Melt blowing is the most common technique to directly produce nonwovens from polymeric resins without any demand for extra bonding steps. The relatively low diameter of meltblown fibers, 1-2 mu m, makes them superior in separations performance to the competing melt processing technique, spunbonding. To further decrease meltblown fiber diameter, the islands-in-the-sea approach was investigated in this study. Immiscible blends of a water-soluble polymer, sulfonated poly(ethylene terephthalate) (SP), and hydrophilic polybutylene terephthalate (PBT) and hydrophobic polyvinylidene fluoride (PVDF) were meltblown and the SP washed away to produce nanofibers of less than 200 nm diameter. Despite a significant increase in blend drop size upon increasing the minor phase fraction,phi, the nanofiber diameter increased only slightly. Thus, we conjecture that nanofiber diameter is mainly controlled by fiber pinch-off, induced by quick stretching during the melt blowing process, rather than being affected by initial drop size. At a high shear viscosity ratio, eta(minor)/eta(matrix)> 3, nanofibers showed a higher level of irregularities, particularly when the polymer flow rate decreased. At phi >= 0.2 fiber bundling was observed but with nanofibers from PVDF, due to its lower surface energy, compared with PBT, bundling decreased. Our work shows that using polymers with relatively low surface energy, and thus low tendency for coalescence, coupled with low viscosity can result in nonwoven mats of fibers with very low average diameter, low bundling, and an acceptable fiber regularity. With PVDF we achieved a record low average diameter of 36 nm. By using a water-soluble polymer matrix, such as sulfonated poly(ethylene terephthalate), concerns about cost and environmental problems related to chemical solvents are alleviated. (C) 2018 Elsevier Ltd. All rights reserved.