Nanocellulose Aerogels Inspired by Frozen Tofu - ACS Publications

toughening after freezing and thawing. It is this toughening mechanism that allows for the formation of nanocellulose aerogels which ordinarily would ...
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Nanocellulose Aerogels Inspired by Frozen Tofu Yinyong Li,† Vijesh A. Tanna,† Yiliang Zhou,† H. Henning Winter,†,‡ James J. Watkins,† and Kenneth R. Carter*,† †

Conte Center for Polymer Research, Department of Polymer Science and Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States ‡ Conte Center for Polymer Research, Department of Chemical Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: Cellulose-based aerogels are reported that were generated using cellulose nanofibril (CNF) gels in an ambient drying process. These nancellulose aerogels were inspired by the preparation of a traditional Chinese food, frozen tofu, which undergoes significant toughening after freezing and thawing. It is this toughening mechanism that allows for the formation of nanocellulose aerogels which ordinarily would collapse upon ambient drying. By freezing hydrated CNF gel dispersions, solvent exchanging, and drying at ambient pressure, monolithic nanocellulose aerogels with high porosity (>98%), low density (as low as 0.018 g cm−3), and high surface area (>30 m2 g−1) were readily generated. Moreover, since no special reactors are required, these structures can be easily created over large areas. For example, a hydrated CNF gel was frozen at −72 °C, and then, the frozen gel was immersed directly in a 2-propanol bath to allow solvent exchange. The resulting alcohol-infused gel was dried at ambient pressure to fabricate monolithic CNF aerogels, enabling the scalable fabrication of nanocellulose aerogels in an energy-efficient and cost-effective manner. KEYWORDS: Aerogel, Nanocellulose, Freezing, Solvent exchange ambient drying

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INTRODUCTION Aerogels rank among the lightest solid materials in the world and have wide applications. When taking into consideration the importance of sustainable development and environmental protection, aerogels made from natural renewable biomass resources have distinct advantages as compared to the conventional aerogels manufactured from nonrenewable and nonbiodegradable materials, such as carbon nanotubes and graphene. Among various biopolymers, cellulose is the most abundant natural resource in the world. Nanocellulose, derived either from trees, crops, or bacteria, has outstanding properties, such as high mechanical strength, light in weight, biocompatibility, and biodegradability.1,2 Reports have appeared on nanocellulose-based aerogels for use in applications such as superabsorbents, water filtration, oil remediation, and thermal insulation.3−9 The majority of these cellulose-based aerogels were prepared using either freeze-drying or supercritical drying methods.10−12 These techniques require either high pressure and temperature or low temperature in a low vacuum, making them energy intensive and cost ineffective. They require special reactors that are incompatible with some continuous processing requirements and suffer from potential safety issues.13,14 Conversely, ambient pressure drying eliminates the need for complicated © 2017 American Chemical Society

and expensive equipment, rendering it a more energy efficient and cost-effective process alternative. However, the intrinsic hydrophilicity of nanocellulose makes it very challenging to prepare pristine nanocellulose aerogels by direct ambient drying, especially when compared to aerogels derived from other hydrophobic materials, such as graphene and hydrophobic silane-modified silica. The strong intermolecular hydrogen bonding and capillary forces present lead to densification of cellulose nanofibrils (CNF) and results in structural collapse of the porous network during ambient drying. Surface modification of cellulose with hydrophobic chemicals, such as anhydrides15 and trityl chloride,16 accompanying solvent exchange has been used to mitigate the influence of capillary force and successfully prepare porous cellulose foam or film with relatively high density of 0.1−0.6 g cm−3. Porous CNF foams have also been prepared from Pickering emulsions of nanocellulose, which is surface modified by octylamine.17 Besides utilizing chemical surface modification (hydrophobilization), enhancing the network’s mechanical strength Received: May 22, 2017 Revised: June 26, 2017 Published: July 5, 2017 6387

DOI: 10.1021/acssuschemeng.7b01608 ACS Sustainable Chem. Eng. 2017, 5, 6387−6391

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. Samples of tofu: (a) Soft tofu. (b) Frozen tofu after frozen in household freezer (−20 °C) and thawed at room temperature. Excess water in the thawed frozen tofu was easily squeezed out and resulted in the formation of a porous network indicated by the arrow. (c) Rheological frequency sweeps of the two tofu samples measured at 25 °C. Square: soft tofu. Circle: frozen tofu. Solid symbol: storage modulus. Open symbol: loss modulus.

