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Reversible Surface Engineering of Cellulose Elementary Fibrils: From Ultralong Nanocelluloses to Advanced Cellulosic Materials
纤维素基本纤维的可逆表面工程：从超长纳米纤维素到先进纤维素材料

Meng Zhou, Dongzhi Chen,* Qianqian Chen, Pan Chen, Guangjie Song, and Chunyu Chang*
Meng Zhou、Dongzhi Chen、* Qianqian Chen、Pan Chen、Guangjie Song 和 Chunyu Chang*

Abstract 摘要

Cellulose nanofibrils (CNFs) are supramolecular assemblies of cellulose chains that provide outstanding mechanical support and structural functions for cellulosic organisms. However, traditional chemical pretreatments and mechanical defibrillation of natural cellulose produce irreversible surface functionalization and adverse effects of morphology of the CNFs, respectively, which limit the utilization of CNFs in nanoassembly and surface functionalization. Herein, this work presents a facile and energetically efficient surface engineering strategy to completely exfoliate cellulose elementary fibrils from various bioresources, which provides CNFs with ultrahigh aspect ratios ( ≈ 1400 ≈ 1400 ~~1400\approx 1400 ) and reversible surface. During the mild process of swelling and esterification, the crystallinity and the morphology of the elementary fibrils are retained, resulting in high yields ( 98 % 98 % 98%98 \% ) with low energy consumption ( 12.4 kJ g − 1 12.4 kJ g − 1 12.4kJg^(-1)12.4 \mathrm{~kJ} \mathrm{~g}^{-1} ). In particular, on the CNF surface, the surface hydroxyl groups are restored by removal of the carboxyl moieties via saponification, which offers a significant opportunity for reconstitution of stronger hydrogen bonding interfaces. Therefore, the resultant CNFs can be used as sustainable building blocks for construction of multidimensional advanced cellulosic materials, e.g., 1D filaments, 2D films, and 3D aerogels. The proposed surface engineering strategy provides a new platform for fully utilizing the characteristics of the cellulose elementary fibrils in the development of high-performance cellulosic materials.
纤维素纳米纤丝（CNFs）是纤维素链的超分子集合体，可为纤维素生物提供出色的机械支撑和结构功能。然而，天然纤维素的传统化学预处理和机械脱纤分别会产生不可逆的表面功能化和对 CNFs 形态的不利影响，从而限制了 CNFs 在纳米组装和表面功能化方面的应用。在此，本研究提出了一种从各种生物资源中完全剥离纤维素基本纤维的简便且能量效率高的表面工程策略，该策略可提供具有超高纵横比（ ≈ 1400 ≈ 1400 ~~1400\approx 1400 ）和可逆表面的 CNFs。在温和的溶胀和酯化过程中，基本纤维的结晶度和形态得以保留，因此产量高（ 98 % 98 % 98%98 \% ），能耗低（ 12.4 kJ g − 1 12.4 kJ g − 1 12.4kJg^(-1)12.4 \mathrm{~kJ} \mathrm{~g}^{-1} ）。特别是在 CNF 表面，通过皂化去除羧基恢复了表面羟基，这为重建更强的氢键界面提供了重要机会。因此，生成的 CNFs 可用作构建多维高级纤维素材料（如一维细丝、二维薄膜和三维气凝胶）的可持续构件。所提出的表面工程策略为充分利用纤维素基本纤维的特性开发高性能纤维素材料提供了一个新平台。

1. Introduction 1.导言

As the most abundant bioresource on Earth, cellulose is a promising candidate for materials science to move from the fossil fuel era to a more sustainable future. [ 1 ] [ 1 ] ^([1]){ }^{[1]} Cellulose elementary fibrils are incredibly strong and stiff one-dimensional nanoscale fibrils composed of extended β β beta\beta-1,4-glucan chains, which exist as structural components in the cell walls of plants, mantles of tunicates, and biofilms secreted by bacteria. [ 2 − 4 ] [ 2 − 4 ] ^([2-4]){ }^{[2-4]} These unique highly crystalline fibrils exhibit high tensile strengths (2-7.7 GPa), [ 5 , 6 ] [ 5 , 6 ] ^([5,6]){ }^{[5,6]} high elastic modulus ( ≈ 150 GPa ≈ 150 GPa ~~150GPa\approx 150 \mathrm{GPa} ), [ 6 , 7 ] [ 6 , 7 ] ^([6,7]){ }^{[6,7]} yet low density ( ≈ 1.6 g cm − 3 ) ≈ 1.6 g cm − 3 (~~1.6(g)cm^(-3))\left(\approx 1.6 \mathrm{~g} \mathrm{~cm}^{-3}\right), so they are seven times stronger but five times lighter than steel. [ 8 ] [ 8 ] ^([8]){ }^{[8]} Owing to its excellent mechanical performance, low density, biodegradability, and renewability, nanocellulose has recently received extensive attention for fabrication of sustainable, light weight, and robust materials. [ 3 , 9 , 10 ] [ 3 , 9 , 10 ] ^([3,9,10]){ }^{[3,9,10]} After isolation from biomass, nanocelluloses have two generic forms, further heterogeneous chemical modification of the surface, the extracted cellulose
作为地球上最丰富的生物资源，纤维素是材料科学从化石燃料时代迈向更可持续发展未来的希望所在。 [ 1 ] [ 1 ] ^([1]){ }^{[1]} 纤维素基本纤维是由延伸的 β β beta\beta -1,4-葡聚糖链组成的一维纳米级纤维，其强度和硬度令人难以置信，是植物细胞壁、鳞藻类外壳和细菌分泌的生物膜的结构成分。 [ 2 − 4 ] [ 2 − 4 ] ^([2-4]){ }^{[2-4]} 这些独特的高结晶纤维具有高拉伸强度（2-7.7 GPa）、 [ 5 , 6 ] [ 5 , 6 ] ^([5,6]){ }^{[5,6]} 高弹性模量（ ≈ 150 GPa ≈ 150 GPa ~~150GPa\approx 150 \mathrm{GPa} ）、 [ 6 , 7 ] [ 6 , 7 ] ^([6,7]){ }^{[6,7]} 低密度 ( ≈ 1.6 g cm − 3 ) ≈ 1.6 g cm − 3 (~~1.6(g)cm^(-3))\left(\approx 1.6 \mathrm{~g} \mathrm{~cm}^{-3}\right) ，因此它们的强度是钢的七倍，而重量却是钢的五倍。 [ 8 ] [ 8 ] ^([8]){ }^{[8]} 由于纳米纤维素具有优异的机械性能、低密度、生物可降解性和可再生性，近来在制造可持续、轻质和坚固材料方面受到广泛关注。 [ 3 , 9 , 10 ] [ 3 , 9 , 10 ] ^([3,9,10]){ }^{[3,9,10]} 从生物质中分离出纳米纤维素后，纳米纤维素有两种一般形式，一种是对其表面进行进一步的异质化学修饰，另一种是提取纤维素。
M. Zhou, Q. Chen, C. Chang
M.Zhou, Q. Chen, C. Chang
College of Chemistry and Molecular Sciences
化学与分子科学学院
Engineering Research Center of Natural Polymer-based Medical
天然聚合物医用材料工程研究中心
Materials in Hubei Province, and Laboratory of Biomedical Polymers of Ministry of Education
湖北省材料研究所和教育部生物医用高分子材料实验室
Wuhan University 武汉大学
Wuhan, Hubei 430072, P. R. China
中国湖北武汉 430072
E-mail: changcy@whu.edu.cn
电子邮件： changcy@whu.edu.cn
D. Chen D.陈
State Key Laboratory of New Textile Materials and Advanced Processing Technology
纺织新材料与先进加工技术国家重点实验室
Wuhan Textile University 武汉纺织大学
Wuhan 430073, P. R. China
中国武汉 430073
E-mail: dzchen@wtu.edu.cn
电子邮件：dzchen@wtu.edu.cn

P. Chen P.陈

School of Materials Science and Engineering
材料科学与工程学院
Beijing Institute of Technology
北京理工大学
Beijing 100081, P. R. China
中国北京 100081
G. Song G. 宋
Beijing National Laboratory for Molecular Sciences
北京分子科学国家实验室
CAS Key Laboratory of Engineering Plastics
中科院工程塑料重点实验室
Institute of Chemistry 化学研究所
Chinese Academy of Sciences
中国科学院
Beijing 100190, P. R. China
中国北京 100190
elementary fibrils are commonly referred to as nanocelluloses, including crystalline cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). [ 1 ] [ 1 ] ^([1]){ }^{[1]} CNCs are obtained by preferentially hydrolyzing the longitudinal periodic amorphous regions of the cellulose with strong acids, resulting in both low yields and aspect ratios. [ 11 ] [ 11 ] ^([11]){ }^{[11]} CNFs produced by defibrillation have high aspect ratios and exhibit gel-like characteristics in water, which retain the native morphologies of the cellulose elementary fibrils. [ 12 , 13 ] [ 12 , 13 ] ^([12,13]){ }^{[12,13]}
基本纤维通常被称为纳米纤维素，包括结晶纤维素纳米晶体（CNC）和纤维素纳米纤维（CNF）。 [ 1 ] [ 1 ] ^([1]){ }^{[1]} CNC 是用强酸优先水解纤维素的纵向周期性无定形区域而获得的，因此产量和纵横比都很低。 [ 11 ] [ 11 ] ^([11]){ }^{[11]} 通过去纤维化生产的 CNF 具有高纵横比，在水中表现出凝胶状特性，保留了纤维素基本纤维的原生形态。 [ 12 , 13 ] [ 12 , 13 ] ^([12,13]){ }^{[12,13]}
Recently, mechanical treatments and chemical modification have been employed to prepare CNFs. Mechanical treatment is always conducted with the never-dried condition using a high-pressure homogenizer, grinder, or ultrasonic irradiation to produce the CNFs. [ 14 − 18 ] [ 14 − 18 ] ^([14-18]){ }^{[14-18]} Chemical or enzymatic pretreatments are required to facilitate defibrillation before the mechanical treatment. The resultant CNFs have abundant hydroxyl groups on their surfaces, which improve the material performance by forming hydrogen bonds among the CNFs. TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation is the most popular oxidation method, and it selectively introduces negative charges to the surfaces of the cellulose elementary fibrils by formation of a carboxylate group at the C6 position of glucose. [ 19 − 21 ] [ 19 − 21 ] ^([19-21]){ }^{[19-21]} After the TEMPO treatment followed by mild magnetic homogenization, CNFs with diameters of 3-5 nm are obtained, which can be evenly dispersed in water due to surface electrostatic repulsion. [ 19 , 22 ] [ 19 , 22 ] ^([19,22]){ }^{[19,22]} Although current methods are effective for the preparation of CNFs with high yields, [ 18 , 23 − 26 ] [ 18 , 23 − 26 ] ^([18,23-26]){ }^{[18,23-26]} some issues still exist; for instance, both high-energy mechanical treatments and hard oxidation processes severely reduce the lengths of the cellulose elementary fibrils, which limit the construction of all nanocellulose materials. [ 5 , 16 , 27 , 28 ] [ 5 , 16 , 27 , 28 ] ^([5,16,27,28]){ }^{[5,16,27,28]} On the other hand, CNFs produced by mechanical treatment need surface modification to improve their dispersibility, while the surface charges of TEMPOoxidized CNFs degrade the mechanical and physical properties of the nanocellulose materials due to the presence of electrostatic repulsion among CNFs. [ 8 , 22 ] [ 8 , 22 ] ^([8,22]){ }^{[8,22]} Contrasting with TEMPO-mediated and periodate oxidation, Beaumont et al. demonstrated that regioselective succinylation approach enabled complete isolation of elementary cellulose fibrils from wood pulp fibers. [ 25 ] [ 25 ] ^([25]){ }^{[25]} Although this impressive method could restore the native supramolecular interaction by removing the succinate groups, the length of resultant CNFs still significantly decreased during the fibrillation of the surface modified cellulose with a high-pressure homogenizer. Therefore, a facile, mild, and energy-efficient method for isolation of the cellulose elementary fibrils must be further developed.
最近，人们采用机械处理和化学改性来制备 CNF。机械处理通常在未干燥的状态下进行，使用高压均质机、研磨机或超声波照射来制备 CNF。 [ 14 − 18 ] [ 14 − 18 ] ^([14-18]){ }^{[14-18]} 在机械处理前需要进行化学或酶预处理以促进去纤维化。生成的 CNF 表面有丰富的羟基，这些羟基可在 CNF 之间形成氢键，从而提高材料的性能。TEMPO（2,2,6,6-四甲基哌啶-1-氧自由基）介导的氧化是最常用的氧化方法，它通过在葡萄糖的 C6 位形成羧酸基，选择性地在纤维素基本纤维表面引入负电荷。 [ 19 − 21 ] [ 19 − 21 ] ^([19-21]){ }^{[19-21]} 经过 TEMPO 处理后，再经过温和的磁性匀浆，可得到直径为 3-5 nm 的 CNFs，由于表面静电排斥作用，这些 CNFs 可均匀地分散在水中。 [ 19 , 22 ] [ 19 , 22 ] ^([19,22]){ }^{[19,22]} 虽然目前的方法可以有效地制备高产率的 CNFs，但 [ 18 , 23 − 26 ] [ 18 , 23 − 26 ] ^([18,23-26]){ }^{[18,23-26]} 仍存在一些问题，例如，高能机械处理和硬氧化过程都会严重降低纤维素基本纤维的长度，从而限制了所有纳米纤维素材料的构建。 [ 5 , 16 , 27 , 28 ] [ 5 , 16 , 27 , 28 ] ^([5,16,27,28]){ }^{[5,16,27,28]} 另一方面，机械处理产生的 CNFs 需要进行表面改性以提高其分散性，而 TEMPO 氧化 CNFs 的表面电荷会降低纳米纤维素材料的机械和物理性能，因为 CNFs 之间存在静电排斥。 [ 8 , 22 ] [ 8 , 22 ] ^([8,22]){ }^{[8,22]} 与 TEMPO 介导的氧化和高碘酸盐氧化不同，Beaumont 等人采用的是 TEMPO 氧化法。 研究表明，区域选择性琥珀酸化方法能够从木浆纤维中完全分离出基本纤维素纤维。 [ 25 ] [ 25 ] ^([25]){ }^{[25]} 尽管这种令人印象深刻的方法可以通过去除琥珀酸基团来恢复原生超分子相互作用，但在使用高压匀浆器对表面修饰的纤维素进行纤化时，所得到的 CNFs 长度仍会显著下降。因此，必须进一步开发一种简便、温和、节能的纤维素基本纤维分离方法。
Herein, for the first time, we report an energetically effective surface engineering strategy that enables the liberation of cellulose elementary fibrils from various cellulosic sources with a two-step process involving swelling and esterification (Figure 1a). We used alkali/dimethyl sulfoxide (DMSO) systems as pseudosolvents that selectively broke the hydrogen bonds and van der Waals (vdW) interactions among the less-accessible surfaces of the cellulose elementary fibrils without dissolving or degrading them, and formed highly swollen networks. Then, esterification of the available hydroxyl groups on both the accessible and less accessible surfaces of the cellulose elementary fibrils was realized by adding cyclic anhydrides into the swollen networks with mild stirring; this yielded CNFs bearing carboxyl moieties (Figure 1a) The two processes are accomplished via a one-pot reaction, and
在此，我们首次报告了一种能量有效的表面工程策略，该策略可通过涉及溶胀和酯化的两步过程从各种纤维素来源中释放出纤维素基本纤维（图 1a）。我们使用碱/二甲基亚砜（DMSO）体系作为假溶剂，在不溶解或降解纤维素基本纤维的情况下，选择性地破坏纤维素基本纤维不易接触表面之间的氢键和范德华（vdW）相互作用，形成高度膨胀的网络。然后，通过在轻度搅拌下向膨胀网络中加入环状酸酐，实现了纤维素基本纤维的可触及表面和不太容易触及表面上可用羟基的酯化；这样就得到了带有羧基的 CNF（图 1a）。
the cellulose pulp was converted into a highly stable CNF suspension due to electrostatic repulsions among the CNFs (Figure 1b). Our strategy preserved the morphologies and sizes of the elementary fibrils without mechanical or chemical damage to the potential longitudinal amorphous region and provided CNFs with ultrahigh aspect ratios ( ≈ 1400 ≈ 1400 ~~1400\approx 1400 ) (Figure 1c). In comparison with other reported methods, such as TEMPO oxidation, periodate oxidation, and acid hydrolysis, [ 21 , 29 , 30 ] [ 21 , 29 , 30 ] ^([21,29,30]){ }^{[21,29,30]} this surface engineering strategy consumed less energy ( ≈ 12.4 kJ / g ≈ 12.4 kJ / g ~~12.4kJ//g\approx 12.4 \mathrm{~kJ} / \mathrm{g} ) and provided higher yields (98%) (Table S1, Supporting Information). Furthermore, the surface charges of the CNFs can be removed easily by saponification, which offers significant opportunities for reconstruction of the hydrogen bonds and vdW interactions among the CNFs and even restoring the surface hydrogen bonding networks of native celluloses. This energy-effective and sustainable method for the isolation of CNFs with ultrahigh aspect ratios provides a new platform for the construction of advanced cellulosic materials by fully utilizing the features of the cellulose elementary fibrils.
由于 CNF 之间存在静电排斥，纤维素浆被转化为高度稳定的 CNF 悬浮液（图 1b）。我们的策略保留了基本纤维的形态和尺寸，而不会对潜在的纵向无定形区造成机械或化学损伤，并提供了具有超高纵横比（ ≈ 1400 ≈ 1400 ~~1400\approx 1400 ）的 CNF（图 1c）。与其他已报道的方法（如 TEMPO 氧化、高碘酸盐氧化和酸水解）相比， [ 21 , 29 , 30 ] [ 21 , 29 , 30 ] ^([21,29,30]){ }^{[21,29,30]} 这种表面工程策略消耗的能量更少（ ≈ 12.4 kJ / g ≈ 12.4 kJ / g ~~12.4kJ//g\approx 12.4 \mathrm{~kJ} / \mathrm{g} ），产率更高（98%）（表 S1，辅助信息）。此外，CNFs 的表面电荷很容易通过皂化去除，这为重建 CNFs 之间的氢键和 vdW 相互作用，甚至恢复原生纤维素的表面氢键网络提供了重要机会。这种高能效、可持续的超高纵横比 CNFs 分离方法充分利用了纤维素基本纤维的特性，为先进纤维素材料的构建提供了一个新平台。

