Verifying the symmetry of differential scanning calorimeters concerning heating and cooling using liquid crystal secondary temperature standards

Abstract

The German Society for Thermal Analysis (Gesellschaft für Thermische Analyse e.V.—GEFTA) recommends verifying the symmetry of differential scanning calorimeters (DSC) between heating and cooling. For this purpose the use of liquid crystal phase transitions is suggested. Merck KGaA, Darmstadt, Germany, offers three certified secondary standards for DSC calibration in heating and cooling mode, which are recommended by ASTM too. In this paper all liquid crystal phase transitions of the three liquid crystals offered are measured using different DSC's in order to verify their applicability for checking the symmetry of the instruments and the suitability for a temperature calibration in heating and cooling mode. The materials under study are: 4-cyano-4′-octylbiphenyl (8OCB, M-24 (Merck)), 4-(4-pentyl-cyclohexyl)-benzoic acid-4-propyl-phenyl ester (HP-53 (Merck)) and 4′-ethyl-4-(4-propyl-cyclohexyl)-biphenyl (BCH-52 (Merck)). The smecticA to nematic transition of M-24 (8OCB) fulfils the requirements for these purposes best. There is no detectable discontinuity in the rate dependence of the transition temperature at zero rates and the material is sufficiently stable to allow several measurements. Because of the weakness of the transition and the neighborhoods of the much stronger crystal to smecticA transition and nematic to isotropic transition instruments with sophisticated control and data treatment algorithms partly fail to detect the rate dependence of the transition correctly. In such cases HP-53 or BCH-52 can be used alternatively. In view of a temperature calibration at different temperatures in heating and cooling mode the recommended liquid crystal phase transitions are suitable. The observed supercooling effects are nearly zero for M-24 and HP-53 and less than 0.25 K for BCH-52. The liquid crystal transitions of the standards are applicable for a calibration in heating mode too.