Entropic Contribution of Flexible Terminals to Mesophase Formation Revealed by Thermodynamic Analysis of 4-Alkyl-4′-isothiocyanatobiphenyl (nTCB)

To understand the role of intramolecular degrees of freedom in forming mesophases, thermodynamic analysis was performed for 4- n -alkyl-4′-isothiocyanatobiphenyl ( n TCB, n is the number of carbon atoms in the alkyl group), which exhibits the crystal E (CrE) phase as a mesophase. The heat capacities of 2TCB and 5TCB were measured by adiabatic calorimetry. Their entropies of transition (Δ trs S ) were compared with those of other n TCBs ( n ≤ 10). Δ trs S of the phase transition from the ordered crystal to CrE phase increased with their alkyl chain length, whereas that of fusion of the CrE phase remained essentially constant. These behaviors clearly show that the alkyl chain of n TCB is fully disordered even in the CrE phase. Through the comparison of Δ trs S among 5TCB, 4-pentyl-4′-cyanobiphenyl (5CB), and pentylbiphenyl, the role of the alkyl chain for the formation of mesophases is discussed.