Cellulose shapes crystal structures

Another way to learn of the likely molecular shapes for cellulose depends on extrapolation of the shapes that are found in crystal structures of molecules such as cellobiose (Chu and Jeffrey 1968), a-cellobiose complexed with Nal and HjO (Peralta-Inga et al. 2002), cellobiose octaacetate (Leung et al. 1976) and related compounds (French and Johnson 2004a). Similar, although less accurate. 

Figure 15-1. The six different chain shapes from the crystal structures of the cellulose polymorphs I, II, and IIIj, superimposed at their Cl, 04, and C4 atoms to show the differences in the molecular shapes. Indicated for the five-residue segments are the linkage torsion angles, N and P. There are two unique chains in both the ip and II structures (with 06 tg) and one each from la and IIIj (with 06gt). The single-chain la structure has two sets of N and P values because of its lower symmetry. Atomic munbering is indicated the reducing end is to the right and the nonreducing end is on the left...

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at 80 °C it is customary, particularly for navy blues, to use a mixture of two or more components of similar hue. If these behave independently, each will give its saturation solubility in the fibre. In practice, certain mixtures of dyes with closely related structures are 20-50% less soluble in cellulose acetate than predicted from the sum of their individual solubilities [87]. Dyes of this kind form mixed crystals in which the components are able to replace one another in the crystal lattice. The melting point depends on composition, varying gradually between those of the components, and the mixed crystals exhibit lower solubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilar molecular shape do not form mixed crystals, the melting point curve of the mixture shows a eutectic point and they behave additively in mixtures with respect to solubility in water and in the fibre. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

Structure modifiers was tested for modification of the crystal form of precipitated LA. The most commonly noted ones include dextrin, carboxymethyl cellulose, and PVA. The presence of dextrin promotes formation of the ot-form [16] while the presence of organic dyes (eosin, erythrosin, or neutral red) at precipitation time enhances the formation of the p-form [15]. The crystals of p lead azide are formed in the shape of long needles (Fig. 4.1). 

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