Mechanically Strong and Thermally Responsive Cellulose Nanofibers

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Mechanically Strong and Thermally Responsive Cellulose Nanofibers/Poly (N-isopropylacrylamide) Hybrid Aerogels Xiaofang Zhang, Yaru Wang, Jiangqi Zhao, Meijie Xiao, Wei Zhang, and Canhui Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00814 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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ACS Sustainable Chemistry & Engineering

Mechanically Strong and Thermally Responsive Cellulose Nanofibers/Poly (N-isopropylacrylamide) composite Aerogels Xiaofang Zhang, Yaru Wang, Jiangqi Zhao, Meijie Xiao, Wei Zhang* and Canhui Lu State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, No. 24, South Section 1, Yihuan Road, Chengdu 610065, P. R. China. *Author for correspondence. E-mail: [email protected]; Phone: 86-28-85460607; Fax: 8628-85402465. KEYWORDS Aerogel, cellulose nanofibers (CNFs), compressive strength, poly (N-isopropylacrylamide) (PNIPAm), thermo-responsive properties.

ABSTRACT

Cellulose nanofibers (CNFs)/poly (N-isopropylacrylamide) (PNIPAm) composite aerogels were successfully fabricated from CNFs aqueous suspension containing PNIPAm solute via lyophilization. PNIPAm was synthesized through free radical polymerization and CNFs were individualized from filter paper cellulose fibers using 2, 2, 6, 6-tetramethylpiperidine-1-oxyl

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radical (TEMPO)-mediated oxidation pretreatment followed by mechanical nanofibrillation. It was discovered that the incorporation of CNFs could remarkably improve the structural integrity of composite aerogels by preventing them from shrinkage during lyophilization. The structure and properties of the obtained aerogels were comprehensively analyzed with various techniques, including infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption analysis, compression testing and water contact angle (CA) measurements. Due to the synergistic effects from the two components, the composite aerogels exhibited attractive thermo-responsive properties with exceptionally high compressive strength. The ultimate compressive strength for the composite aerogel (A4) at 70% strain reached 0.227 MPa, more than 12 times higher than that of neat CNFs aerogel. The CA measurements demonstrated that the hydrophilicity/hydrophobicity of CNFs/PNIPAm composite aerogel could be switched at a certain temperature. The CA sharply increased from 0 to 97° when the temperature was increased from 20 to 35 oC, exhibiting strong temperature-dependent water absorption behaviors.

INTRODUCTION As an important class of highly porous nanostructured materials, aerogels have drawn significant attention for their distinct properties including high porosity, low density, high specific surface area, excellent thermal, acoustic, electrical conductivities, and low dielectric constant.1 They have been pursued for various applications, such as optical materials, carriers for catalysis, drug release, liquid permeation and adsorption, thermal and acoustic insulation.2 Over the past 70 years, inorganic aerogels produced from silica, clay and metal oxide have played a dominating role in both academia and industry.3-5 However, inorganic aerogels usually suffer


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from high fragility with strong tendency to collapse when subjected to small stresses. This problem limits their applications in which mechanical robustness is required. Consequently, increasing interests have been raised to develop organic aerogels with superior properties.6-9 In recent years, aerogels prepared from cellulose have become a hot research topic. Especially, great efforts have been invested on nanocellulose-based aerogels.10-18 Cellulose is one of the most abundant and inexhaustible natural polymers on earth with many fascinating properties such as biodegradability, reproducibility, biocompatibility, low density, high strength and ease for chemical modification.19 However, the high hydrophilicity of cellulose has restrained the application potential. Cellulose aerogels having water repellent properties are very attractive for daily life, biomedical and engineering applications.20 Javadi et al. reported a hybrid organic aerogel comprised of a CNFs skeleton which showed hydrophobicity when CNFs’ surface was functionalized by silane compounds.21 Attractively, Kettunen et al. introduced an inorganic modification of nanocellulose aerogel with TiO2 through chemical vapor deposition (CVD).22 This novel functional material exhibited remarkable photoswitching properties between waterabsorbent and water-repellent states under UV irradiation. The present study was attempted to fabricate a CNFs-based aerogel having temperatureresponsive water absorption properties. To this end, poly(N-isopropyl acrylamide) (PNIPAm) was introduced to the nanocellulose aerogel skeleton as an organic modifier. PNIPAm is a unique temperature-responsive polymer with a lower critical solution temperature (LCST) in aqueous solution at about 32 oC, which is close to the temperature of human body.23 It has been gaining tremendous interests in particular in biomedical research.24-26 Previous study has indicated the grafting of PNIPAm could render cellulose films thermally modulated water permeability.27 Recently, Hebeish et al. reported a thermal-sensitive PNIPAm hydrogel


