Atomistic simulations of cellulose nanofibrils and their aggregates in water

The cellulose nanofibril aggregate is a fundamental hierarchical structure in many man-made cellulosic materials. Such aggregates are formed in pulp fibers, when the lignin-hemicellulose matrix is removed during chemical pulping. Further processing steps such as drying, oxidation and mechanical processing can affect the structure of these aggregates, which is reflected in the macroscopic properties of the finished product. Moreover, the interactions between the nanofibrils can be greatly affected by water penetration into the aggregate structure. We carried out a series of atomistic molecular dynamics simulations to study the aggregation and disintegration properties of cellulose nanofibrils, and the penetration of water into their aggregates. In the first study [1], we considered the interactions between carboxylated (TEMPO-oxidized) cellulose nanofibrils at various functionalization levels. The averaged interaction over multiple charge configurations reflected experimentally measured changes in the disintegration energy of TEMPO-oxidized cellulose fibers. However, the interaction strength turned out to be highly sensitive to the charge configuration. Strong deviations from the average interaction could explain the wide fibril-size distribution observed experimentally at intermediate functionalization levels. In the second study, we simulated water penetration into a compact nanofibril aggregate, with the individual fibrils separated by amorphous cellulose chains. The observed diffusion rate was comparable to that found e.g. in NMR measurements for relatively dry wood fibers. The diffusion causes rapid changes in the inter-fibril morphology, which may affect the strength and dimensional stability of structures comprised of such fibril aggregates.