Thermodynamics of Activated Phase Transitions of 8CB:  DSC and MC Calorimetry

Abstract

The present paper reports the heating rate effect on the phase transitions of a pure liquid crystal octylcyanobiphenyl (8CB) with use of Differential Scanning Calorimetry (DSC) and Modulation Calorimetry (MC) techniques. The DSC runs were taken at various temperature ramp rates from 20 to 0.5 K/min for heating and cooling scans. Well-defined endothermic/exothermic peaks were found at the melting/crystallization, smectic-A to nematic (SmA-N), and nematic to isotropic (N-I) transitions on heating/cooling scans, respectively. All transitions shift in temperature significantly with different ramp rates. The temperature shift of C(p) peaks between heating and cooling scans indicates the order of the transitions. In addition, all transitions follow an Arrhenius behavior. The activation energy of a transition increases as the total energy involved in the transition decreases. The respective enthalpy and entropy change of each transition provides information on the Gibbs free energy. The significance of the results is discussed in terms of the order of transitions. A comparative analysis of MC and DSC techniques highlights the significance of the two techniques. MC is a practicable tool for observing the phase dynamics whereas DSC is a good tool for studying the rate kinematics of the transitions.

Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores

“…The phase transition temperatures can be affected by the ramp rates. The activated dynamics of the transitions of bulk 8CB has been recently investigated by differential scanning calorimetry and modulation calorimetry [29]. In order to quantify rate effects and to disentangle kinetic effects from genuine thermal hysteresis, two different ramp rates have The small angle neutron scattering patterns on the 2D detector of bulk 8CB are displayed in Fig.…”

Effects of copper nanoparticles on the thermodynamic, electrical and optical properties of a disc-shaped liquid crystalline material showing columnar phase

“…Such results have been observed for the other systems as well. [26,27] This happens because of the finite thermal resistance between sample and oven under non-equilibrium condition when the system is dynamic. By the extrapolation technique, transition temperatures obtained at the scan rate of 0 C/min represent true transition temperature under the condition of thermal equilibrium.…”

Thermal cycle test of binary mixtures of some fatty acids as phase change materials for building applications

“…2). Samples were scanned in a cyclic mode with the heating/cooling rate (1 • C/min, 2 • C/min and 4 • C/min) under a constant stream of nitrogen at a flow rate of 20 ml/min to check if there is any significant effect of the heating/cooling rate on onset temperature [18,19]. It can be seen from the figure that onset temperature about remains the same and does not depend on the rate of heating or cooling, as the heating rate decreases, the peak shifts toward lower temperature significantly [17].…”