Polymorphic Transitions of Amylose‐Ethanol Crystalline Complexes Induced by Moisture Exchanges

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Abstract
Vh‐type crystals were prepared by precipitation in pure alcohol of amylose solutions in dimethyl sulfoxide (DMSO). The respective influences of amylose chain length and final equilibrium relative humidity (E.R.H.) on the nature of transitions induced by moisture exchanges were studied extensively using X‐ray diffraction. The nature and kinetics of observed transitions depend on amylose chain length. If the Va form is always observed upon dehydration, A, B or mixtures of A and B forms are obtained according to the amylose chain length when rehydrating over higher water activities. Other specific transitions occurred when the Vh material was preliminary crystallized in mixture of ethanol and water (80/20,v/v). Some results suggest that these transitions occur through a transient disappearance of crystallinity and subsequent reorganization. Such mechanisms would be more conceivable than a solid state conversion from a single helical conformation (Vh type) to a double one (A and B types).
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