Interaction between water and hydrophilic polymers

Various natural and synthetic polymers with hydrophilic groups, such as hydroxyl, carboxyl and carbonyl groups, have either a strong or weak interaction with water. Thermal properties of polymers and water are bath markedly influenced through this interaction. The first-order phase transition of water fractions closely associated with the polymer matrix is usually impossible to observe. Such fractions are called non-freezing water. Less closely associated water fractions exhibit melting/crystallization, showing considerable supercooling and significantly smaller enthalpy than that of bulk water. These water fractions are referred to as freezing bound water. The sum of the freezing bound and non-freezing water fractions is the bound water content, Water, whose melting/crystallization temperature and enthalpy are not significantly different from those of normal (bulk) water, is designated as freezing water. Bound water in the water-insoluble hydrophilic polymers, such as cellulose, lignin and poly(hydroxystyrene) derivatives, breaks hydrogen bonding between the hydroxyl groups of the polymers. The bound water content depends on the chemical and high-order structure of each polymer. Aqueous solutions of water-soluble polyelectrolytes, such as hyaluronic acid, gellan gum, xanthan gum and poly(vinyl alcohol) form gels above a threshold concentration. In the above gels, water mostly exists as the freezing bound water, playing an important role in the junction zone formation. It has also been observed that various kinds of polysaccharide polyelectrolytes with mono-and divalent cations, and other polyelectrolytes, such as polystyrene sulfonate, form thermotropic/lyotropic liquid crystals in the water content, ranging from 0.5 to ca. 3.0 g of water/g of polymer. (C) 1998 Published by Elsevier Science B.V.