States of water in different hydrophilic polymers - DSC and FTIR studies

The structure of water molecules sorbed in different hydrophilic polymers w as studied by DSC and FTIR. The obtained data shows that, first, the sorbed water molecules are directly bound to the hydrophilic site to form non-fre ezable water. Then, beyond a certain water content threshold, the sorbed wa ter molecules become freezable, but with a melting point lower than 0 degre esC, due to their location in the second hydration layer. Bulk-like water w hich can be frozen at 0 degreesC appears at higher water contents, and the two types of freezable water finally merge together at very high water cont ents. The average number of non-freezable water molecules per site depends on the chemical nature of the polar site: ca. 1 for a hydroxyl, and 4.2 for an amide group. For a polymer with carboxylate sites, it increases with th e size of the alkaline counter-ion of the site, due to the increasing abili ty of the carboxylate counter-ion pair to undergo dissociation. (C) 2001 El sevier Science Ltd. All rights reserved.