Supporting Information Chitosan Aerogels: Transparent, Flexible Thermal Insulators Satoru Takeshita* and Satoshi Yoda Detailed experimental procedure Materials: Chitosan (deacetylation rate: > 80%, viscosity: 20 200 mpa s at 5 g L 1 ; 20 C), acetic acid (99.7%), formaldehyde solution (36 38 wt% aqueous solution), and methanol (99.8%) were purchased from Wako Pure Chemical Industries. Liquid CO 2 (99.9%) was purchased from Iwatani Corporation. All reagents were of analytical grade and used without further purification. Synthesis of cross-linked chitosan aerogels: Chitosan powder (1.00 g) was dissolved in 50.0 ml of 2 vol% aqueous acetic acid to prepare a 20.0 g L 1 chitosan solution. This solution was diluted with ultrapure water to prepare 10.0 and 5.0 g L 1 solutions. For a typical synthesis of an aerogel, 6.0 ml of chitosan solution (20.0, 10.0, or 5.0 g L 1 ) was mixed with 1.5 ml of aqueous formaldehyde solution (36 or 9 wt%) in a glass petri dish. This dish was placed in a sealed container and aged at 60 C overnight. After cooling to room temperature, the obtained hydrogel was washed by soaking in ultrapure water for ~6 h under constant stirring at room temperature. The washed hydrogel was then soaked in methanol for at least 4 days with constant stirring at room temperature to complete the solvent exchange. The obtained methanogel was placed in a stainless steel autoclave (Taiatsu Techno, TAS-047, inner volume: 470 ml) with a certain amount of methanol and then the solvent was extracted with supercritical CO 2 at 80 C and ~20 MPa. We also prepared aerogels with different sizes and shapes by using different containers. Characterizations: The density of aerogels was calculated from the diameter, height, and weight of aerogels assuming a cylindrical shape. The porosity was calculated based on the true density of bulk chitosan, ~1.4 g cm 3. S1 The Fourier-transform infrared (FT-IR) transmission spectra were measured in pressed KBr disks on a spectrometer (JASCO, FT/IR-660plus). The thermogravimetry (TG) 1
profiles of aerogels was measured on a TG analyzer (Shimadzu, TGA-51) in a N 2 flow of 20 ml min 1 at a heating rate of 10 C min 1. The fine structure of aerogels was observed with a field-emission scanning electron microscope (SEM; Hitachi, S-4800). The sample for SEM observation was prepared by coating a Pd Pt conductive layer onto the aerogel pieces. The SEM images were trimmed and optimized in brightness and contrast using Microsoft PowerPoint 2010. The specific surface area of the aerogel was determined by the Brunauer Emmett Teller (BET) nitrogen adsorption method using an automatic surface area analyzer (MicrotracBel, BELSORP-max). The X-ray diffraction (XRD) profiles were recorded on an XRD instrument (Rigaku, MiniFlex II) using a Cu K radiation source. The transmission spectra were measured on a UV-visible spectrometer (JASCO, V-570) equipped with a deuterium lamp and a tungsten lamps. The thermal conductivity of aerogels was measured using a thermal conductivity tester (EKO Instruments, HC-074/200) under a constant N 2 flow. The sample for thermal conductivity measurement was prepared by putting an aerogel monolith between two sheets of polyethylene foam (Inoac Corporation, PE light CP-3). The thermal conductivity of aerogels, k ag, was calculated using the following equation: ( Tu Tl ) Q (S1) 2( L k ) ( L k ) pe pe ag ag where T u and T l is the temperatures of upper and lower probes, Q is the average heat flow at the upper and lower probes, k pe and L pe are the thermal conductivity and thickness, respectively, of the polyethylene foam, and L ag is the thickness of the aerogel. The thermal stability of aerogels was evaluated based on the change in their appearance with increasing temperature from room temperature to ~190 C, using a see-through heating cell, in a N 2 atmosphere, and at a heating rate of approximately 10 C min 1. The mechanical properties of aerogels were determined based on the compression stress strain curves recorded on a universal mechanical tester (Shimadzu, Autograph AG-X) equipped with a 10 kn load cell at the compression rate of 1 mm min 1. The compression elastic modulus was calculated from the slope of the linear part in the 0 20% strain region of the stress strain curve. 2
Transmittance (a.u.) Additional Figures C16F7 C16F2 C8F7 C8F2 C4F7 5 Reagent 1 2 3 4 6 7 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) Figure S1. FT-IR spectra of chitosan reagent and aerogels. A broad peak at ~3430 cm 1 (peak 1) is assigned to O H stretching vibration of glucosamine monomers and adsorbed water molecules. S2,S3 Peaks at 2865 2920 cm 1 (peaks 2) and 1590 cm 1 (peak 4) are assigned to C H stretching and NH 2 scissoring vibrations of glucosamine monomers. S2,S3 The 1295 1453 cm 1 region (peaks 6) contains CH 2 scissoring, CH 2 bending, CH 2 wagging, O H bending, and NH 2 twisting of glucosamine monomers. S3 A broad peak at ~1050 cm 1 (peak 7) contains C C and C N vibrations of glucosamine monomers and C O C vibrations of ether bonds between the monomers. S2,S3 The peaks at 1660 cm 1 (peak 3) is assigned to C=O vibration of remaining acetamide groups. S2,S3 These peaks become broader in the aerogel samples, indicating a variety of molecular environments in the aerogels. The weak peak at 1560 cm 1 (peaks 5) observed in aerogel samples is assigned to N=C vibration derived from the Schiff base between NH 2 groups and formaldehyde. S2 No peak from free formaldehyde is observed. 3
Weight loss (wt%) 100 80 60 40 C16F7 C16F2 20 0 C8F7 C8F2 C4F7 100 200 300 400 500 600 Temperature ( O C) Figure S2. TG profiles of chitosan aerogels. The weight loss up to ~200 C is attributed to adsorbed water. The weight loss up to ~600 C corresponds to the decomposition of chitosan chains. S4,S5 No detectable signal from free formaldehyde is observed. 4
Figure S3. Raw SEM image of Figure 2a (16F7). Figure S4. Raw SEM image of Figure 2b (C16F2). 5
Figure S5. Raw SEM image of Figure 2c (C8F7). Figure S6. Raw SEM image of Figure 2d (C8F2). 6
Figure S7. Raw SEM image of Figure 2e (C4F7). Figure S8. Raw SEM image of Figure 2f (C4F7). 7
P/V a (P 0 -P) Quantity adsorbed, V a (cm 3 g -1 ) a 200 150 100 50 0 0 0.1 0.2 0.3 0.4 Relative Pressure P/P 0 b 0.004 0.003 0.002 0.001 0.000 0 0.1 0.2 0.3 0.4 Relative Pressure P/P 0 Figure S9. Adsorption isotherm (a) and BET plot (b) of C4F7 aerogel. 8
Intensity (a.u.) C4F7 Reagent 10 20 30 40 50 60 2 (deg) Figure S10. XRD profiles of chitosan reagent and C4F7 aerogel. 9
23 C 50 C 75 C 100 C 125 C 150 C Figure S11. Change in the appearance of C4F7 with increasing temperature (room temp. 150 C). 10
175 C 185 C 193 C 193 C (after keeping for 15 min) Figure S12. Change in the appearance of C4F7 with increasing temperature (175 193 C). 11
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