Transparent Insulators with a Tough Nanocellulose Skeleton Formed via Freeze-Drying

Abstract

It is important to decrease considerable heat loss in buildings and vehicles to establish a low-carbon society. Transparent insulators applicable to windows can effectively reduce such heat loss. Conventional options for transparent insulators are silica aerogels that provide both optical transparency and thermal insulation benefits. However, silica aerogels are prepared via an unscalable supercritical drying process and are mechanically brittle, which restricts their practical use. In this study, we developed transparent and thermally insulating “cryogels” comprising mechanically strong cellulose nanofibers (CNFs). The term cryogels refers to porous structures prepared via practical freeze-drying of wet gels. In the synthesis, the agglomeration of nanofibers in a wet CNF gel during the freezing step was suppressed by rapidly freezing tert-butyl alcohol (t-BuOH)-exchanged wet gels with a stiffened network skeleton. The visible-light transmittance values of the cryogels ranged from approximately 80–90%, and the thermal conductivity reached as low as 0.023 W/m·K. We also extended this approach to chitin nanofibers (ChNFs) to demonstrate its universality. The resulting ChNF cryogels exhibited similar high transparency (∼80%) and low conductivities (0.022 W/m·K). These cryogels, with the potential for scalable production, are suitable as interspace materials for use in double-glazed windows, offering a promising solution for sustainable transparent insulators in energy-efficient building and vehicle applications.

Main Text

Aerogels are specific porous structures with a high porosity and large internal surface area. These structures are produced via supercritical drying of wet gels and comprise a wide range of components, such as silica, , carbon, and organic polymers. , Among these aerogel compounds, silica is typical. Silica aerogels can exhibit high light transmittance within the visible region, which is not observed in most other aerogels, while they also exhibit thermal insulation properties. , Therefore, silica aerogels could be employed as transparent insulators for heat energy-losing windows in buildings and vehicles. , However, silica aerogels are mechanically brittle, and their handling is difficult. Moreover, the supercritical drying required for aerogel production is a batch process under high pressure, so the size of the resulting aerogel is limited, and the process exhibits significant energy consumption and a high cost.

Xerogels refer to porous structures obtained by ambient-pressure drying of wet gels. While this route is practical and potentially scalable, evaporation-induced capillary pressure tends to collapse the network skeleton of gels, thus lowering visible transmittance compared with supercritically dried aerogels. As another porous structure with favorable insulating properties, “cryogels” or porous structures prepared by freeze-drying wet gels are attracting attention. , Freeze-drying is a process widely employed in the food and medical industries, and scalable production of cryogels, with a reduced energy consumption and a lower cost than those of supercritical drying, is now available.

Cellulose, which is a polysaccharide, is the most abundant biopolymer in nature and exists in plants as a major component of cell walls. Through chemical pretreatment processes of raw cellulose, it can be converted into a water dispersion of approximately 3 nanometre-wide crystalline nanofibers (cellulose nanofibers, CNFs). Examples of such pretreatments include surface oxidation of cellulose crystallites by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation. CNFs are mechanically strong and thermally stable structures and can form a tough mesoporous network via supercritical drying of the dispersion. − The resulting CNF aerogels thus overcome the brittleness problem of the silica skeleton and exhibit optical transparency.

CNF cryogels obtained by freeze-drying water dispersions are also mechanically tough and thermally insulating materials. , However, their optical transparency has not yet been achieved. This opacity problem of cryogels is associated with the microscale formation of crystallites and the subsequent agglomeration of CNFs in the freezing process. To render CNF cryogels suitable for use as insulating window materials, transparency must be achieved. A solution to this problem involves producing smaller crystallites during freezing, thus suppressing CNF agglomeration. Two methods for generating small crystallites have been proposed. One is replacing the solvent in wet gels (water) by tert-butyl alcohol (t-BuOH). When wet CNF gels with a solvent of water are frozen directly, the growth of microscale ice crystals pushes CNFs away and localizes them at the crystallite boundaries, resulting in the formation of film-like CNF aggregates. − These CNF aggregates, which are structurally preserved during the sublimation of ice crystals act as light-scattering centers. By contrast, t-BuOH forms smaller nanoscale crystals, which enables CNFs to preserve their spatial distribution within the wet gels during freezing. The resulting CNF cryogels retain a fine CNF network skeleton with high specific surface areas (SSAs) ranging from approximately 200–300 m² g^–1. ,

The other method is the rapid freezing of wet gels, which enhances the nucleation rate of crystals while limiting the time available for crystal growth. This process further refines the crystallite size and suppresses the CNF aggregation. Previous studies have demonstrated that the skeletal structure of CNF cryogels varies with freezing temperature and that a high freezing speed can prevent the CNFs in wet gels from aggregation. , Direct exposure of wet gels to liquid N₂ is often adopted for this purpose. However, the combination of these two methods only is not sufficient to yield cryogels with optical transparency.

