Physico-Chemical nanomaterials science

Diels-Alder functionalization of multiwalled carbon nanotubes
Diyuk V.E.1, Vakaliuk A.V.1, Mischanchuk O.V.2, and Lisnyak V.V.1
1Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street 64/13, Kyiv 01601, Ukraine.
E-mail: vdiyuk@gmail.com

2O.O. Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kyiv 03164, Ukraine.

Multi-walled carbon nanotubes (MWCNTs) showed high electrical conductivity, biocompatibility, and unique mechanical properties all suggesting the intensive biomedical application. Adhesive and other surface interface parameters of MWCNTs could be improved by surface modification. So, in this work, we studied the ways to functionalize MWCNTs with O-, N-, and S-containing groups. Since MWCNTs have both separate and conjugated double C=C bonds, they can react through the Diels-Alder (D-A) reaction. This reaction is selective and runs between the diene and the unsaturated dienophile. For such purpose, the chemical properties of the active sites of MWCNTs were investigated in two ways, as diene and dienophile. Selected MWCNTs have a BET surface area of 115 m2/g. We used dienes (cyclopentadiene and 1,3-cyclohexadiene) and dienophiles (1,4-benzoquinone maleic anhydride, maleimide, 3-sulfolene, and fumaronitrile) to determine the content and the ratio of isolated and conjugated C=C bonds in terms of D-A reaction passed under barothermal conditions. This took place in an autoclave at a certain temperature in the range 100–250 °C. The resulting samples were examined by nitrogen adsorption, TGA, TPDMS, FTIR and Boehm titration. In fact, we introduced 2 mmol/g of the required functional group into the surface layer of MWCNTs. TGA proved two temperature forms of chemosorbed reagents that are formed after the modifying. The first form is sourced form Diels-Alder grafted residues and decomposed between 150 and 400 °C. Its highest concentration, up to 1 mmol/g, was reached for residues of 1,3-cyclohexadiene, maleimide, and 3-sulfolene. Thermal decomposition of the second form occurs above 350–400 °C. This form is the product of thermal destruction of chemosorbed spices and the partial pyrolysis of modifying agent. 
Under TPDMS control, the pyrolysis of the modified MWCNTs proved the formation of chemisorbed reagents. The products of their partial degradation and dimerization are also found. The D-A process was accompanied by tandem red-ox reactions caused the strong reducing properties of the MWCNTs surface. 
In sum, the Diels-Alder reaction is an effective method for the functionalization of the surface of carbon nanomaterials.