Structure and dispersion of carbon nanotubes Janis M. Brown Air Force Research Laboratory, MLBCO, WPAFB, OH David P. Anderson University of Dayton Research Institute, 300 College Park, Dayton, OH Jian Zhao, Kumar Chokalingham, Max Belfor and Dale W. Schaefer, Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH Jan Ilavsky Advanced Photon Source, Argonne National Laboratory, Argonne, IL Abstract Small-angle light scattering and ultra small-angle X-ray scattering are used to assess the morphology of single-walled (SWNTs) and carbon nanofibers (CNFs). For CNFs, a power-law scattered-intensity profile with a slope of –1.08 is consistent with the rod-like morphology. For SWNTs, however, scattering profiles characteristic of rod-like morphology are not observed on any length-scale from 1 nm to 50  m. Rather, disordered objects are found that we identify as a network of carbon “ropes” enmeshed with polyelectrolyte dispersants. The effectiveness of polyelectrolyte dispersants is assessed using small-angle light scattering in conjunction with exposure to ultrasound. In the presence of an anionic polyelectrolyte, sonication can assist dispersion of both SWNTs and CNFs. In the presence of a cationic agent, however, sonication can induce aggregation. SWNTs respond differently to ultrasound depending on whether residual synthesis catalyst is present. Four dispersants are studied, of which sodium polystyrene sulfonate is the most effective and polyallylamine hydrochloride is the least effective. Bragg Scattering and Length Scale Mass Fractal Dimension = D Surface Fractal Dimension = Ds Real Space Reciprocal Space I ~ MR ~ Q -1 M~R D The Concept of “Dimension” G A = E (a/R) 3+C Aggregate Size G rubber   = a (E/G rubber ) 1/3+C = a (10000) 1/4 = 10 a  1500 Å Silica SWNTS must be dispersed to impart superior performance. Identify the backbone Find the “chemical length,” L Model as a rod with same chemical length And thickness = primary particle diameter G A = E(a/R) 3+C ~ R -4 L ~ R c Bulk Modulus of Silica/Carbon Stiffness of Aggregate a L Witten, Rubenstein, Colby Model Reinforcement by Disordered Fillers Show a Limiting Length Scale SWNT 1-3 nm Conventional Intermediate Modulus Fiber 6000 nm (6 microns) ASI CNFs nm MWNT nm Differences in mechanical properties and processing are expected as one scales down several orders of magnitude CNFs are a good intermediate size fiber to address scaling issues Carbon “Fibers” - Scaling Down Several Orders of Magnitude 1.2 µ ASI Carbon Nanofibers (CNF) 0.5 µ Ropes of single walled tubes (SWNTs) Light +USAXS+SAXS Hollow Tube 0.5 µm 210 Å Wall ASI Carbon Nanofibers 1000 Å 3000 Å Reinforcing element is a “polymer,” not a rod. SWNT Network Rope Network Rope Diameter Rope Mesh Size SWNT Gel Mesh Size Dispersed SWNTs are Not Rod-like at Any Length Scale NH 2 NH H20H20 pH >8 sonicate Polyelectrolyte “Surfactant” Scattering SWNT Polyelectrolyte Dispersion PSSO3 PAAHCl I ~ M Light scattering measures “dispersibility” Agglomeration 20 µm 60 µm Breakup 83 µm 24 µm No effect 13 µm PSSO 3 Good Dispersion PMAA Intermediate PAAHCl Poor Dispersion Sonication Studies Coil Rod Potential Separation Technology Rod-like Remnant in Sonicated “Poor” Dispersions Same Trends as SWNTS Nanofibers (0.1% Fiber) in 3 Dispersants Conclusions The utility of small-angle X-ray and light scattering to measure the dispersion of both SWNTs and CNTs in water suspension was demonstrated. Even well dispersed both forms of carbon exist in an aggregated state. The SWNT aggregates are fractal structures - seem to be the analogue of the network of ropes seen by electron microscopy of dried samples. The ropes in suspension are swollen compared to the dried counterparts. No evidence of a persistence length of order of the diameter of an isolated SWNT. CNTs are also aggregated, but the morphology is side-by-side association of a limited number of tubes. – Morphology remains rod-like. – Porod’s law is observed on length-scales smaller than the radius of a single tube - the surface is presumed smooth and the interface sharp. – Both CNTs and SWNTs respond to dispersion aids in a similar fashion. – Anionic polyelectrolytes are the best dispersants. – There is evidence for phase separation of the dispersant around the tube clusters. – Suspensions respond differently to ultrasound.  In good dispersants ultrasound has minimal effect.  In poor solvents it induces aggregation. Residual catalyst has an effect on sonication. – Clean SWNT suspensions (catalyst removed) have little response to ultrasound regardless of the dispersion aid. – For As Rec’d SWNTs in the poorest dispersant precipitation observed after 10 min of sonication. Acknowledgements This research was funded by the United States Air Force Research Laboratory, partially through contract F D5405 and contract F D The UNICAT facility at the Advanced Photon Source (APS) is supported by the University of Illinois at Urbana-Champaign, Materials Research Laboratory (U.S. Department of Energy, the State of Illinois-IBHE-HECA, and the National Science Foundation), the Oak Ridge National Laboratory (U.S. DOE under contract with UT-Battelle LLC), the National Institute of Standards and Technology (U.S. Department of Commerce) and UOP LLC. The APS is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science under contract No. W ENG-38.