We produced lactate and acetate modified CNCs by a green
solvent-less one-pot concurrent acid hydrolysis/Fischer esterification
processes and compared them to CNCs produced by mineral
acids (HCl or H2SO4). Transmission electron microscopy showed
that the pristine crystal structure of modified CNCs was not
disturbed by using organic acids indicating that modifications
occur primarily at fiber surfaces. The presence of esters by Fischer
esterification of CNCs with organic acids was confirmed by FTIR
through the observation of characteristic ester vibrational
stretching bands with a peak at 1736 cm1. 13C NMR showed unique
peaks for both AA- and LA-CNCs when compared to HCl-CNCs. For
example, the methine carbon for LA-CNCs appears at 66 ppm.Weak
carbonyl signals were observed for AA-CNCs at 173 ppm and LACNCs
at 172 ppm and 174 ppm. By quantitative FTIR analysis, degrees
of substitution values for AA- and LA-CNCs are similar (0.12
and 0.13, respectively). Modification with lactic acid increased the
thermal stability of CNCs by more than 40 C under nitrogen flow
when compared with CNCs produced from H2SO4. Increasing the
thermal stability of CNCs is important because it allows the direct
melt blending of PLA with modified CNCs.