Humidity-responsive thermal conduction properties of bacterial cellulose films

High hygroscopicity is an unavoidable feature of cellulose materials, and it is important for clarifying the humidity dependence of the target function toward precise utilization. Recently, cellulose nanofiber films have been found to have higher thermal conductivity than plastics and glass, and they are expected to be used as flexible thermal conductive films. However, their humidity dependence has not been elucidated. In this study, we investigated the humidity dependence on the thermal conduction properties of bacterial cellulose films by using a custom-made sealing chamber compatible with the laser spot periodic heating radiation thermometry method for measuring thermal diffusivity. Bacterial cellulose films conditioned for long periods of time in constant relative humidity ranging from 11 to 93% regulated by sorbet-like saturated salt solutions showed decreasing in-plane thermal diffusivity with increasing humidity. The moisture content determined by thermogravimetric analysis showed a linear negative correlation with the thermal diffusivity. The decrease in the thermal diffusivity with increasing moisture content was larger than the decrease predicted by the simple law of mixtures, which could be because of an increase in thermal resistance of the interface due to the adsorption of water not only on the open surface of the nanofibers but also on the interface between the nanofibers.