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crosslinked by methylenebis acrylamide could be strengthened with nanocellulose.28 The thermal-responsive properties of PNIPAm can be ascribed to the abrupt transition of molecular chain conformation (linear to coiled) from a hydrophilic to a hydrophobic peripheral structure at the LCST. When the temperature is below the LCST, polymer-solvent interactions are stronger than polymer-polymer interaction and water is a good solvent for the polymer. In this state, hydrogen bonds could be easily established between the stretched amide groups on the PNIPAm molecules and the hydroxyl groups on the surface of CNFs. PNIPAm acted like a binder for CNFs and thus stronger mechanical properties of the resultant CNFs/PNIPAm composite aerogels could be expected. We further demonstrated that the originally hydrophilic CNFs aerogel would be transformed into a water-repellent one upon PNIPAm coating at a temperature higher than 32 oC. This temperature-dependent water absorption property of aerogels is highly desired for many advanced applications. MATERIALS AND METHODS Materials. Commercial bleached filter paper (Filter Paper Factory, Fushan, China) was used as the starting material for CNFs extraction. N-Isopropylacrylamide (NIPAm) was purchased from Shanghai Wing Science and Technology Co., Ltd. 2,2,6,6-tetramethyl-piperidin-1-oxyl (TEMPO) was provided by Shanghai Sun Chemical Technology Co., Ltd. Sodium bromide (NaBr) was supplied by Tianjin Damao Chemical Reagent Co., Ltd. 2, 2'-Azobisisobutyronitrile (AIBN), sodium hypochlorite (NaClO), hydrochloric acid (HCl), sodium hydroxide (NaOH), normal hexane, tertiary butanol, ethyl alcohol were all purchased from Chengdu Kelong Reagent Co., Ltd. Prior to use, NIPAm and AIBN were recrystallized for three times in hexane and ethyl alcohol,


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respectively. No further purification was done for other reagents. All the reagents for PNIPAm synthesis were of analytical grade. Preparation of CNFs. TEMPO-pretreated cellulose fibers were used for CNFs extraction according to the literature.29 Filter paper was pulverized by a crusher. The obtained cellulose powder (30 g) was suspended in distilled water (3000 mL) containing TEMPO (0.468 g, 0.1 mmol per gram of cellulose) and NaBr (3.086 g, 1.0 mmol per gram of cellulose). An 11 wt.% NaClO solution was adjusted to pH 10 by the addition of 0.1 M HCl. TEMPO-mediated oxidation of cellulose was initiated by adding a desired amount of NaClO solution (10 mmol per gram of cellulose) and was continued at room temperature under constant stirring for 1 h. The pH was kept at 10 using 0.5 M NaOH monitored by a pH meter. The TEMPO-oxidized cellulose was thoroughly washed with distilled water. The obtained cellulose slurry was diluted to a concentration of 0.5 wt.%. Finally, the cellulose fiber suspension was physically treated to extract CNFs through sonication (JY99IIDN, Ningbo Scientz Biotechnology Co., Ltd., China) followed by homogenization (T18 basic, IKA, Germany) for 1 h, respectively. The as-prepared CNFs were concentrated to a solid content of approximately 3.0 wt.% by centrifugation (TG16-WS, Xiangyi, Changsha, China) at 10000 r/min for 10 min. The TEMPO pretreatment did not induce remarkable morphology changes of the cellulose fibers. However, the degree of polymerization (DP) of cellulose shapely decreased from 396 to 157. The obtained TEMPO-CNFs had diameters mainly distributed in the range of 15-24 nm and a DP of 147, and they could be well dispersed in water for a long time (at least 30 days), see Figure S1 and Table S1 for details. Synthesis of PNIPAm.


