Elasticity-enhanced and Aligned Structure Nanocellulose Foam-like

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Elasticity-enhanced and Aligned Structure Nanocellulose Foam-like Aerogel Assembled with Cooperation of Chemical Art and Gradient Freezing Yuan Chen, Dongbin Fan, Shaoyi Lyu, Gaiyun Li, Feng Jiang, and Siqun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05085 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Sustainable Chemistry & Engineering

Elasticity-enhanced and Aligned Structure Nanocellulose Foam-like Aerogel Assembled with Cooperation of Chemical Art and Gradient Freezing Yuan Chen†, Dongbin Fan†, Shaoyi Lyu†, Gaiyun Li †*, Feng Jiang‡*, Siqun Wang †,§

†Chinese Academy of Forestry, Research Institute of Forestry New Technology, Hunan Collaborative Innovation Center for Effective Utilizing of Wood & Bamboo Resources, No. 1 Dongxiaofu Xiangshan Road, Haidian District, Beijing, 100091, P. R. China. ‡Department of Wood Science, the University of British Columbia, Vancouver, BC V6T 1Z4, Canada. §Center for Renewable Carbon, University of Tennessee, Knoxville, TN 37996, United States. Author Information Corresponding authors *E-mail: [email protected] *E-mail: [email protected] Email of other authors: Yuan Chen: [email protected] Dongbin Fan: [email protected] Shaoyi Lyu: [email protected] Siqun Wang: [email protected]

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ABSTRACT: Cellulose nanofibrils (CNF) aerogels are renewable and biocompatible materials with high porosity and tunable surface chemistry. However, ultralight and ultraporous aerogels remain a great challenge to obtain high elasticity. This work focused on a scalable strategy to create large-scale lamellar-aligned CNF foam-like aerogels, and the relationship between structure and mechanical property. The morphology and mechanical properties of aerogels assembled by original TEMPO-mediated oxidation CNF crosslinking with 1,2,3,4-butanetetracarboxylic acid were investigated for homogeneous freezer freezing and unidirectional gradient freeze-casting. This study successfully fabricated ultralight foam-like aerogels with centimetre-sized and aligned lamellar/porous structure via cooperation of tunable chemical reaction and unidirectional gradient freezing. The resulting aerogels exhibited yielded flyweight densities of 3~4 mg/cm3, enhanced recovery from 70% strain, over weight-self 82.5 times water adsorption at 20 ℃, and 52.0 times at high temperature (100 ℃) for 20 cycles. Moreover, the aligned aerogel followed by carbonization showed a differential and anisotropic electrical resistivity. KEYWORDS: nanocellulose aerogels, anisotropic structure, elasticity-enhanced, chemical crosslinking

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INTRODUCTION Nanocellulose aerogels are attractive due to their ultralow density, high porosity, large surface area, chemical and thermal inertness, biodegradability, and renewability1. The random three-dimensional (3D) microstructure allows for the above beneficial properties. However, the inefficient network connection and continuity also results in unsatisfactory mechanical properties2-3. Mechanical properties of the nanocellulose aerogels have been improved via strategies including reinforcing with inorganic nanocomponents4-5, complexation with other resilient materials such as graphene oxide6, chemical crosslinking7, and tuning suspension concentration8. However, reinforcing with these approaches leads to other drawbacks such as reduced porosity and specific surface area, lack of strong interactions with inorganic nanocomponents, decreased biodegradability of the synthetics, and high cost. Therefore, it is still an urgent challenge to achieve highly elastic porous nanocellulosic aerogels and improve their reusability. Despite being intrinsic high-porous and simple constituents, some natural materials still exhibit outstanding properties, such as wood9, due to its hierarchical architecture. Biomimicry has proven to be a promising route to develop new and potential materials with superior performance10-11. Structural aerogels from renewable and biocompatible materials that can maintain structural integrity upon a large deformation are highly desirable12. Even though cellulose nanofibril (CNF) aerogels as a famous biomaterial have been generally studied about its intrinsic properties13, previous research seldom focused on geometry optimization, such as cellular or cellular structure14, and high mechanical strength15 and even less on recyclability and anisotropy. Hence, exploring structural differences is significant to the wide application of porous materials, especially biomass aerogels. Freeze-drying is the common technique reported for preparation of porous aerogels or foams16 as it can greatly suppress structural collapse through bypassing 3



