M. Reading

Scanning thermal microscopy: Subsurface imaging, thermal mapping of polymer blends, and localized calorimetry

We have used a platinum/10% rhodium resistance thermal probe to image variations in thermal conductivity or diffusivity at micron resolution and to perform localized calorimetry. The probe is used as an active device that acts both as a highly localized heat source and detector; by generating and detecting evanescent temperature waves, we may control the maximum depth of sample that is imaged. Earlier work has shown that subsurface images of metal particles buried in a polymer matrix are consistent with computer simulations of heat flows and temperature profiles, predicting that a 1 μm radius probe in air will give a lateral resolution of ∼200 nm near the surface, with a depth detection of a few μm. We have a special interest in polymer blends, and we present zero‐frequency mode and temperature‐modulation mode thermal images of some immiscible blends in which the image contrast arises from differences in thermal conductivity/diffusivity between single polymer domains. The behavior of domains is observed i...

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...

The origin and interpretation of the signals of MTDSC

Abstract In Modulated-Temperature Differential Scanning Calorimetry (MTDSC) a conventional heating programme is modulated cyclically. The heat-flow signal is split into an underlying and an approximately periodic part. We summarise the present state of the method with regard to chemical reactions and melting. We also give a more extensive treatment of the glass transition.

Dynamic mechanical analysis at the submicron scale

Dynamic mechanical analysis (DMA) is traditionally performed on bulk samples. However, studies of polymer blends would be enhanced if DMA could be applied on a local scale in order to enable a new form of microthermal analysis. Mounting a sample on a vibrating heating stage and observing the resulting amplitude and phase of the motion of an atomic force microscope cantilever allows the local elastic and visco-elastic properties to be studied. It is demonstrated in this article on samples of polyethersulfone/poly (acryonitrile-co-styrene) and polystyrene/poly(methyl methacrylate) (PS/PMMA) blends, and PMMA, PS and polytetrafluoroethylene homopolymers. Images at a specific temperature and spectroscopic data as a function of temperature of (nominally) a single point were collected. Primary and secondary relaxations were detected; the lateral resolution is better than 100 nm. We discuss the promising and limiting aspects of this new technique.

Progress in near-field photothermal infra-red microspectroscopy

Near‐field photothermal Fourier transform infra‐red microspectroscopy, which utilizes atomic force microscopy (AFM)‐type temperature sensors, is being developed with the aim of achieving a spatial resolution higher than the diffraction limit. Here we report on a new implementation of the technique. Sensitivity of the technique is assessed by recording infra‐red spectra from small quantities of analytes and thin films. A photothermomechanical approach, which utilizes conventional AFM probes as temperature sensors, is also discussed based on preliminary results. Early indication suggests that the photothermal approach is more sensitive than the thermomechanical one.

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.

Two new microscopical variants of thermomechanical modulation: scanning thermal expansion microscopy and dynamic localized thermomechanical analysis

We describe two ways in which thermomechanical modulation may be used in conjunction with scanning thermal microscopy, in order to distinguish between different components of an inhomogeneous sample. The sample is subjected to a modulated mechanical stress, and the heating is supplied locally by the probe itself.

Applications of Micro-Thermal Analysis

Micro-thermal analysis combines the imaging facility of scanning probe microscopy with the ability to characterize, with high spatial resolution, the thermal behavior of materials. A sample may be visualized according to its surface topography and also its relative thermal conductivity. Areas of interest may then be selected and localized thermal analysis (TMA and modulated temperature DTA) performed. Applications of this new technique to study semiconductors, polymer blends and biological specimens are described.

Thermally assisted nanosampling and analysis using micro-IR spectroscopy and other analytical methods

Micro-thermal analysis is a technique in which the probe in a scanning probe microscope is equipped with an ultra-miniature electrical resistor that serves both as a source of heat and a means of measuring temperature. Thermal properties can be imaged and local thermal analysis can be performed by placing the tip on a selected point and linearly ramping its temperature. When a material is softened in this way, the tip often becomes contaminated with the sample. The tip can easily be cleaned by heating it to a temperature high enough to cause the contaminant to decompose. However, prior to this, the contaminant can be analysed by, amongst other techniques, photothermal IR spectroscopy.

Localised Evolved Gas Analysis by Micro-thermal Analysis

Micro-thermal analysis employs a scanning probe microscope fitted with a miniature resistive heater/thermometer to obtain images of the surface of materials and then perform localised thermo analytical measurements. We have demonstrated that it is possible to use the same configuration to pyrolyse selected areas of the specimen by rapidly heating the probe to 600–800°C. This generates a plume of evolved gases which can be trapped using a sampling tube containing a suitable sorbent placed close to the heated tip. Thermal desorption-gas chromatogaphy/mass spectrometry can then be used to separate and identify the evolved gases. This capability extends the normal visualisation and characterisation by micro-thermal analysis to include the possibility of localised chemical analysis of the sample (or a domain, feature or contaminant). The isolation and identification of natural products from a plant leaf are given as an example to illustrate this approach. Preliminary results from direct sampling of pyrolysis products by mass spectrometry are also presented.