CROSS-RELAXATION AND EXCHANGE IN POLY(ACRYLAMIDE) HYDROGEL STUDIED THROUGH H-1 MASS NMR AND 2D NUCLEAR OVERHAUSER ENHANCEMENT SPECTROSCOPY

A combination of two-dimensional (2D) NMR techniques, namely, (H-1-H-1 ) NOESY, and ROESY, has been used for the first time to study cross-re laxation and exchange phenomena in a poly(acrylamide) hydrogel. Intram olecular cross-relaxation interactions are detected for the main-chain protons and are seen to arise from polymer mobility occurring in the long correlation regime (omega(0) tau(c) much greater than 1). Tempera ture-dependent studies show chemical exchange, direct and relayed cros s-relaxation pathways for the magnetization transfer. For amide proton s, cross relaxation dominates over chemical exchange at room temperatu re and below. Intermolecular exchange and cross-relaxation processes a re identified for hydrated water, and evidence has been gathered for i nteracting water molecules with a relatively long reorientational corr elation time. The 2D data also point to the most probable location of water close to the carbonyl group.