the liquid water phase. The freezing modes are actually different, including homogeneous freezing (quick liquid-nitrogen bath freezing17 and slow freezer freezing (FF)16), and unidirectional ice-templating18. Therein, the typical gradient freezing (GF) of aqueous suspension is initially frozen unidirectionally at a constant velocity, followed by unidirectional ice crystals as a template. Then the frozen bulk is subjected to sublimation at a depressurizing system, eventually yielding a monolith with highly ordered porous channels19. And the ordered-structure highly depends on the physical or chemical properties of the precursor suspension. Therefore, ice-templating, especially GF, has been widely applied to the manufacture of a variety of inorganic materials to form a xylem-like or cellular structure. So far, many studies have been published on the lamellar-featured macroporous monolith20-24, such as inorganic particles18, rather than the high aspect ratio of nanocellulose aerogels. Even though the seldom alignment structure of fibril aerogels or foams is investigated by using GF14, the relationship between structure and mechanical properties needs to be further studied. In this study, we reported on a robust strategy to prepare ultralight and elastic enhanced foam-like aerogels from ligno-cellulose. This approach successfully assembled high aspect ratio nanofibrils into ultralight aerogels with a large-scale aligned lamellar/porous structure via a tunable chemical reaction and unidirectional gradient freeze-casting. The objective of this study was to identify an effective method for the manufacture of centimeter-scale aligned porous structures and better understand the structure-performance relationship. These findings could have outstretched applications for other advanced materials (e.g., strain sensors, energy storage, thermal insulation and structurally adaptive form25-26). MATERIALS AND METHODS Materials. Cellulose nanofibrils were derived from the eucalyptus wood pulp purchased from Guangdong Zhanjiang Chenming Paper Co., Ltd in dry form. 4


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1,2,3,4-butanetetracarboxylic acid (BTCA) as crosslinking agents was purchased from Sigma-Aldrich Co., Ltd. The Sodium hypophosphite monohydrate (SHP) were used as catalyst for the reaction of BTCA with TEMPO-mediated oxidation cellulose nanofibrils (TCNF). Preparation of TCNF and BTCA-TCNF aerogels. TCNF aqueous suspension was supplied by employing TEMPO-mediated oxidation according to Saito27, followed by homogenization twice (60 MPa). The original TCNF (0.3 wt%) was mixed by BTCA (10% of dry TCNF)-SHP (10% of BTCA), followed by stirring for 12h to obtain homogenous suspensions. The suspensions were poured into moulds and frozen respectively by freezer (-25 ℃) for 12h or set on a copper surface into liquid nitrogen bath for about 1.5h. The frozen blocks (height of 3 cm) were dried in a lyophilizer (Beijing Boyikang Laboratory Instrument Co., Ltd.) for 2 days to form aerogels. The BTCA-TCNF aerogels were further reacted at 170 ℃ for 3 min. Preparation of carbonized aerogels. A nitrogen tube furnace (GSL/1600X, Hefei Kejing Co. Ltd., China) was used to sinter the carbonized aerogels at 800 ℃ for 2 min. Both of the heating and cooling rate were 5 ℃/min. Characterization. XRD spectra was conducted on an X-ray diffractometer (D8 advance, Bruker Co., Ltd., Germany) with a current of 40 mA from 5°-40° at scanning rate of 4 °/min. Crystallinity index (CrI) was calculated followed by equation28, as, 𝐶𝑟𝐼(%) = (1 ― 𝐼𝑎𝑚 𝐼200) × 100

(1).

FTIR (PE Company, America) was used by transparent KBr and fine ground samples pellets under an accumulation of 128 scans at a resolution of 4 cm-1 resolution in the range of 4000 to 400 cm-1 in the absorbance mode.