Herein, we report CNF cryogels that provide all three features of optical transparency, mechanical toughness, and thermal insulation. These cryogels achieved a visible transmittance of 80–90% at 600 nm, accompanied by a thermal conductivity of 23 mW m^–1 K^–1, which surpasses the properties of previously reported CNF xerogels prepared via an ambient-pressure drying process (∼30% visible transmittance at 600 nm and 60–70 mW m^–1 K^–1 thermal conductivity). The transparency of the prepared CNF cryogels was achieved by gelation of the flowable CNF dispersion to form self-supporting wet gels with a fixed nanofibrous skeleton, followed by rapid freezing of the t-BuOH-exchanged gels to ensure small crystallites (refer to the Methods section for details). The CNFs employed in this study were TEMPO-oxidized fibers, and the corresponding gelation property in response to acid was utilized according to our previous reports. , Briefly, a flowable dispersion of CNFs was formed into a stiff gel by acid in advance, followed by t-BuOH exchange. We also demonstrated the suitability of the CNF cryogels as interspace materials for use in double-glazed windows, showing the superinsulating property superior to air.

Results

Optical Transparency

The appearance of a 6 mm-thick CNF cryogel is shown in Figure a and b. The CNF cryogels prepared in this study were highly light permeable and exhibited an orange appearance under transmitted light, similar to that of silica aerogels. This phenomenon can be explained by the Rayleigh scattering of visible light by the fine CNF network structure (Figure c). In terms of the light transmittance of the prepared cryogels, short-wavelength light exhibited greater scattering than did long-wavelength light. The optical features of the CNF cryogels were hazy, while the total transmittance in the visible-light region was high (Figure d). At a thickness of approximately 1 mm, the transmittance ranged from approximately 70% to 95%, depending on the wavelength. The bulk density (Figure S1) of the CNF cryogels did not significantly influence the light transmittance within a range of approximately 8–26 mg cm^–3 (Figure e; their appearance and light transmittance spectra are shown in Figure S2 and S3a, respectively), and the light transmittance at 600 nm remained at approximately 80–90%.

1.

Transparent CNF cryogel with a networked CNF structure. (a) Photograph of the transparent CNF cryogel with a thickness of 6 mm under scattered light. (b) Photograph of the CNF cryogel with an orange appearance under transmitted light. (c) SEM images of a cross section of the cryogel, showing a fine CNF network structure. (d) Light transmittance and haze spectra of the CNF cryogel with a density of 10 mg cm^–3. The thickness of the cryogels was approximately 1 mm. (e) Light transmittance and haze of the cryogels at 600 nm as a function of the density.

Although a transparent CNF aerogel with sufficient planar dimensions has been demonstrated, this aerogel was achieved using a specially designed lab-made supercritical dryer. Commercially available supercritical dryers typically yield aerogels with a size of a few centimeters. By contrast, the CNF cryogels prepared via freeze-drying were scalable, and the 6 mm-thick disc-shaped sample, with a diameter of 13 cm, was easily produced using a common lyophilizer in the laboratory (Figure a). A cost analysis indicated that CNF cryogel glazing is potentially less expensive than silica aerogel glazing produced from organosilanes via supercritical drying, as discussed in Table S1–S4, which suggests the industrial practicality of the CNF cryogels as scalable transparent insulators.

Processing Effects on Microstructure and Transparency

To clarify the origin of the high transparency described above, we investigated the effects of processing conditions on the network structure and appearance of the CNF cryogels. When CNF hydrogels without solvent exchange were freeze-dried, the resulting cryogels exhibited an aggregated network and an opaque appearance (Figure a), consistent with microscale CNF agglomeration caused by the large ice crystals during freezing. By replacing water with t-BuOH and lowering the freezing temperature from – 20 °C to – 50 °C and then liquid N₂ (−195.8 °C), the network of resulting cryogels progressively refined from partially aggregated to a homogeneous nanofibrillar skeleton (Figure b–d), and the transparency was significantly improved (see Figure S3b for their light transmittance spectra). We further verified that gelation is essential for the transparency of CNF cryogels. When gelation was omitted, the cryogel became opaque, and the total transmittance at 600 nm dropped to approximately 20% (see Figure S4), compared to 86% when gelation was included. These results indicate that prior gelation fixed the nanofibrous network, which was then preserved by t-BuOH solvent exchange and rapid freezing, enabling the formation of transparent cryogels with uniform nanoscale skeletons.

