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

Abstract

Considerable interest in the dynamics and rheology of polyelectrolyte complex coacervates has been motivated by their industrial application as viscosity modifiers. A central question is the extent to which classical Rouse and reptation models can be applied to systems where electrostatic interactions play a critical role on the thermodynamics. By relying on molecular simulations, we present a direct analysis of the crossover from Rouse to reptation dynamics in salt-free complex coacervates as a function of chain length. This crossover shifts to shorter chain lengths as electrostatic interactions become stronger, which corresponds to the formation of denser coacervates. To distinguish the roles of Coulomb interactions and density, we compare the dynamics of coacervates to those of neutral, semidilute solutions at the same density. Both systems exhibit a universal dynamical behavior in the connectivity-dominated (subdiffusion and normal diffusion) regimes, but the monomer relaxation time in coacervates is much longer and increases with increasing Bjerrum length. This is similar to the cage effect observed in glass-forming polymers, but the local dynamical slowdown is caused here by strong Coulomb attractions (ion pairing) between oppositely charged monomers. Our findings provide a microscopic framework for the quantitative understanding of coacervate dynamics and rheology.

DNA dynamics in complex coacervate droplets and micelles

“… 18 This density increase has also been mentioned to explain why the relaxation time in coarse-grained simulations of salt-free polyelectrolyte complexes of equal length showed a stronger dependence on the polyelectrolyte length than the sticky Rouse model. 41 In the latter case, the increase in complex density was also confirmed by the same simulations. For the ssDNA/pLL complex coacervate droplets, an increase in density with increasing chain length might not only explain the stronger dependence of DNA diffusion coefficient on the DNA length, but it might also explain why the pLL chain length affects the DNA diffusion coefficient in the complex.…”

“…First, the reptation regime is only relevant when there are significant entanglements, which only occurs for long polyelectrolyte lengths. 41 Here, we also see a strong length dependence for the short pLL and DNA lengths, which are too short for the reptation regime. In addition, the ionic bonds in (homo)polyelectrolyte complexes are always intermolecular since they are based on the attraction between oppositely charged species and therefore no intramolecular to intermolecular bond transition will occur.…”

Polyelectrolyte Complex Coacervation across a Broad Range of Charge Densities

“…For athermal solvent, this law reads ϕ ≃ ( uf 2 ) (3ν–1)/(2−ν) ≃ ( uf 2 ) 0.54 , where v = 0.588 has been used. 48 , 49 , 65 , 77 The solvent quality for our chains is not currently known, but small-angle neutron studies are underway which provide insights into the structure of PECs and the chain statistics within electrostatic blobs (i.e., solvent quality). TGA of coacervates prepared at 0 M exogenous NaCl reveals a weak dependence of coacervate density—or, more accurately, PE weight fraction ( w P,c )—on charge fraction given by w P,c ∼ f 0.37±0.01 ( Figure 9 A).…”

“…Similar deviations from the scaling predictions to the lower slopes for the ϕ( f 2 ) dependence, 0.41 ± 0.02 vs theoretical 0.54, have been recently reported for athermal solvent. 65 …”

Polyelectrolyte–Dye Interactions: An Overview

“…Due to their excellent water stability and ability to interact with oppositely charged macromolecules and surfaces, polyelectrolytes have been extensively used in various fields, from materials science and colloids to biophysics. Their predominant applications include usage in optoelectronic devices [ 14 ], solar cells [ 15 ], rheology modifiers [ 16 , 17 ], adsorbents [ 18 ], coatings [ 19 , 20 ], biomedical implants [ 21 ], colloidal stabilizers [ 22 ], suspending agents [ 23 ], and for drug delivery and pharmaceutical uses.…”