The Characterization of Fullerene Nanotubes

A conundrum, as defined by the Merriam-Webster dictionary, is an interesting and difficult problem. The proper and cost-effective characterization of fullerene nanotubes is certainly a conundrum. Fortunately, notable national and global institutions (e.g., NIST, JIST, ASTM International, and many others) are tackling this complex problem. At Unidym, we are participating in these activities. However, the requirements for industrial progress often surge ahead of full scientific rigor. In this industry, we must make do with the tools we have and learn from technological history. For example, vulcanized rubber was used industrially to make tires and other useful products, long before the basic structure was understood to be sulfur crosslinked polyisoprene.

The view of SWNTs as being similar to polymers, where the repeating structural units are atoms of carbon, was recognized early.⁵ Based on this view, the use of polymer methods may be the best way to characterize grades of fullerene nanotubes. However, the full application of polymer characterization methods to fullerene analysis has been impeded by their lack of solubility and other difficulties. An example is the difficulty of measuring and representing the molecular weight distribution (“MWD”) of a fullerene sample, something that is now routine for most commercial polymers.

Like polymers, fullerene nanotubes exhibit the general property called polydispersity. For traditional polymers, polydispersity refers to the MWD as well as compositional distributions. Fullerene nanotubes are three-dimensional and have interior space, as shown in Figure 2. Therefore, the polydispersity has an added dimension that we call endotopicity—i.e., something located inside the tube.

Figure 2.
Schematic Diagram of One Chiral Type of SWNT

Because of this added dimension, describing a particular grade in molecular terms is somewhat complex. We use a three-level description of structure: primary, secondary, and tertiary. Furthermore, the dispersion of our grades in matrices, whether they are other polymers, ceramics or concrete, creates a fourth level of structure called quaternary. We will discuss these in turn throughout this article. Table 1 illustrates this hierarchy of structure and some key characteristics that will define commercial “grades”.

Table 1.
Structural Hierarchy and Key
Characteristics of Fullerene Nanotube Grades

Primary structure – Fullerene Carbon Nanotubes	– Length and diameter (molecular weight) * – Chirality* – Endotopic character, * (SWNT, DWNT, “peapods”, metals) – Functional groups (covalent, non-covalent)
Secondary structure – bundles	– Diameter* – Degree of crystallinity – Residual catalyst and non-tubular carbon
Tertiary structure – bulk form	– Fluff, Powder, Pearls, Paper, Fibers – Bulk surface area – Thermal stability
Quaternary structure – in composites	– Individually dispersed tubes – Dispersed ropes, sometimes swollen and exfoliated – 3-dimensional continuity

* usually a polydisperse distribution;
** "Endtopic" description suggested by Prof. Alan Windle, Cambridge University

At CCNI our efforts are focused on developing well-defined commercial grades of fullerene nanotubes in addition to our research grades. The distinguishing feature between a commercial grade and a research grade is the intended use of the nanotubes. A researcher wants to know the description of the chemical in as precise detail as possible. Researchers who are familiar with buying a “chemical” are not always used to the idea of polydispersity. A researcher wants to know if the SWNT sample has a diameter of 1.0 nm and a length of 500nm, or a diameter of 1.2 nm and length of 1000nm, etc. On the other hand, a commercial user of a fullerene nanotube grade wants to know how well it functions in the end product, what it costs, and availability. Commercial users of polymers are familiar with the polydisperse nature of polymer grades and recognize that a grade has a distribution.