An interfacial stress rheometer has been constructed to study the rheology of Langmuir films subjected to time-dependent flows. A magnetized rod resides at the air−water interface and is set into oscillation by applying a sinusoidal magnetic field gradient. Analysis of the amplitude and phase of the resulting rod motion relative to the applied force allows the determination of the dynamic surface modulus, G s*(ω), and measurement of the relative elastic and viscous contributions of the monolayer. Measurements at 22 °C were conducted on eicosanol (C20) and mixtures of a rigid-rod polymer, phthalocyaninatopolysiloxane (PcPS), dispersed in eicosanol. The surface pressure dependence of the rheology for eicosanol reveals the presence of a maximum in the loss modulus, G s‘ ‘(ω), within the L2‘ phase at Π = 6 mN/m. In the LSI phase at pressures above 15 mN/m, the monolayer is Newtonian and has a surface viscosity of 0.03 mN·s/m. The mixtures of PcPS with eicosanol are known to have two-dimensional nematic behavior. The presence of PcPS in the film increased ||G s*(ω)|| 100-fold, creating a non-Newtonian interface with a measurable elasticity. As the polymer rod concentration was increased further, G s*(ω) became less dependent on frequency, and above the isotropic−nematic transition, the storage modulus, G s‘(ω), exceeded the loss modulus, G s‘ ‘(ω). The results on eicosanol and the mixtures of the rigid rod with eicosanol demonstrate that the rheometer is capable of detecting microstructural transitions in a Langmuir monolayer.

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 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.

The effects of blending on the rheology of the individual components in highly entangled miscible blends of 1,4-polyisoprene (PI) and 1,2-polybutadiene (1,2-PB) are investigated. Blend component contributions to the dynamic modulus, G*() contributions indicates that each component's relaxation is governed by a distinct apparent glass transition temperature, Tt, and that at a constant T -Tg blend composition only mildly influences the component relaxation times. A mild increase in the 1,2-PB relaxation time in the blend suggests a possible increase in the 1,2-PB friction factor in the presence of PI.

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