Cellulose structures

R. A. Young and R. M. RoweU, eds.. Cellulose Structure, Modification and Hydrolysis, WUey-Interscience, New York, 1986. 

Although acetylation thus renders the cellulosic structure soluble, cellulose acetate will still decompose below its softening point. It is thus necessary to compound cellulose acetate with plasticisers in order to obtain plastics materials of suitable flow properties. Other ingredients are also added at the same time. 

Whereas cellulose structure is now known in considerable detail [18,19], and the introduction of new solvent systems for dissolving cellulose has intensified the interest in studying point (ii), less is known about point (iii). Such knowledge, however, is a pre-requisite to control the properties of these polymers, hence increase their competitiveness as possible substitutes for synthetic polymers. 

At first glance, the HRC scheme appears simple the polymer is activated, dissolved, and then submitted to derivatization. hi a few cases, polymer activation and dissolution is achieved in a single step. This simplicity, however, is deceptive as can be deduced from the following experimental observations In many cases, provided that the ratio of derivatizing agent/AGU employed is stoichiometric, the targeted DS is not achieved the reaction conditions required (especially reaction temperature and time) depend on the structural characteristics of cellulose, especially its DP, purity (in terms of a-cellulose content), and Ic. Therefore, it is relevant to discuss the above-mentioned steps separately in order to understand their relative importance to ester formation, as well as the reasons for dependence of reaction conditions on cellulose structural features. 

Cellulose activation has been achieved by heating the polymer with dry LiCl, at 110 °C, under reduced pressure, 2 mm Hg, followed by addition of DMAc. It is important to introduce the solvent while the system is maintained under reduced pressure, in order to avoid hornification [56]. As expected, the activation conditions employed were found to be dependent on cellulose structure, samples with high DP and high Ic required pre-treatment, i.e., mer-cerization (cotton linters), and/or longer activation time. This solubilization... 

Fig. 2 Schematic representation of cellulose structures in solution Part A shows the fringed micellar structure. Parts B and C show possible chain conformations of celluloses of different DP. For high molecular weight cellulose, C, intra-molecular hydrogen bonding is possible...

Krassig HA (ed) (1993) In Cellulose Structure, Accessibility, and Reactivity. Gordon and Breach, Yverdon... 

Carbohydrates Sugars, starch, cellulose Structural components of plant cells easily released energy storage in plants and animals Sugars in fruits starch and cellulose in plants glycogen in animals below 1... 

SM), —OCITCOOH (carboxymethyl, CM), which are bonded to the cellulose structure via ether or ester linkages. Anion exchangers are formed by reacting the cellulose with epichlorhydrin and an amine, e g. —OCH2CH2N+(C2H 5)3 (triethylaminoethyl, TEAE). 

It has grown increasingly apparent that the non-crystalline portions of cellulose structures may play as important a role in the properties and behavior of cellulosic materials as the crystalline parts. X-ray diffraction studies have greatly extended knowledge of crystalline cellulose but in the case of the amorphous or disordered fraction the methods of study have necessarily been indirect and not completely reliable. 

Sisson has traced the evolution of current concepts of the crystalline part of cellulose structures. The fiber diagram obtained by X-ray diffraction is now known to be produced by a series of elementary crystals, called crystallites, which have a definite arrangement with respect to the fiber axis. It is also known that the crystallites in regenerated cellulose may be oriented to varying degrees with respect to the fiber axis and that the crystallites in regenerated cellulose and mercerized cotton differ from those in native fibers. These hydrate type crystallites appear to be more reactive chemically than the native type. 

X-ray diffraction methods have been indispensable in the development of the present concept of cellulose structure, but Mark15 has offered the opinion that such methods alone are ill-suited to the quantitative determination of crystalline and non-crystalline fractions. Chemical methods which depend upon the greater reactivity or accessibility of the incompletely ordered regions have offered an alternative approach to the problem. 

Horns and hooves were the raw materials for the early polymer preparations. These materials were ground up and treated in various ways so that they could be fabricated into such items as combs to use for ladies hair, and other specialty things of that sort. The next development was the use of cellulose from cotton or from wood as the raw material which was studied for making films and fibers. Work on the cellulose structure had provided information that it was a hydroxylated product, and by converting the hydroxyls to esters, the natural cellulose could be turned into a soluble material, which was spun into fibers and cast into films to make the first cellulose rayon-type material and cellulose films. 

Then, a few months afterward Meyer repeated and generalized the cellulose structure to include crystalline knots at regular intervals (28). This model was useful in explaining not only the properties of cellulose, but those of rubber as well. Staudinger called the concept unusable, incorrect, and dubbed it "the New Micelle Theory". 

Chanzy, H. (1990). Aspects of cellulose structure. In Cellulose Sources and Exploitation Industrial Utilisation, Biotechnology and Physico-chemical Properties, Kennedy, J.F., Phillips, G.O. and Williams, P.A. (Eds.). Ellis Horwood, Chichestir, UK, pp. 3-12. 

The question of parallel vs. antiparallel chain packing in cellulose I has been a controversial one practically since the first cellulose structure was proposed. A consensus appears to be forming, however, based on both diffraction analysis and other experimental evidence, that one of two possible... 

Further cellulosic structural variation is displayed by DP and crystallite size parameters. Aceiobacter and vascular plant primary wall celluloses are low in DP (2,000-6,000), while siphonocladalean and vascular plant secondary wall celluloses are relatively high in DP (> 10,000) (2). During cotton fiber development, the cellulose IV polymorph is produced during primary wall formation, while in secondary walls, cellulose I is observed... 

In tobacco primary cell wall the cellulose microfibrils observed individually or associated with bundles were also triple-stranded and left-hand helical. These observations are shown in Figure 10. Since cellulose is only 19% of the tobacco cell wall (17), the task of finding and identifying cellulose was complicated. For this reason A. xylinum which produces a pure ribbon of cellulose was used for studying cellulose structure. 

FIGURE 20-29 Cellulose structure. The plant cell wall is made up in part of cellulose molecules arranged side by side to form paracrys-talline arrays—cellulose microfibrils. Many microfibrils combine to form a cellulose fiber, seen in the scanning electron microscope as a structure 5 to 12 nm in diameter, laid down on the cell surface in several layers distinguishable by the different orientations of their fibers. 

Intermediate structure characterized by cell dimensions a — 13.81 A, b = 10.45 A, c = 7.92 A, / = 90°. The structure closely resembles that of cellulose and some cellulosic structural elements are present. The nitrate groups are located between the glucose rings in line with the a-axis. This structure exists between 12.3 and 13.2% N. 

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