Heterogeneous dynamics in a polymer solution revealed through measurement of ultraslow convection

Understanding solution-phase aggregation and dynamics in complex fluids is critical for material processing, yet widely used dynamic light scattering (DLS) is unsuitable for strongly attenuating systems such as conjugated polymers. We use X-ray photon correlation spectroscopy (XPCS) to probe the dynamics of a polymer, PM7, in toluene, revealing unexpected oscillations in the autocorrelation function that show vertical flow during measurement. Despite the relatively low X-ray absorption, measured flow velocities scale with X-ray power and suggest convective transport. Our analyses reveal mobile and static scatterers that together produce oscillatory, heterodyne features in the correlation functions. Finite element simulations predict flow velocities much larger than observed, suggesting that entanglements of the aggregates slow their motion. These results provide a direct measurement of ultraslow convection and highlight the need to account for even modest beam heating in XPCS analysis. Moreover, the observation of distinct scatterer populations underscores the structural complexity of conjugated polymer solutions.