We study amphiphilic diblock copolymers of poly(styrene)-block-poly(acrylic acid), or PS-b-PAA, diblocks. synthesized via an emulsion process thanks to a novel controlled radical polymerization technique called MADIX. Their self-assembling properties in the melt state resulting from the incompatibility between PS and PAA, allow us to prepare different microstructures like spherical, cylindrical and lamellar, simply by varying the PS volume fraction of the diblock (so-called composition). We study by SANS and SAXS, the dispersions of these microstructures in water, a selective solvent for PAA, and at controlled pH. The PAA brush, a polyelectrolyte, becomes charged and swells, while the glassy PS cores are unable to reorganize and conserve their size and shape : they are said to be « frozen in». The hexagonal and lamellar phases prepared in the melt state thus produce in water ordered suspension of nanocylinders and nanoplatelets, which constitute original colloidal systems. Most importantly, these objects bear polyelectrolyte brushes of constant surface density and curvature, as long as the systems are not annealed : when the constrain of glassy cores is suppressed (by heating the dispersions above the glass transition temperature of the PS cores), the objects spontaneously evolve in shape and size. Therefore, the use of these systems is two-fold : first they offer a remarkable and original tool to study the physics of polyelectrolyte brushes, but at the same time, to precisely investigate how brushes can alter the morphology adopted by a diblock copolymer. We will show 2 examples of this : (i) how lamellar mesophases can be used to investigate flat polyelectrolyte brushes; (ii) how the conformation of a polyelectrolyte brush of cylindrical symmetry alters the phase behavior of the dibloc in water.
Rhodia Recherche, Aubervilliers