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For the free radical polymerization of NIPAm, 5.0 g purified NIPAm was added into a threenecked round-bottomed flask containing 100 mL tertiary butanol. Then, 28.5 mg purified AIBN was added as an initiator and the mixture was stirred for 10 min under nitrogen atmosphere. After that, the temperature was raised to 75 oC to initiate the homopolymerization. The reaction lasted for 6 h at 75 oC under constant stirring. The supply of nitrogen gas was maintained throughout the free radical polymerization. The reaction was terminated by cooling to the room temperature. The resulting product was dried at 60 oC because at this temperature the PNIPAm has minimum water absorbency.31 To ensure compete removal of unreacted NIPAm monomer, the product was Soxhlet extracted for about 8 h with water. Finally, the extract was freeze-dried to obtain PNIPAm.14, 31 Gel permeation chromatography (GPC) was performed on a Waters GPC 2410 with a series of Waters Styragel columns in conjunction with a Waters refractive index detector. The mobile phase was THF at a flow rate of 1.0 mL/min. The molecular weight of the PNIPAm was thus determined to be 3.09 × 104. Fabrication of CNFs/PNIPAm composite aerogels. The 3.0 wt.% PNIPAm aqueous solution was prepared at 20 oC. A series of CNFs/PNIPAm composite aerogels were fabricated in parallel as follows: 5, 10, 15 and 20 g CNFs suspensions (3.0 wt.%) were evenly mixed with 5.0 g of 3.0 wt.% PNIPAm solution at 20 oC, which were marked as A1, A2, A3 and A4, respectively. The total mass of each sample was fixed at 30 g by adding distilled water. The CNFs contents in A1, A2, A3 and A4 were 0.5, 1.0, 1.5 and 2.0 wt.%, respectively. Meanwhile, the concentrations of PNIPAm in all samples were 0.5 wt.%. For comparison, neat CNFs aerogel, marked as A0, was prepared from 20 g CNFs suspension (3.0 wt.%) with the addition of 10 g distilled water. The detailed compositions were listed in Table S1. Each sample was transferred into individual 25 mL beaker and mixed uniformly by magnetic


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stirring. After that, they were rapidly frozen in liquid nitrogen (-196 oC) and then placed in a freeze-drying chamber (FD-1A-50, Biocool, China). The freeze-drying process was maintained at -50 oC for 72 h to obtain lightweight sponge-like aerogels with well-defined shapes (see Figure 1). Material characterizations. Characterization of CNFs/PNIPAm composite aerogels The morphologies of the cross-sectional regions of aerogels were observed using the Inspect F50 scanning electron microscopy. Samples were sputter coated with gold at 15 mA for 3 min and then examined at 20 kV. For TEM analysis, a drop of CNFs/PNIPAm dispersion was deposited on a glow-discharged carbon-coated Cu grid and allowed to dry. The sample was observed with a high-resolution transmission electron microscopy (JEOL JEM-100CX). The images were taken under diffraction contrast in the bright-field mode without prior contrast enhancement, using a low-dose exposure. The specific surface area of the aerogels was determined by nitrogen adsorption using a Quantachrome NovaWin instrument and the Brunauer-Emmett-Teller (BET) method at 77 K and various relative vapor pressures (five points 0.01 < P/P0 < 0.3, nitrogen molecular cross-sectional area = 0.162 nm2). The Fourier transform infrared (FTIR) spectra were recorded on a FTIR instrument (MAGNA-IR 560, Nicolet) in the range of 400-4000 cm-1 at a resolution of 4 cm-1. The compression properties of aerogels were examined by an Instron 5567 at a compression rate of 2 mm/min. The results were shown in Table S2. The wettability of CNFs/PNIPAm composite aerogels was characterized using a remote-computer-controlled contact angle (CA) gonimeter system (DSA 30, Kruss). The static water CAs on the aerogels with 2 µL water droplets of different temperatures (from 20 to 45 oC


