Crystal characters

Figure 9-66. Crystal characters from C. Bunn s book [159], Reproduced with permission.

T300, T50 and PI00 are carbon fibers Spectra 1000 is a gel-spun polyethylene and SSE PE is a solid state extruded polyethylene. Other polymers on die plot have stiff backbones and a liquid crystal character],... 

Fig. 1 shows HREM image of as-implanted Si02 layers after Xe ion bombardment. The pictures exhibit 3-4 nm-size dark spots. An increase in the irradiation dose resulted in the growth of number and size of the spots. In small part of them lattice fringes may be distinguished, indicating their crystal character. The interplanar spacing matched those of silicon. 

Special difficulties arise in determining the cell constants of high polymers. Using single crystals instead of crystalline powders is not ordinarily an option because in most cases there are principally no single crystals. In most cases, however, filaments can be drawn from the substance after appropriate heat treatment, such filaments tend to take on a single-crystal character. The reflections are then located on layer lines in flat-film patterns. With some luck, trial and error will lead to not only the translational period in the fiber direction, but also the other cell constants [1, pp. 405 - 406]. 

The properties of many substances, however, exhibit a direction dependence, which is similar to a single crystal, but at the same time indicates that the substance has only partial single- crystal character. Such a solid is said to have texture. The individual crystallites are fully or partly aligned they are said to have preferred orientations. 

The nature of the initial organization of the polymers after cooling is obscure there might be liquid crystal character to the organization. We are studying this at present. 

Among all CPs, there is only one class that appears to display true single-crystal character. This is the poly(diacetylenes) (P(DiAc)s), discussed separately in a subsequent chapter in this book. Large (up to 1 cc) near-single crystals can be grown, with estimated coherence lengths of up to 1,000 monomer units (ca. 1 /tm in length) [325]. 

Such procedures are nevertheless highly empirical in character and until recently the formation of blocky structures could be inferred qualitatively only from observations of thermal transitions and crystal character, and quantitatively only from simple and often insensitive analyses. For wholly amorphous copolymers, only the latter method was available. 

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