Self-Organized Monolayer Films of Stimulus-Responsive Micelles Grant B. Webber, † Erica J. Wanless,* ,† Vural Bu 1 tu 1 n, ‡ Steven P. Armes, ‡ and Simon Biggs † School of EnVironmental and Life Sciences, The UniVersity of Newcastle, Callaghan, N.S.W., 2308 Australia, and School of Chemistry, Physics and EnVironmental Science, UniVersity of Sussex, Falmer, Brighton BN1 9QJ, U.K. Received September 3, 2002; Revised Manuscript Received September 27, 2002 ABSTRACT Weakly charged micelles of poly(2-(dimethylamino)ethyl methacrylate) [10% quaternized]- block-poly(2-(diethylamino)ethyl methacrylate) (10qDMA- DEA) adsorb to form a highly ordered monolayer at the mica-solution interface at pH 8.9. Rinsing with solvent at pH 9 has little effect on the adsorbed layer. Reduction of the pH to 4 results in an irreversible swelling of the thin film, in contrast to a micelle-to-unimer transition seen for the diblock in bulk solution. The resilience of the adsorbed layer opens up potential nanotechnological applications. There is currently a burgeoning interest in the self-assembly of thin films of polyelectrolytes at the solid-liquid inter- face. 1,2 In particular, layer-by-layer (LbL) deposition of polyelectrolyte multilayers presents many opportunities for the preparation of new and highly functional nanomaterials. 3-18 Such LbL-organized polyelectrolyte films have been pro- posed as new materials for drug delivery and catalysis and have also been shown to have applications as nanoreservoirs and as molecular templates. A significant limitation in the preparation of such thin films is the tendency of simple homopolyelectrolytes to generate adsorbed layers of amor- phous morphology. In contrast, simple small-molecule sur- factant systems have been shown to produce self-organized layers at the solid-aqueous interface with a rich variety of morphologies. 19-23 These surfactant systems, however, lack the inherent stability of polyelectrolyte thin films, particularly with regard to changes in solution conditions such as concentration, pH, or temperature or to the removal of the bulk solution. Diblock copolymers, which may themselves be polyelec- trolytes, have recently been shown also to produce adsorbed layers of various, well-organized morphologies on numerous surfaces. 24-42 Adsorbed layer structures reported include hexagonally close-packed spherical aggregates, rodlike ag- gregates, vesicles, ribbonlike aggregates, and large, com- pound micelles. These diblock copolymers can also self- assemble in solution 43-58 and have been shown to solubilize small molecules, thus leading to proposed uses as drug delivery systems, 59,60 nanoreactors, 61 or as templates for nanoparticle formation. 62-64 Importantly, adsorbed layers of diblock copolymers may be produced by either the adsorption and subsequent surface-induced self-assembly of copolymer unimers or by the adsorption of copolymer aggregates directly from solution. Such properties enable the modifica- tion of the adsorbed films through many pathways. For example, changes in the solution aggregate morphology, the nature of the underlying surface, or the architecture of the diblock copolymer itself may all impact upon the morphology of the adsorbed layer. However, the frequent requirement for selective solvents to initiate self-assembly presents a constraint on the potential applications of many diblock copolymers. As a result, greater attention is now turning to aqueous-based systems because of their increased relevance to many natural and industrial processes. 56-58 Herein we report on the adsorption of a diblock copolymer that exhibits pH-induced micellization in aqueous solution. Such a copolymer is of particular interest since its micellar self-assembly is completely reversible, 56-58 leading to po- tential applications in the encapsulation-release of small molecules. Many studies of copolymer thin films have employed techniques such as transmission electron micros- copy (TEM) 26-31,35-37 or atomic (scanning) force microscopy (AFM) in air 38-42,65 to examine the morphology of the adsorbed layer. Such methods are viable only because of the high stability of the copolymeric adsorbed layer, although changes in the adsorbed layer morphology during the annealing process cannot be discounted. To date, there are few studies that have attempted to compare such dried films to the initial, in situ adsorbed layer. 41 These techniques also preclude in situ investigations into the effect of changing solution conditions on the layer of adsorbed diblock copoly- * Corresponding author. E-mail: ewanless@mail.newcastle.edu.au. ² The University of Newcastle. ‡ University of Sussex. NANO LETTERS 2002 Vol. 2, No. 11 1307-1313 10.1021/nl025781b CCC: $22.00 © 2002 American Chemical Society Published on Web 10/24/2002