Effect of bound water on structural change of regenerated cellulose

Phase transition of water sorbed in regenerated cellulose (II), for water contents ranging from 0 to 1,0 (water/dry cellulose, g/g), was investigated by differential scanning calorimetry (DSC). It was found that the amount of water bound to each glucose unit is ca. 3 mol considering that water diffuses only into the amorphous region of cellulose. The increase of this amount by adding excess water to cellulose suggests that the amorphous region of cellulose II increases in the presence of water. The transient state, at a water content ranging from 0,4 to 0,6, where the intermolecular distance expands, is reflected in an intermediate crystallization at a temperature between the crystallization of free water and freezing bound water. Mechanical properties of cellulose II under moist conditions were related to the results obtained by DSC. Further, the strength at break of cellulose II was compared with that of natural cellulose (I). The strength of cellulose II decreases in presence of bound water, whereas that of cellulose I increases. Based on the comparison of the results of DSC and mechanical properties of cellulose I and II, the difference is attributed to the structural change of the amorphous region when water molecules are bound to the hydroxyl groups of cellulose. The defect or internal strain of the amorphous region of cellulose I decreases and the molecular chain is transformed into a more regular arrangement when water molecules break hydrogen bonds. On the other hand, the amorphous chain of cellulose II takes a more released and expanded arrangement in presence of water molecules which break hydrogen bonds in the crystalline region of cellulose II. The decrease of crystallinity was observed by x‐ray diffraction.