From Coils to Rods: Structure and Dynamics of Polyelectrolytes in Water

Correlating the effects of hydration on the structure of polyelectrolytes and their conformational dynamics remains a long-standing fundamental challenge, with significant technological implications. Molecular-level insight, attained from atomistic molecular dynamics simulations of a fully sulfonated polystyrene polyelectrolyte with Na⁺ as a counterion in water, has enabled a direct correlation of the structure across atomistic to mesoscopic length scales with the corresponding dynamics. We find that as the polymer concentration c decreases, the chains extend, as observed by the shifts in the polyelectrolyte signature of the static structure factor, with q_max being the momentum transfer vector at the maximum intensity of this peak. The characteristic length scale that captures the system structure, l_max = 2π/q_max, scales with c^α with α = -0.48. Concurrently, the Na⁺ counterion condensation decreases and the chains transition from coiled to more extended conformations. These structural changes lead to enhanced segmental and mesoscopic mobility, highlighting the coupled roles of hydration, conformation and dynamics in polyelectrolyte systems.