Kinetics of ionic liquid-facilitated cellulose decrystallization by Raman spectral mapping

The decrease in crystallinity of a cotton thread upon exposure to the ionic liquid 1-ethyl-3-methylimidazolium acetate is investigated via two-dimensional Raman spectral mapping of the thread cross-section. By monitoring an intrinsic cellulose I Raman mode, the transition from highly crystalline cellulose I to amorphous cellulose is monitored as a function of treatment time and temperature. At extended treatment times, a heterogeneous decrystallization occurs where only those fibers at the outer edge of the thread decrystallize and dissolve while fibers in the thread center exhibit a constant crystallinity. As a result, a large amorphous cellulose shell develops which surrounds a central bundle of still-distinct and highly crystalline cotton fibers. Observed trends in the temperature-dependent rate of heterogeneous thread decrystallization are attributed to a transition to diffusion-limited decrystallization associated with the development of a solvent-swollen amorphous polymer layer. Raman spectral mapping provides new opportunities to investigate the effect of treatment conditions on the spatial extent and degree of crystallinity as well as the mechanism of decrystallization in cellulose biopolymer materials.