Self-assembly of asymmetric poly(styrene)-block-poly(methacrylic acid) polyelectrolyte-neutral diblock copolymer in aqueous solution by explicit atomistic MD simulations

The micelle mechanism, structure, hydration, and thermodynamics of asymmetric poly(styrene)-block-poly(methacrylic acid) polyelectrolyte-neutral block copolymer chains in salt-free aqueous solution were investigated as a function of copolymer composition (X_PS) for unionized (f = 0) and fully ionized (f = 1) states of PMA block by molecular dynamics simulations. The mechanism of micelle formation is a combined approach of unimer insertion and cluster fusion/fission mechanism, which is examined by the population of unimers and clusters formation across the simulation trajectory. Micelle formation takes a long time for copolymer at charged state in comparison to uncharged with relatively short hydrophobic PS block. The shape of micelle transforms from spherical to ellipsoid with increase in X_PS, the radius of core and entire micelle increased with increase in X_PS. Solvation behavior of micelle increases with the quantity of charge on PMA units and insignificant change by PS fraction of copolymer chains. The interaction of PMA with water becomes more favorable at fully-charged state in comparison to an uncharged state due to the presence of COO¯ groups. The solvent accessible surface area (SASA) of micelle increases with X_PS in both states, commensurate with its shape transformation. The SASA per corona chain shows linear behavior with X_PS. The poly(methacrylic) chains are located on the surface of PS core at uncharged state (f = 0) and attain an extended structure as classical corona at fully charged state (f = 1). The results of atom density profiles, solvation enthalpy, and RDFs of copolymer-Na⁺ ion pairs confirm the existence of the polyelectrolyte micelle in the osmotic regime, at all copolymer compositions, in agreement with results of mean-field theory^.1

Keywords: Self-assembly, Polyelectrolyte​​​​​​.

References:

1. Shusharina, N. P.; Linse, P.; Khokhlov, A. R. Lattice Mean-Field Modeling of Charged Polymeric Micelles. Macromolecules 2000, 33, 8488–8496.