Upscaled Preparation of Trimethylsilylated Chitosan Aerogel

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Materials and Interfaces

Upscaled Preparation of Trimethylsilylated Chitosan Aerogel Satoru Takeshita, and Satoshi Yoda Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02332 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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Industrial & Engineering Chemistry Research

Upscaled Preparation of Trimethylsilylated Chitosan Aerogel Satoru Takeshita,* 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, Ibaraki 305-8565, Japan.

ABSTRACT

Biobased nanofibrous aerogels are ones of the emerging topics in materials chemistry because of their unique properties and environmental friendliness, but their practical potential has not yet been fully clarified. For the first step of industrial feasibility study of biobased nanofibrous aerogel as high-performance thermal insulators, this paper aims to establish a preparation protocol of decimeter-sized monoliths (~120 × 120 mm2) of cross-linked chitosan aerogel and their thermal conductivity evaluation. In contrast to well-studied silica aerogel, the chitosan gel has flexible polymeric network and shows a drastic change in volume during the process, which has to be taken into consideration for the design of solvent exchange protocol. The obtained aerogel shows remarkably low thermal conductivity of 16–17 mW m−1 K−1. This value shows

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that the chitosan aerogel is one of the promising candidates for superinsulation materials in the next-generation thermal energy management.

1. INTRODUCTION Aerogels, open-celled highly porous solids with submicrometer–nanometer scale pores, are ones of the major topics in materials science and chemical process technology. Early stages of aerogel research had focused on supercritically dried silica aerogels and established some practical applications for space dust collectors, Cherenkov detectors, and thermal insulators.1–5 In recent two decades, the variety of materials and preparation routes of aerogels have been extremely expanding. From the industrial view, silica- and polyurethane-based aerogels were successfully commercialized for insulation applications, in which the aerogels are produced by both supercritical and ambient pressure dryings.6,7 Fast and cost-effective preparation of granular and/or powdered silica aerogels is also paid special attention.8,9 From the viewpoint of academic interests, synthetic and physicochemical studies of diverse aerogels have been reported, including silicone-based organic-inorganic hybrids,10,11 nanostructured carbons,12,13 metals and metal oxides,14,15 and biopolymers,16,17 to open up new applications. Among these materials, biobased nanofibrous aerogels such as cellulose,18–21 chitin,22–24 and chitosan are ones of the emerging topics from the aspects of biocompatibility, abundant resources, low costs, and fundamental mechanical toughness, but their industrial potentials have not yet been fully clarified. Chitosan is a biobased polysaccharide obtained from seafood wastes, e.g., crab, shrimp, and lobster shells.25 Owing to its unique physicochemical and physiological properties, chitosan has


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extensively been studied in many fields ranging from waste water treatment to biomedical applications.26–29 As chitosan easily forms gels with a variety of physical and chemical crosslinking bonds in aqueous solvents,30 it has been focused as a material of aerogels featuring drug delivery systems,31,32 catalysts,33–39 biomedical scaffolds,40–42 and environmental remediation.43 Chitosan-based composite aerogels have also been studied as thermal insulators and absorbents by combining with silica,44–46 cellulose,47 clays,48,49 and graphene.50,51 We recently reported synthesis of translucent chitosan aerogel using formaldehyde as a mild cross-linker.52–55 This aerogel consists of three-dimensional network of chitosan nanofibers and exhibits optical transparency, mechanical toughness, and low thermal conductivity of ~22 mW m−1 K−1. These properties are potentially suitable for translucent thermal insulators52 and scaffolds for functional nanoparticles.53 To improve the humidity stability for further practical uses, we also reported hydrophobic modification of the chitosan aerogel by trimethylsilylation.55 In this case, an aprotic supercritical drying solvent, such as acetone/CO2 system, is required instead of conventional alcohol/CO2 systems to prevent decomposition of introduced trimethylsilyl groups. Trimethylsilylated chitosan aerogel has similar nanofibrous microstructure, optical, and mechanical properties with those of the unmodified aerogel, but the effect of trimethylsilylation on thermal conductivity has not been clarified because available samples are not large enough for an accurate thermal conductivity measurement. In this study, we report preparation of large size monoliths of trimethylsilylated, acetone/CO2dried chitosan aerogels. This study has the following two objectives: i) to establish a preparation protocol of decimeter-sized wet gel and aerogel of cross-linked chitosan as the first step of industrial feasibility study. In contrast to well-established preparation methods of silica aerogels, instruments and preparation processes for the chitosan gel meet some dynamic behaviors of the


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flexible polymeric network when the sample size increases. Since the present work is the first report for large size preparation of polysaccharide nanofibrous aerogels to our knowledge, it contains some tips and know-hows that would also be useful for other biobased aerogels. ii) The other objective is to measure the accurate thermal conductivity of trimethylsilylated chitosan aerogels using sufficiently large samples and discuss its potential for high-performance thermal insulators.

