The nanofiber electrospun from the five-membered ring sulfonated polyimide containing the fluorinated group showed an ultrafine and uniform non-beaded structure and the optimized electrospinning conditions produced the sulfonated polyimide nanofibers with several appropriate diameters ranging from 80 to 160 nm. Novel sulfonated copolyimide membranes containing uniaxially aligned ultrafine sulfonated copolyimide nanofibers were prepared for proton exchange membrane fuel cell applications. The proton conductivity of the composite membrane in the parallel direction to the nanofibers alignment increased with the decreasing nanofiber diameter and the increasing amount of nanofibers in the composite membrane and reached 0.3 S cm(-1) at 90 degrees C and 98% RH. In addition, the parallel proton conductivity indicated significantly higher values when compared to that determined for the composite membrane in the perpendicular direction or for the membrane without nanofibers prepared by a conventional solution-casting method. The composite membranes also showed higher chemical stabilities than those of the membrane without nanofibers. Consequently, nanofibers proved to be promising proton exchange materials and the composite membranes containing nanofibers may have potential application for use in fuel cells. (C) 2012 Elsevier B.V. All rights reserved.