2. Results and Discussion
2.结果与讨论

2.1. Selective Disassembly of Cellulose Via Swelling
2.1.通过膨胀选择性分解纤维素

Native cellulose can be dissolved in lithium chloride/ N , N − N , N − N,N^(-)N, N^{-} dimethylacetamide ( LiCl / DMAc LiCl / DMAc LiCl//DMAc\mathrm{LiCl} / \mathrm{DMAc} ) at high temperatures ( 80 ∘ C 80 ∘ C 80^(@)C80{ }^{\circ} \mathrm{C} ) and lithium hydroxide ( LiOH )/urea aqueous solution at low temperatures ( − 5 ∘ C ) . [ 31 , 32 ] − 5 ∘ C . [ 31 , 32 ] (-5^(@)C).^([31,32])\left(-5^{\circ} \mathrm{C}\right) .{ }^{[31,32]} Based on these solvent systems, we developed pseudosolvents to selectively reduce hydrogen bonding and weaken the vdW interactions among the cellulose elementary fibrils while maintaining the crystallinity and morphology of the cellulose elementary fibrils. Unlike dissolution, in which cellulose is dispersed into the solvent at the single chain level, pseudosolvents are expected to efficiently split the packed elementary fibrils into entangled cellulose networks and allow penetration and grafting of reactive reagents on the surfaces of the cellulose elementary fibrils without penetration into the crystalline core. Based on the dissolution mechanism for cellulose, [ 33 − 35 ] [ 33 − 35 ] ^([33-35]){ }^{[33-35]} alkali/DMSO mixtures were used as pseudosolvents because they can disrupt hydrogen bonding and vdW interactions among cellulose elementary fibrils at room temperature.
原生纤维素可在高温下溶解于氯化锂/ N , N − N , N − N,N^(-)N, N^{-} 二甲基乙酰胺（ LiCl / DMAc LiCl / DMAc LiCl//DMAc\mathrm{LiCl} / \mathrm{DMAc} ）中（ 80 ∘ C 80 ∘ C 80^(@)C80{ }^{\circ} \mathrm{C} ），在低温下溶解于氢氧化锂（LiOH）/尿素水溶液中（ ( − 5 ∘ C ) . [ 31 , 32 ] − 5 ∘ C . [ 31 , 32 ] (-5^(@)C).^([31,32])\left(-5^{\circ} \mathrm{C}\right) .{ }^{[31,32]} ）、 ( − 5 ∘ C ) . [ 31 , 32 ] − 5 ∘ C . [ 31 , 32 ] (-5^(@)C).^([31,32])\left(-5^{\circ} \mathrm{C}\right) .{ }^{[31,32]} 在这些溶剂系统的基础上，我们开发出了假溶剂，可选择性地减少纤维素基本纤维之间的氢键和 vdW 相互作用，同时保持纤维素基本纤维的结晶度和形态。在溶解过程中，纤维素会在单链水平上分散到溶剂中，与此不同的是，预计伪溶剂会有效地将包装好的纤维素基本纤丝分割成纠缠在一起的纤维素网络，并允许反应试剂在纤维素基本纤丝表面渗透和接枝，而不会渗透到结晶核心。根据纤维素的溶解机制， [ 33 − 35 ] [ 33 − 35 ] ^([33-35]){ }^{[33-35]} 碱/二甲基亚砜混合物被用作假溶剂，因为它们在室温下可以破坏纤维素基本纤维之间的氢键和 vdW 相互作用。
As shown in Figure 2a, large cellulose fiber bundles (several micrometers in diameter) were observed in native tunicate cellulose which were converted into microfibrils with diameters of tens to hundreds of nanometers after swelling for 8 h in the pseudosolvent comprising DMSO saturated with LiOH ( 2 mg mL − 1 ) LiOH 2 mg mL − 1 LiOH(2mgmL^(-1))\mathrm{LiOH}\left(2 \mathrm{mg} \mathrm{mL}^{-1}\right). When the swelling time was extended to 24 h , the cross-sections of cellulose microfibrils gradually split at the nanoscale, whereas the structures of the cellulose elementary fibrils (several nanometers in diameter) were well preserved in the nanonetworks. The resultant swollen cellulose exhibited gel-like behavior and high viscosity, so we studied the viscosity changes during the swelling process (Figure 2b and Figure S1, Supporting Information) with rheological technology. As the swelling time was increased from 1 to 8 h , the apparent viscosity of the suspension rapidly increased from 19.1 to 450.2 Pa s at a shear rate of 0.1 s − 1 0.1 s − 1 0.1s^(-1)0.1 \mathrm{~s}^{-1} (Figure S1a, Supporting Information), indicating disassembly of the cellulose fiber bundles into microfibrils. The pseudosolvent penetrated the microfibrils and even into the spaces between adjacent elementary fibrils, which formed translucent, gel-like, and highly
如图 2a 所示，在含有 LiOH ( 2 mg mL − 1 ) LiOH 2 mg mL − 1 LiOH(2mgmL^(-1))\mathrm{LiOH}\left(2 \mathrm{mg} \mathrm{mL}^{-1}\right) 的饱和二甲基亚砜组成的伪溶剂中溶胀 8 小时后，原生unicate 纤维素中出现了大纤维素纤维束（直径为几微米），并转化为直径为几十到几百纳米的微纤维。当溶胀时间延长到 24 小时时，纤维素微纤维的横截面逐渐在纳米尺度上分裂，而纤维素基本纤维（直径为几纳米）的结构在纳米网络中得到了很好的保留。因此，我们利用流变学技术研究了溶胀过程中的粘度变化（图 2b 和图 S1，佐证资料）。随着溶胀时间从 1 小时增加到 8 小时，在 0.1 s − 1 0.1 s − 1 0.1s^(-1)0.1 \mathrm{~s}^{-1} 的剪切速率下，悬浮液的表观粘度从 19.1 Pa s 迅速增加到 450.2 Pa s（图 S1a，佐证资料），表明纤维素纤维束分解成了微纤维。伪溶剂渗入微纤维，甚至渗入相邻基本纤维之间的空隙，形成半透明、凝胶状和高度透明的纤维素纤维束。
Figure 1. Surface engineering strategy for the cellulose nanofibrils (CNFs). a) Exfoliation of cellulose elementary fibrils via a two-step approach and advanced cellulose material assembly via surface deionization. The cyclic anhydrides used for esterification of cellulose include: succinic anhydride (SA), maleic anhydride (MA), phthalic anhydride (PA), octenyl succinic anhydride (OSA), and dodecenylsuccinic anhydride (DSA). b) Photographs of tunicate cellulose pulp, swollen tunicate cellulose, and phthalic anhydride esterified tunicate CNF suspensions. c) TEM image of tunicate CNFs with ultrahigh aspect ratio.
图 1：纤维素纳米纤维（CNFs）的表面工程策略。a) 通过两步法剥离纤维素基本纤维，并通过表面去离子法组装高级纤维素材料。用于酯化纤维素的环状酸酐包括：琥珀酸酐（SA）、马来酸酐（MA）、邻苯二甲酸酐（PA）、辛烯基琥珀酸酐（OSA）和十二烯基丁二酸酐（DSA）。b) 鳞片纤维素浆、膨胀的鳞片纤维素和邻苯二甲酸酐酯化的鳞片状 CNF 悬浮液的照片。
entangled fibrils with high viscosity. As the swelling time was increased further to 8 − 24 h 8 − 24 h 8-24h8-24 \mathrm{~h}, the viscosity of the system gradually decreased (Figure S1b, Supporting Information), which was attributed to disruption of the hydrogen bonds among the elementary fibrils. Because the viscosity hardly changed after swelling for 20 h , we speculated that the cellulose was fully swollen, and the elementary fibrils had more individual characteristics rather than being entangled or cross-linked into bundles. Additionally, the pseudosolvent was allowed to freely diffuse among the elementary fibrils, which enabled the transfer and contact of reagents between the elementary fibrils.
缠结的纤维具有高粘度。随着溶胀时间进一步延长至 8 − 24 h 8 − 24 h 8-24h8-24 \mathrm{~h} ，体系的粘度逐渐下降（图 S1b，佐证资料），这是因为基本纤维之间的氢键被破坏了。由于溶胀 20 h 后粘度几乎没有变化，我们推测纤维素已完全溶胀，基本纤维具有更多的个体特征，而不是纠缠或交联成束。此外，假溶剂可以在基本纤维之间自由扩散，这使得试剂可以在基本纤维之间转移和接触。
Small-angle X-ray scattering (SAXS) measurements were used to characterize the structural changes occurring in the cellulose during swelling in LiOH/DMSO (Figure 2c). Before swelling, a nearly straight double logarithmic SAXS curve for the tunicate
小角 X 射线散射 (SAXS) 测量用于描述纤维素在 LiOH/DMSO 中溶胀过程中发生的结构变化（图 2c）。溶胀前，单胞纤维素的 SAXS 曲线几乎是笔直的双对数曲线。
cellulose decayed with a power law of -3.33 , which was related to the smooth surface of the bundled cellulose consisting of elementary fibrils packed along the axis. Because the intimate proximity between cellulose elementary fibrils decreased the electron density contrast, SAXS measurements may not distinguish the widths of the individual fibrils. [ 36 ] [ 36 ] ^([36]){ }^{[36]} Therefore, the scattering intensity of the sample was attributed to the pore structure rather than the interfaces of the elementary fibrils. After swelling for 8 h or 24 h , knee-shaped lines were observed in scattering patterns because of increases in the spacing and electron density fluctuations at the interfaces between the elementary fibrils. The data were fitted and analyzed with the generalized Guinier-Porod model. [ 37 ] [ 37 ] ^([37]){ }^{[37]} The Porod exponent ( n n nn ) describes the surface morphologies of the scattering particles, e.g., n = 4 n = 4 n=4n=4 for particles with smooth surfaces, and n = 3 n = 3 n=3n=3 indicates a rough surface or
纤维素的衰减幂律为-3.33，这与纤维素束的表面光滑有关，纤维素束由沿轴线排列的基本纤维组成。由于纤维素基本纤维之间的紧密性降低了电子密度对比度，SAXS 测量可能无法区分单个纤维的宽度。 [ 36 ] [ 36 ] ^([36]){ }^{[36]} 因此，样品的散射强度归因于孔隙结构，而不是基本纤维的界面。膨胀 8 小时或 24 小时后，散射图案中出现了膝形线条，这是因为基本纤维之间的间距和界面处的电子密度波动增加了。数据采用广义 Guinier-Porod 模型进行拟合和分析。 [ 37 ] [ 37 ] ^([37]){ }^{[37]} 波罗指数（ n n nn ）描述了散射粒子的表面形态，例如， n = 4 n = 4 n=4n=4 表示表面光滑的粒子， n = 3 n = 3 n=3n=3 表示表面粗糙或粗糙的粒子。
Figure 2. Swelling of cellulose in pseudosolvents. a) Schematic diagram and scanning electron microscope (SEM) images of tunicate cellulose in LiOH / LiOH / LiOH//\mathrm{LiOH} / dimethyl sulfoxide (DMSO). b) Apparent viscosity of tunicate cellulose in LiOH/DMSO under a shear rate of 0.1 s − 1 0.1 s − 1 0.1s^(-1)0.1 \mathrm{~s}^{-1}.c) Small-angle X-ray scattering (SAXS) profiles of tunicate cellulose in LiOH/DMSO with different swelling times. d) Raman spectra of DMSO, tunicate cellulose, and the tunicate cellulose/LiOH/DMSO suspension. e) Wide-angle X-ray scattering (WAXS) profiles of tunicate cellulose before and after swelling.
图 2：纤维素在伪溶剂中的膨胀a) LiOH / LiOH / LiOH//\mathrm{LiOH} / 二甲基亚砜（DMSO）中单纤维素的示意图和扫描电子显微镜（SEM）图像。 b) 0.1 s − 1 0.1 s − 1 0.1s^(-1)0.1 \mathrm{~s}^{-1} 剪切速率下 LiOH/DMSO 中单纤维素的表观粘度。c) 不同溶胀时间下单列纤维素在 LiOH/DMSO 中的小角 X 射线散射（SAXS）图谱。 d) DMSO、单列纤维素和单列纤维素/LiOH/DMSO 悬浮液的拉曼光谱。
“collapsed” polymer chains. In our SAXS measurements, the n n nn values of the samples after swelling for 8 h and 24 h were 3.82 and 3.97, respectively, because the highly crystalline elementary fibrils had smooth surfaces and sharp interfaces after swelling. [ 38 ] [ 38 ] ^([38]){ }^{[38]} Furthermore, the s s ss parameter depended on the shapes and dimensions of the particles (rods: s = 1 s = 1 s=1s=1 and lamellae: s = 2 s = 2 s=2s=2 ). The s s ss values of tunicate cellulose were 1.54 and 1.57 after swelling for 8 h and 24 h , respectively, which was related to the ribbon shape of the CNFs. [ 38 , 39 ] [ 38 , 39 ] ^([38,39]){ }^{[38,39]} Moreover, the calculated diameters of the scattering objects ( D D DD ) for the swollen cellulose were 9.36 and 8.82 after 8 h and 24 h , respectively, which reflected the sizes of the elementary fibrils (Table S2, Supporting Information). These results suggested that the interfaces between elementary fibrils were formed after swelling for 8 h and that the networks composed of cellulose elementary fibrils became looser after swelling for 24 h . Based on the rheological and SAXS results, a swelling time of 24 h was used for further investigation.
"塌缩 "聚合物链。在我们的 SAXS 测量中，样品在溶胀 8 小时和 24 小时后的 n n nn 值分别为 3.82 和 3.97，这是因为高结晶的基本纤维在溶胀后表面光滑，界面锐利。 [ 38 ] [ 38 ] ^([38]){ }^{[38]} 此外， s s ss 参数取决于颗粒的形状和尺寸（棒状： s = 1 s = 1 s=1s=1 和片状： s = 2 s = 2 s=2s=2 ）。膨化 8 小时和 24 小时后，簇状纤维素的 s s ss 值分别为 1.54 和 1.57，这与 CNF 的带状形状有关。 [ 38 , 39 ] [ 38 , 39 ] ^([38,39]){ }^{[38,39]} 此外，膨胀纤维素的散射物体（ D D DD ）的计算直径在 8 小时和 24 小时后分别为 9.36 和 8.82，这反映了基本纤维的尺寸（表 S2，佐证资料）。这些结果表明，纤维素基本纤维之间的界面在溶胀 8 小时后形成，而纤维素基本纤维组成的网络在溶胀 24 小时后变得更加松散。根据流变学和 SAXS 的结果，进一步的研究采用了 24 小时的溶胀时间。
Although LiOH was incompletely soluble in DMSO (Figure S2a,b, Supporting Information), the precipitate disappeared after adding cellulose into the LiOH/DMSO mixture. The Raman peak for DMSO ( S = O ) ( S = O ) (S=O)(\mathrm{S}=\mathrm{O}) shifted from 1048 to 1020 cm − 1 1020 cm − 1 1020cm^(-1)1020 \mathrm{~cm}^{-1} in the spectrum of cellulose/LiOH/DMSO (Figure 2d), indicating that
虽然 LiOH 不完全溶于二甲基亚砜（图 S2a、b，佐证资料），但在 LiOH/DMSO 混合物中加入纤维素后，沉淀消失了。在纤维素/LiOH/DMSO 的光谱中，DMSO 的 ( S = O ) ( S = O ) (S=O)(\mathrm{S}=\mathrm{O}) 拉曼峰从 1048 转移到 1020 cm − 1 1020 cm − 1 1020cm^(-1)1020 \mathrm{~cm}^{-1} （图 2d），这表明
the hydroxyl groups of the cellulose also participated in polar coordination with the LiOH. Therefore, we speculated that stable cellulose/LiOH/DMSO complexes were formed through hydrogen bonding, electrostatic interaction, vdW interaction (Figure S2c, Supporting Information), [ 40 ] [ 40 ] ^([40]){ }^{[40]} and the hydrophobic interaction (weakly polar methyl group of DMSO) that also played a significant role in the intercalation between the stacked cellulose 200 crystal planes. Based on the viscosity (Figure S3a, Supporting Information), swelling of the cellulose in the pseudosolvent was affected by the ion diameters, alkalinity, and solvent polarity, and LiOH/DMSO had the best swelling effect on cellulose (Figure S3b, Supporting Information). The crystallinity of cellulose remained almost unchanged after swelling in the pseudosolvent (Figure 2e and Figure S4, Supporting Information). Besides, we measured the molar mass of cellulose before and after the swelling by using size exclusion chromatography (SEC), where the molecular weight of cellulose changed hardly (Figure S5, Supporting Information). These results indicated that the LiOH/DMSO selectively disassembled the hydrogen bonding and vdW interactions among the cellulose elementary fibrils and maintained the original crystal structures and the degree of polymerization of the cellulose. Compared to other solvents
纤维素的羟基也参与了与 LiOH 的极性配位。因此，我们推测稳定的纤维素/LiOH/DMSO 复合物是通过氢键、静电作用、vdW 作用（图 S2c，佐证资料）、 [ 40 ] [ 40 ] ^([40]){ }^{[40]} 和疏水作用（DMSO 的弱极性甲基）形成的，这些作用在堆叠的纤维素 200 晶面之间的插层中也发挥了重要作用。根据粘度（图 S3a，佐证资料），纤维素在假溶剂中的溶胀受离子直径、碱度和溶剂极性的影响，其中 LiOH/DMSO 对纤维素的溶胀效果最好（图 S3b，佐证资料）。在假溶剂中溶胀后，纤维素的结晶度几乎保持不变（图 2e 和图 S4，佐证资料）。此外，我们还利用尺寸排阻色谱法（SEC）测量了溶胀前后纤维素的摩尔质量，发现纤维素的分子量几乎没有变化（图 S5，佐证资料）。这些结果表明，LiOH/DMSO 选择性地分解了纤维素基本纤维之间的氢键和 vdW 相互作用，保持了纤维素原有的晶体结构和聚合度。与其他溶剂相比
Figure 3. Structure and morphology of cellulose and cellulose nanofibrils (CNFs). a) Fourier transform infrared spectrum (FTIR) and b) solid-state 13 C 13 C ^(13)C{ }^{13} \mathrm{C} NMR spectra of tunicate cellulose and the phthalic anhydride esterified CNFs. c) Wide-angle X-ray scattering (WAXS) profile of the tunicate CNFs. d) Morphology of CNFs exfoliated from different cellulose sources. e) Diameter and length distribution of the tunicate CNFs. f) The aspect ratios of the CNFs in this work are higher than those reported in the literature. g) A radar chart illustrates the advantages of our strategy presented in this work. [ 21 , 29 , 30 ] . [ 21 , 29 , 30 ] . ^([21,29,30].){ }^{[21,29,30] .}
图 3：纤维素和纤维素纳米纤维（CNFs）的结构和形态。a) 单胞纤维素和邻苯二甲酸酐酯化 CNFs 的傅立叶变换红外光谱 (FTIR) 和 b) 固态 13 C 13 C ^(13)C{ }^{13} \mathrm{C} NMR 光谱。 c) 单胞 CNFs 的广角 X 射线散射 (WAXS) 图谱。d) 从不同纤维素来源剥离的 CNFs 的形态。 e) 鳞片状 CNFs 的直径和长度分布。 f) 本研究中 CNFs 的纵横比高于文献报道的纵横比。 g) 雷达图说明了我们在本研究中提出的策略的优势。 [ 21 , 29 , 30 ] . [ 21 , 29 , 30 ] . ^([21,29,30].){ }^{[21,29,30] .}
(Table S3, Supporting Information), the LiOH/DMSO system avoided the negative effects of dissolution, degradation, amorphization, heating, and mechanical treatments (e.g., highpressure homogenization or ultrasonication) on cellulose, resulting in the formation of highly accessible nanoscale networks composed of cellulose elementary fibrils.
(表 S3，佐证资料），LiOH/DMSO 系统避免了溶解、降解、变质、加热和机械处理（如高压均质或超声）对纤维素的负面影响，从而形成了由纤维素基本纤维组成的高度可访问的纳米级网络。

