Hemicellulose content affects the properties of cellulose nanofibrils produced from softwood pulp fibres by LPMO

Abstract

Lytic polysaccharide monooxygenase (LPMO)-catalysed oxidation of cellulose has emerged as a green alternative to chemical modifications in the production of cellulose nanofibrils (CNFs) from wood pulp fibres. The effect of the hemicellulose content of the starting pulp fibres on the oxidation capabilities of cellulose-active LPMO is important and has not been investigated previously. In this study, the production of LPMO-oxidised CNFs was evaluated on two commercial softwood pulp fibres with different hemicellulose contents. Thin and colloidally stable CNFs were readily obtained from kraft pulp with a hemicellulose content of 16%. The preserved hemicellulose fraction in the kraft pulp enhanced the access of LPMO into the fibre cell wall, enabling the production of homogeneous CNFs with a thin width of 3.7 ± 1.7 nm. By contrast, the LPMO-oxidised dissolving pulp with a lower hemicellulose content of 4% could only be partially disintegrated into thin CNFs, leaving a large amount of cellulose microfibril aggregates with widths of around 50 to 100 nm. CNFs disintegrated from the LPMO-oxidised kraft pulp could be processed into nanopapers with excellent properties including an optical transmittance of 86%, tensile strength of 260 MPa, and Young's modulus of 16.9 GPa. Such CNFs also showed acid-triggered nanofibril gelation owing to the introduced carboxyl groups on cellulose microfibril surfaces. These results indicate that the inherent hemicelluloses present in the wood cell wall are essential for LPMO-mediated CNF production from wood pulp fibres.