Aerogels from Benign Solution

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Robust Nanofibrillated Cellulose Hydro/Aerogels from Benign Solution/Solvent Exchange Treatment Juanjuan Fan, Shinsuke Ifuku, Mengzhu Wang, Kojiro Uetani, Hai-Wei Liang, Haipeng Yu, Yongming Song, Xiaohe Li, Jiale Qi, Yiqun Zheng, Haigang Wang, Jing Shen, Xianquan Zhang, Qing Li, Shou-Xin Liu, Yixing Liu, Qingwen Wang, Jian Li, Ping Lu, Zhuangjun Fan, and Wenshuai Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00418 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Sustainable Chemistry & Engineering

MS ID: sc-2018-00418a

Robust Nanofibrillated Cellulose Hydro/Aerogels from Benign Solution/Solvent Exchange Treatment

Juanjuan Fan,† Shinsuke Ifuku,‡ Mengzhu Wang,† Kojiro Uetani,§ Haiwei Liang,⊥ Haipeng Yu,† Yongming Song,† Xiaohe Li,† Jiale Qi,† Yiqun Zheng,|| Haigang Wang,† Jing Shen,† Xianquan Zhang,† Qing Li,† Shouxin Liu,† Yixing Liu,† Qingwen Wang,† Jian Li,† Ping Lu,*,# Zhuangjun Fan,*,▽ and Wenshuai Chen*,†

†

Key laboratory of Bio-based Material Science and Technology, Ministry of Education,

Northeast Forestry University, Harbin 150040, P. R. China ‡

Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori

University 4-101 Koyamac-cho Minami, Tottori 680-8552, Japan §

College of Science, Rikkyo University 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo, Japan

⊥

Department of Chemistry, University of Science and Technology of China, Hefei 230026, P.

R. China ||

National Enginerring Research Center for Colloidal Materials, Shandong University, 27 S

Shanda Rd., Jinan, Shandong 250100, P. R. China #

Department of Chemistry and Biochemistry, Long Island University, One University Plaza,

Brooklyn, New York 11201, United States ▽

Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education,

College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001 , P. R. China

Correspondence and requests for materials should be addressed to W. Chen (email: [email protected]), Z. Fan (email: [email protected]) and P. Lu (email: [email protected]).

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ABSTRACT: To fabricate robust nanofibrillated cellulose (NFC) hydro/aerogels, benign solution/solvent exchange treatment was developed by adding five different water miscible solution/solvent into a NFC aqueous suspension. The NFC self-aggregated and formed self-standing gels during the solution/solvent exchange treatment. After a further exchange of solution/solvent inside the gels with water by a thorough water washing followed by freeze-drying, NFC hydrogels and aerogels were obtained. The NaOH-hydrogel demonstrated a decent rheology with a storage modulus of 36.4 kPa and satisfactory mechanical property with a compressive modulus of 37.6 kPa. On the contrary, the acetone-hydrogel was weak due to disaggregation. The NFC aerogels were lightweight and had a characteristic porous structure. The packing density and structure varied among aerogels with different solution/solvent treatments. The NaOH-aerogel had a 2D sheet-like structure with densely packed micrometer-sized pores uniformly distributed within the aerogel network, which demonstrated a high compressive strength. However, the structures of other aerogels were loose, leading to a low compressive strength. These NFC aerogels demonstrated high thermal stability and superior performance for efficient thermal insulation. We believe our work can stimulate interest in the development of NFC hydro/aerogels with multiple structures, properties, and functions for a variety of applications.

