Correlation of Mechanical Properties and Molecular Properties: Investigation of Poly(vinylamine) Polyelectrolyte Hydrogel Formation

¹H. Niess, ²M. Wilhelm

¹Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany, [email protected]

² ITCP, KIT, Karlsruhe, Germany.

Highly swellable polyelectrolyte hydrogels, so-called superabsorbent polymers, are hydrophilic polymer networks that contain charged groups along the polymer backbone. In current studies, three-dimensional chemically crosslinked networks are formed by crosslinking poly(vinylamine) with three different water-soluble bis(N-acylpiperidone) derivates under mild conditions. These crosslinker differ in their electrophilicity of the piperidone carbonyl group and thus in their reactivity towards nucleophiles. [1] By monitoring the time-evolution of the elastic modulus during gelation via oscillatory shear rheology, a clear order of reactivity of the three crosslinker can be established. Besides different gelation times for gels, synthesized with the three crosslinker, a significant difference in gel strength is observable, although crosslinking degree is 1 mol% for all samples. In order to fully understand this macroscopic behavior, NMR relaxometry is further exploited to study the network structure and inhomogeneities of the hydrogels. [2] Coupling of oscillatory shear rheology with time-domain ¹H NMR relaxometry enables the study of the interplay between viscoelastic response and the molecular dynamics and thus a better understanding of structure-properties relationships. [3]

Further investigation of the gelation will focus on in situ measurements of the network formation with combined methods. The unique RheoNMR and RheoIR setups allow a simultaneous measurement of macroscopic mechanical properties (rheology) and molecular properties (spectroscopy). RheoNMR measurements will provide a direct correlation of storage modulus G’ with the transverse relaxation time T₂. To investigate the underlying chemistry  during gelation, the chemical changes will be monitored via RheoIR. Characteristic and IR intense peaks of functional groups such as ketones, aminals and imines will give insight into the crosslinking process. [4]

References

[1] T. Uhlig et al., ACS Macro Lett. 10 (2021), 389-394

[2] J. Höpfner et al., Macromolecules 47 (2014), 4251–4265.

[3] C.Fengler et al., Adv. Sci. 9 (2022), 2104231.

[4] N. W. Radebe et al., J. Rheology. 65 (2021), 681-693