Direct observation of a diblock copolymer‐induced microemulsion at a polymer/polymer interface

We have studied the segregation of a block copolymer of poly(d8‐styrene‐b‐2‐vinylpyridine) (dPS‐PVP) at the interface between polystyrene and a random copolymer of poly(styreneran‐4‐hydroxystyrene) (PS‐r‐PPHS). Forward recoil spectrometry (FRES) was used to measure the equilibrium excess (z*) of the dPS‐PVP chains at the interface as a function of its volume fraction in the bulk PS phase (ϕ∞). It was found that there is a sharp increase in z* at a critical value of ϕ∞. This upturn indicates the formation of a microemulsion of PS and the random copolymer PS‐r‐PPHS due to a vanishing of the interfacial tension caused by the strong adsorption of the block copolymer. Cross‐sectional transmission electron microscopy (TEM) of the interface shows that this microemulsion starts to form at the interface by forming a deeply corrugated structure where the “wavelength” of the corrugations is of the order of 50 nm.