Ester Cross-linking Enhanced Hydrophilic Cellulose Nanofibrils

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Ester Cross-linking Enhanced Hydrophilic Cellulose Nanofibrils Aerogel Yulong Li, Yushang Liu, Yang Liu, Wenchuan Lai, Feng Huang, Anping Ou, Rui Qin, Xiangyang Liu, and Xu Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02284 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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ACS Sustainable Chemistry & Engineering

Ester Cross-linking Enhanced Hydrophilic Cellulose Nanofibrils Aerogel Yulong Li†, Yushang Liu†, Yang Liu†, Wenchuan Lai†, Feng Huang†, Anping Ou†, Rui Qin†, Xiangyang Liu*†, Xu Wang*† † State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu, Sichuan, 610065, P.R. China.

* Corresponding author. *Xu Wang, E-mail: [email protected] *Xiangyang Liu, E-mail: [email protected]

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KEYWORDS: Cellulose Nanofibrils; Ester Crosslinking; Hydrophilicity; Fluorination;Waterdurable

ABSTRACT: The ester cross-linking of cellulose nanofibrils (CNFs) aerogel was carried out by a facile and high effective gas-solid fluorinating reaction without assistance of organic solvent. The fluorination achieved the co-existence of the covalent cross-linking and more hydrophilic groups, simultaneously endowing the fluorine-treated CNFs (F-CNFs) aerogel with enduring water-durable and enhanced hydrophilic property. Hydrogen bond interactions between cellulose nanofibers are replaced by covalent cross-linking of ester bond, which makes the CNFs aggregate more tightly and thus facilitates the enhancement of mechanical properties of F-CNFs aerogel. Meanwhile, the F-CNFs aerogel exhibited ultralight weight, excellent mechanical properties (123.4 KPa at 85% strain) and water absorption (123.65 g/g), while having good shape memory performance. What’s more, there wasn’t any exotic element introduced into F-CNFs, which can thus maximize the possibility for the final materials to maintain the excellent biological properties of original CNFs.

INTRODUCTION With the increasing of environmental pollution and energy crisis, the cellulose nanofibrils (CNFs), as the production of inexhaustible natural biological materials, is attracting more and more interest recently due to its biodegradability and high performances.1 Attributed to its nature of high crystallinity and special structures, CNFs exhibits excellent mechanical properties, which makes it possible to be used as reinforcing agent in the composites without significantly weakening other properties of the matrix materials.2-3 The ample reactive hydroxyl groups within CNFs can be used for surface chemical modification, which thus provides a valuable platform 2 ACS Paragon Plus Environment

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for more applied research, including sewage purification,4-5 filtration,6 oil-water separation,7-8 and catalysis.9 Besides, owing to its low toxicity, biodegradability, biocompatibility and good mechanical properties, cellulose nanofibrils, as natural material, have broad application prospects in biological materials.10-12 It's worth noting that most of these applications of CNFs are applied under ambient conditions of water. However, the excellent hydrophilicity of the CNFs makes the CNFs products (aerogel and film etc.) easily to disintegrate into smaller fragments in water, since the hydrogen bonds between celluloses in CNFs are replaced by the hydrogen bonds between cellulose and water. This will result in the poor water-durable property of CNFs in water environments and its non-recyclability. To overcome this problem, two main solutions have been adopted: i) through using polymer materials to composite with CNFs to maintain the stability of the use of the product,13-14 or ii) through crosslinking cellulose with crosslinking agent.15-18 However, both of these two methods sacrificed some exposed functional groups in the CNFs surface and thus weakened the hydrophilicity of the cellulose material. Some other modification methods even converted cellulose materials into hydrophobic materials for use in aqueous environments,19-20 and the capacity of cellulose for further chemical modifications is also reduced. In addition, these methods often require more complicated steps or more energy consumption. Therefore, it is still a challenging topic in the area of advanced CNFs based materials to improve the water-durable of CNFs materials through a simple and energy-efficient way and simultaneously ensure its hydrophilicity. Meanwhile, for implementing the real-life application in future, it is urgent for CNFs aerogel to improve its mechanical and reusable performance. Here, the covalent crosslinking of CNFs aerogel was realized by the direct contacting between CNFs aerogel and fluorine element gas. It was confirmed that the contents of carboxyl



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and ester bonds in CNFs after fluorination were greatly increased through Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), acid-base titration, etc. Since these reasons, the fluorine-treated CNFs (F-CNFs) aerogel was endowed with excellent water-durable property and enhanced hydrophilic property simultaneously. Herein, its density, water absorption, mechanical properties and reusable performance were characterized. The corresponding performances further confirmed a facile and effective way was provided to expand the application of CNFs products in water environment. In addition, we have given a possible reaction mechanism based on the existing conclusions (As shown in Scheme 1). The aerogel products only need to stay awhile in the fluorine atmosphere for the crosslinking reaction, and the equipment and process requirements are lower by processing in the solid state. Meanwhile, this method would not introduce any other exotic elements into CNFs product, which will help to maintain the non-toxic and environmentally friendly nature of cellulose materials. What’s more, this easy-to-industry method of “post-processing” by fluorine gas is suitable not only for our cellulose aerogels, but also for the cross-linking of other cellulose materials, such as cellulose paper and cellulose fiber.

Scheme 1. The fluorination mechanism of CNFs and its changes in performance.


