Polymorphism and thermodynamic functions of liquid crystalline material 4-cyano-3-fluorophenyl 4-butylbenzoate

Heat capacity measurements were conducted between temperatures of 5 K and 360 K by adiabatic calorimetry for the liquid crystalline material 4-cyano-3-fluorophenyl 4-butylbenzoate to investigate the phase behavior and determine the thermodynamic functions. Five phases are identified: the metastable nematic phase, its glass phase (T-g = 210 K), stable crystal I (T-fus = 288.43 K), a new metastable crystal (crystal II), and the isotropic liquid. The enthalpy of fusion of crystal I is 22.22 kJ . mol(-1), and the entropy is 76.91 J . K-1 . mol(-1). Polarizing microscopy observations were also conducted between T = 100 K and 300 K to confirm the phase behavior. The melting point of crystal II (T-fus = 286 K) and the clearing temperature of the nematic phase (T-c = 279.4 K) were obtained using a microscope. On the other hand, the enthalpy of the clearing transition (526 J . mol(-1)) and the entropy (1.88 J . K-1 . mol(-1)) were obtained from the thermodynamic functions. The Debye temperature of crystal II (55.3 K) is significantly lower than that of crystal I (60.7 K), indicating that the phonon density of states is considerably different between the two crystals. The existence of low-energy excitations in the nematic glass and a low-energy shift of the intramolecular modes in crystal II are also identified. (C) 2012 Elsevier Ltd. All rights reserved.