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 sp2 CC 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 Tg 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 °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.

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