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EXPERIMENT Preparation of Fluorine-treated Cellulose Nanofibrils Aerogel. The 1.05 wt % CNFs hydrogel aqueous dispersion was provided by Tianjin Woodelfbio Cellulose Co., Ltd. The CNFs gel was obtained by oxidizing the fiber material through TEMPO (2, 2, 6, 6tetramethylpiperidine-1-oxyl) and then homogenizing it by a high pressure homogenizer. The specific experimental steps for preparing CNFs raw materials are list in supporting information. The AFM image of CNFs was shown in Figure S1 in supporting information, which indicated that the CNFs with a diameter of several tens of nanometers were obtained.21-24 The CNFs virgin stock was diluted with distilled water to a solids content of 1.0 wt% and 0.5 wt% for later experimentation. A portion of the aqueous CNFs dispersion was frozen at -15 °C in aluminum boxes for the contact angle test and the other portion was dried in Teflon tube at -15 °C for other tests. After being transferred into square aluminum case or PTFE tubes, the CNFs aqueous dispersion was frozen at -15 °C for 12 hours, and then the frozen sample was dried in a freeze dryer for two days. After freeze-drying, the CNFs aerogel can keep its macro size integrity through the interaction created by the hydrogen bonding and entanglement between contiguous CNFs. We got the white CNFs aerogel and kept the sample in the dryer. CNFs aerogel (100 mg) was dried in an electric blast drying oven at 100 °C for 2 hours to remove the absorbed moisture. The fluorination of dried CNFs aerogel was carried out in sealed stainless steel (SUS316) reaction container equipped with a vacuum line. The closed reaction container was vacuumed by vacuum pump and then filled with nitrogen (99.99% purity).25-28 This operation was repeated for three times to remove residual oxygen and moisture in the container. Then 50 KPa F2/N2 (1 vol% for F2) mixture gas was introduced into the container to direct fluorination at room temperature (RT) for 30 minutes. After the fluorination reaction finished, the residual gas in the reactor was



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withdrawn through a vacuum pump to be absorbed by alumina adsorbent and replaced with nitrogen for three times. Thereafter, the sample was taken out and soaked in distilled water for 2 hours. The distilled water was poured out and replaced with fresh distilled water and soaked for 1 hour. Distilled water was then replaced with absolute ethanol. Finally, ethanol was removed by rotary evaporation on a rotary evaporator. Characterization. The composition and chemical state of the surface chemical elements of cellulose nanofibril aerogels were characterized by X-ray photoelectron spectroscopy (XPS) on a Kratos ASAM 800 spectrometer (Kratos Analytical Ltd, UK) using an Al Kα (1486.6 eV) monochromated source (a voltage of 15 kV, a wattage of 250 W). In order to correct the shift due to the effect of charge, the C 1s peak at 284.6 eV of the surface adventitious carbon is used as reference for the binding energies. Spectra were analyzed using CasaXPS software and the treatment of core peaks was carried out using a nonlinear Shirley-type background. The FTIR with the attenuated total reflection mode was achieved on a Nicolet 560 FTIR spectrometer (Thermo Scientific, Waltham, MA, USA) using KBr pellets. The dynamic contact angle of the samples were achieved by pressing the aerogel samples into films, and then the films of aerogel were tested on a surface tension meter (Kruss 100, Hamburg,Germany) with DI water. BET: BET specific surface area and BJH pore size data of the sample were obtained by nitrogen adsorption and desorption isotherms supporting by Quanta chrome Instruments Trading (Shanghai) Co., Ltd. The thermal stability of the aerogel was measured using a thermogravimetric analyzer TGA (PerkinElmer TGA4000，USA) under a nitrogen flow of 60 ml/min. Each sample was heated from 40 °C to 800 °C at a heating rate of 10 °C/min. 6 ACS Paragon Plus Environment

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Mechanical Properties: In order to test the mechanical properties of aerogels, the cellulose aerogels were made into cylinders and then measured their compressive strength in air for a maximum of 90% compression strain on an Universal Material Testing System (INSTRON USA). The cyclic compression tests were performed in water using a compress and release routine with maximum compression of 85% strain, and the number of cycles is 20 times. During stress release, the height at zero stress is recorded as the recovery height (hi), and the origin height of aerogel was marked as ho, then the shape recovery percentage was calculated by the fallowing equation: Shape Recovery Percentage (%) =hi/ho×100%

(1)

Density and Water Absorption Capacity: The F-CNFs aerogel was cut into a cylinder with a diameter of 1cm and a height of 1cm. After drying in an oven at 80 °C for 2 hours, measured its dimensions with a vernier caliper (accuracy of 0.02 mm) at five different positions and calculate its volume (V), then weigh its mass (m1) using an analytical balance (readability 0.1 mg). The density can be calculate by ρ = m1/ V. Each density data is obtained by averaging the density of four different samples. After measured the density, the CNFs aerogel was immersed in 20 ml distilled water for 24 hours, then taken out the water-absorbed sample and weighed its mass (m2). The absorption capacity was calculated according to the following formula: Absorption Capacity (g/g) = (m2- m1)/ m1

(2)

For the cyclic water absorption test, the water-absorbent sample was squeezed through two glass slides to squeeze the absorbed water out. And wipe the surface of extruded sample with water-absorbent paper, and weighed its mass (mi). Then immersed the extruded sample in 20 ml distilled water for 30 minutes and weighted its mass (mi’). Finally dry the surface of the extruded sample with water absorbent paper, and weighed its mass (mi). Guarantee its deformation above 7 ACS Paragon Plus Environment


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90% and reduce its water absorption ability to 10 g/g or less at each extrusion. We performed 20 cycle experiments for each sample and used five samples for each experiment. Shape Recovery Ability: The cylindrical F-CNFs aerogel was immersed in distilled water for 24 hours. Then we squeezed saturated cellulose aerogels by two glass slides to ensure that their deformation reached 90% and their water absorption were also dropped to about 10% (10% means that the mass of water retained by the extruded F-CNFs is 10% of the mass of water retained by saturated F-CNFs aerogels.). The compressed F-CNFs aerogels were then diverted into deionized water and the video is used to record the shape recovery. RESULTS AND DISCUSSION The Water-durable Property and Hydrophilicity of F-CNFs. In order to study the waterdurable property of the CNFs aerogel and F-CNFs aerogel, the CNFs and F-CNFs aerogels were respectively submersed in deionized water as shown in Figure 1a. As we found, the CNFs gel without fluorination easily disintegrated in water by mildly swing motion and formed a stable dispersion system, for the hydrogen bonding interactions between nanofibers are destroyed; however, the F-CNFs gel can still maintain a three-dimensional structure in water even after intensely shaking and even stirring by magnetic stirrer (Figure 1c and Video S1),. In fact, as shown in Figure 1b and Video S2, after boiled in boiling water for two hours F-CNFs gel still maintained its original shape and size, rather than disintegrated in the boiling water. As far as we know, there are two possible reasons contributing to the morphological stability of the cellulose gels, namely, the increase of hydrophobicity of F-CNFs materials due to fluorination, as well as the formation of physical or chemical crosslinking point of F-CNFs gels. To illustrate the exact reason, we measured the hydrophilicity of the cellulose nanofibrils before and after fluorination.


