Structure and Swelling Properties of Biodegradable Cross-Linked Polyurethanes by Means of Nuclear Magnetic Resonance

In this study, the swelling properties and structural changes during the interaction with liquid media of biodegradable cross-linked polyurethanes promising from medical and environmental points of view are reported. To characterize the polyurethane samples, containing sucrose as cross-linking agent in different concentrations, in the dry state, and swollen in water and dimethyl sulfoxide (DMSO), low- and high-field liquid-phase NMR spectroscopic methods, such as relaxometry, diffusometry, and cryoporometry, were applied. Based on the NMR relaxation features of the swelling liquids and the swollen cross-linked polymers, the difference in the interaction with the two liquids was evaluated; furthermore, the rigid and mobile domains in the amorphous part of the polymer were successfully identified. The change of their ratio with the sucrose content and the restricted diffusion of DMSO in the swollen polymer network reflected the cross-link density and highlighted the role of dangling chains. The swelling process in DMSO was followed by the change of the T₂ relaxation of the infiltrating liquid in time and was described by the second-order kinetic model with a suggested, detailed swelling mechanism. Cryoporometry measurements enlightened the effect of swelling on the size change of the liquid domains in the cross-linked structure.