Excitation of surface deformation modes of a phase-separating polymer blend on a patterned substrate

The phase separation kinetics of ultrathin deuterated poly(styrene)/poly(bu tadiene) polymer blend films spun cast onto striped self-assembled monolaye r (SAM) substrates is studied by atomic force microscopy (AFM). Fourier tra nsform analysis of the AFM topographic data at various stages of the film p attern development reveals the presence of quantized surface deformation mo des. These modes are excited by the phase separation process when the scale of phase separation becomes commensurate with the period of the striped su rface pattern. Thus, higher frequency modes become excited at early stages of phase separation, and these excitations decay with time as the phase sep aration pattern further coarsens. The film ultimately self-organizes into a periodic structure in which the fundamental mode has the largest amplitude . The influence of film thickness on the film morphology in this late stage is also investigated. A decrease in the film thickness leads to surface pa tterns that match those of the SAM substrates with increasing resolution. H owever, these film patterns break up into droplet arrays along the SAM stri pes if the films are made too thin. This phenomenon is attributed to a capi llary wave instability.