Can high-molar-mass cellulose molecules be extracted from phosphorylated pulp and TEMPO-oxidized pulps or their nanofibrils using LiCl/DMAc?

Abstract

Phosphorylated (P-) and TEMPO-oxidized (TO-) wood cellulose fibers (pulps), and P- and TO-cellulose nanofibrils (CNFs) were prepared. To extract pure cellulose molecules, all samples were dispersed in water and freeze-dried, and the P-/TO-pulp and P-/TO-CNF samples were stirred in 8 % (w/w) lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). The quantities of the extracted LiCl/DMAc-soluble fractions and molar-masses of the constituents were determined using size-exclusion chromatography. The structures of the LiCl/DMAc-insoluble fractions were analyzed using X-ray diffraction (XRD) and solid-state ¹³C-nuclear magnetic resonance (NMR). The mass ratios of the LiCl/DMAc-extracted fractions obtained from the P-/TO-pulp and P-/TO-CNF samples were in the low range of 2–9 %, consisted with the hypothesis that structurally, all the cellulose molecules in each cellulose microfibril of the P-/TO-pulp and P-/TO-CNF samples possess some phosphate ester groups or C6-carboxy groups. These cellulose molecules containing phosphate ester groups or C6-carboxy groups were insoluble in LiCl/DMAc. XRD and NMR analyses of the LiCl/DMAc-insoluble fractions of the TO-pulp samples showed that the cellulose I crystal structures inside the cellulose microfibrils were transformed into disordered structures by pretreatment with EDA. In contrast, the cellulose I crystal structure of the P-pulp was stable in EDA and was retained after post-treatment.