Communication pubs.acs.org/cm Chitosan Aerogels: Transparent, Flexible Thermal Insulators Satoru Takeshita* and Satoshi Yoda Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan S Supporting Information * T as potential catalysts,13,14 absorbers,16,20,21 and scaffolds for biomedical applications, 17−19 but they have not been considered as potential transparent thermal insulators In this work, we try to open up the application field of thermal insulators by proposing a translucent and ultralight chitosan aerogel prepared without using any catalysts or oxidants The key to achieving transparency in the visible region, i.e., a structural homogeneity at the nanoscale, is the cross-linking conditions employed Chitosan powder was first dissolved in acetic acid to form a homogeneous solution at the molecular level, and then regenerated to form a gel through cross-linking We used formaldehyde as a mild cross-linker Formaldehyde reacts with a NH2 group to form a Schiff base, −NCH2, which reacts with another NH2 group to form an NCN cross-linking bond,22,23 as shown in Figure 1a hermal insulation is a key technology for energy conservation in the 21st century One of the major issues of the current thermal insulation technology is heat flow through transparent parts such as windows and glass walls Silica aerogels, which consist of a three-dimensional network of 10−20 nm silica nanoparticles with a porosity of 90−98%, have attracted much attention as a transparent insulator.1 Although silica aerogels have high transparency in the visible region and extremely low thermal conductivity, their commercial uses are limited by their brittleness and fragile nature A variety of approaches to reinforcing the mechanical properties of silica aerogels have been reported, e.g., coating polymers at the silica surface,2,3 incorporating organic functional groups,4,5 penetrating fibrous polymers into silica aerogel matrices,6,7 and embedding silica aerogel particles in polymer matrices.8 However, increasing organic contents inevitably increases structural inhomogeneity at the nanoscale, which results in low transparency and high thermal conductivity A new category of thermal insulating materials, which has a combination of transparency, flexibility, and a low thermal conductivity, has long been required in this field In contrast to traditional aerogels consisting of nanoparticle skeletons, some researchers have focused on nanofibrous aerogels composed of entangled nanofibers of soft polymers.9−11 These aerogels are expected to have flexibility and become suitable candidates for flexible thermal insulators Most of the nanofibrous aerogels reported so far were opaque because they had structural inhomogeneity at the scale of > ∼ 100 nm Thus, the synthesis of transparent nanofibrous aerogels has been a major challenge in this field A recent breakthrough was made by Isogai’s group, who reported a translucent aerogel of oriented cellulose nanofibers.12 This aerogel had a homogeneous porous structure at the nanoscale and hence a low thermal conductivity However, the use of oxidative catalysts, e.g., 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), in the production of cellulose nanofibers becomes an industrial problem because of its high cost and hazardous properties An alternative approach is therefore needed to synthesize transparent nanofibrous aerogels without using any high-cost catalysts or oxidants Chitosan is a biomass-derived polysaccaride abundant in many raw materials, with a low cost, yet is environmentally friendly, and has high biocompatibility Several gelation approaches to producing chitosan aerogels, such as physical gelation in basic solutions,13,14 cross-linking with hemicellulose citrate,15 and cross-linking with aldehydes,16−19 have been reported Despite the range of available synthetic methods, these previous reports have dealt only with opaque aerogels with high densities The chitosan aerogels have been proposed © 2015 American Chemical Society Figure Preparation of chitosan aerogels (a) Synthesis procedure Photographs of (b) methanogels and (c) aerogels Thicknesses of methanogels and aerogels: 6.2 and 3.9 mm (C16F7), 8.5 and 4.2 mm (C16F2), 7.3 and 4.0 mm (C8F7), 8.6 and 4.8 mm (C8F2), and 8.4 and 3.7 mm (C4F7) Preparation conditions and physical properties of the chitosan aerogels obtained are summarized in Table The concentration of chitosan in the final gelling solution varied from to 16 g L−1, whereas that of formaldehyde was 1.8 or 7.2 wt % The sample C4F2, which was prepared using g L−1 chitosan and 1.8 wt % formaldehyde solutions, did not form any solid gel, probably because of an insufficient amount of cross-links Received: September 14, 2015 Revised: October 22, 2015 Published: November 6, 2015 7569 DOI: 10.1021/acs.chemmater.5b03610 Chem Mater 2015, 27, 7569−7572 Communication Chemistry of Materials Table Preparation Conditions and Physical Properties of Chitosan Aerogels a Sample chitosan [g L−1] formaldehyde [wt %] ρa [g cm−3] Pb [%] C16F7 C16F2 C8F7 C8F2 C4F7 C4F2 16.0 16.0 8.0 8.0 4.0 4.0 7.2 1.8 7.2 1.8 7.2 1.8 0.175 