2.2. Structure and Morphology of Cellulose Nanofibrils
2.2.纤维素纳米纤维的结构和形态

After full swelling in the pseudosolvent, phthalic anhydride was used for surface functionalization of the cellulose elementary fibrils. Esterification was carried out without an added catalyst due to the alkalinity of the pseudosolvent, which activated the hydroxyl groups. The yield and carboxylate content of the tunicate CNFs were tailored by varying the reaction conditions (Figure S6, Note S1, Supporting Information). Besides, four other cyclic anhydrides, including succinic anhydride, maleic anhydride, octenylsuccinic anhydride, and dodecenylsuccinic anhydride, were employed to modify tunicate cellulose (Figure S7a, Note S2, Supporting Information). The yields and carboxylate contents of the resultant CNFs were 65.6-97.6% and 0.565 1.179 mmol g − 1 1.179 mmol g − 1 1.179mmolg^(-1)1.179 \mathrm{mmol} \mathrm{g}{ }^{-1}, respectively (Figure S7b, Supporting Information), and the carboxyl contents of the CNFs were determined by conductometric titration (Figure S7c, Supporting Information). After they were added to the cellulose/LiOH/DMSO sus-
在假溶剂中充分溶胀后，使用邻苯二甲酸酐对纤维素基本纤维进行表面功能化。由于伪溶剂的碱性可激活羟基，因此酯化过程无需添加催化剂。通过改变反应条件，可定制单宁酸盐 CNF 的产率和羧酸盐含量（图 S6，注 S1，佐证资料）。此外，还采用了其他四种环酸酐，包括琥珀酸酐、马来酸酐、辛烯基丁二酸酐和十二烯基丁二酸酐来改性单宁酸纤维素（图 S7a，注 S2，证明资料）。CNFs的产率和羧基含量分别为65.6-97.6%和0.565 1.179 mmol g − 1 1.179 mmol g − 1 1.179mmolg^(-1)1.179 \mathrm{mmol} \mathrm{g}{ }^{-1} （图S7b，佐证资料），CNFs的羧基含量通过电导滴定法测定（图S7c，佐证资料）。将 CNFs 加入到纤维素/LiOH/DMSO 的悬浮液中后，用电导滴定法测定其羧基含量（图 S7c，支持信息）。
pension, the cyclic anhydrides preferentially formed ester bonds with hydroxyl groups through ring opening, which introduced negative charges on the surfaces of the CNFs and prohibited reformulation of the hydrogen bonds between the CNFs. Successful esterification was indicated by the Fourier transform infrared spectrum (FTIR), in which the C=O stretching peaks of the ester groups and carboxylate groups appeared at 1724 and 1562 cm − 1 1562 cm − 1 1562cm^(-1)1562 \mathrm{~cm}^{-1}, respectively (Figures 3a and S8, Supporting Information). [ 25 ] [ 25 ] ^([25]){ }^{[25]} Moreover, esterification of the tunicate CNFs with phthalic anhydride was verified by the NMR signals for the carbonyl ester groups (C7) at 178.3 ppm and the aromatic rings (C8-C13) centered at 125.8 ppm in the solid-state 13 C 13 C ^(13)C{ }^{13} \mathrm{C} NMR spectrum (Figure 3b). [ 41 ] [ 41 ] ^([41]){ }^{[41]} After deconvolution of the C4 peak, peaks at 90 and 85 ppm were observed, which were assigned to the inner crystalline core and externally accessible/less accessible surfaces of the CNFs, respectively. [ 42 ] [ 42 ] ^([42]){ }^{[42]} Compared to the native tunicate cellulose, the crystallinities of the CNFs modified by phthalic anhydride decreased slightly from 85.7 % 85.7 % 85.7%85.7 \% to 84.4 % 84.4 % 84.4%84.4 \% (as shown by NMR), while XRD indicated a lower crystallinity of 70.3 % 70.3 % 70.3%70.3 \% for the CNFs; this was attributed to preferential orientation of the CNFs along the X-ray beam during the X-ray diffraction measurements since the peak intensity increased for the 1-10 planes and decreased for the 200 planes (Figure 3c). These signs indicated that the modifications occurred on the fibrillar surfaces and that the cellulose I crystals remained intact in the nanofibrils. Similarly, the crystallinities of the tunicate CNFs determined by XRD
养恤金方面，环状酸酐通过开环优先与羟基形成酯键，这在 CNF 表面引入了负电荷，阻碍了 CNF 之间氢键的重塑。傅立叶变换红外光谱（FTIR）显示酯化成功，其中酯基和羧基的 C=O 伸展峰分别出现在 1724 和 1562 cm − 1 1562 cm − 1 1562cm^(-1)1562 \mathrm{~cm}^{-1} 处（图 3a 和 S8，佐证资料）。 [ 25 ] [ 25 ] ^([25]){ }^{[25]} 此外，在固态 13 C 13 C ^(13)C{ }^{13} \mathrm{C} NMR 光谱中，以 125.8 ppm 为中心的羰基酯基（C7）和芳香环（C8-C13）的 NMR 信号验证了邻苯二甲酸酐对鳞片状 CNF 的酯化作用（图 3b）。 [ 41 ] [ 41 ] ^([41]){ }^{[41]} 对 C4 峰进行解卷积后，观察到 90 和 85 ppm 的峰，这两个峰分别归属于 CNF 的内部晶核和外部可触及/不可触及表面。 [ 42 ] [ 42 ] ^([42]){ }^{[42]} 与原生单胞纤维素相比，邻苯二甲酸酐修饰的 CNF 结晶度略有下降，从 85.7 % 85.7 % 85.7%85.7 \% 降至 84.4 % 84.4 % 84.4%84.4 \% （如 NMR 所示），而 XRD 显示 CNF 的结晶度较低，为 70.3 % 70.3 % 70.3%70.3 \% ；这归因于在 X 射线衍射测量过程中 CNF 沿着 X 射线束的优先取向，因为 1-10 平面的峰值强度增加了，而 200 平面的峰值强度降低了（图 3c）。这些迹象表明，改性发生在纤维表面，而纤维素 I 晶体在纳米纤维中保持完整。同样，通过 XRD 测定的单胞氯化纤维素的结晶度为
decreased to 51.1-67.6% after modification with other cyclic anhydrides (Figure S9, Supporting Information). The yields and chemical and crystal structures of CNFs from other bioresources, such as softwood, cotton, bagasse, and bacteria, were also characterized (Figures S10 and S11 and Table S4, Supporting Information), revealing that the cyclic anhydrides were homogeneously grafted onto the surface hydroxyls of the CNFs, and the crystalline structures of the elementary fibrils were well preserved.
用其他环酐改性后，CNF 的产率降至 51.1-67.6%（图 S9，佐证资料）。对其他生物资源（如软木、棉花、甘蔗渣和细菌）的 CNF 产量、化学结构和晶体结构也进行了表征（图 S10 和 S11 以及表 S4，"支持信息"），结果表明环酐均匀地接枝到 CNF 的表面羟基上，基本纤维的晶体结构保存完好。
The morphologies of CNFs exfoliated from different cellulose sources via modification with phthalic anhydride are shown in Figure 3d. All of the raw cellulose materials were converted into 1D nanofibrils, and the diameters and lengths were measured by AFM (Figure S12, Supporting Information) and statistically analyzed with “FiberApp” software (Figure S13, Supporting Information). For the tunicate cellulose, the average diameter and length of the CNFs were approximately 7.1 nm and 9.8 μ m 9.8 μ m 9.8 mum9.8 \mu \mathrm{~m}, respectively, demonstrating an ultrahigh aspect ratio of approximately 1400 (Figure 3e). Similarly, the tunicate CNFs esterified with other anhydrides also exhibited ultrahigh aspect ratios of 1000-1300 (Figures S14 and S15, Supporting Information). It is worth noting that the average length of PA esterified CNFs (tunicate) significantly decreased to 3.4 μ m 3.4 μ m 3.4 mum3.4 \mu \mathrm{~m} after being treated under 500 W ultrasound for 20 min (Figure S16, Supporting Information). We also confirmed that the average length of TEMPO oxidized CNFs (tunicate) was 3.8 μ m 3.8 μ m 3.8 mum3.8 \mu \mathrm{~m} (Figure S17, Supporting Information), due to the high-energy mechanical processing. [ 43 ] [ 43 ] ^([43]){ }^{[43]} In the case of bacterial cellulose, the average diameter and length of the CNFs were approximately 18.6 nm and 21 μ m 21 μ m 21 mum21 \mu \mathrm{~m}, respectively, which were much higher than those of the CNFs from plant-derived cellulose. For instance, the CNFs from softwood, cotton, and bagasse had smaller diameters ( 2 − 5 nm 2 − 5 nm 2-5nm2-5 \mathrm{~nm} ) and aspect ratios (300-500), which were consistent with the reported cellulose elementary fibrils. [ 1 , 44 ] [ 1 , 44 ] ^([1,44]){ }^{[1,44]} The difference in the dimensions of various CNFs was attributed to the diversity of cellulose samples synthesized in different biological sources, which also corresponded to the results from wide-angle X-ray scattering (WAXS) characterization (Table S4, Supporting Information). Because strong mechanical disintegration (such as high-pressure homogenization, ball milling, or ultrasound) was avoided, the tunicate CNFs produced with our strategy exhibited higher aspect ratios than those prepared by other methods, such as enzymolysis, carboxymethylation, acid hydrolysis, TEMPO oxidation, and periodate oxidization (Figure 3f and Table S5, Supporting Information). Moreover, the pseudosolvent/reactants and ethanol used for the preparation of CNFs could be recycled with a high recovery ratio, while CNFs could be fabricated by using recycled reagents (Figure S18, Supporting Information). Therefore, this work provides a convenient, flexible, energy-efficient ( 12.4 kJ g − 1 12.4 kJ g − 1 12.4kJg^(-1)12.4 \mathrm{~kJ} \mathrm{~g}^{-1} ), and sustainable method for producing CNFs with high yields ( ≈ 98 % ≈ 98 % ~~98%\approx 98 \% ) and ultrahigh aspect ratios (Figure 3g and Note S3, Supporting Information).
图 3d 显示了从不同纤维素来源经邻苯二甲酸酐改性剥离的 CNFs 的形态。将所有纤维素原料转化为一维纳米纤维，用原子力显微镜测量其直径和长度（图 S12，佐证资料），并用 "FiberApp "软件进行统计分析（图 S13，佐证资料）。对于unicate纤维素，CNFs的平均直径和长度分别约为7.1 nm和 9.8 μ m 9.8 μ m 9.8 mum9.8 \mu \mathrm{~m} ，显示出约1400的超高纵横比（图3e）。同样，与其他酸酐酯化的鳞片状 CNF 也表现出 1000-1300 的超高纵横比（图 S14 和 S15，佐证资料）。值得注意的是，在 500 W 超声波下处理 20 分钟后，PA 酯化 CNFs（unicate）的平均长度明显降低至 3.4 μ m 3.4 μ m 3.4 mum3.4 \mu \mathrm{~m} （图 S16，佐证资料）。我们还证实，由于高能机械加工，TEMPO 氧化 CNFs（unicate）的平均长度为 3.8 μ m 3.8 μ m 3.8 mum3.8 \mu \mathrm{~m} （图 S17，佐证资料）。 [ 43 ] [ 43 ] ^([43]){ }^{[43]} 细菌纤维素的 CNFs 平均直径和长度分别约为 18.6 nm 和 21 μ m 21 μ m 21 mum21 \mu \mathrm{~m} ，远高于植物纤维素的 CNFs。例如，来自软木、棉花和甘蔗渣的 CNFs 的直径（ 2 − 5 nm 2 − 5 nm 2-5nm2-5 \mathrm{~nm} ）和纵横比（300-500）都较小，这与已报道的纤维素基本纤维一致。 [ 1 , 44 ] [ 1 , 44 ] ^([1,44]){ }^{[1,44]} 不同 CNFs 的尺寸差异归因于不同生物来源合成的纤维素样品的多样性，这也与广角 X 射线散射 (WAXS) 表征的结果相符（表 S4，佐证资料）。由于避免了强烈的机械分解（如高压均质、球磨机或超声波），与其他方法（如酶解、羧甲基化、酸水解、TEMPO 氧化和高碘酸盐氧化）相比，用我们的策略制备的鳞片状 CNF 具有更高的纵横比（图 3f 和表 S5，佐证资料）。此外，用于制备 CNFs 的假溶剂/反应物和乙醇可回收利用，回收率高，同时可利用回收试剂制备 CNFs（图 S18，佐证资料）。因此，这项工作为制备高产率（ ≈ 98 % ≈ 98 % ~~98%\approx 98 \% ）和超高纵横比的 CNFs 提供了一种方便、灵活、节能（ 12.4 kJ g − 1 12.4 kJ g − 1 12.4kJg^(-1)12.4 \mathrm{~kJ} \mathrm{~g}^{-1} ）和可持续的方法（图 3g 和注 S3，佐证资料）。