Figure 2. Fabrication of cellulose nanofibril (CNF) aerogel via ambient drying. (a) Schematic illustration of the preparation processes. (b) CNF aqueous dispersion. (c) CNF frozen gel after being cooled to −72 °C. (d) CNF alcohol gel obtained from the frozen gel by solvent exchange. (e) CNF aerogel after ambient drying. (f) SEM image of CNF dried directly from dilute dispersion (arrow shows the larger bundles of CNF (scale bar: 5 μm), and inset is a picture of the CNF dispersion). (g) Cross-sectional SEM image of CNF aerogel (arrow indicates the CNF aggregation due to freezing, and inset is a picture of monolithic aerogel supported by a cotton ball (scale bar: 50 μm). (i) CNF aerogel prepared with freezing and ambient drying. (j) CNF sample dried from alcohol gel without freezing.

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RESULTS AND DISCUSSION Interestingly, the preparation of a traditional Chinese food, frozen tofu, has inspired a promising solution for network strength enhancement, which can contribute to the ambient preparation of CNF aerogel. Frozen tofu is usually prepared by freezing the tofu in the freezer and thawing at room temperature. Similar to the observations that freezing caused graphene oxide flakes agglomeration,19,20 and freezing−thawing induced the gelation reinforcement of poly(vinyl alcohol) (PVA),21 freezing tofu (Figure 1) in a household freezer also induces the generation of ice crystals, which leads to the aggregation of proteins and other molecules in tofu, and results in the formation of porous structures (indicated by the arrow in Figure 1b) after melting. The freezing significantly enhanced the tofu’s strength turning the soft, fragile tofu (Figure 1a) into porous, robust frozen tofu (Figure 1b). Rheology data (Figure 1c) clearly supports these empirical observations. The modulus of tofu increased more than 3 times after being frozen at −20 °C and thawed at room temperature. Guided by the fascinating freezing effect in frozen tofu preparation, we developed a simple method for the preparation of nanocellulose aerogels via

is also effective in preventing structure collapse resulting from capillary forces. Toivonen et al. used vacuum filtration to condense a CNF dispersion into a compact wet gel-cake. The dense interconnection between cellulose fibrils enhanced the network strength and prevented collapsing upon ambient drying after solvent exchange.18 However, the density (0.6 g cm−3) of the resulting aerogel is unfavorably high, which might impair some aerogel properties, such as absorption capacity and thermal insulation. In the present study, a simple yet effective method is developed to generate cellulose nanofibril (CNF) aerogels via ambient drying. Monolithic CNF aerogels were readily obtained by freezing hydrated CNF gel dispersions, solvent exchanging, and drying at ambient pressure. This practical freezing/solvent exchange/ambient drying technique does not involve any low or high pressure processes which are generally used in freeze-drying or supercritical drying, enabling the scalable fabrication of nanocellulose aerogels in an energyefficient and cost-effective manner. 6388

DOI: 10.1021/acssuschemeng.7b01608 ACS Sustainable Chem. Eng. 2017, 5, 6387−6391

Letter

ACS Sustainable Chemistry & Engineering

Figure 3. Rheological behavior of CNF gels. (a) 3.0 wt % and (b) 2.0 wt %. Square: CNF dispersion without freezing. Circle: CNF dispersion after frozen at −72 °C and molten at 25 °C. Solid symbol: Storage modulus (G′). Open symbol: loss modulus (G″).

