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Langmuir 1999, 15, 7752-7761
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-3phosphoethanolamine, 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 airwater 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 (
