The fibrils untwisting limits the rate of cellulose nitration process

The rate of cellulose nitration is lower compared to the low molecular weight substances. Theoretical estimates assess that the rate of the nitrating agents' diffusion cannot provide for the characteristic time of cellulose nitration, as the densely packed regions are too small. However, the electrostatic barrier between the nitrating mixture and the microcrystallites makes the latter inaccessible for the nitronium ion. The cellulose nitration rate decreases and the transformation of elementary fibril structure occurs at the same degree of substitution corresponding to a complete nitration of the fibrils' surface. The supercoiled macromolecules in the elementary fibrils cannot dissociate without untwisting. The fibrils' untwisting as well as their swelling are very slow. Thus, we propose that the nanofibrils' untwisting limits the rate of the nitronium ion transport into the cellulose nanofibrils and, thus, the rate of the nitration reaction as a whole.