(129a) Chemical Determinants of Complexation in Polyelectrolyte Complex Coacervates

Complexes of oppositely-charged polymers exhibit rich phase behavior and materials properties. However, achieving precisely targeted properties under fixed solution conditions remains a significant challenge, in large part because the specific chemical features and chemical interactions that control the properties of these materials are not well-understood.

To address this problem, we used post-polymerization functionalization of poly(N-acryloxy succinimide) to prepare well-defined polyelectrolyte libraries with ionizable groups on 40-100% of the repeat units and either hydrophilic (amide), hydrophobic (alkyl), or aromatic (benzyl) sidechains on the remaining sites. The phase behavior of the resulting polyelectrolyte complexes was characterized via optical turbidity, and the thermodynamics of complexation by isothermal titration calorimetry (ITC). For the polymers with hydrophilic sidechains, we find that the salt resistance of the complexes decreases with decreasing charge density, reflecting phase behavior that is dominated by the entropy of counterion release. For the polymers with hydrophobic alkyl sidechains, on the other hand, we find that the salt resistance increases with increasing hydrophobicity, reflecting dominance of favorable enthalpic interactions between the nonionic monomers, as confirmed by ITC. Finally, for polymers with aromatic sidechains, we observe a pronounced dependence of both the salt resistances and the heat of complexation on the cation of the salt used to set the total ionic strength of the solution, with the strength of complexation increasing as the cation is changed from K+ to Na+ to Li+, consistent with the trend expected if the inorganic salt is competing for and breaking apart inter-chain cation-pi interactions.

These results suggest that both hydrophobic and cation-pi interactions are important drivers of polyelectrolyte complexation, and that changing the chemical composition of polyelectrolytes can be a useful handle for controlling the properties of their complexes even under fixed solution conditions.