Amphiphilic diblock copolymers on mica: Formation of flat polymer nanoislands and evolution to protruding surface micelles

We show that the deposition of poly(isoprene-b-ethylene oxide) block copolymer (PI-PEO) micelles on freshly cleaved mica produced ultraflat, polymer nanoislands of well-defined monomolecular thickness. The islands were partly adsorbed and partly floating on the substrate, and their morphology changed over time (under ambient conditions). Their structural evolution was recorded and analyzed using atomic force microscopy. Larger islands evolved to high globular aggregates while smaller islands behaved in distinctly different manner becoming shorter; in both cases, the final structures had a surface micelle shape and organization. The time-dependent behavior originates from a decrease of the amount of water adsorbed to the mica surface, due to the fact that freshly cleaved mica under ambient conditions changes its surface character from highly hydrophilic to less hydrophilic with increasing exposure time. The combination and competition of strong affinity of the PEO block with the adsorbed water and flexibility of the hydrophobic PI block results in the formation and evolution of these supramolecular structures.