Para-crystallinity

Parachlorobenzotrifluoride, 6 1341 Paracortex, in wool fibers, 22 173 Para-crystalline lattice model, 24 464 Paracyclophane synthesis, 24 38 PARAFAC (PARAllel FACtor analysis),... 

Recently Bourges, Small, and Dervichian (5) reported that a para-crystalline lamellar structure of egg lecithin can solubilize cholesterol up to a maximum of one molecule of cholesterol per molecule of lecithin. However, they conclude that this should not be considered as a molecular association but rather the consequence of the relative arrangement of the molecules in the lamellar structure which is a mutual (solid) solution of lecithin and cholesterol. They also reported that the state of compression in the lamellar structure corresponds to that of a highly compressed mixed monolayer of lecithin-cholesterol. The NMR results of Chapman and Penkett (8) also appear to indicate that solubilization of cholesterol in egg lecithin dispersions results in a highly packed structure in which fatty acyl chains possess little molecular motion. Our results from lecithin-cholesterol monolayers also suggest that these mixed mono-layers are two-dimensional solutions with no specific interaction and that the apparent condensation in some instances is caused by the steric factors of the fatty acyl chains and not by the interaction or association between lecithin and cholesterol. 

Four major computational steps are necessary to separate the individual peaks and the different profile-broadening components (i) correction and normalisation of the diffraction data, (ii) resolution of the total peak scattering from the so-called background scatter, and resolution of crystallographic, para-crystalline, and amorphous peaks from each other, (iii) correction of the resolved profiles for instrumental broadening, (iv) separation of the corrected profiles into size and distortion components. In this paper we will discuss these steps in turn, but most attention will be paid to the hitherto largely neglected step of profile resolution. 

Another relationship, based on Hosemann s theory of para-crystallinity, is (17)... 

Although initially the model has been developed for well-oriented para-crystalline fibres, experimental results have shown that it can also be applied to well-oriented semicrystalline fibres. 

Microfibrils made up Solution-spinning Para-crystalline of rigid rodlets with gas gap and microfibrils... 

TABLE 19.11 Physical properties of para-crystalline fibres in comparison with other reinforcing materials... 

Disorder is a central issue for CPs. Real materials are almost always highly disordered the crystalline regions—in fact, better described as para-crystalline, for instance—are small (lateral dimensions ca. 10 nm). Accurate description of the structures at all scales and understanding of their influence on the electronic properties have not been achieved. Dopingundoping cycles, corresponding to large-scale motion of often bulky ions, usually increase the disorder irreversibly. 

It is the heteropoiysaccharides of plants that bestow cellulosic composites with the ability to absorb impact, the ability to absorb moisture, and the ability to create pores in the form of free volume in amorphous (disordered or para-crystalline) materials [58,59]. Modification by reducing molecular regularity has the additional benefit of creating a transition from a focus on mechanical (structural) functions to an emphasis on energy storage and gel formation. Reduction in order translates into ease of hydrolysis, enzyme accessibility, rate of nutrient release for decay organisms, water absorption and swelling. Reduction in order is achieved by the introduction of monosaccharide units, and of bond types, which differ from those of cellulose. The principal monosaccharides involved in the heteropoiysaccharides of plants are shown in O Fig. 12. 

The images indicate the predominance of an amorphous matrix in all samples and the presence of some residual granular structures. The residual granular structures were quite scarce in the gel-T sample and more numerous in both gel-HP and gel-HP-T samples. Moreover, the residual granules observable in the gel-T sample appeared to have maintained a crystalline/para-crystalline structure (as observable under polarized light conditions) while the granules in the both gel-HP and gel-HP-T samples... 

The weak reflections between 10 and 20 °20 observed for the C-Fe-pp aggregate also suggest the incipient formation of Fe-polymer para-crystalline phases. 

Figure 5.4. Schematic drawing of the structure of segmented polyurethanes stretched approximately 500%. The hard segments have turned into the direction of elongation and form para-crystalline layer lattices. The elongation crystallization of the soft segments has been reduced or has disappeared. On relaxation, the orientation of the hard segments is largely maintained. (Bonart, 1968.)...

Isotactic polypropylene is known to crystallize when cooled quickly from the melt into a crystal form which was called a smectic, mesomorphic form. The X-ray diffraction pattern resembled clearly that of a smectic material. Later, this structure was also called para-crystalline to indicate the poor crystalline order. It was found that this structure is metastable below 335 K. At room temperature it has been reported to persist for over 18 months so that it should be called a CD-glass. The proposed transition mechanism to the stable crystal form involves intramolecular helix... 

The sketch includes amcrphcus areas (defects), chain ends, rotated crystals, loose folds, sharp folds, areas of para-crystalline structure, extended-chain sections, voids, sheared regions, chain-kinks, single fibrils, migrating folds, and single-crystal regicns. 

Application of the negative staining-carbon film procedure to the much smaller human erythrocyte catalase (relative molecular mass 256 kDa) has resulted in the formation of several different para-crystalline and truly crystalline 2D forms, an occurrence also encountered with the E. colt chaperone GroEL. One of the catalase 2D crystal forms is... 

The anisotropic cokes and carbon blacks partially consist of para-crystalline structures. The degree of orientation of the graphitic carbons can be determined by the spacing of the graphite layers for the ideal crystal, it is 0.3354 nm, for coke 0.34 to 0.35 nm, and for carbon blacks 0.36 nm. Figure 13.1 shows the crystal lattice of hexagonal graphite. 

Kulicke WM, Kull AH, Kull W, Thielkmg H, EngeUiart J, Pannek JB (1996) Characterization of aqueous carboxymethylcellulose solutions in terms of their molecular structure and its influence on rheological behavior. Polymer 37(13) 2723-2731 Kulicke WM, Reinhardt Fuller UGG, Arendt O (1999) Characterization of the flow properties of sodium carboxymethylcellulose via mechanical and optical techniques. Rheol Acta 38 26-33 Kulshreshtha AK, Dweltz NE (1973) Para crystalline lattice disorder in cellulose - 1. Reappraisal of the application of the two-phase hypothesis to the analysis of powder x-ray diffractograms of native and hydrolyzed cellulosic materials. J Polym Sci 11 487 97 Mathur NK, Mathur V (2001) Chemical Weekly, July Edition, 155... 

Marsh H, O Hair TE, Wynne-Jones Lord, The carbon-atomic oxygen reaction - surface-oxide formation on para-crystalline carbon and graphite. Carbon 1969 7(5) 555-558. 

Figure 2. Features of the SFAM. A diagrammatic picture showing haw the SFAM models a cross-section through a fibril aggregate (16fibrils surrounded by water). White squares are cross-sections through the individual cellulose I fibrils, gray color represents water. Polyglucan chains (not shown explicitly) are perpendicular to the plane cf the pcper. Annotations within the figure show some example positions of the different cellulose forms, C= crystalline cellulose, PC=para-crystalline cellulose, AS accessible fibril surface, 11 inaccessible...

These different crystallization behaviors can be related, in the case of SEP treatment, to a reorganization at high temperatures of amorphous and/or para-crystalline cellulose regions, releasing the strains existing in the native cellulose, mainly due to the interactions with hemicelluloses and lignin... 

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