Insight into thermal stability of cellulose nanocrystals from new hydrolysis methods with acid blends

This study provides insight into the thermal degradation of cotton cellulose nanocrystals (CNCs) by tuning their physico-chemical properties through acid hydrolysis using blends of phosphoric and sulfuric acid. CNCs isolated by sulfuric acid hydrolysis are known to degrade at lower temperatures than CNCs hydrolyzed with phosphoric acid; however, the reason for this change is unclear. Although all CNCs are inherently relatively thermally stable, their application in polymer composites and liquid formulations designed to function at high temperatures could be extended if thermal stability was improved. Herein, thermogravimetric analysis was carried out on six types of CNCs (in both acid and sodium form) with different surface chemistry, surface charge density, dimensions, crystallinity and degree of polymerization (DP) to identify the key properties that influence thermal stability of nanocellulose. In acid form, CNC surface charge density was found to be the determining factor in thermal stability due to de-esterification and acid-catalyzed degradation. Conversely, in sodium form, surface chemistry and charge density had a negligible effect on the onset of thermal degradation, however, the DP of the cellulose polymer chains highly influenced stability. The presence of more reducing ends in lower DP nanocrystals is inferred to facilitate thermally-induced depolymerization and degradation. Degree of crystallinity did not significantly affect CNC thermal stability. Studying CNCs produced from single or blends of acids (and changing the counterion) elucidated the thermal behavior of cellulose and furthermore demonstrated new routes to tailor CNCs thermal and colloidal stability.