Synthesis and NMR-based network structure analysis of cationic hydrogels for seawater applications

Superabsorbent polymers (SAPs) are hydrophilic polymer networks (i. e., hydrogels) that contain charged monomer units along the polymer backbone. Poly(sodium acrylate) (PSA), which has negatively charged carboxylate groups, is the most prominent chemical structure of SAPs. Recent studies have shown that PSA can be used as a separation medium for the desalination of salt water. This approach, however, is limited to NaCl solutions because the divalent Mg$^{2+}$ and Ca$^{2+}$ cations in seawater interact electrostatically with the anionic polymer backbone of PSA, inducing thereby a network collapse. To overcome this limitation, this dissertation explores the swelling and desalination capacity of cationic SAPs in seawater. Two cationic SAP model systems with distinct functional groups were synthesized. The first one is based on a poly(acrylamide) (PAM) derivative with a trimethyl quaternary ammonium group as the positively charged monomer unit. The second model system is based on poly(vinyl amine) (PVAm) bearing positively charged ammonium groups. Swelling capacity measurements reveal that divalent SO$_{4}^{2-}$ anions in seawater induce a network collapse of PVAm similar to the PSA analogue. ... mehrIn contrast, the swelling behavior of PAM hydrogels is fully unaffected by seawater, suggesting that the quaternary ammonium moiety is essential to provide seawater resistant swelling properties of SAPs. In addition to swelling capacity studies, this dissertation aims to advance our understanding of the intriguing interplay between macroscopic, mechanical properties and molecular dynamics of the hydrogel network. Due to the inherent multi-length scale structural complexity of hydrogels, the quantitative correlation of mechanical properties with molecular dynamics remains a longstanding challenge. An advanced rheometer setup consisting of a portable low-field NMR unit that is integrated into a rheometer was used to study the gelation kinetics of acrylic acid (AAc) hydrogels. The elastic modulus G′ was studied by small amplitude oscillatory time sweeps whereas the local molecular mobility of polymer network chains was probed by T$_{2}$ relaxation measurements. From the in-situ G′ and T$_{2}$ correlation plots, it can be concluded that the elastic plateau modulus is inversely proportional to the T$_{2}$ relaxation time of the hydrogel. Consequently, the mechanical strength of hydrogels can be predicted based on the segmental mobility of polymer network chains, which has important implications for the further development of non-invasive and forceless mechanical characterization techniques.