Viscoelastic Properties of Lipopolymers at the Air-Water Interface: A Combined Interfacial Stress Rheometer and Film Balance Study C. A. Naumann, † C. F. Brooks, † G. G. Fuller, † W. Knoll, †,‡ and C. W. Frank* ,† Department of Chemical Engineering, Stanford University, Stanford, California 94305 and MPI f. Polymerforschung, Mainz, Germany Received March 4, 1999. In Final Form: June 2, 1999 Poly(ethylene glycol) (PEG) is a molecule that exhibits unique behavior when compared with polymers in its homologous family. Depending on its environment, it may show hydrophilic, hydrophobic, or amphiphilic properties. We have studied several PEG lipopolymers, where a PEG chain with a molecular weight (MW) of 2000 g/mol or 5000 g/mol is covalently attached to 1,2-dipalmitoyl- or 1,2-distearoyl-sn-glycero-3- phosphoethanolamine, with a Langmuir film balance and a recently developed interfacial stress rheometer. In particular, we have determined how the rheological properties of PEG molecules anchored at the air- water interface change when the polymer chains are forced into highly stretched brush conformations. Pressure-area isotherms of monolayers of PEG lipopolymers exhibit two phase transitions: a desorption transition of the PEG chains from the air-water interface at 10 mN/m and a high film pressure transition at 20-40 mN/m, but the nature of the latter transition is still poorly understood. We have observed a remarkable change of the viscoelastic properties in the range of the high-pressure transition. The monolayer is fluid below the transition, with the surface loss modulus, Gs′′, being larger than the surface storage modulus, Gs′, but becomes remarkably elastic above, with Gs′ > Gs′′. This indicates that a strong correlation exists between the reversible, first order-like high-pressure transition and the formation of a physical gel. Our surface rheological experiments indicate that formation of a physical network can be understood if water intercalates mediate the interaction between adjacent PEG chains via hydrogen bonding. Introduction Poly(ethylene glycol) (PEG) is a molecule with unique behavior. Unlike other polyethers, PEG is water-soluble, which is related to a specific structuring of water molecules along the polymer chain. 1 Beyond such a hydrophobic interaction, PEG has also been suggested to form H-bonds with surrounding water molecules via its ether oxygen in a hydrophilic effect. 1,2 PEG is not just a simple hydrophilic polymer, however, because it is also highly soluble in organic solvents and forms a monolayer at the air-water interface at moderate film pressures (<10 mN/m), thus revealing an amphiphilic character. 3-6 Spectroscopic experiments have shown that the PEG chain conformation is more ordered in aqueous solutions than in organic solvents. 3,4 It is very likely that the observed behavior of PEG is caused by a complex interplay between hydrophobic interactions, H-bonding between water and PEG, and PEG-PEG interactions. This unusual behavior of PEG has been linked to its unique biological inertness. 7-9 Even though the polymer chain has an internal struc- ture in aqueous solution, classical scaling arguments provide a reasonably good description for understanding the interacting surfaces of end-grafted PEG chains in water in the surface forces apparatus and osmotic stress measurements. 10,11 Although both techniques force the PEG chains into constrained conformations, they are insensitive to changes in their in-plane shear rheological properties because of such constraints. In the context of these findings, we will address the question: How will the rheological properties of grafted PEG chains change when they are forced into highly stretched brush confor- mations? To investigate the surface rheological properties of monolayers of lipopolymers at the air-water interface, we use a recently developed interfacial stress rheometer which uses an oscillating rod at the air-water interface. 12 There is a wide variety of methods used to measure the rheological properties of amphiphilic molecules at inter- faces including deep canal devices, channel flow devices, and rotating disks and rings. 13,14 Surface rheology experi- ments go beyond the information that can be obtained from classical pressure-area isotherms, because they provide a relationship between mechanical and confor- mational properties of amphiphiles. Our model system, lipopolymers at the air-water interface, gives us the * Corresponding author: Curtis W. Frank, Department of Chemical Engineering, Stanford University, Stanford, CA 94305- 5025. E-mail: curt@chemeng.stanford.edu. † Stanford University. ‡ MPI f. Polymerforschung. (1) Kjellander, R.; Florin, E. J. Chem. 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Interfacial Transport Processes and Rheology; Butterworth-Heinemann: Boston, 1991; Chapter 7. 7752 Langmuir 1999, 15, 7752-7761 10.1021/la990261q CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999