2.

SEM images and appearances of CNF cryogels prepared under different processing conditions. (a) CNF cryogels freeze-dried from hydrogels (no t-BuOH exchange). (b–d) CNF cryogels formed from t-BuOH-exchanged gels freezed at −20 (b) and −50 °C (c), and in liquid N₂ (−195.8 °C) (d).

Mechanical Properties

Figure a shows compressive stress–strain (σ–ε) curves of the CNF cryogels. The CNF cryogels were mechanically tough and unbroken, even at a high strain of 80%. This feature is unlike that of silica aerogels, which are brittle and fracture at a low strain of less than 10%. The CNF cryogels exhibited linear elasticity at low strains of up to approximately 10%, and Young’s modulus, E, was calculated from this range (Figure b). In general, Young’s modulus of aerogels increases exponentially with increasing bulk density, following a power law:

$$E\sim {\rho }^{\alpha }$$

where the power α is determined by the skeletal homogeneity of the aerogel, and the α value of silica aerogels ranges from approximately 2.0–3.6. The lower the skeletal homogeneity of the aerogel is, the greater the value of α. For CNF cryogels, α = 1, within a density range of approximately 8–26 mg cm^–3, indicating that the skeletal homogeneity of the CNF cryogels is high. The obtained skeletal homogeneity was also supported by the approximately constant SSAs of the CNF cryogels, within a narrow range of 400–500 m² g^–1, as a function of the bulk density (Figure S5b). The kinetic energy absorption in the compression until a strain of 70% also scaled with density in a roughly linear manner (Figure c). These mechanical features of the cryogels arise from the tough CNF skeleton and are comparable to those of supercritical-dried CNF aerogels and commercially available polystyrene (PS) foams, surpassing polyethylene (PE) foams, as shown in Figure S6. In contrast, silica aerogels exhibit much lower energy absorption due to their brittleness. ,, As a reference, direct freeze-drying of CNF hydrogels without solvent exchange to t-BuOH produced opaque cryogels with markedly reduced SSAs and weaker mechanical properties at comparable densities (see Figure S7), underscoring the importance of t-BuOH exchange for retaining mesoporosity and mechanical robustness of the cryogels.

3.

Mechanical properties of the CNF cryogels. (a) Compressive stress–strain (σ–ε) curves of the CNF cryogels. (b) Young’s modulus (E) of the CNF cryogels as a function of the density. (c) Energy absorption of the CNF cryogels calculated from the σ–ε curves. (d) Flexibility of the CNF cryogels.

The flexibility of the CNF cryogels should be considered. As shown in Figure d, the cryogels were easily bent without fracture, which is consistent with the σ–ε curves shown in Figure a. When slightly bent to the extent of the second image shown in Figure d, the cryogel could spring back to its original flat shape. To assess the elastic recovery of the CNF cryogels under repeated loading, cyclic compression–release tests were performed for three cycles within the linear elastic region for each loading step (Figure S8a,b). The loading–unloading curves were reproducible over the three cycles, and small permanent strains were observed after unloading (0.7%, 1.2%, and 1.6% after cycles 1, 2, and 3, respectively; Figure S8b). The Young’s modulus of each loading step also remained nearly unchanged (approximately 110–120 kPa). In contrast, the compression beyond the linear region led to irreversible densification of the cryogels. When compressed to a strain of 80%, the CNF cryogels exhibited plastic deformation and could not recover to their original thickness upon unloading (Figure S8c).

Thermal Properties

The through-plane thermal conductivity of the CNF cryogels is shown in Figure a. Significantly low thermal conductivities ranging from 23–25 mW m^–1 K^–1 were measured at densities of approximately 8–10 mg cm^–3 by means of temperature wave analysis, which are lower than those of common insulators, such as expanded and extruded polystyrene (EPS and XPS, respectively) and mineral wool (∼30–60 mW m^–1 K^–1), as shown in Figure b. ,,− Another measurements using the transient plane source method were conducted to further validate the reliability of the thermal conductivity value, which produced almost identical results to those of the temperature wave method (Figure S9). This low thermal conductivity of the CNF cryogels is a critical feature, as specific insulators potentially applied to windows. The thermal conductivity (λ_total) of porous structures can be expressed as follows:

