Thickness-mass contrast

The transmission electron microscope (TEM) image from crazes in such films, the negative from which Fig. la was printed, can be analyzed not only to reveal dimensions of the craze but also to yield the local craze fibril volume fraction v by using a microdensitometer to measure the local density of the image, its mass thickness contrast The extension ratio of the craze fibrils, X equals i. ... 

Diffraction and mass-thickness contrast are both caused by an intensity change of a diffracted beam over the field of view. Since the intensity, specifically the amplitude, of a beam causes these types of image contrast, diffraction and mass-thickness contrast are referred to as amplitude contrast. ... 

The three commonly encountered contrast mechanisms in TEM imaging are (i) mass-thickness contrast which occurs due to greater absorption or scattering of incident electrons from denser or thicker parts of the specimen (ii) diffraction contrast where crystalline regions of different orientation exhibit different contrast due to the orientational dependence of Bragg diffraction and (iii) phase contrast where phase-shifted waves from the undiffracted and diffracted beams are allowed to interfere and generate lattice fringes. 

As in the field of condensed matter sciences, polymer research has made extensive use of conventional transmission electron microscopy (CTEM) ever since its invention. Polymer research focuses on materials that mainly consist of carbon and other light elements. These materials are relatively weak electron scatterers and therefore give low mass thickness and phase contrast. Mass thickness contrast may be enhanced using an objective aperture, staining, and/or low... 

A quantitative analysis of craze shape and mass thickness contrast within the craze allowed Lauterwasser and Kramer [382] to derive the stress profile existing along a polystyrene craze. Kramer and his coworkers have extended this study to many other polymers, relating the mean density of craze material to entanglement density in the polymer glass and to toughness [395] without a basic change of preparation technique. 

Mass thickness contrast is generally weak in polymers. Staining, shadowing or decoration methods (Chapter 4) have to be applied to enhance this contrast. Diffraction contrast, produced by the scattering of diffracted beams outside the objective aperture, is limited by radiation damage of crystallinity. This leaves phase contrast, where scattered beams (inside the objective aperture) are phaseshifted and recombined with the unscattered beam. 

The mass-thickness contrast is the reason why denser regions with higher scattering probability of the beam appear dark and light regions appear bright in TEM [112]. 

Figure 3.11 Illustration of the mass-thickness contrast in TEM The influence of locally increased specimen thickness and density on scattering of electrons is shown thicker specimen and more density material scatter more electrons, resulting in darker area on the micrograph.

In practice, most TEM investigations of polymers make use of mass-thickness contrast, that is, specimen parts containing elements with a higher atomic number and/or which are thicker scatter electrons stronger, but not contributing to image formation. Therefore, such parts appear darker than the surroundings (see Fig. 3.11). 

Figure 3.12 Mass-thickness contrast in TEM micrographs of isotactic PP ultrathin sections due to selectively chemical staining of the amorphous regions and the boundaries between the lamellae (a) a - iPP with the typical cross-hatched structure of lamellae and (h) - iPP with a bundle of parallel lamellae (compare also with Fig. 3.9).

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