SEM is a broad field of analysis that has improved dramatically over the years . Resolution of electron microscopy rivals that of TEM and AFM. An electron beam from a heated filament is focused and scans the surface of a sample in a raster pattern. The electrons interact with the materials of the surface and are absorbed, scattered, or reflected. Reflected or secondary electrons generated from the specimen as a result of the electron beam collision, or electromagnetic radiation, can be subsequently detected depending on the microscope hardware. A representation of the surface is generated after electron surface collisions are detected and correlated with an x, y coordinate on a display. The resulting images can have tremendous verisimilitude and produce three-dimensional depth of high fidelity.
SEM is very commonly used in polyurethane research, but because the signal is primarily a representation of surface features, there are limitations. SEM is often used to image gross images of polyurethane structures such as foam cell structure that can be more difficult with optical techniques due to incident light scattering. Figure 5.5 shows an SEM of an open-cell foam. Notice that the magnification is not particularly high, but the same picture by optical microscopy would typically not contain the observed level of detail. SEM is also commonly utilized to visualize polyurethane 2-phase composites such as fiber- and mineral-reinforced structures and their interfacial features. Such an image of a urethane composite is shown in Figure 5.5b showing high levels of adhesion of urethane to the embedded glass fiber.
Hard segment and soft segment do not scatter electrons with sufficient distinction to be separately observable since PU samples are normally coated with a conductive layer such as gold to conduct electrons away from the surface and reduce deresolution
Figure 5.5 Illustrative scanning electron micrographs of (a) open-cell polyurethane foam and (b) glass-filled polyurethane insulation foam composite.
of the image caused by electron scattering from a negatively charged insulating PU sample. Gross changes in fracture surfaces at different hard segment contents have been rationalized as demonstrating the effect of hard segment, but do not reveal the hard segment structure observed by other techniques.
Figure 5.6 Illustrative transmission electron microscopy images of polyurethanes. The image on the right is a TEM image of the elastomer shown as an optical image in Figure 5.4. The image on the left shows oriented rodlike hard segments. The orientation is probably an artifact of the compression molding process. Images courtesy of Robert Cieslinski and Justin Virgili. Reprinted with permission from Ref. . © John Wiley & Sons, Inc.