Molecular dynamics study on stiffness and ductility in chitin-protein composite

Chitin-protein composite is the structural material of many marine animals including lobster, squid, and sponge. The relationship between mechanical performance and hierarchical nanostructure in those composites attracts extensive research interests. In order to study the molecular mechanism behind, we construct atomistic models of chitin-protein composite and conduct computational tensile tests through molecular dynamics simulations. The effects of water content and chitin fiber length on the stiffness are examined. The result reveals the detrimental effect on the stiffness of chitin-protein composite due to the presence of water molecules. Meanwhile, it is found that the chitin-protein composite becomes stiffer as the embedded chitin fiber is longer. As the tensile deformation proceeds, the stress-strain curve features a saw-tooth appearance, which can be explained by the interlocked zigzag nanostructure between adjacent chitin fibers. These interlocked sites can sacrificially break for energy dissipation when the system undergoes large deformation, leading to an improvement of ductility.