KEYWORDS: nanofibrillated cellulose, hydrogel, aerogel, solution/solvent exchange, porous structure, thermal insulation


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INTRODUCTION Hydrogels and aerogels with 3D cross-linked networks have drawn considerable interest in recent years.1-5 They have been widely used in multiple areas such as catalysis,6-8 biomedical engineering,9-12 energy storage,13-18 and environmental purification.19-21 Hydro/aerogels can be synthesized from a wide variety of precursors, including organic materials,22,23 inorganic materials24-26 and metallic particles.27-29 The precursors and synthetic strategies determine the structure and property of the resulting hydro/aerogels. In order to develop green building blocks for advanced materials, more attention has been paid to the renewable nanomaterials such as nanofibrillated cellulose (NFC) for constructing multi-functional hydro/aerogels.19,30-35 NFC is mainly exfoliated from higher plants, which exhibits advantageous mechanical and thermal properties, including a high Young’s modulus of 138 GPa,36 an estimated mechanical strength of 2 to 3 GPa, and a very low coefficient of thermal expansion of 10−7 K−1.37 Furthermore, NFC having a high aspect ratio and can easily form entangled networks. Therefore, the fabrication of hydro/aerogels from NFC can lead to a series of novel bulk materials by combining the remarkable performance of NFC with the unique structure and property of hydro/aerogels. NFC with a high specific surface area can be dispersed in water. The NFC suspension loses its liquidity and exhibits gel behavior even though the NFC content is only around 1 wt%.38 Aerogels can be obtained from the NFC suspension by freeze-drying, critical point drying, and supercritical drying, from suspensions with NFC contents as low as 0.018 wt%.38 The density, porosity, morphology, and mechanical property of hydro/aerogels can be manipulated by adjusting the NFC content in the initial suspension as well as the type of NFC. However, the mechanical strength of the gels prepared by the above methods is usually weak because these NFCs are weakly interconnected. On the other hand, solution/solvent exchange treatment is a simple but highly effective way to make strong and free-standing gels.39 During the solution/solvent exchange, the ions/molecules in the solution/solvent replace water molecules by diffusing into the NFC suspension, resulting in the self-aggregation of the NFC and subsequent formation of strongly interconnected networks. Upon further removal of the exchange solution/solvent by water washing and appropriate drying, self-standing NFC hydro/aerogels with high mechanical strength can be obtained. Recently, several hydro/aerogels were developed through solution/solvent exchange method. For example, an aqueous suspension of tunicate cellulose nanofibers was 3 ACS Paragon Plus Environment


solvent-exchanged with a water miscible solvent (e.g., acetone) to generate a self-standing gel.39 The gel was further developed as a self-assembled nanofiber template for the processing of polymer nanocomposites.39 By adjusting the pH of TEMPO-oxidized NFC aqueous suspensions to approximately 2 by 0.1 M hydrochloric acid, the suspensions lost their fluidity and formed free-standing and transparent gels, even at an NFC concentration of 0.1% w/v.40 Ultralow-density aerogels with large surface areas were accordingly prepared by freeze-drying or supercritical drying of these NFC hydrogels.34,40 Hydrogels were also prepared from NFC aqueous suspensions by alkaline treatment41,42 and cation inducing.43 Solution/solvent treatment can produce hydro/aerogels with various structures. Although the mechanism on the self-aggregation behavior of nanocellulose and the formation of hydro/aerogels has been proposed in several works, comparative study on how solution/solvent exchange treatment affects the structure and property of hydro/aerogels has not been reported. In this work, we used wood nanofibrillated cellulose (NFC) as a model nanocellulose building block for the fabrication of hydro/aerogels. The NFC was obtained by chemical pretreatment of wood pulp followed by high intensity ultrasonication. Five different solutions/solvents including two types of dilute acids, alkali, salt, and organic solvent were used for the exchange treatment. We systematically studied the robust NFC hydro/aerogels from these benign solution/solvent exchange treatments, which included the comparison of their structures, densities, porosities, rheology behaviors, mechanical and thermal properties of the exchanging gels, hydrogels and aerogels. EXPERIMENTAL SECTION Materials. The raw material for NFC was poplar (populus ussuriensis) wood powders. Sodium chlorite, acetic acid (HAc), potassium hydroxide, sodium hydroxide, sodium chloride, hydrochloric acid, acetone and other laboratory grade chemicals were used without further purification. Fabrication of NFC. Poplar wood powders were purified by a series of cyclic chemical pretreatments.44 First, the lignin in the sample was removed with acidified sodium chlorite at 75 °C for 1 h, which was repeated 5 times. Then, the sample was treated by 5 wt% potassium hydroxide at 90 °C for 2 h to remove hemicellulose. The sample was further subjected to acidified sodium chlorite at 75 °C for 2 h and 5 wt% potassium hydroxide at 90 °C for 2 h. 4 ACS Paragon Plus Environment

