Mechanism of TEMPO-oxidized cellulose nanofibril film reinforcement with poly(acrylamide)

Abstract

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) and poly(acrylamide) (PAM) with sodium carboxylate groups (TOCN-COONa and PAM-COONa, respectively) were converted to TOCN and PAM with protonated carboxyl groups (TOCN-COOH and PAM-COOH, respectively). Transparent and flexible PAM-COOH/TOCN-COOH, PAM-COONa/TOCN-COOH, and PAM-COOH/TOCN-COONa composite films were prepared by mixing aqueous PAM-COOH or PAM-COONa solutions and aqueous TOCN-COOH or TOCN-COONa dispersions with various PAM/TOCN weight ratios, and successive casting/drying of the mixtures. In all cases, the Young’s modulus and tensile strength of the composite film were highest when the PAM content of the composite film was in the range 10–25 %. PAM therefore has the potential to reinforce TOCN films, irrespective of the sodium carboxylate groups or protonated carboxyl groups of TOCN or PAM. Although the FT-IR spectra of the PAM-COOH/TOCN-COOH composite films with various PAM/TOCN weight ratios indicated the formation of hydrogen bonds between PAM-COOH and TOCN-COOH, the formation of these hydrogen bonds could not explain the reinforcing effect achieved by PAM addition to TOCN-COOH. Because the TOCN elements form nematic-ordered or self-aligned domain structures in aqueous dispersions, PAM molecules at the corresponding PAM contents are probably present around the boundary regions of TOCN domains, and fill or cover the defects present between the TOCN domains.