Effects of Surface Chemistry and Counterion Selection on the Thermal Behavior of Carboxylated Cellulose Nanocrystals

Researchers have sought to improve the thermal performance of cellulose nanocrystals (CNCs) via new production routes, often by incorporating carboxylate groups on CNC surfaces. The CNC properties responsible for increased thermal stability, however, are not well understood. This study investigated carboxylated CNCs with varying physicochemical properties and benchmarked their thermal performance with two counterions (H+ and Na+) against typical sulfated CNCs. Carboxylated CNCs were more thermally stable in acid form than sodium form (the opposite of sulfated CNCs), highlighting that CNCs with different surface chemistries cannot be compared with the same counterion when making claims about thermal behavior. Thermogravimetric analysis and solid-state NMR spectroscopy were used to evaluate five types of CNCs. Overall, sulfate group content affected CNC thermal stability more than carboxylate contentsulfated CNCs showed both the highest and lowest onsets of thermal degradation in sodium and acid form, respectively. Carboxylated CNCs displayed a variety of “intermediate” thermal behaviors: CNCs with higher carboxylate contents and larger specific surface areas had lower degradation temperatures and tended toward one main pyrolysis step (instead of two) as surface area increased. The catalytic effect of sodium in highly charged 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized carboxylated CNCs, and the formation of sodium carbonate in moderately charged, esterified CNCs, led to more structural degradation of cellulose in sodium form than acid form. Due to the generated sodium carbonate, decarboxylation reactions upon heating decreased for esterified CNCs in sodium form but increased in acid form. This new understanding will allow optimized performance of CNC-based materials at high temperatures and may enable the development of bionanomaterials with targeted thermal properties.