Dynamics of polyzwitterions in salt-free and salt solutions

Abstract

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 (C_s) 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 C_s ∼ 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 C_s increases, the sedimentation coefficient exhibits a maximum at C_s ∼ 0.1 M whereas the diffusion coefficient has a minimum at C_s ∼ 10⁻³ M and a maximum at C_s ∼ 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.