Thermal and electrical properties of nanocellulose films with different interfibrillar structures of alkyl ammonium carboxylates

Transparent films of cellulose nanofibrils (CNFs) with high moduli and low thermal expansion coefficients have attracted significant attention for use as packaging films or as substrates for flexible electronics. In the present study, the thermal and electrical properties of 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized CNF (T-CNF) films with quaternary alkyl ammonium (QAs) carboxylates were characterized. As the alkyl chain length of the QAs was increased, the thermal decomposition temperature and thermal conductivity of the T-CNF films decreased. The surface resistivities of T-CNF films with tetrapropylammonium and tetrabutylammonium carboxylates were increased up to 1.2 × 10⁹ and 1.5 × 10¹⁰ Ω sq⁻¹, respectively. These results indicate that the mobility of the QAs was increased with an increase in their alkyl chain length. The distances between the T-CNFs inside the films, evaluated using small-angle X-ray scattering, increased as the alkyl chain length of the QAs was increased. Solid-state ¹³C cross-polarization/magic-angle spinning nuclear magnetic resonance analyses of the T-CNF films showed that the strength of the ionic bonds between the carboxylates and the QAs decreased with increasing QA alkyl chain lengths. The relaxation times of the carbons of the methyl and methylene groups of the QAs increased as the alkyl chain length of the QAs was increased. This study shows that the thermal, electrical, and structural properties of the T-CNF films can be controlled by the interfibrillar structures of the T-CNFs using a carboxylate ion-exchange process.