A. Hammiche

Micro-thermal analysis: techniques and applications

The terms micro-thermal analysis and micro-spectroscopic analysis are used to include any form of localized characterization or analysis combined with microscopy that uses a near-field thermal probe to exploit the benefits of using thermal excitation. Individual regions of a solid sample are selected by means of surface or sub-surface imaging (atomic force microscopy and/or scanning thermal microscopy), so as to add spatial discrimination to four well-established methods of chemical fingerprinting, namely thermomechanometry, calorimetry, spectroscopy and analytical pyrolysis. We begin by describing the state of the art of scanning microscopy that uses resistive thermal probes, followed by an account of the various techniques of micro-thermal analysis. Modern materials technology is increasingly concerned with the control of materials at the mesoscale. The ability to add an extra dimension of, say, chemical composition information to high-resolution microscopy, or microscopic information to spectroscopy, plays an increasingly useful part in applied research. Micro-thermal analysis is now being used commercially to visualize the spatial distribution of phases, components and contaminants in polymers, pharmaceuticals, foods, biological materials and electronic materials. This review outlines various applications that have been described in the literature to date, the topics ranging from multi-layer packaging materials and interphase regions in composites, to the use of the technique as a means of surface treatment.

Localized thermal analysis using a miniaturized resistive probe

We describe a novel thermal characterization technique based on a differential arrangement, which achieves spatially localized calorimetric analysis. It involves the use of an active probe which acts both as a highly localized heat source and a thermometer. This ability opens the way for the implementation of scanning calorimetric microscopy where image contrast will be created from thermal analysis data. For a number of polymers we have recorded events such as glass transitions, meltings, recrystallizations and thermal decomposition within volumes of material estimated at a few μm3. The data obtained are compared with those obtained from conventional calorimetry and the events registered in both cases are found to match. For a full quantitative analysis of the results obtained, mathematical modelling of the operation of the technique, taking account of physical and other changes in materials, is required.

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...

Fourier Transform Infrared Microspectroscopy Identifies Symmetric PO2− Modifications as a Marker of the Putative Stem Cell Region of Human Intestinal Crypts

Complex biomolecules absorb in the mid‐infrared (λ = 2–20 μm), giving vibrational spectra associated with structure and function. We used Fourier transform infrared (FTIR) microspectroscopy to “fingerprint” locations along the length of human small and large intestinal crypts. Paraffin‐embedded slices of normal human gut were sectioned (10 μm thick) and mounted to facilitate infrared (IR) spectral analyses. IR spectra were collected using globar (15 μm × 15 μm aperture) FTIR microspectroscopy in reflection mode, synchrotron (≤10 μm × 10 μm aperture) FTIR microspectroscopy in transmission mode or near‐field photothermal microspectroscopy. Dependent on the location of crypt interrogation, clear differences in spectral characteristics were noted. Epithelial‐cell IR spectra were subjected to principal component analysis to determine whether wavenumber‐absorbance relationships expressed as single points in “hyperspace” might on the basis of multivariate distance reveal biophysical differences along the length of gut crypts. Following spectroscopic analysis, plotted clusters and their loadings plots pointed toward symmetric (νs)PO2− (1,080 cm−1) vibrations as a discriminating factor for the putative stem cell region; this proved to be a more robust marker than other phenotypic markers, such as β‐catenin or CD133. This pattern was subsequently confirmed by image mapping and points to a novel approach of nondestructively identifying a tissues stem cell location. νsPO2−, probably associated with DNA conformational alterations, might facilitate a means of identifying stem cells, which may have utility in other tissues where the location of stem cells is unclear.

Modulated differential scanning calorimetry: 6. Thermal characterization of multicomponent polymers and interfaces

A quantitative thermal method of determining the weight fraction of interface and the extent of phase separation in polymer materials is described. It is based on the differential of heat capacity signal from modulated-temperature differential scanning calorimetry. By measurement of the increment of heat capacity at the glass transition temperature, the total interface content can be determined. The method assumes that the interface and the rest of the system can be modelled as a series of discrete fractions each with its own glass transition temperature. Several examples, including block copolymers, block copolymers blended with homopolymer, and two-phase and four-phase systems are given to illustrate the range of the method. The calculated results were close to the experimental data for two-phase and four-phase systems.

