Transient target patterns in phase separating filled polymer blends

Recent simulations suggest that the presence of filler particles in a phase separating blend can induce the development of composition waves having th e symmetry of the filler particles. We investigate these predictions throug h atomic force microscopy (AFM) measurements on ultrathin (L approximate to 100 nm) polystyrene and poly(vinyl methyl ether) blend films containing a low concentration of model filler particles (silica particles having a nomi nal diameter approximate to 100 nm). The filled blend films were spun-cast on acid-cleaned silica wafers, and phase separation was induced by a temper ature jump into the two-phase region (T approximate to 145 degrees C) of th e bulk polymer blend. By rinsing off the polymer film with solvent, we show that the silica particles are associated with the substrate so that the fi ller particles represent a quenched disorder perturbation of the film phase separation. The presence of the filler particles leads to the development of circular composition waves ("target patterns") about the filler particle s during the intermediate stage of phase separation. These target patterns disintegrate as the background spinodal phase separation pattern becomes mu ch larger than the filler particles. Our observations are consistent with i dealized two-dimensional Cahn-Hilliard-Cook simulations of the phase separa tion of polymer blends having a small concentration of filler particles.