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 differen t degree of cross-link and different amount of added water. The spin-lattic e relaxation time T-1, the spin-spin relaxation time T-2, and the free indu ction decay (FID) were measured in the 200-320 K temperature range. An aper iodic saturation recovery sequence was applied for measuring T-1, while a C PMG sequence was used for measuring T-2. A full deconvolution of FIDs allow s a plethora of information. Lightly cross-linked chitosan chains form supe r adsorbing hydrogels. The amount of water in these gels at the swelling eq uilibrium is so high to hide completely the properties of bound water. Thus , dried samples of chitosan and of cross-linked chitosan derivatives were p artially rehydrated and studied as a function of added water and of tempera ture. Data obtained from a H-1 NMR relaxation study allowed us to measure t he number of water molecules in different shells of solvation of macromolec ules. Only about four water molecules per repeat unit are tightly coordinat ed by chitosan while the number of water molecules tightly coordinated mark edly increases for cross-linked chitosans and is correlated with the maximu m swelling properties of the networks.