2. EXPERIMENTAL SECTION 2.1. Materials. Chitosan (degree of deacetylation: >80%, viscosity: 20–200 mPa s at 5 g L−1; 20 °C), aqueous formaldehyde solution (36.5 wt%), 1,1,1,3,3,3-hexamethyldisilazane (HMDS, 96.0%), acetic acid (99.7%), methanol (99.8%), and acetone (99.0%) were purchased from Wako Pure Chemical Industries. Chitosan powder (5.00 g) was dissolved in ultrapure water (995.0 mL) and acetic acid (5.0 mL, 0.5 vol%) to make 5 g L−1 stock solution of chitosan. This stock solution was stored for at least 1 week before use in order to stabilize the solvation. 2.2. Synthesis of Chitosan Hydrogels. Scheme 1 shows a schematic representation of gel preparation and solvent exchange procedures. The 5 g L−1 chitosan solution was diluted to be 4.75 g L−1 by adding ultrapure water. This solution was mixed with aqueous formaldehyde solution (36.5 wt%) at a volume ratio of chitosan:formaldehyde = 4:1 (glucosamine unit:formaldehyde is approximately 1:147 in molar ratio) and then poured into a stainless steel mesh–plate double mold (Figure 1). Two types of molds were used: i) a square mold consists of a 330 mm × 330 mm (outer size) × 35 mm height inner mesh tray in a 331 mm × 331 mm (inner size) × 33 mm height outer plate tray and ii) a circular mold consists of a 260 mmφ (outer size) ×


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32 mm height inner mesh tray in a 261–262 mmφ (inner size) × 30 mm height outer plate tray. The heights of the inner mesh trays had to be 2 mm higher than those of corresponding outer plates for smooth detaching of the plates after the gelation. The mesh pore size was 2 mmφ for both molds. For the square mold, 1744 mL of chitosan solution (4.75 g L−1) was mixed with 436 mL of formaldehyde solution. For the circular mold, 852 mL of chitosan solution (4.75 g L−1) was mixed with 213 mL of formaldehyde solution. After mixing the solutions, the mold was sealed in a polypropylene airtight container and aged at 60 °C for 24 h in a chamber (Yamato Scientific, IG-420) to complete the gelation.

Scheme 1. Schematic illustration of gel preparation process.


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Figure 1. Photographs of stainless steel molds.

2.3. Solvent Exchange. After the complete gelation, the mold was cooled to room temperature and then the outer plate was detached from the inner mesh. The obtained hydrogel in the mesh was soaked in pure methanol, methanol/acetone mixed solvent, and pure acetone in the following order: methanol:acetone = i) 1:0 (pure methanol), ii) 1:1, iii) 1:3, iv) 1:4, v) 1:5, v’) 1:7 (square gels only), vi) 1:9, vii) 0:1 (pure acetone), viii) 0:1, and ix) 0:1. The solvent was changed twice a day. Between processes ii) and iii), the outer ~5 mm edge of the gel was cut and removed to prevent crack formation caused by expansion of the gel. At the same time, the bottom of the gel was scratched using a bar (for the square mold, Figure S1) and a curved wire (for the circular mold, Figure S2) to peel off the gel from the bottom mesh (see also supporting videos). After the


