Nanoscale Modelling of Natural Fibres

The main objective of this research plan is to investigate how the mechanical properties of the plant cell wall and its constituents are controlled at the atomistic levels through molecular dynamics simulations. In particular, different constituents such as crystalline cellulose, hemicellulose, dislocated cellulose, lignin and water are simulated and studied, through molecular dynamics methods, aiming at a better understanding of the molecular/atomistic level interactions. The thesis uses enhanced state-of-the-art methodologies, such as umbrella sampling to calculate interaction energy values between different constituents.

The results of the thesis can be summarized as below:

- The tensile behaviour of cellulose nanocrystals and cellulose microfibrils was assessed thoroughly. The Young’s modulus of both models was computed and compared. It was elucidated how energy dissipates in the models and how this energy dissipation is controlled through the changes happening at the glycosidic linkage. The effect of water on the mechanical properties of individual fibrils was assessed. Water showed no significant influence on the mechanical properties.

- The ultimate properties of cellulose nanocrystals were studied. Ultimate stress of 9.2 GPa and ultimate strain of 8.5% was estimated for a 36-chain model. It was shown that the C4-O4 bond is responsible for the failure of cellulose nanocrystals, and that the failure primarily starts at the surface and propagates inwards.

- The applicability of macroscopic rules, i.e., the inverse rule of mixtures, was assessed at the nanoscale. It was shown that the inverse rule of mixtures could be used to estimate the effective elastic modulus of cellulose microfibrils, given that the degree of crystallinity of the fibrils is known.

- Size distribution of dislocated segments in cellulose microfibrils was estimated. By combining results of the molecular dynamics simulations and experimental results from the literature, it is shown that dislocations could have 6-11 anhydroglucose units, equal to 3.1-5.8 nm in length.

- Five different hemicellulose models were simulated; namely, Glucomannan, Galactoglucomannan, Xyloglucan, Glucuronoxylan, and Glucuronoarabinoxylans, and their interaction energies when adsorbed onto cellulose were computed and compared. Results showed that Glucuronoarabinoxylan shows the highest interaction energies while xyloglucan is the weakest. Moreover, regioselectivity of substituents on the backbone of the hemicellulose was found to be the determining parameter in defining the adsorption properties. Failure in the shear mode was shown to primarily happen at the interface of cellulose and hemicellulose. Water was shown to deteriorate the interactions within the hemicellulose matrix and the interface.