TEMPO-Mediated Oxidation of Native Cellulose. The Effect of Oxidation Conditions on Chemical and Crystal Structures of the Water-Insoluble Fractions

TLDR

Cellulose cotton linter was oxidized with NaClO, catalytic NaBr, and TEMPO under varied conditions, and the resulting water‑insoluble fractions were isolated and characterized for functional groups, crystallinity, polymerization, morphology, and water retention. The TEMPO oxidation introduced up to 0.7 mmol g⁻¹ carboxylate and 0.3 mmol g⁻¹ aldehyde groups, retained >80 % of the water‑insoluble fraction, left cellulose I crystallinity and crystal size essentially unchanged, and increased water retention from 60 % to ~280 % while converting fibrous morphology to short fragments.

Abstract

Cellulose cotton linter was oxidized with sodium hypochlorite with catalytic amounts of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) under various conditions. After this TEMPO-mediated oxidation, water-insoluble fractions were collected and characterized in terms of carboxylate and aldehyde contents, crystallinities and crystal sizes, degrees of polymerization, morphology, and water retention values. Carboxylate and aldehyde groups were introduced into the water-insoluble fractions up to about 0.7 and 0.3 mmol/g, respectively, by the oxidation, where recovery of the water-insoluble fractions were generally higher than 80%. Crystallinities and crystal sizes of cellulose I were nearly unchanged during the oxidation, and thus, carboxylate and aldehyde groups were introduced selectively on crystal surfaces and in disordered regions of the water-insoluble fractions. Water retention values of cotton linter can be increased from 60% to about 280% through the introduction of hydrophilic carboxylate groups and morphological changes from fibrous forms to short fragments by the TEMPO-mediated oxidation.