Effect of an interactive surface on the equilibrium contact angles in bilayer polymer films

Scanning transmission X-ray microscopy (STXM), atomic force microscopy (AFM ), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and pho toemission electron microscopy (PEEM) were used to obtain the three-dimensi onal concentration profiles and the late-stage morphology of liquid bilayer thin films of the two imiscible polymers polystyrene (PS) and (polybromost yrene) (PBrx=0.79S), where x fraction of monomers brominated) as a function of the lower PS film thickness. The results showed that the apparent conta ct angle at the polymer/air interface decreases exponentially with increasi ng PS film thickness with a constant that scales with the PS radius of gyra tion (R-g) In contrast, the Neuman angle, as determined from the PBrS STXM images, increases with substrate thickness. NEXAFS and PEEM data show that the droplets consist of a PBrS core fully encapsulated by PS for substrate thicknesses greater than R-g. Only partial encapsulation is seen for substr ates less than R-g. These results could not be explained with existing mode ls developed for bulk viscoelastic fluids. We found that, for the case of e ntangled polymers, other factors such as the interfacial energy with the su bstrate, the film thickness, and the molecular weights must be considered. A model which includes a restoring force characterizing the change in stiff ness of the substrate layer due to surface interactions provided good agree ment with the experimental observations.