Morphological properties of nanofibrillated cellulose produced using wet grinding as an ultimate fibrillation process

Nanofibrillated cellulose (NFC) aqueous suspensions were produced from once-dried bisulfite softwood pulp using enzymatic or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation pretreatments, followed by wet grinding, as an ultimate fibrillation technique. Two commercial enzyme solutions: cellulase, with the major activity of endoglucanase and exoglucanase, and monocomponent endoglucanase, were compared to facilitate the nanofibrils isolation from cellulose fibers. The influence of their concentrations, as well as the other processing conditions, was analyzed. The morphology of the produced NFC was characterized using optical microscopy, atomic force microscopy (AFM), field emission gun-scanning electron microscopy (FEG-SEM), and morphological fiber analyzer (MorFi). Nanofibrils with a wide size distribution were produced. The average lateral dimensions of 12 ± 7 nm for the most disintegrated enzymatically hydrolyzed NFC and 4 ± 2 nm for TEMPO-oxidized NFC were determined from the AFM height images. The degree of polymerization (DP) decreased and the crystallinity index (CI) increased with an increase of the concentration of both enzyme solutions. TEMPO-oxidation did not have a significant impact on the cellulose CI; however, the DP was strongly affected. The monocomponent endoglucanase solution was found to have a better effect on the nanofibrils isolation rather than their depolymerization.