Cellulose forms crystalline fibrils, via biosynthesis, that can be just a few nanometers wide. The crystallinity is a structural factor related to material performance. Recently, many routes to isolate these fibrils as nanocellulose have been developed, and there exist various types of nanocellulose with different crystallinities. Quantitative assessment of the crystallinity of nanocellulose is thus essential to advance knowledge in the high performance and functionality of such materials. Solid-state 13C cross-polarization/magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy is a strong tool to investigate the structural features and dynamics of solid cellulose. The crystallinity is often evaluated by using the NMR signal ratio of the C4 crystalline and noncrystalline regions as a crystallinity index (CI) value. To calculate the CI value, it is necessary to examine the dependence of the contact time (CT) for CP on the signal intensity and set the optimum CT at a maximum of the signal intensity. However, the dependence has not been investigated for evaluation of the CI value of various cellulose samples with different crystal sizes. Here, we optimized the CT for evaluation of the CI value of cellulose with different crystal sizes. The error in the CI at the optimized CT was ~ 3%. At the optimized CT, the structural change after surface modification by TEMPO-oxidation was also analyzed from the NMR spectra of the C6 region. The relationship between the CI value and the degree of oxidation shows that it is possible to evaluate the degree of oxidation from the NMR spectra irrespective of the crystallinity of cellulose. Furthermore, the C4-based CI value was linearly correlated with the C6-based trans-gauche (tg) ratio, which is approximated by a function, CI = 0.9 tg ratio.

中文翻译：

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使用固态¹³ C NMR的各种尺寸的纤维素微晶的交叉极化动力学和构象研究

纤维素通过生物合成形成仅几纳米宽的结晶原纤维。结晶度是与材料性能有关的结构因素。近来，已经开发了许多分离这些原纤维作为纳米纤维素的途径，并且存在具有不同结晶度的各种类型的纳米纤维素。因此，对纳米纤维素结晶度的定量评估对于提高此类材料的高性能和功能性知识至关重要。固态13C交叉极化/魔角旋转（CP / MAS）核磁共振（NMR）光谱是研究固体纤维素的结构特征和动力学的强大工具。通常通过使用C4结晶区和非结晶区的NMR信号比作为结晶度指数（CI）值来评估结晶度。为了计算CI值，有必要检查CP的接触时间（CT）对信号强度的依赖性，并将最佳CT设置为最大信号强度。但是，尚未研究依赖关系来评估具有不同晶体尺寸的各种纤维素样品的CI值。在这里，我们优化了CT，以评估具有不同晶体尺寸的纤维素的CI值。在优化的CT上CI的误差约为3％。在优化的CT上，还根据C6区的NMR光谱分析了通过TEMPO氧化进行表面改性后的结构变化。CI值与氧化度之间的关系表明，与纤维素的结晶度无关，可以从NMR光谱评价氧化度。此外，