(413f) Coarse-Grained Model for Polyelectrolyte Complexation

Under appropriate circumstances, a mixture of oppositely charged polyelectrolytes can separate into two distinct liquid phases: a polymer-rich complex coacervate phase and a polymer-deficient supernatant phase.
The coacervation process and the structural properties of the resulting phases can be strongly influenced by variations in pH, the type and concentration of added salt, temperature, and ionic strength.
We have developed a coarse-grained molecular model to describe and predict polyelectrolyte complexation.

For fully charged homopolymers, we present three key findings. First, we
show that the model is capable of capturing the dependence of polymer
concentration, molecular weight, and salt concentration on coacervation. The results of simulations
compare favorably with available experimental data. Second, we show that the concentration of salt
in the coacervate complexes is generally different than in the supernatant phase. This observation also
supports several recent experimental reports. Third, our model allows us to calculate the dynamic properties,
including moduli, of the coacervate. The shapes of the predicted dynamic moduli also compare well with
experimental measurements.

Building on our results for homopolymers, we next apply the model to mixtures of complexating polymers consisting of short charged blocks and long uncharged blocks.
For diblock and triblock copolymers, our model reproduces the mean and distribution of micelle aggregation numbers observed in experiments. Notably, for triblocks we predict the formation of low-polymer concentration gels whose structure is confirmed in neutron scattering experimental data. By relying on extensive calculations, we are able to identify several key features of these gels, as well as several distinguishing characteristics that do not arise in conventional amphiphilic gels.