Crystalline fraction

Simmons, A.H., Michal, C.A., Jelinski, L.W. Molecular orientation and two-component nature of the crystalline fraction of spider dragline silk. Science 271 84-87, 1996. 

Similar results were obtained when o-phenylenediamine was reacted with mb. A range of low boiling compounds were separated from the reaction mixture after 10-12 hr heating at 220°C. These have been identified as diazadimethylsilaindane Vb (m.p. 76-78°C lit. (321 m.p. 75-83°C) and bis(phenylamino)dimethylsilane VIb (m.p. 46-48°C lit. (331 m.p, 45 2°C). They are probably formed by thermal decomposition of the intermediate unsymmetrical disilazane IVb". At 190-225°C/0.3 torr, a crystalline fraction with... 

X-ray diffraction methods have been indispensable in the development of the present concept of cellulose structure, but Mark15 has offered the opinion that such methods alone are ill-suited to the quantitative determination of crystalline and non-crystalline fractions. Chemical methods which depend upon the greater reactivity or accessibility of the incompletely ordered regions have offered an alternative approach to the problem. 

It is obvious that acid hydrolysis methods leave a number of unsolved problems and many minor disagreements to be ironed out. In general, however, the available results suggest that the natural celluloses consist chiefly of crystalline material which is only slowly eroded by acids. The non-crystalline fraction appears to be relatively more susceptible to hydrolysis than the crystalline fraction and to have a greater capacity to absorb moisture. In other words, the non-crystalline fraction is probably more reactive than the crystalline material, as Mark14 has suggested. In this connection the fact should not be overlooked that the surface layer of the crystallites is probably amorphous and hence relatively more reactive than the underlying layers. 

The evidence presented fails to suggest the causes for the large variations in crystallinity estimates which have been reported for similar cellulosic materials. There is a possibility that the different methods may not measure precisely the same characteristic of the material. It also may be that relative crystallinity is not a fixed quantity in any case but depends on circumstances involved in the measurement, such as the amount of swelling. The estimates reached by different methods need to be reconciled. At present, crystallinity estimates which depend wholly or in part on X-ray diffraction seem to be much higher than those obtained by chemical methods. The fact is that X-ray diffraction methods are ideal for studies of the crystalline fraction but are necessarily indirect in application to the non-crystalline fraction. The converse is true for the chemical approach. Apparently a combination of diffraction and chemical methods may adjust the existing differences. 

Figure 30 Variable-temperature solid-state NMR spectra (relative intensities distorted due to different relaxation rates) (a) 29Si (referenced to Q8M8 (Bruker) (b) 2H (isotropic peak truncated to show hem and crystalline fractions).285 Reprinted with permission from Mueller, C. Schmidt, C. Frey, H. Macromolecules 1996, 29, 3320-3322. 1996 American Chemical Society.

Solid PET feedstock for the SSP process is semicrystalline, and the crystalline fraction increases during the course of the SSP reaction. The crystallinity of the polymer influences the reaction rates, as well as the diffusivity of the low-molecular-weight compounds. The crystallization rate is often described by the Avrami equation for auto-accelerating reactions (1 — Xc) = cxp(—kc/"), with xc being the mass fraction crystallinity, kc the crystallization rate constant and n a function of nucleation growth and type. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

With semi-crystalline polymers we should always carefully distinguish between the behaviour below Tg and above Tg. Below Tg (such as with PEEK) the crystalline fraction, which is somewhat stiffer than the amorphous glass, dominates, so that E is somewhat higher. Above Tg, such as with PE and PP, the amorphous fraction, which is in the rubbery condition, is responsible for a significantly lower E. 

The BA is relatively well crystallized, which tends to lower hydrolysis when considering a long-term perspective. Dissolution rates are generally lowered if the crystalline fraction is higher and the growth of less resistant phases like meli-lites can be reduced. This is mainly the case for bulk compositions with a higher Si/Ca ratio. 

The atactic amorphous portion (9-16% of the total) contained in the obtained polypropylene has been separated by treating the raw polymer with n-heptane at room temperature. When operating in such a way, we have not separated the stereoblock fraction (extractable in boiling n-hep-tane) from the isotactic (not extractable in boiling n-heptane) fraction of polymer. The results reported in this paper are generally referred to the crystalline fraction, named non-atactic, which contains also some stereoblock polymers (at the considered polymerization temperatures, the latter generally correspond only to 5-7 % of the total) (9)... 

The higher crystalline fraction of the radiation cross-linked polyethylene even after a melt-freeze cycle has great technological merit for the heat-shrink packaging and electrical connector products. ... 

Compared with chemical cross-linking of PE, radiation curing produces a different product in many respects. The chemical cross-linking is done at temperatures near 125°C (257°F), where the polymer is in the molten state. Consequently, the cross-link density in the chemically cross-linked polyethylene is almost uniformly distributed, while there are relatively few cross-links in the crystalline fraction of the radiation cross-linked PE. The crystalline fraction of the radiation-processed polyethylene is greater than that in the chemically cured product. ... 

Fig. 69. Saturation magnetostriction, Ag, versus crystalline fraction, p, in (a) Fe89Zr7B4 and (b) FegjZr7B6Cu2 nanocrystalline alloys. After Slawska-Waniewska et al. (1996).

Thus, 60% of the total radioactive particle mass is found in the aerial cloud population. This is partitioned between the early, glass fraction and the late, crystalline fraction in accordance with the areas under the two lognormal curves which combine to produce the observed size distribution with mass. 

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