Influence of "Hydrophobicity" on the Composition and Dynamics of Polyelectrolyte Complex Coacervates

Various types of specific interactions are believed to supplement the major entropic driving forces responsible for spontaneous liquid-liquid phase separations in mixtures of oppositely charged polyelectrolytes. Among these interactions, hydrophobicity has recently been probed experimentally via the synthesis and complex formation of polyelectrolytes bearing hydrophobic pendant groups or backbones. In this work, poly(4-vinylpyridines), P4VP, were N-alkylated with chains from one to six carbons in length. The fully alkylated polycations were complexed with poly(sodium methacrylate) to yield polyelectrolyte complexes or coacervates (PECs). Counterintuitively, PECs made with N-methyl to N-butyl P4VP were less stable to the addition of salt the longer the alkane chain, being easier to dope and having a lower critical salt concentration for dissolution. In contrast, the linear viscoelastic response of these PECs varied little. A transition in doping and properties was observed with N-pentyl and N-hexyl chains, the latter having a much higher modulus and much less sensitivity to salt concentration. Small-angle X-ray scattering suggested a new morphology for the N-pentyl and N-hexyl P4VP PECs, with interacting/phase separating alkane chains providing a transition into hydrophobicity-dominated PECs.