Salt Counterion Valency Controls the Ionization and Morphology of Weak Polyelectrolyte Miktoarm Stars
Abstract
The properties of weak polyelectrolyte polymer blocks vary as a function of the ionic strength and salt counterion valency. However, the specific conformational and ionization behaviors and overall morphologies of star-like micelles formed by triblock copolymers containing hydrophobic (e.g., PS), weak polyelectrolyte (e.g., PAA), and nonionizable hydrophilic (e.g., PEO) blocks remain unknown.
In order to predict how these block polymers respond to variations in ionic strength and salt counterion valency, we used coarse-grained models and performed Hamiltonian Monte Carlo simulations in the reaction ensemble, assuming that hydrophobic blocks form the micellar core and describing the micelles as miktoarm stars with nonionizable and dynamically ionizable parts (weak polyelectrolyte). By sequentially varying the order of the blocks in the unimer chains, the pH, the salt concentration, and the salt counterion valency, we found that the degree of ionization of the ionizable arms strongly depends on the order of the blocks in the unimer chains.
Furthermore, the star is able to capture all divalent counterions from the solution until its salt capacity. At low pH values, the radius of gyration of the star, R(g), increases with the salt concentration similarly to the degree of ionization of the ionizable region.
Conversely, at high pH values, R(g) increases until peaking at the salt capacity concentration but then returns to values similar to those of the neutral polymer star. The star morphologies vary as a function of pH, salt concentration, and salt counterion valency.
The stars resemble core-shell and octopus-like particles at a high pH and a high concentration of the monovalent salt but form core-shell, Janus, and patchy particles at a high pH and a high concentration of divalent salt. Combined, our findings demonstrate that the morphological type of a star-like micelle can be controlled by changing the salt counterion valency.