Spinodal decomposition in a subsurface layer of a polymer blend film

We have studied phase separation in off-critical mixtures of poly(ethylene propylene) (hPEP) and perdeuterated poly(ethylene propylene) (dPEP) inside the metastable and unstable region of the phase diagram but under the influence of a surface that attracts the majority component (dPEP). In the adjacent depletion region nucleation barriers are eliminated, and the film starts to phase separate long before phase separation occurs in the bulk. The hPEP-rich domains are confined to grow anisotropically within the unstable region. The growth gives rise to the formation of a regular roughness pattern on the surface when the domains relax into a more circular shape driven by interfacial tension. The volume fraction versus depth profiles are obtained using time-of-flight forward recoil spectrometry (TOF-FRES) while the roughness pattern that mirrors the phase morphology in the depletion layer can be monitored by scanning force microscopy (SFM). Depending on initial dPEP volume fraction the lateral bicontinuous phase pattern inside the unstable region either develops into a dPEP-rich layer containing hPEP-rich droplets or forms a continuous hPEP-rich layer with dPEP-rich droplets. The lateral growth of the pattern follows a λ(t) = λ(t=0) + bt^0.80 law where λ(t=0) and λ(t) are the pattern wavelength initially and at time t after the quench.