Yu Shihua, Liu Zhigang, Xu Na, Chen Jie, Gao Yan
College of Chemical and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin, 132022, China.
Center of Analysis and Measurement, Jilin Institute of Chemical Technology, Jilin, 132022, China.
Anal Sci. 2020 Aug 10;36(8):947-951. doi: 10.2116/analsci.19P427. Epub 2020 Feb 7.
Full width at half maximum (FWHM) of diffraction peaks of native cellulose is larger than 1°. The diffraction peaks of the crystalline phase largely coincide with those of the amorphous phase of cellulose, leading to the low resolution. Therefore, when calculating the crystallinity of natural cellulose by fitting the peaks, only relying on the single evaluation factor of Rwp (R-weighted Pattern) may lead to great randomicity of calculation results. Due to the special crystal structure of natural cellulose and the characteristics of crystallinity determination by peak separation method, the XRD Rietveld fitting method is adopted to determine the crystallinity of native cellulose. Through limiting the convergence conditions of fitting functions, we firstly discuss the effects of peak shape functions, scanning range, and the positions of amorphous peaks on crystallinity determination, which helps to reduce the randomness of XRD in solving crystallinity of cellulose and improve the precision of calculation. Then, three evaluation indexes (Rwp, FWHM, and RSD) are used to evaluate the rationality of crystallinity calculation results. Moreover, the reproducibility and precision of the crystallinity of three kinds of natural cellulose are tested under the optimized conditions. At the same time, the optimized fitting method is adopted to calculate the content of cellulose I and II, which can be used to guide the selection of alkali concentration and alkali treatment time in the spinning process of native cellulose.
天然纤维素衍射峰的半高宽（FWHM）大于1°。结晶相的衍射峰与纤维素非晶相的衍射峰基本重合，导致分辨率较低。因此，在通过峰拟合计算天然纤维素的结晶度时，仅依靠Rwp（R加权图谱）这一单评价因子可能会导致计算结果的随机性很大。鉴于天然纤维素特殊的晶体结构以及峰分离法测定结晶度的特点，采用XRD全谱拟合法测定天然纤维素的结晶度。通过限定拟合函数的收敛条件，首先探讨了峰形函数、扫描范围以及非晶峰位置对结晶度测定的影响，这有助于减少XRD在求解纤维素结晶度时的随机性并提高计算精度。然后，使用三个评价指标（Rwp、FWHM和RSD）来评估结晶度计算结果的合理性。此外，在优化条件下测试了三种天然纤维素结晶度的重现性和精度。同时，采用优化后的拟合方法计算纤维素I和II的含量，可用于指导天然纤维素纺丝过程中碱浓度和碱处理时间的选择。
