Simultaneous strengthening and toughening lignin/cellulose nanofibril composite films: Effects from flexible hydrogen bonds

Simultaneous strengthening and toughening lignin/cellulose nanofibril composite films were prepared using phenolated lignins with improved phenolic hydroxyls and uniform nanoparticle morphology, revealing the hydrogen bonding effects are vital for enhancing the mechanical strength of composite films. • Regulated LNPs with uniform morphology could be obtained by phenolated lignins. • CNF composite film could be strengthened and toughened by adding regulated LNPs. • Phenolated lignin/cellulose nanofibril exhibits stronger hydrogen bonding effect. • The enhancement effect is proportional to the content of phenolic hydroxyl in lignin. • Phenolated lignin/polyvinyl alcohol exhibits the same strengthening effect. Inspired by the lignin-cellulose network in natural wood, we developed a strategy to produce cellulose nanofibril (CNF) composite films using lignin nanoparticles (LNPs) with varied contents of phenolic hydroxyl (Ph-OH) groups as a reinforcing agent. Regulated LNPs with lower molecular weight, higher Ph-OH content, and uniform morphology were successfully prepared by phenolation and anti-solvent precipitation. CNFs mixed with phenolated LNPs of high Ph-OH content (8.23 mmol/g) exhibited a higher tensile strength (∼190 MPa), increased toughness (∼15 MJ/m 3 ), and enhanced UV-blocking ability (∼99%) compared to lignin-free CNFs and CNFs composites with lower Ph-OH content. Thus, hydrogen bonds between Ph-OH groups on lignin and hydroxyl groups on CNFs are vital for enhancing the mechanical strength of composite films. Similar enhanced mechanical properties were found by adding phenolated LNPs into polyvinyl alcohol as a fortifier. This study provides novel insights into producing lignin-reinforced cellulose composite films with UV shielding, thermal stability, and biodegradability.