A novel strategy to synthesize amphiphilic surface-compartmentalized nanoparticles based on linear ABC triblock copolymers is presented. These so-called Janus micelles consist of a cross-linked core and a corona with a "northern" and a "southern" hemisphere. Selectively cross-linking spherical domains of the polybutadiene middle block in a well-ordered bulk morphology of a polystyrene-blockpolybutadiene-block-poly(methyl methacrylate) triblock copolymer (SBM) leads to the conservation of the compartmentalization of the outer blocks after dissolution of the material. Multi-angle laser light scattering gel permeation chromatography, fluorescence correlation spectroscopy, small-angle neutron scattering, and static and dynamic light scattering, as well as scanning force microscopy, indicate the existence of an equilibrium between molecularly dissolved Janus micelles (unimers) and aggregates (multimers), so-called supermicelles.

We describe the synthesis and the solution properties of Janus micelles containing a polybutadiene (PB) core and a compartmentalized corona consisting of a poly(methacrylic acid) (PMAA) and a polystyrene (PS) hemisphere. The Janus micelles were obtained via cross-linking the middle block of a microphase-separated PS-block-PB-block-PMMA triblock copolymer in the bulk state, followed by alkaline hydrolysis of the poly(methyl methacrylate) (PMMA) ester groups. Results of fluorescence correlation spectroscopy, field flow fractionation, light scattering, cryogenic transmission electron microscopy, scanning electron microscopy, and scanning force microscopy indicate that above a critical aggregation concentration of about 0.03 g/L spherical supermicelles are formed from about 30 PS-PMAA micelles in aqueous solution in the presence of NaCl. These supermicelles have radii of 40-60 nm, significantly increasing on ionization (pH >6). In addition, very large spherical objects are observed with radii of 100-250 nm.

Janus micellessasymmetric star block copolymers resulting from cross-linking of the polybutadiene middle blocks of polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock copolymerss and the corresponding non-cross-linked ABC triblock copolymer precursors form monolayers on a water surface. Analysis of the lateral pressure/area isotherms and scanning force microscopy images of monolayers spread from dilute chloroform solutions reveal a lateral structure: In monolayers of both polymers, one observes elevated nanometer-sized domains separated by flat regions. However, the monolayers made from the two copolymers differ in morphology and size of the observed domains. Janus micelle monolayers are predominantly composed of arrays of circular domains with height of 16-18 nm and diameter of 70-80 nm, while the monolayers of the precursor triblock copolymer are mainly composed of significantly smaller elongated domains of approximately 3 nm height, 40 nm width, and nonuniform length. The formation of domains and the shape of the lateral pressure/area isotherms are in accordance with the assumption that the poly(methyl methacrylate) chains spread out on the water surface, while the hydrophobic polystyrene and polybutadiene chains dewet from the water surface. Differences in domain morphology are probably due to the different chain architectures of the two polymers but may as well be influenced by the preaggregation in the spreading solvent.