Thermal desorption analysis:

Experiments on water desorption from silicate glass powders of different chemical compositions were carried out by means of DSC. In order to establish the desorption energy distributions, a model of the heterogeneous surface was applied to the thermoanalytical desorption curves, and the results are discussed together with contact angle measurements on flat polished glass samples.The maxima in the desorption curves shift to lower temperature with increasing hydrophobic character; this is caused by divalent cations such as Pb²⁺, Ca²⁺ and Zn²⁺, while alkali metal ions are responsible for an increase in the hydrophilic behaviour.

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Zusammenfassung Wasserdesorptionsexperimente an Silikatglaspulvern unterschiedlicher Zusammensetzung wurden mittels DSC ausgeführt. Für die Berechnung von Desorptionsenergieverteilungen wurde ein Modell der heterogenen Oberfläche genutzt und zusammen mit Randwinkelmessungen an glatten, polierten Glasoberflächen diskutiert. Das Maximum der Desorptionskurven verschiebt sich mit wachsendem hydrophoben Verhalten nach tieferen Temperaturen, wofür divalente Kationen wie Pb²⁺ Ca²⁺ Zn²⁺ verantwortlich sind, während Alkaliionen das hydrophile Verhalten begünstigen.

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A simple and effective method for smoothing DSC curves

All known procedures of smoothing are limited by the beginning of falsification of the signals. We suggest a simple modification of the moving average of the polynomial smoothing procedure. The signal curve is smoothed to the best degree and independently of the special problem within an interval calculated from then-th multiple of the standard deviation of the noise.

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Zusammenfassung In einigen Fällen (Bildung von Ableitungen, Entschmierungsrechnungen, Auswertung schwacher Effecte) ist auch bei der DSC-Technik eine Glättung des primären Signals angebracht. Alle bekannten Verfahren beinhalten immer einen Kompromiß zwischen erreichter Glättung und beginnender, nicht mehr zu vernachlässigender Verfälschung der Analysenergebnisse. Zudem erfordert jedes Problem seine eigenen optimalen Glättungsparameter. Wir schlagen eine einfache und universell anwendbare Modifikation des Verfahrens mit gleitender Mittelwertsbildung bzw. unter Verwendung der Golay-Sawitzky-Filter vor. In einem Streifen, der aus einem vorgebbaren Vielfachen der Standardabweichung des Rauschens gebildet wird, erhält man so eine optimale Glättung.

     

Kinetics and mechanism of the epoxide-amine polyaddition

The mechanism and kinetics of the epoxide-amine polyaddition reaction have been studied by isothermal and scanning DSC measurements. The initial concentrations of the reactants (epoxides: bisphenol-A-diglycidyl ether (DGEBA) and phenyl glycidyl ether (PGE), amines: N,N′-dibenzylethylenediamine (DBED) and aniline) in our model systems have been strongly varied. The suggested kinetic model describes the reaction behavior of mixtures with any initial epoxide/amine ratios over the whole range of cure by a single parameter set. To find the optimum kinetic parameters, we have solved the set of differential equations numerically by the technique of multivariate non-linear regression (Mult-NLR). Excellent agreement was obtained between calculated and experimental curves.     

DSC Investigations on a Commercial Photocuring Sheet Material

The photopolymerization of a commercial sheet material as used in the dental laboratory was investigated by using the photocalorimeter DSC7/DPA7 (Perkin-Elmer). Using normal reaction conditions (light wavelength 400 nm, light intensity 1 mW cm^–2 and sample thickness 2.5 mm), the curing was completed within few min. The reaction rate was nearly independent on the light wavelength in the range between =300 and 380 nm, but decreased distinctively at longer wavelengths. After desmearing the DSC curves, the dependence of the reaction rate on light intensity and sample thickness could be described in very good approximation by an equation derived by Tryson [1]. Comparable with pure acrylates or methacrylates, a pronounced dark reaction was found after interruption of the illumination.This revised version was published online in November 2005 with corrections to the Cover Date.     

