Twisted pseudo-tetragonal orthorhombic lamellar crystal in cellulose/ionic liquid spherulite

Abstract

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 h00, 0k0, and 00l 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.

Graphic abstract