[link] shows a typical TGA for a functionalized SWNT. In this case it is polyethyleneimine (PEI) functionalized SWNTs prepared by the reaction of fluorinated SWNTs (F-SWNTs) with PEI in the presence of a base catalyst.
The TGA of SWNTs functionalized with polyethyleimine (PEI) under air showing the sequential loss of complexed CO
2 and decomposition of PEI.
In the present case the molecular weight of the PEI is 600 g/mol. When the sample is heated, the PEI thermally decomposes leaving behind the unfunctionalized SWNTs. The initial mass loss below 100 °C is due to residual water and ethanol used to wash the sample.
In the following example the total mass of the sample is 25 mg.
The initial mass, M
i = 25 mg = mass of the SWNTs, residues and the PEI.
After the initial moisture has evaporated there is 68% of the sample left. 68% of 25 mg is 17 mg. This is the mass of the PEI and the SWNTs.
At 300 °C the PEI starts to decompose and all of the PEI has been removed from the SWNTs at 370 °C. The mass loss during this time is 53% of the total mass of the sample. 53% of 25 mg is 13.25 mg.
The molecular weight of this PEI is 600 g/mol. Therefore there is 0.013 g / 600 g/mol = 0.022 mmole of PEI in the sample.
15% of the sample is the residual mass, this is the mass of the decomposed SWNTs. 15% of 25 mg is 3.75 mg. The molecular weight of carbon is 12 g/mol. So there is 0.3125 mmole of carbon in the sample.
There is 93.4 mol% of carbon and 6.5 mol% of PEI in the sample.
Determination of the mass of a chemical absorbed by functionalized swnts
Solid-state
13 C NMR of PEI-SWNTs shows the presence of carboxylate substituents that can be attributed to carbamate formation as a consequence of the reversable CO
2 absorption to the primary amine substituents of the PEI. Desorption of CO
2 is accomplished by heating under argon at 75 °C.
The quantity of CO
2 absorbed per PEI-SWNT unit may be determined by initially exposing the PEI-SWNT to a CO
2 atmosphere to maximize absorption. The gas flow is switched to either Ar or N
2 and the sample heated to liberate the absorbed CO
2 without decomposing the PEI or the SWNTs. An example of the appropriate TGA plot is shown in
[link] .
The TGA results of PEI(10000)-SWNT absorbing and desorbing CO
2 . The mass has been normalized to the lowest mass recorded, which is equivalent to PEI(10000)-SWNT.
The sample was heated to 75 °C under Ar, and an initial mass loss due to moisture and/or atmospherically absorbed CO
2 is seen. In the temperature range of 25 °C to 75 °C the flow gas was switched from an inert gas to CO
2 . In this region an increase in mass is seen, the increase is due to CO
2 absorption by the PEI (10000Da)-SWNT. Switching the carrier gas back to Ar resulted in the desorption of the CO
2 .
The total normalized mass of CO
2 absorbed by the PEI(10000)-SWNT can be calculated as follows;
Solution outline
Minimum mass = mass of absorbant = M
absorbant
Maximum mass = mass of absorbant and absorbed species = M
total
Absorbed mass = M
absorbed = M
total - M
absorbant
% of absorbed species= (M
absorbed /M
absorbant )*100
1 mole of absorbed species = MW of absorbed species
Number of moles of absorbed species = (M
absorbed /MW of absorbed species)
The number of moles of absorbed species absorbed per gram of absorbant= (1g/M
total )*(Number of moles of absorbed species)
Solution
M
absorbant = Mass of PEI-SWNT = 4.829 mg
M
total = Mass of PEI-SWNT and CO
2 = 5.258 mg
M
absorbed = M
total - M
absorbant = 5.258 mg - 4.829 mg = 0.429 mg
% of absorbed species= % of CO
2 absorbed = (M
absorbed /M
absorbant )*100 = (0.429/4.829)*100 = 8.8%
1 mole of absorbed species = MW of absorbed species = MW of CO
2 = 44 therefore 1 mole = 44g
Number of moles of absorbed species = (M
absorbed /MW of absorbed species)= (0.429 mg / 44 g) = 9.75 μM
The number of moles of absorbed species absorbed per gram of absorbant =(1 g/M
total )*(Number of moles of absorbed species) = (1 g/5.258 mg)*(9.75)= 1.85 mmol of CO
2 absorbed per gram of absorbant
Bibliography
I. W. Chiang, B. E. Brinson, A. Y. Huang, P. A. Willis, M. J. Bronikowski, J. L. Margrave, R. E. Smalley, and R. H. Hauge,
J. Phys. Chem. B , 2001,
105 , 8297.
E. P. Dillon, C. A. Crouse and A. R. Barron,
ACS Nano , 2008,
2 , 156.
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