Investigations on the thermal decomposition of 1,2-dioxetanes via DSC

Reproducible measurements, reliable results and 1^st order kinetics for the whole reaction are obtained during the thermal decomposition of dioxetanes only, if inclusions of impurities in commercial sample pans are blocked by additional thick (magnitude of 100 ?m) and close Al₂O₃ protective layers. As a rule, nearly the same activation parameters are then found both for the decomposition of solvent-free dioxetanes and diluted solutions in several solvents. Mixtures of different dioxetanes in the same solvent contribute independently to the overall heat flow rate.

     

Physikalisch-chemische Untersuchungen am System Natriumbenzoat/Benzoesäure

Journal of Thermal Analysis and Calorimetry - Mittels der Differential Scanning Calorimetry und der temperaturabhängigen Röntgenbeugung wurde das Phasenverhalten des Systems...     

The cis-trans Isomerization of Azobenzene in the Molten State: A useful test reaction for the kinetic evaluation of DSC measurements

Contrary to the situation in the field of temperature, heat and heat flow rate calibration, so far no generally accepted and easily practicable chemical reaction exists with regard to a kinetic evaluation. A possible reaction would be the well-known first-order cis-trans isomerization of subcooled liquid azobenzene. Surprisingly, the evaluation of measurements performed with a power compensated calorimeter yields activation parameters, which are dependent on the heating rate. The desmearing of the curves does not produce any improvements. However, constant activation parameters are obtained, if a small self-heating of the sample during the exothermic reaction is taken into account.This revised version was published online in November 2005 with corrections to the Cover Date.     

Investigation of thermally activated or photoinitiated polymerization by DSC

Differential scanning calorimetry is undoubtedly one of the most powerful techniques for investigating the reactions leading to thermoplastic or thermosetting materials. The conditions for the obtaining of reliable experimental results are summarized. A mechanism for the curing of epoxyamine systems is suggested, which is capable of describing the behaviour of stoichiometric and nonstoichiometric mixtures up to high reaction degrees. The advantage of coupling a DSC instrument with a device for the photochemical activation of polymerizations is demonstrated by some examples.

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Zusammenfassung Die Differential-Scanning-Kalorimetrie (DSC) gehört ohne Zweifel zu den leistungsfähigsten Methoden der Untersuchung von Bildungsreaktionen thermoplastischer und thermisch härtbarer Substanzen. Die Bedingungen zur Gewinnung zuverlässiger experimenteller Ergebnisse werden zusammengestellt. Für die Härtung von Epoxid-Amin-Systemen wird ein Mechanismus vorgeschlagen, der das Verhalten stöchiometrischer und nichtstöchiometrischer Mischungen bis zu hohen Umsätzen beschreiben kann. Die Vorzüge der Kopplung eines DSC-Instruments mit einer Einrichtung für die photochemische Aktivierung von Polymerisationen wird an einigen Beispielen demonstriert.

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Investigations of photopolymerization by means of intermittent illumination

An apparatus is described for intermittent illumination of the sample in a DSC-2C (Perkin-Elmer) modified with some additional optical equipment. A slotted disk is used as a light chopper. This is driven by a computer-controlled stepping motor. The various frequencies needed for investigations of this kind are derived from a computer quartz-controlled clock frequency by means of software. The operator dialogue is achieved through the Perkin-Elmer Data Station to simplify the use of the equipment.

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Zusammenfassung Mit Hilfe einiger optischer Ergänzungsausrüstungen wurde ein DSC-2C (Perkin-Elmer) Gerät modifiziert, um Proben unterbrochen belichten zu können. Als Lichtunterbrecher diente eine Lochscheibe, die von einem computerkontrollierten Schrittmotor getrieben wurde. Die verschiedenen Frequenzen, die zu Untersuchungen dieser Art notwendig sind, werden softwaregestützt aus der quarzgesteuerten Uhrenfrequenz des Computers gewonnen. Zur Vereinfachung der Kommunikation mit dem System geschieht diese über die Perkin-Elmer Data Station.

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