drying and contributes to the retention of highly porous structures without failure. The resulting aerogel maintains its monolithic shape without noticeable shrinkage (Figure 2i). In comparison, the aerogel dried from alcohol gel without freezing shows significant shrinkage due to structural collapse (Figure 2j). To verify that the freezing induced nanofiber aggregation strengthens the network, rheological measurements were performed on a frozen−thawed gel (FTG) and pristine gel (PG) of CNF. The FTG was prepared by freezing a CNF aqueous dispersion in the same previously mentioned mixture of ethanol/dry ice (−72 °C) for 10 min. The sample was then fully thawed at room temperature for 2 h after which the rheology measurement was performed. As shown in Figure 3, both PG and FTG exhibited solid-like behavior demonstrated by the clear plateau observed in both samples’ storage modulus (G′). In addition, G′ was also much higher than loss modulus (G″). We attribute this behavior to the strong entanglements of high aspect-ratio nanofibrils. A significant enhancement in both storage and loss modulus was observed in the FTG in comparison to the PG, with a 2-fold and 1.9-fold increase in G′ and G″, respectively, indicating a solid-to-solid transition. The enhancement of gel mechanical properties is due to the freezing-induced nanofibril agglomeration. When comparing the moduli of the gels with different concentrations of CNF (Figure 3a,b), we found that gels with higher concentration (3.0 wt %) were about 2.5 times stronger than those with lower concentration (2.0 wt %). This suggests that by increasing the solids concentration the nanofibrils have more interfibril entanglements, resulting in higher moduli. The same solid-to-solid transition upon freezing was observed for gels with lower concentration (2.0 wt %), except that the effect of freezing was reduced (60% increase of modulus). This is rationalized by the reduced opportunity for fibril entanglement at lower concentration. The entanglement interaction between cellulose nanofibrils shows good thermal stability; even after heating to 75 °C, the FTG modulus did not drop significantly (Figure S1). For a comparison, the modulus of a poly(vinyl alcohol) solution (PVA, 10 wt %) was measured. As expected, freezing increased the modulus of the PVA solution and resulted in a liquid-tosolid transition (Figure S2). This transition arises from freezing-induced hydrogen bonding and localized crystallization generating physical networks of PVA. The modulus of frozen− thawed PVA gel reduced sharply when heated above 50 °C (Figure S1), which is a consequence of the breakdown of the

freezing/solvent exchange/ambient drying processes. This technique allows fabrication of nanocellulose aerogels with densities as low as 0.018 g cm−3 without the involvement of demanding conditions, such as high pressure or low vacuum. Neither complicated instruments nor surface modification are required in the process, therefore rendering the technique energy efficient and cost effective. Figure 2a illustrates the fabrication processes. An aqueous gel of CNF was frozen in an ethanol/dry ice bath (−72 °C), after which the frozen gel was immersed in a 2-propanol bath (−20 °C) to conduct solvent exchange. As the frozen gel melted in 2propanol, water was gradually displaced by 2-propanol, resulting in an alcohol-infused gel. The alcohol gel was dried at 70 °C under ambient pressure to generate CNF aerogels. Figure 2b−e shows the images of the CNF gel, frozen gel, alcohol gel, and corresponding aerogel. Figure 2g, inset, shows a lightweight aerogel sample supported by a piece of cotton. It is worth noting that the freezing temperature (−72 °C) was chosen due to convenience factors. Further study of the effect of freezing temperature (−20 to −196 °C) on the properties of the aerogel is currently under investigation. The solvent involved in the solvent exchange process is not limited to 2propanol. Any solvent that has a much lower surface tension than water and is completely miscible with water is feasible for solvent exchange, including but not limited to tetrahydrofuran, acetone, ethanol, and t-butanol. The influence of the formation of azeotrope for a water−alcohol mixture is negligible since sufficient solvent (>10 times more of solvent than water, exchanged for three cycles) is used to replace most of the water from the CNF gel. The CNF, prepared by mechanical grinding, was obtained from the Process Development Center at the University of Maine. The individual fibers have diameters of less than 50 nm, and most fibers are longer than 5 μm (Figure 2f). This high length-to-width aspect ratio (>100:1) ensures high interfiber interaction, which gives rise to the dense entanglements of fibers and contributes to the formation of an interconnected network upon drying. The ambient-dried aerogel exhibits threedimensional open-cell porous structures with the pore size varying from a few microns to tens of microns. The ice crystals, formed during the freezing of the aqueous CNF gel, push the nanofibrils onto the ice crystal grain boundaries, leading to the nanofibril aggregation, as indicated by the sheet-like structures shown in Figure 2g. Nanofibril agglomeration is expected to reinforce the mechanical strength of the network structures, which allows it to withstand capillary forces during ambient 6389

