Coarse-Grained Explicit Solvent Molecular Dynamics Simulations of Polyzwitterion-Polyelectrolyte Complexes

We investigate solutions containing equal proportions of polyzwitterions (PZ) and polyanions (PA) across the dilute to semi-dilute solution concentration regimes using coarse-grained molecular dynamics simulations with explicit dipolar solvent. These polymers share the same degree of polymerization, with a PA chain featuring a pendant negative charge and a free counterion, while a PZ chain is represented by a pendant dipole with the positive charge attached to the backbone. The form factors P(q) of the dilute solutions containing pure PZs and PAs exhibit scaling of q^-5/3 and q^-1, respectively, suggesting that the PZ chains behave like a neutral chain in a good solvent while the PA chains assumes a rod-like structure. The scattering function S(q) of the pure PA displays the polyelectrolyte peak, mostly absent in pure PZ, but a similar peak appears at much higher concentrations in the solutions containing the PZs. However, a peak analogous to the polyelectrolyte peak is observed in the S(q) of the solutions containing the mixture of the PZ and the PA chains from the scattering of PZ, albeit at a higher q, indicating that complexation of the PZ and the PA chains leads to an effective polyelectrolyte. Analysis of radial distribution functions reveals that counterion condensation is more pronounced in the complex compared to the PA, while solvent arrangement around the PZ is disrupted in the complex. Simulated scattering functions qualitatively matched small-angle scattering experiments, validating our findings.