Amorphous fraction, cellulose sample

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,... 

Deuteration has also been used as a gravimetric method for determining accessibility, the results for cotton being in exact agreement with those of Valentine. The linear relationship which exists between moisture sorption and the fraction of material found to be amorphous to infrared has been confirmed for thirteen different t5qies of cellulose, and it is suggested that a difference in the ratio of moisture regain to amorphous content between samples of cellulose I and cellulose II reflect differences in their sorption behavior and, hence, in the structure of their amorphous... 

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. 

Zeronian and co-workers (3) proposed that a better estimate of the fraction of amorphous material (F) in a cellulose sample can be obtained from its moisture regain (M ) if the moisture regain of the microcrystalline cellulose produced from hydrolysis (M ) and the moisture regain of amorphous cellulose (M ) are taken inEo consideration iy using equation The preparation of the microcrystalline cellulose is critical in order for it to be used as a reasonable facsimile of the crystalline regions present in the fiber (3). 

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. 

Conqparison of the values for F and A for a cellulosic sanple then will yield an estimate of the contribution of the disordered (or amorphous) regions relative to that of crystallite surfaces towards the accessibility of the sample. The fraction of crystalline material (X) is given in equation 16. 

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 oxide method. Hailwood and Horrobin [254] developed an equation for water sorption of cellulose based on a solution theory that allowed the calculation of the fraction of the sample inaccessible to water. 

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