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Figure 1. (a) Images of CNFs aerogel disintegrated in water and F-CNFs aerogel remained intact in water; (b) F-CNFs aerogel remained intact in boiling water and (c) in stirring water; (d) the time-dependent water contact angle of CNFs and F-CNFs.

To test the hydrophilicity of CNFs aerogel and F-CNFs aerogel, we pressed the aerogels into flakes with glass slides and then measured their dynamic contact angles through the drop method. The time-dependent of contact angles is shown in Figure 1d. As we can see, the initial contact angle (the time of the water drop contact sample surface is zero) of F-CNFs is 39.2° which is almost 50% lower than that of CNFs, whose initial contact angle is 76.4°. Correspondingly, the contact angle of F-CNFs drops to 10.6° in 2.0 s and 4.4° in just 6.8 s, while the CNFs still has a contact angle of 45.9° in 2.0 s and 25.8° after 16.0 s. It’s obvious that after 9 ACS Paragon Plus Environment


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fluorination, the hydrophilicity of F-CNFs is not weakened but greatly increased compared to that of CNFs. The increase in the hydrophilicity of F-CNFs means that the dimensional stability of F-CNFs in water cannot be attributed to an enhancement of the hydrophobicity. Surface Morphology. It can be seen from the SEM images (Figure S2) that both CNFs and F-CNFs present honeycomb-like porous structure with a few hundred micrometers. These macropores were caused by the formation of ice crystals and phase separation during freezing process.29 It’s obvious that F-CNFs aerogel still remains a honeycomb-like porous structure on large dimensions as the same as CNFs. Figure S2c shows closed-up image of the wall on the honeycomb-like structure in CNFs aerogel. The honeycomb walls of CNFs aerogel which was caused by squeezing and concentrating the cellulose nanofibril solid components by ice crystals exhibit a rough planar structure with many dozens of nanometers of holes and fibrous structure. These holes and fibrous structure were formed by random stacking of cellulose nanofibrils during freezing.30 The coexistence of these macropores and mesopores conforms to their large specific surface area which is conducive to making them hydrophilic material. Compared to CNFs, the structure of F-CNFs didn’t change significantly and retained the structure of macropores and mesopores coexistence. The specific surface area and pore diameter were calculated by the BET and BJH methods, respectively, as shown in Table S1. As we can see, the specific surface area of F-CNFs is 97 m2/g, which is lower than that of CNFs who owns a specific surface area of 139 m2/g. The pore diameter of F-CNFs increased from 3.1 nm of CNFs to 5.6 nm, which is supposed to be the reason leading to the decrease in specific surface area of F-CNFs. In general, there was no macroscopical structural change occurred in the cellulose nanofibril aerogels after fluorination, but the pore size of F-CNFs increased on the microstructure.


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ATR-FTIR. In order to explore the changes of chemical composition of fluorine-treated cellulose, the ATR-FTIR spectroscopy was introduced. As shown in Figure 2a, the bands at 3420 cm-1, 2920 cm-1, 1422 cm-1, 1375 cm-1, 1161 cm-1, 1110 cm-1, 1057 cm-1 and 894 cm-1 are typical absorption peaks of CNFs.31-32 The absorption peaks at 1161 cm-1, 1110 cm-1, 1057 cm-1 and 894 cm-1 are attributed to C-O-C asymmetric stretching vibrations, in-plane ring stretching, C-O stretching at C-3 and β-glycosidic linkages, respectively. In fact, the peaks at 1161 cm-1, 1110 cm-1 and 894 cm-1 are related to the main chemical linkages of the cellulose molecular skeleton. As we can see, those typical absorption peaks of CNFs are all appeared in the spectroscopies of F-CNFs and most of them don’t show obvious changes, which means that F-CNFs maintained the main chemical structure of cellulose. Although F-CNFs preserved the main structure of CNFs, there are still some changes in their absorption peak. The absorbency at 1375 cm-1 and 1422 cm-1, assigned as C-H bending and symmetric -CH2 bending respectively.33 Compared to CNFs, the absorbency at 1422 cm-1 of F-CNFs decreased obviously and the value of I1422/I1375 decreased from 1.102 to 0.897 (As shown in the insert table in Figure 2a), which reviewed that the decrease of CH2 coming from C-6. In addition, the ATR-FTIR spectroscopy of F-CNFs appears an obvious absorption at 1730 cm-1 compared to that of CNFs who shows a tiny absorbency at 1730 cm-1, indicating the increase of the carbonyl (C=O) in F-CNFs. The new absorption peaks appeared at 1730 cm-1 in the spectroscopy of F-CNFs may rose from C=O vibrations of aldehyde group, carboxyl and ester functions.34-37 Subsequently, we also determined the increase of carbonyl in F-CNFs by solid-state NMR (as shown in Figure S3). The peak at 3420 cm-1 is assigned to the stretching vibration of hydroxyl groups (-OH) in CNFs, while for F-CNFs, the position of the hydroxyl stretching vibration peak shifted to 3432 cm-1. As we have known, the formation of hydrogen bonds makes the electron density of -OH decrease and the bond length of



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hydroxyl group increase, which results in the stretching vibration absorption of -OH shifting to a lower wave number. Therefore, the fact that the absorption peak of the hydroxyl group in the FCNFs shifted to higher wavenumber of 3432 cm-1 indicates that the number of hydrogen bonds in the sample of F-CNFs decreased.38-39 A decrease in the number of hydrogen bonds causes more free hydroxyls to be exposed, meanwhile, the carboxyl groups embedded in the cellulose nanofibrils by hydrogen bonds will also be exposed, thereby increasing the hydrophilicity of the material. In addition, it is necessary to point out that there were no obvious absorption peaks assigned to C-F bonds or any other F-containing groups observed in the IR spectra.

Figure 2. (a) The ART-FTIR spectra of CNFs and F-CNFs; (b) TGA and DTG curves of CNFs and F-CNFs; C 1s spectra of CNFs (c) F-CNFs (d). The table inserted in Figure 2a is the value of I1422/I1375 calculated from the ATR-FTIR data, and the molecular structure inserted in the Figure 2a is a fragment of cellulose molecule.