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XPS with a Thermo escalab 250Xi System (Thermo Electron Scientific Instruments Co. Ltd., USA) was conducted with an Al Ka X-ray source (1486.6 eV). The exposure conditions of Micro-CT (SkyScan1172, Brucker Co., Ltd.) were as follows: instrument S/N 12Q01223, source voltage 40 kV, source current 250 μA and exposure 1520 ms. SEM (Hitachi S-4800) was used by operation voltage 2 kV and working distance 15 mm. The cyclic compression tests of aerogel specimens (20×20×30 mm) were measured using an Instron 5848 instrument (Instron Co., USA) equipped with a 50 N load cell at a compression speed of 5 mm/min for 70% strain, followed by the same releasing speed until zero loading for 2 min. The same aerogel was tested for 5 cycles and repeated at least 5. The ρad of aerogels were estimated via weighting the specimens and measuring their volumes. The porosity was calculated according to the equation as follows: Porosity(%) = (1 ― 𝜌𝑎𝑑 𝜌𝑠𝑐) × 100

(2)，

where the ρad and ρsc are respective the apparent and skeletal density of specimens. The skeletal density is calculated based on the weighted average of densities of all components. The SBET and pore volumes (Vtotal and Vmicro) were performed by Brunaure-Emmet-Teller (BET) method (NOVA 1200e Quantachrome, USA) and the mercury intrusion porosimetry method (AutoPore IV 9500 V1.09, USA). The electrical resistivity was measured using a ST 2253 4-point probe resistivity measurement system (Suzhou Jingge Electronic Co., Ltd, China). 6


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RESULTS AND DISCUSSION To fabricate mechanically robust foam-like aerogel with large-scale aligned microchannels at extreme low concentration, it is essential to chemically cross-link the individual cellulose nanofibrils into an intact cellulose sheet. A cross-linking reaction was carried out by using 1,2,3,4-butanetetracarboxylic acid (BTCA) with the original TEMPO-mediated oxidation cellulose nanofibrils (TCNF). Fig. 1 showed the proposed mechanism for the formation of BTCA-TCNF. The cross-linking between cellulose nanofibrils and BTCA occurs via esterification reactions of cellulose hydroxyl and BTCA carboxylic acid groups29. Ultimately, they were forming a cross-linking network structure, and the aqueous suspension maintains uniform dispersion and the visual aspect of TCNF (Fig. S1). The characterizations of TCNF prior and after cross-linking were presented in Fig. 2. X-ray diffractometry spectra (XRD) was used to analyze the crystallinity of TCNF and BTCA-TCNF. The patterns (Fig. 2a) illustrated that the cross-linking reaction maintained the original crystalline structure, but significantly varied in crystallinity index (CrI). The CrIs of TCNF and BTCA-TCNF were 68.3% and 64.9%, respectively. Clearly, BTCA increased the amorphous area. Fourier transform infrared spectroscopy (FTIR) spectra of neat TCNF and BTCA-TCNF freezing-dried aerogels (before and after heating, Fig. 2b) exhibited a broad scope from 500 to 2500 cm-1, which were typical peaks of cellulose. In addition, a new absorption peak at 1726 cm-1 in both of the BTCA-TCNF aerogels (before and after heating) was assigned to the C=O stretching vibration. However, this finding could not prove the successful cross-linking of BTCA and TCNF. To further validate the successful ‘bridge’ construction of BTCA-TCNF, elemental analysis was performed via X-ray Photoelectron Spectroscopy (XPS) (Fig. 2c-d). The wide survey scan spectra presented pronounced C 1s and O 1s peaks respective at binding energies of approximately 286 and 532 eV. The C 1s peak for two aerogels was composed of four peaks with binding energies of 289.0 eV (O-C=O), 288.7 eV (C=O), 286.2 eV (C-O), 7



