By utilizing electrospun blend fibers of polystyrene (PS) and poly(ethylene oxide) (PEO) with diameters in sub-microns, nanodroplets of the minor component (PEO) were obtained by annealing the blend fibers above the glass transition temperature (T(g)) of the matrix polymer (PS), as a result of the Rayleigh-Plateau instability in the melt. However, direct thermal annealing of the PS/PEO blend fibers led to poor Rayleigh breakup of the PEO fibers in the PS matrix, and fractionated crystallization with both homogeneous and heterogeneous nucleation was observed, probably due to a broad size distribution of PEO particles. On the contrary, after confining the PS/PEO blend fibers with a high T(g) polymer, poly(4-tert-butyl styrene) (P4tBS, T(g) similar to 143 degrees C), well-defined Rayleigh breakup of the PEO fiber was achieved by annealing the P4tBS-coated PS/PEO blend fibers at 150 degrees C. Consequently, exclusive homogeneously nucleated PEO crystallization was observed at -20 degrees C. This report could provide a universal method to achieve nanosized droplets for the study of nanoconfinement effect by utilizing electrospun immiscible polymer blend fibers without addition of any compabitilizers. (C) 2011 Elsevier Ltd. All rights reserved.