Synthesis of water-soluble multiwalled carbon nanotubes with grafted temperature-responsive shells by surface RAFT polymerization

Water-soluble multiwalled carbon nanotubes (MWNTs) with temperature-responsive shells were synthesized by grafting poly(N-isopropylacrylamide) (PNIPAAM) from sidewall of MWNT via surface reversible addition and fragmentation chain transfer (RAFT) polymerization using RAFT agent functionalized MWNT as chain transfer agent. Carboxylic groups on MWNT were formed by nitric acid oxidation. Then bromoisobutyrate groups were covalently attached to the MWNT by esterification of 2-hydroxyethyl-2'-bromoisobutyrate with carboxylic groups, forming bromoisobutyrate functionalized MWNT (MWNT-Br). RAFT agent functionalized MWNT was produced by substitute reaction of MWNT-Br with PhC(S)SMgBr. H-1 NMR, FT-IR, and thermogravimetric analyses (TGA) results showed that PNIPAAM chains grew from MWNT by surface reversible addition fragmentation chain transfer polymerization. The molecular weight of PNIPAAM on MWNT increased linearly with monomer conversion, and its PDI is narrow (around 1.3). TGA showed that the amount of PNIPAAM grown from the MWNT increased with the increase of polymerization time. The MWNT-g-PNIPAAM has good solubility in water, chloroform, and tetrahydrofuran. TEM images also showed the MWNT-g-PNIPAAM was dispersed individually, indicating that the bundles of original MWNT were separated into individual tubes by surface RAFT polymerization. The produced MWNT-g-PNIPAAM has a PNIPAAM shell, which is very sensitive to change of temperature. This method can also be employed to graft other functional polymer chains onto MWNTs.