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Abstract  

The results of the StepScan DSC obtained for 15Na2O⋅xMgO⋅(10–x)CaO⋅75SiO2 glasses were described in the frame of the commonly accepted theory of the glass transition. A new simplified model of the reversible part of StepScan DSC record was developed on the basis of the Tool Narayanaswamy Moynihan relaxation theory. Equivalence between the formal activation energy of enthalpy relaxation process on one side, and the viscous flow activation enthalpy on the other side, was found.

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Journal of Thermal Analysis and Calorimetry
Authors: Marek Liška, Zdenek Černošek, Mária Chromčíková, Jana Holubová, Eva Černošková, and Libor Vozár

Abstract  

The results of StepScan DSC obtained for various oxides, chalcogenides, and organic glasses are discussed in connection with the commonly accepted theory of the glass transition. The new experimental features supporting the apparent idea of a reversible equilibrium being a part of the glass transition that is commonly interpreted as purely kinetic-relaxation phenomenon are discussed. Two alternative methods of the description of the reversible part of StepScan DSC record are compared:the empirical one using the exponential-power function [1 − exp(T/T g)n], and the second one based on the van’t Hoff’s equation describing the temperature dependence of equilibrium constant in terms of reaction enthalpy, ΔH. The adequacy of the empirical description is rationalized in the framework of the Tool–Narayanaswamy–Moynihan’s relaxation theory.

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Abstract  

The crystallisation kinetics, melting behaviour and morphology, of bacterial poly(3-hydoxybutyrate) (PHB) have been investigated by using differential scanning calorimetry (DSC), step-scan DSC (SDSC), wide angle X-ray diffraction (WAXRD) and hot stage polarised optical microscopy (HSPOM). DSC imparted isothermal crystallisation thermal history. The subsequent melting behaviour revealed that all PHB materials experienced secondary crystallisation during heating and the extent of secondary crystallisation varied depending on the crystallisation temperature. PHB samples were found to exhibit double melting behaviour due to melting of SDSC scan-induced secondary crystals, while considerable secondary crystallisation or annealing took place under the modulated heating conditions. The overall melting behaviour was rationalised in terms of recrystallisation and/or annealing of crystals. Interestingly, the PHB materials analysed by SDSC showed a broad exotherm before the melting peak in the non-reversing curve and a multiple melting peak reversing curve, verifying that the melting-recrystallisation and remelting process was operative. HSOM studies supported the conclusions from DSC that the radial growth rate of the PHB spherulites was significantly varied upon the crystallisation conditions. One form of crystals was shown by WAXRD from isothermally crystallised PHB.

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data. StepScan DSC, as a relatively new calorimetric method, can give similar results to those obtained by other methods of MDSC group techniques. By this method it is possible to split a global calorimetric signal in two signals that can be

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T 0 is the initial temperature and β 0 is the average heating rate. Another mT-DSC method, the called step-scan DSC (SDSC), has recently become available [ 12 ]. SDSC utilizes a heating-isothermal (or cooling-isothermal) program, where the

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Abstract  

A method was developed for the quantification of low levels of amorphous content in maltitol with StepScan DSC. The method was based on the fact that the change of specific heat at the glass transition is linearly proportional to the amorphous content. The influence of different measurement parameters of StepScan DSC was evaluated and two different calibration heating rates were tested. Synthetic mixtures with various proportions of crystalline and amorphous maltitol were prepared. Two different measurement methods were compared and the linear regression between ΔC p and amorphous content was obtained. The limit of detection (LOD) and the limit of quantification (LOQ) values were for the fictive temperature 0.24% (amorphous content) and 0.81% and for the half point temperature 0.27 and 0.92%, respectively, (method 1) and for the fictive temperature 0.18 (amorphous content) and 0.61% and for the half point temperature 0.16 and 0.52%, respectively (method 2). Very low determination limits for the quantification of amorphous content could be attained with the StepScan DSC method. However, the realistic limit of quantification was somewhat higher (about 3%) because of noise in the StepScan measurement. The main advantage of the StepScan DSC method for quantification of amorphous content was that the glass transition and relaxation peaks are separated into different curves and the interpretation becomes easier.

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Abstract  

Two enantiomeric forms of xylose were identified as α-D-xylopyranose and α-L-xylopyranose by powder diffraction. Their melting behaviour was studied with conventional DSC and StepScan DSC method, the decomposition was studied with TG and evolved gases were analyzed with combined TG-FTIR technique. The measurements were performed at different heating rates. The decomposition of xylose samples took place in four steps and the main evolved gases were H2O, CO2 and furans. The initial temperature of TG measurements and the onset and peak temperatures of DSC measurements were moved to higher temperatures as heating rates were increased. The decomposition of L-xylose started at slightly higher temperatures than that of D-xylose and L-xylose melted at higher temperatures than D-xylose. The differences were more obvious at low heating rates. There were also differences in the melting temperatures among different samples of the same sugar. The StepScan measurements showed that the kinetic part of melting was considerable. The melting of xylose was anomalous because, besides the melting, also partial thermal decomposition and mutarotation occurred. The melting points are affected by both the method of determination and the origin and quality of samples. Melting point analysis with a standardized method appears to be a good measure of the quality of crystalline xylose. However, the melting point alone cannot be used for the identification of xylose samples in all cases.

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Abstract  

Differential scanning calorimetry (DSC) is one of the most frequently used techniques for analyzing small concentrations of amorphous phases in a crystalline matrix. In recent years novel enhanced DSC approaches have been intensively looked for to improve parameters such as sensitivity, accuracy, and detection limit of the technique. Low levels of amorphous phases can be quantitatively determined in DSC by measuring the heat capacity change associated with the glass transition. In this short review the potentials provided by the HyperDSC and StepScan DSC techniques are discussed. Examples illustrate the advantages and disadvantages of the techniques and compare their abilities to detect small glass transitions and determine low contents of amorphous phases in samples which are mostly crystalline.

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.71073 Å). Thermoanalytical properties were recorded by power-compensated differential scanning calorimeter Diamond (Perkin-Elmer) using both conventional and StepScan DSC modes. Temperature and enthalpy calibrations of the calorimeter were

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Journal of Thermal Analysis and Calorimetry
Authors: Martin Tjahjono, Martin Karl Schreyer, Liangfeng Guo, and Marc Garland

measurements were carried out using a step-scan temperature increase from 297.65 to 298.65 K with a slow scanning rate of 0.083 K h −1 . The slow scanning rate was used to minimize thermal gradients occurring in the samples. The baselines were taken 1800 s

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