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After each chemical pretreatment process, the sample was filtered and rinsed with distilled water. Next, the never-dried cellulose pulps were suspended in distilled water (1.0 wt%) and were nanofibrillated by a high-intensity ultrasonicator (JY99-IIDN, Ningbo Scientz Biotechnology Co., Ltd, China) with an output power of 1200 W for 30 min, and 1 wt% NFC suspension was generated. Fabrication of Hydrogels and Aerogels. The 1 wt% NFC suspension (10 g) was placed in a 25-mL beaker with 31.73 mm inner diameter. Then, 15 g of exchange solution/solvent (i.e., 15 wt% NaOH, 15 wt% NaCl, 1 M HCl solution, HAc and acetone) was carefully and slowly poured into the NFC suspension. All samples were maintained at room temperature for 24 h and gels were formed for the NaOH-, NaCl-, HCl-, and HAc-treated samples. For the acetone-treated sample, the acetone was mainly distributed at the top region owing to its low density (0.78 g cm−3). The obtained sample was weak probably due to incomplete solvent exchange. Thus, we repeated the acetone exchange process six more times and a self-standing gel was obtained eventually. Next, all the gels were thoroughly washed with distilled water for eight times over 2 days, and NFC hydrogels were produced. To fabricate aerogels, these hydrogels were placed in a refrigerator at -18 °C overnight. Then, the samples were freeze-dried by a freeze-dryer (Scientz-10N, Ningbo Scientz Biotechnology Co., Ltd, China) to obtain NFC aerogels. For simplicity, the exchanged gels, hydrogels, and aerogels obtained by NaOH solution treatment are coded as NaOH-exchanging gels, NaOH-hydrogels, and NaOH-aerogels, respectively. This naming convention also applies to the NaCl-, HCl-, HAc-, and acetone-treated samples. Hydrogel Characterization. Shrinkage. The changes in diameter, height, and volume of the gels with different solution/solvent treatments were determined by measuring the diameter and height of the NFC suspensions and those of the exchanged gels and hydrogels. Water Content. The hydrogels were immersed into distilled water for approximately 3 days at room temperature. Then, the excess water on the hydrogel surface was removed with a filter paper. The weight of hydrogels was recorded as Wh. The hydrogels were freeze-dried, and the weight of the as-prepared aerogels was recorded as Wd. The water content was calculated by using Equation (1): 5 ACS Paragon Plus Environment


Water content =

× 100%

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(1)

Dynamic Rheological Property. Dynamic rheology property of the NFC-hydrogels was measured using a rheometer (AR2000ex, TA instruments, America) with a diameter of 25 mm flat parallel plate, and the gap was 1.5 mm. The frequency sweeps were performed over the range of 0.01–100 rad s−1 at a controlled strain amplitude of 1 % and the temperature was 25 °C. The shear viscosity was studied at a shear rate of 0.01–1 s−1. The hydrogels with a height of 1.5 mm, which were prepared by cross-cutting their partner hydrogels, were used for dynamic rheological experiments. Mechanical Property. The compressive test of the different NFC exchange gels and hydrogels was performed with an AI-7000S TC160701511 testing machine, equipped with two flat-surface compression stages. The strain ramp rate was maintained at 1 mm min−1. The as-prepared gels were directly used for compressive tests without any treatment. Aerogel Characterization. To investigate the microstructure of the obtained NFC aerogels, the morphology of top surface, cross-section and vertical section was acquired with a scanning electron microscope (SEM, Quanta200, FEI, USA); XRD diffractogram of the samples was obtained on an X-ray diffractometer (D/max 2200, Rigaku, Japan) with Ni-filtered CuKα radiation (λ = 1.5406 Å) at 40 kV and 30 mA. Scattered radiation was detected in the range of 2θ = 5°~40° at a scan rate of 4° min−1; The thermal stability of the aerogels was determined with a thermogravimetric analyzer (Pyris 6, Perkin-Elmer, USA) in a nitrogen environment with a heating rate of 10 °C min−1; The thermal conductivity of the aerogels was measured through the transient hot-wire method with a thermal conductivity tester (TC3020, Xi’an Xiatech Electronic Technology Co., Ltd, China). Bulk Density and Porosity. The bulk density of the NFC aerogels was determined from their dimension and weight. The porosity of aerogels was calculated by using Equation (2): Porosity = (1−

