N.N. Semenov Institute of Chemical Physics RAS, Moscow, Russia.
N.N. Semenov Institute of Chemical Physics RAS, Moscow, Russia; Biological Faculty of M.V. Lomonosov Moscow State University, Moscow, Russia.
Carbohydr Polym. 2019 Jan 15;204:232-237. doi: 10.1016/j.carbpol.2018.10.019. Epub 2018 Oct 10.
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.
与低分子量物质相比，纤维素的硝化速率较低。理论估计表明，硝化剂扩散的速率不能提供纤维素硝化的特征时间，因为致密的区域太小。然而，硝化混合物和微晶之间的静电障碍使得后者无法接触硝酰离子。纤维素硝化速率降低，并且在相同的取代度下发生基本原纤维结构的转化，对应于原纤维表面的完全硝化。在没有解扭的情况下，基本原纤维中的超螺旋大分子不能解离。原纤维的解扭以及它们的溶胀非常缓慢。因此，我们提出纳米原纤维的解扭限制了硝酰离子向纤维素纳米原纤维中的传输速率，从而限制了硝化反应的整体速率。
