Microcrystalline regions

Nylon fibers are semicrystalline, that is, they consist of crystallites separated by amorphous regions. Hydrogen bonding is an important secondary valence interaction in nylon-6 and nylon-6,6. Individual chains in the microcrystalline regions of nylons are held together by hydrogen bonds. Nylons are resistant to aqueous alkali but deteriorate more readily on exposure to mineral acids. 

Figure 1. High-resolution electron micrograph and corresponding optical transform (inset) of an x-ray amorphous zeolite-Y specimen that has undergone ion-exchange with a solution containing U022+ ions. The microcrystalline regions are rendered visible by the locally ordered U022+ ions. ( See text.)...

Most biomolecules, such as polysaccharides, simple sugars, lipids, and proteins, are crystalline (International Centre for Diffraction Data, 2006). If HS consist merely of associations of biological residues, they should have characteristic crystal structures that can be rigorously studied and identified by X-ray diffraction analysis. However, the research evidence clearly shows that environmental organic matter has to be considered as highly amorphous material, which additionally contains microcrystalline regions like polymethylene crystallite (Hu et al., 2000 Schaumann, 2006b). 

FIGURE 6.22 Highly schematic picture of microcrystalline regions in a mass of a linear polymer at very high concentration (little or no solvent). The reader should remember that a two-dimensional picture cannot give a fully realistic representation of a three-dimensional structure in a case like this. 

FIGURE 46.11. o-Dc (7)for crosslinked PAN-ES, PAN-CSA (CHCI3), and PAN-CSA (m-cresol) samples (from Ref. [73]). The dashed lines are based upon the quasi-1 D VRH model. Note here crosslinks refers to physical crosslinks (microcrystalline regions) not chemical crosslinks (covalent bonds). 

TFE and monomer (I) were also copolymerized in supercritical carbon dioxide using a free radical initiator such as bis(perfluoro-2-propoxypropionyl)peroxide (III) at 35°C (the half-life time of the initiator is 40 minutes at 35°C) [9], The decomposition of the initiator proceeds through a single-bond homolysis mechanism [10], resulting in the formation of perfluorinated end group that yields thermally stable polymers [9] (Scheme 16.1). The reaction conditions and properties of the copolymers of (I) and TFE obtained in supercritical carbon dioxide are shown in Table 16.1. PTFE is crystalline, so that when the amount of TFE increases in copolymers, the polymer has some microcrystalline regions. The polymers obtained in carbon dioxide have similar properties with the commercial polymers. 

Polyolefin thermoplastic elastomers are generally blends of polypropylene with up to 65% ethylene-propylene rubber and it is supposed that short propylene blocks in the latter co-crystallize with segments of the polypropylene chains to give microcrystalline regions which act as cross-links. A recent development in this field has been the use of highly cross-linked ethylene-propylene rubbers (and other rubbers) in the blends to give so-called thermoplastic vulcanizates (TPVs). In these blends the rubber is present as finely dispersed particles in a polypropylene matrix. Compared to the simple blends, these materials have generally enhanced properties. 

Carbon membranes used for gas separation usually have a turbostralic structure (Pierson, 1993, Fig. 23.2a) with very little long-range order and are considered essentially isotropic. Figure 23.2a illustrates that there are also lattice vacancies in the sp hybridized carbon sheets, and pores are formed from packing imperfections between microcrystalline regions in the material, as shown in Figure 23.2b. Therefore, the pore structure in carbon... 

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