Mercerization of cellulose. III. Changes in crystallite sizes

When cellulose is mercerized slowly, the initial conversion is from the cellulose I crystal structure to that of Na–cellulose I. The conversion is a crystal‐to‐crystal phase transformation, without passage through an amorphous state. The analysis of crystallite sizes of cellulose I and Na‐cellulose I during this transformation has shown that the process takes place in two steps. The first is a rapid step, occurring in approximately 1/7 (or less) of the total conversion time, resulting in a conversion ratio of ca. 65%. The measured crystallite sizes of both cellulose I and Na–cellulose I remain constant during this stage, at ∼ 62 and ∼ 35 Å, respectively. In the following slow step, complete conversion to Na–cellulose I takes place, and the crystallite size of cellulose I decreases steadily until disappearance. The crystallite size of Na–cellulose I increases steadily at the same time, to a final value of ∼ 50 Å. Simultaneously, the unit cell parameters of cellulose I change significantly. The observation of a two‐step conversion is consistent with a proposed mercerization mechanism in which the conversion is assumed to begin in the amorphous phase of the cellulose fiber.