Surface Chemical Composition and Structure. The changes of the surface chemical structure of F-CNFs relative to that of CNFs were studied by XPS. As Table S2 shows, the content of F element in both CNFs and F-CNFs is 0, which means that F element has not been 12 ACS Paragon Plus Environment

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introduced into the material. In addition, compared with CNFs, it can be seen that the O/C of FCNFs has decreased from 0.61 to 0.53. The decrease in oxygen content may be due to dehydration of cellulose during fluorination.40 These indications indicate that the chemical composition of cellulose may have changed after fluorination. To further explore the changes in the chemical structure of cellulose before and after fluorination, we introduced the C 1s spectrum analysis of CNFs and F-CNFs as shown in Figure 2c, 2d and Table S2.The peak of 288.1 eV with an amount of 12.99% in the C1s spectrum of the CNFs may represent carbonyl groups (C*=O) or hemiacetal groups (O-C*-O), while in the C1s spectrum of F-CNFs, the content of 288.1 eV representing C*=O or O-C*-O increased to 17.94%.41 This is consistent with the increase in the absorption peak intensity at 1730 cm-1 in ATR-FTIR, that is, the carbonyl content in F-CNFs increased after fluorination. After fluorination, the peak at 286.3 eV assigned to C*-OH shows a decrease from 48.77% to 40.70%, which means that the content of C-OH decreased.

42

Furthermore, the content of the C-C was 41.08% and 41.36% in CNFs and F-CNFs respectively. The content of C-C in CNFs and F-CNFs were basically the same, which indicated that the skeleton in F-CNFs is well preserved. This is consistent with the results of ATR-FTIR. As we all know, carbonyl generally represents the three structures of aldehyde group, carboxyl and ester functions. Although the results of both ATR-FTIR and XPS demonstrate an increase in the carbonyl content, it is difficult for the two characterization methods to distinguish the specific structure of the carbonyl group. To further demonstrate the specific structure of the carbonyl group in F-CNFs, we hydrolyzed F-CNFs in an alkaline solution, and the hydrolysis of F-CNFs in an alkaline solution indicated the possible presence of ester groups in F-CNFs (Figure S4 and Figure S5). Then we determined the content of carboxyl and ester groups in F-CNFs by



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titration (Figure S6). These evidences indicate that both the carboxyl and ester groups in F-CNFs are greatly increased compared to CNFs. Thermal Stability of CNFs and F-CNFs. The TGA and the corresponding DTG curves of CNFs and F-CNFs are shown in Figure 2b and the characteristic values of TGA and DTG are listed in Table S3. The weight loss before 100 °C for CNFs and F-CNFs is attributed to the evaporation of moisture absorbed by the cellulose nanofibril aerogels.43 The Tmax and T10% represent the temperatures at which the maximum degradation of cellulose and 10% weight loss of cellulose occurred, respectively. The Tmax and T10% of CNFs aerogels are 311 °C and 230 °C, respectively. For F-CNFs aerogels, the Tmax and T10% are 342°C and 239 °C respectively. Obviously, the Tmax and T10% of F-CNFs aerogels are higher than those of CNFs, which means that the thermal stability of F-CNFs was improved. The increase of thermal stability can be attributed to the formation of chemical cross-linking in F-CNFs. Although F-CNFs aerogel have better thermal stability, its char yield (CY) is lower than that of CNFs aerogel. The decrease of CY from 20.4% to 17.2% may be due to the higher carbonyl content in F-CNF. During the thermal degradation of cellulose, hydroxyl groups typically leave the cellulose in a dehydrated manner while forming double bonds on the cellulose, in which case the carbon element remains on the cellulose skeleton.44 The carboxyl groups leave the cellulose molecules in the form of a decarboxylation reaction, which removes carbon while removing oxygen. That is, the increase of carbonyl content of F-CNFs and the decrease of hydroxyl content after treatment decrease the residual carbon rate of F-CNFs. Mechanical Properties and Shape Recovery. Unlike conventional inorganic silicone aerogel, F-CNFs gels exhibit high ductility and toughness under water. After 90% compression set, water absorbing F-CNFs can quickly recover to their original state in an anhydrous


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environment with no significant damage (Video S3). Figure 3a shows the compressive stress−strain curves of CNFs and F-CNFs. The compressive stresses of both CNFs and F-CNFs aerogels shown a slowly increase of before 70% compressive strain and a rapid increase after 70% compressive strain. The compressive stress showed an exponentially increasing when the compressive strain exceeded 80%. At low compressive deformation, the deformation of the aerogels is mainly due to the bending and destruction of the hydrogen bonds and the physical crosslinking points,45 while as the compression ratio increases, the material becomes more and more compact, and the contact points between the fibers increase. The formation of a large number of new contact points promotes a sharp increase in the compressive strength of the material when compressive strain is over 80%.46 When the compressive strain is greater than 80%, the compressive stress-strain curves of FCNFs aerogels were always higher than those of CNFs. Specific stress-strain data for CNFs and F-CNFs aerogels were shown in Figure 3b. The stress values of F-CNFs at 80%, 85% and 90% strain were 66.6 KPa, 123.5 KPa and 299.2 KPa, respectively, while the corresponding stress values of CNFs were 59.0 KPa and 96.9 KPa, 193.7 KPa respectively. During the compression process, physical cross-linking points (such as entanglement or hydrogen bonding) may slip or untangle, while chemical cross-linking points are relatively more stable and not easy to change. As proved by alkaline hydrolysis and titration experiments of F-CNFs, a large number of ester crosslinking points have been introduced into F-CNFs, while the contact points between the fibers in CNFs were mainly hydrogen bond interactions or entanglement. Therefore, the F-CNFs have more contact points than CNFs under the same compression condition. Meanwhile the chemical cross-linking points in F-CNFs will make the materials more compact, thereby increasing the compressive strength of F-CNFs.



