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Abstract  

The response of temperature-modulated differential scanning calorimetry (TMDSC) to irreversible crystallization of linear polymers was investigated by model calculations and compared to a number of measurements. Four different exotherms were added to a typical modulated, reversible heat-flow rate in order to simulate irreversible crystallization. It was found that the reversing heat-flow rate of the TMDSC in response to such irreversible crystallization exotherms is strongly affected by tbe shape of the transition and the phase-angle where the exotherm occurs. A comparison with the experimental data gave valuable insight into the transitions, as well as the nature of the TMDSC response which is usually limited to an analysis of the first harmonic term of the Fourier series that describes the heat-flow rate.

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The results from temperature modulated DSC in the glass transition region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent glass transition temperature can be fitted with a VFT-equation. The transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The glass transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.

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Abstract  

ADSC with its periodical temperature programs combines the features of DSC measured at high heating rate (high sensitivity) with those at low heating rate (high temperature resolution). In addition, the “reversing” cp effects can be separated from the “non-reversing” latent heat effects. Various periodical temperature programs can be applied. This paper compares the different possible temperature programs and their algorithms for the cp determination for metal, metal oxide and polymer of various properties. Simulated and measured results for various wave shapes and samples are presented. The relevant sample properties and their influence on the measurements are identified and guiding rules for the proper choice of the various experimental parameters are given. Measurements with different samples, performed with the new METTLER TOLEDO STARe-System, are shown and compared with the simulation results. The simulations and the measurements clearly show that the alternating techniques can yield new information about sample properties, but are susceptible to the proper choice of the various experimental parameters.

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Abstract  

Electron paramagnetic resonance (EPR, ST-EPR) and differential scanning calorimetry(DSC) were used in conventional and temperature modulated mode to study internal motions and energetics of myosin in skeletal muscle fibres in different states of the actomyosin ATPase cycle. Psoas muscle fibres from rabbit were spin-labelled with an isothiocyanate-based probe molecule at the reactive sulfhydryl site (Cys-707) of the catalytic domain of myosin. In the presence of nucleotides (ATP, ADP, AMP⋅PNP) and ATP or ADP plus orthovanadate, the conventional EPR spectra showed changes in the ordering of the probe molecules in fibres. In MgADP state a new distribution appeared; ATP plus orthovanadate increased the orientational disorder of myosin heads, a random population of spin labels was superimposed on the ADP-like spectrum. In the complex DSC pattern, higher transition referred to the head region of myosin. The enthalpy of the thermal unfolding depended on the nucleotides, the conversion from a strongly attached state of myosin to actin to a weakly binding state was accompanied with an increase of the transition temperature which was due to the change of the affinity of nucleotide binding to myosin. This was more pronounced in TMDSC mode, indicating that the strong-binding state and rigor state differ energetically from each other. The different transition temperatures indicated alterations in the internal microstructure of myosin head region The monoton decreasing TMDSC heat capacities show that C p of biological samples should not be temperature independent.

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Modulated differential scanning calorimetry (MDSC) uses an abbreviated Fourier transformation ≼r the data analysis and separation of the reversing component of the heat flow and temperature signals. In this paper a simple spread-sheet analysis will be presented that can be used to better understand and explore the effects observed in MDSC and their link to actual changes in the instrument and sample. The analysis assumes that instrument lags and other kinetic effects are either avoided or corrected for.

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Abstract  

Calorimetry deals with the energetics of atoms, molecules, and phases and can be used to gather experimental details about one of the two roots of our knowledge about matter. The other root is structural science. Both are understood from the microscopic to the macroscopic scale, but the effort to learn about calorimetry has lagged behind structural science. Although equilibrium thermodynamics is well known, one has learned in the past little about metastable and unstable states. Similarly, Dalton made early progress to describe phases as aggregates of molecules. The existence of macromolecules that consist of as many atoms as are needed to establish a phase have led, however, to confusion between colloids (collections of microphases) and macromolecules which may participate in several micro- or nanophases. This fact that macromolecules can be as large or larger than phases was first established by Staudinger as late as 1920. Both fields, calorimetry and macromolecular science, found many solutions for the understanding of metastable and unstable states. The learning of modern solutions to the problems of materials characterization by calorimetry is the topic of this paper.

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Abstract  

Contributions of modern, temperature-modulated calorimetry are qualitatively and quantitatively discussed. The limitations are summarized, and it is shown that their understanding leads to new advances in instrumentation and measurement. The new thermal analysis experiments allow to separate reversing from irreversible processes. This opens the irreversible states and transitions to a description in terms of equilibrium and irreversible thermodynamics. Amorphous systems can be treated frommacroscopic to nanometer sizes with weak to strong coupling between neighboring phases. Semicrystalline, macromolecular systems are understood on the basis of modulated calorimetry as globally metastable, micro-to-nanophase-separated systems with locally reversible transitions.

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Modulated differential scanning calorimetry in the glass transition region

VI. Model calculations based on poly(ethylene terephthalate)

Journal of Thermal Analysis and Calorimetry
Authors: B. Wunderlich and I. Okazaki

Abstract  

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.

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.1016/0040-6031(95)90466-2 . 9. Wunderlich , B , Okazaki , I , Ishikiriyama , K , Boller , A . Melting by temperature-modulated calorimetry . Thermochim Acta . 1998 ; 324 : 77 – 85 . 10.1016/S0040

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