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

Abstract

Dynamics of sodium ions and water in swollen superabsorbent polymer (SAP) hydrogels are studied by 23Na‐ and 1H‐NMR, respectively. The apparent diffusion coefficients of water in swollen SAPs, probed by 1H 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. 23Na‐NMR relaxation and diffusion reveal the 23Na+ 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.

Structure of Superabsorbent Polyacrylate Hydrogels and Dynamics of Counterions by Nuclear Magnetic Resonance

“…Other alkali ions in PAAM hydrogels exhibit similar behavior; their diffusion coefficients do also not change significantly in this small range of Δ, see also ref. [34]. Compared to a reference value 0, D Li + of a dilute LiCl aqueous solution (1.03 × 10 -9 m 2 s -1 , solid line in Figure 4a), [56,57] D Li + in PAALi at a swelling degree of 1:5 is approximately five times smaller.…”

“…To further explore the translational mobility of M + in PAAM hydrogels probed by NMR, one possible influencing factor, the hydration of M + in a PAAM network, is taken into consideration and discussed on the basis of previous experimental and numerical studies . Different interactions are known to appear between water, counterions and polymers . Polar water molecules form hydration shells on M + due to electrostatic forces.…”

“…With increasing swelling, networks expand and more water molecules form hydration shells with alkali ions and carboxylic groups. Due to the electrostatic interaction and/or hydrogen bonds, water molecules will continue to form the hydration shell gradually . Water molecules undergo 1 H exchange with “free” water molecules.…”

“…Water molecules undergo 1 H exchange with “free” water molecules. An alternative interpretation of the translational 1 H mobility considered interactions such as hydration and steric interaction which can be described within a distribution model, that is, the mobility of water in swollen hydrogels is approximated by a diffusion distribution function . Compared to the diffusion of various alkali ions in PAAM gels (Figure a,c), mean 1 H diffusion coefficients decrease with diffusion time on the larger diffusion time scale (Figure b).…”

“…[19][20][21][22][23][24][25][26][27][28][29] Nuclear magnetic resonance (NMR) has proven to be a powerful tool for quantitative characterization of gels and the dynamics of polymer chains, guest molecules, and ions in swollen gels. [30][31][32][33][34][35][36][37] Recently, Volkov et al studied the ionic conductivity, the NMR line width and chemical shift of Li + , Na + , and Cs + ions in perfluorinated sulfocationic and carboxylic membranes by NMR. [38] Moreover, NMR was used in biological and medical sciences, for example, for quantification of the intracellular monovalent cation concentrations (Li + , Na + , K + , Rb + ) in an isolated perfused rat heart, [39] cation movements across cell walls of intact tissues, [40] as well as for in vitro study of cellular sodium metabolism.…”