title: Structural studies on nitrocellulose: direct investigation and structural model
Creators: name: Morris, Edmund Alan
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: nitrocellulose
subject: fibre X-ray diffraction
subject: cellulose nitrate

description: Nitrocellulose (NC) is a structurally important material with applications ranging from biomedical test membranes to propellants. As a polymer binder for double base propellants (DBPs), NC gives mechanical strength to solid formulations together with excellent combustion characteristics. The mechanical and chemical properties of NC make DBPs a versatile propellant system found in both rocket motors and gun propellants. Despite the widespread use of NC as a propellant, there is only limited and empirical understanding of how its structure affects mechanical properties. This is exacerbated further by the properties of cellulose - its precursor, a natural product that is subject to many variations. Poor understanding of why current DBP formulations perform well mechanically, combined with difficulties reliably producing these formulations, illustrates the need for better characterisation of their structure. Despite the relatively high crystallinity found in NC compared to other polymers, there remains no credible atomistic crystal structure for NC. This research therefore explores a selection of approaches to gain reliable structural information about NC through two general approaches: direct diffraction experiments and small molecule models. First, fibre X-ray diffraction (FXRD) was used to probe NC directly. To achieve this, samples were optimised for high crystallinity and uniaxial orientation to give a high signal-to-noise ratio and strong well-defined diffraction signals. Various nitration methods, cellulose sources and annealing methods were evaluated through diffraction experiments to obtain optimal samples for FXRD. Meridional diffraction signals on the 7th, 10th and 15th lines of the fibre diffraction patterns were found, however a missing signal on the 5th line remains unexplained and casts doubt upon the previously proposed helical structure for cellulose making the possibility of simple 5- or 2-fold helical structures unlikely. The shortest repeat fitting the fibre XRD data determined here is now a 7-fold helical structure suggesting this as a plausible alternative worthy of investigation. This is the first reported evidence that the helical structure of cellulose does not take on the widely reported simple helical structures with low numbers of repeats. The overall fibre repeat, c-axis, length was found to be close to literature reported values at 25.6 Å. The second approach looks to reduce NC chain length to increase crystallinity. The influence of NC polymer chain length on crystallinity of NC was first evaluated, utilising simultaneous hydrolysis and nitration of cellulose. The selectivity of hydrolysis was investigated finding that hydrolysis preferentially occurred in amorphous regions to give high crystallinity NC of short chain length. With short chain lengths, recrystallisation of this material revealed additional Bragg peaks in diffraction experiments. Based on this, small-molecule analogues of NC were prepared, starting with cellobiose octanitrate (CON), the dimer of NC. The structure of the dimer is structurally characterised using single crystal X-ray diffraction and compared with published unit cell dimensions for NC, finding significant similarities. Packing and intermolecular interactions were investigated through non-covalent interaction plots and Hirschfeld surfaces. The dominant interaction was found to be non-classical H-bonding at C-H…O-N; this contrasts with the strong classical H-bonding seen for the non-nitrated dimer. Further crystal structures for isomers of CON: maltose octanitrate (MON), lactose octanitrate (LON) and trehalose octanitrate (TON) were obtained. Comparison of these structures highlights the influence of the polysaccharide structural features on molecular structure and packing. This thesis builds up a more comprehensive structural understanding of NC and rationalises the properties of NC through this structural model. Experiments presented here, especially the initial FXRD results and determination of NC dimer crystal structure, provide a foundation to determine a fully atomistic crystal structure of NC. Ultimately, reliable structural models for NC could enable better control of NC material properties. Through such models the characterisation of individual NC samples would be possible, along with the development of structure property relationships. This could potentially provide a route to NC without the variability found in naturally derived materials.