Pasquali, Matteo2024-05-222024-05-222023-052024-04-19May 2023Ginestra, Cedric. Phase behavior, flow dynamics, and solution processing of boron nitride nanotubes and carbon nanotubes. (2024). PhD diss., Rice University. https://hdl.handle.net/1911/116220https://hdl.handle.net/1911/116220EMBARGO NOTE: This item is embargoed until 2025-05-01Carbon nanotubes (CNTs) have the potential to combat climate change by displacing innately high CO2-intensive materials like metals and carbon fiber, through synthesis of CNTs by natural gas pyrolysis and solution fiber spinning. Despite having been synthesized shortly after CNTs, structurally similar boron nitride nanotubes (BNNTs) are at a somewhat nascent technological phase, marred by stagnation from early technical obstacles. Continued improvements to BNNT synthesis and purification are still required to produce high aspect ratio, pure BNNTs. In this work, we process the first BNNT liquid crystalline solutions into neat BNNT structures using scalable methods. We find that high material purity is required to achieve the liquid crystalline phase preferred for solution processing of rigid rods into macroscopic objects, such as films and fibers. We find that BNNT fiber properties are ~10x lower than expected from results obtained with early CNT fibers composed of similar aspect ratio nanotubes, and we attribute this performance gap to low BNNT purity, which leads to poor solution quality. To better understand the behavior of nematic nanotubes, we study solutions of CNTs and BNNTs using quantitative polarized optical microscopy (Q-POM) and find that the purity of different BNNT grades might possibly be determined semi-quantitatively by measuring the birefringence intensities of these solutions. If so, this approach will significantly enhance the optimization of future BNNT purification with corresponding advancements expected in macroscopic material performance. By QPOM of BNNT and long CNT solutions, we show that flow-induced alignment created by sample preparation does not relax on experimental timescales due to low rotational diffusivity - this behavior is distinct from solutions of short CNTs, which lose their flow-induced order on relatively short time scales, manifesting as a reduction in birefringence intensity. The existence of a pseudo-equilibrium for long CNTs is of particular importance for materials characterization, because the isotropic-nematic phase transition concentration (commonly measured by POM) is used to estimate CNT aspect ratios and predict CNT fiber performance. We find POM to be ill-suited to determine this transition concentration for long CNTs and recommend against using these POM-based measurements to inform changes in CNT synthesis or fiber spinning processes.application/pdfengCopyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.Carbon nanotubes (CNTs)boron nitride nanotubes (BNNTs)polarized optical microscopy (POM)liquid crystalsphase behaviorrotational diffusionrigid rodsThesis2024-05-22