Size effects in continuous polyacrylonitrile-based polymer, composite, and carbon nanofibers

Nanotechnology is expected to produce the next revolution in structural materials and composites. However, despite significant efforts worldwide, a supernanocomposite reinforced by carbon nanotubes or graphene is yet to be demonstrated. The problems are well documented and stem from the discontinuous nature of nanoreinforcement. Continuous nanofibers produced by electrospinning represent an emerging class of nanomaterials with critical advantages for structural and functional applications. However, until recently they were considered weak, and their mechanical properties and structure have not yet been sufficiently studied. The goal of this dissertation was systematic analysis of size effects on mechanical properties and structure of polyacrylonitrile-based polymer and carbon nanofibers.;Control of nanofiber morphology, average diameter and diameter distribution was achieved through process and solution parameters. Individual polyacrylonitrile nanofibers were fabricated in a wide range of diameters and tested through failure. Simultaneous increases of two orders of magnitude in strength and modulus as well as three orders of magnitude in toughness (area under the stress/strain curve) were observed for ultrafine (<250nm) nanofibers for the first time. Structural investigation and comparison with mechanical behavior of annealed nanofibers allowed attribution of the observed simultaneous size effects to improved polymer chain alignment coupled with low crystallinity due to rapid solvent evaporation from fine jets in electrospinning process. This structural hypothesis was further verified and supported by modification of the nanofiber structure and properties through change in solvent and addition of plasticizer.;Examination of the structure of polyacrylonitrile-based carbon fibers (CNFs) revealed generally poor graphitic structure (compared to commercial carbon fibers) with some improvements in the structure and crystal orientation for thinnest nanofilaments. Graphitic structure and crystal orientation were successfully improved through addition of small amounts of carbon nanotubes and graphene oxide. These structural modifications hold the potential for improvements in mechanical and transport properties of CNFs.;Results of this work and the proposed structural explanations constitute a possible paradigm shifting approach in the fiber science and technology for the development of the next generation of advanced fibers.