Amphiphilic Diblock Copolymers on Mica: Formation of Flat Polymer Nanoislands and Evolution to Protruding Surface Micelles Emmanouil Glynos, † Stergios Pispas, ‡ and Vasileios Koutsos* ,† Institute for Materials and Processes, School of Engineering and Electronics & Centre for Materials Science and Engineering, UniVersity of Edinburgh, King’s Buildings, Edinburgh EH9 3JL, United Kingdom, and Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou AVe., 11635 Athens, Greece ReceiVed NoVember 27, 2007; ReVised Manuscript ReceiVed April 1, 2008 ABSTRACT: 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. Introduction Block copolymers have received considerable attention as the building blocks for bottom-up, self-assembled nanostructures 1 which can be used in a variety of enabling nanotechnologies 2 associated with a diverse number of applications. In particular, thin and ultrathin films of block copolymers 3 promise unique solutions for various technology sectors spanning from bio- technology 4 to nanoelectronics. 5 Unique opportunities for mod- ifying surfaces to achieve tunable properties arise if one uses appropriate block copolymer nanostructures responsive to certain external stimuli such as pH, 6,7 annealing, and exposure to solvents. 8 Block copolymer nanostructures adsorbed and/or self-as- sembled on solid surfaces have been extensively studied by atomic force microscopy (AFM) 7,9–25 sa technique which offers unprecedented spatial resolution and real space images in three dimensions. The vast majority of these studies are concerned with stable equilibrium structures of multichain aggregates and supramolecular assemblies, in solution or more usually in dry state, which in many cases take the form of stripelike or spherically shaped micellar structures. 18,25 Diblock copolymers form spherical micelles in a solution (for concentrations above the critical micelle concentration, cmc) within a selective solvent 26,27 and in many cases could remain more or less intact after deposition on a solid substrate. 23 Alternatively, surface micelles can be formed due to the selective adsorption of one of the two blocks onto the substrate forming a thin monolayer while the other blocks aggregate on top or in this layer forming spherically shaped domes. 9,22,24,28 Amphiphilic block copoly- mers consisting of a hydrophobic and a hydrophilic block have attracted considerable interest lately, owing to their ability to form micelles in watersa crucial property for biomedical and environmental applications. 29,30 Micelle surface relaxation, reorganization, spreading, and some partial dissociation have been reported in the case of hydrophobic substrates which attract the hydrophobic micellar core. 23,24,28,31 In this study we show that (a) the presence of an ultrathin surface water film can instigate the formation of flat, polymer, brushlike islands of well-defined thickness which are sensitive to the surface conditions and (b) the progressiVe depletion of the ultrathin water film can induce the structural evolution of these flat islands to protruding nanostructures of surface micelle shape and organization. To our knowledge, it is the first time that these ultraflat polymer islands were observed and imaged, and their progressive evolution to surface micellar structures has been captured in real time. Furthermore, we show that there is a very important size effect and islands of different size behave in a distinctively different manner. Of paramount importance in the interpretation of our results is the behavior of water on freshly cleaved mica in ambient conditions. Mica is one of the most usual model substrates for AFM studies of polymer nanostructures, adsorbed polymers, and biomolecules on surfaces. Clean, contamination-free, and atomically flat mica surfaces can be prepared by cleavage. However, the role of the water layer always present on mica in ambient conditions has been usually ignored in these studies. In general, a thin film of water grows on hydrophilic surfaces in ambient conditions via water vapor adsorption and condensa- tion on the solid surface. 32,33 This is an important phenomenon as it can alter the interactions between the surface and adsorbed molecules, especially if these molecules have strong affinity with water. The amount of water adsorbed on a hydrophilic surface is determined by the environmental conditions. Mica has been proven to be a popular substrate for studying the physicochem- ical properties of adsorbed water layers. Freshly cleaved mica in ambient conditions adsorbs water from the atmosphere because of its high hydrophilicity, which results in an ordered or partially ordered water layer. The existence of the adsorbed water layer on mica has been established in many studies using various experimental techniques. 34–40 These studies revealed that at ambient temperature and relative humidity, RH ≈ 50%, two layers of water are formed. There is a first structured/ordered and rigid layer of water due to its strong bonding with the mica * To whom correspondence should be addressed: Ph +44 (0)131 6508704; Fax +44 (0)131 6506551; e-mail vasileios.koutsos@ed.ac.uk. † University of Edinburgh. ‡ National Hellenic Research Foundation. 4313 Macromolecules 2008, 41, 4313-4320 10.1021/ma702630c CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008