Temperature calibration of temperature-modulated differential scanning calorimeters

Calibration of differential scanning calorimeters (DSC) is essential because it is not possible to calculate heat-how rate and sample temperature directly from the quantities measured. The question arises whether the calibration used in the conventional DSC mode is also valid for temperature-modulated DSC (TMDSC). To prove this, a well-defined transition is needed which does not disturb the temperature profile within the sample during the modulation, the response of the whole system (calorimeter, sample, transition) must be linear. Some low-energy liquid-crystal transitions behave in this way under certain conditions and can be used for temperature calibration in TMDSC. The procedure is demonstrated for a smectic-A-nematic transition in a cyanobiphenyl liquid crystal (80CB). Limitations of the method are discussed - in particular, rate effects. The TMDSC measurements also give some information about the order of phase transitions (80CB:SN - second-order; NI - first-order). (C) 1997 Elsevier Science B.V.