Cellulose stmcture

The chemical and physical properties of cellulose depend ia large measure on the spatial arrangements of the molecules. Therefore, cellulose stmctures have been studied iatensively, and the resulting information has been important ia helping to understand many other polymers. Despite the extent of work, however, there are stiU many controversies on the most important details. The source of the cellulose and its history of treatment both affect the stmcture at several levels. Much of the iadustrial processiag to which cellulose is subjected is iatended to alter the stmcture at various levels ia order to obtain desired properties. 

H.A. Krassig, "Cellulose - Stmcture, Accessibility and Reactivity", Polymer Monographs, ed. M.B. Huglin, Gordon and Breach Science Publishers, Amsterdam, 1993. 

Cellulose stmcture and conformation understand structure and properties... 

Rayon is an ester made by treating cotton with acetic acid. Use the cellulose stmcture given in Section 22.3 to draw a Lewis structure for a short rayon molecule. Hint An acid-base reaction occurs randomly.)... 

According to reports, SMPU treatment contributes to physical surface modifications of textile materials, while DMDHEU and LA treatments cause chemical modifications to the cellulose stmcture (Greenwood, 1987 Han and Chen, 1998 Jang et ai, 1993 Liu etal, 2005 Trask-Morrell et al, 1996 Yanai, 2001 Yanai et al, 2005, 2006 Yanai and Shimizu, 2006 Zeronian et al, 1990). As will be described below, there are significant differences to the working principles of the SMPU, DMDHEU and LA treatments respectively, when they are applied to cellulose materials. 

SMPU is a long polymer chain, and with the exception of some special cases, its molecular weight is generally larger than 10 000. It is impossible for such a large chain to penetrate into the cellulose stmcture and react with the inner part of the cellulose fibers. Therefore, SMPU must be coated or grafted onto textile materials. 

Dimethyloldihydroxylethyleneurea (DMDHEU) molecules penetrate into the cellulose stmcture, cross-link with the hydroxyl groups in the amorphous region and thus reduce the slipping of cellulose chains. Macroscopically, fabric appearance, crease retention and wrinkle recovery performance can be increased by DMDHEU treatment, while the fabric strength can be significantly reduced. 

Amorphous areas in the cellulose fiber are responsible for the finest capillaries formation that is a sub-microscopic space inside the cellulose stmcture is formed. The presence of the sub-microscopic system of capillaries in the cellulose fibers is of paramount significance, because it is the channel of chemical reactions by which water-soluble reagents penetrate deep in the cellulose stmcture. More active hydroxyl groups interacting with various substances are also disposed here [15-18],... 

H. A. Krassig, Cellulose - Stmcture, Accessibihty and Reactivity , Polymer Monographs, 11, Gordon and Breach, London, 1993. 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

Fully modified yams had smooth, all-skin cross sections, a stmcture made up of numerous small crystaUites of cellulose, and filament strengths around 0.4 N/tex (4.5 gf/den). They were generally known as the Super tire yams. Improved Super yams (0.44—0.53 N/tex (5—6 gf/den)) were made by mixing modifiers, and one of the best combiaations was found to be dimethylamine with poly-(oxyethylene) glycol of about 1500 mol wt (25). Ethoxjlated fatty acid amines have now largely replaced dimethylamine because they are easier to handle and cost less. 

The bulk properties of regenerated cellulose are the properties of Cellulose II which is created from Cellulose I by alkaline expansion of the crystal stmcture (97,101) (see Cellulose). The key textile fiber properties for the most important current varieties of regenerated cellulose are shown in Table 2. Fiber densities vary between 1.53 and 1.50. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

Fig. 9. SEM photographs of cellulose acetate membranes cast from a solution of acetone (volatile solvent) and 2-meth5l-2,4-pentanediol (nonvolatile solvent). The evaporation time before the stmcture is fixed by immersion ia water is shown (24).

Other blends such as polyhydroxyalkanoates (PHA) with cellulose acetate (208), PHA with polycaprolactone (209), poly(lactic acid) with poly(ethylene glycol) (210), chitosan and cellulose (211), poly(lactic acid) with inorganic fillers (212), and PHA and aUphatic polyesters with inorganics (213) are receiving attention. The different blending compositions seem to be limited only by the number of polymers available and the compatibiUty of the components. The latter blends, with all natural or biodegradable components, appear to afford the best approach for future research as property balance and biodegradabihty is attempted. Starch and additives have been evaluated ia detail from the perspective of stmcture and compatibiUty with starch (214). 

Structure of the Cell Wall. The iaterior stmcture of the ceU wall is shown in Figure 6. The interfiber region is the middle lamella (ML). This region, rich in lignin, is amorphous and shows no fibnUar stmcture when examined under the electron microscope. The cell wall is composed of stmcturaHy different layers or lamellae, reflecting the manner in which the cell forms. The newly formed cell contains protoplasm, from which cellulose and the other cell wall polymers are laid down to thicken the cell wall internally. Thus, there is a primary wall (P) and a secondary wall (S). The secondary wall is subdivided into three portions, the S, S2, and layers, which form sequentially toward the lumen. Viewed from the lumen, the cell wall frequendy has a bumpy appearance. This is called the warty layer and is composed of protoplasmic debris. The warty layer and exposed layer are sometimes referred to as the tertiary wad. 

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