2.3. Colloidal Stability and Processability of Cellulose Nanofibrils
2.3.纤维素纳米纤维的胶体稳定性和可加工性

Due to the presence of negative charges, the CNFs were homogeneously dispersed in water. For example, the benzoic acid groups provided negative charges in the CNFs, which could be protonated at low pH ( < 6 ) pH ( < 6 ) pH( < 6)\mathrm{pH}(<6) and saponified at high pH ( > 11 ) pH ( > 11 ) pH( > 11)\mathrm{pH}(>11) (Figure 4a).
由于存在负电荷，CNF 在水中均匀分散。例如，CNF 中的苯甲酸基团可提供负电荷，在低 pH ( < 6 ) pH ( < 6 ) pH( < 6)\mathrm{pH}(<6) 时可质子化，在高 pH ( > 11 ) pH ( > 11 ) pH( > 11)\mathrm{pH}(>11) 时可皂化（图 4a）。
The former process was reversible and rapidly completed within 1 s , while the latter process was irreversible and slow (maintain fluidity within 4 h and complete gelation after 12 h at pH 13 ). For pH < 6 pH < 6 pH < 6\mathrm{pH}<6 or > 11 > 11 > 11>11, the CNF suspensions were unstable and gelation even occurred with a low concentration of 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% because the electrostatic repulsions among the CNFs were eliminated. Under stable conditions, the zeta potential of the CNF suspension decreased from -24.9 to -40.0 mV as the pH was increased from 6 to 11 (Figure 4b). All hydrophilic CNFs ( 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ) were highly stable in water, and their optical transmittance changed only slightly after storage for 120 days (Figure S19a,b, Supporting Information). In addition, the CNFs were dispersed in other polar solvents, such as DMSO, N, N-dimethylformamide (DMF), and DMAc (Figure S19c, Supporting Information).
前一个过程是可逆的，并在 1 秒内迅速完成，而后一个过程是不可逆的，且速度较慢（在 pH 值为 13 的条件下，4 小时内保持流动性，12 小时后完全凝胶化）。对于 pH < 6 pH < 6 pH < 6\mathrm{pH}<6 或 > 11 > 11 > 11>11 ，CNF 悬浮液不稳定，甚至在 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% 浓度较低时也会发生凝胶化，因为 CNF 之间的静电排斥被消除了。在稳定条件下，随着 pH 值从 6 升至 11，CNF 悬浮液的 zeta 电位从 -24.9 mV 降至 -40.0 mV（图 4b）。所有亲水性 CNF（ 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ）在水中都非常稳定，储存 120 天后其透光率仅有轻微变化（图 S19a、b，佐证资料）。此外，CNFs 还分散在其他极性溶剂中，如 DMSO、N, N-二甲基甲酰胺 (DMF) 和 DMAc（图 S19c，佐证资料）。
To exploit shear-thinning (Figure S20a, Supporting Information) and acid-induced gelation of PA esterified tunicate CNF suspensions, various patterns comprising protonated CNFs were fabricated by printing the CNF inks onto the surfaces of fabrics wetted with 0.1 m HCl (Figure 4c). Flexible gels formed rapidly after the CNF inks contacted the acid, which could be bent or peeled from the substrate. The chemical conversions of − COO − − COO − -COO^(-)-\mathrm{COO}^{-} to - COOH groups on the CNF surface were confirmed by a shift of the C-O peak from 1562 to 1730 cm − 1 1730 cm − 1 1730cm^(-1)1730 \mathrm{~cm}^{-1} (Figure S21a, Supporting Information). The crystallinity of the protonated CNFs increased slightly to 74.4 % 74.4 % 74.4%74.4 \%, possibly due to the preferred orientation of the crystalline 110 planes (Figure S21b and Table S4, Supporting Information). On the other hand, CNF hydrogels with different shapes were fabricated by injecting the suspension ( pH = 13 ( pH = 13 (pH=13(\mathrm{pH}=13 ) into various molds (Figure 4d). The negatively charged ester groups on the surfaces of the CNFs were completely removed by saponification, which was confirmed by the disappearance of the C-O peaks for carboxyl, carboxylate, and ester groups in the FTIR spectrum of the saponified CNFs (Figure S21a, Supporting Information). Moreover, the WAXS profile of saponified CNFs showed high crystallinity without a visible amorphous region, which was comparable to that of native tunicate cellulose (Figure S21b and Table S3, Supporting Information). After ≈ 4 h ≈ 4 h ~~4h\approx 4 \mathrm{~h}, translucent and tough hydrogels consisting of saponified CNFs were obtained by washing with distilled water, and they maintained stable shapes in acidic, alkaline, and salty environments. These results revealed that saponification formed a physically cross-linked hydrogel and restored the surface hydrogen bonding and vdW interactions of the native celluloses.
为了利用 PA 酯化鳞片状 CNF 悬浮液的剪切稀化（图 S20a，佐证资料）和酸诱导凝胶化，我们将 CNF 油墨印刷到用 0.1 m HCl 润湿的织物表面，从而制作出了由质子化 CNF 组成的各种图案（图 4c）。CNF 油墨与酸接触后迅速形成柔性凝胶，可以弯曲或从基材上剥离。CNF表面的 − COO − − COO − -COO^(-)-\mathrm{COO}^{-} 基团转化为- COOH基团的化学反应通过C-O峰从1562移动到 1730 cm − 1 1730 cm − 1 1730cm^(-1)1730 \mathrm{~cm}^{-1} 得到证实（图S21a，佐证资料）。质子化 CNF 的结晶度略有增加，达到 74.4 % 74.4 % 74.4%74.4 \% ，这可能是由于结晶 110 平面的优先取向所致（图 S21b 和表 S4，佐证资料）。另一方面，将 ( pH = 13 ( pH = 13 (pH=13(\mathrm{pH}=13 )悬浮液注入不同的模具中，可制成不同形状的 CNF 水凝胶（图 4d）。通过皂化，CNF 表面带负电荷的酯基被完全去除，皂化后 CNF 的傅立叶变换红外光谱中羧基、羧酸基和酯基的 C-O 峰消失也证实了这一点（图 S21a，佐证资料）。此外，皂化氯化萘纤维素的 WAXS 图谱显示出较高的结晶度，没有可见的无定形区，与原生鳞片纤维素的结晶度相当（图 S21b 和表 S3，佐证资料）。 ≈ 4 h ≈ 4 h ~~4h\approx 4 \mathrm{~h} 后，用蒸馏水洗涤可得到由皂化CNFs组成的半透明且坚韧的水凝胶，它们在酸、碱和盐环境中都能保持稳定的形状。 这些结果表明，皂化形成了物理交联的水凝胶，并恢复了原生纤维素的表面氢键和 vdW 相互作用。

