Molecular Dynamics (MD) Simulations of Soil-Strengthening Nanocomposite-Polyelectrolyte Hydrogels

Nanocomposite-polyelectrolyte hydrogels are gels formed from a mixture of charged polymer chains and nanofillers. They can absorb large amounts of water, mimic biological fluids/tissues, and more recently, they can potentially serve as a sustainable alternative to concrete for soil strengthening purposes. Despite the large number of studies on polyelectrolytes and their adsorption onto oppositely charged particles, little is known about the mechanism of network formation and structure of such charged hydrogels. Thus, we use molecular dynamics (MD) simulations to investigate the effect of charge on the gelation of our system and related properties including viscosity, network formation, cluster geometry, and polymer adsorption onto filler particles.<br/><br/>We use a coarse-grained approach, where nanofillers are modeled as rigid bodies of disk-like shapes. Polyelectrolyte chains are modeled as beads connected by finitely extensible nonlinear elastic (FENE) springs. All pairwise interactions are defined by the truncated and shifted 12-6 Lennard-Jones (LJ) potential. Systems also included uncharged solvent particles and free-floating univalent counterions for both nanofillers and polymer chains to ensure electroneutrality.<br/><br/>We vary the charge of both the filler particles and the polyelectrolyte beads independently. Upon reaching equilibrium, we use the Green-Kubo relation to calculate viscosity by integrating the stress-autocorrelation function (SAF), the step-strain test (SST) to determine the mechanical stability, the radial distribution function (RDF) of the polymers with respect to the fillers in order to measure adsorption, and finally, a network clustering algorithm (NCA) to test for cluster formation. If clustering occurs, we quantify cluster geometry by calculating the gyration tensor, allowing us to determine asphericity, acylindricity, and relative shape anisotropy.<br/><br/>Our initial results for systems with charge magnitudes of q=1 for both the polymer and filler beads show both the SAF and SST decaying to zero, implying gelation was not induced, while the RDF and NCA suggested that adsorption and clustering, respectively, also did not occur. We did observe increasing viscosity with charge magnitude, suggesting that larger charge magnitudes could induce gelation. Results also show that the cluster geometry of nanofillers is an important determinant of the final properties of the hydrogel where the assembly is mediated by polymer and nanofiller net charges. Our work will be able to determine structural information and conditions for hydrogel design. They will also enable experimental teams to manufacture and test nanocomposite hydrogels with the specific charge properties that our models predicted would exhibit enhanced behaviors.<br/><br/>We would like to thank the Morin Charitable Trust for funding this project as well as Prof. Miriam Rafailovich and the Garcia Research Program for making this work possible. We would also like to acknowledge the mentorship of Shoumik Saha and Prof. Dilip Gersappe for their guidance and support. All simulations were run on the Stony Brook University SeaWulf supercomputer.