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Figure 3. (a) Compressive stress−strain curves and (b) histogram of stress contrast of F-CNFs and CNFs aerogels at different strains; (c) cyclic compressive stress–strain hysteresis and (d) shape recovery percentage of F-CNFs gels under water at ε=85% after 20 cyclic compressions; (e) cyclic water absorption of F-CNFs with density of 10.39 mg/cm3; images of F-CNFs before (f) and after (g) compressing water out in the cyclic water absorption experiment.

In addition, we found that the mechanical properties of the cellulose aerogels prepared under our process conditions are better than those of the reported cellulose nanofiber aerogels. The specific compressive strength of the F-CNFs aerogel prepared by us at 80% strain is 6.41 MPa cm3 g-1. While the specific compressive strength of TEMPO-oxidized cellulose nanofibril 16 ACS Paragon Plus Environment

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aerogels at 80% strain is 3.12 MPa cm3 g-1 and the specific compressive strength of nanofibrillated cellulose aerogels prepared by solvent exchange at 70% strain was less than 2.27 MPa cm3 g-1.47-48 To evaluate the compressibility and shape recovery performance of aerogels under water in the recycling process the cyclic compression tests were undertaken (Video S4). As can be seen from the cyclic compressive stress–strain hysteresis curves (Figure 3c), F-CNFs gels still had good cycling performance even at maximum compressive strain of 85%, especially in compression strain between 0 and 60% range. Shape recovery percentage of F-CNFs gels under water at ε=85% after 20 cyclic compressions are shown in Figure 3d. In all 20 cycles of compression experiments, the shape recovery percentage of F-CNFs gels remained above 95%. In fact, except for the first cycle, in which the shape recovery rate of the material decreased significantly (decreased from 100% to 97.2%), the shape recovery percentage of the F-CNFs gels materials showed only a slight downward trend within the range of the 2nd to 20th cycles. The Video showing the ability of F-CNFs gels to recovery shape in water can be found in Supporting Information, Video S4. Excellent shape recovery ability can be attributed to the cross-linked structure within the material and its good hydrophilic properties. Water Absorption Performance. To detect the water absorption properties of the F-CNFs aerogel material, we prepared some cylindrical samples about 1 cm in diameter and about 0.8 cm in height. Before immersing the F-CNFs aerogels in deionized water, we measured the mass and volume of each sample separately and calculated their density. We have prepared two different densities of F-CNFs aerogel by suspension with different concentrations, namely F-LCNFs and F-HCNFs. As shown in Table S4, F-LCNFs and F-HCNFs coming from the suspension with concentrations of 0.5 wt% and 1 wt% own low density of 10.39 mg/cm3 and 17.31 mg/cm3



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respectively. The difference in water absorption of these two different density aerogels indicates that we can adjust the water absorption rate by adjusting the density of the material. Correspondingly, the water absorption of F-LCNFs can reach 123.65 g/g which is comparable to the reported density of cellulosic aerogels by others.47-48 Different from the law of water absorption, we found that the higher the density of fluorine-treated cellulose nanofibrils aerogels, the faster the shape recovery rate of the aerogels when immersing in water. We have found that the shape recovery of F-LCNFs is about 4 s (Video S5) and the time of F-HCNFs is about 2 s (Video S6). The faster shape recovery rate of F-HCNFs may be due to the higher cellulose content inside it, which can increase the rigidity of the gel and lead to a faster shape recovery rate. Up to now, we have found that F-CNFs has a good water absorption and shape recovery rate while whether it can be reused is another important basis for testing the quality of the material. The F-CNFs aerogel performed very good cyclic water absorption as shown in Figure 3e. Even after 90% compression set (Figure 3f and 3g) and 30-minute water absorption every time, FCNFs aerogel still retained better water absorption (from 123.65 g/g to 92.80 g/g) after 20 cycles. The Chemistry Reaction during Fluorination. Due to the highly reactive nature of the fluorine gas, the reaction between the fluorine gas and the organic molecule is difficult to be controlled, and the organic matter will be seriously damaged.49-50 In order to control the reaction between F2 and organic molecule, an inert gas is often used to dilute fluorine gas.51 In many cases, the application properties of polymer product are primarily determined by its surface properties.52 So the application of F2 in polymer surface modification is outstanding when the fluorination reaction can be controlled. The direct fluorination method has now been considered as an effective modification method for polymer.53-55 In addition to polymers, surface modification using fluorine gas is also used in many other materials, such as carbon materials56


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and metal oxides.57 However, since the fluorine gas is too active, it can react with various structures in the material, so the selectivity of the fluorine gas fluorination reaction is poor. Herein, fluorine gas was diluted and fluorinating conditions were optimized in order to obtain a moderate reaction (Figure S8). The chemically crosslinked F-CNFs aerogels we obtained have excellent water resistance, hydrophilicity, mechanical properties, water absorption capacity and reusability. The results of FTIR and XPS confirmed the increase of carbonyl content in F-CNFs, and the alkaline hydrolysis and titration experiments of F-CNFs proved the formation of carboxyl groups and ester groups in F-CNFs. The water resistance and hydrophilicity of FCNFs indicated the formation of chemical crosslinks in F-CNFs. Therefore, we infer that the ester bond cross-linking points formed in F-CNFs endow the F-CNFs enduring dimensional stability in water, while the carboxyl groups formed in F-CNFs increase the hydrophilicity. In the results of XPS, the decreases of O/C and -OH in F-CNFs indicate that dehydration reaction of FCNFs occurred during fluorination treatment. The F2 is likely to oxidize hydroxyl groups in cellulose due to the strong oxidizing properties of fluorine gas.50 The results of ATR-FTIR showed that the C-6 content was significantly reduced while the CNFs skeleton remained intact, so we concluded that the oxidation site mainly occurred at the C-6 position. Through the above structural analysis, we speculate on the chemical structure of CNFs which undergo cross-linking during fluorination as shown in Figure 4a. There are dehydration, acylation, and esterification reactions of cellulose molecule during the fluorination of cellulose. First, the hydroxyl groups on C-6 of the cellulose molecule (A) in the atmosphere of fluorine convert to an acyl fluoride bond (A’). Besides the hydroxyl group on C-2 will desorb with the hydrogen of the hydroxyl group on C-3 (A) to form water and carbonyl group located in C-3 (A’).



