Surface Rheology of Monolayers of Triblock Copolymers of PEO and PPO:  Surface Light Scattering Studies at the Air/Water Interface

Static and dynamic viscoelastic studies of monolayers of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, are performed at the air/water interface. These copolymers are better known by their trade names of Pluronics and Polaxamers. The technique of surface light scattering (SLS) with the Wilhelmy plate method enables us to probe both static and dynamic viscoelastic properties of the monolayers. The effect of varying the molecular architecture on the surface viscoelastic response is examined with two different copolymers, PEO−PPO−PEO (2.20 kg/mol) and its symmetric counterpart, PPO−PEO−PPO (1.95 kg/mol), over the temperature range 7−32 °C. SLS experiment reveals that the elastic contribution dominates the viscoelastic behavior of both copolymers over the entire temperature range. Further, there is a clear correspondence between the static elasticity and the dynamic viscoelasticity relative to monolayer surface density. When the static elasticity deduced from the surface pressure−surface density isotherm reaches a maximum, the corresponding behavior is observed for the dynamic elasticity and viscosity deduced from SLS.