$${\lambda }_{\mathrm{total}}={\lambda }_{\mathrm{solid}}+{\lambda }_{\mathrm{gas}}+{\lambda }_{\mathrm{rad}}$$

where λ_solid, λ_gas, and λ_rad denote the contributions of the solid phase, gas phase, and radiation, respectively. The cryogels with a fine CNF network skeleton exhibited low densities of approximately 8–26 mg cm^–3 and high SSAs of 400–500 m² g^–1 (Figure S5b), so solid conduction (λ_solid) was assumed to be low. Furthermore, the CNF cryogels possessed mesopores with sizes ranging from approximately 5–30 nm (Figure S5c), which is below the mean free path of air molecules (∼70 nm). Such small pores restrict the elastic collision between air molecules within the cryogels, resulting in the inhibition of heat transfer in the gas phase (λ_gas) according to the Knudsen effect. These structural characteristics explain why the thermal conductivity is even lower than that of air (∼26 mW m^–1 K^–1 at 23 °C under the experimental conditions in this study). Consistent with a prior study of CNF porous structures, their through-plane thermal conductivity remains essentially unchanged after moderate uniaxial compression, supporting their tolerance for practical handling and assembly as insulating glazing interlayers in buildings. Additionally, the combination of ultralow thermal conductivity and mechanical toughness is beneficial for interlayer materials in automobile glazing, where vibration demands their mechanical integrity.

4.

Thermal properties of the CNF cryogels. (a) Thermal conductivity of the CNF cryogels as a function of the density. (b) Thermal conductivity of the CNF cryogels and other porous materials. (c) Heat resistance test of the CNF cryogel and polystyrene foam. (d) Illustrations, planar-view photographs, and edge-view photographs of CNF glass and single-pane glass. (e) Infrared images of CNF glass and single-pane glass after placement on a hot plate for 2 and 150 s.

As another thermal property of the CNF cryogels, their heat resistance was also tested. Here, we used a commercially available insulator, PS foam, as a reference sample. A CNF cryogel with a density of approximately 10 mg cm^–3 and PS foam were placed on a hot plate heated to approximately 135 °C for 120 s. The CNF cryogel was stable and showed no dimensional change, whereas the PS foam melted rapidly (Figure c). The thermal stability of the CNF cryogels was further evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere (Figure S10). The decomposition onset temperature of 5% weight loss, T_5%, for the CNF cryogel was approximately 230 °C, which is consistent with the typical decomposition profile of TEMPO-oxidized CNFs, indicating that the cryogels possess sufficient thermal stability to serve as glazing interlayer materials.

CNF cryogel with a density of 8 mg cm^–3 was sandwiched between two pieces of single-pane glass to assemble a double-glazed window with a 4 mm-thick CNF cryogel interlayer (CNF glass). The CNF cryogel was in the center and was surrounded by air. The edge of the CNF glass was sealed with PE foam, as shown in Figure d. The CNF glass and single-pane glass were placed on a hot plate heated to a temperature of approximately 50 °C. The infrared images obtained after 2 and 150 s of heating are shown in Figure e. The center temperature reached a plateau at 90 s for CNF glass and 20 s for single-pane glass during hot-plate heating (Figure S11), indicating that the 150 s infrared images in Figure e were in equilibrium for both. The CNF glass exhibited a significantly better thermal insulation performance than did the single-pane glass. After 150 s of heating, the CNF cryogel area was cooler than the surrounding air, which further confirms its lower thermal conductivity than that of air.

Hydrophobic Modification

A challenge in the practical use of CNF cryogels is their low resistance to water. CNFs are hydrophilic due to the presence of many surface hydroxy groups. This inherent hygroscopicity renders the CNF cryogels sensitive to moisture, and they severely shrink upon contact with water. We thus conducted a hydrophobic surface modification on the CNF cryogels with methyltrimethoxysilane (MTMS) via a chemical vapor deposition route. , The successful incorporation of MTMS onto the CNF skeleton was confirmed by elemental analysis using energy-dispersive X-ray spectroscopy (EDX). The EDX spectrum of the MTMS-modified cryogel showed a clear Si peak at approximately 1.7 keV, whereas no Si signal was detected for the unmodified cryogel at this energy level (Figure S12).

