Structure and properties of nitrocellulose: approaching 200 years of research

Abstract

This review brings together almost 200 years of fragmented research on the structure of nitrocellulose to give an overview that covers production to application in composite materials. As a mouldable plastic, energetic rocket propellant and biomolecular binding membrane, nitrocellulose still finds widespread practical application today despite the inception of synthetic plastics. The influence of different cellulose source materials affects the structure and properties of nitrocellulose in ways that are not fully understood, and so this review brings together relatively recent developments in the understanding of cellulose nanostructures to highlight where the gaps in understanding now reside. The influence of nitration conditions on the material properties of nitrocellulose is described, together with the proposed mechanisms and equilibria associated with these synthetic routes. The reported crystal structures of nitrocellulose are also reviewed, and the confirmed structural features are separated from those yet to be proven. We also consider how nitrocellulose interacts with other compounds, to help explain the distinct properties of its composite materials. This review points to further work that is required to obtain well founded structural models of nitrocellulose, while highlighting opportunities to control and direct its structure to improve its material properties.

Fig. 1. Repeating dimer units of (a) cellulose and (b) NC. The dimer illustrates the intra-chain interactions in cellulose, while the glucose monomer is the more accurate chemical repeating unit. OH-1 and OH-2 represent the two most accessible hydroxyl groups on cellulose for substitution, while OH-3 is the least accessible.

Fig. 2. Illustrations of (a) a cellulose elementary fibril (CEF), (b) a macrofibril and (c) a microfibrils compared with their respective atomic force microscopy (AFM) images (d), (e) and (f). In (d) to (f) the green arrows highlight the structural feature illustrated. Illustrations adapted from ref. with permission from American Chemical Society, copyright 2006 and AFM images reproduced from ref. with permission from Springer Nature B.V., copyright 2014.

Fig. 3. Illustration of CSCs for plant and BAT sources; the grey dots indicate where cellulose chains are extruded from the cell membrane. The plant CSC in (a) is compared with the CEF to show the relationship between their size and shape. Algal cellulose CSCs of (b) green algae (Micrasterias), (c) yellow-green algae (Vaucheria) and (d) red algae (Erythrocladia). The CSCs for (e) tunicate (Metandroxarpa uedai) cellulose and (f) bacterial (Acetobacter) cellulose. Illustration adapted from ref. with permission from Taylor and Francis, copyright 1996.

Fig. 4. Scanning electron microscopy (SEM) images of (a) a bacterial cellulose pellicle and (b) plant cell wall surface of cotton cellulose. Images reproduced form ref. with permission from MDPI, copyright 2019.

Fig. 5. General mechanism for the nitration of cellulose.

Fig. 6. Crystal structures of (a) cellulose I_α viewed along a axis and (b) cellulose I_β viewed along c axis, with unit cells marked by black box. Plotted in mercury with structures from ref. and respectively.

Fig. 7. The first published fibre diffraction pattern of NC with DoN of 12.6%, derived from nitration of ramie plant fibres. Reproduced with permission from ref. with permission from John Wiley and Sons, copyright 1927.

Fig. 8. X-ray diffraction patterns of (a) a ramie cellulose fibre, compared with (b) with di-substitution NC and (c) tri-substitution NC. Diffraction patterns reproduced from ref. , copyright 1955.

Fig. 9. Crystallographic models of NC proposed by (a) Miles in 1955 (orthorhombic) and (b) Happey et al. in 1978 (monoclinic, showing the now disputed five-fold axis). Illustrations reproduced from (a) ref. , copyright Imperial Chemicals Industry 1955 and (b) ref. with permission from Elsevier, copyright 1978.

Fig. 10. Multiple overlapping NC powder diffraction patterns from ref. reproduced with permission from Fraunhofer ICT, copyright 2014, with Miller index planes marked from indexing of NC by Miles et al. from ref. .

Fig. 11. Schematic summarising the solubility of NC across a variety of functional groups in relation to DoN. Solubility data from ref. .

Fig. 12. (a) Polarised light microscope images and (b) X-ray diffraction pattern of extruded fibres from mixed isotropic and anisotropic phases of NC/tetrahydrofuran/ethanol mixture. (c) Polarised light microscope images and (d) X-ray diffraction pattern of extruded fibres from completely anisotropic phase of NC/tetrahydrofuran/ethanol mixture. Reproduced from ref. with permission from Taylor and Francis, copyright 1986.