Rheological properties of acylated chitosans with various acyl lengths were measured in the melt state. The acylated chitosans have two rheological transitions corresponding to rather abrupt decreases in viscoelastic functions and an abrupt drop in the height of plateau of the storage modulus. These transition temperatures become lower with increasing length of side chains. The time-temperature superposition principle was applied to dynamic viscoelastic functions for acylated chitosans. The time-temperature superposition fails for acylated chitosans with shorter acyl length. However, for an acylated chitosan with the longest acyl length, the viscoelastic data can be superposed into three separated master curves in different temperature ranges. Wide-angle X-ray diffraction measurements indicate that acylated chitosans have a layered structure. Temperature dependencies of d-spacing attributable to this layered structure indicate two transitions, whose temperatures reasonably correspond to those of the rheological transitions, exist. When the temperature exceeds the lower transition temperature, the d-spacing increases with increasing temperature, and the transition temperatures decrease with increasing length of side chains. The decreases of dynamic viscoelastic functions are associated with the layer spacing changes. It is suggested that the abrupt decrease of the plateau height is caused by the interchain movement of molecules or reorganization of the structure.
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