Dynamics of Sodium Ions and Water in Swollen Superabsorbent Hydrogels as Studied by 23Na- and 1H-NMR

Dynamics of sodium ions and water in swollen superabsorbent polymer (SAP) hydrogels are studied by Na-23- and H-1-NMR, respectively. The apparent diffusion coefficients of water in swollen SAPs, probed by H-1 pulsed field gradient NMR, decreases with increasing diffusion time. The degree of hindrance depends on structural and synthesis parameters. It is quantified within a tortuosity model. Based on the results, the swelling degree has the highest impact on the ion mobility, apart from synthesis parameters leading to different levels of physical and chemical crosslinks. Na-23-NMR relaxation and diffusion reveal the Na-23(+) mobility in swollen SAPs. A higher degree of neutralization leads to faster relaxation and to a smaller apparent diffusion coefficient. Surface crosslinking restricts water mobility, but has a smaller impact on the dynamics of sodium ions. The experimental results indicate an influence of SAP structure on the dynamics of ions and water molecules.