Nanocrystalline cellulose membranes functionalized with imidazole and doped with trifluoroacetic acid: Thermal properties, proton conductivity, and studies of molecular dynamics

The performance of trifluoroacetic acid (TFA) as a dopant in membranes (CNC-Im) made of cellulose nanocrystals (CNCs) and imidazole (Im) was studied. Two composites with the same concentration of imidazole but different amounts of acid were synthesized. Their thermal properties, conductivity, and molecular dynamics were investigated and compared with those of the analog CNC-Im without TFA. The obtained results confirmed the positive effect of TFA on the process of thermal degradation: the maximum imidazole evaporation rate in CNC-Im TFA was shifted by 20 degrees towards a higher temperature (240 °C). TFA was found to have a neutral effect on the dynamics of the protons and the reorientation of imidazole as follows from solid-state NMR. The effect of TFA on the conductivity is negative: the composites exhibit a proton conductivity of about half an order of magnitude lower than that of the corresponding acid-free composite.