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with a temperature interval of 5 oC) were measured by the sessile drop method. The values reported are averages of five measurements made on different areas of the surfaces at 5s after deposition of the droplets. RESULTS AND DISCUSSION Drying shrinkage of the aerogels Aerogels from CNFs/PNIPAm dispersions with various CNFs contents (0.5 to 2.0 wt.%) were prepared via freeze-drying. Figure 1 shows that all specimens (A0-4) had a well-defined shape analogous to the mold beaker. For the neat CNFs aerogel A0 and the composite aerogel A4, both of which had a CNFs concentration of 2.0 wt.%, they did not shrink noticeably. However, the shrinking degree of the composite aerogels increased with the decrease of CNFs content. Notably, the neat PNIPAm solution could not form a three-dimensional (3D) aerogel even with a much higher PNIPAm content (3.0 wt.%), indicating CNFs played a crucial role as the skeleton of the 3D structured aerogels. Structural characteristics of the aerogels The influence of PNIPAm and CNFs content on the microscopic morphology of the composite aerogels was examined by SEM. The corresponding SEM images at different magnifications are compared in Figure 2. All the aerogels exhibited a porous structure with pore sizes ranging from nanometers to micrometers. Compared with neat CNFs aerogel A0 (Figure 2A), more obvious sheet-like structures were formed in the composite aerogels. When cellulose concentration was low (aerogel A1, 0.5 wt.%), the CNFs were coagulated to form extended sheet-like structures (Figure 2B). This was attributed to the nucleation and growth of large ice crystals within the


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network, which pushed out the CNFs from their original location during the freeze-drying process.30 At higher cellulose concentrations (aerogels A2 and A4, 1.0 and 2.0 wt.%, respectively), the pore size of aerogels was relatively smaller (Figure 2C and 2D). This might be resulted from the denser CNFs network in the system that hindered the growth of ice crystals.30 Under higher magnifications, the coating layer of PNIPAm on CNFs’ surface can be clearly distinguished (Figure 2F). In addition, it seemed that the PNIPAm could bind CNFs together, leading to the more obvious sheet-like structures (Figure 2F-H). The molecular interaction between CNFs and PNIPAm was also schemed in Figure 2. When mixing the PNIPAm solution with CNFs suspension, the temperature was lower than the LCST of PNIPAm, and the stretched amide groups on the PNIPAm molecules could form hydrogen bonds with the hydroxyl groups on CNFs to generate a relatively uniform core-shell composite structure. To further elucidate the composite structure formation, the samples were examined by TEM. From the acquired TEM image (Figure S2), the PNIPAm-coated CNFs were presented as black filaments. As indicated with arrows, the PNIPAm was apparently attached to CNFs. The specific surface area of the aerogels was measured by N2 adsorption/desorption, and the porosity and each component’s volume fraction in the aerogels were calculated. The results were listed in Table S1. As expected, the specific surface area decreased with increasing density of the aerogels.9 From aerogel A1 to A4, the density gradually increased from 0.018 to 0.034 g/cm3, and the BET surface area decreased from 55.42 to 44.05 m2/g which was in accordance with the variation of porosity. The density of composite aerogel A4 (0.034 g/cm3) was much higher than that of neat CNFs aerogel A0 (0.020 g/cm3) though both had the same CNFs content (2.0 wt.%). However, the specific surface area of A4 (44.05 m2/g) only decreased slightly as compared to that of A0 (44.51 m2/g).