0.128 0.103 0.081 0.042 86 91 93 94 97 ODc [mm−1] 0.10 0.08 0.14 0.13 0.15 did not form a gel kd [W m−1 K−1] Ee [MPa] 0.029 0.022 38.9 34.9 21.2 2.26 0.35 Density bPorosity cOptical density at 800 nm dThermal conductivity eElastic modulus than those of chitosan cryogels24,25 and comparative to those of nanocellulose aerogels.12 Assuming a cylindrical shape, the nanofibers are calculated to be 5.2 nm in diameter from the specific surface area This is consistent with the SEM observations, showing the small contact areas between nanofibers The X-ray diffraction profiles (Figure S10) show that chitosan nanofibers in the aerogels are almost amorphous The UV−visible transmission spectra of the chitosan aerogels (Figure 3a) consist of two wavelength regions: a broad The other samples formed transparent hydrogels and methanogels (Figure 1b) After extraction with supercritical CO2, the obtained aerogels (Figure 1c) were translucent with a yellowish color that originated from −NCH2 groups.22,23 The color became darker with increasing chitosan and formaldehyde concentrations, reflecting the increase in the amount of cross-links According to the Fourier-transform infrared spectroscopy and thermogravimetry (Figures S1 and S2), the aerogel samples did not contain a detectable amount of free formaldehyde The apparent density of the aerogels decreased from 0.175 to 0.042 g cm−3 in the order of C16F7, C16F2, C8F7, C8F2, and C4F7 These results mean that the apparent density depends on the amount of cross-links, which can be controlled by changing the chitosan and formaldehyde concentrations As shown in the scanning electron microscopy (SEM) images (Figures and S3−S8), the cross-linked chitosan Figure Optical properties of chitosan aerogels (a) Transmission spectra with thicknesses of 4.0 mm (C16F7), 4.4 mm (C16F2), 3.5 mm (C8F7), 4.6 mm (C8F2), and 4.6 mm (C4F7) (b) Optical densities per mm thick at 800 and 400 nm absorption in the 300−500 nm blue region corresponding to −NCH2 bonds and a gradual increase in transmittance in the 500−800 nm visible region corresponding to the light scattering The apparent optical densities per mm (Figure 3b) were calculated from these spectra with an assumption of negligible surface reflection The optical density at 400 nm roughly increases with increasing density, indicating that the amount of −NCH2 bonds increases as the cross-links increase On the contrary, the optical density at 800 nm decreases with increasing density We suggest that the microstructure of chitosan aerogels has a small inhomogeneity at the nanoscale, which causes light scattering, particularly in low density samples The largest optical density at 800 nm per mm, 0.15, of C4F7 corresponds to 71% in transmittance This value is still sufficiently high to be used in transparent thermal insulation applications The thermal insulation property of the chitosan aerogels was evaluated based on a thermal conductivity measurement using the axial heat flow method The thermal conductivities of C4F7 and C8F2 are 0.022 and 0.029 W m−1 K−1, respectively These values are lower than those of commercial flexible thermal insulators, e.g., mineral wools (0.033−0.05 W m−1 K−1), cellulose foams (0.046−0.054 W m−1 K−1), and polystyrene foams (0.03−0.04 W m−1 K−1),26,27 and comparative to those of nanocellulose aerogels (0.018−0.038 W m−1 K−1).12 In Figure SEM images of chitosan aerogels: (a) C16F7, (b) C16F2, (c) C8F7, (d) C8F2, and (e,f) C4F7 aerogels consist of entangled nanofibers of 5−10 nm in diameter with mesopores of 10−50 nm in size These pores are small enough to cause negligible scattering of visible light The area of pores observed in the SEM images increases as the apparent density decreases The specific surface area determined by the Brunauer−Emmett−Teller method is 545 m2 g−1 for C4F7 (Figure S9) This value is 4−30 times larger 7570 DOI: 10.1021/acs.chemmater.5b03610 Chem Mater 2015, 27, 7569−7572 Communication Chemistry of Materials general, the thermal conductivity of a porous material, k, is given by the following equation,28,29 k = kg,cond + kg,conv + ks,cond + k rad (1) where kg,cond, kg,conv, ks,cond, and krad are the thermal conductivity factors for gas conduction, gas convection, solid conduction, and radiation, respectively Because the gas convection, kg,conv, is negligible for pores smaller than mm,28 the total thermal conductivity strongly depends on the gas and solid conductions The gas conduction, kg,cond, can be dramatically suppressed when pores are smaller than the average mean free paths of air components, e.g., 68 nm for N2 Such a nanoporous structure is called a “nano insulation material (NIM)”,29 and enables us to achieve a lower thermal conductivity than that of stationary air, 0.0262 W m−1 K−1 at 300 K.30 As the sample C4F7 has a thermal conductivity of 0.022 W m−1 K−1, the entangled structure of chitosan nanofibers acts like a NIM On the other hand, the solid conduction, ks,cond, is proportional to the square of the density, because the area of solid in a cross section increases with increasing density This explains the large thermal