Polymorphy, crystalline

Structural studies of amylose have, in turn, revealed a wide range of crystalline polymorphy, both in chain conformation and in crystalline packing. An example is the group of V-amyloses that exist as complexes with small organic molecules, water, or iodine. The latter complex is particularly interesting because it displays an intense blue color. The V-amyloses can be prepared by precipitation or drying from solution, and they crystallize readily. Consequently, their crystal structures are of interest in connection with any regenerated form of starch material. 

Our approach to investigating the multiple crystalline form hypothesis in native celluloses is fourfold. First, 0 spectra of additional native celluloses are examined and further variations are evaluated in the light of the polymorphy hypothesis. Second, variations in the ratio of to Ig arising from chemical or mechanical treatments are studied. The crystalline forms may have different sensitivities to chemical attack or mechanical stress so that the to lo ratios could be... 

Figure 9 raises an Important question concerning crystalline polymorphy in the native celluloses. The hypothesis that higher plant celluloses like cotton and ramie are crystalline composites was suggested, in part, by the outer doublet and the sharper central feature of the Cl resonance in spectrum 9A. However, comparison of spectra 9A and 9C shows the central peak at Cl to be less intense at 200s compared with 1 ms, while the shape of the crystalline resonance at C4 remains constant. Since carbons C1-C5 relax at very nearly the same rates (32) in a T experiment, it is very likely that the signals for each of these carbons in... 

In order to demonstrate crystalline polymorphy more fully. Figure 12 shows the results of three Rhizoclonium cellulose experiments, like those of Figure 11, in which three different lines were initially perturbed. Spectrum 12A is the parent spectrum (like spectrum HA). The remaining six spectra, in pairs, consist of profiles of an original population perturbation (like spectrum HE) caused by the Dante inversion followed by the... 

A survey of several native celluloses reinforced the similarity of the higher plant celluloses to one another, although limits of resolution and questions of chemical purity in the cellulose chains make comparison difficult and less meaningful. A parallel survey of NMR spectra from the more chemically pure algal celluloses and the bacterial cellulose, Acetobacter xylinum, indicated a general uniformity, albeit these spectra were distinct from the spectra of the higher plant celluloses. These algal cellulose spectra, however, showed small variations, outside of experimental error, which were taken as evidence for crystalline polymorphy. 

The highly stereoregular syndiotactic polystyrene (s-PS) has received considerable interest recently. Depending on the thermal history, s-PS possesses several polymorphie erystalline structures. Helical (TTGG) conformation is formed in solution reeovered s-PS, whereas the thermodynamically favored all-trans (TTTT) eonformations are formed from either melt or annealing at an elevated temperature. The extent to which thermal history affects the crystalline strueture and erystallization kinetics for s-PS has been extensively studied. ... 

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