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solvent exchange with pure acetone, the organogel was transferred into a smaller mold (Figure 1): i) a square mesh tray of 250 mm × 250 mm (outer size) × 25 mm height and ii) a circular mesh tray of 190 mmφ (outer size) × 22 mm height. 2.4. Trimethylsilylation. The obtained organogel filled with acetone was trimethylsilylated by soaking in HMDS/acetone mixed solvent for 24 h at room temperature under constant stirring. For the square gel, a total 3200 mL solution containing 160 mL (5 vol%) or 240 mL (7.5 vol%) of HMDS was used. For the circular gel, a total 1600 mL solution containing 80 mL (5 vol%) or 120 mL (7.5 vol%) of HMDS was used. After the trimethylsilylation, the gel was washed by soaking in acetone for at least 2 days with a constant change of acetone twice a day. 2.5. Supercritical Drying. The mesh trays holding trimethylsilylated organogels were piled in a stainless steel cage (Figure S3) and soaked in acetone in a stainless steel container. Then this container was transported from our laboratory to Mohri Oil Mill Co., Ltd. Matsusaka Factory on a truck equipped with an air suspension. Supercritical drying was carried out in a 140 L autoclave (cylindrical shape with an inner diameter of 530 mmφ, a height of 672 mm, and an available height of 510 mm for the drying rack) in this factory. First, approximately 84 kg of acetone was poured into the autoclave. The cage containing organogels was soaked into it, and the autoclave was sealed. As shown in Figure 2, the pressure of the autoclave was increased by introducing CO2 up to 15 MPa, while the temperature was set to be 60 °C. The acetone was extracted at 60 °C and 15 MPa for 45 h. After the extraction, the pressure and temperature were gradually decreased to be ambient conditions. The obtained aerogels were packed in a vacuum desiccator and transported to our laboratory by train, and then thermal conductivity measurement was carried out on the very day to avoid any temporal change during storage.


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Figure 2. Set temperature, pressure, and CO2 flow rate for supercritical drying.

2.6. Characterization. The apparent density of aerogel was calculated from the diameter, height, and weight of the monolithic sample. The Fourier-transform infrared (FT-IR) transmission spectrum was measured on a FT-IR spectrometer (JASCO, FT/IR-4600) using a pressed KBr disk. The water contact angle was measured on an automatic contact angle analyzer (Kyowa Interface Science, DMs-401). The microstructure of the sample was observed with a field-emission scanning electron microscope (SEM, Hitachi, SU9000) with a thin coating of conductive Pd–Pt. The samples for FT-IR, contact angles, and SEM were obtained by picking up a tiny piece from the edge of the aerogel. Photographs were taken with a digital camera (Canon, IXY210), and the brightness was optimized on PowerPoint 2013. Video files were recorded on a digital video camera (Sony, Handycam FDR-AX40) and edited with VideoStudio X9 (Corel).


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The thermal conductivity of aerogel was measured by means of heat flow method using an axial heat flow meter (EKO Instruments, HC-074). The upper and lower probes were set to be 15 and 35 °C, respectively, and the measurement was carried out at ambient pressure. The thermal conductivity, λ, was calculated from equation (1):

λ = c × Q× d/∆T

(1)

where ∆T is the temperature difference, 20 K, d is the thickness of the sample, Q is the heat flux under the steady state, and c is calibration factor of the heat flux sensors. The actual Q value was obtained from the average of heat fluxes of upper and lower heat flux sensors. The sensors were calibrated using a standard fibrous glass board (NIST 1450d, λ = (1.10489 × 10−4) × temperature ± 1.0% W m−1 K−1 in −7.5–62.5 °C range) and an expanded polystyrene standard (EKO Instruments, 08080634). Before the measurement, the accuracy of low-thermal conductivity region of the instrument was confirmed using a silica-aerogel-polypropylene composite with a thermal conductivity of 15 mW m−1 K−1.56 2.7. Hazardous Materials. As the amounts of formaldehyde solution and organic solvents are large, they should be handled with particular caution in a fume hood.

3. RESULTS ANS DISCUSSION 3.1. Size Change during Solvent Exchange. In contrast to silica hydrogel and alcogel, which have rigid nanoparticle skeletons, the chitosan gel drastically changes its volume depending on the inner solvent composition because of flexible skeleton of the polymeric network. This dynamic feature enhances the flexibility in handling the gel, however, the volumetric change


