Complementary techniques fiber studies

External surface features of fibers such as contours, defects and damagp, chemical treatments, and polymer coatings are normally observed in the SEM (Figure 5.4). Recent developments in atomic force microscopy (AFM) should expand the potential to provide ultrahigh-resolution data about surface features at the atomic and the molecular levels. Thus, a complementary array of techniques is now available for detailed surface studies. 

The stereocomplex formed from complementary strands of isotactic and syndiotactic poly(methyl methacrylate)s (it- and st-PMMAs) with an it st-stoichiometry of 1 2 represents another class of unique, polymer-hased helical supramolecules suitable for AFM studies. Although this stereocomplex has heen known for half a century, the molecular basis of the stracture and the mechanism of complex formation are still under dehate. In 1989, Schomaker and Challa proposed a double-stranded helix model for the stereocomplex based on the wide angle X-ray scattering (WAXS) analysis of the stretched fiber, which is composed of a 9i it-PMMA helix (nine repeating MMA units per turn) strrrormded hy an 18i st-PMMA helix, resulting in a double-stranded helix with a helical pitch of 1.84 nm. To verify the stmcture of the PMMA stereocomplex by the AFM technique, Kumaki et al. prepared a mixed monolayer of it- and... 

Complementary microscopic techniques are useful in the elucidation of polymer film microstructures. Optical techniques provide information relating to the orientation and crystallinity, while SEM can be used for surface detail relevant to end uses. TEM techniques, similar to those used in model film studies and in fibers, are useful in describing the internal structures, especially of spherulites and their deformed counterparts, microfibrils. TEM studies of films and fibers continue to provide fundamental observations relating the structure to properties and applications. 

Characterization of fiber microstructure normally requires several microscopy techniques, as was shown in the simple example in the last section. An optical cross section of a fiber may have a dogbone shape (Fig. 5.2D), and yet this image does not reveal much about the internal fiber structure. On the other hand, a fracture surface of a fiber may reveal the presence of internal detail when viewed in the SEM (Fig. 5.11), and yet still not provide a complete picture of the structure. Clearly, complementary microscopy techniques and nonmicroscopy techniques must be applied to solving structural problems. Specific problem solving examples are described here which are representative of the wide range of studies conducted and documented in the many journals that publish polymer research. 

X-ray scattering techniques are the most commonly applied complementary discipline to microscopy for structural studies. The type of information that is obtained by x-ray scattering experiments includes phase identification and quantification, crystallinity, crystallite size, lattice constants, molecular orientation and structure, molecular packing and order, and amorphous structure [26-30]. Diffraction techniques that will be described include powder diffraction, wide angle x-ray scattering (WAXS), and fiber diffraction. Small angle x-ray scattering (SAXS) will be described in Section 7.4.4. 

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