Monolayers of Symmetric Triblock Copolymers at the Air-Water Interface. 1. Equilibrium Properties Mercedes G. Mun ˜ oz, † Francisco Monroy,* ,‡ Francisco Ortega, † Ramo ´n G. Rubio, † and Dominique Langevin ‡ Departamento de Quı ´mica Fı ´sica I, Facultad de Ciencias Quı ´micas, Universidad Complutense, E28040 Madrid, Spain, and Laboratoire de Physique des Solides, LPS CNRS, Ba ˆ timent 510, Universite ´ Paris-Sud, F91405 Orsay, France Received February 10, 1999. In Final Form: October 1, 1999 Surface pressure isotherms and ellipsometric measurements of monolayers of two triblock symmetric copolymers, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), at the air-water interface have been carried out. These copolymers are water-soluble, and the difference in hydrophobicity between the blocks is small. This represents a different scenario for brush formation than for most of the hydrophobic-hydrophilic block copolymers reported so far. The surface pressure curves show two different phase transitions. The ellipsometric measurements indicate a thickness transition when the monolayer saturates, which supports the hypothesis for brush formation. The experimental data have been analyzed in terms of the scaling theory of adsorption of polymer brushes. Despite the possibility of diffusion from the interface, the PPO block acts as an efficient anchoring element in the formation of an adsorbed brush, once the adsorption sites at the interface are fully occupied. This is analogous to what has been reported for diblock copolymers with a much larger difference in the hydrophobicity of the blocks. 1. Introduction Since the pioneering works of Langmuir, 1 mono- molecular layers at fluid interfaces have been the subject of many experimental and theoretical studies. The earliest studies on small amphiphilic insoluble monolayers re- vealed the existence of phase transitions in these bidi- mensional systems, usually known as Langmuir mono- layers. 1,2 Recent experimental techniques, such as Brewster angle and fluorescence microscopy and small-angle X-ray or neutron reflectivity, have evidenced a rich phase behavior in this type of monolayers. In particular, gas and liquid expanded phases have been observed in the low surface pressure and low density regions of the phase diagram, while a wide variety of highly structured condensed phases have been found at higher pressures. 3 On the other hand, since diffusive exchange with the bulk phase is possible, high-density condensed states are not generally observed in adsorbed monolayers of soluble amphiphiles, also called Gibbs monolayers. The condensed states are unstable to compression, forcing the surfactant in excess to dissolve in the adjacent bulk solution. This is well illustrated by the fact that the collapse area per molecule of insoluble fatty alcohols (C n -OH, n > 12) at the air-water interface, A ∞ ≈ 19 Å 2 /molecule, is smaller than the saturation area of the adsorbed monolayers of the smaller soluble homologous (n < 10), A ∞ ≈ 29 Å 2 / molecule. 4 Only a few studies describe the coexistence of two condensed phases in adsorbed or Gibbs monolayers of small surfactants. 5,6 In the earlier ones, the observed structures are believed to be caused by insoluble or sparingly soluble impurities. Only very recently, Melzer et al. 7 have clearly pointed out a first-order transition in adsorbed monolayers of a soluble n-alkyl amide, from a typical liquid expanded phase to a highly ordered con- densed phase. The combined use of Brewster angle microscopy and grazing X-ray diffraction from a synchro- tron source has allowed the comparison of this phase with that of an insoluble homologous surfactant at the same surface pressure Π. The morphological and structural equivalence between the two crystalline phases has been inferred from this study, which is the first clear evidence for phase coexistence in Gibbs monolayers. In general, quasi-two-dimensional layers of long- polymer chains at the air-water interface do not show the complex phase behavior of small surfactants. This is due to the fact that crystalline order is not usually found unless strong stereoregularity constraints are fulfilled. However, it is also well accepted that adsorbed polymer layers can be obtained in several different configurations depending on polymer-interface and polymer-polymer interactions. 8 Consequently, one may expect phase tran- sitions to occur between different states in films made of soluble hydrophilic polymers. In particular, weak surface adsorption, driven by polymer-interface hydrophobic forces, is expected to occur at low bulk concentrations. However, a brush phase made of chains grafted to the interface is expected at higher concentrations where the scarcity of adsorption sites in the interface favors a tridimensional arrangement stabilized by chain-chain and solvent-chain interactions. 9 This scenario is sup- ported by many theoretical 9-12 and experimental studies. These focus mainly on the characteristic of the study of * To whom correspondence should be addressed. E-mail: monroy@eucmos.sim.ucm.es. † Universidad Complutense. ‡ Universite ´ Paris-Sud. (1) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Harkins, W.; Boyd, E. J. Chem. Phys. 1941, 45, 20. (3) Knobler, C. M. Adv. Chem. Phys. 1990, 77, 397. (4) Lucassen-Reynders, E. H. Prog. Surf. Membr. Sci. 1976, 10, 253. (5) Berge, B.; Faucheux, L.; Schwab, K.; Libchaber, L. Nature 1991, 350, 322. (6) Rivie `re, S.; He ´non, S.; Meunier, J. Phys. Rev. E 1994, 49, 1375. (7) Melzer, V.; Volhardt, D.; Weidemann, G.; Brezesinski, G.; Wagne, R.; Mo ¨hwald, H. Phys. Rev. E 1998, 57, 901. (8) Fleer, G. J.; Cohen-Stuart, M. A.; Cosgrove, T.; Vincent, B. Polymer at Interfaces; Chapmann and Hall: London, 1993. (9) Halperin, A. Soft Order in Physical Systems; Plenum: New York, 1994. (10) Milner, S. T. Science 1991, 251, 905. (11) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1991, 100, 31. 10.1021/la990142a CCC: $18.00 © xxxx American Chemical Society PAGE EST: 10.9 Published on Web 00/00/0000