Preparation and characterization of TEMPO-oxidized cellulose nanofibril films with free carboxyl groups

A 2,2,6,6-tetramehylpiperidine-1-oxy radical (TEMPO)-oxidized wood cellulose with sodium carboxylate groups was completely converted to individual TEMPO-oxidized cellulose nanofibrils with free carboxyl groups (TOCN-COOH) dispersed in water at pH 4.6. Self-standing films prepared by casting and drying of the TOCN-COOH/water dispersion were flexible and highly transparent. Fourier transform infrared (FT-IR) spectra and sodium content determination confirmed that the sodium carboxylate groups of TOCN-COONa were almost completely converted to free carboxyl groups in TOCN-COOH. The TOCN-COOH and TOCN-COONa films had similar densities and tensile strengths, but the former had lower moisture content, higher Young's modulus and lower elongation values than those of the latter. The oxygen permeability of the TOCN-COOH films was 0.049 mL μm m−2 day−1 kPa−1, which was clearly lower than that (15.5 mL μm m−2 day−1 kPa−1) of poly(ethylene terephthalate) films.