Large-scale atomistic simulations of cellulose and nitrated derivatives

Abstract

Nitrocellulose is a reactive derivative of cellulose, one of the most commonly occurring natural materials. Nitration of cellulose decreases the stability of the polymer, meaning there is less understanding of the structure and reactions. Insight into the reaction mechanisms can be important as it can provide information about the reaction constants, which can help increase understanding of the conditions needed for storage or use. Despite the importance of the chemical, the literature about nitrocellulose reactions available is limited; as the instability increases as the nitration increases, making it difficult to study the reaction at an atomic level. This thesis used several different forms of computational chemistry to investigate nitrocellulose and to develop methods that could be used to increase understanding even further. It began by using molecular dynamic simulations to create large-scale structures of different nitration levels, varying from 0–14.14 wt% nitrogen content. Cellulose is often found in fully crystalline forms, and nitrocellulose is more commonly found to be paracrystalline or amorphous. A protocol has been presented for creating realistic structures of nitrocellulose, focusing on the crystallinity of the systems being simulated. Results are presented and analysed for the creation of structures containing over 52,000 atoms. Comparisons of the different nitration structures meant that some hypotheses could be made as to what causes some different structures to be more likely to become paracrystalline than others. The actual degradation reactions required quantum mechanical methods, specifically density functional theory (DFT) to study them. First of all these methods were used on nitrate esters, being used as a small analogue for nitrocellulose. Transition state searching methods and minimum energy pathway searches were used on nitrate ester structures to find potential mechanisms for the reaction. Although there were issues getting the calculations to fully converge, some suggested transition states were found for these, and some possible mechanisms were suggested. Finally, these methods were then considered for larger systems, created through the methods already developed. The ring structure of nitrocellulose, not present in the nitrate esters tested, meant that more care needed to be taken when setting up the products and the reactants for the calculations. This meant that before insightful results could be reached, additional processes for setting up the correct structures needed to be defined. Algorithms were developed for generating these structures.

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Identifiers

ORCID for Chris Skylaris: orcid.org/0000-0003-0258-3433

ORCID for Graeme Day: orcid.org/0000-0001-8396-2771

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