Origin of Apparent Slow Solvent Dynamics in Concentrated Polymer Solutions

Due to their size disparity, the dynamics of solvent molecules in concentrated polymer solutions can be faster by several orders of magnitude compared to the polymer matrix. Despite these observations, past research suggests that a fraction of the solvent contributes to the slow dynamics on the timescale of the macromolecules. By combining depolarized dynamic light scattering and molecular dynamics simulations, we show that long-lived solvent-solvent cross-correlations cause these slow solvent contributions. They originate from the energy landscape that the solute imprints onto the neighboring solvent, leading to enhanced correlations between the positions and orientations of different solvent molecules until the solute relaxes. This mechanism explains the discrepancies found between the results of techniques probing collective and single-particle dynamics, such as dielectric and nuclear magnetic resonance spectroscopy. Our findings shed new light on how solvents behave in the vicinity of macromolecules and are of relevance for materials ranging from polymer-plasticizer systems to hydrated proteins.