Specific interfacial interactions in multi-component systems such as composites improve the chemical compatibility between the filler and the matrix, prevent the filler from aggregating and strengthen the interface. Carbon nanotube (CNT) based polyacrylonitrile (PAN) composites were prepared either by extrusion or electrospinning to yield fibers with diameters on two different scales - micro- and nanometric. Introduction of a third component, ethylene carbonate, a plasticizer which is capable of forming strong dipolar interactions with the nitrile group of the polymer, had a four-fold effect. It enabled melt-processing of the polymer, caused a structural transition in the crystalline matrix, improved the uniformity and decreased the diameter of the electrospun fibers. It also indirectly revealed a preferred interaction between the pi-electrons of the CNT and the nitrile groups of PAN, as was identified based on synchrotron microbeam WAXD, DSC, C-13 NMR and FTIR of CNT-filled and unfilled PAN fibers. The nature of CNT and PAN interaction was modeled by charge-transfer of CNT pi-electrons from the highest occupied molecular orbital to the empty nitrile pi* orbital of PAN. (c) 2007 Elsevier Ltd. All rights reserved.