) × 100%

(2)

where represents the bulk density of the NFC aerogels, and represents the density of the crystalline cellulose nanofibers ( ≈ 1.6 g cm−3). Mechanical Property. The compressive 6 ACS Paragon Plus Environment

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test of NFC aerogels was performed with an AI-7000S TC160701511 testing machine, equipped with two flat-surface compression stages. The strain ramp rate was maintained at 1 mm min−1.

RESULTS AND DISCUSSION Wood NFC containing entangled cellulose nanofiber bundles with high aspect ratio and individualized cellulose nanofibers, 3–4 nm in width (Figure 1a), was obtained by chemical pretreatment combined with high-intensity ultrasonication. The NFC was then used as building block to fabricate hydro/aerogels. The fabrication of NFC hydro/aerogels is schematically illustrated in Figure 1b. Five different solutions/solvents, including acid, alkali, salt, and organic solvent were slowly added into the NFC aqueous suspensions. During the addition of exchange solution/solvent, the ions/molecules of the solution/solvent diffused into the NFC suspension. Simultaneously, the water molecules contained inside the NFC suspension diffused out and was replaced by the exchange solution/solvent. Thus, the dispersion environment of NFC was altered by the mutual diffusion of ions/molecules and water. The NFC self-aggregated and formed interconnected gel networks. The mechanism of gel formation was intriguing and worth exploring. The large amount of hydroxyl groups on the surface of NFC led to the formation of diffused electric double layer and negatively charged surface in water. The repulsion between individual NFC, which was derived from the osmotic pressure by overlapping the electric double layers, promoted the uniform dispersion of NFC in water. When the exchange solutions/solvents were added, the newly introduced ions/molecules diffused into the NFC suspension and replaced the water molecules to a great extent. These ions/molecules interacted with the negatively charged NFC surfaces through electric charge neutralization or formation of hydrogen bonds. Thus, the charge balance of the whole NFC water suspension system was lost and the repulsive forces between the NFC were greatly weakened. As a result, the NFC self-aggregated, leading to the volume shrinkage and formation of gels. Owing to different interactions among various exchanging ions/molecules and hydroxyl groups on the surface of NFC, the shrinkage ratio of 7 ACS Paragon Plus Environment


the exchanging gels and hydrogels varied with the solution/solvent exchange systems. The change in shrinkage of gels played a critical role in the structure, density, porosity, rheology behavior, and mechanical and thermal property of the resulting exchanging gels, hydrogels and aerogels. For the 15-wt% NaOH-treated sample, the NFC immediately self-aggregated with a notable volume shrinkage; the height and diameter shrinkage ratios were approximately 37% and 28% (Table 1), respectively, compared with the original NFC aqueous suspension (Figure 2a,b). For the 15-wt% NaCl-, 1-mol L−1 HCl-, and HAc-treated samples, the NFC also self-aggregated quickly but with only slight volume shrinkage; the height and diameter shrinkage ratios were approximately 6%-8% and 3%-6%, respectively (Table 1), and finally formed self-standing gels (Figure 2a,b). After the addition of solutions/solvents, the above-mentioned gels floated on the top part of the suspensions (Figure 2a). However, for acetone-treated sample, the volume shrinkage was not obvious (Figure 2a). Although a gel-like sample was formed and the sample lost its fluidity, the mechanical strength of the sample was very weak. It was hard to pick up the sample from the beaker without damaging its structural integrity. Thus, we repeated the acetone exchange for 6 extra times to produce a self-standing gel with height and diameter shrinkage ratio of 34% and 10%, respectively (Table 1), which formed at the bottom of the beaker (Figure 2b). To obtain hydrogels, all the gels were thoroughly washed with distilled water to replace the exchanged solutions/solvents with water. The water content of all the hydrogels was higher than 95.5% (Table 1). During the water washing process, the HAc-, NaCl-, and HCl-treated hydrogels underwent further shrinkage with a volume shrinkage ratio of ca. 20% (Figure 2d) to finally form stable and self-standing hydrogels (Figure 2c). The NaOH-treated sample underwent further severe shrinkage with a height and diameter shrinkage ratio of approximately 47% and 40%, resulting in a hydrogel with high mechanical strength (Figure 2c). The high shrinkage in preparing hydrogels using the NaOH system was related to the mercerization and irregular aggregation of NFC during the NaOH treatment.45,46 The longitudinal shrinkage of NFC during the alkaline treatment also contributed to the high shrinkage of NFC.41 As such, the hydrogels obtained from the NaOH treatment showed a higher shrinkage ratio compared with the hydrogels fabricated by using other solution/solvent 8 ACS Paragon Plus Environment

