Heat capacities and phase transitions of the antiferroelectric liquid crystals MHPOBC and MHPOCBC

Heat capacities of the antiferroelectric liquid crystals 4-(1-methylheptyloxycarbonyl)phenyl-4'-octyloxybiphenyl-4-carboxylate (MHPOEC) and 4-(1-methylheptyloxycarbonyl)phenyl-4'-octylcarboxybiphenyl-4-carboxylate (MHPOCBC), have been measured with an adiabatic calorimeter between 350 and 460 K. MHPOBC showed three smectic subphases (ferrielectric C-gamma*, ferroelectric C* and a fancy phase C-alpha*) between antiferroelectric smectic C-A* and paraelectric smectic A, while MHPOCBC exhibited only one subphase (smectic C-alpha*). These phases are clearly discriminated by the existence of phase transitions. The enthalpies and entropies gained at the respective phase transitions were very small. A much larger phase transition from smectic A to isotropic liquid was also observed in both compounds.