Comparative characterization of TEMPO-oxidized cellulose nanofibril films prepared from non-wood resources

Three non-wood celluloses, hemp bast holocellulose, and commercial bamboo and bagasse bleached kraft pulps, were oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation in water at pH 10. The water-insoluble TEMPO-oxidized celluloses thus obtained were converted to aqueous dispersions of TEMPO-oxidized cellulose nanofibrils (TOCNs) and then to self-standing TOCN films. Weight recovery ratios of the TEMPO-oxidized celluloses decreased to 70-80% and their carboxylate contents reached 1.5-1.7 mmol g−1 by the TEMPO-mediated oxidation. The viscosity-average degrees of polymerization remarkably decreased from 800-1100 to 200-480 by partial depolymerization occurring during the oxidation, depending on the non-wood celluloses used as the starting materials. The average lengths and widths of the TOCNs were estimated to be 500-650 nm and 2.4-2.9 nm, respectively, from their atomic force microscopy images. The self-standing TOCN films had high light-transparencies (>87% at 600 nm), high tensile strengths (140-230 MPa), high Young's moduli (7-11 MPa), low coefficients of thermal expansion (4-6 ppm K−1) in spite of low densities of 1.4-1.7 g cm−3. In particular, the TOCN films prepared from the hemp bast holocellulose had clearly high works of fracture (~30 MJ m−3), whereas those prepared from other two non-wood celluloses had 2-8 MJ m−3.