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Then a part of the acyl fluoride bonds are hydrolyzed by water to a carboxyl group (A’’). Those formed carboxyl groups will be retained to improve the hydrophilicity of F-CNFs gel. For other acyl fluoride bonds, they react with adjacent hydroxyl groups in another cellulose molecule (B) to form ester bonds by the reaction of alcoholysis which links different cellulose molecules together (A’’’ and B’). Meanwhile, hydrogen fluoride (HF) is released as by-product, making fluorine atom be absent in the fluorine-treated CNF aerogel. These reactions occurred mainly in the amorphous region of the cellulose nanofibrils as there was no significant change in the crystallization of cellulose nanofibrils aerogel before and after the reaction (Figure S7). From the existing results, we speculated a change in the physical structure that could lead to an increase in the F-CNFs nanopore (the specific analysis process can be found in Supporting Information). As shown in Figure 4b, the cellulose nanofibrils in CNFs are mainly bound together by hydrogen bonding interactions and entanglements. After treating by fluorine, the formation of the covalent ester linking between cellulose nanofibers will reduce the distance between fibers, since the length of the covalent cross-linking point of the ester group is shorter than that of the hydrogen bond interaction. The local tight aggregation of CNFs facilitates the enhancement of mechanical properties of final CNF aerogel, such as compressive stress and resilience. Simultaneously, outside of the sites where CNFs are aggregated, the space between cellulose nanofibers became bigger. In addition, compared to hydrogen bonding interactions, the formation of chemical crosslinking sites enhances the interaction between cellulose nanofibers. The chemical crosslinking formed at the fiber lap joint ensures that the aerogel material remains intact in the aqueous solvent rather than being disintegrated in water.


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Figure 4. (a) Possible esterification crosslinking reaction of cellulose molecules and (b) schematic of structural changes for cellulose Nano-fibers during fluorination.

CONCLUSIONS In summary, we treated the CNFs aerogel by the direct fluorination method, which improved the resistance of the material to disintegrate in water and improved the hydrophilicity of the material simultaneously. Compared with the untreated CNFs aerogel, the pore size of FCNFs increased from 3.1 nm to 5.6 nm, and there were no significant change on the larger size (> 20 nm) between CNFs and F-CNFs. Our research shows that after fluorination the contents of 21 ACS Paragon Plus Environment


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carboxyl and ester functional groups were significantly increased and the hydrogen bonding interactions within the CNFs aerogel were destroyed. The increase of carboxyl groups can improve the hydrophilicity of the material, and the ester group can form the chemical crosslinking point between the cellulose molecules to improve the ability for materials to resist breaking in water and even in boiling water. Due to the formation of chemical crosslinking points, F-CNFs aerogel showed good mechanical properties both in air and in water. The synergistic enhancement of hydrophilia and morphological stability of CNFs in water makes it behaved the excellent water absorption and shape recovery rate. More importantly, these good properties of the F-CNFs aerogel were maintained at a high level after 20 cycles, which will provide the potential of material practical application. ASSOCIATED CONTENT Supporting Information AFM image of CNFs; SEM and XRD images of CNFs and F-CNFs; tables of Specific surface area and hole size analyses, element analyses by XPS,TGA analyses of CNFs and F-CNFs; tables of comparison of strength between F-CNFs and other cellulose Nano aerogels, the water absorption performance of F-CNFs aerogels with different densities. (PDF) Videos of F-CNFs aerogels keeping intact aggregated structure in stirring water and boiling water; videos of water absorbing F-CNFs for quickly recover in an anhydrous environment and under water; videos of shape recovery of F-CNFs with different densities. (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: *[email protected] (X. Wang); [email protected] (X. Liu) 22 ACS Paragon Plus Environment

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Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51633004 and Grant No. 51573105), the Joint fund of National Defense Science & Technology Committee and Ministry of Education (614A02033219) and the National Key Research and Development Plan (2017YFB0404701) ABBREVIATIONS CNFs, Cellulose Nanofibrils; F-CNFs, Fluorine-treated Cellulose Nanofibrils; F-LCNFs, Fluorine-treated Cellulose Nanofibrils coming from a low suspension concentration of 0.5 wt%; F-HCNFs, Fluorine-treated Cellulose Nanofibrils coming from a high suspension concentration of 1.0 wt%. REFERENCES 1. Takaichi, S.; Saito, T.; Tanaka, R.; Isogai, A., Improvement of nanodispersibility of ovendried TEMPO-oxidized celluloses in water. Cellulose 2014, 21 (6), 4093-4103. 2. Diddens, I.; Murphy, B.; Krisch, M.; Müller, M., Anisotropic Elastic Properties of Cellulose Measured Using Inelastic X-ray Scattering. Macromolecules 2009, 41 (24), 9755-9759. 3. Siró, I.; Plackett, D., Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose 2010, 17 (3), 459-494. 4. Zhu, C.; Soldatov, A.; Mathew, A. P., Advanced microscopy and spectroscopy reveal the adsorption and clustering of Cu(ii) onto TEMPO-oxidized cellulose nanofibers. Nanoscale 2017, 9 (22), 7419-7428.



