Modulated differential scanning calorimetry

Modulated-temperature differential scanning calorimetry was used to measure the glass transition temperature,T _g, the heat capacity relaxation in the glassy state and the increment of heat capacity, C_p, in the glass transition region for several polymers. The differential of heat capacity with respect to temperature was used to analyseT _g and C_p simply and accurately. These measurements are not affected by complex thermal histories.     

Modulated differential scanning calorimetry

The Modulated Differential Calorimetry (MDSC) is applied to the determination of the reversibility in the cholesteryl chloride, which presents a cholesteric monotropic phase between the isotropic and crystalline states. The experimental modulation parameters that govern this method i.e. frequency, amplitude and heating/cooling rate, are determined. MDSC curves and complementary thermomicroscopical observations assign melting, crystallization and liquid cholesteric transition as non reversing, and clarification as reversing.     

Multicycle differential scanning calorimetry

Multicycle Differential Scanning Calorimetry (MCDSC) is a procedure where repeated temperature cycles are executed and the measured data are superimposed for a selected number of cycles. Temperature cycles with a single sample are executed under selected experimental conditions in one of these procedures, namely, the MCDSC_s. The second one, MCDSC_m is a procedure in which every identical temperature cycle starts with a new sample of the same substance of a similar mass. The procedure MCDSC_s using the same sample for a number of cycles is only applicable for substances and materials which are chemically and physically stable under the selected experimental conditions. The application of MCDSC enhances two extremely important qualities of a DSC measurement, namely, the sensitivity and the statistical base, both qualities with respect to the final data elucidated. Another possibility by MCDSC also related to the enhanced sensitivity can lead the discovery of a phenomenon which hitherto has not been observed. The most important result of any MCDSC application is the determination of the mean DSC curve within the temperature interval of interest by superimposing the single curves point by point and by the division of the calorimetric values obtained with the number of scans evaluated. The signal-to-noise-ratio (SNR) for the mean curve can be compared with the value determined for one or even for all the single curves measured yielding the improvement factor achieved with a MCDSC measurement. This experimentally determined improvement of the SNR can be compared with the value given on a statistical consideration by Gauss as the square root of the number of cycles evaluated. The main aims of this article are to prove the practical application of the procedure and the efficiency in case of rather small sample masses. Substances were selected with known enthalpy transitions and, in addition, polystyrene was taken for a determination of the data for the glass transition by MCDSC. Rather small sample masses in the order of micrograms as well as the experimental conditions have been selected for the measurements with 4,4′-azoxyanisole and n-hexatriacontane with the expectation to get a value of SNR for the single curves of about unity or even below. Two aims should be achieved with these experiments. First, the multicycle procedures and the data evaluation developed should be capable of establishing, after performing of a certain number of cycles, a mean curve showing an improvement over the SNR with respect to the single curves. Second, we should be able to get a rough estimation of the lower limit of the SNR for a single curve, below the instrumental noise level of the DSC used, necessary to achieve with a MCDSC experiment a mean curve with a clearly visible peak.     

Modulated differential scanning calorimetry

Modulated DSC^TM (MDSC) is a new, patent-pending extension to conventional DSC which provides information about the reversing and nonreversing characteristics of thermal events, as well as the ability to directly measure heat capacity. This additional information aids interpretation and allows unique insights into the structure and behaviour of materials., A number of examples of its use are described.     

Thermal gradients in differential scanning calorimetry

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%.     

Modulated temperature differential scanning calorimetry

Modulated temperature differential scanning calorimetry (MTDSC) is used to study simultaneously the evolution of heat flow and heat capacity for the isothermal and non-isothermal cure of an epoxy-anhydride thermosetting system. Modelling of the (heat flow related) chemical kinetics and the (heat capacity related) mobility factor contributes to a quantitative construction of Temperature-Time-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams for the thermosetting system.     

Quantitative aspects of differential scanning calorimetry

The quantitative performance of differential scanning calorimeters is reviewed. Temperature calibration is discussed in terms of an isothermal correction plus a contribution from thermal lag, this can be derived from individual curves and is valid in both, heating and cooling. It is emphasised that baselines that are drawn to thermal events, such as melting and transition phenomena, must have thermodynamic significance and a general procedure is suggested. When this is used, a power compensation calorimeter calibrated for heat-capacity work can reproduce heats of fusion and transition for a diverse range of materials to better than 1%.     

