Single-walled carbon nanotubes and multiwalled carbon nanotubes functionalized with poly(L-lactic acid): A comparative study

Spectroscopic (Raman) and gravimetric (acid titration, TGA) techniques were compared to determine the functionalization degree of multiwalled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) with poly(L-lactic acid) (PLLA), following a two-step oxidation/esterification process. After oxidation with HNO₃, carbon nanotubes (CNTs) were activated with thionyl chloride to the corresponding acid chlorides and then grafted with PLLA. FTIR spectroscopy confirmed the formation of very similarly grafted CNT-PLLA materials for both SWNT and MWNT. But, according to chemical titration and TGA results, oxidized and esterified SWNT showed a significantly higher degree of functionalization than their MWNT counterparts. Contrary to these observations, the increase of the Raman I_D/I_G ratio was higher for MWNT-derived materials than for the SWNT counterparts. Therefore, it was concluded that this spectroscopic technique is unsuitable for the quantitative determination of the degree of functionalization when MWNT and SWNT are compared.