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 (Tg = 210 K), stable crystal I (Tfus = 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 (Tfus = 286 K) and the clearing temperature of the nematic phase (Tc = 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.

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► Five phases of the compound are identified by adiabatic calorimetry and polarizing microscopy.
► Thermodynamic functions are determined for all phases.
► The compound forms a glass of the metastable nematic phase (Tg = 210 K).
► The clearing temperature (Tc = 279.4 K) is lower than the melting point of crystal I (Tfus = 288.43 K).
► The melting point of new metastable crystal II is Tfus = 286 K.