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system. Conversely, the acetone-treated sample became dis-aggregated during the washing process and eventually generated a bulk material without uniform structure (Figure 2c).

Figure 1. Fabrication of NFC hydro/aerogels. (a) TEM image of NFC showing the building blocks of the hydro/aerogels. (b) Schematic representation of the synthesis of NFC hydro/aerogels through solution/solvent exchange treatment.



Figure 2. Photographs and volume shrinkage ratios of the NFC gels. (a) Photographs of the NFC suspensions obtained immediately after the addition of exchange solution/solvent. (b) Photographs of the exchanging gels. (c) Photographs of the hydrogels. (d) Volume shrinkage ratio of the exchanging gels and hydrogels.


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Table 1. Height and diameter shrinkage ratios of the NFC exchanging gels and hydrogels (Numbers in parentheses represent standard deviations) Exchanging gel

Hydrogel

Shrinkage ratio (%)

Shrinkage ratio (%)

Solution/Solvent

Water content (%) Height

Diameter

Height

Diameter

15 wt% NaOH

36.7 (1.2)

28 (2.2)

46.8 (1.8)

39.7 (1.2)

95.7

15 wt% NaCl

6.3 (4.1)

3.7 (2.3)

18 (0.8)

1.9 (0.6)

98.9

1 mol/L HCl

8.3 (6.8)

3.9 (2.4)

21 (3.5)

2.3 (0.8)

98.9

HAc

7.0 (3.8)

6.7 (3.5)

18 (3.6)

3.1 (0.8)

98.8

Acetone

34 (0.8)

10.2 (0.8)

-

-

-

Dynamic rheology data of the NFC hydrogels are shown in Figure 3. The storage modulus (G′) of all the hydrogels was several times greater than the loss modulus (G″) over the entire frequency range (Figure 3a, b), indicating a substantial elastic response of the NFC hydrogels. In addition, the G′ and G″ values of these hydrogels were almost independent of the angular frequency (Figure 3a, b), indicating that the hydrogels had stable entangled networks. The G′ and G′′ values of all the obtained hydrogels were apparently higher than the original 1 wt% NFC hydrogel, indicating that the solution/solvent exchange treatments improved the mechanical property of hydrogels. The G′ values of the HCl-hydrogel (1.5 kPa), HAc-hydrogel (3.4 kPa), and NaCl-hydrogel (1.1 kPa) at ~1 Hz were greater than 1.0 kPa, revealing that the introduction of acid and salt solution facilitated the aggregation of the NFC and formation of free-standing network structures. The NaOH-hydrogel showed a significant volume shrinkage during the formation process, which had an apparent high modulus with G′ of 36.4 kPa at 100 rad/s, indicating that the introduction of alkali solution led to considerable NFC self-aggregation and enhanced the gel network strength. To the best of our knowledge, the G′ value of our NaOH-hydrogel is one of the highest values among bio-based nanofiber hydrogels compared with those of TEMPO-oxidized NFC hydrogel (~20 kPa at 1 Hz),40 NFC-Ag+ hydrogel (~a plateau value of 6.8 kPa),47 Fe3+-induced NFC hydrogel (~31kPa at 0.1 rad/s),43 chitin nanofiber hydrogel (~13 kPa),48 and cellulose nanocrystal/poly(acrylic 11 ACS Paragon Plus Environment


