Calorimetric Investigations of the Liquid Crystalline Material Heptyloxybenzylidene-Heptylaniline (70.7)

Abstract

Using an adiabatic scanning calorimeter we have studied the thermal behavior of the liquid crystalline material heptyloxybenzylidene-heptylaniline in the temperature range between 52°C and 86°C. In this temperature range a rich variety of liquid crystalline and plastic crystalline phases and phase transitions could be studied. The isotropic to nematic (NI) transition and the nematic to smectic A (NA) transition (only 0.3°C below T_NI) are both first-order with a latent heat of 2.1 (± 0.1) kJ/mol and 2.2 (± 0.1) kJ/mol respectively. The first-order nature of the NA transition is in accordance with recent observations for systems with similarly small nematic ranges. The smectic A to smectic C (AC) transition is second-order within the experimental resolution. The observed anomaly in the specific heat C_p for the AC transition is not consistent with the theoretically expected XY behavior. It can, however, be described rather well with a mean field theory. The transition from the smectic C to the plastic B phase is first-order with a latent heat of 2.9 (± 0.1) kJ/mol. The transition between the plastic B and G phases is also first-order but with a rather small latent heat of 0.135 (± 0.015) kJ/mol. In the B phase we have observed several thermal anomalies, which can be associated with different restacking transitions.