Property-Thickness Correlations of Transparent All-Nanocellulose Laminates

Abstract

Millimeter-thick laminated materials comprised solely of cellulose nanofibers (CNFs) combine optical transparency, high strength, and thermal anisotropy. These CNF laminates were fabricated by stacking thin CNF sheets and applying CNFs dispersed in a water/ethanol mixture as binder, followed by hot pressing. The number of stacked CNF sheets can control the laminate thickness within a wide range of dimensions. Here, we report the correlation between the material properties and laminate thickness of this all-CNF plate material. CNF laminates with different thicknesses (~30-500 μm) were fabricated using 1-15 CNF sheets. The crystallinity of the CNFs in the laminates was greatly enhanced by increasing the number of stacked sheets. This trend in crystallinity is explained by the longer period of time required for drying thicker laminates in the hot press process. The visible-light transmittance of the laminates was well described by the Beer–Lambert law; scattering and/or reflection at the binding interface between sheets were estimated to be negligibly small. The elastic modulus of the laminates increased with increasing number of stacked sheets and reached a maximum of 36 GPa. This increasing trend was explained by the enhanced crystallinity of the CNFs in the laminates. The tensile strengths of the laminates varied greatly, and no clear trend as a function of the number of stacked sheets was found. The thermal conductivity increased with increasing number of stacked sheets, which was attributed to the enhanced crystallinity.