Topological Insight into Superabsorbent Hydrogel Network Structures: a ¹H Double‐Quantum NMR Study

Abstract

Superabsorbent polymer (SAP) hydrogels have pronounced water‐absorbing and water‐storing capacities, which are essential for numerous potential applications. It remains a challenge to better understand the network topology because of their amorphous and anisotropic structures. Synthesis parameters such as monomer concentration, degree of neutralization and crosslinking, and surface crosslinking are varied to correlate structural changes in the network with low‐field proton double‐quantum (1H DQ) NMR results. 1H DQ‐NMR data are processed by a reliable, user‐independent analysis approach to determine the fractions of network defects, of mobile sol components, and of network chains as well as the residual dipolar coupling distribution in SAPs. In addition, results obtained by applying different distributions to describe the DQ buildup curves are quantified and compared. The correlation between topological and synthesis parameters as well as the impact of temperature, swelling, and solvent of SAP on DQ signals is investigated and discussed.

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

“…1 H‐ 1 H residual dipolar couplings (RDC) D res measured by 1 H DQ‐NMR depend on polymer specific parameters like geometry, distance, and chain dynamics. D res is related to synthesis parameters . An increase in D res is in parallel with an increase in σ/ D H,mean ( Q ,Δ max ) (Figure b).…”

“…1 H double‐quantum NMR indicates that guest moieties may be distributed in a nonuniform way. Water molecules in agarose and alginate gels also experience hindered diffusion.…”

“…a) Signal decays S / S 0 for 1 H diffusion in a swollen hydrogel (SAP75|20|0.3|RSXL) at different diffusion times (▲: 40; ◊: 80; ○: 120; ∆: 160; □: 240; ●: 480 ms); b) 1 H relative distribution width of diffusion σ/ D H,mean,Δmax and D res (residual 1 H‐ 1 H dipolar coupling determined by 1 H DQ‐NMR). Adapted with permission . Copyright 2018, Wiley‐VCH; c) schematic representation of a SAP hydrogel, hydrogen bonds, hydration of carboxylic group and 23 Na + , and the release of 23 Na + during swelling; d) simplified model of water‐mobility in the area between two chains in a swollen hydrogel.…”

“… 23 Na‐NMR diffusion and relaxation rates at = 1:9 together with 1 H D res . Adapted with permission . Copyright 2018, Wiley‐VCH.…”

“…Translational motion of 23 Na + and water in swollen SAP hydrogels is influenced by the often inhomogeneous polymer network, which is closely related to the synthesis parameters such as degree of neutralization, monomer concentration, degree of crosslinking, and surface crosslinking as well as particle size and shape . Thus water and ions, that is, diffusing moieties, may be distributed throughout the swollen polymer networks in a nonuniform way.…”