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must be taken into consideration when designing the process for large size gel preparation. The solvent exchange protocol in the present study is as follows: water in as-prepared hydrogel is once changed with methanol to stabilize the gel structure and remove the solute species, and then methanol is gradually changed with acetone, which is the solvent for supercritical drying. We cannot use methanol as a supercritical drying solvent because methanol/CO2 system under high pressure forms an acidic atmosphere, which detaches trimethylsilyl groups introduced on OH groups of chitosan.55 We also note that direct exchange from water to acetone causes fast shrinkage and distortion of the gel, which was confirmed with small samples in the previous work.55 Trimethylsilylation is carried out on organogel filled with sufficiently pure acetone to prevent any unexpected reactions between methanol and HMDS. Figures 3 and 4 show the changes in appearances of square and circular samples during the solvent exchange process. At the first step, the gel expands in methanol, and wrinkles are formed at the surface. This expansion is mainly attributed to osmotic pressure due to the solutes of crosslinking reaction, e.g., acetate ions and unreacted formaldehyde, in the as-prepared hydrogel. We note that the expansion is not attributed to the intrinsic volumetric phase transition from hydrogel to methanol-gel, because clean chitosan hydrogel becomes much larger than clean methanol-gel. The wrinkles gradually fade out after being soaked in methanol:acetone = 1:1. The edge of the gel should be cut and removed at this moment, otherwise the gel occasionally breaks by expansion (Figure 5). At the same time, the bottom of the gel physically sticks to the bottom mesh unless using a Teflon-lined mold. This sticky bottom causes some cracks (Figure 5) when the gel shrinks in acetone. As shown in Figures S1 and S2, the bottom of the gel needs to be scratched to peel off the gel from the mesh (see also supporting videos).


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After the edge removal and bottom scratching, the solvent is gradually exchanged with acetone. The gel now shrinks with increasing acetone content, and organogel filled with pure acetone is ~70% in diameter to the initial size. The mesh mold should be changed with a smaller one at this moment for the efficient use of space in the autoclave. This mold change can be smoothly done by floating the gel on the solvent. Trimethylsilylation does not affect the size of the gel, regardless of the HMDS concentration. The final organogel is fundamentally crack-free, because if there was a crack the gel would be separated into small pieces. As the present work is the first try for large size preparation, we applied the time-consuming stepwise protocol for solvent exchange from methanol to acetone, i.e., total 6–7 steps, to avoid any damage caused by a sudden size change. Taking mechanical flexibility of the polymeric network into consideration, there is still room for shortening the solvent exchange process significantly as long as any part of the gel is not stuck to the mold. 3.2. Aerogel Appearance and Microstructure. As summarized in Table 1, we prepared four square aerogels and six circular aerogels with different HMDS concentrations. These aerogels show translucent yellowish appearance with smooth surface (Figures 6, 7, and S4). The yellowish color becomes weak when the HMDS concentration increases. The main origin of this color is Maillard reaction caused by heat during supercritical drying.52–54 We applied 3 days long drying process in the present study to avoid any possible troubles related to incomplete drying, but such a long process resulted in deeper yellowish color than that of small samples. We could have shortened the drying process to be 1–1.5 day because the extraction monitor (Figure 8) showed that the acetone extraction almost finished in ~15 h. In Figure 8, the total amount of extracted acetone (~73 kg) was smaller than the initial amount (~84 kg), because a small fraction


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of acetone was dissipated into the gaseous phase together with CO2 inside the CO2/acetone separation chamber.

Figure 3. Change in appearance of square gel (sample S4) during solvent exchange.


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Figure 4. Change in appearance of circular gel (sample C3) during solvent exchange.

Figure 5. Broken gel samples without edge removal or bottom scratching.


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Table 1. Preparation conditions and final properties of aerogels.

Final size of aerogel

Final weight of aerogel [g]

Apparent density −3 [g cm ]

Unmodified

108 mm × 108 mm × 6.4 mmt

7.711

0.103

S2

5 vol%

120 mm × 116 mm × 6.5 mmt

7.974

0.088

S3

5 vol%

126 mm × 119 mm × 5.6 mmt

8.007

0.095

S4

7.5 vol%

124 mm × 120 mm × 6.3 mmt

8.708

0.093

Circle C1

Unmodified

83 mmφ×5.9 mmt

3.337

0.105

C2

Unmodified

81 mmφ×5.1 mmt

3.129

0.119

C3

5 vol%

89 mmφ×5.8 mmt

3.825

0.106

C4

5 vol%

87 mmφ×6.0 mmt

3.322

0.093

C5

7.5 vol%

92 mmφ×5.3 mmt

4.188

0.119

C6

7.5 vol%

93 mmφ×6.2 mmt

4.003

0.095

Mold

Sample

Square S1

HMDS concentrati on


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Figure 6. Photographs of square aerogel samples.

Figure 7. Photographs of circular aerogel samples.


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Figure 8. Integrated amount of extracted acetone during extraction.

