Solving the Cellulose I Polymorphic Structural Riddle: Disorder in Hydrogen Bond Networks Activates Diagnostic Terahertz Dynamics

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 3D structure of two crystalline polymorphs of cellulose, I_alpha and I_beta, are currently still unknown due to the presence of disorder in the intermolecular hydrogen bond networks, hampering an in-depth understanding of the structure-property relation of this crystalline material. Disorder in the hydrogen bond networks of cellulose I polymorphs was investigated with terahertz spectroscopy, powder X-ray diffraction, and solid-state density functional theory in order to reveal previously unresolved atomic-level detail about the crystal structures. We show that the nature of the (dis)order of the hydrogen-bonded layers of has a direct effect on the terahertz vibrational spectra, providing contrast that enables the ability to differentiate between various structures spectroscopically. Comparison between theoretical and experimental data indicates that these two static networks coexist spatially throughout cellulose I polymorphs.