2.4. Cellulosic Materials Reconstructed by Surface Engineering of Cellulose Nanofibrils
2.4.通过纤维素纳米纤维的表面工程重建纤维素材料

Due to the ultrahigh aspect ratios and reversible surface functionalizations, PA esterified CNFs (tunicate) can serve as advanced building blocks in constructing various cellulosic materials (such as 1D filaments, 2D films, and 3D aerogels) through surface engineering (saponification). As shown in Figure 5a, strong filaments were fabricated by wet spinning, in which the CNF suspension ( 1.5 wt % 1.5 wt % 1.5wt%1.5 \mathrm{wt} \% ) was spun into acetone for coagulation, followed by saponification in an alkaline aqueous solution (Figure S22a, Supporting Information). After stretching and air-drying processes, the ordered CNFs aligned along the axis direction were retained in the filament through reconstruction of the hydrogen bonds
由于 PA 酯化 CNFs（tunicate）具有超高的长径比和可逆的表面功能化特性，因此可作为先进的构建模块，通过表面工程（皂化）构建各种纤维素材料（如一维细丝、二维薄膜和三维气凝胶）。如图 5a 所示，强力纤维丝是通过湿法纺丝制造的，将 CNF 悬浮液（ 1.5 wt % 1.5 wt % 1.5wt%1.5 \mathrm{wt} \% ）纺入丙酮中进行凝固，然后在碱性水溶液中进行皂化（图 S22a，佐证资料）。经过拉伸和风干过程后，沿轴线方向排列的有序 CNF 通过氢键重建保留在丝状物中。
Figure 4. Colloidal stability and processability of tunicate cellulose nanofibrils (CNFs). a) Schematic diagram of the surface structure changes of the CNFs under different pH values. For the different structures of CNFs, they are defined as CNF ( COONa ) , CNF ( COOH ) CNF ( COONa ) , CNF ( COOH ) CNF_((COONa)),CNF_((COOH))\mathrm{CNF}_{(\mathrm{COONa})}, \mathrm{CNF}_{(\mathrm{COOH})}, and CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} under ionized state, acid protonation, and alkali saponification treatments, respectively. b) Zeta potential of phthalic anhydride esterified CNF suspension ( 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ) under different pH values. c) Various profiles formed by printing the CNF suspension. d) CNF hydrogels prepared by saponification.
图 4.a) 不同 pH 值下 CNFs 表面结构变化示意图。不同结构的 CNF 在离子态、酸质子化和碱皂化处理下分别定义为 CNF ( COONa ) , CNF ( COOH ) CNF ( COONa ) , CNF ( COOH ) CNF_((COONa)),CNF_((COOH))\mathrm{CNF}_{(\mathrm{COONa})}, \mathrm{CNF}_{(\mathrm{COOH})} 和 CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 。 b) 邻苯二甲酸酐酯化的 CNF 悬浮液（ 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ）在不同 pH 值下的 Zeta 电位。 c) 印刷 CNF 悬浮液形成的各种轮廓。
between the saponified CNFs (Figure 5b). The Herman’s orientation factors ( f c ) f c (f_(c))\left(f_{c}\right) of the 1 − 10 , 110 1 − 10 , 110 1-10,1101-10,110, and 200 planes of cellulose I in the filament were 0.57 , 0.51 0.57 , 0.51 0.57,0.510.57,0.51, and 0.52 , respectively (Figure S23, Supporting Information), indicating the uniaxial orientation of the CNFs. The resulting filaments exhibited high mechanical strengths and good flexibilities, and they could lift a weight of 100 g and undergo complex deformations (Figure S22b, Supporting Information). The effects of the spinning rate and stretching ratio used during the preparation process on the mechanical performance of the CNF filaments were also investigated. The CNF filaments showed the highest tensile strength ( 331 ± 331 ± 331+-331 \pm 22 MPa ) and modulus ( 14.5 ± 1.1 GPa 14.5 ± 1.1 GPa 14.5+-1.1GPa14.5 \pm 1.1 \mathrm{GPa} ) with a spinning rate of 100 mm min − 1 100 mm min − 1 100mmmin^(-1)100 \mathrm{~mm} \mathrm{~min}^{-1} (Figure S24a, Supporting Information). Additionally, the tensile strength and modulus of the CNF filament with a stretching ratio of 30 % 30 % 30%30 \% were 641 ± 15 MPa 641 ± 15 MPa 641+-15MPa641 \pm 15 \mathrm{MPa} and 52.9 ± 4.2 GPa 52.9 ± 4.2 GPa 52.9+-4.2GPa52.9 \pm 4.2 \mathrm{GPa}, respectively (Figure S24b, Supporting Information), which surpassed the mechanical data for filaments composed of unsaponified CNFs or protonated CNFs (Figure 5c,d). The arrangement of CNFs contributed significantly to the mechanical strengths, i.e., if these nanofibrils with high aspect ratios could be oriented to an even higher extent, much higher strength values would be obtained [ 45 − 47 ] [ 45 − 47 ] ^([45-47]){ }^{[45-47]} Furthermore, denser morphologies on the sur-
皂化的 CNF 之间（图 5b）。纤维丝中纤维素 I 的 1 − 10 , 110 1 − 10 , 110 1-10,1101-10,110 和 200 平面的赫尔曼取向因子 ( f c ) f c (f_(c))\left(f_{c}\right) 分别为 0.57 , 0.51 0.57 , 0.51 0.57,0.510.57,0.51 和 0.52（图 S23，佐证资料），表明 CNFs 具有单轴取向。所制备的长丝具有很高的机械强度和良好的柔韧性，可承受 100 克的重量并发生复杂的变形（图 S22b，佐证资料）。此外，还研究了制备过程中使用的纺丝速率和拉伸比对 CNF 长丝机械性能的影响。纺丝速率为 100 mm min − 1 100 mm min − 1 100mmmin^(-1)100 \mathrm{~mm} \mathrm{~min}^{-1} 时，CNF 长丝的拉伸强度（ 331 ± 331 ± 331+-331 \pm 22 MPa）和模量（ 14.5 ± 1.1 GPa 14.5 ± 1.1 GPa 14.5+-1.1GPa14.5 \pm 1.1 \mathrm{GPa} ）最高（图 S24a，佐证资料）。此外，拉伸率为 30 % 30 % 30%30 \% 的CNF长丝的拉伸强度和模量分别为 641 ± 15 MPa 641 ± 15 MPa 641+-15MPa641 \pm 15 \mathrm{MPa} 和 52.9 ± 4.2 GPa 52.9 ± 4.2 GPa 52.9+-4.2GPa52.9 \pm 4.2 \mathrm{GPa} （图S24b，佐证资料），超过了由未皂化CNF或质子化CNF组成的长丝的力学数据（图5c,d）。CNF 的排列对机械强度有很大的影响，也就是说，如果这些高纵横比的纳米纤维能够定向到更高的程度，就能获得更高的强度值 [ 45 − 47 ] [ 45 − 47 ] ^([45-47]){ }^{[45-47]} 。
faces of CNF filaments were observed in comparison with other CNF filaments (Figure S25, Supporting Information). These results suggested that interfacial cohesion between the CNFs was significantly improved after saponification due to recovery of the native interfacial supramolecular interactions, which also maximized the bulk density.
与其他 CNF 细丝相比，CNF 细丝的表面更光滑（图 S25，佐证资料）。这些结果表明，由于恢复了原生界面超分子相互作用，皂化后 CNF 之间的界面内聚力得到了显著改善，同时也最大限度地提高了体积密度。
In addition to anisotropic 1D filaments, isotropic 2D films consisting of interwoven tunicate CNFs were prepared through vacuum-assisted filtration (Figure S26a, Supporting Information). The resultant films exhibited both high transparency and high haze values as well as excellent flexibilities (Figure 5e and Figure S26b,c, Supporting Information). Compared to unsaponified CNF films, the densification of the CNF films was significantly increased by mild posttreatment (Figure 5f and Figure S26d, Supporting Information), which improved the mechanical strengths of the films. As shown in Figure 5g, the tensile strength of the CNF film was 372 ± 6.8 MPa 372 ± 6.8 MPa 372+-6.8MPa372 \pm 6.8 \mathrm{MPa}, which was higher than those of an unsaponified CNF film ( 126 ± 4.6 MPa 126 ± 4.6 MPa 126+-4.6MPa126 \pm 4.6 \mathrm{MPa} ) or protonated CNF film ( 194 ± 20 MPa 194 ± 20 MPa 194+-20MPa194 \pm 20 \mathrm{MPa} ), and it was also superior to those of other isotropic films made from natural polymers such as cellulose, chitin, and silk fibroin (Table S6, Supporting Information). The CNF film consisting of saponified CNFs exhibited
除了各向异性的一维细丝外，还通过真空辅助过滤制备了由交织的鳞片状 CNF 组成的各向同性二维薄膜（图 S26a，佐证资料）。制备出的薄膜具有高透明度、高雾度值和优异的柔韧性（图 5e 和图 S26b、c，《证明资料》）。与未皂化的 CNF 薄膜相比，温和的后处理显著提高了 CNF 薄膜的致密性（图 5f 和图 S26d，佐证资料），从而改善了薄膜的机械强度。如图 5g 所示，CNF 薄膜的拉伸强度为 372 ± 6.8 MPa 372 ± 6.8 MPa 372+-6.8MPa372 \pm 6.8 \mathrm{MPa} ，高于未皂化的 CNF 薄膜（ 126 ± 4.6 MPa 126 ± 4.6 MPa 126+-4.6MPa126 \pm 4.6 \mathrm{MPa} ）或质子化的 CNF 薄膜（ 194 ± 20 MPa 194 ± 20 MPa 194+-20MPa194 \pm 20 \mathrm{MPa} ），也优于由纤维素、甲壳素和蚕丝纤维素等天然聚合物制成的其他各向同性薄膜（表 S6，佐证资料）。由皂化的 CNF 组成的 CNF 薄膜表现出

such as bending and twisting (Figure S28c, Supporting Information). The resultant aerogels exhibited hierarchical porous structures (Figure 5 j 5 j 5j5 j and Figure S29, Supporting Information), and the porosity and density were easily regulated by tuning the concentration of the CNF suspension (Figure 5k). Benefiting from the ultrahigh aspect ratios and mechanical features of the cellulose elementary fibrils, the porosities and densities of the CNF aerogels reached 99.9 % 99.9 % 99.9%99.9 \% and 1.95 mg cm − 3 1.95 mg cm − 3 1.95mgcm^(-3)1.95 \mathrm{mg} \mathrm{cm}^{-3}, respectively. In addition, unlike aerogels composed of unsaponified or protonated CNFs, denser layered structures were observed on the pore walls of the saponified CNF aerogel (Figure S30, Supporting Information).
例如弯曲和扭曲（图 S28c，佐证资料）。由此产生的气凝胶呈现出层次分明的多孔结构（图 5 j 5 j 5j5 j 和图S29，《证明资料》），而且孔隙率和密度很容易通过调节CNF悬浮液的浓度来调节（图5k）。得益于纤维素基本纤维的超高长径比和机械特性，CNF气凝胶的孔隙率和密度分别达到了 99.9 % 99.9 % 99.9%99.9 \% 和 1.95 mg cm − 3 1.95 mg cm − 3 1.95mgcm^(-3)1.95 \mathrm{mg} \mathrm{cm}^{-3} 。此外，与未皂化或质子化 CNF 组成的气凝胶不同，在皂化 CNF 气凝胶的孔壁上观察到了更致密的层状结构（图 S30，佐证资料）。
Based on these advantages, the resultant CNF aerogels were used for thermal insulation in extreme environments, and they exhibited better thermal stabilities than CNFs prepared by TEMPO oxidation. In particular, the saponified CNFs showed thermal behavior similar to that of native cellulose (Figure S31a, Supporting Information). To evaluate their thermal insulation capacities, the CNF aerogels were placed on a hot stage ( 120 ∘ C ) 120 ∘ C (120^(@)C)\left(120{ }^{\circ} \mathrm{C}\right) and cold plate ( − 196 ∘ C ) − 196 ∘ C (-196^(@)C)\left(-196^{\circ} \mathrm{C}\right) for 60 min , and the temperature variations were recorded and plotted (Figure S31b, Supporting Information). In the hot stage, the surface temperature of the CNF
基于这些优点，所制备的氯化萘纤维气凝胶可用于极端环境下的隔热材料，与通过 TEMPO 氧化制备的氯化萘纤维相比，它们表现出更好的热稳定性。特别是，皂化的 CNF 表现出与原生纤维素相似的热行为（图 S31a，佐证资料）。为了评估它们的隔热能力，将 CNF 气凝胶放在热台 ( 120 ∘ C ) 120 ∘ C (120^(@)C)\left(120{ }^{\circ} \mathrm{C}\right) 和冷板 ( − 196 ∘ C ) − 196 ∘ C (-196^(@)C)\left(-196^{\circ} \mathrm{C}\right) 上 60 分钟，记录并绘制温度变化曲线（图 S31b，佐证资料）。在热阶段，CNF 的表面温度为
the highest elastic modulus ( 12.1 ± 0.83 GPa 12.1 ± 0.83 GPa 12.1+-0.83GPa12.1 \pm 0.83 \mathrm{GPa} ) and toughness ( 11 ± 0.26 MJ m − 3 ) 11 ± 0.26 MJ m − 3 (11+-0.26MJm^(-3))\left(11 \pm 0.26 \mathrm{MJ} \mathrm{m}^{-3}\right) (Figure 5h). Due to the surface engineering, the CNF film showed good water resistance, with a tensile strength, elastic modulus, and toughness of 49.1 ± 0.22 MPa 49.1 ± 0.22 MPa 49.1+-0.22MPa49.1 \pm 0.22 \mathrm{MPa}, 0.58 ± 0.024 GPa 0.58 ± 0.024 GPa 0.58+-0.024GPa0.58 \pm 0.024 \mathrm{GPa}, and 2.45 ± 0.25 MJ m − 3 2.45 ± 0.25 MJ m − 3 2.45+-0.25MJm^(-3)2.45 \pm 0.25 \mathrm{MJ} \mathrm{m}^{-3}, respectively, in the wet state (Figure S27a, Supporting Information). Moreover, the thickness of the CNF film changed only slightly after immersion in water for 1 h , but the unsaponified CNF film swelled like a gel cake, and the thickness increased five times (Figure S27b, Supporting Information). The cross-sectional morphologies of the CNF films were also compared, and only the saponified CNF film still exhibited a dense structure after swelling (Figure S27c, Supporting Information).
弹性模量（ 12.1 ± 0.83 GPa 12.1 ± 0.83 GPa 12.1+-0.83GPa12.1 \pm 0.83 \mathrm{GPa} ）和韧性 ( 11 ± 0.26 MJ m − 3 ) 11 ± 0.26 MJ m − 3 (11+-0.26MJm^(-3))\left(11 \pm 0.26 \mathrm{MJ} \mathrm{m}^{-3}\right) 最高（图 5h）。由于表面工程的作用，CNF 膜表现出良好的耐水性，在湿态下的拉伸强度、弹性模量和韧性分别为 49.1 ± 0.22 MPa 49.1 ± 0.22 MPa 49.1+-0.22MPa49.1 \pm 0.22 \mathrm{MPa} 、 0.58 ± 0.024 GPa 0.58 ± 0.024 GPa 0.58+-0.024GPa0.58 \pm 0.024 \mathrm{GPa} 和 2.45 ± 0.25 MJ m − 3 2.45 ± 0.25 MJ m − 3 2.45+-0.25MJm^(-3)2.45 \pm 0.25 \mathrm{MJ} \mathrm{m}^{-3} （图 S27a，佐证资料）。此外，CNF 薄膜在水中浸泡 1 小时后厚度变化不大，但未皂化的 CNF 薄膜像凝胶饼一样膨胀，厚度增加了五倍（图 S27b，佐证资料）。我们还比较了 CNF 薄膜的横截面形态，只有皂化的 CNF 薄膜在膨胀后仍然呈现出致密的结构（图 S27c，佐证资料）。
Based on the ultrahigh aspect ratios of the tunicate CNFs and the high storage modulus of the CNF suspension (Figure S20a,b, Note S4, Supporting Information), 3D lightweight aerogels were fabricated by sequential saponification, tert-butanol exchange, and freeze-drying of the CNF suspension (Figure 5i and Figure S28a, Supporting Information). Although the concentrations of the CNF suspensions were as low as 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \%, robust aerogels with different shapes were obtained (Figure S28b, Supporting Information), and they underwent large deformations,
基于单列CNF的超高长径比和CNF悬浮液的高储存模量（图S20a,b，注S4，佐证资料），通过对CNF悬浮液进行连续皂化、叔丁醇交换和冷冻干燥，制备出了三维轻质气凝胶（图5i和图S28a，佐证资料）。虽然CNF悬浮液的浓度低至 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ，但还是得到了不同形状的坚固气凝胶（图S28b，佐证资料），而且它们发生了很大的变形、
Figure 5. Cellulosic materials reconstructed by surface engineering of cellulose nanofibrils (CNFs). a) Photograph and b) scanning electron microscope (SEM) image of the tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} filaments. c) Stress-strain curves and d) the corresponding mechanical parameters of various tunicate CNF filaments. e) Photograph and f) SEM image of the tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} film. g) Stress-strain curves and h) the corresponding mechanical parameters of various tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} films. i) Photograph and j) SEM image of the tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} aerogel. k) Density and porosity of aerogels as a function of tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} concentrations. I) Surface temperature-time curves of the tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} aerogels on the hot stage ( 120 ∘ C ) 120 ∘ C (120^(@)C)\left(120^{\circ} \mathrm{C}\right) and the cold plate ( − 196 ∘ C ) − 196 ∘ C (-196^(@)C)\left(-196^{\circ} \mathrm{C}\right) for 60 min , respectively.
图 5.通过纤维素纳米纤丝（CNFs）表面工程重构的纤维素材料。 a) tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 细丝的照片和 b) 扫描电子显微镜（SEM）图像。g) 各种unicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 薄膜的应力-应变曲线和 h) 相应的机械参数。 i) tunicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 气凝胶的照片和 j) SEM 图像。）气凝胶的密度和孔隙率与unicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 浓度的函数关系。I) 分别在热平台 ( 120 ∘ C ) 120 ∘ C (120^(@)C)\left(120^{\circ} \mathrm{C}\right) 和冷板 ( − 196 ∘ C ) − 196 ∘ C (-196^(@)C)\left(-196^{\circ} \mathrm{C}\right) 上 60 分钟的unicate CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 气凝胶表面温度-时间曲线。
aerogel increased rapidly in the first 5 min and then remained at 55 ∘ C 55 ∘ C 55^(@)C55^{\circ} \mathrm{C} for 60 min (Figure 51). Similarly, the surface temperature of the CNF aerogel decreased rapidly to 20 ∘ C 20 ∘ C 20^(@)C20^{\circ} \mathrm{C} in the first 5 min and then hardly changed within 60 min in the cold stage. These results indicated that CNF aerogels with both high porosities and good thermal stabilities were excellent thermal insulators at extreme temperatures ( − 196 ∘ C − 196 ∘ C (-196^(@)C:}\left(-196^{\circ} \mathrm{C}\right. or 120 ∘ C ) 120 ∘ C {:120^(@)C)\left.120^{\circ} \mathrm{C}\right).
气凝胶的表面温度在最初 5 分钟内迅速升高，然后在 60 分钟内保持在 55 ∘ C 55 ∘ C 55^(@)C55^{\circ} \mathrm{C} （图 51）。同样，CNF 气凝胶的表面温度在最初 5 分钟内迅速降至 20 ∘ C 20 ∘ C 20^(@)C20^{\circ} \mathrm{C} ，然后在冷阶段的 60 分钟内几乎没有变化。这些结果表明，在极端温度 ( − 196 ∘ C − 196 ∘ C (-196^(@)C:}\left(-196^{\circ} \mathrm{C}\right. 或 120 ∘ C ) 120 ∘ C {:120^(@)C)\left.120^{\circ} \mathrm{C}\right) 下，具有高孔隙率和良好热稳定性的CNF气凝胶是极好的热绝缘体。

