Elucidating the Effects of Metal Complexation on Morphological and Rheological Properties of Polymer Solutions by a Dissipative Particle Dynamics Model

KP Santo and A Vishnyakov and R Kumar and AV Neimark, MACROMOLECULES, 51, 4987-5000 (2018).

DOI: 10.1021/acs.macromol.8b00493

When a salt is added to a polymer solution, metal cations may coordinate with polymer ligands forming interchain and intrachain links. Metal coordination leads to drastic changes of polymer morphology, formation of clusters, and, ultimately, a sol gel transition that affect the solution rheology. Although metal coordination is ubiquitous in polymeric systems, the physical mechanisms of coordination-induced morphological and rheological changes are still poorly understood due to the multiscale nature of this phenomenon. Here, we propose a coarse- grained dissipative particle dynamics (DPD) model to study morphological and rheological properties of concentrated solutions of polymers in the presence of multivalent cations that can coordinate the polymer ligands. The coordinating metal is introduced as a 3D complex of planar, tetrahedral, or octahedral geometry with the central DPD bead representing the metal cation surrounded at the vertices by either four or six dummy beads representing coordination sites, some of which are occupied by counterions to provide electroneutrality of the complex. Coordination is modeled as the dynamic formation and dissociation of a reversible link between the vacant coordination site and a ligand described by the Morse potential. The proposed model is applied to study the specifics of the equilibrium morphology and shearing flow in polyvinylpyrrolidone dimethylformamide solutions in the presence of metal chlorides. Coordination leads to interchain and intrachain cross- links as well as to metal cations grafted onto polymer chains by a single link. The interchain cross-links induce a sol gel transition to a weak gel phase as the metal concentration increases. Because of the reversible nature of interchain cross-links, the weak gel phase behaves as a viscoelastic fluid, the viscosity of which gradually increases with the metal concentration and decreases as the shear rate increases. The change of viscosity due to interchain coordination cross-links scales with the interchain cross-link density and the metal concentration according to the power law with the exponent v approximate to 1.15. The influence of the grafted metal atoms on the viscosity is found to be much weaker, while the effect of the intrachain cross-links is found to be negligible. The simulation results are in qualitative agreement with available literature data. The proposed DPD model provides a physical insight into the morphological features of polymer solutions in the presence of multivalent slats and can be extended to other coordinating systems such as metal-substituted polyelectrolytes.

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