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IR analysis The FTIR spectra of CNFs, PNIPAm and CNFs/PNIPAm composite aerogel (A4) are shown in Figure 3. The TEMPO-oxidized CNFs display a typical FTIR spectrum of cellulose (Figure 3a) and there is no characteristic absorption peak for the carboxyl groups because their carboxyl content was rather low (0.38 mmol/g from titration, see ESI). In the spectrum of PNIPAm (Figure 3b), the two sharp absorption peaks at 1550 and 1650 cm-1 are attributed to the N-H bending vibration and the N-C=O stretching vibration, respectively.32 The spectrum of the composite aerogel exhibits characteristic features of both CNFs and PNIPAm. Compared with the spectra of CNFs and PNIPAm, no new absorption peak was observed for CNFs/PNIPAm aerogel (Figure 3c), indicating there was no chemical reaction happened between cellulose and PNIPAm.33 The characteristic peak at 3300 cm-1 assigned to the N-H asymmetric stretching for PNIPAm was weakened while the peak at around 3440 cm-1 became stronger after compositing with CNFs. These two peaks also trended to merge into one broad peak due to the possible hydrogen bond formation between PNIPAm and cellulose chains.34 Compression Testing The mechanical properties of aerogels are crucial for many applications. A lot of factors such as composition, microstructure, and relative density can influence these properties of aerogels.21 As demonstrated in Figure 4, the neat CNFs aerogel A0 deformed considerably under a load of 100 g, whereas the composite aerogel (A4) almost did not show any deformation. The strain-stress curves for all the aerogels are also shown in Figure 4 and the obtained data were listed in Table S2. Notably, no crack was observed for those aerogels during the tests even at 70% compressive strain. For the neat CNFs aerogel A0, the yield stress was not detected, which was in accordance


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with the early study on CNFs aerogel 35 Compared with A0, aerogel A4 exhibited improved mechanical properties with a compressive strength of 0.227 MPa, a Young’s modulus of 0.223 MPa and a yield stress of 27.47 KPa. The compressive strength and Young’s modulus were more than 12 times and 9 times higher than those of A0, respectively. Comparing the mechanical properties of aerogels A0 and A1, both of which had similar densities, the compressive strength and the Young’s modulus of A1 were obviously higher than those of A0. For the composite aerogels A1-4, which had the same PNIPAm content, the compressive strength, Young’s modulus and yield stress were all enhanced with the increase of CNFs content. This could be explained by several facts. On one hand, the increasing cellulose content gave rise to a higher overall bulk density, which was favorable for the mechanical properties of aerogels.36 On the other hand, there were more two-dimensional extended sheet-like structures formed in aerogels at a higher CNFs content (see Figure 2), stabilizing their initial shapes upon external forces.37 Considerable stiffening at higher strains was observed for aerogels A2, A3 and A4 due to the densification of the porous structure.36,38 Numerous early studies were attempted to improve the mechanical properties of CNFs aerogel. For instance, Alireza et al. reported the ultimate compressive stress of the CNFs/graphene oxide nanosheets composite aerogel (0.014 MPa) at 80% strain was more than three times higher than that of the CNFs aerogel (0.004 MPa).21 Herein, for the CNFs/PNIPAm aerogel, the remarkable enhancement of the mechanical properties were attributed to PNIPAm’s long polymeric chains resulting in high density hydrogen bonds with CNFs. Surface wettability Owing to the abundant hydroxyl groups coupled with the high porosity, CNFs aerogels are frequently regarded as superabsorbents for water.21 In this study, the incorporation of PNIPAm