Figure a shows the Fourier transform infrared (FTIR) spectra of the CNF cryogel, MTMS-modified cryogel, and pristine MTMS reagent. As shown in Figure b, the characteristic absorptions of MTMS were identified at 1,269 cm^–1 (δ­(C–H) of the methyl group), 1,193 cm^–1 (ν­(C–O) of the methoxy group), 848 cm^–1 (ν­(Si–O–C)), and 794 cm^–1 (ν­(Si–C)). Pristine MTMS exhibits high activity. The original methoxy groups of MTMS demonstrated hydrolyzation or polymerization between individual MTMS molecules, which was indicated by the absence of ν­(C–O) of the methoxy group in the spectrum of the MTMS-modified CNF cryogel. For the MTMS-modified cryogel, a new peak of ν­(Si–OH) was observed at 920 cm^–1, , indicating the hydrolysis of the methoxy groups of MTMS. These hydroxy groups likely formed hydrogen bonds with those on the CNF surface. In addition to hydrogen bonding, it was suggested that MTMS produced covalent bonds with the hydroxyl groups on the CNF surface since a small yet new ν­(Si–O–C) peak at approximately 859 cm^–1 was observed in the spectrum of the modified cryogel. Furthermore, considering the lack of ν­(Si–C) in the spectrum of the MTMS-modified cryogel, the absorption at 776 cm^–1 corresponding to the combination of ν­(Si–C) and ν­(Si–O–Si) indicated the presence of ν­(Si–O–Si) in the modified cryogel. This result supported the polymerization of MTMS molecules on the CNF surface. A summary of the plausible interactions between the CNF surface and polysiloxane is shown in Figure c.

5.

Hydrophobic modification of the CNF cryogels. (a) FTIR spectra of the CNF cryogel, MTMS-modified cryogel, and pristine MTMS reagent. (b) Chemical structure of MTMS and characteristic FTIR bands of the CNF cryogels and MTMS liquid. (c) Presumed interactions between the CNF surface and polysiloxane. (d) Photograph of water droplets dripping onto the surfaces of the CNF cryogels with and without MTMS modification.

Water was dropped onto the surfaces of the CNF cryogels with and without MTMS modification (Figure d). The water droplet remained on the surface of the modified CNF cryogel with a water contact angle of 128°, whereas the unmodified cryogel absorbed the water and severely shrunk. The modified cryogels maintained a water contact angle of 114° after 2 months of ambient storage, confirming the long-term hydrophobic stability (Figure S13). The dimensional stability of the cryogels under repeated humidity conditioning was also evaluated (see Figure S14). A conditioning cycle was initiated with moisture exposure at 40 °C and 80% RH, followed by drying at 105 °C. Even after 3 cycles of conditioning, the MTMS-modified cryogels fully preserved their shape, whereas the unmodified ones shrunk markedly. This hydrophobic modification resulted in a slight decrease in the total light transmittance at 600 nm, from approximately 86% before modification to 76% (Figure S15a). The thermal conductivity of the MTMS-modified CNF cryogel also slightly increased from 25 to 28 mW m^–1 K^–1. These changes are likely due to the thickening of the CNF skeleton under the influence of MTMS modification. Moreover, the MTMS-modified CNF cryogels showed remarkable improvements in their mechanical properties (Figure S15b). Young’s modulus increased from approximately 92 to 165 kPa, and the energy absorption increased from approximately 12 to 25 kJ m^–3. The hydrophobic MTMS layers deposited on the surface of the CNF skeleton might also promote robust binding between CNFs, thereby enhancing the structural integrity of the cryogels. These results demonstrate that the CNF cryogels can be hydrophobic without significant material function loss.

Application to Other Biopolymers

Like cellulose, chitin is an abundant biopolymer in nature, existing as crystallite nanofibers in crustaceans, insects, and fungi. We prepared cryogels via the same process using chitin nanofibers (ChNFs) as the raw material to demonstrate the universality of this freeze–drying method. The AFM observation demonstrated the uniform fibrillar morphology of ChNFs (Figure a), which was comparable to the morphology observed for CNFs (Figure S16). The resulting ChNF cryogel (Figure b inset) exhibited a similar appearance to that of the above CNF cryogel. The total light transmittance in the visible region of the ChNF cryogel was high and reached approximately 77% at 600 nm (Figure b). Moreover, the thermal conductivity of the ChNF cryogel was also extremely low (22 mW m^–1 K^–1 at a density of 7 mg cm^–3), namely, it was also lower than that of air.

6.

ChNF cryogels. (a) Atomic force microscopy (AFM) image of ChNF and the appearance of ChNF dispersion (inset). (b) Light transmittance spectra and the appearance (inset) of the ChNF cryogel.

