Structural information sources electron microscopy

There are three potential methods by which a protein s three-dimensional structure can be visualized X-ray diffraction, NMR and electron microscopy. The latter method reveals structural information at low resolution, giving little or no atomic detail. It is used mainly to obtain the gross three-dimensional shape of very large (multi-polypeptide) proteins, or of protein aggregates such as the outer viral caspid. X-ray diffraction and NMR are the techniques most widely used to obtain high-resolution protein structural information, and details of both the principles and practice of these techniques may be sourced from selected references provided at the end of this chapter. The experimentally determined three-dimensional structures of some polypeptides are presented in Figure 2.8. 

Transmission electron microscopy (TEM) is a powerful and mature microstructural characterization technique. The principles and applications of TEM have been described in many books [16 20]. The image formation in TEM is similar to that in optical microscopy, but the resolution of TEM is far superior to that of an optical microscope due to the enormous differences in the wavelengths of the sources used in these two microscopes. Today, most TEMs can be routinely operated at a resolution better than 0.2 nm, which provides the desired microstructural information about ultrathin layers and their interfaces in OLEDs. Electron beams can be focused to nanometer size, so nanochemical analysis of materials can be performed [21]. These unique abilities to provide structural and chemical information down to atomic-nanometer dimensions make it an indispensable technique in OLED development. However, TEM specimens need to be very thin to make them transparent to electrons. This is one of the most formidable obstacles in using TEM in this field. Current versions of OLEDs are composed of hard glass substrates, soft organic materials, and metal layers. Conventional TEM sample preparation techniques are no longer suitable for these samples [22-24], Recently, these difficulties have been overcome by using the advanced dual beam (DB) microscopy technique, which will be discussed later. 

The use of electron microscopy as an aid in structure studies of catalysts is illustrated in the case of another form of silica namely, diatomaceous earth. Many other techniques such as chemisorption for surface composition and surface acidity, x-ray and electron diffraction, thermal analysis and magnetic susceptibility are gaining wider application in catalyst research. Elevated temperature adsorption studies of reactants and products are still in their infancy. The thorough investigation of adsorption properties along with information from other sources will improve our understanding of catalysts and will eventually provide a means for the design and preparation of catalytic materials which possess the desired properties. 

Although the base sequence of DNA remains to be elucidated, Watson and Crick [90, 91] proposed a model for its tridimensional structure. Several models had been proposed prior to that enounced by Watson and Crick, but they were all discarded because they were not compatible with the established physical and chemical properties of the DNA molecule. Modern concepts of the tridimensional structure of the DNA molecule are based on four different sources of factual information (1) the base composition of the DNA molecule, (2) the physicochemical information suggesting the existence of hydrogen bonds (3) the electron microscopy data, which indicates that the molecule is a long, extended structure about 30,000 A long and 30 A wide and (4) X-ray diffraction studies of crystalline and paracrystalline DNA molecules from which the molecule is known to have a helicoidal shape. (The term spiral should not be used to describe DNA. A spiral winds around a cone and a helix winds around a cylinder.)... 

To obtain real-space information about the morphology of polymeric materials, various optical microscopic methods such as OM and CLSM are available (cfr. Chp. 5.3). Use of electrons as a light source for microscopy opens other perspectives [124]. Electron microscopy (EM) provides structural information in both the real and reciprocal space. Electron... 

SAXS and WAXS are particularly efficient in the study of amorphous polymers including microstructured materials, hence their use in block copolymers (see also Chapters 6 and 7). The advent of synchotron sources for X-ray scattering provided new information, particularly on the evolution of block copolymer microstructures with time resolution below one second. In particular, the morphology of TPEs is most often studied with these techniques Guo et al. [108] applied SAXS to the analysis of the phase behavior, morphology, and interfacial structure in thermoset/thermoplastic elastomer blends. WAXS is often associated with SAXS and some other methods, such as electron microscopy, and various thermal and mechanical analyses. It is mainly used in studies of the microphase separation [109,110], deformation behavior [111], and blends [112]. 

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