3. Conclusion 3.结论

We demonstrated that surface engineering is a facile method for exfoliation of cellulose elementary fibrils from various cellulosic sources and reconstruction of the advanced cellulosic materials via restoration of the hydrogen bonds and vdW networks among the CNFs. After swelling in the LiOH/DMSO system and reacting with a cyclic anhydride, the elementary fibrils were liberated from the cellulose fibers to produce CNFs with ultrahigh aspect ratios ( ≈ 1400 ≈ 1400 ~~1400\approx 1400 ) and reversibly functionalized surfaces. Because strong chemical and mechanical treatments were not required, this strategy for isolating CNFs exhibited high yields, energy efficiency, scalability, fewer equipment requirements, and mild conditions. The resultant CNFs served as the basic building blocks for 1D filaments, 2D films, and 3D aerogels through saponification, and the hydroxyl groups on the surfaces of the CNFs were recovered and the hydrogen bonding networks and raw interfacial vdW interactions were reconstructed, resulting in strong cellulosic materials. Reversible surface engineering of the cellulose elementary fibrils offers a novel strategy for preparing ultralong CNFs and provides a platform for developing strong cellulosic materials.
我们证明了表面工程是一种简便的方法，可从各种纤维素来源中剥离纤维素基本纤维，并通过恢复 CNFs 之间的氢键和 vdW 网络重建高级纤维素材料。在LiOH/DMSO体系中溶胀并与环酸酐反应后，基本纤维从纤维素纤维中释放出来，生成具有超高纵横比（ ≈ 1400 ≈ 1400 ~~1400\approx 1400 ）和可逆功能化表面的CNFs。由于不需要强烈的化学和机械处理，这种分离 CNFs 的策略具有产量高、能效高、可扩展性强、设备要求少和条件温和等优点。通过皂化，CNFs 表面的羟基得以恢复，氢键网络和原始界面 vdW 相互作用得以重建，从而形成了强纤维素材料。纤维素基本纤维的可逆表面工程为制备超长 CNFs 提供了一种新策略，并为开发强纤维素材料提供了一个平台。

