This work applies a molecular theory to study the formation of lateral self-assembled aggregates in mixed brushes composed of polyanion and polycation chains. To overcome the well-known limitations of mean-field electrostatics to capture polyelectrolyte complexation, the formation of ion pairs between anionic and cationic groups in the polyelectrolytes is explicitly modeled in our theory as an association reaction. Polyanions and polycations mixed in solution are known to form interpolyelectrolyte complexes stabilized by ion pairs between the charged groups in the polyelectrolyte chains.The formation of these ion pairs results in the expulsion of small counterions that are condensed on the polyelectrolytes. This entropic process is now recognized to be a main driving force behind polyelectrolyte complexation. A mixture of polycations and polyanions end-grafted to a surface, is also expected to form interpolymer complexes on the surface, but it cannot macroscopically phase-separate because of the constraints introduced by grafting. Polyelectrolyte complexation is a key phenomenon for applications in different areas like nano/microencapsulation. We theoretically study whether the complexion of oppositely charged polyions in a mixed polyelectrolyte brush can lead to the formation of lateral aggregates via a microphase-separation process triggered by polyelectrolyte complexation. The effects of solution pH and ionic strength, surface coverage, and chain length on the morphology of the mixed brush are systematically explored. It is shown that increasing salt concentration leads to the rupture of polyelectrolyte complexes and the stabilization of the homogeneous, non-aggregated brush, providing that the formation of ion pairs between the polyelectrolytes and the salt ions in solution is explicitly accounted for by the theory. The inclusion of ion-pairing association reactions between oppositely charged polyelectrolytes within a mean-field description of electrostatics emerges from this work as a useful and simple theoretical approach to capture the formation of polyelectrolyte complexes and their responsiveness to solution ionic strength and pH.

Microphase separation of oppositely charged polyelectrolytes brushes triggered by ion pairing

ACS Spring Meeting 2021

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