Semicrystalline polymers reflections

SEM micrographs of two members of these polymers (HB and HBIB-50) are shown in Figure 7 to provide further evidence for superstructure on the micron level within the solution cast films. One can directly observe the surface of the spherulitic structure of the HB homopolymer as well as in that of the copolymer HBIB-50. Clearly, the level of structure (-5 pm) is well above that of the individual domains of either HB or HI and reflects the possible primary nucleation and subsequent growth behavior common to spherulitic semicrystalline polymers. The Hv patterns shown in... 

The accuracy of a structure deduced from such spacings depends on the number of reflections observed. For a semicrystalline polymer showing only three or four reflections, only a rough estimate of the structure can be obtained. However, the structure of a metal or inorganic salt, which may show more than a thousand reflections, can be determined with very high reliability. 

As the CP efficiency in the crystalline region is in general larger than that in the noncrystalline region in the CP/MAS NMR spectra of semicrystalline polymers, the chemical shift changes of resonances which appear in the spectra are shown in Fig. 21.2 and Table 21.1 and should reflect predominantly the differences in the crystalline environment, that is, the crystalline... 

More detailed aspects of transport in heterogeneous media have been given in the excellent reviews of the subject by Barrer (55) and Petropolus (51) The models described by these authors and others include the effects of size, shape, and anisotropy of the crystalline phase on the tortuosity. Models of highly ordered anisotropic media have been demonstrated to have tortuosities in the range of 30, which reflects the rather dramatic role that orientation can have on the barrier properties of semicrystalline polymers. 

The diffraction peaks obtained with a perfect crystal are in theory expected to be infinitely sharp. The finite widths of the observed diffraction peaks as seen in Figure 3.2 reflect the fact that crystallites in semicrystalline polymers are not perfect, and the analysis of the line widths can tell us about the nature and degree of imperfection in the polymer crystal lattices and the size of the polymer crystallites if they are small. 

For most of the semicrystalline polymers that have been synthesized and studied to any extent to date the unit cell parameters are known at least approximately, and a good tabulation of such data is found in reference books such as Polymer Handbook.11 When the unit cell geometry is entirely unknown, indexing the Bragg reflections is a more involved process. Having a fiber diagram rather than a powder diagram alone is then useful, since the two-dimensional information about the x and y coordinates of... 

It is well known that chitin is a semicrystalline polymer. Consequently, it is necessary to verify whether chitin s crystalline volume fraction changes with heat treatment and its influence on relaxation behaviors. The diffraction pattern of purified a-chitin powder is shown in Figure 2.8. The five characteristic crystalline reflections for a-chitin are present [51]. They are indexed as (020), (110), (120),... 

Isotactic polypropylene (iPP) is generally regarded as a semicrystalline polymer since its wide angle X-ray diffraction patterns show the characteristics of both a crystalline phase (as indicated by the sharp reflections) and an amorphous phase (as indicated by a diffuse halo). 

At temperatures above their Tg, the resonance spectrum of noncrystalline polybutadiene (PB) (Fig. 8a) is clearly different from that of the semicrystalline polyethylene (Fig. 8c). Amorphous PB exhibits a narrow Lorentzian line shape with a width of 0.2 G. In contrast, the PE spectrum comprises two components, ie, narrow and broad line shapes. When the spectra of semicrystalline polymers are recorded in the glassy state (Figs. 8b and 8d), only abroad component is observed. This indicates that the line shape corresponds to molecular mobility and the line width reflects a correlation (or relaxation) time. Therefore, the broad and narrow components of semicrystalbne PE (Fig. 8c) are related to protons of methylene groups in rigid and mobile (amorphous) environments, respectively. On the basis of this, it was proposed that the degree of crystallinity could be determined by resolving the area of the broad component (rigid phase) from the spectrum. 

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