介质诱导的纤维素晶面选择性重构及其对纤维素纳米晶形成路径的启示

Medium-induced selective reconstruction of cellulose crystal facets: insights into cellulose nanocrystals (CNCs) formation pathway

摘要

【目的】纤维素纳米晶（CNCs）的形成通常被解释为酸水解过程中无定形区的优先降解与去除，但该观点难以解释无定形纤维素在水分或水热条件下发生的自发再结晶现象。本研究聚焦非晶化纤维素在不同介质中实现晶面重构的过程，旨在揭示再结晶过程中晶面构成比例的变化规律，并阐明介质条件对晶型转变及晶面恢复顺序的影响。【方法】首先比较不同球磨时间下各晶面的变化特征，分析球磨对纤维素晶体结构的选择性破坏行为。在此基础上，以无定形纤维素为实验材料，研究其在水热与草酸体系中的再结晶过程。结合X射线衍射（XRD）高斯分峰拟合与傅里叶变换红外光谱（FTIR）分析（200）、（1¯10）、（110）和（004）晶面的演化特征。【结果】球磨使纤维素I特征峰显著展宽并减弱，相对结晶度由80.1%降至28.9%；晶面分析表明（1¯10）晶面衰减速率快于（110）晶面，（004）晶面明显展宽，表现出侧向氢键网络优先破坏的选择性解构特征。随后，将球磨后的样品分别在100 ℃下水热处理和草酸溶液中处理2 h后，其相对结晶度分别恢复至65.5%和68.2%，且均出现纤维素II特征峰。晶面构成比例分析显示，再结晶并未复原原始结构，而是在纤维素I/II共存条件下建立了新的比例平衡。其中，水热体系优先恢复（200）晶面，而草酸体系在较低温阶段促进（004）晶面恢复，高温下两者均趋向典型纤维素II分布。【结论】球磨非晶化改变了晶面构成比例，再结晶过程实质为“选择性破坏-比例重分配-新平衡形成”的晶面重构路径；不同介质通过调控链段溶胀与氢键重组影响晶面恢复顺序，但不改变晶型转变方向。该研究结果表明CNCs的形成除传统的“非晶区去除”路径外，还可能来源于无序链段的介质诱导重排。本研究为低酸耗绿色制备CNCs提供了理论依据。

Abstract

[Objective] The formation of cellulose nanocrystals (CNCs) is conventionally attributed to the preferential degradation and removal of amorphous regions during acid hydrolysis. However, this prevailing view fails to account for the spontaneous recrystallization of amorphous cellulose observed under aqueous or hydrothermal conditions. To address this gap, this study focuses on the crystal facet reconstruction of amorphous cellulose across diverse media. We aim to elucidate the evolving proportion of specific crystallographic planes during recrystallization and to clarify how medium conditions govern polymorphic transformations and the restoration sequence of crystal facets. [Method] First, the evolution of specific crystal facets under varying ball-milling durations was compared to analyze the selective destructuring of the cellulose crystalline architecture. Subsequently, using amorphous cellulose as the starting material, its recrystallization behavior was investigated in both hydrothermal and oxalic acid systems. The dynamic evolution of the (200), (1¯10), (110), and (004) crystallographic planes was systematically tracked via Gaussian multi-peak fitting of X-ray diffraction (XRD) patterns combined with Fourier transform infrared (FTIR) spectroscopy. [Result] Ball milling significantly broadened and attenuated the characteristic peaks of Cellulose I, reducing the relative crystallinity from 80.1% to 28.9%. Crystallographic analysis revealed that the (1¯10) plane degraded more rapidly than the (110) plane, while the pronounced broadening of the (004) plane indicated a facet-selective deconstruction dominated by the preferential disruption of lateral hydrogen-bonding networks. Subsequently, the milled samples were subjected to separate treatments at 100 ℃ for 2 h-either under hydrothermal conditions or in an aqueous oxalic acid solution-which restored the relative crystallinity to 65.5% and 68.2%, respectively, and induced the emergence of characteristic peaks of the Cellulose II. Quantitative analysis of facet proportions demonstrated that recrystallization did not restore the original crystalline architecture; rather, it established a new equilibrium between Cellulose I and II. Specifically, the hydrothermal system preferentially restored the (200) plane, whereas the oxalic acid system facilitated the recovery of the (004) plane at lower temperatures. At elevated temperatures, however, the facet distributions in both systems converged toward a typical Cellulose II profile. [Conclusion] In conclusion, ball-milling-induced amorphization altered the proportional composition of crystallographic planes, revealing that recrystallization essentially follows a “selective disruption–proportional redistribution–new equilibrium formation” pathway of crystal facet reconstruction. While the reaction medium governed the restoration sequence of specific facets by modulating chain segment swelling and hydrogen-bond reformation, it did not alter the ultimate direction of the polymorphic transition. These findings suggest that, in addition to the conventional “amorphous region removal” pathway, CNC formation can also originate from the medium-induced rearrangement of disordered chains. This work provides a theoretical basis for the low-acid, environmentally benign preparation of CNCs.

