Size and solvent effects on cellulose transverse anisotropy and toughening design

Cellulose nanocrystals (CNCs) are a promising class of materials derived from the most abundant natural polymer resource on Earth. Hydroxyl-induced polarity is a crucial advantage of CNCs, making them promising for advanced design and applications. Side chain hydroxyls, hydrogen bonds, and particular crystal structures of CNCs lead to unique anisotropy. However, the nuanced anisotropy in the transverse section is not sufficiently stressed and cannot be precisely described experimentally. Although partially covered by previous studies, a quantitative explanation of size dependency and a systematic comparison of solvent influences are still lacking. The manufacturing of cellulose materials requires a better understanding of anisotropy, size dependency, and solvent influences. In this study, the anisotropic performance of CNCs in specific directions and a diverse array of solvent environments was carefully inspected and compared using molecular simulations. Furthermore, a data-supported explanation for the size dependency and transverse arrangement toughness-enhanced designs were both proposed. These systematic comparisons and unique transverse arrangements could aid future applications of cellulose.

This study employed all-atom molecular dynamics (MD) simulations using the GROMACS software alongside the CHARMM36 force field, with force field files generated via CHARMM-GUI tools. To overcome the challenge of applying shear loads in MD, the models corresponding to the characteristic directions were constructed and subjected to cell deformation at a constant velocity. All simulations were conducted under the NPT ensemble at 0.1 MPa. The reliability of the observed anisotropy and fracture behaviors was confirmed via replica simulations at stretching velocities ranging from 0.1 to 40.0 nm/ns. Finally, explicit solvent models were utilized to evaluate solvent exposure effects.