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could drastically change the surface properties of the aerogels. To figure out how temperature could affect the surface wettability of the composite aerogels, the CA measurements were conducted at different temperatures and the results are illustrated in Figure 5. It is evident that the CAs were kept at nearly 0° when the water temperature increased from 20 to 30 oC. Interestingly, a drastic increase of CA was recorded as the temperature increased from 30 to 35 o

C and it continued to rise slowly when the temperature went even higher. The temperature-dependent hydropilicity/hydrophobicity of the composite aerogels was

demonstrated intuitively in Figure 6. For the neat CNFs aerogel A0, water droplets were absorbed by the aerogel in less than 1s and the three-dimensional (3D) architecture of aerogel also collapsed (Figure 6A and B). Aerogel A0 exhibited a fast vanishing water CA on the surface. When put in water, the neat CNFs aerogel immediately disintegrated into small irregular pieces no matter whether the water was 20 or 35 oC. Early investigations had indicated that the CNFs aerogels are constructed though entanglements of CNFs and hydrogen bonds formed between adjacent CNFs.39 Such weak interactions are vulnerable in water environment. Besides, when water contacts with CNFs aerogels, strong capillary forces will be generated in those micro or nano pores, which suppress the pore structure and consequently destroy the aerogels. In contrast, the CNFs/PNIPAm composite aerogel A4 well kept its structural integrity during CA measurements and exhibited remarkable temperature-responsive water affinity. When tested at 35 oC (Figure 6C), the aerogel was hydrophobic with a water CA of 97°. The water droplets maintained their shape and were not absorbed by the aerogel. On the other hand, when the temperature was 20 oC (Figure 6D), the water CA was nearly 0°. The water droplets could be absorbed by the aerogel but with no damage to its appearance. To be more vivid, the composite aerogel could rest stably on the surface of hot water (35 oC), showing distinct water repellency. It


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is noteworthy that after soaking at 35 oC for 24 h, the transparency of water did not change(see Figure S3C), suggesting that PNIPAm could be hardly dissolved in hot water. However, when put in cold water (20 oC), the aerogel sank down rapidly to the beaker bottom while still preserved its structural integrity. After 24 h soaking at this temperature, the solvent water was transparent but would turn to milk white in color if heated to 35 oC (Figure S3A and S3B), indicative of the dissolution of PNIPAm in water at a lower temperature. It was calculated that 83.47 wt% of PNIPAm in A4 aerogel was dissolved in cold water (see ESI for details). The water uptake behaviors of aerogels were also studied and the results were listed in Table S2. The neat CNFs aerogel could absorb water 30 times its dry weight at the water temperature of both 20 or 35 oC. However, for all the composite aerogels, their water uptake capacity at 20 oC was distinctly higher than that at 35 oC. By increasing the CNFs content, the water absorption of aerogel declined owing to the decrease of its porosity and specific surface area. The cold water (20 oC) absorption capacity of aerogel A4 was 14.5 g/g, and it dropped to 1.9 g/g for hot water (35 oC), demonstrating strong temperature-dependent water absorption behaviors. The mechanism for the thermal responsive behavior was depicted in Figure 7. This phenomenon comes from the intrinsic properties of PNIPAm. When the temperature is below LCST (e.g. 20 oC), the coating layer of PNIPAm is hydrophilic due to its extended chain conformation, which exposes the amide groups. Those hydrophilic functional groups can easily establish hydrogen bonds with water molecules, leading to considerable absorption of water. By contrast, when the temperature is above the LCST (e.g. 35 oC), the chain conformation of PNIPAm suddenly changes. The stretched molecular chain becomes coiled. The hydrophilic amide groups will be shelled by the hydrophobic isopropyl-methyl groups. As a consequence, the whole aerogel exhibits hydrophobic characteristics.


