X-ray crystallinity

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... 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

Crystallinity, X-ray, Cu/ZnO/Al O, 31 293 Crystallites, see also Alloy films electron micrographs of, 22 135-138 Crystallography... 

In several instances 0.5-mm. thick sheets of Surlyn A were hung over concentrated aqueous hydrochloric acid in a desiccator for about one week, then dried in vacuum and irradiated. Infrared examination of thin films conditioned in this way for 3 days indicated little or no increase in crystallinity x-ray examination indicated the presence of small amounts of crystalline sodium chloride. 

Determination of the proportions of crystalline and amorphous material in partially crystalline polymers. Knowledge of the unit cell dimensions in high polymer crystals leads to a knowledge of the density of the crystalline regions. If the density of amorphous regions is also known, either by measurement of the density of an entirely amorphous specimen (if this can be obtained) or by extrapolation of the liquid density/temperature curve, it is possible to calculate, from the measured density of any partially crystalline specimen, the proportions of crystalline and amorphous material. Since the physical properties of polymer specimens are profoundly influenced by the degree of crystallinity, X-ray determinations of crystallinity are much used in such studies (see Bunn, 1957). 

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. 

The M(DLD)3" complexes (M = Co, Rh, Cr) can be isolated as crystalline, x-ray isomorphous salts of the Me3PhN+ cation with two waters of hydration. The diamagnetism and electronic spectra of the Co(lII) complex are consistent with a d6 octahedrally coordinated ion. Cyclic voltametry shows quasi-reversible oxidation at +0.14 V (in CH2C12 versus Ag AgI). A separation between the anodic and cathodic waves of 0.60 V suggests that a reversible rearrangement occurs upon oxidation of the Co(DED)3 anion. An investigation of the oxidation product is currently underway. 

Polyoxybenzoate is a stiff chain, lyotropic liquid crystalline material, as was discussed on the basis of its copolymers with ethylene terephthalate (see Sect. 5.1.4). The crystal structure of the homopolymer polyoxybenzoate was shown by Lieser 157) to have a high temperature phase III, described as liquid crystalline. X-ray and electron diffraction data on single crystals suggested that reversible conformational disorder is introduced, i.e. a condis crystal exists. Phase III, which is stable above about 560 K, has hexagonal symmetry and shows an 11 % lower density than the low temperature phases I and II. It is also possible to find sometimes the rotational disorder at low temperature in crystals grown during polymerization (CD-glass). 

Electron diffraction crystallinity X-Ray crystallinity (RULAND s Method)... 

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. 

Three groups of materials can be distinguished (a) non-crystalline (X-ray amorphous) (b) zeolite type and (c) crystalline, non zeolite type. 

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