DOI: 10.1021/acssuschemeng.7b01608 ACS Sustainable Chem. Eng. 2017, 5, 6387−6391

Letter

ACS Sustainable Chemistry & Engineering

Figure 4. Preparation of different aerogels. (a) Large CNF aerogel (diameter of 18 cm). (b) Rolled-up sample of a thin CNF aerogel sheet (thickness of 1.0 mm). (c) Aerogel prepared using CNF derived from recycled paper. (d) Pectin-enhanced CNF aerogel.

TEMPO-oxidized CNF after freezing at −20 °C (10.9 m2 g−1)7 but lower than CNF aerogels obtained by freeze-drying enzyme-hydrolyzed CNF in liquid propane at −180 °C (66 m2 g−1).25 While the source and size of the nanofibril may influence the surface area, the surface area variation is considered to be primarily due to the differences in the freezing process. It is believed that lower freezing temperature causes a higher freezing rate, which leads to smaller ice crystals, resulting in smaller pore sizes and higher surface areas. Conversely, freeze-drying from t-butanol gives much higher surface area (254 m2 g−1), due to smaller crystal size and a lower surface tension effect.23 Overall, the aerogel fabricated by the ambient drying method in this report shows a similar specific surface area in comparison with samples made by conventional freeze-drying techniques. It may be possible to reduce the pore size further by utilizing lower freezing temperatures, and this is the subject of ongoing studies. In addition to the advantages of simplicity, energy efficiency, and cost effectiveness, this ambient drying method does not require any complicated reactor design. Therefore, the sample size is not limited by the dimensions of a freeze-dryer chamber or supercritical drying facilities. Figure 4a shows a large piece of CNF aerogel sample with a diameter around 18 cm prepared by ambient drying. Potentially, aerogels with dimensions of tens of meters can be easily fabricated by using large storage freezer warehouses. Moreover, CNF can be roller-coated on an aluminum web for fabricating a thin aerogel sheet, which is flexible and can be rewound into a roll (Figure 4b). The successful aerogel preparation by roller coating suggests that this technique is compatible with roll-to-roll processing for the continuous fabrication of aerogel rolls in a cost-effective way. Additionally, this aerogel preparation method is not only limited to the fabrication of nanocellulose aerogel. We have prepared aerogels from recycled paper (Figure 4c), as well as other polysaccharides such as pectin (Figure 4d), both of which maintained the monolithic shape and did not experience significant shrinkage during drying. To summarize, this work presents a method for the fabrication of ultralight, highly porous, nanocellulose aerogels via a freezing/solvent exchange/ambient drying technique. The morphology and mechanical properties of the aerogels have been closely examined and demonstrate the effectiveness of the process to make high quality aerogels from readily available feedstocks. The process demonstrates the potential for largescale, continuous manufacturing of nanocellulose aerogel, potentially at lower cost and lower energy consumption than any previously reported method. We believe, with further investigation, this method will open the door for the utilization