关键词

纤维素纳米晶 (CNCs) / 晶面重构 / 再结晶 / 介质效应 / 球磨法 / 绿色制备

Key words

cellulose nanocrystals (CNCs) / crystal facet reconstruction / recrystallization / medium effects / ball milling / green preparation

参考文献

[1] Shojaeiarani J, Bajwa D S, Chanda S. Cellulose nanocrystal based composites: a review[J].Composites Part C: Open Access, 2021, 5: 100164.

[2] Wang C, Bai J, Tian P, et al. The application status of nanoscale cellulose-based hydrogels in tissue engineering and regenerative biomedicine[J].Frontiers in Bioengineering and Biotechnology, 2021, 9: 732513.

[3] Wang F, Hu Z, Ouyang S, et al. Application progress of nanocellulose in food packaging: a review[J].International Journal of Biological Macromolecules, 2024, 268: 131936.

[4] Sun B, Zhang M, Hou Q, et al. Further characterization of cellulose nanocrystal (CNC) preparation from sulfuric acid hydrolysis of cotton fibers[J].Cellulose, 2016, 23(1): 439-450.

[5] Zulnazri Z, Dewi R, Muarif A, et al. Effect of hydrochloric acid hydrolysis under sonication and hydrothermal process to produce cellulose nanocrystals from oil palm empty fruit bunch (OPEFB)[J].Polymers, 2024, 16(13): 1866.

[6] 金晟, 谢淑婷, 李树立, 等. 纤维素纳米晶的性质、制备及应用研究进展[J].生物质化学工程, 2024, 58(6): 60-72.

[7] Jin S, Xie S T, Li S L, et al. Research progress of the properties, preparation, and application of cellulose nanocrystal[J].Biomass Chemical Engineering, 2024, 58(6): 60-72.

[8] Wang Y, Liu H, Wang Q, et al. Recent advances in sustainable preparation of cellulose nanocrystals via solid acid hydrolysis: a mini-review[J].International Journal of Biological Macromolecules, 2023, 253: 127353.

[9] Aziz T, Rohullah, Ullah A, et al. Advancements in cellulose nanocrystals: a review of functionalization, applications, and challenges[J].International Journal of Biological Macromolecules, 2025, 315: 144552.

[10] Habibi Y, Lucia L A, Rojas O J. Cellulose nanocrystals: chemistry, self-assembly, and applications[J].Chemical Reviews, 2010, 110(6): 3479-3500.

[11] Lan L, Chen H, Lee D, et al. Effect of ball-milling pretreatment of cellulose on its photoreforming for H₂ production[J].ACS Sustainable Chemistry & Engineering, 2022, 10(15): 4862-4871.

[12] Kugo Y, Nomura S, Isono T, et al. Elucidating the structural changes of cellulose molecules and dynamics of Na ions during the crystal transition from cellulose I to II in low temperature and low concentration NaOH solution[J].Carbohydrate Polymers, 2024, 332: 121907.

