Molecular dynamics simulations of PAA-PMA polyelectrolyte copolymers in dilute aqueous solution: Chain conformations and hydration properties

Atomistic molecular dynamics simulations of copolymers of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in dilute aqueous solution were performed as a function of charge density, in explicit solvent medium and counterions. The studied polyelectrolytes follow a general behavior of chain expansion with charge density until a point where the repulsion between the electrostatic charges between the anionic residues is effectively neutralized by the counterions. The average persistence length is found to increase and levels off at higher charge densities, and the values imply the chains to be flexible. With increase in PMA content in the chain, counterions show increased correlation with chain backbone and a systematic reduction in the number of water molecules in the first hydration shell. The intermittent hydrogen-bond correlation function for the hydrogen bonds between the chain residues and water decays faster for PAA chain as compared to PMA, indicating a rigid hydration layer for the latter. The shorter H-bond lifetimes coupled with the slower relaxation indicate that MA-water H-bonds break more easily than those of AA-water H-bonds, but the water molecules remain in the vicinity of the chain because of slow diffusivity and can easily reform the bonds. © 2012 American Chemical Society.