The role of intefacial diffuseness on surface segregation from polymer blends

We present two mean-field theories for the surface segregation from polymer mixtures at surfaces with variable interface diffuseness. We are motivated to study this issue since surface segregation can dramatically alter the behavior of polymers at surfaces, e. g., the wetting behavior of films, surface roughness, and phase separation in thin films. In particular, we are motivated by two results in the literature. First, experimentally it has been determined that surface segregation at the air surface is always significantly higher than at hard substrates. Second, Klein and his coworkers have prepared a series of cross-linked polymer films as substrates and found that the segregation from an overlayer polymer blend to these surfaces is directly correlated to the crosslink density. Our theories show that the surface segregation is strongly coupled to interface diffuseness, with the magnitude of the segregation increasing with increasing diffuseness. These results, which are in good agreement with experiments, arises since the blend prefers to place the component with the less favorable polymer-polymer energy at the surface. This effect is magnified at interfaces with increasing diffuseness. Coupled with findings that interface penetrability can give rise to new phenomena, such as phase transition induced surface roughening, we stress that surface diffuseness can play a critical role in polymer surface behavior.