Computational Studies on Cellulose: Pyrolysis, Nanostructure and Hydrodynamic Behaviour: Dissertation

Cellulose, the major component of plant matter, has a complex hierarchical structure that extends from the scale of cells down to the molecular level. Knowledge of the structural fundamentals of cellulose is relevant, not only for an understanding of plant life, but also for numerous technologies that use it as a raw material. The methods of computational physics are increasingly used to support experimental efforts in cellulose research. This thesis reports molecular and fluid dynamics simulations that address questions related to the pyrolytic degradation of cellulose and the aggregation and deaggregation of cellulose microfibrils.
Cellulose pyrolysis involves hundreds of chemical reactions and volatile products, the description of which remains a formidable challenge. Here, we demonstrate the use of reactive force field methods for predicting mechanisms and kinetics of cellulose pyrolysis. We show that reactive molecular dynamics simulations can reproduce essential features of the degradation process, most notably its onset via glycosidic bond cleavage, and thus offer a means to complement quantum chemistry methods and experimental analytics.
The aggregation of microfibrils is fundamental to the structural hierarchy of native cellulose and has direct implications for its processing into nanostructured forms. Here, we use atomistic simulations to elaborate on the effects of chemical modification on microfibril interactions. Our simulations reveal the sensitivity of the interaction to non-uniform substitution patterns, a feature that is not captured by continuous theoretical models. Our findings suggest a connection between uneven charge distribution and heterogeneity observed in disintegration experiments.
We also investigate the structure of microfibril bundles, and their relationship to the bound water of the cell wall, using molecular dynamics simulations. The simulations predict the spontaneous formation of a twisted ribbon-like bundle with a twist rate compatible with recent experimental evidence. This also leads to a reasonable prediction for the amount of bound water, which consists of molecular water layers surrounding the fibrils, along with several other experimental indicators.
Microfibril interactions also manifest themselves in the rheology of aqueous cellulose nanofibril suspensions. Here, we demonstrate the coordinated use of rheometry, printing experiments and computational fluid dynamics simulations in the development of cellulose-based hydrogels for wound dressing applications. One of our key findings is the inadequacy of rotational rheometry as a basis for models of printer head flow, and the consequent need for an alternative model building strategy.