conductivity of 0.029 W m−1 K−1 for C8F2 with a density of 0.081 g cm−3 We also note that the thermal conductivity of chitosan aerogels is much lower than those of nanocellulose aerogels when they are compared at the same apparent density, although their true densities are almost the same.31,32 This can be attributed to the difference in microstructures Cross-linked chitosan aerogels consist of entangled nanofibers with a random orientation In contrast, the cellulose nanofibers tend to form oriented domains.12 Such an oriented structure probably has a longer continuous length of pores along the orientation, which makes the gas conduction easier along that direction According to a thermal stability test using a see-through heating cell (Figures S11 and S12), C4F7 did not show any change in appearance during heating from room temperature to 175 °C This result means that the chitosan aerogel has a sufficient thermal stability for residential and vehicular applications, such as house windows, glass walls of buildings, and car windows Figure 4a shows the compression stress−strain curves for the chitosan aerogels They were compressed without forming fractures or cracks up to ∼95% strain The compression curves are similar to those of nonbrittle cellular polymer foams,33 and consist of the following three parts The first linear region up to ∼15% strain is attributed to elastic deformation The elastic region is followed by a gradual increase in the stress with a large strain up to 60−90% This is probably attributable to a plastic deformation through breaking the cross-links After the second region, a steep increase in the stress is observed at strain >60− 90% This might be attributable to elimination of pores through densification of nanofibers We point out two mechanical characteristics of the chitosan aerogels: (i) The entangled nanofibrous structure has high mechanical toughness compared to nanoparticle skeletons, such as silica aerogels, which usually break at low strains ∼10%.34 (ii) The nanofibrous structure also shows a strong density dependence in elasticity, e.g., the elastic modulus, yield stress, and stress at 50% strain (Figure 4b) This might be attributed to the increase in the cross-links, which harden the structure The thin sample of C4F7 aerogel (Figure 4c) is even bendable by hand without a fracture Judging from these results, the entangled nanofibrous structure of the chitosan aerogels has a great advantage in terms of mechanical Figure Mechanical properties of chitosan aerogels (a) Compression stress−strain curves Inset: expanded curves in the small stress region (b) Changes with density in elastic modulus, stress at 50% strain, and yield stress The yield stress of C4F7 is not shown because of an ambiguous yield point (c) Photographs of a 0.6 mm-thick sample of C4F7 toughness and flexibility, and these aerogels have the potential to be used as flexible thermal insulators In conclusion, we have prepared translucent chitosan aerogels by the cross-linking gelation method without using any catalysts or oxidants The aerogels consisted of entangled nanofibers with a high porosity up to ∼97% The aerogel microstructure exhibits visible transparency and a low thermal conductivity of ∼0.022 W m−1 K−1, which is lower than that of stationary air The aerogels also showed flexibility and higher mechanical toughness than conventional silica aerogels The characteristic mechanical properties associated with a low thermal conductivity enable us to realize a new category of thermal insulators: environmentally friendly, transparent, and flexible ■ ASSOCIATED CONTENT S Supporting Information * The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03610 Detailed experimental procedure, FT-IR spectra, TG profiles, SEM images, BET and XRD profiles, and a thermal stability test (PDF) ■ AUTHOR INFORMATION Corresponding Author *S Takeshita E-mail: s.takeshita@aist.go.jp Notes The authors declare no competing financial interest ■ ■ ACKNOWLEDGMENTS The mechanical measurement was performed by Global Application Development Center of Shimadzu Corporation REFERENCES (1) Pierre, A C.; Pajonk, G M Chemistry of Aerogels and Their Applications Chem Rev 2002, 102, 4243−4266 7571 DOI: 10.1021/acs.chemmater.5b03610 Chem Mater 2015, 27, 7569−7572 Communication Chemistry of Materials (21) Zhao, L.; Dong, P.; Xie, J.; Li, J.; Wu, L.; Yang, S.-T.; Luo, J Porous Graphene Oxide−Chitosan Aerogel for Tetracycline Removal Mater Res Express 2014, 1, 015601 (22) Singh, A.; Narvi, S 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insulation applications The thermal insulation property of the chitosan aerogels was evaluated based on a thermal conductivity measurement... large thermal conductivity of 0.029 W m−1 K−1 for C8F2 with a density of 0.081 g cm−3 We also note that the thermal conductivity of chitosan aerogels is much lower than those of nanocellulose aerogels. .. toughness and flexibility, and these aerogels have the potential to be used as flexible thermal insulators In conclusion, we have prepared translucent chitosan aerogels by the cross-linking gelation