Coassembly of Poly(ethylene oxide)-block-poly(methacrylic acid) and N‑Dodecylpyridinium Chloride in Aqueous Solutions Leading to Ordered Micellar Assemblies within Copolymer Aggregates

Formation of polyelectrolyte–surfactant (PE–S) complexes of poly(ethylene oxide)-block-poly(methacrylic acid) (PEO705–PMAA476) and N-dodecylpyridinium chloride (DPCl) in aqueous solution was studied by static and dynamic light scattering (SLS, DLS), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). While it was found previously [Macromolecules 1997, 30, 3519] by microcalorimetric titration that in a similar system (PEO176–PMAA186) crystallization of aliphatic tails of N-dodecylpyridinium bromide did not occur, in our system it was evidenced by SAXS that upon addition of DPCl to fully ionized PEO705–PMAA476 the ordered arrangement of the surfactant occurs in a certain range of PEO705–PMAA476 concentrations and surfactant-to-polyelectrolyte charge molar ratio (Z). Our data suggest a four-step process in the behavior of the PEO705–PMAA476/DPCl system: (i) coexistence of loose aggregates of electrostatically bound surfactants to PMAA block with free and almost unperturbed copolymer coils at Z ≪ 1, (ii) formation of aggregates containing ill-defined cores formed by DPCl micelles attached to coiled PMAA chains (beads-on-a-string nanoparticles) in the range around Z = 0.5, (iii) formation of compact core–shell nanoparticles with a core formed by densely packed ordered (crystalline) DPCl micelles and PEO shell starting slightly before charge equimolarity (Z = 1), and (iv) the region of coexistence of the core–shell nanoparticles with free DPCl micelles in excess above equimolarity (Z ≫ 1). In the region around Z = 0.5, the nanoparticles with nonordered cores coexist in a mixture either with a fraction free chains and large swollen nanoparticles decorated by surfactant micelles (at lower Z) or with the core–shell nanoparticles (at higher Z). PE–S complexes were characterized in detail in terms of molar mass, size, shape, and internal structure.