Calorimetric study of the relationship between molecular structure and liquid-crystallinity of rod-like mesogens I. Heat capacities and phase transitions of 4′-propylbiphenyl-4-carbonitrile andtrans, trans-4′-propylbicyclohexyl-4-carbonitrile

The heat capacities of the rod-like compounds 4′-propylbiphenyl-4-carbonitrile (3-BBCN) and trans, trans-4′-propylbicyclohexyl-4-carbonitrile (3-CCCN) have been measured with an adiabatic calorimeter between 12 and 383 K. 3-BBCN is not mesogenic and melts into an isotropic liquid at 338·77 K, whereas 3-CCCN is mesogenic and its melting and clearing points are 330·73 K and 353·80 K, respectively. The enthalpy and entropy of fusion of 3-BBCN are 22·7 kJ mol−1 and 67·0 J K−1 mol−1, respectively, while those of 3-CCCN are 27·0 kJ mol−1 and 81·7 J K−1 mol−1, respectively. The enthalpy and entropy gain due to the nematicisotropic transition of 3-CCCN are 1·8 kJ mol−1 and 5·0 J K−1 mol−1. 3-CCCN shows a mesomorphic transition from a smectic to the nematic state at 329·62 K, which occurs as a metastable state, its transition enthalpy and entropy are 5·4 kJ mol−1 and 16·4 J K−1 mol−1, respectively. The temperature dependence of the molar entropies of both compounds shows that molecular arrangement in the crystal is more ordered in 3-CCCN than in 3-BBCN. This fact may be related to the stability of mesophases. Finally, Eidenschink's theoretical model for the nematicisotropic transition has been applied to 3-CCCN. As far as the present mesogen is concerned, the transition enthalpy estimated according to this model agrees well with the observed value.