Crystal Packing and Molecular Conformation of Nitrocellulose from Fiber X-ray Differaction and Molecular Dynamics Simulations

We present an improved fiber diffraction pattern and interpretation for nitrocellulose, obtained using state-of-the-art facilities available at Diamond Light Source. Beamlines I24 and VMXm permitted unprecedented data collection for delicate nitrated ramie plant fibers that are highly susceptible to radiation damage, allowing correction of historical misassignments of crystal system and unit-cell parameters. We reassign the previously labeled monoclinic or orthorhombic unitcell to a new low-symmetry triclinic unit cell (a = 9.10, b = 8.62, c = 25.80 ˚A, α = 108, β = 116, and γ = 107◦). Containing a single polymer chain, our structure is closely related to the nitrated analogue of the Iα polymorph of cellulose. Model building indicates that the most feasible polymer geometry commensurate with the given c-axis and density measurements (1.78 g cm−3) is the 52 righthanded helix first proposed in 1978. Molecular dynamics simulations performed on a 3 x 3 x 1 supercell representation confirm the helix structure is stable and 1generates a sterically sound side-chain conformation distribution. The simulated diffraction pattern aligns well with the experimental pattern, giving confidence that the correct unit-cell parameters for this polymorph have now been obtained.