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

Polyacrylate hydrogels PAAM (M = Li-7(+), Na-23(+), K-39(+), Rb-87(+), and Cs-133(+)) are synthesized by free-radical copolymerization and are studied by Nuclear Magnetic Resonance (NMR) over wide time and length scales to get an insight into the polymer network structure and the dynamics of alkali ions and water. H-1 double-quantum NMR (H-1 DQ-NMR) indicates that neutralization by different alkali ions influences the polymer structure in swollen PAAM hydrogels. Relaxation rates of alkali ions are found to be dominated by quadrupolar relaxation. H-1 relaxation varies with the chain fraction obtained from H-1 DQ-NMR. Pulsed field gradient NMR reveals diffusion coefficients of alkali ions and water for different swelling degrees. Network structure and dynamics as well as the interactions between water, alkali ions, and polymer chains are discussed.