weak physical bonding and localized crystallization of PVA molecules.22 A comparison of the distinctive rheological behaviors of PVA and CNF gels upon thermal treatment demonstrates that the CNF physical entanglements are very robust, and the aggregation of CNF is irreversible, which is beneficial for the preparation of aerogels via ambient drying. CNF dispersions with variable concentrations (1.0−3.0 wt %) were used to prepare aerogels. Aerogel densities as low as 0.018 g cm−3 were achieved with a corresponding porosity of approximately 98.8%. As the concentration was decreased from 3.0 to 1.5 wt %, the aerogel volume shrinkage during drying was observed to increase from 6.2% to 16.7% (Table S1). Further decreasing the dispersion concentration to 1.0 wt % resulted in severe network shrinkage, deforming the final aerogel into an irregular shape. The high shrinkage at low concentrations can be ascribed to the low number of entanglements and lower modulus of the dilute dispersion, which cannot resist the capillary forces generated during drying that drive structural collapse. The mechanical properties of CNF aerogels were determined by compression testing with 80% strain (Figure S4). As the density increased from 0.018 to 0.032 g cm−3, the modulus increased from 24.0 to 75.7 kPa (Table S1). The specific modulus (modulus/density, around 2.4 MPa cm3 g−1) of the aerogel with a density of 0.032 g cm−3 is around 1/3 of the value (6.8−6.9 MPa cm3 g−1) of previously reported aerogels prepared from TEMPO-oxidized CNF.4,23 This difference in modulus can be ascribed to the morphological differences between the two types of CNF: the nanofibers used in this work are only mechanically ground and not fully fibrillated (demonstrated by the large fibril bundles indicated by the arrow in Figure 2f), while the TEMPO-oxidized cellulose is fully fibrillated into nanofibrils by high pressure homogenization. It is known that the highly fibrillated TEMPO-oxidized cellulose nanofibers have stronger mechanical properties.24 The surface area of the aerogel was determined by collecting nitrogen adsorption−desorption isotherms using the Brunauer−Emmett−Teller (BET) method (Figure S5). Aerogel prepared with 3.0 wt % concentration has a specific surface area of 30.8 m2 g−1, and the pore sizes range from 5 to 50 nm, with a wide distribution, suggesting irregular pore structures within the aerogel. Generally speaking, the apparent pores of nanocellulose aerogels, even those prepared by freeze-drying, have been found to be in the range of a few microns to tens of microns, which are much larger than those observed by silica aerogels.4,8 The specific surface area of the ambient dried aerogel is comparable to the values reported for CNF aerogels prepared by conventional freeze-drying methods. It is higher than that of CNF aerogel prepared by freeze-drying of 6390

DOI: 10.1021/acssuschemeng.7b01608 ACS Sustainable Chem. Eng. 2017, 5, 6387−6391

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ACS Sustainable Chemistry & Engineering