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acid) nanocomposite hydrogel (~28 kPa at 10 Hz).49 The G′ value of the acetone-hydrogel was as low as 300 Pa, suggesting a weak NFC interconnection among the hydrogel after the replacement of acetone by water. The loss tangent (tanδ) of all the hydrogels was lower than 0.3 (~0.1 for NaOH-hydrogel, in the range of 0.16–0.17 for the HCl-, HAc-, and NaCl-hydrogels, and ~0.24 for the acetone-hydrogel) (Figure 3c). This finding further indicated that all the hydrogels were predominantly elastic. Figure 3d shows the changes in the shear viscosity of the hydrogels as a function of shear rate. All the hydrogels showed a large decrease of viscosity with increasing shear rate owing to shear thinning. Thus, these hydrogels could be considered to be pseudoplastic materials. At a shear rate of 0.01 s−1, the viscosity of NaOH-hydrogel was ~105 Pa·s, which was higher than that of the HCl-hydrogel (28450

Pa·s),

HAc-hydrogel

(22360

Pa·s),

NaCl-hydrogel

(11540

Pa·s),

and

acetone-hydrogel (512 Pa·s). Because the strongly entangled network prevented the collapse of the structure and release of individual NFC upon shearing, the viscosity of NaOH-hydrogel had a high value of ~ 332 Pa·s even at high shear rate of 1 s-1.


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Figure 3. Dynamic viscoelasticity of the NFC hydrogels. (a) Storage modulus (G′), (b) loss modulus (G″) and (c) loss tangent (tanδ) as a function of frequency for the hydrogels. (d) Viscosity of the hydrogels as a function of shear rate.

Figure 4. Mechanical property of the NFC exchanging gels and hydrogels. (a) Compressive stress-strain curves of the exchanging gels. (b,c) Compressive stress-strain curves of the hydrogels. (d) Compressive modulus of exchanging gels and hydrogels.

Compressive stress-strain curves of the NFC gels before and after the water exchange treatment are shown in Figure 4. Most solution/solvent exchanging gels exhibited a compressive stress less than 2 kPa at a compressive strain of 20% (Figure 4a). The compressive modulus values were less than 7 kPa (Figure 4d). However, the acetone exchanging gel had a high strength with a compressive stress of ~5.6 kPa at a compressive 13 ACS Paragon Plus Environment


strain of 20% (Figure 4a). The compressive modulus of the acetone exchanging gel was 25.6 kPa (Figure 4d). The high mechanical strength of the acetone exchanging gel was probably contributed by the repeated acetone exchanging treatment (7 times). The multiple acetone exchanging treatment promoted the self-aggregation of NFC and the formation of hydrogen bonds between the oxygen atom of acetone molecules and the hydroxyl groups on the surface of NFC. These results illustrated that solution/solvent exchange treatment promoted the shrinkage of the NFC and the formation of self-standing gels. The structure, shrinkage ratio and mechanical property of the gels were mainly dependent on the interactions of the NFC and the ions and molecules of the exchange solutions/solvents. After further exchange of the solutions/solvents inside the gels with water, most of the hydrogels were weakened and the acetone hydrogel collapsed and could not form a self-standing bulk structure. However, the NaOH-treated hydrogel with a high-shrinkage ratio maintained good mechanical property with a compressive stress of ~48 kPa at a compressive strain of 60% (Figure 4b, c), which was higher than that of the chitin nanofiber hydrogel (~10 kPa at 1 wt%).48 The compressive modulus of the NaOH-treated hydrogel was as high as 37.6 kPa (Figure 4d), which was higher than that of bacterial cellulose hydrogel (~7 kPa),50 regenerated chitin hydrogel (~5 kPa),51 and Z-directions cellulose nanocrystal/poly (oligoethylene glycol methacrylate) hydrogel (~20 kPa at a 1:1 ratio).52 The high mechanical property of the NaOH-treated hydrogels was ascribed to the NaOH treatment, which led to considerable NFC aggregation and the formation of strong inter-connected entangled networks.