Conclusions

Transparent and thermally insulating cryogels were prepared via acid-induced gelation of CNF dispersions, followed by a rapid freeze-drying of t-BuOH-exchanged CNF wet gels. The resulting cryogels formed a uniform CNF network with large specific surface areas (∼ 400–500 m² g^–1). These cryogels were mechanically strong, exhibiting flexibility and maintaining structural integrity under compressive strains up to 80%. Their thermal conductivity was notably low (23 mW m^–1 K^–1 at a density of 8 mg cm^–3), even below that of air, enabling superior thermal insulation properties compared to conventional building materials. Furthermore, hydrophobic modification by methyltrimethoxysilane improved the water resistance and mechanical strength of the cryogels, while preserving their transparency and low thermal conductivity. The method was also extended to chitin nanofibers, confirming the universality of this freeze-drying approach for other biopolymers. These features render the cryogel an appealing material that satisfies the low-cost and sustainability requirements of insulators in the building and automobile industries.

Methods

Materials and Chemicals

All the laboratory-grade chemicals were purchased from FUJIFILM Wako Pure Chemical Co. (Osaka, Japan) and used as received. Proton-type TEMPO-oxidized cellulose pulp provided by DKS Co., Ltd. (Kyoto, Japan), was used as the starting material for the CNFs. The oxidized pulp was kept in a water/ethanol mixture to prevent the growth of bacteria, so it was first rinsed with distilled water 3 times to remove ethanol. A 1 M NaOH solution was subsequently added to the proton-type pulp for neutralization. The neutralized pulp was suspended in distilled water at a concentration of 0.5 wt % and disintegrated into CNFs by passing it through a high-pressure water jet system (HJP–25005X, Sugino Machine Limited) at 150 MPa 3 times. To remove any residual unfibrillated pulp, the CNF dispersion was passed through a nylon mesh filter with a pore size of 7 μm twice using a vacuum pump.

The CNF cryogels prepared from the dispersion at a concentration lower than 0.3 wt % are very weak, whereas the concentrated dispersion at a concentration higher than 1.0 wt % exhibits low flowability. Therefore, concentrations of 0.3, 0.4, 0.7, and 1.0 wt % were used to prepare the CNF cryogels. Specifically, CNF dispersions with concentrations of 0.3 and 0.4 wt % were prepared by diluting the initial 0.5 wt % dispersion with distilled water, and dispersions with concentrations of 0.7 and 1.0 wt % were concentrated using a rotary evaporator at 40 °C under a reduced pressure. All the above dispersions were stored at 4 °C.

Preparation of the CNF Cryogels

The CNF dispersions (9 g) with different concentrations were poured into plastic molds and degassed using a vacuum pump for 30 min. We then placed both the CNF dispersion and glacial acetic acid in an airtight container, allowing vapor diffusion to form a thin gel layer on the dispersion surface. Such a surface gelation before pouring the HCl solution ensured a flat surface and yielded uniform hydrogels. The gelation process was then completed by adding a 0.1 M HCl solution (5 mL) to the dispersion, and the dispersion was allowed to stand for 1 h to prepare the hydrogels. The resulting hydrogels were then cut into pieces approximately 10 mm × 10 mm × 1.5 mm in size using a sharp blade and shaken in a 0.01 M HCl solution (50 mL) for 1 day on a rotary shaker at 30 rpm.

The CNF cryogels freeze-dried from the hydrogels with a solvent of water exhibited an aggregated internal structure and an opaque appearance. To prepare transparent CNF cryogels, the solvent of the CNF hydrogels was exchanged with t-BuOH. The solvent of the CNF hydrogels was first substituted with a mixture of water and t-BuOH at a volume ratio of 1:1 to suppress hydrogel shrinkage. The hydrogels containing the t-BuOH-water mixture as a solvent were then shaken in pure t-BuOH (50 mL) at 40 °C for 6 h, which was repeated 3 times to efficiently replace the solvent of the CNF hydrogels with t-BuOH.

The t-BuOH-exchanged gels were frozen in liquid N₂ to preserve the fine CNF skeletal structure. The frozen gels were then transferred to a Scanvac CoolSafe freeze-dryer (LaboGene) to remove t-BuOH. The drying temperature was set to – 50 °C to ensure that no recrystallization occurred in the drying process. The CNF cryogels were then obtained by sublimation of t-BuOH for 16 h and secondary drying at 40 °C for 2 h. The prepared CNF cryogels were conditioned at 23 °C and a 50% relative humidity for more than 1 day before use.