Use of cyclic differential scanning calorimetry

Formation of an activated cellulose (Cellulose) species $$CELLULOSE\xrightarrow[{air}]{{heat}}CELLULOSE*$$ is the designated first stage of cellulose degradation in air [1]. Little is known about either the process or the nature of CELLULOSE^*. The transition, designatedT ₂, is observed as an exotherm around 300°C as the sample temperature is raised. No corresponding endotherm is observed on cooling. The process is therefore not reversible but is repeatable as subsequent reheating results in the exotherm being observed again. The exotherm is also found to be oxygen dependant. The effect of all the flame retardant treatments studied was to reduceT ₂ compared to the value for the untreated cotton.     

Purity determination by differential scanning calorimetry

A review of the literature on the DSC method for purity determination is presented, with a discussion of the most important aspects, i.e. theory, sample handling, calibration of the instrument, evaluation of melting curves, and the conditions and accuracy of the measurement of eutectic impurities.     

Design and development in self-reference differential scanning calorimetry

An intermediate range (50–1000°C) self-referencing differential scanning calorimeter (SR-DSC) has been built and its performance evaluated. The SR-DSC measures heat flow across a heat flow metal plate, and any changes to the heat flow caused by a thermal transition occurring in a centrally placed sample is monitored by a temperature difference across the plate. The criteria for high sensitivity are that the circular plate should be as thin as possible and have a low thermal conductivity. The best sensitivity conducive with robust behaviour was achieved with an inconel thermal plate of uniform thickness, 75 m, this gave reproducible results, and the enthalpy of the thermal transition was proportional to sample mass. Calorimeter sensitivity decreased with increasing temperature and a sloped baseline was observed. Both of these effects can be corrected mathematically. An example of the use of the SR-DSC in polymer characterisation was limited to a study of the physical ageing of PET.This revised version was published online in November 2005 with corrections to the Cover Date.Paper was read at the TAC2001 Conference in Liverpool.     

Origins of endothermic peaks in differential scanning calorimetry

The origins of multiple peaks observed by differential scanning calorimetry have been examined in detail for a linear polyethylene fraction crystallized from dilute solution and for bulk-crystallized copolymers of ethylene. At least two major bases are demonstrated. One is melting–recrystallization; the other a consequence of the distribution of crystallite sizes. The melting–recrystallization process, however, often only yields one endothermic peak. This peak is therefore not characteristic of the original crystallites present. In these situations complementary methods need to be used to determine the melting temperature. We have complemented the calorimetric studies with measurement of the crystallite size distribution as determined from the Raman low frequency acoustical mode spectra. A major increase occurs cooperatively in the crystallite size distribution during the initial melting. Most importantly, we have also been able to make a direct comparison between the interfacial free energies of thin crystallites formed from dilute solution and from bulk-crystallized linear polyethylene.     

The limit of detection in differential scanning calorimetry

A procedure is described to determine the limit of detection of DSC instruments by using tiny signals from spontaneous polymorphic transitions of CsCl, K₂Cr₂O₇ and Na₂SO₄. It is shown how such signals can be found well-resolved in DSC diagrams of powder samples. To distinguish them from the baseline noise they should exhibit a height at least twice that of the baseline width. For the instrument employed the corresponding smallest amount of heat, i.e., the limit of detection, was found to be 0.1 mJ.The authors thank Mr. H. Maltry for technical help and the Deutsche Forschungsgemeinschaft for support.     

Problems of non-linear theory in differential scanning calorimetry

The problems of DSC non-linear theory, connected with the dependence of the thermophysical parameters upon the temperature, are discussed. The changes in thle form of the thermal energy curve recorded by the device as results of the thermaconductivity jump and heat capacity shift taken into account in the course of transformation are shown. A correct non-linear DSC model is formulated. The mather matical apparatus and some simplifying notions for calculation with the non-lineamodel are suggested. The interpretation of a calorimetric curve of an adiabatic scan. ning microcalorimeter over a pretransition range of temperatures is given as an example

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Zusammenfassung Der Beitrag erörtert das Problem der nicht-linearen DSC-Theorie im Zusammenhang mit der Abhängigkeit der thermophysikalischen Parameter von der Temperatur. Die Änderungen der durch das Gerät registrierten Form der Wärme-Kraft-Kurve rühren, wie gezeigt wird, von einer jähen Änderung der Wärmeleitfähigkeit her und Änderungen der Wärmekapazität spielen im Laufe des Prozesses ebenfalls mit. Ein korrektes nicht-lineares DSC-Modell wird formuliert. Der mathematische Apparat und einige vereinfachende Begriffe zur Errechnung des nicht-linearen Modells werden vorgeschlagen. Als Beispiel wird die Interpretation einer kalorimetrischen Kurve eines adiabatischen Scanning-Mikrokalorimeters gegeben, das den Vor-Übergangs-Bereich von Temperaturen erfaßt.

