Water in hydrogels. An NMR study of water/polymer interactions in weakly cross-linked chitosan networks

A pulsed low-resolution H-1 NMR relaxation study at 75 MHz was performed on chitosan and on two lightly cross-linked chitosan networks with a different degree of cross-link and different amount of added water. The spin-lattice relaxation time T-1, the spin-spin relaxation time T-2, and the free induction decay (FID) were measured in the 200-320 K temperature range. An aperiodic saturation recovery sequence was applied for measuring T-1, while a CPMG sequence was used for measuring T-2. A full deconvolution of FIDs allows a plethora of information. Lightly cross-linked chitosan chains form super adsorbing hydrogels. The amount of water in these gels at the swelling equilibrium is so high to hide completely the properties of bound water. Thus, dried samples of chitosan and of cross-linked chitosan derivatives were partially rehydrated and studied as a function of added water and of temperature. Data obtained from a H-1 NMR relaxation study allowed us to measure the number of water molecules in different shells of solvation of macromolecules. Only about four water molecules per repeat unit are tightly coordinated by chitosan while the number of water molecules tightly coordinated markedly increases for cross-linked chitosans and is correlated with the maximum swelling properties of the networks.