Dissolved Cellulose Couples with Flow and Capillarity to Affect Ionic Liquid Crystallization: An X-ray Diffraction and Raman Spectroscopy Study

Cellulose is a semicrystalline polymer with domains controlled by a cooperative hydrogen-bond network and by solvophobic forces that do not energetically favor solubility. Ionic liquids dissolve such domains in a nonderivatizing way, and many ionic liquids also meet the criteria of green solvents. The crystallization behavior of 1-butyl-3-methylimidazolium chloride (bmimCl) was investigated in the presence of cellulose and added water. It was hypothesized correctly that the structural transitions in bmimCl during low-temperature treatment would reflect the interactions with either solute. Mixtures with cellulose at concentrations of 1 or 3 wt % and with added water concentrations of either 0.0, 0.6, or 2.7 wt % were subjected to two low-temperature treatments, i.e., repeated exposure at either -25 °C or -17 °C with room temperature in between. Such mixtures were investigated by Fourier-transform or dispersive Raman spectroscopy and wide-angle X-ray scattering. The two different treatments induced fluid flow with either more laminar or more turbulent character. The more laminar flow led to the extended AA butyl chain conformation in bmimCl and to the orthorhombic crystal structure O; the more turbulent flow led to the GA conformation and either the metastable stressed monoclinic M or the stable monoclinic M0 crystal structure. The presence of cellulose can shift the butyl chain conformation toward the more extended one; such a shift might be an effect of interactions, or cellulose macromolecules might be acting indirectly. By containing the mixtures in two types of glass vessels with different internal diameters (2 mm or 9 mm), the coupling of the fluid flow to capillarity phenomena was confirmed. Such findings are important for ionic liquid phase transitions, for cellulose-ionic liquid interactions, and for structuring of materials by flow, thus attaining better control of material properties across length scales, starting at nanodimensions through to micro- and macrodimensions.