Self-assembled nanostructures in mixed anionic-neutral double hydrophilic block copolymer/cationic vesicle-forming surfactant solutions

The self-assembled nanostructures formed in mixed solutions of a double hydrophilic anionic-neutral block copolymer, poly[(2-sulfamate-3-carboxylate)isoprene-b-ethylene oxide] (SCIEO), and the vesicle-forming surfactant, didodecyldimethylammonium bromide (DDAB), are investigated. In these solutions electrostatic interactions exist between the anionic poly[(2-sulfamate-3-carboxylate) isoprene] block and the cationic surfactant. Combined static and dynamic light scattering measurements indicate that at low copolymer concentration vesicles of DDAB with adsorbed block copolymer chains are present in the solutions. As block copolymer concentration increases the structure of the nanoassemblies transforms to a core-shell, micellar-like structure, with a poly(ethylene oxide) corona and a core formed by the complex of poly[(2-sulfamate-3-carboxylate)isoprene] chains and DDAB molecules. This transformation of global aggregate structure should be attributed to a collapse of the DDAB vesicle bilayer as the number of complexed surfactant head groups to anionic polymeric segments increases. This happens in order to satisfy the conformational requirements of the complexing polymeric block and the steric requirements in the PEO corona, which apparently overcome the existing interactions between surfactant molecules in the ordered bilayer of initial DDAB vesicles. Block copolymer decorated vesicles show lower stability to an increase in solution temperature compared to micellar-like aggregates. Both nanostructures are found to be stable to changes in solution ionic strength, due to the combination of electrostatic and hydrophobic interactions acting within the mixed aggregates.