clearing points were 340.27 and 371.17 K, respectively. The sum of all of the solid-to-solid phase transition entropies amounted to 44.87 J K−1 mol−1 (25.8 per cent of the total transition entropy). These solid-solid phase transitions can be ascribed to successive conformational melting of the alkyl-chains attached to the central triphenylene core. The present result strongly supports the prediction that a rich solid polymorphism is a necessary condition for favourable formation of the columnar structure in the discotic mesophases. The entropy change at the phase transition from the columnar mesophase to the isotropic liquid was very small in comparison to that observed for the discotic mesogens having a benzene core. This small transition entropy is discussed in relation to the wider temperature range for the discotic mesophase in the tri-phenylene derivatives compared to the benzene-core mesogens. Thermal properties of the discotic mesogens are compared to those of classical rod-like liquid crystals.