Polyelectrolyte complex coacervates are a fascinating class of materials formed by associative phase separation of oppositely-charged polymers in solution. These materials have a wide range of potential applications in drug delivery, adhesion, and separations, and are also similar to many of the biological materials that undergo phase separation in living cells. Understanding how different chemical features of the polymers control their phase behavior and viscoelasticity is not only important for developing a better fundamental understanding of these materials, but also for guiding design of polyelectrolyte complexes with precisely targeted properties for each application.
In our group, we are interested in understanding how specific molecular-scale interactions between the polymer chains affect the materials properties. We use two primary approaches to achieve this goal.
First, to investigate how specific molecular-scale interactions affect the phase behavior of the complexes, we use post-polymerization functionalization to synthesize well-defined polymer libraries in which we systematically vary the mixture of functional groups on the polymer sidechains while leaving all other features of the polymer the same, and then characterize the resulting complexes.
Using this approach, we showed that there is an important trade-off between charge density and hydrophobicity in these materials. Decreasing the charge density of the polymers, by substituting some of the charged sites with small, nonionic, hydrophilic functional groups, decreases the stability of the complexes, as the entropic driving force for complexation is reduced. If these small, hydrophilic functional groups are replaced with larger, hydrophobic groups however, the complexes become significantly more stable as favorable hydrophobic interactions compensate for the decreased entropic favorability of complexation.
Increasing the length of alkyl sidechains incorporated into the charged polymers that make up the complexes drives a transition from charge-density-dominated behavior, in which stability decreases with decreasing charge content, to hydrophobicity-dominated behavior, in which stability increases with increasing hydrophobic content.
Recently, we extended this idea to investigate how cation-pi interactions between the polymer chains impact the complexation process. We compared complexes of polymers that contained aromatic sidechains with complexes of polymers in which the aromatic rings were replaced with cyclohexane rings. We found that cation-pi interactions affected the phase behavior and enthalpy of complexation only when new cation-pi interactions could be formed between the chains. This work suggests that cation-pi interactions can play an important role in these materials, and should be taken into account when designing and analyzing their properties.
Second, to understand how different molecular-scale interactions affect the viscoelastic properties of the complexes, we characterize commercial polymers and simple synthetic polymers in the presence of high salt concentrations. These experiments let us decouple changes in the polymer chemistry, salt identity, and salt concentration from changes in the polymer concentration in the materials, which otherwise complicate interpretation of measurements of the materials properties.
Using this approach, we found that the dependence of the relaxation time of these materials on the polymer concentration is significantly stronger than has been assumed in much of the work to date. This insight is now enabling us to tease apart the role of other chemical and physical interactions in determining the relaxation times and viscoelastic properties of coacervate materials, as well.
Together, these efforts are helping our group build a comprehensive picture of how molecular-scale features of charged polymers affect the bulk properties of their complexes. If you are interested in learning more about our work in this area, check out the individual papers linked above, or any of our other publications on coacervates, below!
Our work on complex coacervates is currently supported by the National Science Foundation under award no. 2203857 (CHE-MSN). Our past work in this area has also been supported by the Petroleum Research Fund of the American Chemical Society under award 58034-DNI7, and by start-up funding from the University of Pittsburgh.