4. Experimental Section 4.实验部分

Materials: Tunicate (Halocynthia roretzi Drasche) was purchased from Weihai Evergreen Marine Science and Technology Co., Ltd. (Shandong, China). Bagasse (cellulose 87.7%, hemicellulose 9.8 % 9.8 % 9.8%9.8 \% and lignin 2.5%) was purchased from Wuda Ziqiang supermarket at Wuhan University. Both tunicate and bagasse cellulose were isolated according to the previous work. [ 48 ] [ 48 ] ^([48]){ }^{[48]} Bacterial cellulose was received from Guilin Qihong Technology Co., Ltd. (Guangxi, China). The pine pulp (cellulose 87.2 % 87.2 % 87.2%87.2 \%, hemicellulose 12.1 % 12.1 % 12.1%12.1 \% and lignin 0.7 % 0.7 % 0.7%0.7 \% ) was obtained from the State Key Laboratory of Pulp and Paper Engineering (Guangzhou, China). The cotton pulp (cellulose 93.8 % 93.8 % 93.8%93.8 \% and hemicellulose 6.2%) was commercial, supplied by Hubei Chemical Fiber Co., Ltd. (Hubei, China). DMSO, DMAc, DMF, lithium hydroxide ( LiOH ) ( LiOH ) (LiOH)(\mathrm{LiOH}), sodium hydroxide ( NaOH ) ( NaOH ) (NaOH)(\mathrm{NaOH}) potassium hydroxide ( KOH ) ( KOH ) (KOH)(\mathrm{KOH}), calcium hydroxide (CaOH), lithium chloride ( LiCl ), lithium bromide ( LiBr ), succinic anhydride (SA), maleic anhydride (MA), phthalic anhydride (PA), octenyl succinic anhydride (OSA) and dodecenylsuccinic anhydride (DSA), ethanol, acetone, and tert-butanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China) and were used as received.
材料褐藻（Halocynthia roretzi Drasche）购自威海长青海洋科技有限公司（中国山东）。(中国山东）购买。蔗渣（纤维素 87.7%，半纤维素 9.8 % 9.8 % 9.8%9.8 \% 和木质素 2.5%）购自武汉大学武大自强超市。根据前人的研究，分离出鳞茎纤维素和蔗渣纤维素。 [ 48 ] [ 48 ] ^([48]){ }^{[48]} 细菌纤维素购自桂林奇宏科技有限公司（中国广西）。(中国广西）。松浆（纤维素 87.2 % 87.2 % 87.2%87.2 \% 、半纤维素 12.1 % 12.1 % 12.1%12.1 \% 和木质素 0.7 % 0.7 % 0.7%0.7 \% ）来自制浆造纸工程国家重点实验室（中国广州）。棉浆（纤维素 93.8 % 93.8 % 93.8%93.8 \% 和半纤维素 6.2%）为商品浆，由湖北化纤股份有限公司（中国湖北）提供。(中国湖北）提供。DMSO、DMAc、DMF、氢氧化锂 ( LiOH ) ( LiOH ) (LiOH)(\mathrm{LiOH}) 、氢氧化钠 ( NaOH ) ( NaOH ) (NaOH)(\mathrm{NaOH}) 、氢氧化钾 ( KOH ) ( KOH ) (KOH)(\mathrm{KOH}) 、氢氧化钙（CaOH）、氯化锂（LiCl）、溴化锂（LiBr）、丁二酸酐（SA）、琥珀酸酐（SA）、马来酸酐（MA）、邻苯二甲酸酐（PA）、辛烯基琥珀酸酐（OSA）和十二烯基琥珀酸酐（DSA）、乙醇、丙酮和叔丁醇购自国药集团化学试剂有限公司。Ltd. (中国北京)购得。(这些试剂购自国药集团化学试剂有限公司（中国北京），按原样使用。
Preparation of Cellulose Nanofibrils: Raw cellulose ( 1 g ) was dispersed in 100 mL DMSO containing LiOH ( 2 mg mL − 1 2 mg mL − 1 2mgmL^(-1)2 \mathrm{mg} \mathrm{mL}^{-1} ) under stirred violently. After swelling for 24 h , cyclic anhydride (anhydride/AGU molar ratio = 3: 1) was added into the system and reacted at room temperature for 1 h to achieve self-defibrillation of cellulose. After adding a total of 400 mL of ethanol in four batches, the products were purified by centrifugations ( 10000 rpm , 10 min , 4 10000 rpm , 10 min , 4 10000rpm,10min,410000 \mathrm{rpm}, 10 \mathrm{~min}, 4 times) to remove pseudosolvent and unreacted anhydride. The gel-like wet precipitation was moved to the NaOH aqueous solution ( pH = 10 pH = 10 pH=10\mathrm{pH}=10 ) under mild magnetic stirring for 1 h . The CNF suspensions were obtained by centrifugation for 10 min under 10000 rpm and dialysis in deionized water.
制备纤维素纳米纤维：将未加工的纤维素（1 克）分散在 100 毫升含有 LiOH ( 2 mg mL − 1 2 mg mL − 1 2mgmL^(-1)2 \mathrm{mg} \mathrm{mL}^{-1} ) 的二甲基亚砜中，剧烈搅拌。溶胀 24 小时后，向体系中加入环酸酐（酸酐/AGU 摩尔比 = 3:1），并在室温下反应 1 小时，以实现纤维素的自振荡。分四次共加入 400 mL 乙醇后，通过离心（ 10000 rpm , 10 min , 4 10000 rpm , 10 min , 4 10000rpm,10min,410000 \mathrm{rpm}, 10 \mathrm{~min}, 4 次）纯化产物，除去假溶剂和未反应的酸酐。在温和的磁力搅拌下，将凝胶状湿沉淀转移到 NaOH 水溶液（ pH = 10 pH = 10 pH=10\mathrm{pH}=10 ）中 1 小时。在 10000 rpm 转速下离心 10 分钟，然后在去离子水中透析，得到 CNF 悬浮液。
The carboxylate content ( mmol g − 1 mmol g − 1 mmolg^(-1)\mathrm{mmol} \mathrm{g}^{-1} ) of CNFs was determined by conductivity titration and calculated by the following Equation (1),
CNF 的羧酸盐含量（ mmol g − 1 mmol g − 1 mmolg^(-1)\mathrm{mmol} \mathrm{g}^{-1} ）通过电导滴定法测定，计算公式如下（1）、
Carboxylate content = ( V 1 − V 0 ) × C NaOH / m 0 = V 1 − V 0 × C NaOH / m 0 =(V_(1)-V_(0))xxC_(NaOH)//m_(0)=\left(V_{1}-V_{0}\right) \times C_{\mathrm{NaOH}} / m_{0}
羧酸含量 = ( V 1 − V 0 ) × C NaOH / m 0 = V 1 − V 0 × C NaOH / m 0 =(V_(1)-V_(0))xxC_(NaOH)//m_(0)=\left(V_{1}-V_{0}\right) \times C_{\mathrm{NaOH}} / m_{0}
where V 0 ( mL ) V 0 ( mL ) V_(0)(mL)V_{0}(\mathrm{~mL}) and V 1 ( mL ) V 1 ( mL ) V_(1)(mL)V_{1}(\mathrm{~mL}) are the consumption volumes of NaOH before and after no change in electrical conductivity during titration, m 0 ( g ) m 0 ( g ) m_(0)(g)m_{0}(\mathrm{~g}) is the solid content of CNFs suspension, C NaOH ( mmol mL − 1 ) C NaOH mmol mL − 1 C_(NaOH)(mmolmL^(-1))C_{\mathrm{NaOH}}\left(\mathrm{mmol} \mathrm{mL}^{-1}\right) is the concentration of NaOH of 0.01 mmol mL − 1 0.01 mmol mL − 1 0.01mmolmL^(-1)0.01 \mathrm{mmol} \mathrm{mL}^{-1}, and M AGU ( g mol − 1 ) M AGU g mol − 1 M_(AGU)(gmol^(-1))\mathrm{M}_{\mathrm{AGU}}\left(\mathrm{g} \mathrm{mol}^{-1}\right) is the molar mass of anhydroglucose unit of 162 g mol − 1 162 g mol − 1 162gmol^(-1)162 \mathrm{~g} \mathrm{~mol}^{-1}. The yield of CNFs was obtained by gravimetric analysis, which was calculated by Equation (2),
式中： V 0 ( mL ) V 0 ( mL ) V_(0)(mL)V_{0}(\mathrm{~mL}) 和 V 1 ( mL ) V 1 ( mL ) V_(1)(mL)V_{1}(\mathrm{~mL}) 为滴定过程中电导率不变前后的 NaOH 消耗量； m 0 ( g ) m 0 ( g ) m_(0)(g)m_{0}(\mathrm{~g}) 为 CNFs 悬浮液的固体含量； C NaOH ( mmol mL − 1 ) C NaOH mmol mL − 1 C_(NaOH)(mmolmL^(-1))C_{\mathrm{NaOH}}\left(\mathrm{mmol} \mathrm{mL}^{-1}\right) 为 0.01 mmol mL − 1 0.01 mmol mL − 1 0.01mmolmL^(-1)0.01 \mathrm{mmol} \mathrm{mL}^{-1} 的 NaOH 浓度； M AGU ( g mol − 1 ) M AGU g mol − 1 M_(AGU)(gmol^(-1))\mathrm{M}_{\mathrm{AGU}}\left(\mathrm{g} \mathrm{mol}^{-1}\right) 为 162 g mol − 1 162 g mol − 1 162gmol^(-1)162 \mathrm{~g} \mathrm{~mol}^{-1} 的无水葡萄糖单位摩尔质量。CNF 的产率通过重量分析得出，计算公式为 (2)、
Yield = ( m CNFs − m AH ) / m c × 100 % = m CNFs − m AH / m c × 100 % =(m_(CNFs)-m_(AH))//m_(c)xx100%=\left(m_{\mathrm{CNFs}}-m_{\mathrm{AH}}\right) / m_{\mathrm{c}} \times 100 \% 产量 = ( m CNFs − m AH ) / m c × 100 % = m CNFs − m AH / m c × 100 % =(m_(CNFs)-m_(AH))//m_(c)xx100%=\left(m_{\mathrm{CNFs}}-m_{\mathrm{AH}}\right) / m_{\mathrm{c}} \times 100 \%
where m CNFs ( g ) m CNFs  ( g ) m_("CNFs ")(g)m_{\text {CNFs }}(\mathrm{g}) is the total mass of CNFs, m AH ( g ) m AH  ( g ) m_("AH ")(g)m_{\text {AH }}(\mathrm{g}) is the weight of the grafted anhydride, and m c ( g ) m c ( g ) m_(c)(g)m_{c}(\mathrm{~g}) is the total mass of cellulose.
其中 m CNFs ( g ) m CNFs  ( g ) m_("CNFs ")(g)m_{\text {CNFs }}(\mathrm{g}) 为 CNF 的总重量， m AH ( g ) m AH  ( g ) m_("AH ")(g)m_{\text {AH }}(\mathrm{g}) 为接枝酸酐的重量， m c ( g ) m c ( g ) m_(c)(g)m_{c}(\mathrm{~g}) 为纤维素的总重量。
Preparation of CNF Filaments: The PA-esterified CNF (tunicate) suspension ( 1.5 wt % 1.5 wt % 1.5wt%1.5 \mathrm{wt} \% ) in a syringe ( φ 8.5 , 80 mm φ 8.5 , 80 mm varphi8.5,80mm\varphi 8.5,80 \mathrm{~mm} ) was extruded through a needle ( D : 0.034 mm , L : 50 mm D : 0.034 mm , L : 50 mm D:0.034mm,L:50mmD: 0.034 \mathrm{~mm}, L: 50 \mathrm{~mm} ) into acetone coagulation baths using a single-pump injector at a spinning rate of 10 − 1000 mm min − 1 10 − 1000 mm min − 1 10-1000mmmin^(-1)10-1000 \mathrm{~mm} \mathrm{~min}^{-1}. The gel filaments were immersed in various baths ( 0.1 M HCl , 0.1 M NaOH ( 0.1 M HCl , 0.1 M NaOH (0.1MHCl,0.1MNaOH(0.1 \mathrm{M} \mathrm{HCl}, 0.1 \mathrm{M} \mathrm{NaOH}, or DI water) for 24 h , thoroughly washed with ethanol aqueous solution (50 wt % wt % wt%\mathrm{wt} \% ) and dried at 50 ∘ C 50 ∘ C 50^(@)C50^{\circ} \mathrm{C} for 1 h under loading.
制备 CNF 纤维：在注射器（ φ 8.5 , 80 mm φ 8.5 , 80 mm varphi8.5,80mm\varphi 8.5,80 \mathrm{~mm} ）中的 PA 酯化 CNF（tunicate）悬浮液（ 1.5 wt % 1.5 wt % 1.5wt%1.5 \mathrm{wt} \% ）通过针头（ D : 0.034 mm , L : 50 mm D : 0.034 mm , L : 50 mm D:0.034mm,L:50mmD: 0.034 \mathrm{~mm}, L: 50 \mathrm{~mm} ）挤入丙酮凝固浴（使用单泵注射器，旋转速度为 10 − 1000 mm min − 1 10 − 1000 mm min − 1 10-1000mmmin^(-1)10-1000 \mathrm{~mm} \mathrm{~min}^{-1} ）。将凝胶丝浸泡在各种浴槽 ( 0.1 M HCl , 0.1 M NaOH ( 0.1 M HCl , 0.1 M NaOH (0.1MHCl,0.1MNaOH(0.1 \mathrm{M} \mathrm{HCl}, 0.1 \mathrm{M} \mathrm{NaOH} 或去离子水中 24 小时，用乙醇水溶液（50 wt % wt % wt%\mathrm{wt} \% ）彻底清洗，并在 50 ∘ C 50 ∘ C 50^(@)C50^{\circ} \mathrm{C} 负载下干燥 1 小时。
Preparation of CNF Films: The PA esterified CNF (tunicate) suspensions ( 0.1 wt % 10 mL 0.1 wt % 10 mL 0.1wt%10mL0.1 \mathrm{wt} \% 10 \mathrm{~mL} ) were deposited onto a PVDF membrane ( φ 0.22 μ m φ 0.22 μ m varphi0.22 mum\varphi 0.22 \mu \mathrm{~m} ) by vacuum-assisted filtration ( 0.05 bar ). The resultant CNF layers were immersed in various baths ( 0.1 m HCl , 0.1 m NaOH ( 0.1 m HCl , 0.1 m NaOH (0.1mHCl,0.1mNaOH(0.1 \mathrm{~m} \mathrm{HCl}, 0.1 \mathrm{~m} \mathrm{NaOH}, or DI water). After that, the samples were washed with ethanol aqueous solution ( 50 wt % 50 wt % 50wt%50 \mathrm{wt} \% ) and dried overnight in an oven at 50 ∘ C 50 ∘ C 50^(@)C50^{\circ} \mathrm{C}.
制备 CNF 薄膜：通过真空辅助过滤（0.05 巴）将 PA 酯化 CNF（tunicate）悬浮液（ 0.1 wt % 10 mL 0.1 wt % 10 mL 0.1wt%10mL0.1 \mathrm{wt} \% 10 \mathrm{~mL} ）沉积到 PVDF 膜（ φ 0.22 μ m φ 0.22 μ m varphi0.22 mum\varphi 0.22 \mu \mathrm{~m} ）上。将得到的 CNF 层浸入不同的水浴 ( 0.1 m HCl , 0.1 m NaOH ( 0.1 m HCl , 0.1 m NaOH (0.1mHCl,0.1mNaOH(0.1 \mathrm{~m} \mathrm{HCl}, 0.1 \mathrm{~m} \mathrm{NaOH} 或去离子水中。）然后，用乙醇水溶液（ 50 wt % 50 wt % 50wt%50 \mathrm{wt} \% ）清洗样品，并在 50 ∘ C 50 ∘ C 50^(@)C50^{\circ} \mathrm{C} 烘箱中干燥过夜。
Preparation of CNF Aerogels: The gelation of PA esterified CNF (tunicate) suspensions was achieved by treating with HCl ( 0.1 m , 1 h ) HCl ( 0.1 m , 1 h ) HCl(0.1m,1h)\mathrm{HCl}(0.1 \mathrm{~m}, 1 \mathrm{~h}) and NaOH ( 0.1 m , 24 h ) NaOH ( 0.1 m , 24 h ) NaOH(0.1m,24h)\mathrm{NaOH}(0.1 \mathrm{~m}, 24 \mathrm{~h}), respectively. Then, the CNF ( COOH ) CNF ( COOH ) CNF_((COOH))\mathrm{CNF}_{(\mathrm{COOH})} and CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} aerogels were obtained by tert-butanol exchange ( 30 wt % 30 wt % 30wt%30 \mathrm{wt} \% ), frozen ( − 50 ∘ C − 50 ∘ C -50^(@)C-50^{\circ} \mathrm{C}, 12 h ) and freeze-drying ( 10 Pa , − 50 ∘ C 10 Pa , − 50 ∘ C 10Pa,-50^(@)C10 \mathrm{~Pa},-50^{\circ} \mathrm{C} ). The CNF ( COONa ) CNF ( COONa ) CNF_((COONa))\mathrm{CNF}_{(\mathrm{COONa})} aerogel was obtained by freeze-drying of CNF suspensions ( 30 % 30 % 30%30 \% tert-butanol aqueous solution). Thermal insulation of CNF aerogels was tested on a 120 ∘ C 120 ∘ C 120^(@)C120^{\circ} \mathrm{C} heating plate or − 196 ∘ C − 196 ∘ C -196^(@)C-196^{\circ} \mathrm{C} cooling plate.
制备 CNF 气凝胶：分别用 HCl ( 0.1 m , 1 h ) HCl ( 0.1 m , 1 h ) HCl(0.1m,1h)\mathrm{HCl}(0.1 \mathrm{~m}, 1 \mathrm{~h}) 和 NaOH ( 0.1 m , 24 h ) NaOH ( 0.1 m , 24 h ) NaOH(0.1m,24h)\mathrm{NaOH}(0.1 \mathrm{~m}, 24 \mathrm{~h}) 处理PA酯化的CNF（tunicate）悬浮液，使其凝胶化。然后，通过叔丁醇交换（ 30 wt % 30 wt % 30wt%30 \mathrm{wt} \% ）、冷冻（ − 50 ∘ C − 50 ∘ C -50^(@)C-50^{\circ} \mathrm{C} ，12 h）和冷冻干燥（ 10 Pa , − 50 ∘ C 10 Pa , − 50 ∘ C 10Pa,-50^(@)C10 \mathrm{~Pa},-50^{\circ} \mathrm{C} ）得到 CNF ( COOH ) CNF ( COOH ) CNF_((COOH))\mathrm{CNF}_{(\mathrm{COOH})} 和 CNF ( OH ) CNF ( OH ) CNF_((OH))\mathrm{CNF}_{(\mathrm{OH})} 气凝胶。通过冷冻干燥 CNF 悬浮液（ 30 % 30 % 30%30 \% 叔丁醇水溶液）获得了 CNF ( COONa ) CNF ( COONa ) CNF_((COONa))\mathrm{CNF}_{(\mathrm{COONa})} 气凝胶。在 120 ∘ C 120 ∘ C 120^(@)C120^{\circ} \mathrm{C} 加热板或 − 196 ∘ C − 196 ∘ C -196^(@)C-196^{\circ} \mathrm{C} 冷却板上测试了 CNF 气凝胶的隔热性。
