Viscoelastic Properties of Lipopolymers at the Air-Water Interface: A Combined Interfacial Stress Rheometer and Film Balance Study

Poly(ethylene glycol) (PEG) is a molecule that exhibits unique behavior when compared with polymers in its homologous family. Depending on its environment, itmayshowhydrophilic, 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 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.