Origins of toroidal micelle formation through charged triblock copolymer self-assembly

The toroidal micelle morphology has been produced by the self-assembly of poly(acrylic acid)-block-poly(methyl acrylate)-block-polystyrene (PAA-b-PMA-b-PS) triblock copolymer via interaction with organic diamines in mixed THF/water solution. Formation of toroidal, or ring-like, micelles was controlled by the ratio of THF to water, the chain length of the polystyrene block, the type and amount of diamino counterion as well as the solution preparation procedure. Two distinct mechanisms for toroidal micelle formation are proposed. Under the appropriate solution condition, toroids can be constructed either through elimination of high-energy spherical micelles and/or cylindrical micelle endcaps, or through perforation of disc-like micelles. Slow chain dynamics of triblock copolymers in solution associated with their high molecular weight plays an important role in toroidal micelle construction. Toroidal nanostructures were characterized with both transmission electron microscopy (TEM) and cryogenic transmission electron microscopy (cryo-TEM).