Twisted pseudo-tetragonal orthorhombic lamellar crystal in cellulose/ionic liquid spherulite
Résumé
Cellulose and Ionic liquids can form new types of complex crystals growing in form of spherulites. Microfocus X-ray diffraction on selected location of a spherulite, formed from cellulose solution in 1-allyl-3-methylimidazolium chloride (AmimCl) by water vapor diffusion, gave diffraction patterns of fiber-type texture that could be used to index the diffraction spots. The unit cell dimension was tetragonal with a = b = 12.92 Å, c = 10.4 Å, at room temperature and a = b = 12.8 Å and c = 10.45 Å at 100 K with α = β = γ = 90˚. However, the symmetry of the molecule and stoichiometry suggests a space group P212121, compatible with chiral chain molecule with the systematic absence of odd-order h 0 0, 0 k 0, and 0 0 l reflections. The spherulite consists of lamellar crystallites rotating along the b axis, the latter lying in the radial direction. The half pitch of the order of 12 µm according to polarized microscopy, was smaller than the beam size leading to a fiber-like pattern. The crystallite sizes estimated from the diffraction broadening were at least of the order of 150 nm along a and b direction, limited by the instrument resolution, and about 15~30 nm along c direction. The 10.4 Å of c-axis is the typical length of two-fold helix of cellulose suggesting that cellulose chain is parallel to the c-axis. The unit cell would contain 4 glucose residues, 4 ionic liquid molecules and 4 - 8 water molecules
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