Internal Interface of a Compressed PEE-PEO Diblock Copolymer Monolayer Alexander Wesemann, † Heiko Ahrens, † Roland Steitz, ‡,§ Stephan Fo ¨rster, | and Christiane A. Helm* ,† Angewandte Physik, Universita ¨ t Greifswald, Friedrich-Ludwig-Jahn-Str. 16, D-17487 Greifswald, Germany, Stranski-Laboratorium, TU Berlin, Strasse des 17. Juni 112, D-10623 Berlin, Germany, BENSC, Hahn-Meitner Institut, Glienicker Strasse 100, D-14109 Berlin, Germany, and Institut fu ¨ r Physikalische Chemie, Universita ¨ t Hamburg, Bundesstr. 45, D-20146 Hamburg, Germany Received May 16, 2002. In Final Form: November 4, 2002 Amphiphilic block copolymers consisting of a fluid hydrophobic (poly(ethyl ethylene), PEE) and a hydrophilic (poly(ethylene oxide), PEO) block form monolayers at the air/water interface. With X-ray and neutron reflectivity, the density profile of PEE432-PEO484 was investigated. It was found that the polymer adsorption layer consists of a homogeneous PEE and a solubilized PEO block, which can be laterally compressed by a factor of 3. The PEE thickness increases in inverse proportion to the molecular area, and the PEO brush follows the scaling law predicted for a brush in a good solvent. However, the stretching of the two blocks roughens the PEE-PEO interface, causing a transition from a PEO monolayer adsorbed to a hydrophobic interface (0.8-1.1 nm thick) to a PEE-PEO/water interfacial layer of 3 nm thickness. This transition of the interfacial layer highlights the rich phase behavior of amphiphilic block copolymers, which resembles that of lipids and nonionic surfactants. Introduction Poly(ethylene oxide) (PEO) is one of the few neutral, water-soluble, and biodegradable polymers. Therefore, it is discussed for numerous biological and medical applica- tions. Sensors are an intriguing application for polymer- supported bilayers, 1,2 since a polymer cushion between the lipid membrane and the solid support allows for both the lateral mobility and biofunctionality of the membrane- bound proteins. PEO provides a suitable cushion. 3 Another possible application is drug delivery, with the drug enclosed either in a lipid vesicle stabilized by lipopolymers 4 or directly in a vesicle made of amphiphilic diblock copolymers. 5 In the latter case, for the hydrophobic block, a fluid polymer like poly(ethyl ethylene) (PEE) is neces- sary, to ensure mobility as well as equilibrium configu- ration. Being the focus of research, PEO proved to be both more interesting and more complicated than originally assumed. Even though water soluble, 6 PEO adsorbs on the water surface, 7 a behavior well-known for amphiphilic molecules 8 consisting of hydrophilic and hydrophobic groups but unexpected for a water-soluble polymer. Indeed, “different faces” have been attributed to PEO. 6 Actually, water is a very good solvent for PEO at dilute concentrations but becomes less good at high polymer concentrations. This phenomenon is attributed to changes in the hydrogen- bonding interaction of water and PEO through either intra- or intermolecular bonds. 9 Lipids with PEO chemically attached to their head- groups, so-called lipopolymers, were considered as a suitable model system to investigate PEO brushes. However, they showed an unexpected behavior. 10-12 A nanostructure consisting of frozen alkyl chains embedded in solubilized PEO was observed; the stability of this structure suggests that solubilized PEO does adsorb onto the hydrophobic alkyl chains. Furthermore, the lipopoly- mer monolayer elasticity and viscosity are dramatically increased. 13 Therefore, kinetically trapped polymer con- figurations cannot be excluded. A low-viscosity, fluid anchoring system should be more suitable to investigate PEO equilibrium configuration. Fluid hydrophobic chains with small hydrophilic anchoring groups have been investigated at the air/water interface. Such monolayers exhibit the structural properties of a nano- meter-thick melt of bulk density, and their thickness increases linearly with the anchoring density. 14 If the monolayer is compressed, the lateral repulsion causes a pressure increase, which is due to the entropic repulsion of the chains trapped in the solvent-free brush. 15 Fluid † Universita ¨ t Greifswald. ‡ TU Berlin. § Hahn-Meitner Institut. | Universita ¨ t Hamburg. (1) Sackmann, E. Science 1996, 271, 43-48. (2) Frank, C. W.; Naumann, C. A.; Knoll, W.; Brooks, C. F.; Fuller, G. G. Macromol. Symp. 2001, 166,1-12. (3) Otsuka, H.; Nagasaki, Y.; Kataoka, K. Curr. Opin. Colloid Interface Sci. 2001, 6,3-10. (4) Lasic, D. D.; Papahadjopoulos, D. Science 1995, 267, 1275-1276. (5) Discher, B. W.; Won, Y. 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Macromolecules 1996, 29, 8912. 709 Langmuir 2003, 19, 709-716 10.1021/la0204592 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/04/2003