Transparent Thermally Conductive CNC / PVA Composites Prepared via Cation‐Promoted Network Formation and Matched Refractive Index

Introducing inorganic thermal fillers into organic polymers is the major solution to increase thermal conductivity (TC), but typically at the expense of optical transparency. Fabrication of transparent thermally conductive polymers remains a great challenge today. In this work, negatively charged and highly crystalline cellulose nanocrystals (CNCs) are introduced into polyvinyl alcohol (PVA). Owing to their closely matched refractive indices, the prepared CNC/PVA composites exhibit enhanced TC and reduced loss of optical transparency. The CNC/PVA composite with 75 wt% CNC loading shows cross‐plane TC of 0.33 W m −1 K −1 (1.74 times that of pristine PVA) while retaining 77% of the original transparency of PVA at 600 nm. Addition of cetyltrimethylammonium bromide (CTAB) further improves the thermal and optical properties of CNC/PVA composites. With 0.5 wt% CTAB, the composite has a significantly enhanced cross‐plane TC of 0.92 W m −1 K −1 (4.84 times that of pristine PVA), while preserving 96% of original PVA transparency at 600 nm. Comparative studies using neutral and anionic surfactants suggest that the improvement in TC by CTAB is probably due to the promoted CNC network formation by cations.