and 284.8 eV (C-C)30. Furthermore, the content of C-O increases from 48.73% (TCNF) to 70.13% (BTCA-TCNF after heating), which finally confirmed the successful combination of BTCA with TCNF after heating due to the esterification. Further, two freezing-dried aerogels at an extremely low concentrations (0.01 wt%, freezer freezing) were used to characterize the morphology of original TCNF and chemical modified cellulose via scanning electron microscopy (SEM, Fig. 2e). The TCNF presented an obvious micro-fibrils morphology, and the fibre surface was smooth (magnified images). However, the BTCA-TCNF exhibited laminar structure, which might result from the strong bonding interactions and entanglements between BTCA and TCNF. These results further demonstrated that the TCNF was successful in crosslinking with BTCA. Freeze-casting is one of most commonly used method to prepare low-density nanocellulose aerogels or foams. Freezing mode plays an important influence on the aerogel structure31. Fig. 3a distinguished between homogeneous freezing in freezer (FF, -25 ℃) and unidirectional gradient freeze-casting (GF, -25℃ on the bottom of mould) techniques and their corresponding optical images. The growth of the ice crystals in FF mode was uncontrolled due to the same environment temperature. However, the orientation of ice growth in unidirectional GF mode was aligned because of the copper temperature gradient from bottom to up, resulting in a structured frozen growth. The optical images of BTCA-TCNF in FF and GF modes also presented different surface roughness (the front view and planform morphology of aerogels in Fig. S2). Obviously, the apparent morphology of GF was more orderly than FF aerogel. Micro-computed tomography (CT) was performed to scan the 3D geometric arrangement via a cross-sectional plane detection for the perpendicular axis of BTCA-TCNF-FF/GF aerogels (0.3 wt%, 2 mm in diameter and 7.5 mm in height, Fig. 3b). The BTCA-TCNF-FF exhibited an irregular appearance, due to the nucleation of ice crystals randomly occurring. The cross-section showed a more strip-shape, which was subjected to the ordered sheet of ice crystals. Dissimilarly, the 8


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GF foam-like aerogel formed the regular branch-like morphology from bottom to top. The horizontal cross-section exhibited more uniform point diagram, resulting from the aligned orientation of BTCA-TCNF sheets (3D morphology in Videos S1-2). Fig. 4a-b illustrated the unidirectional gradient freeze-casting method and SEM images of three directions (YZ 、 XZ and XY) of BTCA-TCNF aerogel. The nanofibril suspension was poured into a Teflon mould and directly placed in contact with a copper plate, which was immersed in the liquid nitrogen. The simple approach provided a vertical temperature gradient and forces ice crystals to preferentially grow from the bottom to the top (Fig. 4a). Similar with inorganic particles ice-tempreating32, both of the long fibrils (TCNF and BTCA-TCNF) formed a large-scale anisotropic structure (about 10 mm, in Fig. S3). The 3D SEM images showed that the foam-like aerogel fabricated by cooperation of chemical crosslinking and gradient freeze-casting presented an aligned array from bottom to top corresponding to the orientations of ice crystals. Obviously, the YZ and XZ directions presented aligned tubular and sheet structure, and the XY presented cellular morphology. Fig. 4c suggested the BTCA-TCNF-GF aerogel had a more clearly aligned array than the TCNF-GF. The TCNF-GF still contained heterogeneously shaped pores with sizes widely varied from one to few hundred micrometers, and encased in irregular thin walls, resulting in the sheet breakage. The crosslinking reaction (BTCA-TCNF-GF) led to a more complete sheet structure. Conspicuously, a noticeable sheet-conjunction-sheet structure existed in BTCA-TCNF-GF foam-like aerogels (magnified images in red-box). However, the aerogel fabricated in the freezer presented random morphology (Fig. 4d and Fig. S4). The details of apparent density (ρad), porosity, Brunaure-Emmet-Teller (BET) surface areas (SBET) and pore volumes including the total and micro pores volume (Vtotal and Vmicro) were listed in Table 1. The ρad and porosity of all the aerogels demonstrated the aerogels assembled from cellulose nanofibrils owns ultralight (3.17-4.42 mg/cm3) and ultraporous (99.70-99.78%) properties. The tubular shaped 9



