Dynamics of polyzwitterions in salt-free and salt solutions

Dynamics of polyzwitterions remains largely unclear. We have prepared zwitterionic poly[1-(3-sulphopropyl) betaine-2-vinylpyridinium] (PSB) and investigated its dynamics in aqueous solution as a function of added salt (NaCl) concentration (Cs) by dynamic laser light scattering (DLS) and sedimentation velocity (SV) in analytical ultracentrifugation (AUC). A fast and a slow mode can be observed by DLS in a salt-free and low-salt solution, where the latter exhibits a maximum intensity at Cs ∼ 10(-3) M. SV measurements demonstrate that the fast mode corresponds to the diffusion of individual chains and the slow mode arises from the dynamic inhomogeneity due to interchain electrostatic repulsion. As Cs increases, the sedimentation coefficient exhibits a maximum at Cs ∼ 0.1 M whereas the diffusion coefficient has a minimum at Cs ∼ 10(-3) M and a maximum at Cs ∼ 0.1 M. Namely, PSB shows a complex dynamics in a salt-free and low-salt solution and anti-polyelectrolyte behavior in a high-salt solution. Our studies reveal that the dynamics of polyzwitterions is mediated by the long range interchain electrostatic repulsion and short range intrachain attraction, which are determined by effective charges on the chains.