Cellulose Disordered regions

The proportions of ordered and disordered regions of cellulose vary considerably depending on the origin of the sample (cf. Table 9-1). Cotton cellulose is more crystalline than cellulose in wood. 

Another AFM study of Valonia cellulose I showed 06 to be in thegt orientation [213]. Work with NMR spectrometry has gone further. Difference spectra show that the surfaces have extensively disordered regions as well as gt and gg 06 orientations [214]. When cellulose I is in water, NMR studies have indicated that its surface hydroxyl groups are also iagt positions [215]. 

Acid hydrolysis is usually carried out with mineral acids at elevated temperatures. A portion of the cellulose reacts much faster than the remainder under these conditions. It is believed that the initial reaction occurs in the disordered regions and later extends to the ordered regions. The chain cleavage occurs and the products are glucose, soluble oligosaccharides, and an undissolved residue designated hydrocellulose. The weight of hydrocellulose is plotted... 

A complete definition of the fine structure of cellulose would entail a knowledge of the exact distribution of the size and shape of these ordered and disordered regions, a position which has not yet been achieved. However, a report claims that four types of material which differ in the degree of orderly arrangement of molecular chains can be distinguished in both native and regenerated celluloses, although quantitative measurements have only been made on one viscose-rayon fiber. These four types of material are ... 

It will be observed that the moisture regain and deuteration determinations yield lower values for degree of order than the chemical reaction techniques. As will be discussed later Zeronian et al. p) have demonstrated it is feasible to use the moisture regain technique in a manner which will permit differentiation between sorption on crystallite surfaces and sorption in the disordered regions of the cellulose. When allowance was made for the sorption on the crystallite surfaces they found the degree of order for cotton and mercerized cotton was similar to those found by the acid hydrolysis technique. 

These techniques are based on the premise that seme reactions occur preferentially in the disordered regions of the cellulose. Thus, initially, it is thouc t, reaction occurs in the disordered regions and extends later to the ordered regions. 

Periodate Oxidation. Hiis method is based on the preferential oxidation of the disordered regions of cellulose by sodium metc riodate (12,13). Conditions are selected so that the reaction is confined as far as possible to the Malaprade course resulting in the formation of 2,3 dialddiyde units. The course of the reaction is followed measuring oxidant consumption from the amount of periodate consumed. From plots of log oxidant consumption against time, a measure of the fraction of ordered material can be calculated in a manner analogous to that used with the acid hydrolysis method. 

Cellulase-producing bacteria include CelMomonas, Clostridium, Pseudomonas, Streptomyces, Thermonospora, and Ruminococcus, but bacterial cellulases digest cotton fibers to lower levels compared to amorphous celluloses (Ray et al., 2006). Cellulases have two functionally distinct domains in their structure, namely catalytic domain and cellulose substrate binding domain, linked by an interdomain, glycosylated linker peptide at either the N or the C terminal of the protein cellulose binding domains exhibit different affinities, specificities, some binding to crystalline cellulose, while others restrict themselves to the disordered regions. Catalytic domain has an active site in the shape of a tunnel or cleft where hydrolytic reactions take place (Cavedon et al., 1990 Lee et al., 1996). 

As noted above, prior to the development of the LiCl/DMAC cellulose solvent, permitting a homogeneous cellulose reaction medium, all cellulose reactions were heterogeneous in nature, at least initially, due to the two-phase nature of cellulose. That is, solid cellulose is composed of crystaline and disordered regions. The determination of the crystallinity of a cellulose sample will not be treated here. The interested reader is referred to T ripp s paper (9). 

The acid degradation occurs in the amorphous or disordered regions of cellulose (admittedly some removal occurs on the surfece of the crystallites but this does not result in a change in D.P.) Cleavage of the cellulose chain, in the amorphous or disordered... 

Notice that the small peak at 16.78° started to appear after the electrical activation. This means that some structural change took place during the electrical actuation, which might be associated with the crystallization of disordered region. In other words, recrystallization of disordered cellulose of EAPap sample could be accelerated by molecular rearrangement during the electrical... 

The final conclusion non-crystalline domains of cellulose are heterogeneous and can consist of fulty disordered regions, low-ordered regions and mesomorphous clusters. 

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