BTCT-TCNF aerogels with 1.8 cm diameter and 2.5 cm length could be absorbed by static electricity (Fig. S5). Moreover, the ρad of aerogels in GF mode, whichever TCNF and BTCA-TCNF, were lower than the FF aerogels, suggesting the decreasing volume shrinkage for GF aerogels (Fig. S2). In addition, the mode of freezing-casting had a more pronounced influence on the SBET and Vtotal. For GF freeze-casting, both of SBET and Vtotal were increasing compared with the FF specimens. And the Vmicro/Vtotal of all aerogels declined from 5.59% to 2.94% and 35.23% to 24.03% for FF and GF, respectively. The results implied that the unidirectional gradient freezing could increase the specific surface area and decrease the micropore volume. Furthermore, the mercury intrusion porosimetry method was used to measure the macropore and mesopore volume, including total pore area (A), total intrusion volume (Vtotal), median pore diameter (D), and apparent density (Dad) of BTCA-TCNF in both FF and GF mode (Table S1). The A increased from 1.419 to 4.686 m²/g and the D decreased from 90.039 to 25.454 μm for FF and GF. The results with Vtotal and Vmicro/Vtotal suggested that the GF could increase the mesopore volume and help macropore volume uniform, consequently, which was conducive to reduce the macroscopic volume collapse in drying process. Besides the properties of ultralight and ultraporous, another positive improvement of the foam-like aerogels in GF mode with chemical modification was extraordinary flexibility. Fig. 5 showed plots of fatigue cyclic compressive stress and strain for TCNF, BTCA-TCNF in FF and GF at a set maximum deformation of 70% for 5 cycles. The compression direction was respectively perpendicular to the cross-section corresponding to FF and GF1 (XY, Fig. 5a-b and d-e), and perpendicular to the ice crystal growth direction (XZ, GF2, the schematic images in Fig. 5c and f). For the first cycle, three distinct stages were observed in all of FF, GF1 and TCNF-GF2 aerogels: a steep-linear elastic and a gentle region followed by a densification region. However, the BTCA-TCNF-GF2 presented a distinct two-stage pattern (only a steep-linear elastic and densification region), suggesting better 10


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flexibility. The release curves to the second cycle suggested the maximum degradation of compressive strength of all the aerogels. The values decreased ranging from 23.6% to 5.6% (FF), 18.6% to 3.3% (GF1), and 3.1% to 1.5% (GF2), suggesting that cross-linking can improve the flexibility of aerogels. Moreover, The GF2 aerogels, both of TCNF and BTCA-TCNF, showed a substantial decrease as compared with the FF. They were ranged from 23.6% to 3.1% and 5.6% to 1.5% for TCNF and BTCA-TCNF, respectively (Video S3-5). From the TCNF-FF to the BTCA-TCNF-GF2, the flexibility increases 93.6%. Furthermore, the area integral was subjected to characterize the storage modulus of the aerogels and loss ratio in cyclic compression (Table S2). The maximum loss in storage modulus occurred between the first and second cycle for all aerogels. And the loss ratio continued to decrease followed by cycle 2-5. For each cycle, the plastic deformation of GF2 aerogel was lower than the others, particularly BTCA-TCNF-GF2. The SEM images of two aerogels following GF2 compression (Fig. S6) showed that un-chemically modified aerogel presents an irreversible mechanical fracture after compression. All the results implied the extraordinary elastic recovery of aerogels benefits from the cooperation of chemical modification and unidirectional gradient freezing. The mechanical results demonstrated that the cooperation of crosslinking and GF could improve the elasticity of porous aerogels. Fig. 6 showed the schematic and SEM images of BTCA-TCNF aerogel cross-section structure deformation in both of FF and GF mode during the compression process. For the BTCA-TCNF-FF aerogel (Fig. 6a), the interior structure was disorder with different-size parallel-sheets and tilt-sheets. They formed random 3D network by ice crystal growth and BTCA-TCNF crosslinking. When a compressive force acted on the aerogel surface, the interior parallel-sheets extruded the pores and deform. However, the tilt-sheets were hindering between parallel-sheets, causing pore unrecovered difficulty. But it was helpful for the stress increasing, which was in good agreement with the experimental observation. Whereas, the aligned lamellar structures, cellular-like shape, owned 11



