Thermal expansion of anhydrous chitosan crystal

Chitosan is a renewable polysaccharide derived from chitin. However, its structure-property relationships remain poorly understood. Specifically, the thermal expansion of anhydrous chitosan crystals has not yet been fully elucidated. In this study, thermal expansion of highly crystalline anhydrous chitosan was investigated using X-ray diffraction and molecular dynamics (MD) simulations. X-ray diffraction measurements from 303 to 523 K revealed linear, anisotropic thermal expansion in the orthorhombic lattice, with coefficients of α_a = 1.79 × 10^-5, α_b = 7.16 × 10^-5, and α_c = 1.06 × 10^-5 K^-1, and a volumetric coefficient of β = 10.21 × 10^-5 K^-1. MD simulations reproduced the same order of anisotropy (α_b > α_a > α_c) and showed good quantitative agreement with the experimental volumetric expansion. Further analysis revealed that the observed anisotropy is closely associated with anisotropic phonon-like lattice vibrations: power spectral analysis of chain center-of-mass motions showed pronounced softening of the a-axis mode alongside disappearance of the b-axis mode with increasing temperature, even as the hydrogen-bonding network and overall molecular conformation remained essentially unchanged. These directional differences in vibrational dynamics, together with the directionally distinct nature of intermolecular hydrogen-bonding interactions along each crystallographic axis, are considered to underlie the observed thermal expansion anisotropy, which was thereby elucidated through the combined use of experimental and computational approaches.