Hydrophobic Modification

We conducted hydrophobic surface modification of the CNF cryogels via a chemical vapor deposition route. , The CNF cryogels were placed in an airtight container together with an open glass vial containing 2 mL of MTMS reagent. Then, vapor deposition of the MTMS molecules on the CNF cryogels was performed for 12 h under a reduced pressure at 95 °C, followed by degassing the container for 1 min to remove any residual MTMS. To investigate the effect of the MTMS modification to moisture exposure, the MTMS-modified and unmodified CNF cryogels were conditioned at 40 °C and 80% RH for 3 h, followed by drying at 105 °C for 3 h. The dimensions of the cryogels were measured with a caliper, and the volume (V) was calculated accordingly. The volume retention was determined using the following equation:

$$\mathrm{Volume}\mathrm{retention}(\%)=\frac{V}{V_0}\times 100$$

Preparation of ChNF Cryogels

A ChNF dispersion with a concentration of 0.25 wt % was prepared as reported previously by our group. The dispersion was concentrated to 0.4 wt % using a rotary evaporator at 40 °C under reduced pressure. The ChNF cryogels were prepared in the same way as the CNF cryogels, except that the gelation agent was a 1 M NaOH solution.

Characterizations

Scanning electron microscopy (SEM) images of the internal structure of the CNF cryogels were observed using a Hitachi S–4800 field–emission microscope at 1 kV with pretreatment utilizing a Meiwafosis Neo osmium coater at 5 mA for 10 s (2.5 s each time, 4 times in total). The porosity values (Figure S1) of the CNF cryogels were calculated as follows:

$$\mathrm{Porosity}=(1-\frac{\rho }{{\rho }^{*}})$$

where ρ is the bulk density (g cm^–3) of the cryogels calculated from the sample weight and dimensions, and ρ* (1.64 g cm^–3) is the true density of the CNF measured with a MicrotracBEL gas pycnometer BELPycno. The weights of the samples were measured after drying at 105 °C for 3 h. Nitrogen adsorption–desorption isotherms (Figure S5a) were obtained with a Quantachrome NOVA 4200e instrument at – 195.8 °C after degassing the CNF cryogels for 3 h at 105 °C. According to the Brunauer–Emmett–Teller (BET) theory and the Barrett–Joyner–Halenda (BJH) model, the SSA values (Figure S5b) and pore size distributions (Figure S5c) of the cryogels were estimated from the isotherms. , The total light transmittance spectra of the cryogels were measured using a JASCO UV–vis V670 spectrophotometer. The set wavelength ranged from 300 to 800 nm. The compression tests were performed at 23 °C and a 50% relative humidity using a Shimadzu EZ–SX instrument equipped with a 500-N load cell. The samples were uniaxially compressed at 0.25 mm min^–1 with a preload of 0.05 N. The permanent strain in cyclic compression was calculated using the following equation:

$$\mathrm{Permanent}\mathrm{strain}=1-(t/t_0)$$

where t ₀ is the initial thickness of the hydrogel and t is the thickness after each cycle. Young’s modulus was determined from the slope of the initial linear region of the obtained stress–strain curve. The energy absorption of the CNF cryogels was calculated as the area below the stress–strain curve until a strain of 70%. The thermal diffusivity (α) of the cryogels was determined in the through-plane direction by temperature wave analysis with an ai–Phase Mobile 1u device at 23 °C and 50% relative humidity. The thickness of the cryogels was approximately 1 mm. The heater and sensor plates were placed on either side of the cryogel at a distance of approximately 1 mm. The space between the two plates was adjusted to provide solid contact with the surface of the cryogel. The applied voltage was set to 1.4 V, and eight points were used to measure the phase delay of the temperature wave. The integration time of each point was 50 s, and the device was shielded with an optically opaque windscreen during the measurement. The thermal conductivity (λ) of the CNF cryogels was then calculated as follows:

