Amorphous fraction, cellulose

A cellulose sample, prepared physically from pulp, dissolved by as much as 58 % (at c = 5 %) in alkali and a significant increase in amorphous content was observed, although Mv was as low as 1.9 x 104. Accordingly, an increase in the solubility of cellulose having crystal form I seems to reflect an increase in amorphous content or a decrease in Mv. However, no physical treatment can produce cellulose having a solubility Sa (at c = 5 wt%) >0.85. Kamide et al. 29) disclosed the relationship between Sa and the amorphous fraction. Table 15 summarizes the crystal form, Mv, Sa,... 

Polymer crystallization may also complicate the transition from the amorphous state to the intermediate liquid crystalhne state. Even small crystalline fractions in a polymer prevent free movement of macromolecules This is the case, for example, with cellulose. Cellulose fibres, obtained via cellulose xanthate (viscose), in the process of forming partially crystallize. Although the amorphous fraction in these fibres is large (up to 70-75 %), at a short-term heating of the fibres above the glass-transition temperature (240-260 °C) only slight self-elongation of fibres is observed, which can be attributed to the transition to the liquid crystalline state at the expense of the amorphous fraction... 

Early studies of cellulose degradation revealed for the first time that hydrolytic agents selectively attacked the amorphous fraction (1) of the polymer, breaking and reordering accessible chain segments (2). Later work on both poly(ethylene terejhthalate) (3,4) and polyethylene (d) confirmed that localized reactions were characteristic of all polymers with impervious crystalline regions. 

A variety of methods have been developed to relate accessibility to microstructure. Almost all of them begin with the premise that the cellulose can be regarded as having a crystalline fraction and a disordered or amorphous fraction. It is then assumed that the amorphous or disordered fraction is accessible while the... 

Another example of the confusion which can arise is in equating the accessibility of the cellulose to its amorphous fraction. The amorphous fraction may determine the accessibility of the hydroxyl groups to water or the glucosidic bonds to dilute mineral acid but is not necessarily related to the accessibility of the cellulose to a large molecule such as an enzyme. For example, the amorphous material in wood is accessible to water and hydrogen ions but is completely inaccessible to an enzyme. If, therefore, a correlation is found between the amount of amorphous cellulose in a sample and its susceptibility to attack by cellulase, this is coincidental, and the true reason for the increase in reactivity must be caused by some other change which has occurred simultaneously with the change in crystallinity. 

Moisture sorption was measured for each cellulose sample as described in the previous paper (70). It is known that moisture regain of cellulose is proportional to the amorphous fraction, or the readily accessible portion of cellulose. Figure 2 shows that moisture regain increases with the milling time which indicates the increase of the amorphous fraction. 

Clearly, the crystallinity increased by 5-15% it is believed to be due to loss of lignin and hemicellulose during alkali treatment. The number of water molecules sorbed on each glucose unit at 66% RH appear to decrease with x (%), suggesting an increase in the accessibility of water due to increase in the amorphous fraction of cellulose. 

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. 

It is obvious that acid hydrolysis methods leave a number of unsolved problems and many minor disagreements to be ironed out. In general, however, the available results suggest that the natural celluloses consist chiefly of crystalline material which is only slowly eroded by acids. The non-crystalline fraction appears to be relatively more susceptible to hydrolysis than the crystalline fraction and to have a greater capacity to absorb moisture. In other words, the non-crystalline fraction is probably more reactive than the crystalline material, as Mark14 has suggested. In this connection the fact should not be overlooked that the surface layer of the crystallites is probably amorphous and hence relatively more reactive than the underlying layers. 

Highly ordered crystalline cellulose has a density as high as 1.63 g cm whereas highly disordered amorphous cellulose has a density as low as 1.47 g cm 3. High-molecular-weight native cellulose, which is insoluble in 17,5% aqueous sodium hydroxide solution, is called alpha cellulose. The fraction that is soluble in 17.5% sodium hydroxide solution but insoluble in 8% solution, is called beta cellulose, and that which is soluble in 8% sodium hydroxide solution is called gamma cellulose. 

Adsorption of water is accompanied with swelling of wood. Since cellulose constitutes almost 50% of the wood substance, its fractional contribution to sorption is the highest. Swelling of wood is thus manifested by the adsorption of water by hydroxyl groups located on surface of micells and amorphous regions of cellulose, crystalline zones being impenetrable. Reduction in... 

Water vapor at room temperature will not penetrate well-defined crystallites but will be adsorbed in the amorphous regions. Consequently, moisture sorption measured gravimetrically at a given relative vapor pressure and temperature has been used to determine order in cellulosic materials. In the case of Valentine [252] and Jeffries [253], the fraction of ordered material was obtained by correlating moisture sorption with values obtained by the deuterium... 

Zeronian et al. [261] hypothesized that if microcrystalline cellulose is prepared that is a facsimile of the crystalline regions present in the fiber then the fraction of amorphous material F) of the fiber can be obtained from the relation... 

At the molecular level, portions of cellulose assume a highly structured crystalline form, while other parts are amorphous. Amorphous cellulose is more easily digested by enzymes than the crystalline parts. The fraction of crystalline cellulose is called the relative crystallinity, an index of digestibility. The crystallinity of commercially available substrates lies between 85 and 90%. Even so-called non-crystalline cellulose has a relative crystallinity of 65%. 

---