Kinetic and thermodynamic studies of phase transition from hexagonal to discotic liquid crystal in metal-free 1,4,8,11,15,18,22,25-octahexylphthalocyanine

Abstract

ABSTRACT The objective of this work is to examine the Phase changes in the metal-free 1,4,8,11,15,18,22,25-octahexylphthalocyanine compound under non-isothermal and predicted isothermal conditions by Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM). The kinetic and thermodynamic parameters of the two endothermic phases observed were reported. The reaction of these two changes appeared to be the transition from hexagonal columnar mesophase to a less ordered crystalline phase and then to the transition to the discotic columnar mesophase. The isoconversional models, namely, Friedman and Flynn-Wall-Ozawa (FWO) were employed to calculate the effective activation energy and pre-exponential factor. The reaction model involved in the phase change was proposed by employing the z-master plot for nonisothermal results and the integral form for isothermal results. Additionally, by using the activation energy obtained from Friedman and FWO methods the thermodynamic parameters, including the changes of enthalpy, the Gibbs free energy and entropy were determined. The change of enthalpy showed a quite consistent tendency with the activation energy, whereas the change of Gibbs free energy always kept positive during the whole process, this implies that the reaction is non-spontaneous and thus requires external heat supply.