Crossover from Rouse to Reptation Dynamics in Salt-Free Polyelectrolyte Complex Coacervates

The dynamics and rheology of polyelectrolyte complex coacervates has attracted considerable interest due to their industrial application as viscosity modifiers. A central question is whether the classical Rouse and reptation models can be applied to these systems. By using molecular simulations, we directly demonstrate the crossover from Rouse to reptation dynamics in salt-free coacervates as a function of chain length. Our results show that this crossover shifts to short chain length with increasing electrostatics, which attributes to the formation of denser coacervates. To clarify the roles of Coulomb interactions and density, we further compare coacervates to those of neutral, semidilute solutions at the same density and chain conformations. A universal dynamical behavior is observed for both systems in the sub-diffusion and normal diffusion regimes, but the monomer relaxation time in coacervates is much longer and increases as Bjerrum length increases. Similar phenomenon called cage effect exists in glass-forming polymers, but the local dynamical slowdown in our system is caused by strong Coulomb attractions (ion pairing) between oppositely charged monomers. Our findings contribute to the fundamental understanding of coacervate dynamics and rheology in microscopic level.