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5. Zhang, N.; Zang, G. L.; Shi, C.; Yu, H. Q.; Sheng, G. P., A novel adsorbent TEMPOmediated oxidized cellulose nanofibrils modified with PEI: Preparation, characterization, and application for Cu(II) removal. J Hazard Mater 2016, 316, 11-8. 6. Nemoto, J.; Saito, T.; Isogai, A., Simple Freeze-Drying Procedure for Producing Nanocellulose Aerogel-Containing, High-Performance Air Filters. ACS Appl Mater Interfaces 2015, 7 (35), 19809-15. 7. Zhang, Z.; Sèbe, G.; Rentsch, D.; Zimmermann, T.; Tingaut, P., Ultralightweight and Flexible Silylated Nanocellulose Sponges for the Selective Removal of Oil from Water. Chemistry of Materials 2014, 26 (8), 2659-2668. 8. Cervin, N. T.; Aulin, C.; Larsson, P. T.; Wågberg, L., Ultra porous nanocellulose aerogels as separation medium for mixtures of oil/water liquids. Cellulose 2011, 19 (2), 401-410. 9. Jang, K. H.; Kang, Y. O.; Park, W. H., Functional cellulose-based nanofibers with catalytic activity: effect of Ag content and Ag phase. Int J Biol Macromol 2014, 67, 394-400. 10. Jorfi, M.; Foster, E. J., Recent advances in nanocellulose for biomedical applications. Journal of Applied Polymer Science 2015, 132 (14), 41719. 11. Barua, S.; Das, G.; Aidew, L.; Buragohain, A. K.; Karak, N., Copper–copper oxide coated nanofibrillar cellulose: a promising biomaterial. RSC Advances 2013, 3 (35), 14997. 12. Alexandrescu, L.; Syverud, K.; Gatti, A.; Chinga-Carrasco, G., Cytotoxicity tests of cellulose nanofibril-based structures. Cellulose 2013, 20 (4), 1765-1775. 13. Goetz, L.; Foston, M.; Mathew, A. P.; Oksman, K.; Ragauskas, A. J., Poly(methyl vinyl ether-co-maleic acid)-polyethylene glycol nanocomposites cross-linked in situ with cellulose nanowhiskers. Biomacromolecules 2010, 11 (10), 2660. 14. Spoljaric, S.; Salminen, A.; Luong, N. D.; Seppälä, J., Stable, self-healing hydrogels from nanofibrillated cellulose, poly(vinyl alcohol) and borax via reversible crosslinking. European Polymer Journal 2014, 56, 105-117. 15. Cai, H.; Sharma, S.; Liu, W.; Mu, W.; Zhang, X.; Deng, Y., Aerogel microspheres from natural cellulose nanofibrils and their application as cell culture scaffold. Biomacromolecules 2014, 15 (7), 2540-7. 16. Zhang, W.; Zhang, Y.; Lu, C.; Deng, Y., Aerogels from crosslinked cellulose nano/microfibrils and their fast shape recovery property in water. Journal of Materials Chemistry 2012, 22 (23), 11642. 17. Yang, X.; Cranston, E. D., Chemically Cross-Linked Cellulose Nanocrystal Aerogels with Shape Recovery and Superabsorbent Properties. Chemistry of Materials 2014, 26 (20), 6016-6025. 18. Erlandsson, J.; López Durán, V.; Granberg, H.; Sandberg, M.; Larsson, P. A.; Wågberg, L., Macro- and mesoporous nanocellulose beads for use in energy storage devices. Applied Materials Today 2016, 5, 246-254. 19. Korhonen, J. T.; Kettunen, M.; Ras, R. H.; Ikkala, O., Hydrophobic nanocellulose aerogels as floating, sustainable, reusable, and recyclable oil absorbents. ACS Appl Mater Interfaces 2011, 3 (6), 1813-6. 20. Khanjani, P.; King, A. W. T.; Partl, G. J.; Johansson, L. S.; Kostiainen, M. A.; Ras, R. H. A., Superhydrophobic Paper from Nanostructured Fluorinated Cellulose Esters. ACS Appl Mater Interfaces 2018, 10 (13), 11280-11288. 21. Stelte, W.; Sanadi, A. R., Preparation and Characterization of Cellulose Nanofibers from Two Commercial Hardwood and Softwood Pulps. Industrial & Engineering Chemistry Research 2009, 48 (24), 11211-11219. 24 ACS Paragon Plus Environment

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


22. Ahola, S.; Salmi, J.; Johansson, L. S.; Laine, J.; Osterberg, M., Model films from native cellulose nanofibrils. Preparation, swelling, and surface interactions. Biomacromolecules 2008, 9 (4), 1273-82. 23. Wang, B.; Sain, M., Isolation of nanofibers from soybean source and their reinforcing capability on synthetic polymers. Composites Science and Technology 2007, 67 (11-12), 25212527. 24. Abdul Khalil, H. P. S.; Bhat, A. H.; Ireana Yusra, A. F., Green composites from sustainable cellulose nanofibrils: A review. Carbohydrate Polymers 2012, 87 (2), 963-979. 25. Wang, X.; Dai, Y.; Wang, W.; Ren, M.; Li, B.; Fan, C.; Liu, X., Fluorographene with high fluorine/carbon ratio: a nanofiller for preparing low-kappa polyimide hybrid films. ACS Appl Mater Interfaces 2014, 6 (18), 16182-8. 26. Li, B.; Li, M.; Fan, C.; Ren, M.; Wu, P.; Luo, L.; Wang, X.; Liu, X., The wear-resistance of composite depending on the interfacial interaction between thermoplastic polyurethane and fluorinated UHMWPE particles with or without oxygen. Composites Science and Technology 2015, 106, 68-75. 27. Liu, Y.; Zhang, C.; Huang, B.; Wang, X.; Li, Y.; Wang, Z.; Lai, W.; Zhang, X.; Liu, X., Skin-core structured fluorinated MWCNTs: a nanofiller towards broadband dielectric material with high dielectric constant and low dielectric loss. Journal of Materials Chemistry C 2018, 6 (9). 28. Lai, W.; Wang, X.; Li, Y.; Liu, Y.; He, T.; Fan, K.; Liu, X., The particular phase transformation during graphene fluorination process. Carbon 2018, 132, 271-279. 29. Pan, Z. Z.; Nishihara, H.; Iwamura, S.; Sekiguchi, T.; Sato, A.; Isogai, A.; Kang, F.; Kyotani, T.; Yang, Q. H., Cellulose Nanofiber as a Distinct Structure-Directing Agent for Xylemlike Microhoneycomb Monoliths by Unidirectional Freeze-Drying. ACS Nano 2016, 10 (12), 10689-10697. 30. Han, J.; Yue, Y.; Wu, Q.; Huang, C.; Pan, H.; Zhan, X.; Mei, C.; Xu, X., Effects of nanocellulose on the structure and properties of poly(vinyl alcohol)-borax hybrid foams. Cellulose 2017, 24 (10), 4433-4448. 31. Sun, Y.; Lin, L.; Deng, H.; Li, J., Structural changes of bamboo cellulose in formic acid. Bioresources 2008, 3 (2), 297-315. 32. Oh, S. Y.; Yoo, D. I.; Shin, Y.; Kim, H. C.; Kim, H. Y.; Chung, Y. S.; Park, W. H.; Youk, J. H., Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy. Carbohydr Res 2005, 340 (15), 2376-91. 33. Colom, X.; Carrillo, F., Crystallinity changes in lyocell and viscose-type fibres by caustic treatment. European Polymer Journal 2002, 38 (11), 2225-2230. 34. Habibi, Y.; Chanzy, H.; Vignon, M. R., TEMPO-mediated surface oxidation of cellulose whiskers. Cellulose 2006, 13 (6), 679-687. 35. Benítez, A. J.; Walther, A., Counterion Size and Nature Control Structural and Mechanical Response in Cellulose Nanofibril Nanopapers. Biomacromolecules 2017, 18 (5), 1642. 36. Gopakumar, D. A.; Pasquini, D.; Henrique, M. A.; de Morais, L. C.; Grohens, Y.; Thomas, S., Meldrum’s Acid Modified Cellulose Nanofiber-Based Polyvinylidene Fluoride Microfiltration Membrane for Dye Water Treatment and Nanoparticle Removal. ACS Sustainable Chemistry & Engineering 2017, 5 (2), 2026-2033.