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CONCLUSION Thermo-responsive CNFs-based aerogels with high strength were fabricated by adding PNIPAm into the CNFs suspension followed by freeze-drying. After the mixture, a thin PNIPAm layer was coated on the CNFs’ surface to form a uniform core-shell structure. The strong interaction between PNIPAm and CNFs through massive hydrogen bonds made the aerogel largely strengthened. The ultimate compressive stress of the composite aerogel A4 at 70% strain was more than 12 times higher than that of neat CNFs aerogel. Interestingly, the composite aerogel exhibited a novel thermo-responsive behavior. By tuning the water temperature, its water affinity could be drastically altered. It was able to float on hot water without wetting, but would sink down in cold water. We envisage that this unique aerogel is very promising in biomedical research, particularly in cell culture on a 3D scaffold, which allows temperature-controllable cell proliferation and cell detachment. ASSOCIATED CONTENT Supporting Information The characterizations of the CNFs are included in the Supporting Information. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (W. Zhang), [email protected] (C. Lu); Phone: 86-28-85460607; Fax: 86-28-85402465. ACKNOWLEDGMENT


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The authors would like to thank National Natural Science Foundation of China (51303112, 51473100 and 51433006), Excellent Young Scholar Fund of Sichuan University (2015SCU04A26) and State Key Laboratory of Polymer Materials Engineering (sklpme2016-309) for the financial support of this work. REFERENCES (1) Aaltonen, O.; Jauhiainen, O. The preparation of lignocellulosic aerogels from ionic liquid solutions. Carbohydr. Polym., 2009, 75, 125-129. (2) Hüsing, N.; Schubert, U. Aerogele–luftige Materialien: Chemie, Struktur und Eigenschaften. Angew. Chem. Int. Edit., 1998, 37, 22-47. (3) Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev., 1997, 97, 2373-2420. (4) Davis, M. E. Ordered porous materials for emerging applications. Nature, 2002, 417, 813821. (5) Dubinin, M. M. The potential theory of adsorption of gases and vapors for adsorbents with energetically nonuniform surfaces. Chem. Rev., 1960, 60, 235-241. (6) Meador, M. A. B.; Malow, E. J.; Silva, R.; Wright, S.; Quade, D.; Vivod, S. L.; Guo, H.; Guo, J.; Cakmak, M. Mechanically strong, flexible polyimide aerogels cross-linked with aromatic triamine. ACS Appl. Mater. Inter., 2012, 4, 536-544.


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Figure Captions Figure 1. Samples of cylindrically shaped aerogels obtained from CNFs or CNFs/PNIPAm dispersions and the shapeless PNIPAm aerogel. Figure 2. The SEM images of the inner structures of aerogels A0 (A, E), A1 (B, F), A2 (C, G) and A4 (D, H) under different magnifications. A scheme was shown to explain the possible interaction between CNFs and PNIPAm chains. Figure 3. The FTIR spectra of CNFs (a), PNIPAm (b) and aerogel A4 (c). Figure 4. The compressive stress-strain curves for different aerogels and the qualitative comparison of compressive properties between aerogels A0 and A4. Figure 5. The CAs on aerogel A4 tested at temperatures from 20 to 45 oC. The insert photographs show the equilibrium state of water droplets on the aerogel. Figure 6. Photographs showing the water droplet shapes on aerogels A0 and A4 at different temperatures, and the insert images were captured during water CA measurements. The photos of aerogels A0 and A4 in water are shown on the right column. Figure 7. The schematic for the thermal responsive mechanism of the composite aerogel.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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Figure 7.


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For Table of Contents Use Only Mechanically Strong and Thermally Responsive Cellulose Nanofibers/Poly (Nisopropylacrylamide) composite Aerogels Xiaofang Zhang, Yaru Wang, Jiangqi Zhao, Meijie Xiao, Wei Zhang* and Canhui Lu*

Composite aerogels with unique temperature-dependent water absorption properties were prepared by coating PNIPAm on the renewable cellulose nanofibers’ surface.


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217x162mm (96 x 96 DPI)


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Mechanically Strong and Thermally Responsive Cellulose Nanofibers

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