Surface carboxylation of cellulose nanowhiskers using mPEG-TEMPO: its recovery and recycling

The surface carboxylation of cellulose nanowhiskers (CNWs) prepared by hydrochloric acid hydrolysis was performed via the oxidation of the primary hydroxyl groups on the surface using mPEG2000-TEMPO, a 2,2,6,6-tetramethyl-1-piperidinyloxy radical moiety grafted to monomethoxy poly(ethylene glycol) 2000, to yield carboxylated CNWs (CCNWs). After the oxidation step, the used mPEG2000-TEMPO was recovered by extraction with dichloromethane (DCM) and reused for further oxidation cycles. mPEG2000-TEMPO could be used for the surface oxidation of CNWs for at least three repeated cycles, although the degree of oxidation in the second and third cycles was reduced to 60% of that in the first cycle. The recovery of mPEG2000-TEMPO after every cycle was not complete (similar to 80%), thus suggesting an irreversible adsorption of mPEG2000-TEMPO on the surface of the CCNWs. Soxhlet extraction of the obtained CCNWs with DCM, however, did not achieve a recovery of mPEG2000-TEMPO.