Ion-stabilized block copolymer micelles: Film formation and intermicellar interaction

Thin films of poly[styrene-block-(2-vinylpyridine)] with equally long blocks were prepared by casting micellar toluene solutions on a mica or carbon substrate. The structure of the films was studied by scanning force microscopy and transmission electron microscopy. Structural variations were observed depending on the concentration of the solution from which the films were obtained and depending on the neutralization of the pyridine units by HAuCl4. Scanning force microscopy as well as dynamic light-scattering experiments demonstrated an increasing stability of the inverse micelles upon ionization of the poly(2-vinylpyridine) core. While the micelles of polystyrene block poly(2-vinylpyridine) transformed into wormlike and lamellar structures upon film formation, the micellar structure was preserved when the poly(2-vinylpyridine) block was partly or fully neutralized by the gold acid. Scanning force microscopy revealed formation of an adsorbed, brushlike first layer on which the micelles assembled to form the following layers. The kinetic stability of the block ionomer micelles is reflected in the scanning force micrographs by the fact that surface coverage of the adsorbed block copolymer brush was reduced when the toluene solution was diluted directly before film preparation. During the given time, the concentration of the free chains (critical micelle concentration) could not recover by dissociation of micelles. Increasing degrees of neutralization, i.e., increasing loading of the micelle core by AuCl4-, resulted in growing diameters of the core as well as of the polystyrene shell, which is explained by the enhanced segregation parameter chi(core-shell) and stretching of both blocks. Remarkable repulsion between the micelles was observed in monolayer films at high degrees of ionization which is temptatively explained by electrostatic repulsion caused by transfer of AuCl4- ions to the substrate-bound brush.