Thermal gradients in differential scanning calorimetry

Abstract

A combined static and dynamic temperature calibration is described. The static calibration corrects the instrumental dial temperature reading. The dynamic calibration has instrumental and material components and therefore varies from specimen to specimen. It is obtained from individual DSC curves and so removes uncertainties in sample temperature due to varying mass, geometry, and heating rate. The instrumental performance is improved and specific heats may be obtained to an accuracy of ~1%. Cp, Tf List of symbols used steady initial and final temperatures, respectively, in a calorimetric experiment; dial temperature reading; true temperature corresponding to To after all corrections have been made; melting point; correction to T D to give Tunder isothermal, 'static' conditions; additional correction to TD, due to thermal lag, to give T under programmed temperature conditions; contributions to 6T originating in the instrument and sample material respectively; total correction to T D to give T under programmed temperature conditions; calorimeter heating rate; enthalpy at T~ and Tf respectively; mean heat capacity over the temperature range T i to T~; heat capacity at Tf. T~,TfDifferential thermal analysis has long been a useful tool in the fields of geology and metallurgy [1]. The recent introduction of commercial apparatus, based on either differential thermal or differential enthalpic methods, has led to many applications far removed from these areas. In particular the technique has proved

Dependence of the glass transition temperature of poly (methyl methacrylates) on their tacticity

“…The baseline with empty cells and the calibration curve with a 45 mg sapphire were measured in the same r~gime. The sample temperature was corrected for thermal inertia calculated from the descending part of the measuring curve after completion of the programmed rise in temperature [21].…”

Determination of solidus and liquidus temperatures by means of a Perkin-Elmer 1B differential scanning calorimeter

“…The solidus temperature, T~, may be determined in any way, for instance according to the procedure given by the manufacturer or by that proposed by us in the previous section, or by means of an isothermal temperature calibration method [7]. It is important to note that the T~ determination must be done in just the same way as was used during the correction of the indicated temperature scale of the instrument.…”

Determination of some parameters to be utilized in correcting DSC curves for effects due to thermal lag

“…Their melting points were determined in the "static mode" [5] so as to obtain temperature measurements free of dynamic thermal lag. For the determination of the thermal lag parameters the caIorimeter was always used in the "specific heat mode" [23,24].…”

“…The aim of this paper is to show that, when the above conditions are met, it is possible to correct for dynamic thermal lag in DSC curves with the aid of some parameters: the assumption that they are instrumental parameters is enhanced by comparison of the data obtained with those reported by other authors [5].…”

“…Consequently, as a relative temperature accuracy from point to point on the curve is required, the thermal lag must be constant or change in a known manner along the portion of the curve on which measurements are made. It has been shown [5] that, when the heat capacity of the sample changes rather rapidly with the temperature, meaningfu/ errors are made in the determination if effects of thermal lag are not properly considered. Similar errors are possible in purity measurements, since the melting Curve of an impure substance can be described by a characteristic specific heat function [7,11 ].…”