Coarse-grained model for polyelectrolyte complexation

Self-assembly based on co-polyelectrolyte complexation offers new opportunities for materials design. Here, oppositely charged polyelectrolytes undergo an associative process of phase separation, driven by the entropy gain associated with the release of counter-ions. Homopolymers typically self-assemble into coacervates, while block copolyelectrolytes form micelles or hydrogels. In this study, we aim to quantitatively reproduce experimental results for the phase behavior and rheology of coacervates with a coarse-grained model. Specifically, we focus on three features of the coacervate. First, there is a strong dependence of polymer concentration on the added salt concentration. Second, we show that salt is generally expelled from the coacervate phase and that our model is capable of capturing the salt partition coefficient between the coacervate and supernatant phases. Third, the model enables calculation of the mechanical response and it compares well with experimental measurements. Moreover, it captures changes in the relaxation time as a function of added salt concentration, a phenomenon referred to as "time-salt superposition". We explore the effects of model parameters on the above features of coacervates, and explain the role of short range interactions and ionic strength.