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The modulated differential scanning calorimetry (MDSC) technique superimposes upon the conventional DSC heating rate a sinusoidally varying modulation. The result of this modulation of the heating rate is a periodically varying heat flow, which can be analysed in various ways. In particular, MDSC yields two components (‘reversing’ and ‘non reversing’) of the heat flow, and a phase angle. These each show a characteristic behaviour in the glass transition region, but their interpretation has hitherto been unclear. The present work clarifies this situation by a theoretical analysis of the technique of MDSC, which introduces a kinetic response of the glass in the transition region. This analysis is able to describe all the usual features observed by MDSC in the glass transition region. In addition, the model is also able to predict the effects of the modulation variables, and some of these are discussed briefly.

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Abstract  

Radium-228 was separated from aged thorium nitrate by liquid-liquid, two-phase, extraction and extensively purified, principally by ion-exchange chromatography. The radioactivity concentration of the purified radium-228 was measured by means of liquid-scintillation β-particle measurements of the228Ac daughter (corrected for progeny ingrowth). The results were confirmed by Ge(Li)well-detector intercomparison with radium-228 in equilibrium with its thorium-232 precursor which had been measured gravimetrically. Three hundred ampoules were provided to the U.S. Environmental Protection Agency, Las Vegas, for distribution.

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Vitrification during the isothermal cure of thermosets

Part I. An investigation using TOPEM, a new temperature modulated technique

Journal of Thermal Analysis and Calorimetry
Authors: I. Fraga, S. Montserrat, and J. Hutchinson

Abstract  

The process of vitrification that occurs during the isothermal cure of a cross-linking system at temperatures below T g∞, the glass transition temperature of the fully cured resin, has been studied by TOPEM, a new temperature modulated DSC (TMDSC) technique based upon the use of stochastic temperature pulses. A comparison is made between TOPEM and another TMDSC technique, and some advantages of TOPEM are considered. The TOPEM technique is used to show that the mobility factor is not always a reliable approach to predicting the cure rate during vitrification, in view of its frequency dependence. Also, the dependence of the apparent vitrification time on frequency is examined. There appears to be a non-linear relationship between the apparent vitrification time and log(frequency), which is further discussed in the second part of this series.

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TOPEM, a new temperature modulated DSC technique

Application to the glass transition of polymers

Journal of Thermal Analysis and Calorimetry
Authors: Iria Fraga, S. Montserrat, and J. Hutchinson

Abstract  

TOPEM is a new temperature modulated DSC technique, introduced by Mettler-Toledo in late 2005, in which stochastic temperature modulations are superimposed on the underlying rate of a conventional DSC scan. These modulations consist of temperature pulses, of fixed magnitude and alternating sign, with random durations within limits specified by the user. The resulting heat flow signal is analysed by a parameter estimation method which yields a so-called ‘quasi-static’ specific heat capacity and a ‘dynamic’ specific heat capacity over a range of frequencies. In a single scan it is thus possible to distinguish frequency-dependent phenomena from frequency-independent phenomena. Its application to the glass transition is examined here.

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The use of tomographic image reconstruction techniques for the creation of three-dimensional images of the distribution of activity in solid samples has been examined. The aim has been to determine whether the method offers a finish to neutron activation procedures wich is capable of providing detailed position-sensitive information. Investigations with a phantom have shown that good images can be produced, even with relatively low-cost equipment.

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Abstract  

The processes of vitrification and devitrification that occur in an epoxy resin when it cures non-isothermally with a hardener are studied in terms of their frequency dependence and as a function of the heating rate. A novel modulated DSC technique, TOPEM, has been used which permits the evaluation of the frequency dependence for a single sample in a scan at constant underlying heating rate, thus avoiding errors arising from the composition of the sample. The effects of both frequency and heating rate on vitrification and devitrification are investigated. Some advantages of this technique are observed and discussed.

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Abstract  

The procedure for the fabrication of epoxy-based polymer layered silicate nanocomposites is important in respect of the nanostructure that is developed. To further our understanding of this, the influence of an organically modified clay (montmorillonite, MMT) on the curing kinetics of an epoxy resin has been studied by differential scanning calorimetry. Clay loadings of 10 and 20 mass% are used, and isothermal as well as dynamic cures have been investigated. For both cure schedules the effect of the MMT is to advance the reaction. Kinetic analysis yields values for the activation energy, but shows that the reaction cannot be described simply by the usual autocatalytic equation. The glass transition of the cured nanocomposites is lower than that for the cured neat resin, a result that is attributed to homopolymerisation taking place in addition to the epoxy–amine reaction.

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Abstract  

Tomographic techniques exploiting the measurement of transmitted gamma-radiation have been investigated as a means of providing information about the state of selected mechanical components. In this exploratory study, all samples were taken to a specially constructed laboratory rig for examination. Examples are given of the images generated.

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Abstract  

An analysis developed in previous work has been further refined in order to study the effect of heat transfer on the heat capacity and phase angle measurements by TMDSC. In the present model, a temperature gradient within the sample has been taken into account by allowing for heat transfer by thermal conduction within the sample. The influence of the properties of the sensors, the heat transfer conditions between the sensor and sample,and the properties of the sample have been investigated by varying each parameter in turn. The results show that heat capacity measurements are reliable only within a restricted frequency range, for which the experimental conditions are such that the heat transfer phase angle depends linearly on the modulation frequency.

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