Figure 9 shows FT-IR spectra of the aerogels. Trimethylsilyl groups introduced in silylated aerogels are confirmed by the peaks from Si–C vibrations at 1254 and ~850 cm−1 and CH3 bending at 753 cm−1 (see Table S1 for the detailed peak assignments).57 The relative intensity of these peaks increases when the HMDS concentration increases from 5 (samples S2, S3, and C3) to 7.5 vol% (samples S4 and C5). According to the previous work, the upper limit of degree of silylation, i.e., fraction of trimethylsilylated OH, is ~50% of the OH groups in chitosan, which might correspond to the ratio of surface/internal OH groups of the nanofibers.55 By comparing the relative intensities of IR peaks of trimethylsilyl groups and chitosan chains, the degrees of silylation in the present work are roughly estimated to be ~20% and 30–40% for 5 and 7.5 vol% of initial HMDS concentrations, respectively (see Supporting Iinformation for details). As shown in Figures 10 and S5–S7, the water contact angle increases with increasing HMDS concentration. Penetration of water droplet into the aerogel is also suppressed for high HMDS concentrations, indicating that the hydrophobic trimethylsilyl groups effectively repel the water.


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Figure 9. FT-IR spectra of aerogel samples.

Figure 10. Contact angle measurement. a) Change in contact angle with time of samples C1 (unmodified), C3 (HMDS 5 vol%), and C5 (HMDS 7.5 vol%). Water droplets just after being dropped on samples b) C5, c) C3, and d) C1.


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As shown in Table 1, the final weights of silylated aerogels (samples S4, C5, and C6) are 10– 20% higher than those of unmodified aerogels (samples S1, C1, and C2) owing to the heavy Si(CH3)3 groups. On the other hand, apparent densities of these samples are around 0.09–0.1 g cm−3 regardless of trimethylsilylation, because the final size of trimethylsilylated aerogels are slightly larger than those of unmodified ones. We suggest that the hydrophobic nature of Si(CH3)3 groups suppresses the shrinkage of the gel in hydrophobic supercritical CO2. In general, supercritical drying does not affect the size of aerogel because of the lack of surface tension, while the chitosan gel changes its size by almost half in diameter during supercritical drying. This shrinkage is not caused by drying itself but is probably attributed to volumetric phase transition triggered by the change of inner solvent from acetone to liquid and/or supercritical CO2.53 This is another dynamic feature of cross-linked chitosan gel, and we note that detailed investigation on the volumetric phase transition behavior will be needed for the efficient use of space in the autoclave.

Figure 11. SEM images of sample S2.


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Figure 11 shows SEM images of sample S2. The aerogel consists of nanofibrous network as well as the small samples reported in our previous report.55 Taking consideration of a surface Pd– Pt layer of ~3 nm thick, we estimated the width of the nanofibers to be around several–20 nm. The surface of the nanofibers is not smooth because of the presence of some ambiguous nanoparticulate structures. Some nanofibers form branch structures (marked in Figure 11) and lengths of individual nanofibers are not clearly distinguishable. This is the major difference between cross-linked chitosan and other biobased polysaccharides, e.g., cellulose and chitin. Aerogels constructed from cellulose and chitin are fundamentally nanofibrous because these polysaccharides tend to form nanofibers with well-defined shapes and sizes, which are primary building blocks of plant and animal shells in the nature.58–60 On the other hand, chitosan is once dissolved in the acetic acid, and then regenerated as hydrogel in the presence of cross-linkers. Thus, molecular chitosan spontaneously assembles into the nanofibrous structure during gelation and/or drying processes. 3.3. Thermal conductivity. Thermal conductivity measurement methods can be divided into non-steady state methods and steady state methods. Since the former methods cannot guarantee the accuracy in low thermal conductivity region, evaluation of thermal insulators normally conducted by the latter methods, e.g., guarded hot plate and heat flow methods, however, these methods require large samples. In our first try on thermal conductivity measurement of unmodified chitosan aerogel,52 we assembled several small samples to make a large disk, sandwiched it with polyurethane foams, and measured the thermal conductivity indirectly from the heat flux of the sandwiched structure. In the present study, we directly measured the thermal conductivity of the aerogel thanks to the large sample size. As shown in Figure 12, the


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measurement was carried out on square aerogels, the size of which completely covers those of the heaters. The sample was surrounded with a polyurethane spacer (λ = 49.4 mW m−1 K−1) to avoid any unexpected convection. The measurements were carried out at least 6 times for each sample (see Supporting Information for measurement logs). First three measurements were for stabilizing the upper and lower temperatures and heat fluxes, and the thermal conductivity value was calculated from the average of last three measurements.