excellent elasticity due to the large number of well-organized air-channels. When compressed, the cellular-like pore transformed into an oval and then quickly recovers as releasing. The SEM images of TCNF-GF cross-section structure were shown in Fig. S7. The anisotropic aligned structure of TCNF-GF aerogel contained incalculable and heterogeneous micropores due to weak binding force of monofilament, resulting in an inferior elasticity. The ultralight and high-porous foam-like aerogels had superior water absorption capacities at 20 ℃, ranging from an average 48.1-82.5 g/g for 20 cycles (Fig. 7a). The highest average absorption value was from the TCNF-GF aerogel, remarkably, even with over weight-self 82.5 times and 1650 times for 20 cycles. The water-saturated aerogels retained the same dimension as the dry state after 20 cycles (inset optical images in Fig. 7a), which demonstrated the aerogels could be compressed and reused for more compression-absorption cycles. And this value was expected to continue to increase for additional cycles. Moreover, all the aerogels in GF mode had more absorption capacity than in FF. One reason was because of richer pore volume and smaller macroscopic volume contraction; and the other reason was probably due to the anisotropic property. In the transfer process of the aerogel with absorbing water, the anisotropic aerogels could hold more water due to aligned lamellar macropore tubes. The ordered lamellar acted as a water barrier to prevent the water flowing away. But the aerogels in FF mode allowed absorbing water to squeeze out in transfer process due to the omnibearing disordered macro/micro pores, resulting in water desorption (Fig. S8). We also investigated the effect of temperature on water absorption. The water absorbency at 100 ℃ was less than the value at 20 ℃ (Fig. 7b) because of large bubbles in the boiling water preventing absorption into macropores. The highest absorption value was from the BTCA-TCNF-GF aerogel, with over weight-self 1000 times for 20 cycles, still an exhilarating absorbency. All the results displayed that foam-like aerogels with anisotropic structure had more excellent repetitive cyclic water absorption-desorption and water-locking capacities. 12


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The anisotropic structure also led to superior electrical property after carbonization. A four-probe method was conducted to test the electrical resistivity (ρ) of the carbonized aerogels from both of the longitudinal and cross orientations of aligned lamellar (the inset images in Fig. 8). The results showed a tremendous difference between FF and GF modes for longitudinal measurement (Fig. 8a). The ρ of aerogels in GF mode were obviously lower than the FF mode, indicating the aligned interconnected structure could decrease the electrical resistivity. In addition, the ρ of BTCA-TCNF aerogels was less than TCNF materials, particularly comparing BTCA-TCNF-FF and TCNF-FF. This result is likely due to the more heterogeneously shaped pores in TCNF aerogel. All aerogels with cross-section orientation were similar in ρ measurement (Fig. 8b), indicative of the undifferentiated electrical resistivity due to isotropic cross-section structures. These results showed that aerogels could be optimized with chemical modification and unidirectional freeze-casting to improve the electrical property and further to conduct or store energy. CONCLUSIONS In summary, ultralight (3~4 mg/cm3), ultra-porpous (≥ 99.7%), and elasticity-enhanced foam-like aerogels were successfully manufactured from eucalyptus wood pulp cellulose via a two-step process of tunable chemical modification and unidirectional gradient freeze-casting. The nanocellulose fibrils were derived from coupled TEMPO oxidation and twice high pressure homogenization treatment and then followed by crosslinking reaction with BTCA. As a comparison, nanocellulose suspensions (0.3 wt%) were assembled into monolithic forms of a highly porous structure through freezer freezing (-25 ℃) and unidirectional gradient liquid nitrogen freeze-casting (-25 ℃ at the bottom of mould). The BTCA-TCNF in GF mode indicated a large-scale anisotropic porous structure, which could provided superior porosity, compressibility, shape recovery, as well as excellent water absorbency and water-locking properties. And the anisotropic lamellar structure also provided unidirectional low electrical resistivity. This nanocellulose material can 13