Modulated differential scanning calorimetry: 1. A study of the glass transition behaviour of blends of poly(methyl methacrylate) and poly(styrene-co-acrylonitrile)

Abstract The glass transition and the effect of specific interactions on this transition process in a binary polymer blend of poly(methyl methacrylate) (PMMA) and poly(styrene- co -acrylonitrile) (SAN), which have very similar glass transition temperatures, have been investigated by means of modulated differential scanning calorimetry. The blends investigated were miscible blends of PMMA and SAN and a physical mixture of the two constituent polymers. Using the differential of heat capacity versus temperature signal, the technique has been shown to be able to resolve the two glass transitions so long as their difference is not less than about 5°C. The value of heat capacity of the miscible blend does not satisfy simple linear addition of the heat capacities of the two components polymers over the glass transition region. It is believed that this enhancement of heat capacity in the glass transition region results from specific interactions between segments of the two polymers.

Thermochemical nanopatterning of organic semiconductors

Patterning of semiconducting polymers on surfaces is important for various applications in nanoelectronics and nanophotonics. However, many of the approaches to nanolithography that are used to pattern inorganic materials are too harsh for organic semiconductors, so research has focused on optical patterning and various soft lithographies. Surprisingly little attention has been paid to thermal, thermomechanical and thermochemical patterning. Here, we demonstrate thermochemical nanopatterning of poly(p-phenylene vinylene), a widely used electroluminescent polymer, by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 microm, and achieve write speeds of 100 microm s(-1). Experiments show that a resolution of 28 nm is possible when the tip-sample contact region has dimensions of approximately 100 nm and, on the basis of finite-element modelling, we predict that the resolution could be improved by using a thinner resist layer and an optimized probe. Thermochemical lithography offers a versatile, reliable and general nanopatterning technique because a large number of optical materials, including many commercial crosslinker additives and photoresists, rely on chemical mechanisms that can also be thermally activated.

Sub-surface imaging by scanning thermal microscopy

Scanning probe thermal microscopy has been used to achieve sub-surface imaging of metallic particles embedded in a polymer matrix, using a probe which can act as both ohmic heater and thermometer. A lateral resolution of the order of a micron and a depth detection of a few microns were achieved. Together with the description of the technique and the experimental results obtained, a basic theoretical framework is presented which describes heat flow and temperature distributions within a sample consisting of inclusions buried within a bulk material. Computer models have been developed to give theoretical heat flows and temperature profiles: these are compared here with the experimental data. The theoretical lateral resolution was found to be in good agreement with the experimental observation. We show that theoretical modelling can be used to calibrate the instrument for specific investigations. For example, the technique could be used quantitatively to determine and map thermal conductivity variations across heterogeneous samples, or to determine the depth at which inclusions are located in the case where the thermal conductivities of both the inclusions and the enclosing material are known as well as the geometry of the inclusions.

FTIR micro-spectroscopy identifies symmetric PO2- modifications as a marker of the putative stem cell region of human intestinal crypts.

Complex biomolecules absorb in the mid‐infrared (λ = 2–20 μm), giving vibrational spectra associated with structure and function. We used Fourier transform infrared (FTIR) microspectroscopy to “fingerprint” locations along the length of human small and large intestinal crypts. Paraffin‐embedded slices of normal human gut were sectioned (10 μm thick) and mounted to facilitate infrared (IR) spectral analyses. IR spectra were collected using globar (15 μm × 15 μm aperture) FTIR microspectroscopy in reflection mode, synchrotron (≤10 μm × 10 μm aperture) FTIR microspectroscopy in transmission mode or near‐field photothermal microspectroscopy. Dependent on the location of crypt interrogation, clear differences in spectral characteristics were noted. Epithelial‐cell IR spectra were subjected to principal component analysis to determine whether wavenumber‐absorbance relationships expressed as single points in “hyperspace” might on the basis of multivariate distance reveal biophysical differences along the length of gut crypts. Following spectroscopic analysis, plotted clusters and their loadings plots pointed toward symmetric (νs)PO2− (1,080 cm−1) vibrations as a discriminating factor for the putative stem cell region; this proved to be a more robust marker than other phenotypic markers, such as β‐catenin or CD133. This pattern was subsequently confirmed by image mapping and points to a novel approach of nondestructively identifying a tissues stem cell location. νsPO2−, probably associated with DNA conformational alterations, might facilitate a means of identifying stem cells, which may have utility in other tissues where the location of stem cells is unclear.

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.