ABSTRACT

The efficacy of surfactants for stabilizing interfaces between immiscible fluids depends on both thermodynamic and dynamic driving forces.  Dynamic driving forces are especially important when the fluids of interest are immiscible high molecular weight polymers.  The surfactants that are used to stabilize interfaces between immiscible polymers are block copolymers.  The chain-like character of polymers leads to molecular entanglement, which, in turn, leads to extremely slow dynamics.  We have exploited this slow dynamics to prepare two surfactant-bearing polymeric interfaces that are initially out of equilibrium.  The distance between the interfaces was varied from 50 to 600 nm, and the transport of the surfactant molecules one interface to another was measured by dynamic secondary-ion mass spectroscopy. This transport depends on traditional diffusion coefficients and the depth of the thermodynamic potential wells that trap the surfactant molecules at the interfaces.  The diffusion coefficients of our system were measured in independent experiments.  The depth of the thermodynamic potential well was obtained from self-consistent field theory (SCFT).  The Flory-Huggins interaction parameters and the statistical segment lengths of the polymers needed to complete the SCFT calculations were measured by small angle neutron scattering. This enables a comparison of our experimental interfacial transport measurement and theoretical predictions with no adjustable parameters. We note in passing that such a comparison is not possible in the case of traditional surfactants at oil/water interfaces due to the experimental difficulty of arranging parallel fluid interfaces at controlled distances, and the lack of predictive thermodynamic models for multicomponent systems comprising hydrophilic and hydrophobic moieties.  We demonstrate the importance of dynamical considerations in the rational design of effective polymeric surfactants.