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Figure 5. (a,b) Photographs of the NFC aerogels obtained from the corresponding hydrogels by freeze-drying: (a) top view; (b) side view. (c) Density and porosity of the NFC aerogels.

After freeze-drying the hydrogels, NFC aerogels were obtained (Figure 5a, b). The shape of the hydrogels was well preserved during the freeze-drying process. Owing to the considerable shrinkage during the formation of hydrogel, the NaOH-aerogel exhibited the largest bulk density of 0.112 g cm−3 (Figure 5c). However, it still showed a high porosity of 93% (Figure 5c). Other aerogels that showed less shrinkage during the formation of the hydrogels had low densities (less than 0.015 g cm−3) and high porosity (greater than 99%) (Figure 5c).



Figure 6. SEM images of the NFC aerogels obtained from top surface, cross-section and vertical section: (a-c) NaOH-aerogel, (d-f) NaCl-aerogel, (g-i) HCl-aerogel, (j-l) HAc-aerogel, and (m-o) acetone-aerogel.

The microstructures of aerogels are shown in Figure 6 for comparison. All the aerogels exhibited porous network structures with hierarchically interconnected 2D sheets, which were assembled by entangled high-aspect ratio cellulose nanofibers and nanofiber bundles. The direct SEM observation confirmed that the NFC self-aggregated during solvent exchange and freeze-drying processes, and supported the skeletons of the aerogels. However, the density of the packing and the micrometer-sized pore structures varied among the different aerogels. For the NaOH-aerogel, 2D sheet-like structures were densely packed (Figure 6a) and micrometer-sized pores were uniformly distributed within the aerogel networks (Figure 6c). The pore sizes were also smaller compared with those of other NFC aerogels. In contrast, the structures of other aerogels were relatively loose. Several large-sized pores were also occasionally observed. For example, large-sized pores, with diameters greater than 100 µm, 16 ACS Paragon Plus Environment

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could be observed in the acetone-aerogel (Figure 6m). Pores with different sizes were non-uniformly distributed within the aerogel, indicating that the acetone-aerogel was irregular throughout the whole aerogel network.

Figure 7. Mechanical property of NFC aerogels. (a,b) Compressive stress-strain curves of the 17 ACS Paragon Plus Environment


NFC aerogels. (c) Compressive modulus and specific compressive modulus of the NFC aerogels.

We further compared the mechanical and thermal properties of the NFC aerogels. As shown in Figure 7a, b, owing to the dense packing of the NFC and the strong entangling of the networks, the NaOH-aerogel demonstrated high mechanical strength with a compressive stress up to 1461 kPa at a compressive strain of 80%. However, the acetone-aerogel showed a low compressive stress of 29 kPa at a compressive strain of 80%, mainly because of their loose structure and anisotropic pore distribution. The compressive modulus of the NaOH-aerogel was 401 kPa, which was apparently higher than those of the other aerogels (Figure 7c). However, the specific compressive modulus of the NaOH-aerogel (3651 kPa g−1 cm3) was lower than those of the original NFC aerogel (5000 kPa g−1 cm3), HAc-aerogel (4818 kPa g−1 cm3), HCl-aerogel (4357 kPa g−1 cm3) and NaCl-aerogel (3800 kPa g−1 cm3). The results further confirmed that the high mechanical strength of the NaOH-aerogel could be mainly attributed to the dense and uniform packing of high-aspect ratio NFC within the aerogel networks. The X-ray diffraction (XRD) patterns of aerogels were shown in Figure 8a. The HAc-, HCl-, NaCl-, and acetone-aerogels had diffraction peaks at 14.6°, 16.5° and 22.6° which were assigned to the (1-10), (110) and (200) planes, respectively. These peaks are characteristic crystal patterns of cellulose I, confirming that the native crystal structure of cellulose was preserved after the solution/solvent exchange treatment. For the NaOH-aerogel, the cellulose I crystal structure was mostly preserved even though small but noticeable change was observed at the peaks around 14.6° and 16.5° owing to the alkalization of NFC. The NaOH concentration reached 9% in our experiments which was close to the threshold concentration that could start to convert the cellulose I crystal structure to cellulose II.45 The thermal stability of NFC aerogels was compared to evaluate their performance in high-temperature environments. As the crystal structure of the NFC was unchanged during the solvent/solution exchange process (Figure 8a), all the NFC aerogels showed a high thermal stability because of the high crystallinity of NFC. The TG and DTG curves of all five aerogels had similar 18 ACS Paragon Plus Environment

