Coupling Nanofibril Lateral Size and Residual Lignin to Tailor the Properties of Lignocellulose Films

Lignocellulosic nanofibrils (LCNF) are produced from a single source of unbleached, oxidized wood fibers by serial disintegration, high-pressure microfluidization, and homogenization. Sequential centrifugation enables fractionation by fibril width (≈5, ≈9, and ≈18 nm). LCNF residual lignin of high molecular mass reports together with the finest fraction (LCNF-fine), whereas the more strongly cellulose-bound lignin, of relatively lower molecular mass, associates with the coarsest fraction (LCNF-coarse). Hot pressing softens the amorphous lignin, which fills the interstices between fibrils and acts as an in-built interfacial cross-linker. Thus, going from the LCNF-fine to the LCNF-course films, it is possible to obtain a range of values for the structural consolidation (density from 0.9 to 1.2 g cm⁻³ and porosity from 19% to 40%), surface roughness (RMS from ≈6 to 13 nm), and strength (elastic modulus from 8 to ≈12 GPa). The concentration of free hydroxyl groups controls effectively the direct surface interactions with liquids. The apparent surface energy dispersive component tracks with the total surface free energy and appears to be strongly influenced by the higher porosity as the fibril lateral size increases. The results demonstrate the possibility to tailor nanofibril cross-linking and associated optical and thermo-mechanical performance of LCNF films.