Discovering principles to tune the heat-transport properties of cellulose nanofibril (CNF) films will open the door for the development of biomass-derived heat-transfer materials and break away from existing petroleum-based polymer composites. In this study, we added various multivalent metal ions to CNF films with surface carboxy groups formed by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation and measured their thermal diffusivities in the dry state by an original method to verify the tunability of the thermal diffusivity. We found that the in-plane thermal diffusivity of the film is inversely proportional to the ionic radius and directly proportional to the Pauling electro-negativity. The CNF film with proton-neutralized carboxyl groups showed the highest level of thermal diffusivity among the films with various metal ions. Molecular dynamics simulations clarified that the spatial distribution of the introduced ions is determined by the closest distance between the cation and carboxylate oxygen atom of the TEMPO-oxidized CNF surface.
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