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

The structure of water molecules sorbed in different hydrophilic polymers was 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-freezable water. Then, beyond a certain water content threshold, the sorbed water molecules become freezable, but with a melting point lower than 0°C, due to their location in the second hydration layer. Bulk-like water which can be frozen at 0°C appears at higher water contents, and the two types of freezable water finally merge together at very high water contents. 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 the size of the alkaline counter-ion of the site, due to the increasing ability of the carboxylate counter-ion pair to undergo dissociation.