Chromatographic Separation and Analysis of Dispersed SWNT
Introduction
Single
wall carbon nanotubes (SWNT) are produced as a variety of chiral
structures, each having a set of specific electronic properties.
They also have a variety of lengths and diameters which affect
physical properties. Common dispersion methods also contain
significant contaminations of large clusters of tubes. Chromatography
offers an efficient means of eliminating clustered species and
sorting SWNTs by length, diameter, and chiral structure
Experimental Approach
Structure
Viscosity power law
scattering power law
rod
~1.3
1.0
self avioding walk
0.8
1.67
random walk
0.5
2.0
branched
<0.5
2.0 to 3.0
dense
0
4.0
The viscosity power law, [h] = KMa measures SWNT structure
type. The results can be compared to independent scattering
measurements, I ~ q-P.
Three approaches are possible: ion
chromatography (IC), high performance liquid chromatography
(HPLC), and size exclusion chromatography (SEC). Initial work
has been published on IC of SWNT/DNA complexes and on SEC of
aqueous/surfactant and organic complexes. The Polymers Division
has a long history of polymer chromatography covering all aspects
from basic research on mechanisms to production of standards,
and has SEC equipment capable of performing in situ rheological
measurements.
SEC Universal Calibration enables calibration of columns
for SWNT length, and viscosity and concentration detectors
give intrinsic viscosity [h]. The Mark Houwink equation, [h]
= KMa is used to determine molecular mass, M. The chromatography
can be scaled up to preparatory size, producing milligram
quantities of SWNTs sorted by length. Other methods such as
IC or HPCL can sort SWNTs by chirality and diameter.
Results
SEC separates by molecular size, with the largest molecules
eluting first. A concentration detector such as refractive
index determines the concentration. An in-line capillary
viscometer measures the viscosity. A combination of
5 narrow distribution polystyrenes shows that the higher
molecular weights are more viscous.
The ratio of the viscosity detector to that of the
concentration detector gives the intrinsic viscosity
as a function of molecular weight. A calibration curve
is established over a wide size range.
Dispersions are made by reacting acid treated SWNT
with octadecyl amine and are then sonicated in tetrahydrofuran.
Multiple detectors map the viscosity change with molecular
size.
Metrology
Dispersion of SWNT in solvents is
a crucial step in using them as components in nanomanufacturing.
A practical laboratory metrology for measuring dispersion effectiveness
is crucial to further developments.
Production
Sorting of SWNTs into their components is necessary
for specific applications. IC is capable of sorting by chirality
and diameter while SEC is capable of sorting by length. High
level applications such as electronics may only be possible
through such purifications.
Contributors:
Barry J. Bauer, William R. Blair, Charles M. Guttman,
Erik K. Hobbie
Processing Characterization Group
Polymers Division
Materials Science and Engineering Laboratory
