Multi-Angle DLS-Microrheology of Poly(acrylic acid): Separating Chain-Charge and Added-Salt Contributions to Viscoelasticity

Polyelectrolyte solution dynamics are governed by several coupled variables, making it difficult to isolate the distinct roles of chain charge and added salt. Here we employ multi-angle passive dynamic-light-scattering (DLS) microrheology, complemented by bulk rheology, to probe semidilute poly(acrylic acid) as a function of degree of neutralization (α) and with added monovalent salt. By resolving the wavevector dependence of embedded-probe dynamics using multiple probe sizes, we separate two largely independent contributions to the linear viscoelastic response. Increasing chain charge rescales a single relaxation spectrum, shifting the modulus and characteristic timescale while preserving its overall shape. In contrast, adding salt at fixed charge alters the shape of the response, introducing a slower, low-frequency feature consistent with transient interchain association. We further examine how chain charge modulates this salt-induced response. Across conditions, multi-angle microrheology resolves features that are inaccessible to bulk rheology alone for these low-viscosity viscoelastic liquids. These results demonstrate that charge and salt-driven contributions to polyelectrolyte viscoelasticity can be disentangled within a single model system, providing a framework for interpreting the dynamics of charged macromolecular solutions.