Layer-multiplying coextrusion was used to fabricate assemblies of 257 layers, in which polypropylene (PP) nanolayers alternated with thicker polystyrene (PS) layers. The PP layer thickness was varied from 12 to 200 nm. When the assembly was heated into the melt, interfacial instability-driven breakup of the thin PP layers produced a dispersion of PP particles in a PS matrix. Particle size analysis indicated that breakup of PP microlayers produced a bimodal particle size distribution. A population of submicron particles formed because of the Rayleigh instability, and a second population of large particles formed by relaxation. Breakup of 12-nm layers resulted in primarily submicron particles. The fraction of PP as submicron particles dropped dramatically as the layer thickness increased to 40 nm. The particle dispersion was characterized by thermal analysis and wide angle X-ray diffraction. Fractionated crystallization gave rise to four crystallization exotherms at 40, 60, 85, and 100 degrees C. The exotherm at 40 degrees C was identified with homogeneous nucleation of the submicron particles in the smectic form. Exotherms at higher temperatures represented fractionated crystallization of the large micron-sized particles in the PP alpha-form. (c) 2007 Wiley Periodicals, Inc.