Solving the cellulose I polymorphic structural riddle: disorder in hydrogen bond networks activates diagnostic terahertz dynamics

Abstract

Cellulose is a common polymer found in natural sources, with the potential to be used in a wide variety of green and technologically relevant applications. Despite years of effort, the precise three-dimensional structures of two crystalline polymorphs of cellulose, Iα and Iβ, are currently still unknown due to the presence of disorder in the intermolecular hydrogen bond networks, hampering the in-depth understanding of the structure–property relationship of this crystalline material. Disorder in the hydrogen bond networks of cellulose I polymorphs was investigated using terahertz spectroscopy, powder X-ray diffraction, and solid-state density functional theory in order to reveal previously undiscovered atomic-level details about the crystal structures. We show that the nature of the (dis)order in the hydrogen-bonded layers has a direct effect on the terahertz vibrational spectra, providing contrast that allows differentiating between various structures spectroscopically. Comparison between theoretical and experimental data indicates that these two static networks coexist spatially throughout cellulose I polymorphs.