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trends (Figure 8b, c). The thermal degradation temperature and the highest thermal degradation temperature of all the aerogels were >300 oC and >340 °C, respectively. The high thermal stability of the NFC aerogels illustrated that the aerogels could be employed at high temperature environment, in which other types of chemically modified nanocellulose aerogels (such as TEMPO-oxidized NFC aerogels53 and sulfuric acid-hydrolysis CNC aerogels53) cannot be applied.

Figure 8. Thermal property of the NFC aerogels. (a) XRD diffractogram of the NFC aerogels. (b) TG curves and (c) DTG curves of the NFC aerogels. (d) Thermal conductivity of the NFC aerogels.

Thanks to their high porosity and high thermal stability, NFC aerogels showed a great potential to be used as thermal insulators. The thermal conductivity of the aerogels was determined by using the transient hot-wire method to be 0.032 W m-1 K-1, 0.033 W m-1 K-1, and 0.033 W m-1 K-1 (Figure 8d) for HAc-, NaCl- and acetone-aerogel, respectively. These values were similar to the values of pure chitosan foam-1% (0.032 W m−1 K−1)54 and 19 ACS Paragon Plus Environment


polyurethane foam (0.03 W m−1 K−1), and lower than those of Styrofoam (0.036 W m−1 K−1)55 and foam glass (0.038–0.05 W m−1 K−1).56 The low thermal conductivity of these NFC aerogels could be ascribed to their low density (approximately 0.01 g cm−3) and high porosity, as shown in Figure 5c. For the NaOH aerogel, it exhibited a relatively high thermal conductivity of 0.044 W m-1 K-1 (Figure 8d), which was related to the dense packing of NFC in the aerogels.

CONCLUSIONS Five types of NFC hydro/aero-gels were obtained from different solution/solvent exchange treatment. Thanks to the swift NFC aggregation and subsequent formation of strongly interconnected and entangled networks, the NaOH-hydrogel demonstrated a satisfactory rheology with a storage modulus of 36.4 kPa and an impressive mechanical property with a compressive modulus of 37.6 kPa. In contrast, the acetone-hydrogel was weak due to disaggregation. The as-prepared aerogels were lightweight and highly porous. The packing density and pore structure varied among different aerogels. The NaOH-aerogel had a 2D sheet-like structure with densely packed micrometer-sized pores uniformly distributed within the aerogel network. As a result, it demonstrated a high mechanical property. In contrast, the structures of other aerogels were loose, leading to a weak mechanical strength. These aerogels showed a high thermal stability and demonstrated a great potential in thermal insulation applications. Although the required water washing could be tedious, it was still considered convenient to fabricate robust NFC hydro/aerogels by employing the solution/solvent exchange systems. We expect the NFC gels derived from the solution/solvent exchange systems can stimulate interest in a wide range of research fields such as intelligent hydro/aero-gels, nanocomposites, carbon materials and energy storage applications.

Conflicts of Interest 20 ACS Paragon Plus Environment

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There are no conflicts of interest to declare.

ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (No. 31770594), Natural Science Foundation of Heilongjiang Province, China (No. C2017006), Young Elite Scientists Sponsorship Program by CAST (No. 2017QNRC001), Funds supported by the Fok Ying-Tong Education Foundation, China (No. 161025), Overseas Expertise Introduction Project for Discipline Innovation, 111Project (No. B08016), and the Startup Funds and Development Funds from Long Island University.



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Robust nanofibrillated cellulose hydro/aerogels were obtained from benign solution/solvent exchange treatment.


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Aerogels from Benign Solution

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