Development of a novel, scalable fabrication procedure to produce permeable, vertically aligned carbon nanotube membranes
Creators: name: Castellano, Richard
Publisher: name: No Publisher Supplied
Year: 2021

description: Membranes with well-controlled nanopores are of interest for applications as diverse as high-throughput filters, molecular sieves, as well as “green” energy like solar panels, and “blue” power generation owing to their fast water transport and ion selectivity. Membranes incorporating vertically aligned (VA) carbon nanotubes (CNTs) as through-pores have been shown to transport fluids at rates orders-of-magnitude faster than predicted by theory, offering promise as highly permeable membranes for desalination membranes or breathable garments while their nanometer size allows for protection from bacteria, viruses, and large toxins. However, there is a need for cost-effective and scalable methods for fabricating large-area VA-CNT membranes, ideally starting from bulk nanotubes. In fact, macroscopic, VA-CNT membranes with sub-nanometer pores have not been previously fabricated due to the difficulty in growing aligned forests of such small CNTs.Herein, we describe a new, solution-based fabrication technique for creating polymer composite membranes with aligned CNTs starting from bulk-produced CNT powder. While CNT membranes from less scalable fabrication techniques (like chemical vapor deposition) have been produced on the lab scale for over a decade, we have developed a facile, scalable, and cost-effective solution to producing such membranes compatible with large-scale, roll-to-roll fabrication. The process is applicable to any type of nanotube with the sole limitation that the nanotubes’ electrical properties differ from that of the solution. To accomplish the fabrication procedure, the carbon nanotubes are suspended in a fluid, either liquid prepolymer or a solvent. In suspension, the CNTs are aligned with an electric field and electrophoretically concentrated to produce an aligned forest of CNTs on one electrode surface. We then replace the solvent with UV curable pre-polymer, and use extinction-limited curing to produce a thin polymeric membrane encapsulating the vertically aligned CNTs.We experimentally measure the kinetics of the electric-field alignment process and compared with theory, demonstrating understanding of the physics of CNT alignment in electric fields under viscous, thermal, and shear forces. Additionally, in this thesis, we develop a novel technique to measure the ensemble length distribution of CNTs in suspension, applicable to any polarizable, high-aspect-ratio particle. This measurement is key to selecting CNTs which are long enough to span the thickness of the polymer membrane, and developing the dispersion procedure so as not to break the CNTs into sections shorter than the membrane thickness.The fabricated vertically-aligned CNT-pore membranes are subjected to a barrage of tests to count the number of CNTs, measure flow enhancement, overall breathability, and rule out defects in the membranes. We fabricated membranes with various nanotube diameters, with the highest water-vapor flow enhancements of 5 × 10^5 coming from CNTs of diameter 0.8 nm, and the pressure-driven N2 flux enhancement up to 300× for 1.78 nm CNTs. Such membranes with CNT pores offer promise as breathable, protective layer for garments to protect soldiers from chemical and biological hazards.