Modulated Differential Scanning Calorimetry

Abstract

The reproducibility and reliability of the TA Instruments Modulated Differential Scanning Calorimeter (MDSC) was tested over a range of conditions. The equipment base line was found to be fairly constant with a very small fluctuation (10 μW), which means a 0.1 % fluctuation on the scale of a normal polymer MDSC curve. The excellent stability of the base line and the reasonable reproducibility of the curves (5%) suggest that frequent calibration is not required.

The heat capacities calculated from the modulated response to the variable temperature depend on the frequency for a given cell constant. The heat capacity cell constant is a unique function of the modulation frequency:k _c =K ^o_c p/(p−6.3) wherep is the time of the periodicity expressed in seconds and K ^o_c is the heat capacity cell constant measured on a standard material and reduced to zero frequency. The cell constants depend on the flow rate of the helium according to:K(He)=K ^o(1.298−0.004424He+1.438·10⁻⁵ He ²) whereHe is the flow rate of helium in ml min⁻¹ andK ^o represents a constant at 100 cm³ min⁻¹. There is a strong dependence of cell constant on the flow rate ranges from 10 to 80 cm³ min⁻¹, while above this rate (up to 135 ml min⁻¹) the cell constant approaches a plateau.