Characterization: The morphology of CNFs was characterized by transmission electron microscopy (JEM-2010, JEOL, Japan) and atomic force microscopy (Asylum Research Cypher system, Oxford, UK). Scanning electron microscope (SEM) measurements of swollen cellulose and cellulosic materials were carried out using a field emission scanning electron microscope (Zeiss, Germany) with an accelerating voltage of 5-10 kV. The molar mass measurements of cellulose before and after swelling were performed by size exclusion chromatography (SEC) (PL-GPC50, Agilent, USA) with the refractive index detector. The column was PLgel 10 μ m 10 μ m 10 mum10 \mu \mathrm{~m} MIXED-B. The cellulose samples were dissolved in LiCl / D M A c ( 0.9 % , w / v ) LiCl / D M A c ( 0.9 % , w / v ) LiCl//DMAc(0.9%,w//v)\mathrm{LiCl} / D M A c(0.9 \%, w / v) with a concentration of 1 mg mL − 1 1 mg mL − 1 1mgmL^(-1)1 \mathrm{mg} \mathrm{mL}^{-1} referring to previous studies. [ 27 , 28 ] [ 27 , 28 ] ^([27,28]){ }^{[27,28]} The eluent was LiCl / D M A c ( 0.5 % , w / v ) LiCl / D M A c ( 0.5 % , w / v ) LiCl//DMAc(0.5%,w//v)\mathrm{LiCl} / D M A c(0.5 \%, w / v) at a flow rate of 1 mL min − 1 1 mL min − 1 1mLmin^(-1)1 \mathrm{~mL} \mathrm{~min}^{-1} and the run time was 45 min . The data was treated with Cirrus GPC (Version 3.4). Raman spectroscopy was performed on a Raman imaging microscope (Thermo Fisher Scientific, Fitchburg, USA). FTIR spectra were measured on a NICOLET 5700) using the KBr method. Solid-state 13 C 13 C ^(13)C{ }^{13} \mathrm{C} NMR measurements were performed on Bruker Avance III 400 MHz spectrometer (Bruker, Switzerland) equipped with a 4 mm HXMAS probe and ZrO 2 ZrO 2 ZrO_(2)\mathrm{ZrO}_{2} rotors. ID WAXS measurements of samples were carried out on an XPert Pro spectrometer (PANalytical, Netherlands) using an X X XX beamline with wavelength λ = 1.54 Å λ = 1.54 Å lambda=1.54"Å"\lambda=1.54 \AAÅ. The samples were freeze-dried ( − 50 ∘ C , 10 Pa ) − 50 ∘ C , 10 Pa (-50^(@)C,10(Pa))\left(-50^{\circ} \mathrm{C}, 10 \mathrm{~Pa}\right) and tablet-pressed before the test. Scattering peaks of each crystal plane were determined by deconvolution of WAXS profiles, and the crystallinity ( Cr ) and crystallinity index (CI) were calculated in equation:[49]
表征：使用透射电子显微镜（JEM-2010，日本 JEOL）和原子力显微镜（Asylum Research Cypher 系统，英国牛津）对 CNFs 的形态进行表征。使用加速电压为 5-10 kV 的场发射扫描电子显微镜（德国蔡司）对溶胀的纤维素和纤维素材料进行了扫描电子显微镜（SEM）测量。纤维素溶胀前后的摩尔质量测量是通过尺寸排阻色谱法（SEC）（PL-GPC50，美国安捷伦公司）和折射率检测器进行的。色谱柱为 PLgel 10 μ m 10 μ m 10 mum10 \mu \mathrm{~m} MIXED-B。参照以前的研究，将纤维素样品溶解在浓度为 1 mg mL − 1 1 mg mL − 1 1mgmL^(-1)1 \mathrm{mg} \mathrm{mL}^{-1} 的 LiCl / D M A c ( 0.9 % , w / v ) LiCl / D M A c ( 0.9 % , w / v ) LiCl//DMAc(0.9%,w//v)\mathrm{LiCl} / D M A c(0.9 \%, w / v) 中。 [ 27 , 28 ] [ 27 , 28 ] ^([27,28]){ }^{[27,28]} 洗脱液为 LiCl / D M A c ( 0.5 % , w / v ) LiCl / D M A c ( 0.5 % , w / v ) LiCl//DMAc(0.5%,w//v)\mathrm{LiCl} / D M A c(0.5 \%, w / v) ，流速为 1 mL min − 1 1 mL min − 1 1mLmin^(-1)1 \mathrm{~mL} \mathrm{~min}^{-1} ，运行时间为 45 分钟。数据用 Cirrus GPC（3.4 版）处理。拉曼光谱在拉曼成像显微镜（Thermo Fisher Scientific, Fitchburg, USA）上进行。傅立叶变换红外光谱在 NICOLET 5700 上使用 KBr 法进行测量。固态 13 C 13 C ^(13)C{ }^{13} \mathrm{C} NMR 测量在配备了 4 毫米 HXMAS 探头和 ZrO 2 ZrO 2 ZrO_(2)\mathrm{ZrO}_{2} 转子的布鲁克 Avance III 400 MHz 光谱仪（Bruker，瑞士）上进行。样品的 ID WAXS 测量是在 XPert Pro 光谱仪（PANalytical，荷兰）上进行的，使用的是波长为 λ = 1.54 Å λ = 1.54 Å lambda=1.54"Å"\lambda=1.54 \AAÅ 的 X X XX 光束线。测试前，样品经过 ( − 50 ∘ C , 10 Pa ) − 50 ∘ C , 10 Pa (-50^(@)C,10(Pa))\left(-50^{\circ} \mathrm{C}, 10 \mathrm{~Pa}\right) 冻干和压片处理。通过对 WAXS 曲线进行解卷积确定每个晶面的散射峰，并按以下公式计算结晶度（Cr）和结晶度指数（CI）[49]。
Cl = ( I 200 − I 18.5 ∘ ) / I 200 Cl = I 200 − I 18.5 ∘ / I 200 Cl=(I_(200)-I_(18.5^(@)))//I_(200)\mathrm{Cl}=\left(I_{200}-I_{18.5^{\circ}}\right) / I_{200}
Cr = A cry / A total Cr = A cry  / A total  Cr=A_("cry ")//A_("total ")\mathrm{Cr}=A_{\text {cry }} / A_{\text {total }}
where I 200 I 200 I_(200)I_{200} and I 18.5 ∘ I 18.5 ∘ I_(18.5^(@))I_{18.5^{\circ}} were the scattering intensity of the 200 plane peaks and the peak at the 2 θ = 18.5 ∘ 2 θ = 18.5 ∘ 2theta=18.5^(@)2 \theta=18.5^{\circ}, respectively. A cry A cry  A_("cry ")A_{\text {cry }} was the sum of crystalline band areas, and A total A total  A_("total ")A_{\text {total }} was the total area under the diffractograms.
其中 I 200 I 200 I_(200)I_{200} 和 I 18.5 ∘ I 18.5 ∘ I_(18.5^(@))I_{18.5^{\circ}} 分别为 200 平面峰和 2 θ = 18.5 ∘ 2 θ = 18.5 ∘ 2theta=18.5^(@)2 \theta=18.5^{\circ} 处峰的散射强度。 A cry A cry  A_("cry ")A_{\text {cry }} 是结晶带面积之和， A total A total  A_("total ")A_{\text {total }} 是衍射图下的总面积。
The lattice spacing (d) was calculated using the Bragg equation of Equation (5) [ 50 ] [ 50 ] ^([50]){ }^{[50]}
晶格间距 (d) 是通过公式 (5) [ 50 ] [ 50 ] ^([50]){ }^{[50]} 的布拉格方程计算得出的。
2 d × sin ⁡ θ = x × λ 2 d × sin ⁡ θ = x × λ 2d xx sin theta=x xx lambda2 d \times \sin \theta=x \times \lambda
where θ θ theta\theta was the Bragg angle corresponding to the plane; x x xx was the diffraction order of 1 and λ λ lambda\lambda was the X X XX-ray wavelength ( 0.154 nm ).
其中， θ θ theta\theta 是与平面相对应的布拉格角； x x xx 是衍射阶数 1， λ λ lambda\lambda 是 X X XX 射线波长（0.154 纳米）。
The apparent crystallite size (ACS) was calculated using the Scherrer equation of Equation (6) [ 49 ] [ 49 ] ^([49]){ }^{[49]}
表观晶粒大小（ACS）是通过公式（6） [ 49 ] [ 49 ] ^([49]){ }^{[49]} 中的舍勒方程计算得出的。
ACS = ( K × λ ) / ( β × cos ⁡ θ ) ACS = ( K × λ ) / ( β × cos ⁡ θ ) ACS=(K xx lambda)//(beta xx cos theta)\mathrm{ACS}=(K \times \lambda) /(\beta \times \cos \theta)
where K K KK was a constant of value 0.89 and β β beta\beta was the half-height width of the diffraction band.
其中， K K KK 是值为 0.89 的常数， β β beta\beta 是衍射带的半高宽度。
2D small angle X -ray scattering (SAXS) and WAXS tests with a 2D sensor were performed using an X-beamline with wavelength λ = 1.54 Å λ = 1.54 Å lambda=1.54"Å"\lambda=1.54 \AAÅ at Xeuss 2.0 (Xenocs, France). The sample-to-detector distances for SAXS/WAXS measurement were set to be 2514.02 and 60.0 mm , respectively. All 1D curves were calculated by integrating the 2D profiles. SAXS curves were fitted and analyzed using the generalized Guinier-Porod model of equation:[37]
使用波长 λ = 1.54 Å λ = 1.54 Å lambda=1.54"Å"\lambda=1.54 \AAÅ 为 Xeuss 2.0 的 X 光束线（Xenocs，法国）进行了带有二维传感器的二维小角 X 射线散射（SAXS）和 WAXS 测试。用于 SAXS/WAXS 测量的样品到探测器的距离分别设定为 2514.02 毫米和 60.0 毫米。所有 1D 曲线都是通过对 2D 曲线进行积分计算得出的。SAXS 曲线的拟合和分析采用公式为[37] 的广义 Guinier-Porod 模型。
I ( Q ) = G Q s exp ⁡ ( − Q 2 R g 2 3 − s ) + B I ( Q ) = G Q s exp ⁡ − Q 2 R g 2 3 − s + B I(Q)=(G)/(Q^(s))exp((-Q^(2)R_(g)^(2))/(3-s))+BI(Q)=\frac{G}{Q^{s}} \exp \left(\frac{-Q^{2} R_{g}^{2}}{3-s}\right)+B for Q ≤ Q 1 Q ≤ Q 1 Q <= Q_(1)Q \leq Q_{1}  I ( Q ) = G Q s exp ⁡ ( − Q 2 R g 2 3 − s ) + B I ( Q ) = G Q s exp ⁡ − Q 2 R g 2 3 − s + B I(Q)=(G)/(Q^(s))exp((-Q^(2)R_(g)^(2))/(3-s))+BI(Q)=\frac{G}{Q^{s}} \exp \left(\frac{-Q^{2} R_{g}^{2}}{3-s}\right)+B 代表 Q ≤ Q 1 Q ≤ Q 1 Q <= Q_(1)Q \leq Q_{1} 。
I ( Q ) = A Q n + B I ( Q ) = A Q n + B I(Q)=(A)/(Q^(n))+BI(Q)=\frac{A}{Q^{n}}+B for Q ≥ Q 1 Q ≥ Q 1 Q >= Q_(1)Q \geq Q_{1}  I ( Q ) = A Q n + B I ( Q ) = A Q n + B I(Q)=(A)/(Q^(n))+BI(Q)=\frac{A}{Q^{n}}+B 代表 Q ≥ Q 1 Q ≥ Q 1 Q >= Q_(1)Q \geq Q_{1} 。
where Q Q QQ was the scattering variable; I ( Q ) I ( Q ) I(Q)I(Q) was the scattered intensity; G G GG and A A AA were the Guinier and Porod scale factors; R g R g R_(g)R_{g} was the radius of gyration; s s ss was the shape parameter; n n nn was the Porod exponent, and B B BB was the background baseline, respectively.
其中， Q Q QQ 为散射变量； I ( Q ) I ( Q ) I(Q)I(Q) 为散射强度； G G GG 和 A A AA 分别为吉尼埃尺度因子和波罗德尺度因子； R g R g R_(g)R_{g} 为回旋半径； s s ss 为形状参数； n n nn 为波罗德指数， B B BB 为背景基线。
Applying the same continuity of the Guinier and Porod functions and their derivatives yields in equation: [ 37 ] [ 37 ] ^([37]){ }^{[37]}
应用 Guinier 和 Porod 函数及其导数的相同连续性，可得出等式： [ 37 ] [ 37 ] ^([37]){ }^{[37]}
Q 1 = 1 R g [ ( n − s ) ( 3 − s ) 2 ] 1 / 2 Q 1 = 1 R g ( n − s ) ( 3 − s ) 2 1 / 2 Q_(1)=(1)/(R_(g))[((n-s)(3-s))/(2)]^(1//2)Q_{1}=\frac{1}{R_{g}}\left[\frac{(n-s)(3-s)}{2}\right]^{1 / 2}
A = G exp ⁡ ( − Q 1 2 R g 2 3 − s ) Q 1 ( n − s ) = G R g ( n − s ) exp ⁡ [ − ( n − s ) 2 ] [ ( n − s ) ( 3 − s ) 2 ] ( n − s ) / 2 A = G exp ⁡ − Q 1 2 R g 2 3 − s Q 1 ( n − s ) = G R g ( n − s ) exp ⁡ − ( n − s ) 2 ( n − s ) ( 3 − s ) 2 ( n − s ) / 2 A=G exp((-Q_(1)^(2)R_(g)^(2))/(3-s))Q_(1)^((n-s))=(G)/(R_(g)^((n-s)))exp[-((n-s))/(2)][((n-s)(3-s))/(2)]^((n-s)//2)A=G \exp \left(\frac{-Q_{1}^{2} R_{g}^{2}}{3-s}\right) Q_{1}^{(n-s)}=\frac{G}{R_{g}^{(n-s)}} \exp \left[-\frac{(n-s)}{2}\right]\left[\frac{(n-s)(3-s)}{2}\right]^{(n-s) / 2}
In WAXS, Herman’s orientation parameters ( f c ) ( f c ) (fc)(f c) were calculated from the azimuthal-intensity distribution curves of the X-ray scattering profiles according to equation:
在 WAXS 中，赫尔曼的取向参数 ( f c ) ( f c ) (fc)(f c) 是根据公式从 X 射线散射剖面的方位角-强度分布曲线计算得出的：
f c = 3 ⟨ cos 2 ⁡ φ ⟩ − 1 2 f c = 3 cos 2 ⁡ φ − 1 2 f_(c)=(3(:cos^(2)varphi:)-1)/(2)f_{c}=\frac{3\left\langle\cos ^{2} \varphi\right\rangle-1}{2}
⟨ cos 2 ⁡ φ ⟩ = ∫ 0 π 2 I ( φ ) cos 2 ⁡ φ sin ⁡ φ d φ ∫ 0 π 2 I ( φ ) sin ⁡ φ d φ cos 2 ⁡ φ = ∫ 0 π 2   I ( φ ) cos 2 ⁡ φ sin ⁡ φ d φ ∫ 0 π 2   I ( φ ) sin ⁡ φ d φ (:cos^(2)varphi:)=(int_(0)^((pi)/(2))I(varphi)cos^(2)varphi sin varphi(d)varphi)/(int_(0)^((pi)/(2))I(varphi)sin varphidvarphi)\left\langle\cos ^{2} \varphi\right\rangle=\frac{\int_{0}^{\frac{\pi}{2}} I(\varphi) \cos ^{2} \varphi \sin \varphi \mathrm{~d} \varphi}{\int_{0}^{\frac{\pi}{2}} I(\varphi) \sin \varphi \mathrm{d} \varphi}
where φ φ varphi\varphi was azimuthal angle; I ( φ ) I ( φ ) I(varphi)I(\varphi) was the 1D intensity distribution along with φ φ varphi\varphi and ⟨ cos ⁡ 2 φ ⟩ ⟨ cos ⁡ 2 φ ⟩ (:cos 2varphi:)\langle\cos 2 \varphi\rangle was calculated by integrating the intensity of specific 2 θ 2 θ 2theta2 \theta diffraction peak along φ φ varphi\varphi.
其中， φ φ varphi\varphi 为方位角； I ( φ ) I ( φ ) I(varphi)I(\varphi) 为沿 φ φ varphi\varphi 的一维强度分布； ⟨ cos ⁡ 2 φ ⟩ ⟨ cos ⁡ 2 φ ⟩ (:cos 2varphi:)\langle\cos 2 \varphi\rangle 通过沿 φ φ varphi\varphi 的特定 2 θ 2 θ 2theta2 \theta 衍射峰的强度积分计算得出。
The transmittance of suspension ( 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ) was obtained on a Mapada P7 UV-Vis spectrophotometer (Mapada, Shanghai) using a quartz colorimetric dish with a 1 cm optical path. Zeta potential of CNF suspension was measured on a Nanotrac Wave II (Microtrac, Germany) at 25 ∘ C 25 ∘ C 25^(@)C25^{\circ} \mathrm{C}. Rheological property of suspension was characterized by a TADHR-2 (TA Instruments, USA) using a parallel-plate measuring geometry (diameter: 40 mm ). The mechanical test of filament and film samples was performed
悬浮液的透射率（ 0.1 wt % 0.1 wt % 0.1wt%0.1 \mathrm{wt} \% ）是在 Mapada P7 紫外可见分光光度计（上海 Mapada）上用 1 厘米光程的石英比色皿测得的。CNF 悬浮液的 Zeta 电位在 Nanotrac Wave II（Microtrac，德国）上以 25 ∘ C 25 ∘ C 25^(@)C25^{\circ} \mathrm{C} 测量。悬浮液的流变特性由 TADHR-2 （美国 TA 仪器公司）使用平行板测量几何形状（直径：40 毫米）进行表征。对长丝和薄膜样品进行了机械测试
on an Instron model 5967 instrument (Instron, USA) with a speed of 5 mm min − 1 5 mm min − 1 5mmmin^(-1)5 \mathrm{~mm} \mathrm{~min}^{-1} at room temperature. Thermogravimetric analysis of dried samples ( 5 mg ) was carried out on a Q500 system (TA Instruments) under an air atmosphere in the temperature range of 80 − 800 ∘ C 80 − 800 ∘ C 80-800^(@)C80-800^{\circ} \mathrm{C} at a heating rate of 10 ∘ C min − 1 10 ∘ C min − 1 10^(@)Cmin^(-1)10^{\circ} \mathrm{C} \mathrm{min}^{-1}.
在 Instron 5967 型仪器（Instron，美国）上以 5 mm min − 1 5 mm min − 1 5mmmin^(-1)5 \mathrm{~mm} \mathrm{~min}^{-1} 的速度在室温下进行。干燥样品（5 毫克）的热重分析在 Q500 系统（TA 仪器公司）上进行，在空气环境下，温度范围为 80 − 800 ∘ C 80 − 800 ∘ C 80-800^(@)C80-800^{\circ} \mathrm{C} ，加热速度为 10 ∘ C min − 1 10 ∘ C min − 1 10^(@)Cmin^(-1)10^{\circ} \mathrm{C} \mathrm{min}^{-1} 。
Statistical Analysis: The statistical data presentation follows the mean ± ± +-\pm SD type for the measurements of yield, zeta potential, and mechanical performance, where n = 3 n = 3 n=3n=3 for each sample. The statistical data presentation follows the mean ± ± +-\pm SD type for all size measurements of CNF diameter and length, where n = 100 n = 100 n=100n=100 for each sample. All statistical analysis experiments were processed using Origin.
统计分析：对于每个样品的产量、ZETA电位和机械性能测量值（ n = 3 n = 3 n=3n=3 ），统计数据采用平均 ± ± +-\pm SD类型。对于 CNF 直径和长度的所有尺寸测量，每个样品的统计数据均采用 ± ± +-\pm SD 类型，其中 n = 100 n = 100 n=100n=100 为平均值。所有统计分析实验均使用 Origin 进行处理。

Supporting Information 辅助信息

Supporting Information is available from the Wiley Online Library or from the author.
辅助信息可从 Wiley 在线图书馆或作者处获取。

Acknowledgements 致谢

This work was financially supported by the National Natural Science Foundation of China (52373 104), and State Key Laboratory of New Textile Materials and Advanced Processing Technologies (FZ2022015).
这项工作得到了国家自然科学基金（52373 104）和纺织新材料与先进加工技术国家重点实验室（FZ2022015）的资助。

Conflict of Interest 利益冲突

The authors declare no conflict of interest.
作者声明没有利益冲突。

Data Availability Statement
数据可用性声明

The data that support the findings of this study are available in the supplementary material of this article.
支持本研究结果的数据见本文的补充材料。

Keywords 关键词

advanced cellulosic materials, cellulose elementary fibrils, nanocellulose, reversible surface engineering, ultrahigh aspect ratio
先进纤维素材料、纤维素基本纤维、纳米纤维素、可逆表面工程、超高纵横比
Received: November 15, 2023
收到：2023 年 11 月 15 日
Revised: January 24, 2024
已修订：2024 年 1 月 24 日
Published online: February 28, 2024
在线发表：2024 年 2 月 28 日
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