Figure 12. Thermal conductivity measurement system and values for stationary air61 and aerogel samples.

As shown in Figure 12, thermal conductivities of unmodified, HMDS 5 vol%, and HMDS 7.5 vol% samples are 16.9, 16.3, and 15.8 mW m−1 K−1, respectively. These values are smaller than


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thermal conductivities of major commercial insulators (34–45 mW m−1 K−1 for mineral wools, 29–55 mW m−1 K−1 for expanded polystyrene, and 20–29 mW m−1 K−1 for polyurethane foams)5 and stationary air (26.2 mW m−1 K−1 at 300 K)61 but slightly larger than those of silica aerogels (12–20 mW m−1 K−1)5 and recently reported polyvinylpolymethylsiloxane aerogels (~15 mW m−1 K−1).11 The values are also lower than that of the unmodified, methanol/CO2-dried chitosan aerogel in our previous report,52 but we cannot directly compare the values because the sample size and density are different. In general, thermal conductivity of a porous material can be described as a sum of the four components: conduction in gaseous phase, convection in gaseous phase, conduction of solid phase, and radiation. In conventional silica aerogel, the conduction in solid phase is quite low reflecting its low density. In addition, the homogeneous porous structure suppresses the convection and conduction in gaseous phase because the characteristic length of the nanoporous structure is shorter than the mean free paths of gas molecules, ~70 nm, which is so-called Knudsen effect.62 We suggest that the cross-linked chitosan aerogel is also sufficiently homogeneous to exhibit this effect as it shows much lower thermal conductivity than that of stationary air. According to the comparison of the unmodified (sample S1) and trimethylsilylated aerogels (samples S2–S4), the thermal conductivity slightly decreases with increasing HMDS concentration.

Since

the

apparent

density

does

not

systematically

change

with

trimethylsilylation, this would be related to a microscopic structural change of nanofiber surfaces by introducing large trimethylsilyl groups. But, we cannot conclude that the tendency is obvious because i) the number of samples is not enough so far and ii) heat flow method is not perfect in such low conductivity region to discuss the difference in 0.1 mW m−1 K−1 order. Although further works will be needed for detailed investigation on the relation between microstructure and


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thermal conductivity, we conclude that the chitosan aerogel shows sufficiently high insulating performance, and it is worth keep investigating on this material as a superinsulation material for the next-generation thermal energy management.

4. CONCLUDING REMARKS We prepared large size monolithic samples (up to ~120 × 120 mm2) of trimethylsilylated chitosan aerogel. The preparation process consists of the following four steps: cross-linking gelation by mixing chitosan and formaldehyde solutions, solvent exchange with methanol and acetone, trimethylsilylation using HMDS, and supercritical drying in acetone/CO2 system. In contrast to silica aerogel, the preparation protocol and molds for the chitosan aerogel have to be designed to tolerate the dynamic size change of the gel during solvent exchange. This dynamic change does not form any cracks as long as the gel is separated from the mold, indicating high flexibility of the polymeric network. The obtained aerogel has a density of 0.09–0.1 g cm−3 and consists of three-dimensional nanofibrous structure. The thermal conductivities of the aerogels are ~17 and ~16 mW m−1 K−1 for unmodified and trimethylsilylated samples, respectively. These values are in the superinsulation region and smaller than the conductivity of stationary air, showing that the nanoporous structure of the aerogel suppresses the thermal conduction in gaseous phase. These results suggest that the cross-linked chitosan aerogel is one of the suitable candidates for superinsulation materials in the near future. As a concluding remark, we suggest that further studies should focus on shortening the solvent exchange and supercritical drying processes in addition to systematic evaluation of physicochemical stability to improve the industrial feasibility of this material.


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ASSOCIATED CONTENT Supporting Information. Video files for edge removal and bottom scratching (qt), additional photographs, assignments for IR spectra, water contact angle measurements, estimation of degree of silylation, and thermal conductivity measurement logs (pdf).

AUTHOR INFORMATION Corresponding Author * [email protected] Notes The trimethylsilylated chitosan aerogel is the subject of a Japanese patent application by AIST. After this paper is published, the samples would possibly be provided to other institutes or private companies for seeking collaboration opportunity.

ACKNOWLEDGMENT The authors thank A. Nakamura and Y. Kawazoe at Mohri Oil Mill Co., Ltd. for supercritical drying and Dr. Matsuzawa at AIST for water contact angle measurement.

REFERENCES


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