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 whe n compared with polymers in its homologous family. Depending on its environ ment, it may show hydrophilic, hydrophobic, or amphiphilic properties. We h ave studied several PEG lipopolymers, where a PEG chain with a molecular we ight (MW) of 2000 g/mol or 5000 g/mol is covalently attached to 1,2-dipalmi toyl- or 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, with a Langmuir f ilm balance and a recently developed interfacial stress rheometer. In parti cular, we have determined how the rheological properties of PEG molecules a nchored at the air-water interface change when the polymer chains are force d into highly stretched brush conformations. Pressure-area isotherms of mon olayers of PEG lipopolymers exhibit two phase transitions: a desorption tra nsition of the PEG chains from the air-water interface at 10 mN/m and a hig h film pressure transition at 20-40 mN/m, but the nature of the latter tran sition is still poorly understood. We have observed a remarkable change of the viscoelastic properties in the range of the high-pressure transition. T he monolayer is fluid below the transition, with the surface loss modulus, G(s)", being larger than the surface storage modulus, Gs', but becomes rema rkably elastic above, with G(s)' > G(s)". This indicates that a strong corr elation exists between the reversible, first order Like high-pressure trans ition and the formation of a physical gel. Our surface rheological experime nts indicate that formation of a physical network can be understood if wate r intercalates mediate the interaction between adjacent PEG chains via hydr ogen bonding.