Large Anisotropy of Thermal Conductivity in Oriented Cellulose–Clay Composites

This study characterized the anisotropic thermal conductivity of clay/cellulose nanocomposites, an eco-friendly functional flame-retardant material exhibiting excellent mechanical properties, gas barrier properties, and biodegradability. Thermal conductivity anisotropy is important for flame-retardant materials. Low thermal conductivity in the through-thickness direction serves as a thermal barrier, whereas high thermal conductivity in the in-plane direction prevents local heat accumulation. We prepared a series of membranes of nanocomposites of montmorillonite clay platelets and cellulose nanofibrils via vacuum filtration/drying and measured their directional thermal conductivities as a function of the montmorillonite clay/cellulose nanofibril content. The results indicate that the through-thickness and in-plane thermal conductivities depend nonmonotonically on the clay content. The highest in-plane thermal conductivity reached 7.5 W m^-1 K^-1, exhibiting a maximum anisotropy of 30 for a clay content of 50%. Structural investigation via Raman spectroscopy revealed an enhanced planar alignment of the cellulose nanofibrils and indicated alignment of the clay platelets. The correlation between the degree of alignment and thermal conductivity anisotropy suggests that alignment increases the contact area between the cellulose nanofibrils and clay platelets, which enhances in-plane heat conduction by increasing the phonon transport path and impedes through-thickness heat conduction by enhancing phonon boundary scattering.