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

Area of Science:

• Polymer physics
• Soft matter physics
• Materials science

Background:

• Polyelectrolyte complex coacervates are used as viscosity modifiers.
• Understanding their dynamics and rheology is crucial for industrial applications.
• Classical Rouse and reptation models are often used, but their applicability in electrostatically driven systems is questioned.

Purpose of the Study:

• To analyze the crossover from Rouse to reptation dynamics in salt-free complex coacervates.
• To investigate the influence of chain length and electrostatic interactions on dynamics.
• To differentiate the effects of Coulomb interactions and density on coacervate dynamics.

Main Methods:

• Molecular simulations were employed.
• Analysis focused on salt-free complex coacervates across varying chain lengths.
• Comparison was made with neutral, semidilute polymer solutions at equivalent densities.

Main Results:

• The crossover from Rouse to reptation dynamics shifts to shorter chain lengths with stronger electrostatic interactions, correlating with denser coacervates.
• Both coacervates and neutral solutions show universal dynamics in subdiffusion and normal diffusion regimes.
• Monomer relaxation time in coacervates is significantly longer, increasing with Bjerrum length due to Coulomb attractions.

Conclusions:

• Strong Coulomb attractions between oppositely charged monomers cause a local dynamical slowdown, analogous to the cage effect in glass-forming polymers.
• The findings offer a microscopic understanding of coacervate dynamics and rheology.
• This research provides a framework for optimizing polyelectrolyte complex coacervates for industrial uses.