44861-AC7
Counterion Condensation and Collapse of Polyelectrolyte Chains in a Poor Solvent: Computer Simulations and Theoretical Study
In aqueous environment, polystyrene sulfonate (PSS) is known to form a necklace globule of dense beads connected by strings of monomers. This structure appears as a result of the optimization of both electrostatic and short-range interactions. Using molecular dynamics simulations we have studied counterion condensation and NaPSS chain conformation as a function of fraction of charged monomers on the polymer backbone (f=1/4, 1/3, 1/2, 1), chain degree of polymerization (N=16-64) and the effect of solvent structure. We have performed two sets of MD simulations: i) simulations with implicit solvent, which was modeled by a dielectric continuum with dielectric constant 77.73; ii) explicit solvent simulations of NaPSS in water (TIP3P). All simulations were done at T=300 K. In particular we have studied the effect of the solvent structure on the counterion condensation. This was achieved by comparing the radial distribution function between the sulfonate groups and the sodium counterions for two sets of simulations and for chains with different fraction of charged monomers. In simulations with explicit water we have established that there is a solvation cage around Na+ ions which presence is manifested in the weaker than linear dependence of the ion mean-square displacement. The analysis of the distribution of the sulfonate groups indicates that the ionized groups are located on the surface of the collapsed hydrophobic regions. The structure of the chain was investigated by plotting the radius of gyration as a function of degree of polymerization and fraction of charged monomers on the polymer backbone. Our simulations have shown that NaPSS chains adopt a more elongated conformation as both the fraction of the charged monomers and the chain’s degree of polymerization increases.
Using combination of the molecular dynamics simulations and scaling analysis we have studied the effect of the solvent quality for the polymer backbone, strength of the electrostatic interactions, chain degree of polymerization and brush grafting density on the conformations of planar polyelectrolyte brushes in salt-free solutions. Our simulations have shown that the polyelectrolyte brush could form: (1) hemispherical micellar aggregates, (2) vertically oriented cylindrical micelles, (3) maze-like aggregate structures, or (4) thin polymeric layer uniformly covering the substrate. These different brush morphologies appear as a result of the fine interplay between electrostatic and short-range monomer-monomer attractive interactions. The brush thickness shows nonmonotonic dependence on the value of the Bjerrum length determining the strength of the electrostatic interactions. It first increases with increasing the value of the Bjerrum length than begins to decrease. This behavior can be explained by counterion condensation within brush volume.
