Microscopy of radiation sensitive materials

Question (2). It is possible that optical microscopy or one of the many other characterization methods (Chapter 6) could give the required information. The problems of radiation sensitivity are more severe in the TEM. Use of the SEM may thus be an alternative involving less difficulty. 

Question (1). This defines the radiation sensitivity of the material in an operational way that depends very strongly on the sample and the information required from it. Thus a fluoro-polymer may degrade rapidly and have no known staining procedure. When the microstructure of the homopolymer is studied, this will be a very sensitive material. No information will be available after a large radiation dose. The same polymer could be a minority second component in a blend, with the TEM used to determine the 

Question (3). A shadowed carbon replica of an etched sample contains little or no polymer. If it contains the information required, it is an excellent way of producing a less sensitive sample. Stains by definition increase the contrast of the specimen, but do not necessarily increase the stability of the image. Some stains, such as iodine, are rapidly driven off by irradiation in vacuum while others are completely stable. 

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It is usually necessary to use a higher beam current than is normal for microscopy, which le ds to increased contamination of the specimen and/or to radiation dainage of beam-sensitive materials. 

The microstructure observed for thick films shows fibrils, about 4-10 nm in diameter for polystyrene, in agreement with SAXS measurements on the crazes in the bulk polymer. Very thin films of polystyrene (100 nm) show modification in the craze structure as there is no plastic restraint normal to the film [397]. Deformation zones have also been studied in polycarbonate, polystyrene-acrylonitrile and other polymers [398]. Crazes in thermosets can be studied in thin films spun onto NaCl substrates which can be washed away when the film has been cured. Mass thickness measurements are difficult to make in radiation sensitive materials that is why most TEM work has been done on polystyrene and least on PMMA. After developing the techniques described above for TEM Donald and Kramer [398] applied similar methods in optical microscopy to study radiation sensitive materials and the kinetics and growth of deformation zones. Thin films were strained on grids in situ in a reflecting OM. Change of interference color, which depends on the film thickness, was a very sensitive method for observing film deformation. 

PMMA. After developing the techniques described above for TEM, Donald and Kramer [526] applied similar methods in optical microscopy to study radiation sensitive materials and the kinetics and growth of deformation zones. Thin films were strained on grids in situ in a reflecting OM. Change of interference color, which depends on the film thickness, was a very sensitive method for observing film deformation. 

In transmission electron microscopy (TEM), a beam of electrons is passed through a thin sample, such that an image is formed as a result of absorption or diffraction contrast. In the case of polymers, a combination of disorder and radiation sensitivity means that, of these, absorption contrast is most important, in which case, high resolution images can be generated where image contrast is based on the spatial variation in electron density. In the case of materials such as nanocomposites, the distribution of the nanoparticles can therefore easily be imaged, as a result of the difference in the atomic number between the nanoparticles and the matrix polymer, as shown in Eig. 2.18. 

Infrared and Raman spectroscopy, coupled with optical microscopy, provide vibrational data that allow us to chemically characterise geochemical sediments and weathered samples with lateral resolutions of 10-20 pm and 1-2 pm respectively. Fourier transform infrared spectroscopy involves the absorption of IR radiation, where the intensity of the beam is measured before and after it enters the sample as a function of the light frequency. Fourier transform infrared is very sensitive, fast and provides good resolution, very small samples can be analysed and information on molecular structure can be obtained. Weak signals can be measured with high precision from, for example, samples that are poor reflectors or transmitters or have low concentrations of active species, which is often the case for geochemical sediments and weathered materials. Samples of unknown... 

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