[13] Wohlert M, Benselfelt T, Wågberg L, et al. Cellulose and the role of hydrogen bonds: not in charge of everything[J].Cellulose, 2022, 29(1): 1-23.

[14] Salem K S, Kasera N K, Rahman M A, et al. Comparison and assessment of methods for cellulose crystallinity determination[J].Chemical Society Reviews, 2023, 52(18): 6417-6446.

[15] Podgorbunskikh E, Kuskov T, Bukhtoyarov V, et al. Recrystallization of cellulose, chitin and starch in their individual and native forms[J].Polymers, 2024, 16(7): 980.

[16] Dahlström C, Eivazi A, Nejström M, et al. Regenerated cellulose properties tailored for optimized triboelectric output and the effect of counter-tribolayers[J].Cellulose, 2024, 31(4): 2047-2061.

[17] Segal L, Creely J J, Jr Martin A E, et al. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer[J].Textile Research Journal, 1959, 29(10): 786-794.

[18] Scherrer P. Bestimmung der inneren Struktur und der Größe von Kolloidteilchen mittels Röntgenstrahlen[M]. Berlin, Heidelberg: Springer,1912: 387-409.

[19] Lee Y J, Lee D Y, Lee T J, et al. Cellulose I crystallinity estimation using a combination of infrared spectroscopy and machine learning approaches[J].Carbohydrate Polymers, 2025, 368: 124210.

[20] Maurer R J, Sax A F, Ribitsch V. Molecular simulation of surface reorganization and wetting in crystalline cellulose I and II[J].Cellulose, 2013, 20(1): 25-42.

[21] Kirui A, Ling Z, Kang X, et al. Atomic resolution of cotton cellulose structure enabled by dynamic nuclear polarization solid-state NMR[J].Cellulose, 2019, 26(1): 329-339.

[22] Ouajai S, Shanks R A. Solvent and enzyme induced recrystallization of mechanically degraded hemp cellulose[J].Cellulose, 2006, 13(1): 31-44.

[23] Shao B, Han Z, Pang R, et al. The crystalline structure transition and hydrogen bonds shift determining enhanced enzymatic digestibility of cellulose treated by ultrasonication[J].Science of the Total Environment, 2023, 876: 162631.

[24] Nelson M L, O’Connor R T. Relation of certain infrared bands to cellulose crystallinity and crystal latticed type. Part I. spectra of lattice types I, II, III and of amorphous cellulose[J].Journal of Applied Polymer Science, 1964, 8(3): 1311-1324.

[25] Stanciu M C, Tanasă F, Teacă C A. Crystallinity changes in modified cellulose substrates evidenced by spectral and X-ray diffraction data[J].Polysaccharides, 2025, 6(2): 30.

[26] Jin E, Guo J, Yang F, et al. On the polymorphic and morphological changes of cellulose nanocrystals (CNC-I) upon mercerization and conversion to CNC-II[J].Carbohydrate Polymers, 2016, 143: 327-335.

[27] French A D, Santiago Cintrón M. Cellulose polymorphy, crystallite size, and the Segal Crystallinity Index[J].Cellulose, 2013, 20(1): 583-588.

[28] Yao W, Weng Y, Catchmark J M. Improved cellulose X-ray diffraction analysis using Fourier series modeling[J].Cellulose, 2020, 27(10): 5563-5579.

[29] Ahvenainen P, Kontro I, Svedström K. Comparison of sample crystallinity determination methods by X-ray diffraction for challenging cellulose I materials[J].Cellulose, 2016, 23(2): 1073-1086.

[30] Jicsinszky L, Bucciol F, Chaji S, et al. Mechanochemical degradation of biopolymers[J].Molecules, 2023, 28(24): 8031.

[31] 何奥希, 陈晋, 李毅恒, 等. 机械活化在矿物浸出过程中的应用研究[J].矿产综合利用, 2018, 39(4): 1-6.

[32] He A X, Chen J, Li Y H, et al. Application and research on mechanical activation in mineral leaching[J].Multipurpose Utilization of Mineral Resources, 2018, 39(4): 1-6.

基金资助

国家外国专家项目(H20200568)