Evidence for capillary waves at immiscible polymer/polymer interfaces

The width of the interface between two immiscible polymers, deuterated polystyrene and poly(methyl methacrylate), has been measured using neutron reflectivity as a function of the thickness of the deuterated polystyrene layer. A logarithmic dependence of interface width on film thickness is observed, characteristic of an interface broadened by thermal induced capillary waves, whose spectrum is cut off by dispersive interactions across the polymer layer. Reasonable agreement is obtained with the results of self-consistent field theory when suitably modified to account for capillary waves, resolving a long-standing discrepancy between theory and experiment.