Thermal Analysis of Liquid Crystal Phase Transition Using FTIR Spectroscopy

Abstract

Thermal spectra of liquid crystal, p-n-dodecyloxy benzoic acid were collected at 26-150 'C by FTIR microspectroscopy. Conformational apparent enthalpy differences ΔH in phase transitions have been determined by van't Hoff equation at constant pressure and plotted with temperature. Compared to DSC, these enthalpies plot with temperature could be divided into three types. The first one was long alkyl chain group which has only one apparent endothermic enthalpy peak at 95 'C. The second one was partial carbonyl and aromatic ether groups has three endothermic enthalpy peaks in the phase transition. The third type including benzoyl group which has one exothermic enthalpy peak at 95 'C and a broad endothermic enthalpy peak in the range of 128-140 'C. These differences indicated that oxy-benzoic acid part of dodecyloxy benzoic acid recrystallized to cycle dimer during alkyl chain melting in crystal-smectic phase transition.