Micellization and adsorption behavior of a near-monodisperse polystyrene-based diblock copolymer in nonpolar media.

The micellar self-assembly behavior of a near-monodisperse polystyrene-hydrogenated polyisoprene (PS-PEP) diblock copolymer is examined in non-polar media (either n-heptane or n-dodecane). Direct dissolution of this diblock copolymer leads to the formation of relatively large polydisperse colloidal aggregates that are kinetically frozen artifacts of the solid-state copolymer morphology. Dynamic light scattering (DLS) and transmission electron microscopy studies indicate that heating such copolymer dispersions up to 90-110 °C leads to a structural rearrangement, with the generation of relatively small, well-defined spherical micelles that persist on cooling to 20 °C. Variable temperature (1)H NMR studies using deuterated n-alkanes confirm that partial solvation (plasticization) of the polystyrene micelle cores occurs on heating. This increased mobility of the core-forming polystyrene chains is consistent with the evolution from a kinetically-trapped to a thermodynamically-favored copolymer morphology via exchange of individual copolymer chains, which are observed by DLS. These micellar self-assembly observations are also consistent with small-angle X-ray scattering (SAXS) studies, which indicate the formation of star-like micelles in n-heptane, with a mean polystyrene core diameter of about 20 nm and an overall diameter (core plus corona) of about 80 nm. Micelle dissociation occurs on addition of chloroform, which is a good solvent for both blocks. Finally, physical adsorption of this PS-PEP diblock copolymer onto a model colloidal substrate (carbon black) has been confirmed using X-ray photoelectron spectroscopy. A Langmuir-type adsorption isotherm has been constructed using a supernatant depletion assay based on UV spectroscopy analysis of the aromatic chromophore in the polystyrene block. Comparable results were obtained using thermogravimetric analysis to directly determine the amount of adsorbed copolymer. Based on the maximum adsorbed amounts observed at 20 °C, these studies strongly suggest that individual PS-PEP copolymer chains adsorb onto carbon black from chloroform, whereas micellar adsorption occurs from n-alkanes.