(10) Mi, Q. Y.; Ma, S. R.; Yu, J.; He, J. S.; Zhang, J. Flexible and Transparent Cellulose Aerogels with Uniform Nanoporous Structure by a Controlled Regeneration Process. ACS Sustainable Chem. Eng. 2016, 4 (3), 656−660. (11) Zhang, X. F.; Wang, Y. R.; Zhao, J. Q.; Xiao, M. J.; Zhang, W.; Lu, C. H. Mechanically Strong and Thermally Responsive Cellulose Nanofibers/Poly(N-isopropylacrylamide) Composite Aerogels. ACS Sustainable Chem. Eng. 2016, 4 (8), 4321−4327. (12) Zhou, S. K.; Liu, P. P.; Wang, M.; Zhao, H.; Yang, J.; Xu, F. Sustainable, Reusable, and Superhydrophobic Aerogels from Microfibrillated Cellulose for Highly Effective Oil/Water Separation. ACS Sustainable Chem. Eng. 2016, 4 (12), 6409−6416. (13) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Silica aerogel films prepared at ambient-pressure by using surface derivatization to induce reversible drying shrinkage. Nature 1995, 375 (6530), 431−431. (14) Gurav, J. L.; Jung, I.-K.; Park, H.-H.; Kang, E. S.; Nadargi, D. Y. Silica aerogel: synthesis and applications. J. Nanomater. 2010, 2010, 23−33. (15) Sehaqui, H.; Zimmermann, T.; Tingaut, P. Hydrophobic cellulose nanopaper through a mild esterification procedure. Cellulose 2014, 21 (1), 367−382. (16) Pour, G.; Beauger, C.; Rigacci, A.; Budtova, T. Xerocellulose: lightweight, porous and hydrophobic cellulose prepared via ambient drying. J. Mater. Sci. 2015, 50 (13), 4526−4535. (17) Cervin, N. T.; Andersson, L.; Ng, J. B. S.; Olin, P.; Bergstrom, L.; Wagberg, L. Lightweight and strong cellulose materials made from aqueous foams stabilized by nanofibrillated cellulose. Biomacromolecules 2013, 14 (2), 503−511. (18) Toivonen, M. S.; Kaskela, A.; Rojas, O. J.; Kauppinen, E. I.; Ikkala, O. Ambient-dried cellulose nanofibril aerogel membranes with high tensile strength and their use for aerosol collection and templates for transparent, flexible devices. Adv. Funct. Mater. 2015, 25 (42), 6618−6626. (19) Yang, H. S.; Zhang, T. P.; Jiang, M.; Duan, Y. X.; Zhang, J. M. Ambient pressure dried graphene aerogels with superelasticity and multifunctionality. J. Mater. Chem. A 2015, 3 (38), 19268−19272. (20) Li, C. W.; Qiu, L.; Zhang, B. Q.; Li, D.; Liu, C. Y. Robust vacuum-/air-dried graphene aerogels and fast recoverable shapememory hybrid foams. Adv. Mater. 2016, 28 (7), 1510−1516. (21) Ricciardi, R.; D’Errico, G.; Auriemma, F.; Ducouret, G.; Tedeschi, A. M.; De Rosa, C.; Laupretre, F.; Lafuma, F. Short time dynamics of solvent molecules and supramolecular organization of poly (vinyl alcohol) hydrogels obtained by freeze/thaw techniques. Macromolecules 2005, 38 (15), 6629−6639. (22) Yang, X.; Liu, Q.; Chen, X.; Yu, F.; Zhu, Z. Investigation of PVA/ws-chitosan hydrogels prepared by combined γ-irradiation and freeze-thawing. Carbohydr. Polym. 2008, 73 (3), 401−408. (23) Sehaqui, H.; Zhou, Q.; Berglund, L. A. High-porosity aerogels of high specific surface area prepared from nanofibrillated cellulose (NFC). Compos. Sci. Technol. 2011, 71 (13), 1593−1599. (24) Kumar, V.; Bollstrom, R.; Yang, A.; Chen, Q. X.; Chen, G.; Salminen, P.; Bousfield, D.; Toivakka, M. Comparison of nano- and microfibrillated cellulose films. Cellulose 2014, 21 (5), 3443−3456. (25) Paakko, M.; Vapaavuori, J.; Silvennoinen, R.; Kosonen, H.; Ankerfors, M.; Lindstrom, T.; Berglund, L. A.; Ikkala, O. Long and entangled native cellulose I nanofibers allow flexible aerogels and hierarchically porous templates for functionalities. Soft Matter 2008, 4 (12), 2492−2499.

of a variety of different materials for aerogel fabrication, in particular, a variety of composite aerogels, which could have outstanding performance for applications in filtration, catalysis, insulation, absorption, etc. Potentially, this technique will help increase the feasibility of turning organic-based aerogels into widely used commercialized products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01608. Materials and methods, figures of rheological behavior of frozen−thawed CNF dispersion and PVA solution, images of CNF aerogels, figure of stress−strain curve of aerogels, figure of nitrogen adsorption isotherms of CNF aerogel, and table of aerogel properties. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1-4135771416. E-mail: [email protected]. ORCID

Kenneth R. Carter: 0000-0002-7081-2296 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was kindly supported by the NSF DMRPOLYMERS grant (DMR 1506968). REFERENCES

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DOI: 10.1021/acssuschemeng.7b01608 ACS Sustainable Chem. Eng. 2017, 5, 6387−6391