Micro-thermal analysis and evolved gas analysis

doi:10.1016/S0040-6031(00)00675-4

Thermochimica Acta

Volumes 367–368, 8 March 2001, Pages 195-202

https://doi.org/10.1016/S0040-6031(00)00675-4 Get rights and content

Abstract

Micro-thermal analysis employs an active heated tip in a scanning probe microscope. This allows topographic and thermal imaging of surfaces to be carried out and permits localised thermal analysis of selected areas of material. In order to provide a means for spatially resolved chemical characterization, a method of trapping the gases that are evolved when the thermal probe is used to pyrolyze the surface has been developed. The trapped gases can then be thermally desorbed and passed into a GC–MS system for separation and identification. With this system it is possible to ablate a small $(<10 μm ×10 μm)$ area of a sample (or a domain, feature or contaminant) and elucidate its composition. This paper presents initial results for a homogeneous polymer (poly(methyl methacrylate)) — and a heterogeneous polymer blend (polystyrene/poly(α-methyl styrene)).

Introduction

Micro-thermal analysis (micro-TA) combines the imaging capabilities of atomic force microscopy with the physical characterization capabilities of thermal analysis [1], [2]. Specimens may be visualized by means of their topography and ability to conduct heat. The images so obtained can then be used as a means of selecting positions on the sample to carry out the spatially resolved equivalents of the “macro-TA” techniques such as TMA and modulated temperature DSC. With some knowledge of the components in the sample and their likely thermal responses (e.g. melting point, softening temperature, etc.) it is possible to use the results from such experiments to elucidate the nature and distribution of different phases within the bulk. Where the chemistry of the sample is unknown however, one may be forced to resort to the more traditional methods of surface analysis capable of providing localized compositional information (such as X-ray photoelectron spectroscopy (XPS) or secondary ion mass spectrometry (SIMS)) [3]. However, XPS and SIMS are often inconvenient techniques to employ since they require the sample to be analyzed under high vacuum. The chemical information that they produce can be limited — especially in the case of complex mixtures.

In the “macro-world” there is a long history of using analytical pyrolysis methods to probe the structure of polymers [4]. Samples are simply heated so as to decompose them into small fragments which are then analyzed by mass spectrometry (MS) or capillary gas chromatography–mass spectrometry (GC–MS). A number of systems have been described whereby the sample is heated in a thermobalance and the evolved gases trapped and analyzed [5]. In the realms of micro-TA it is easy to use the resistively heated probe as a means of locally ablating material from the surface. We have now combined this into a system whereby the evolved gases are captured and then analyzed by GC–MS. Furthermore, the ability to perform localized pyrolysis experiments within a small volume of material enhances the value of the technique.

Section snippets

Experimental

Measurements were carried out on a TA Instruments 2990 Micro-Thermal Analyzer based on a ThermoMicroscopes Explorer Scanning Probe Microscope with the addition of an active thermal probe and associated electronics. Imaging and localised thermal analysis can be carried out in the usual way. For pyrolysis experiments the probe is placed in contact with the region of interest and rapidly heated to 600°C. The evolved gases are trapped in a specially designed tube packed with a suitable sorbent such

Results and discussion

An example of a pyrolysis crater on a poly(methyl methacrylate) sheet (ICI Perspex) obtained using the thermal probe is shown in Fig. 2. Line profiles horizontally and vertically across the image indicate that the pit is 6 μm in diameter and 1.7 μm deep (Fig. 3). Not all of this volume was pyrolized since some of the material flows away from the probe on heating. Fig. 4 shows the total ion chromatogram obtained from the material trapped in the sorbent tube. A mass spectrum taken from peak at a

Conclusions

The combination of scanning thermal microscopy and localized thermal analysis presents a powerful new form of analytical microscopy, which is able to elucidate the organization and constitution of materials. The addition of a localized form of chemical analysis by pyrolysis-GC–MS to this technique provides an independent means of identification and which is essential in cases where there is no a priori knowledge of the specimen’s makeup or there is unclear discrimination between phases. This

Acknowledgements

DMP would like to thank the UK Engineering and Physical Sciences Research Council for financial support.

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