The effect of waveforms of rotor motion in a chaotic mixing device on the development of morphology in the blending of polypropylene (PP) and polyamide 6 (PA6) was investigated. The rotor motion was delivered in a steady and time-periodic manner and the mixing effectiveness was evaluated in terms of Poincaré maps and stretching distribution, the speed of conversion of PP-phase into droplets, and droplet size distribution. The pathway of morphology development, such as deformation of the PP-phase into lamella, fibrils, and droplets, was found not to depend on the nature of the waveform. However, the makeup of each morphological form depended strongly on the particular waveform used. The best results were obtained when the rotor motion was delivered using a sine waveform. In this case, the PP-phase turned most rapidly into fibrils and droplets and the droplets were of more uniform size. The square waveform, although produced higher mean stretching and showed more uniform mixing in Poincaré maps, yielded large-size droplets, purportedly through premature breakup of the fibrils in the zones of weak flow, such as in the vicinity of the stationary rotors. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2346–2358, 2004