$$\lambda =\rho \alpha c$$

where ρ and c are the bulk density (g cm^–3) and specific heat capacity (J g^–1 K^–1) of the CNF cryogel, respectively. The specific heat capacity c (1.155 J g^–1 K^–1) at approximately 23 °C was measured using a Perkin–Elmer DSC 8500 instrument. In addition to the temperature-wave analysis described above, we performed another thermal conductivity measurement using the transient plane source method with a Hot Disk TPS 2500s instrument (Hot Disk AB, Sweden) at 23 °C and 50% RH. We applied a Kapton-insulated sensor (7577) with a radius of 2.0 mm in the isotropic mode with an output power of 20 mW. A commercially available insulator, polystyrene (PS) foam, was adopted as a reference sample to evaluate the heat resistance of the CNF cryogel. The CNF cryogel with a CNF concentration of 0.4 wt % and PS foam, with sizes of 10 mm × 10 mm × 5 mm, were placed on a hot plate heated to approximately 135 °C for 120 s. Thermogravimetric analysis (TGA) was carried out using TG-50 (Shimadzu, Japan). Approximately 5 mg of the sample was set in an aluminum pan. The specimens were dried in advance at 105 °C for 3 h. Analysis was conducted in a nitrogen atmosphere from room temperature to 500 °C at a heating rate of 20 °C min^–1. CNF glass and single-pane glass were placed on a hot plate heated to approximately 50 °C for 150 s. The infrared images after 2 and 150 s of heating were recorded using a Fluke Ti480 PRO Infrared Camera. The EDX spectra of the MTMS-modified and unmodified cryogels were collected on a Shimazu EDX-8000 instrument under vacuum conditions. FTIR spectroscopy was employed to investigate the chemical structure of the CNF and MTMS-modified cryogels. The instrument used was an FT/IR-6100 device (JASCO Co., Ltd.) equipped with an attenuated total reflection (ATR) stage with a resolution of 4 cm^–1. The transmission mode was applied to acquire the FTIR spectrum of the pristine MTMS reagent. Spectral operations, such as baseline correction, smoothing, and normalization, were performed using the Jasco Spectra Manager software. The water contact angles of the CNF and MTMS-modified cryogels were measured using a Drop Master 500 instrument (Kyowa Interfacial Science).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.5c19203.

• Density and porosity of the CNF cryogels as a function of the dispersion concentration (Figure S1), appearances of the CNF cryogels with different densities (Figure S2), light transmittance spectra of the CNF cryogels with different bulk density and freezing temperature, and their light transmittance at 600 nm (Figure S3), light transmittance spectrum and appearance of CNF cryogels without pregelation (Figure S4), structural properties of the CNF cryogels (Figure S5), mechanical properties of the CNF cryogels and other porous insulators (Figure S6), effect of solvent exchange on the properties of CNF cryogels (Figure S7), cyclic compression of the CNF cryogels to reach 5% strain and compression–release test of the CNF cryogels up to a compressive strain of 80% (Figure S8), thermal conductivity of the CNF cryogels as measured by an ai-Phase and a Hot Disk TPS 2500s (Figure S9), thermogravimetric curve of the CNF cryogel (Figure S10), the center temperature as a function of time during hot-plate heating of the CNF glass and single-pane glass (Figure S11), EDX spectra of MTMS-modified and unmodified CNF cryogels (Figure S12), water contact angles of MTMS-modified CNF cryogels measured immediately after modification and after 2 months of ambient storage (Figure S13), dimensional stability of MTMS-modified and unmodified CNF cryogels under humidity conditioning (Figure S14), light transmittance spectra and compressive stress–strain (σ–ε) curves of the CNF and MTMS-modified CNF cryogels (Figure S15), appearances, atomic force microscopy (AFM) images, and height distributions of the fibers of the CNF cryogel (Figure S16), assumed freeze-drying capacity for industrial-scale CNF cryogel production (Table S1), materials and chemicals consumption and cost for producing 1 m² of CNF cryogel (Table S2), electricity consumption and cost per freeze-drying batch (Table S3), and total production cost for 1 m² CNF cryogel glazing and the comparison with silica aerogel glazing (Table S4) (PDF)

T.S. conceived the concept of this study. X.H., J.K., W.S., and T.S. designed the experiments. X.H. and J.K. performed the experiments and analyzed the data with help provided by W.S., T.I., Y.Kaku, Y.Kobayashi, and Z.S., X.H., and T.S. wrote the manuscript with contributions from all the authors.

The authors declare no competing financial interest.

This manuscript was previously submitted to the preprint server ChemRxiv on September 11, 2025. The preprint version can be found under the following: Hou, X.; Kotsuka, J.; Sakuma, W.; Ito, T.; Kaku, Y.; Sun, Z.; Fujisawa, S.; Saito, T. Transparent Insulators with a Tough Nanocellulose Skeleton Formed via Freeze-Drying. 2025, chemrxiv-2025-q6vtk. ChemRxiv. 10.26434/chemrxiv-2025-q6vtk (accessed January 7, 2026). The authors declare no competing financial interest.