Colloidal complexes obtained from charged block copolymers and surfactants:: A comparison between small-angle neutron scattering, Cryo-TEM, and simulations

We report on small-angle neutron scattering and cryo-transmission electron microscopy of complexes made from polyelectrolyte-neutral block copolymers and surfactants. Two block copolymer/surfactant systems have been investigated. In the first system, the polyelectrolyte block is negatively charged (poly(sodium acrylate), molecular weight 5 000 g/mol) and the neutral block is a poly(acrylamide) chain of molecular weight 30 000 g/mol. This copolymer is studied in solution in the presence of a cationic surfactant, dodecyltrimethylammonium bromide (DTAB). In the second system, the polyelectrolyte block is positively charged (poly(trimethylammonium ethylacrylate), molecular weight I 1000 g/mol) and the neutral block is again a poly(acrylamide) of molecular weight 30 000 g/mol. This copolymer is studied in solution with an anionic surfactant, sodium dodecyl sulfate (SDS). We show that the diblocks copolymers associate with oppositely charged surfactants into colloidal complexes which have a core-shell microstructure. For the two systems investigated, we have found that the core is constituted from densely packed surfactant micelles, presumably connected by the polyelectrolyte chains. Within the complexes, the DTAB or SDS micelles have the same aggregation number as in aqueous solutions above the cmc. The outer part of the complex is a corona formed by the neutral poly(acrylamide) chains. The microstructure of the core has been inferred from a strong forward neutron scattering and the appearance of a structure peak at high wave vectors (q(0) similar to 0.16 Angstrom(-1)). Using a model of aggregation of colloids developed for latex-silica nanocomposites and based on a Monte Carlo algorithm, we have simulated the internal structure of the aggregates. The model assumes spherical cages containing from one to several hundreds of micelles in a closely packed state. The agreement between the model and the data is remarkable. This allows us to conclude that the structure peak at q(0) similar to 0.16 Angstrom(-1) is associated with the hard-sphere interactions between micelles in the core.