Multihydroxy Polymer-Functionalized Carbon Nanotubes:  Synthesis, Derivatization, and Metal Loading

Multiwalled carbon nanotubes (MWNTs) were oxidized with concentrated HNO3 and H2SO4 to introduce carboxylic groups onto MWNT surfaces. The oxidized MWNTs were reacted subsequently with thionyl chloride, glycol, and 2-bromo-2-methylpropionyl bromide, producing MWNT-based macroinitiators, MWNT-Br, for the atom transfer radical polymerization (ATRP) of glycerol monomethacrylate (GMA). In the presence of N,N,N‘,N‘ ‘,N‘ ‘-pentmethyldiethylenetriamine (PMDETA)/Cu(I)Br, hydroxyl-functional polyGMA, with weight concentration of ca. 50−90%, was successfully grown from the MWNT surface. Comparative experiments, with MWNT-Br and ethyl 2-bromoisobutyrate as co-initiators, showed that (1) the presence of free initiator has no significant effect on the nanotube-surface initiating polymerizations, (2) both the polymer content grafted on the nanotubes (46−88 wt %) and the number-average molecular weight (Mn) of free polymer (3000−17 000) can be efficiently controlled by adjusting the feed ratio of monomer to co-initiators, and (3) the polydispersity index (PDI) of the free polymer increased when either the feed ratio or the molecular weight was increased. Further derivatization of the grafted polyGMA with succinic anhydride converted the hydroxyl groups into carboxylic acid groups. The resulting polymer-functionalized MWNTs were characterized with FTIR, 1H NMR, 13C NMR, TGA, SEM, and TEM. Furthermore, the poly(carboxylic acid)-modified MWNTs were used to sequester metal ions such as Ag+, Co2+, Ni2+, Au3+, La3+, and Y3+, forming MWNT-polymer/metal hybrid nanocomposites, nanowires, or necklace-like nanostructures, depending on the grafted polymer content and the nature of the captured metal. SEM and TEM studies combined with X-ray analyses confirmed the structure and elements of the novel hybrid nanoobjects. Interestingly, polymer enwrapped-silver nanobeads or nanodots with a diameter of ca. 3−10 nm decorating the convex surface of MWNTs were obtained. The synthesis, derivatization, and metal loading of functionalized MWNTs suggests a new route for rational molecular design and augurs well for future applications of functionalized nanomaterials, including device fabrication.