Crystalline X-ray diffraction

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

In 1955, Pruitt and Baggett (4,5) reported the polymerization of propylene oxide catalyzed by the reaction product of ferric chloride and propylene oxide, i.e., Pruitt-Baggett catalyst . Polypropylene oxide obtained from DL-monomer could be fractionated into two parts, one rubbery and another resinous. The latter fraction gave a discrete crystalline X-ray diffraction pattern. 

Let s summarize what we have just learned. Polymer crystallization is incomplete (i.e., polymers are semi-crystalline ). X-ray diffraction experiments tell us the chain conformation and the mode of packing. 

Pyrolysis of one of the products whose NMR spectrum and elemental analysis indicated a constitution of [(CH3SiH)o.73(CH3Si)o.i[(CH3)-SiCH2CH2Si(CH3)NH] 117] indicated a formal composition of 1 SiC + 0.033 Si3N4 + 0.04 C. Thus the stoichiometry and reaction conditions in this experiment gave a high yield of a ceramic product that was silicon carbide contaminated with only minor amounts of silicon nitride and free carbon. When such a pyrolysis was effected to 1500 C, the ceramic product was at least partly crystalline. X-ray diffraction showed lines due to P-SiC only. 

Although polyvinyl acetate is amorphous, PVA derived from the former in most cases gives a crystalline x-ray diffraction pattern. When PVA fiber is drawn, an x-ray fiber diagram is... 

Degree of crystallinity % X-ray diffraction from collagen fibers 20-40 (11)... 

X-ray diffraction has also been used to measure crystallinity. X-ray diffraction analysis showed that the polychloroprene unit cell is orthorhombic, a = 0.88 nm, b = 1.02 nm, and c = 0.48 nm (128). 

An amorphous polymer does not exhibit a crystalline X-ray diffraction pattern, and it does not have a flrst-order melting transition. If the structure of crystalline polymers is taken to be regular or ordered, then by difference, the structure of amorphous polymers contains greater or lesser amounts of disorder. 

There are four basic methods in wide use for the study of polymer crystallinity X-ray diffraction, electron diffraction, infrared absorption, and Raman spectra. The first two methods constitute the fundamental basis for crystal cell size and form, and the latter two methods provide a wealth of supporting data such as bond distances and intermolecular attractive forces. These several methods are now briefly described. 

Apparently the only exception is N,N -tetramethyl sulphonyl diamine in which both gas electron diffraction and crystalline X-ray diffraction determined larger than tetrahedral N-S-N bond angles (110 5+0 4 and 112.6+0.4 , respectively). 

Vinyl acetate content (mol%) Melting range (°C) Degree of crystallinity (X-ray diffraction) (%)... 

Polymer A showed a crystalline X-ray diffraction pattern and an endothermic peak at 31-42 C in differential scanning thermograms. The polymer had birefringence below 90 C under a polarizing microscope. It is of interest to speculate as follows. Both of the hydrocarbon side chains and the polysaccharide main chains are crystalline and the polymer A exhibits a unique two-stage melting process of both crystalline regions. The polymer possibly forms a liquid-crystalline mesophase between these two transition temperatures. 

Such retrograded a. are insoluble in water below 150 °C. They are structured in elementary cells of double helices which exhibit the typical crystalline x-ray diffraction pattern of the B-polymorph. This type is found in native starches of roots and tubers and always in gelatinized and retrograded systems as the crystalline component. When retrogradation is forced above 50 °C, the A-polymorph is formed. Association of linear a-l,4-linked polyglucan chains into double-helix structures is the functional origin of the typical behavior of a. as well as a.-containing starches ... 

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