Duncan M. Price

Vapor pressure determination by thermogravimetry

A method for measuring the vapor pressures of a wide range of materials using a conventional thermobalance and standard sample holders is described. The equipment is calibrated using pure reference materials of known vapor pressure and exploiting the relationship between volatilization rate and vapor pressure based on the Langmuir equation for free evaporation. Enthalpies of vaporization and sublimation can be determined, in some cases, the melting temperature and enthalpy of fusion can be obtained directly from thermogravimetry. Applications to the study of plasticizers and UV absorbers are described. Poor correlation of experimental results with predicted values obtained by molecular modeling is found. The application of modulated temperature thermogravimetry for the determination of enthalpies of sublimation and vaporization is also explored.

Micro-thermal analysis: scanning thermal microscopy and localised thermal analysis

Micro-thermal analysis combines the imaging capabilities of atomic force microscopy with the ability to characterise, with high spatial resolution, the thermal behaviour of materials. The conventional AFM tip is replaced by a miniature heater/thermometer which enables a surface to be visualised according to its response to the input of heat (in addition to measuring its topography). Areas of interest may then be selected and localised thermal analysis (modulated temperature calorimetry and thermomechanical analysis) carried out. Localised dynamic mechanical measurements are also possible. Spatially resolved chemical analysis can be performed using the same basic apparatus by means of pyrolysis gas chromatography-mass spectrometry or high-resolution photothermal infrared spectrometry.

Calorimetry of two disperse dyes using thermogravimetry

A method for studying the volatilisation rate of materials using a conventional thermobalance and standard sample holders is described. This was used to study two dyes; CI Disperse Yellow 54 and CI Disperse Red 60. Using pure reference materials, a relationship between volatilisation rate and vapour pressure based on the Langmuir equation for free evaporation was used to calibrate the system. Thus, the vapour pressures of the dyes could be determined. Heats of sublimation and vaporisation were calculated from a plot of the logarithm of the vapour pressure against reciprocal absolute temperature. Extrapolation of the vapour pressure vs. temperature curve outside the experimentally measured region for Red 60 was made using the melting point and heat of fusion found by DSC. Good agreement was found with literature data. In favourable cases, the melting temperature and heat of fusion can be obtained directly from thermogravimetry.

Highly localized thermal, mechanical, and spectroscopic characterization of polymers using miniaturized thermal probes

In this article, we demonstrate the versatility of use of cantilever-type resistive thermal probes. The probes used are of two kinds, Wollaston wire probes and batch-microfabricated probes. Both types of probe can be operated in two modes: a passive mode of operation whereby the probe acts as a temperature sensor, and an active mode whereby the probe acts also as a highly localized heat source. We present data that demonstrate the characterization of some composite polymeric samples. In particular, the combination of scanning thermal microscopy with localized thermomechanometry (or localized thermomechanical analysis, L-TMA) shows promise. Comparison with data from conventional bulk differential scanning calorimetry shows that inhomogeneities within materials that cannot be detected using conventional bulk thermal methods are revealed by L-TMA. We also describe a new mode of thermal imaging, scanning thermal expansion microscopy. Finally, we outline progress towards the development of localized Fourier tr...

Micro‐thermal analysis of polymers: current capabilities and future prospects

The current state of development of micro-thermal analysis (micro-TA) and related techniques are briefly reviewed. Results for a PET/epoxy resin composite and a bilayer polymer film are given as illustrations. Details are given of a new interface that enables the micro-TA unit to be placed inside a conventional FTIR spectrometer to carry out photothermal IR microscopy. New results are presented for a micro-pyrolysis-mass spectroscopy technique. The limitations of the current instrumentation are discussed in terms of the overriding problem being one of spatial resolution. Images obtained using pulsed force mode AFM with a high-resolution heated tip indicate the scope for future development of this technique. The possibility of even higher spatial resolution with other forms of probe are discussed along with the potential for imaging micro-pyrolysis time of flight mass spectroscopy and even tomography. It is concluded that these methods offer excellent prospects for characterising a wide range of polymer systems.

Sublimation properties of x, y-dihydroxybenzoic acid isomers as model matrix assisted laser desorption ionisation (MALDI) matrices

Abstract The enthalpies of sublimation of the six isomers of dihydroxybenzoic acid were determined by thermogravimetry as part of a study to investigate their performance as model matrices in matrix assisted laser desorption/ionisation-mass spectrometry. The enthalpy of sublimation could be determined to within an error of ±7%. In the MALDI mechanism, sublimation is thought to play an important part, however in the present study no positive correlation between this parameter and the performance of the material as a matrix was found.

New Adventures in Thermal Analysis

This paper describes recent advances in thermal analysis instrumentation which combine the high resolution imaging capabilities of the atomic force microscope with physical characterisation by thermal analysis. Images of the surface may be obtained according to the specimens thermal conductivity and thermal expansivity differences in addition to the usual topographic relief. Localised equivalents of modulated temperature differential scanning calorimetry, thermomechanical and dynamic mechanical analysis have been developed with a spatial resolution of a few micrometres. A form of localised thermogravimetry-evolved gas analysis has also been demonstrated. The same instrument configuration can be adapted to allow IR microspectrometry at a resolution better than the optical diffraction limit.

Volatilisation, Evaporation and Vapour Pressure Studies Using a Thermobalance

There is considerable interest in performing volatilisation and evaporation measurements by thermogravimetry. A quick and simple method for determining vapour pressure using a conventional thermobalance and standard sample holders has been developed. These yield meaningful thermodynamic parameters such as the enthalpies of sublimation and vaporisation. Under favourable conditions the melting temperature and enthalpy of fusion of such compounds can be obtained. This technique has been used for the study of dyes, UV absorbers and plasticisers. The use of modulated- temperature programs for such work is also described.

Vapour pressures of hydroxybenzophenone UV absorbers

A thermogravimetric method was used to measure the vapour pressures of a series of hydroxybenzophenone UV stabilisers. Enthalpies of vaporisation were determined from the slope of a plot of the logarithm of the vapour pressure against reciprocal absolute temperature. Enthalpies of sublimation were measured either directly by the same method or by summing the enthalpy of vaporisation with the enthalpy of fusion determined by differential scanning calorimetry. Values of the vapour pressure at the melting temperature are given along with extrapolated values at 25°C and the estimated normal boiling temperature. Decreasing the hydrogen bonding potential within a series of compounds increases the volatility whereas increasing molecular weight has the opposite effect.

Novel methods of modulated-temperature thermal analysis

Abstract Two novel methods of Modulated-Temperature Thermal Analysis are described. These are loosely-termed “Constrained Rate DSC” and “Modulated-Temperature TMA”. In the first, the sample response is used to control the temperature program of a DSC in a method analogous to Controlled Rate Thermal Analysis. This results in a temperature modulation which is variable in period, amplitude and underlying heating rate depending on the sample behaviour and user defined limits. Deconvolution of the signals into thermally “reversing” and “non-reversing” heat flows is still possible. Secondly, the application of Modulated-Temperature Programming to Thermomechanical Analysis is discussed.