CALORIMETRIC STUDY OF THE RELATIONSHIP BETWEEN MOLECULAR-STRUCTURE AND LIQUID CRYSTALLINITY OF ROD-LIKE MESOGENS II - HEAT-CAPACITIES AND PHASE-TRANSITIONS OF 4-(TRANS-4-PROPYLCYCLOHEXYL)BENZONITRILE

The molar heat capacity of the rod-like compound 4-(trans-4-propylcycl ohexyl)benzonitrile (3-CBCN), purity of 99.8 mol%, has been measured w ith an adiabatic calorimeter at temperatures between 15 and 385 K. 3-C BCN is a nematogenic mesogen, whose melting and clearing points are 31 6.33 and 319.09 K, respectively. The enthalpy and entropy gained at fu sion are 20.4 kJ mol(-1) and 64.4 J K-1 mol(-1), respectively; those f or the nematic-to-isotropic transition are 1.1 kJ mol(-1) and 3.5 J K- 1 mol(-1) respectively. 3-CBCN exhibits a supercooled nematic phase, w hose molar heat capacities have been measured from 25 K below the melt ing point. The molar and transition entropies of 3-CBCN are discussed in relation to those of 4'-propylbiphenyl-4-carbonitrile (3-BBCN) and trans,trans-4'-propylbicyclohexyl-4-carbonitrile (3-CCCN). There seems to exist a correlation between these values and mesophase stability. Finally, Eidenschink's theoretical model for the nematic-to-isotropic transition has been applied to 3-CBCN; the transition enthalpy estimat ed according to this model agrees well with the observed value.