The effect of chain length and solvent interactions on the dissolution of the B-type crystalline polymorph of amylose in water

To characterize the effectiveness of water as a diluent in depressing the melting temperature of amylose crystals, maltooligomer/water interactions were probed, as a function of composition, by calorimetry and vapour sorption measurements. These studies, in part analysed using the Flory-Huggins theory of polymer solutions, showed that although there is a strong energetic interaction between water and the oligomeric chain, this is balanced by an opposing, first neighbour entropic contribution. At room temperature, water was characterized as a poor solvent for the amylose chain. The dissolution in water of the B-type crystalline polymorph of amylose, crystallized from fractions of limited polydispersity, ranging in chain length from 12 to 55 residues, was examined by scanning calorimetry. With increasing chain length in this range, the dissolution temperature, at a volume fraction of water of 0.8, increased from 57 to 119 °C. The extrapolated dissolution temperature for the high molecular weight polymer at this water content was 147 °C.