Hydrogen bridges, cellulose

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

Some polymers, such as cellulose, although linear in structure, have such a strong molecular interaction, mostly due to hydrogen bridges and polar groups that they do not soften or melt. Consequently, the transition temperatures as such are less important to this class of polymers. Normally they are highly crystalline, with a crystalline melting point (far) above the decomposition temperature. Their physical behaviour - except for... 

Fig. 5.4 Formation of hydrogen bridges (...) between butyl amine and cellulose [6,11]...

Cellulose and methyl cellulose Dry or moist Hydrogen bridges ... 

As the hydroxyl group content in the cellulose acetate increases, its solubility in solvents increases as does its tendency to form hydrogen bridges. At a substitution level near 0.8, it even achieves solubility in water. Table 5.94 [656]. 

The single glucose molecules are rotated at 180° (syndiotactic arrangement) (Fig. 2.25) within the polysaccharide. Multiple hydrogen bridge bonds act between the polymer chains. Because of this intense sterical gearing, the final cellulosic fibers lack plasticity. Therefore, cellulosic fibers cannot be texturized and are difficult to iron. 

According to the solubility parameters blends of NBR with cellulose acetate, propionate, or acetate butyrate should give a homogeneous phase. Owing to the formation of hydrogen-bridge bonds, however, the theory fails the polymer blends consist exclusively of two phases and they have no resistance to ozone, even when the fluxing has been carried out at temperatures of around 180°C. 

It has been proposed that in alkali cellulose, alkali (NaOH) and water bridge the cellulose molecules (23). However, it does not seem likely that all direct intermolecular hydrogen bonds in alkali cellulose are disrupted, inasmuch as the fibrous morphology is retained. Additionally, there have not been any reports of any precipitates of con lexes between oligomers and alkali in aqueous solutions. 

The constants for H-bridging complex formation between D-glucose, cellobiose, xylose, and phenol as models for cellulose and different 0-basic dipolar molecules such as N-methyl-caprolactam HMPA, and DMSO in chloroform and ethanol have been determined. Evidence of hydrogen bonding in D-fructose as shown in (28) has been obtained from variable temperature high field n.m.r. spectroscopy. The preponderance of the -furanose form in DMSO is attributed to this feature. The kinetics of the tautomeric equilibria were studied. 

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