Cellulose nanofibers vs. cellulose nanocrystals: a comparative study on their influence on the properties of imidazole-doped proton-conducting nanocomposites

Abstract

Nanocellulose-based nanocomposites exhibit properties dependent on nanocellulose morphology, affecting thermal, electrical, and molecular characteristics. The study compares imidazole-doped nanocomposites from cellulose nanofibers (CNFs) and nanocrystals (CNCs). Scanning electron microscopy shows CNFs form entangled networks, while CNCs form compact grains, influencing the imidazole’s interaction within the matrix. CNC-based composites have higher electrical conductivity (0.326 S/m at 150 °C) due to weaker imidazole hydrogen bonds and compact structure, facilitating proton transport. In contrast, CNF-based composites, with a disordered structure and stronger hydrogen bonds between imidazole and polymer matrix, exhibit lower conductivity (0.021 S/m at 140 °C) but enhanced thermal stability, degrading above 220 °C. Solid-state nuclear magnetic resonance (NMR) spectroscopy revealed two imidazole species: slowly and rapidly reorienting and exchanging protons. The activation energy for proton exchange and reorientation of imidazole is lower in the CNC-based composite (0.39 eV) than CNF-based (0.44 eV), indicating weaker hydrogen bonds. The CNF matrix is inhomogeneous, and the imidazole molecules bond in different environments. The wider activation energy distribution in CNF composites supports this conclusion. Heteronuclear correlation (1H-15N HETCOR NMR) spectroscopy identified imidazole bonding to cellulose OH groups and residual water. Proton transport involves imidazole reorientation and exchange via cellulose OH groups and water, contributing to conductivity.