Page 26 of 27

37. Liu, L.; Sun, J.; Li, M.; Wang, S.; Pei, H.; Zhang, J., Enhanced enzymatic hydrolysis and structural features of corn stover by FeCl3 pretreatment. Bioresour Technol 2009, 100 (23), 5853-8. 38. Cao, Y.; Tan, H., Structural characterization of cellulose with enzymatic treatment. Journal of Molecular Structure 2004, 705 (1–3), 189-193. 39. Cao, Y.; Tan, H., Effects of cellulase on the modification of cellulose. Carbohydrate Research 2002, 337 (14), 1291-1296. 40. Zhang, M.; Geng, Z.; Yu, Y., Density Functional Theory (DFT) Study on the Dehydration of Cellulose. Energy & Fuels 2011, 25 (6), 2664-2670. 41. Li, J.; Wan, Y.; Li, L.; Liang, H.; Wang, J., Preparation and characterization of 2,3dialdehyde bacterial cellulose for potential biodegradable tissue engineering scaffolds. Materials Science and Engineering: C 2009, 29 (5), 1635-1642. 42. You, J.; Zhao, C.; Cao, J.; Zhou, J.; Zhang, L., Fabrication of high-density silver nanoparticles on the surface of alginate microspheres for application in catalytic reaction. J. Mater. Chem. A 2014, 2 (22), 8491-8499. 43. Fukuzumi, H.; Saito, T.; Iwata, T., Transparent and High Gas Barrier Films of Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation. Biomacromolecules 2009, 10 (1), 162-165. 44. Wang, S.; Dai, G.; Yang, H.; Luo, Z., Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review. Progress in Energy and Combustion Science 2017, 62, 33-86. 45. Sehaqui, H.; Salajková, M.; Zhou, Q.; Berglund, L. A., Mechanical performance tailoring of tough ultra-high porosity foams prepared from cellulose I nanofiber suspensions. Soft Matter 2010, 6 (8), 1824. 46. Ashby, M. F., The properties of foams and lattices. Philosophical Transactions Mathematical Physical & Engineering Sciences 2006, 364 (1838), 15-30. 47. Jiang, F.; Hsieh, Y.-L., Super water absorbing and shape memory nanocellulose aerogels from TEMPO-oxidized cellulose nanofibrils via cyclic freezing–thawing. J. Mater. Chem. A 2014, 2 (2), 350-359. 48. Jiang, F.; Hsieh, Y.-L., Amphiphilic superabsorbent cellulose nanofibril aerogels. J. Mater. Chem. A 2014, 2 (18), 6337-6342. 49. Rozen, S., Elemental fluorine as a legitimate reagent for selective fluorination of organic compounds. Accounts of Chemical Research 1988, 21 (8), 307-312. 50. Chambers, R. D.; Darren, H.; Rees, A. J.; Graham, S., Microreactors for oxidations using fluorine. Journal of Fluorine Chemistry 2003, 119 (1), 81-82. 51. Purrington, S. T.; Kagen, B. S.; Patrick, T. B., Application of elemental fluorine in organic synthesis. Cheminform 1987, 18 (23), 997-1018. 52. Kharitonov, A. P.; Kharitonova, L. N., Surface modification of polymers by direct fluorination: A convenient approach to improve commercial properties of polymeric articles. Pure and Applied Chemistry 2009, 81 (3), 451-471. 53. Li, B.; Zhang, J.; Ren, M.; Wu, P.; Liu, Y.; Chen, T.; Cheng, Z.; Wang, X.; Liu, X., Various surface functionalization of ultra-high-molecular-weight polyethylene based on fluorineactivation behavior. Rsc Advances 2015, 5 (96), 79081-79089. 54. Du, B. X.; Li, J.; Du, W., Dynamic behavior of surface charge on direct- fluorinated polyimide films. IEEE Transactions on Dielectrics & Electrical Insulation 2013, 20 (3), 947-954. 55. Liu, Y.; An, Z.; Cang, J.; Zhang, Y.; Zheng, F., Significant suppression of surface charge accumulation on epoxy resin by direct fluorination. IEEE Transactions on Dielectrics & Electrical Insulation 2012, 19 (4), 1143-1150. 26 ACS Paragon Plus Environment

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56. Liu, Y.; Zhang, Y.; Wang, Z.; Lai, W.; Zhang, X.; Wang, X.; Liu, X., Investigation of the dispersion behavior of fluorinated MWCNTs in various solvents. Physical Chemistry Chemical Physics Pccp 2017, 19 (32), 21565. 57. Liu, Y.; Zhang, Y.; Zhang, C.; Huang, B.; Li, Y.; Lai, W.; Wang, X.; Liu, X., Low temperature preparation of highly fluorinated multiwalled carbon nanotubes activated by Fe3O4 towards enhanced microwave absorbing property. Nanotechnology 2018.

Abstract Graphic：：

A facile and energy-efficient way is provided for preparing cross-linking CNFs aerogel with enduring water-durable and enhanced hydrophilic property.


Ester Cross-linking Enhanced Hydrophilic Cellulose Nanofibrils

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