Thermal stabilization of TEMPO-oxidized cellulose

A partially C6-carboxylated cellulose with carboxylate content of 1.68 mmol/g was prepared by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation of a softwood bleached kraft pulp. Thermogravimetric analyses of the TEMPO-oxidized cellulose (TOC) and its related materials were studied to improve thermal stability of the TOC. Thermal decomposition (T_d) points of the TOC with sodium carboxylate groups, alkali-treated TOC with free carboxyl groups of 0.23 mmol/g and the original cellulose were 222 °C, 264 °C and 275 °C, respectively. Thus, the anhydroglucuronic acid units formed by TEMPO-mediated oxidation of the native wood cellulose and present in the TOC cause the decrease in T_d point by decarbonation during heating process. When carboxyl groups in the TOC were methylated with trimethylsilyl diazomethane (TMSCHN₂), the T_d point increased from 222 °C to 249 °C, and the peak temperature in its derivative thermogravimetric (DTG) curve increased from 273 °C to 313 °C, which was almost equal to that of the original cellulose. Thus, the methyl esterification of carboxyl groups in the TOC is effective in improving thermal stability. When sodium ions present in the TOC as counter ions of carboxylate groups were exchanged to some other metal ions, thermal stability was improved to some extent. Especially, when CaCl₂, Ca(OAc)₂, Ca(NO₃)₂ and CaI₂ solutions were used in the ion-exchange treatments, the peak temperatures in the DTG curves increased to approximately 300 °C. MgCl₂, NiCl₂, SrCl₂ and Sr(OAc)₂ solutions were also effective to some extent in increasing the peak temperatures of DTG curves. Thus, thermal stability of the fibrous TOC can be improved to some extent by methyl esterification of the sodium carboxylate groups present in the original TOC with TMSCHN₂ or ion-exchange treatments with some metal salt solutions.