Quantifying the Contribution of the Dispersion Interaction and Hydrogen Bonding to the Anisotropic Elastic Properties of Chitin and Chitosan

The
elastic tensors of chitin and chitosan allomorphs were calculated
using density functional theory (DFT) with and without the dispersion
correction and compared with experimental values. The longitudinal
Young’s moduli were 114.9 or 126.9 GPa for α-chitin depending
on the hydrogen bond pattern: 129.0 GPa for β-chitin and 191.5
GPa for chitosan. Furthermore, the moduli were found to vary between
17.0 and 52.8 GPa in the transverse directions and between 2.2 and
15.2 GPa in shear. Switching off the dispersion correction led to
a decrease in modulus by up to 63%, depending on the direction. The
transverse Young’s moduli of α-chitin strongly depended
on the hydroxylmethyl group conformation coupled with the dispersion
correction, suggesting a synergy between hydrogen bonding and dispersion
interactions. The calculated longitudinal Young’s moduli were,
in general, higher than experimental values obtained in static conditions,
and the Poisson’s ratios were lower than experimental values
obtained in static conditions.