High-speed/high performance differential scanning calorimetry (HPer DSC): Temperature calibration in the heating and cooling mode and minimization of thermal lag

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Abstract

The temperature calibration of high performance DSC in the heating and cooling mode is discussed. Several primary and secondary calibration standards are studied at different sample masses – from 0.4 μg to 10 mg – and various heating and cooling rates – from 1 to 500 °C/min and from 1 to 300 °C/min, respectively. The experimental onset and peak temperatures of Indium with different sample masses are measured at different heating rates and the two related correction factors are presented. The symmetry of the HPer DSC with respect to the cooling and the heating modes is checked and found to be good. The liquid crystals M24, HP-53 and BCH-52, being substances with no or very small supercoolings, are recommended as secondary standards for temperature calibration in both the cooling and heating mode. In order to verify whether the proposed correction factors of Indium can also be used in the cooling mode, the melting behavior of Indium and the phase transition temperatures of the secondary standards obtained in heating are compared, and it turns out that the latter are usable as well. Finally, recommendations for both an extensive temperature calibration and a rather quick calibration procedure of scanning calorimeters in the heating and the cooling mode for various sample masses and various rates are given. The calibration procedure developed for HPer DSC facilitates making the right choices to minimize the thermal lag with respect to the sample mass and scan rates at the start of the measurement, instead of just making corrections afterwards.

Introduction

A relatively new high-speed calorimetry technology, high performance DSC (HPer DSC; see http://www.scite.nl), has recently been discussed for practical use and since then marketed by PerkinElmer under the name HyperDSC™, see http://www.hyperdsc.com. Mathot and co-workers published the detailed characteristics and use of this new mode of measurement, which represents a major step forward in high-speed calorimetry, as compared to standard DSC [1]. Controlled and constant scan rates of hundreds of degrees per minute and combinations thereof both in the cooling and heating mode are possible. Heats of transition, heat capacities, temperature-dependent crystallinities, etc. can be established at the extreme rates applied. The short measuring times also provide the high throughput needed in combinatorial chemistry.

Application fields concerning HPer DSC are situated in the study of the kinetics and metastability of macromolecular and pharmaceutical systems, particularly in the analysis of rate-dependent phenomena at real temperature–time conditions. HPer DSC is very much suited to investigate kinetics of processes like crystallization, cold crystallization, recrystallization, annealing, and solid-state transformations in polymers, pharmaceuticals and liquid crystals. (Sub)milligram amounts of material can be investigated at very high, controlled cooling and heating rates of hundreds of degrees per minute, which facilitates the analysis of films, expensive and extraordinary products, inhomogeneities in materials, etc. High cooling rates need to be applied to simulate processing conditions like in film blow molding, injection molding and extrusion. It turns out that for some processing techniques the cooling rate can be mimicked by HPer DSC. Measurements concerning metastability and kinetics are also necessary to (re)connect heating behavior with cooling history.

This article discusses the fundamental requirements for a universally applicable and unified correct temperature calibration in the heating and cooling mode of high performance differential scanning calorimeters. High-speed calorimeters need a new approach for calibration, in order to deal with different high heating and cooling rates and the adjustment needed with respect to the sample masses. An important additional goal is to provide procedures to optimize experimental conditions – especially to minimize thermal lag – before starting the measurement.

Calibration has to be performed to correct for the thermal lag: increasing sample mass/size increases the thermal lag of sample temperature versus sensor temperature and in addition gives risk to temperature gradients within the sample resulting in peak broadening, while increasing the heating and cooling rates increases the thermal lag of the sensor temperature with respect to the furnace temperature. To characterize the thermal lags in the system and sample, it is useful to differentiate between an outside thermal resistance: Ro, and a thermal resistance within the sample: Rs. The outside thermal resistance consists of a thermal resistance related to the instrument, Ri, and a thermal contact resistance between the container and the instrument, Rc. The total thermal resistance, R, is defined as the sum of Ro and Rs. The temperature calibration of HPer DSC will be done in subsequent steps: first a regular calibration will be performed with primary standards, and then a calibration matrix with respect to scan rate and sample mass will be developed in the heating mode. The symmetry of the HPer DSC furnaces will be checked using secondary standards, by working both in the cooling and heating mode. Finally, the results of the secondary calibrants obtained in heating are compared to the melting characteristics of Indium.

Section snippets

Experimental

Measurements were performed with a modified PerkinElmer Pyris 1 calorimeter having software version 7.0. This power-compensation DSC was selected to be used as an HPer DSC because its furnace has low mass and small dimensions, ensuring a much faster heat transfer than in the existing commercial heat-flux calorimeters. The small gap between the furnace and the aluminum cooling system, which is even reduced by guard ring inserts, promotes effective cooling. The HPer DSC was cooled with a cryofill

Temperature calibration

A correct way to perform temperature calibration for the HPer DSC starts with a regular calibration over a wide temperature range using primary standards, which are selected on the basis of their phase transition temperatures situated in the temperature range of interest. Thereafter, a calibration matrix is set-up for different heating rates and various sample masses on the basis of one primary standard. Once the calibration in the heating mode has been performed, the symmetry of the

Conclusions

In this article temperature calibration of scanning calorimeters in the heating and the cooling mode for various sample masses and various scan rates is discussed. Besides primary calibration standards like Indium, usable for calibration in heating, three secondary calibration standards, M24, HP-53 and BCH-52, were studied and found to be suitable for calibration in the cooling mode.

Finally, recommendations are postulated concerning the temperature calibration of scanning calorimeters in both

Acknowledgements

This work was performed in the framework of a European Union Marie Curie Industry Host Fellowship of Geert Vanden Poel: contract No. HPMI-CT-2002-00180. All measurements were performed at DSM Research in The Netherlands. The authors appreciate the contribution of Tom Hillegers of the Mathematical and Statistical group of DSM Research, and the discussions with Thijs Pijpers, Katholieke Universiteit Leuven, België.

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