Interpolyelectrolyte Complexes of Polycationic Micelles and Linear Polyanions: Structural Stability and Temporal Evolution

The complexation of poly(dimethylaminoethyl methacrylate)-block-poly(styrene) micelles with poly(styrenesulfonate) homopolymers was investigated in aqueous buffer at pH 4.5 as a function of ionic strength. The complexation process was monitored by turbidimetric titration, and the structure and stability of the complexes were assessed by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryoTEM), and small-angle X-ray scattering. When complexes were formed by slow titration of one polyelectrolyte solution into the other, soluble complexes could be formed with either polyelectrolyte in excess as long as the mixture did not pass through the charge-neutral point. The initial complexes exhibited bimodal size distributions by DLS, with one population similar in size to or slightly smaller than the bare micelles, and the other significantly larger. The former correspond to individual micelles with complexed polyelectrolytes leading to a contracted corona; the latter reflect multimicelle aggregates that were directly observed by cryoTEM. At low ionic strength (e.g., 10 mM), these aggregates were stable on weeks-to-months time scales, but at high ionic strength (e.g., 500 mM), the aggregates rapidly annealed toward structures whose size and solubility depended on which polyelectrolyte was present in excess. These results are discussed in terms of the kinetics and thermodynamics of the polyelectrolyte complexation process and allow a detailed description of the interplay between kinetic and thermodynamic factors in this system. This work will inform design of polyelectrolyte complexes with tunable structure and stability for future applications.