We report on the formation of colloidal complexes resulting from the electrostatic self-assembly of polyelectrolyte−neutral diblock copolymers and oppositely charged surfactant. The copolymers investigated are asymmetric and characterized by a large neutral block. Using light, neutron, and X-ray scattering experiments, we have shown that the colloidal complexes exhibit a core−shell microstructure. The core is described as a dense microphase of micelles connected by the polyelectrolyte blocks, whereas the shell is a diffuse brush made from the neutral chains. For all copolymer/surfactant systems, we show the existence of a critical charge ratio Z C (∼1) above which the formation of hierarchical structures takes place. Copolymers of different molecular weight and polyelectrolyte blocks have been studied in order to assess the analogy with another type of core−shell aggregates, the polymeric micelles made from amphiphilic copolymers. The present results indicate that the radius of the core depends essentially on the degree of polymerization of the polyelectrolyte block and not on that of the neutral chain. On the other hand, the size of the overall colloid increases with increasing molecular weights of the copolymers. Taking advantage of the resolution of X-ray scattering, we have also shown that the micelles in the core of the aggregates are structurally disordered.
We report on the phase behavior and scattering properties of colloidal complexes made from block copolymers and surfactants. The copolymer is poly(sodium acrylate)-b-poly(acrylamide), hereafter abbreviated as PANa-PAM, with molecular weight 5000 g/mol for the first block and 30000 g/mol for the second. In aqueous solutions and neutral pH, poly(sodium acrylate) is a weak polyelectrolyte, whereas poly(acrylamide) is neutral and in good-solvent conditions. The surfactant is dodecyltrimethylammonium bromide (DTAB) and is of opposite charge with respect to the polyelectrolyte block. Combining dynamical light scattering and small-angle neutron scattering, we show that in aqueous solutions PANa-PAM diblocks and DTAB associate into colloidal complexes. For surfactant-to-polymer charge ratios Z lower than a threshold (Z(C) approximately 0.3), the complexes are single surfactant micelles decorated by few copolymers. Above the threshold, the colloidal complexes reveal an original core-shell microstructure. We have found that the core of typical radius 100-200 A is constituted from densely packed surfactant micelles connected by the polyelectrolyte blocks. The outer part of the colloidal complex is a corona and is made from the neutral poly(acrylamide) chains. Typical hydrodynamic sizes for the whole aggregate are around 1000 A. The aggregation numbers expressed in terms of numbers of micelles and copolymers per complex are determined and found to be comprised between 100-400, depending on the charge ratio Z and on the total concentration. We have also shown that the sizes of the complexes depend on the exact procedure of the sample preparation. We propose that the driving mechanism for the complex formation is similar to that involved in the phase separation of homopolyelectrolyte/surfactant systems. With copolymers, the presence of the neutral blocks prevents the macroscopic phase separation from occurring.
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 11 000 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 ∼ 0.16 Å-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 ∼ 0.16 Å-1 is associated with the hard-sphere interactions between micelles in the core.
A quaternary system made of an ionic surfactant (SDS), octanol, water and sodium chloride has been investigated. We present experimental results that demonstrate the existence of a phase of vesicles at thermal equilibrium. We show that vesicles can be prepared both in a dilute regime leading to an isotropic liquid phase of low viscosity and in a concentrated regime leading to a phase of close packed vesicles (probably multilayered) exhibiting high viscosity and viscoelasticity
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