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Résumé L'article discute les problèmes concernant la théorie de l'analyse calorimétrique différentielle (DSC) non linéaire, en rapport avec le fait que les paramètres thermophysiques dépendent de la température. On montre que les variations de forme de la courbe enregistrée par l'instrument résultent d'un saut de conductibilité thermique et d'un changement de chaleur spécifique lors de la transformation. On donne un modèle correct de DSC non linéaire. On propose un traitement mathématique et quelques notions simplificatrices pour le calcul du modèle non linéaire. On donne comme exemple l'interprétation d'une courbe; calorimétrique fournie par un analyseur microcalorimétrique adiabatique, dans le domaine de température précédant la transition.

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Starch-lipid interactions studied by differential scanning calorimetry

The amylose-lipid complex shows an endothermic transition around 100 °C in excess water. Complexes were prepared by adding lipids to an amylose-solution, and the precipitated complex was studied in the DSC during a heating and cooling sequence. The thermal stability of the complex depends on the lipid part, and the reversibility during cooling depends on presence of excess lipids.The influence of lipids on the gelatinization of starch was studied by adding lipids to wheat and potato starch, respectively, before the DSC-analysis. Depending on the lipid, an earlier as well as a delayed gelatinisation could be obtained.     

Gas evolution as an artefact in differential scanning calorimetry

With CuSO₄·5H₂O as test system, it is demonstrated that the evolution of a gaseous product with thermal conductivity much lower than that of the purge gas in differential scanning calorimetry, leads to artefacts in the form of intense exotherms. This is due to a reduction of the heat flow from the sample to the environment.We are grateful to the Forschungsförderungsfonds of Austria for financial support.     

Modulated differential scanning calorimetry in the glass transition region

Temperature-modulated differential scanning calorimetry (TMDSC) is based on heat flow and represents a linear system for the measurement of heat capacity. As long as the measurements are carried out close to steady state and only a negligible temperature gradient exists within the sample, quantitative data can be gathered as a function of modulation frequency. Applied to the glass transition, such measurements permit the determination the kinetic parameters of the material. Based on either the hole theory of liquids or irreversible thermodynamics, the necessary equations are derived to describe the apparent heat capacity as a function of frequency.Presented in part at the 24th Conference of the Northamerican Thermal Analysis Society, San Francisco, CA, September 10–13, 1995.     

Modulated differential scanning calorimetry in the glass transition region

Temperature-modulated calorimetry (TMC) allows the experimental evaluation of the kinetic parameters of the glass transition from quasi-isothermal experiments. In this paper, model calculations based on experimental data are presented for the total and reversing apparent heat capacities on heating and cooling through the glass transition region as a function of heating rate and modulation frequency for the modulated differential scanning calorimeter (MDSC). Amorphous poly(ethylene terephthalate) (PET) is used as the example polymer and a simple first-order kinetics is fitted to the data. The total heat flow carries the hysteresis information (enthalpy relaxation, thermal history) and indications of changes in modulation frequency due to the glass transition. The reversing heat flow permits the assessment of the first and higher harmonics of the apparent heat capacities. The computations are carried out by numerical integrations with up to 5000 steps. Comparisons of the calculations with experiments are possible. As one moves further from equilibrium, i.e. the liquid state, cooperative kinetics must be used to match model and experiment.On leave from Toray Industries, Inc., Otsu, Shiga 520, Japan.This work was supported by the Division of Materials Research, National Science Foundation, Polymers Program, Grant # DMR 90-00520 and the Division of Materials Sciences, Office of Basic Energy Sciences, U. S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U. S. Department of Energy, under contract number DE-AC05-96OR22464. Support for instrumentation came from TA Instruments, Inc. Research support was also given by ICI Paints, and Toray Industries, Inc.     

Morphology characterization of emulsions by differential scanning calorimetry

This article is a review of some results obtained by Differential Scanning Calorimetry (DSC) for characterizing the morphology of emulsions. In a classical DSC experiment, an emulsion sample is submitted to a regular cooling and heating cycle between temperatures that include freezing and melting of the dispersed droplets. By using the thermograms found in the literature for various emulsions, how to get information about the solidification and melting, the presence of solute, the emulsion type, the transfer of matter, the stability and the droplet size is shown.