Chawla, Sneha Anil (2017-05). Microstructure-Mechanical Property Relationships in Carbon Nanofibers. Doctoral Dissertation.
Carbon nanofibers, though radially more homogeneous compared to carbon fibers, currently do not possess mechanical properties as high as carbon fibers. By principles of size effect, carbon nanofibers are expected to possess considerably higher strengths than carbon fibers. Theoretically, CNFs are expected to have strengths as high as 14GPa. However, at present, CNFs possess strengths much lower than expected. The gap in theoretical and experimental work points to three main reasons: graphitic alignment in the nanofiber, radial structure of the nanofiber and presence of surface defects. The work presented in this dissertation aims at closing the gap via relating the microstructure and mechanical properties of carbon nanofibers. Graphitic alignment in carbon nanofibers imparts high modulus and strength to the fibers. This alignment of graphitic domains arises from the induced molecular alignment in precursor fiber. The precursor is polyacrylonitrile (PAN) fiber obtained from electrospinning of PAN in Dimethylformamide (DMF) solution. Limited molecular alignment is achievable with electrospinning, which creates the need to use other methods to improve molecular alignment. The research uses a method for hot drawing, which takes place at temperatures above the Tg of the polymer. The temperature aids chain mobility in the fiber, allowing it to stretch. The molecular alignment obtained in the hot drawing process facilitates the improvement in graphitic alignment in the carbon nanofiber formed. The effect of this enhanced alignment on single carbon nanofibers is studied via mechanical tests performed on single carbon nanofibers, with diameters of 250nm-700nm, using a microelectromechanical system (MEMS) device in conjunction with digital image correlation (DIC). It has been observed that improvement in the molecular alignment of the precursor fiber leads to improvement in strength and modulus of carbon nanofibers. This increase can be related to improvement in graphitic orientation and size of crystallites in the CNF. In summary, it has been observed that molecular alignment in the PAN fiber prior to the stabilization stage is crucial in the evolution of graphitic domains, which was achieved via hot drawing. This effort presents a systematic study of molecular alignment and its effect on the mechanical properties of CNFs. Qualitative assessment of the morphology of the fibers is accomplished using Fourier Transform Infrared (FTIR), X-Ray diffraction (XRD), Selected Area Electron Diffraction (SAED), and Transmission Electron Microscopy (TEM).