Man-made high-performance fibers produced a revolution in structural materials in the twentieth century. Fiber properties are known to increase with the decrease of their diameters. Ultrafine sub-micron and nanometer-sized fibers attract rapidly growing interest due to their unique advantages for diverse applications. Electrospinning is a straightforward low-cost top-down nanomanufacturing technique producing continuous nanofibers. Electrospinning allows direct fabrication of macroscopic nanofilamentary assemblies for structural and functional applications. Recently, high-performance polyimide nanofibers have been produced by electrospinning of polyamic acid precursors followed by thermal curing. However, curing resulted in fusion of nanofilaments at contacts. While fusion might be beneficial for some applications, it usually results in deterioration of the ordered molecular structure and mechanical properties of fibers in bundles and tows. In this report, we investigated the effect of residual solvent dimethylacetamide (DMAc) on the fusion of nanofibers in electrospun nanofiber bundles during thermal curing of polyimide. Feasibility of removal of the residual DMAc by aging in vacuum and by washing with other solvents was explored. The effect of an additive, dodecylethyldimethylammonium chloride (DEDAC) on fusion was investigated. It was found that the removal of residual DMAc by aging in vacuum reduced the nanofilament fusion during imidization. The presence of DEDAC in the absence of DMAc was shown to further reduce the fusion. DEDAC may represent a new class of additives with a potential to preserve nanostructure of polyamic acid precursors during thermal conversion to polyimides. The results of this report can be used in the development of ultrahigh-performance nanofilamentary fibers. (C) 2011 Elsevier Ltd. All rights reserved.