possibly be used for applications such as, bioengineering, energy storage and thermal insulation. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: XXXX The Supporting Information includes figures (Fig. S1-S8), table (Table S1-S2) and videos (Video S1-S5). The aqueous suspension optical images, morphology optical images, SEM of TCNF and BTCA-TCNF aerogels in FF and GF mode, and schematic illustration of water desorption (Fig. S1-S8), more details of aerogel pore area, volume, pore diameter and apparent density, and storage modulus and loss ratio of the aerogels (Table S1-S2). 3D morphology and compression and resilience of BTCA-TCNF-FF/GF aerogels (Video S1-S5). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We gratefully acknowledge support from National Nonprofit Special Fund for Fundamental Research from Chinese Academy of Forestry (CAFYBB2017SY038), the National Natural Science Foundation of China (31700480). Moreover, we thank Xu Xiuping from Plant Science Facility of the Institute of Botany, Chinese Academy of Sciences for their excellent technical assistance on Micro-CT imaging system. REFERENCES 1 Jiang, F.; Hsieh, Y. L. Amphiphilic superabsorbent cellulose nanofibril aerogels. J. Mater. Chem. A. 2016, 2, 6337-6342, DOI 10.1039/c4ta00743c. 14


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Figure 1.

Fig. 1 The mechanism of the formation of BTCA-TCNF by cross-linking reaction between cellulose nanofibrils and BTCA.

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Figure 2.

Fig. 2 The characterizations of TCNF prior and after cross-linking. (a) XRD spectra, (b) FTIF spectra, (c) XPS spectra and (d) high-resolution XPS C 1s spectra of TCNF and BTCA-TCNF were investigated on the crystalline structure and chemical reaction. (e) The SEM images showed the morphology and surface structure of two specimens.

21



Figure 3.

Fig. 3 (a) Schematic illustration of the difference between FF and GF techniques and corresponding optical images of BTCA-TCNF aerogels. (b) The geometric arrangement of BTCA-TCNF-FF and BTCA-TCNF-GF aerogels via micro-CT scanning.

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Figure 4.

Fig. 4 (a) The demonstration of the unidirectional gradient freezing and (b) SEM images of three directions (YZ 、 XZ and XY) of BTCA-TCNF-GF aerogel. (c)The crosslinking foam-like aerogel BTCA-TCNF-GF had more obvious aligned array than the TCNF-GF. (d) The SEM image presented obvious random morphology of aerogel fabricated in freezer.

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Table 1. The details of apparent density (ρad), porosity, BET surface areas (SBET) and pore volumes (total and micro pores volume Vtotal and Vmicro) of aerogels. Aerogels

ρada) (mg/cm3)

Porosi ty

SBETb)

Vtotalc)

Vmicrod)

(m²/g )

(cm³/ g)

(cm³/g )

(%)

Vmicro/ Vtotal (%)

TCNF-FF

4.42±0.28

99.70

9.870

0.048

0.003

5.58

TCNF-GF

3.17±0.26

99.78

53.497

0.432

0.013

2.94

BTCA-TCNF-FF

3.93±0.22

99.73

10.905

0.011

0.004

35.23

BTCA-TCNF-GF

3.59±0.35

99.76

13.872

0.013

0.003

24.03

24


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Figure 5.

Fig. 5 The plots of cyclic compressive stress and strain for TCNF, BTCA-TCNF in FF and GF at set maximum deformation of 70% for 5 cycles. The aerogels of (a-c) TCNF, and (d-f) BTCA-TCNF were in FF and GF modes. The compress direction was perpendicular to the cross-section (XY, a-b and d-e, FF and GF1) and to the ice crystal growth direction (XZ, c and f, GF2), respectively.

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Figure 6.

Fig. 6 The schematic and SEM images of cross-section structure deformation of BTCA-TCNF in FF and GF. (a) The disorder tilt-sheets formed location hinder when an axial compressive force acted on the surface, resulting unrecovered difficulty for FF mode. (b) The aligned air-channels in GF were un-devastated when lamellar structures are compressed and recovered.

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Figure 7.

Fig. 7 The water absorption capacities of all aerogels for 20 cycles at (a) 20 ℃ and (b) 100 ℃.

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Figure 8.

Fig. 8 The electrical resistivity (ρ) of the carbonized aerogels were from (a) longitudinal and (b) cross orientations of TCNF and BTCA-TCNF aerogels, respectively.

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TOC/Abstract Graphic:

Synopsis: We present a straightforward strategy to generate foam-like aerogels with elasticity-enhanced and aligned structure, which uses the renewable and biodegradable nanocellulose.

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Elasticity-enhanced and Aligned Structure Nanocellulose Foam-like

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