Crystallinity of Cellulose and Its Significance for the Fiber Properties

The subject of this paper is treated mainly from the standpoint of cotton cellulose. The concept of crystallinity is discussed not simply in the narrow sense of degree of crystallinity but in the broader sense, which involves the whole fine structure of cellulose. With respect to crystallinity in the latter sense, there are four main factors, each involving a mean value and a distribution of values, both mathematical and spatial. These parameters, independent within certain limits, are (1) the nature of the crystal lattice, (2) the percentage of crystalline ma terial, (3) the size of the crystallites, and (4) the orientation of the crystallites. A well- planned analysis of covariance between these factors and the important physical properties of cotton would be desirable. This would be a task of considerable magnitude even if other variables not directly related to crystallinity, such as molecular size and morphology, were not active.

In the absence of such an analysis it is still possible to show the marked influence of some of these parameters on certain of the important physical properties of cotton. The relation of physical properties to the crystalline modification has been pointed out frequently by a com parison of the properties of the native fibers with those of regenerated fibers or with those of mercerized fibers. However, the changes in the percentage of crystalline material which usually occur in these transformations have not been evaluated exactly, and the conclusions drawn from previous studies should probably be checked.

Decreased crystallinity and higher content of amorphous cellulose is symbatic with in creased elongation, increased moisture regain, and increased chemical reactivity. Of particular interest in the last category is the special case of hydrolysis, a reaction in which it is possible that crystallization may occur.

Data on the third parameter, the size of the crystallite, are too scanty to permit any con clusions to be drawn. A study of this factor will have to wait on improved methods for meas uring the size and size-distribution of the crystalline areas.

The increase of fiber strength and of refractive index with improved orientation has been well established. A decrease in elongation usually accompanies this.