Mechanical tests on individual carbon nanofibers reveals the strong effect of graphitic alignment achieved via precursor hot-drawing

Electrospun carbon nanofibers (CNFs) processed via carbonization of electrospun precursors are an emerging class of nanoscale carbon-based materials with abundant sp(2) C-C bonds which can offer significant opportunities for structural light-weighting in multifunctional materials. In this work, we have studied the effect of graphitic alignment on mechanical properties of CNFs. Graphitic alignment was achieved by hot-drawing polyacrylonitrile (PAN) nanofiber precursors at temperatures above the T-g of PAN which induces chain alignment. We studied several states of PAN chain alignment by varying electrospinning take-up velocity and hot-drawing ratios. Chain alignment and orientation induced crystallization was studied via polarized Fourier Transform IR spectroscopy and X-ray diffraction. IR spectroscopy revealed that the formation of crystals delays thermal stabilization and cyclization in hot drawn PAN nanofibers. Thus, we modified the stabilization process to transform PAN chains into a ladder-like structure suitable for carbonization. The carbonization was carried out at 1100 degrees C. MEMS-based mechanical characterization of individual CNFs revealed over 100% improvement in average strength and over 70% improvement in modulus of CNFs as a result of graphitic alignment. The CNFs obtained from hot-drawn samples demonstrated strength as high as 5.4 GPa, which is among the highest reported for this class of material. (C) 2017 Elsevier Ltd. All rights reserved.