Cellulose amorphous regions

Another finding from Fig. 3 is that PMMA extracted from the cellulose structure shows a melting behavior at the highest temperature (350 °C) among the PMMA samples. This indicates that the PMMA formed within the cellulose amorphous region has a very high molecular weight. 

Figure 4.3.1 Schematic representation of the enzymatic degradation of starch from plants, fungi and bacteria, and cellulose from plants, (o), Reducing glucosyl residues ending with a half-acetal unit ( ), nonreducing glucosyl residues corresponding to full ac-etals f—), crystalline regions of cellulose ( ), amorphous regions of cellulose. (From Kennedy, 1988.)...

The cellulose amorphous regions have lower density compared to the crystalline regions, thus are more susceptible to the action of the acid during the acid hydrolysis process, which breaks the structure into individual crystallites, while the crystalline region remains intact [25]. 

Cellulose is the main component of the wood cell wall, typically 40—50% by weight of the dry wood. Pure cellulose is a polymer of glucose residues joined by 1,4-P-glucosidic bonds. The degree of polymerization (DP) is variable and may range from 700 to 10,000 DP or more. Wood cellulose is more resistant to dilute acid hydrolysis than hemiceUulose. X-ray diffraction indicates a partial crystalline stmcture for wood cellulose. The crystalline regions are more difficult to hydrolyze than the amorphous regions because removal of the easily hydrolyzed material has Htde effect on the diffraction pattern. 

Cellulose crystallinity has been shown to affect pyrolysis rates and Ea s (2,26,27). The initial low temperature decomposition is reported to occur first in the amorphous region (5,26,27). Also,... 

As water swells cellulose in an intercrystalline way (i.e. only within the non-crystalline amorphous regions), a relationship would be expected between accessibility and moisture uptake, and this is indeed found (Figure 5.5). Refining causes cellulosic fibres to swell and it would therefore be expected to cause a change in the water adsorption isotherm. This is indeed observed (Figure 5.6). 

It has been claimed that the cryst(or ordered) regions of cellulose resist the penetration of reagents while the amorphous regions are more reactive. This definition is only approximate... 

Treatment of cellulose with acids results in preferential hydrolysis in the more accessible amorphous regions and produces a product known as microcrystalline cellulose (MCC). MCC is used to prepare fat-free or reduced-fat food products, to strengthen and stabilize food foams, as a tableting aid, and as a noncaloric bulking agent for dietetic foods. It has GRAS status. 

However, with naturally occurring macromolecules, such as cellulose, the older fringed micelle concept is believed to apply. This represents a crystalline polymer, (it would be more correct to speak of semicrystalline or partially crystalline polymers since a material consisting of chain molecules can never be completely ordered), made up of ordered (crystalline) domains interspersed with disordered (amorphous) domains, so that each polymer chain passes through several crystalline and amorphous regions (Figure 4). 

The problem of modeling the hydrolysis kinetics is complicated by the fact that cellulose is a solid substrate consequently, the reaction can be surface limited (5,12). Furthermore, some sites are more susceptible to hydrolysis—e.g., the amorphous regions as well as specific regions of the crystalline cellulose such as edges, corners, and dislocations. Several investigators (17,20,36) have suggested that the kinetic model should be based on a shrinking site model in which the number of susceptible... 

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