Cellulose gels, films and composite materials containing the gels, and methods of forming same

Abstract

To provide: cellulose-based flexible aerogels and xerogels which have tunable optical, heat-transfer and stiffness properties, and methods of forming the gels; and highly transparent and flexible cellulose nanofiber-polysiloxane composite aerogels characterized by enhanced mechanical robustness, tunable optical anisotropy and low thermal conductivity.SOLUTION: A method comprises: a) oxidizing alcohol units of bacterial cellulose to form bacterial cellulose containing a plurality of carboxylate groups and / or carboxylic acid groups; b) reacting the oxidized bacterial cellulose carboxylate groups with a surface modifying agent to form surface modified bacterial cellulose; and c) reacting in a solvent the surface modified bacterial cellulose with a crosslinking agent to form a bacterial cellulose aerogel.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 62 / 684,670, filed June 13, 2018, entitled Method for Producing Nanocellulose Xerogel. [Background technology]

[0002] Federally supported research This invention was made with government support under Grant No. DE-AR0000743 awarded by the Department of Energy and Grant DMR-1410735 awarded by the National Science Foundation. The government has certain rights in this invention. Summary of the Invention

[0003] The present disclosure relates to cellulose-based gels, such as (e.g., flexible) aerogels and xerogels. Exemplary cellulose-based gels include cellulose nanorods, ribbons, fibers, etc., where the gels can have tunable properties, such as optical, thermal, and mechanical properties. Further disclosed are highly transparent, flexible cellulose nanofiber-polysiloxane composite aerogels characterized by improved mechanical properties, such as robustness, tunable optical anisotropy, and low thermal conductivity. [Brief explanation of the drawings]

[0004] [Figure 1A] 1A-1E illustrate transparency characteristics during the process of forming the disclosed xerogel: Figure 1A shows the gel before solvent exchange with isopropyl alcohol; [Figure 1B] 1A-1E show transparency characteristics during the course of the disclosed xerogel formation, and FIG. 1B shows post-alcogel in the first 8 hours with isopropyl alcohol. [Figure 1C]1A-1E show the transparency characteristics during the course of the disclosed xerogel formation, and FIG. 1C shows the alcogel's shrinkage behavior during the second 8-hour exchange with isopropyl alcohol. [Figure 1D] 1A-1E show the transparency characteristics during the course of the disclosed xerogel formation, and FIG. 1D shows that the alcogel regains transparency upon a third 8-hour exchange with isopropyl alcohol. [Figure 1E] 1A-1E show transparency characteristics during the formation of the disclosed xerogels, and FIG. 1E shows the final polyvinylmethyldimethoxysilyl (PVMDMS) ambient-dried aerogel. [Figure 2] During ambient drying, exemplary alcogels (a) and (b) shrink and turn white. Once dry, they recover to their original size and become transparent (c and d). [Figure 3] Figures 3A-3C show bacterial cellulose films obtained from beer wort: Figure 3A shows the WBW after autoclaving in the culture chamber, Figure 3B shows the film after 2 weeks, and Figure 3C shows the film removed for purification. [Figure 4] Figures 4A-4C show bacterial cellulose films at various stages of purification. Figure 4A shows material treated with 1% NaOH at 80°C. Figure 4B shows material treated with DI water. Figure 4C shows the final purified bacterial cellulose. [Figure 5] FIG. 1 shows an FTIR comparing the spectrum of TEMPO-oxidized bacterial cellulose with APTMS-functionalized bacterial cellulose. [Figure 6]Figures 6A-6F illustrate the fabrication of large aerogels of MTMS / CNF-APTMS using glass molds. Figure 6A illustrates the mold for the polycondensation reaction chamber made of glass. Figure 6B illustrates the organogel fabrication setup in a heated water bath. Figure 6C illustrates the organogel in DI water. Figure 6D illustrates the organogel in water / isopropanol. Figure 6E illustrates the organogel in isopropanol. Figure 6F illustrates the final aerogel. [Figure 7] Figures 7A-7C show a large-scale production sample of CNF-APTMS / MTMS aerogel, 16.51 cm (6.5 in) in diameter. [Figure 8] 1 is a graph showing transmittance and haze measurements of the disclosed aerogels. [Figure 9] 9A and 9B show IR images of cellulose aerogel on a hot (FIG. 9A) and cold surface (FIG. 9B). [Figure 10] Figure 10A is a photograph taken through the aerogel plate (dotted red box), and Figure 10B is an IR image of the same photograph showing the insulating properties of the aerogel. [Figure 11] FIG. 1 illustrates a procedure in accordance with at least one embodiment of the present disclosure. [Figure 12] FIG. 1 illustrates an example method for preparing a xerogel in accordance with at least one embodiment of the present disclosure. [Figure 13] FIG. 1 illustrates a proposed reaction mechanism in accordance with at least one embodiment of the present disclosure. [Figure 14] FIG. 1 shows silanols on the surface of our substrate. [Figure 15] FIG. 1 illustrates a gel bonded to a modified glass substrate in accordance with at least one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0005] As used herein, a "gel" is understood to be a substantially dilute crosslinked system that does not exhibit flow when in a steady state. The primary component of a gel is the surrounding fluid, which may be in liquid or gas form. Prefixes and variations such as "aero," "organo," and "hydro" are understood to refer to the surrounding fluid and the major component of the gel material in the crosslinked gel matrix.

[0006] The disclosed gels can contain cellulose nanocomposites that may be in an aligned liquid crystalline phase. Thus, the disclosed gels allow formulators to tailor the optical properties of the gel from opaque to transparent by adjusting the gel's optical transparency. Additionally, these properties can be tailored to interact with a wide range of electromagnetic radiation, e.g., from the visible to the infrared spectrum. In one embodiment, the thermal conductivity of the gel can be tailored. The bulk properties of the disclosed gels, e.g., their level of stiffness or flexibility, can be tailored by the selection of the constituent cellulose materials, e.g., nanorods, ribbons, fibers, etc., as well as the concentration of these materials within the resulting gel.

[0007] As used herein, "film" and variations refer to a lamella which may range in thickness and any lateral extent, for example, from about 1 μm to about 10 cm or from about 10 nm to 1 mm.

[0008] As used herein, the term "cross-section" refers to width, and these terms are used interchangeably. The disclosed cellulose nanomaterials have widths of about 10 nm to about 500 nm, or less than 1 nm, or even less than 0.1 nm. The length of the nanomaterial can be at least 10 times its width.

[0009] The term "composition," as used herein, can refer to the disclosed cellulose nanomaterial aqueous dispersions, hydrogels, organogels, aerogels, and liquid crystalline gels. The composition can be a single layer of nanomaterial-containing material, or the composition can be a single layer of nanomaterial-containing material, with each layer being made up of one type of material. It may be formed from two or more separate layers of material only. As a non-limiting example, one layer may consist of an ordered nematic cellulose gel onto which a second layer of aligned cholesteric cellulose film is applied. This layer configuration thereby forms an integrated composite material comprising the separate layers.

[0010] The term "hydrogel" as used herein refers to a network of cellulosic material as a colloidal gel dispersed in a carrier. In one embodiment, the carrier is water. In another embodiment, the carrier is a mixture of water-compatible (miscible) organic solvents. The cellulosic material may be crosslinked or non-crosslinked.

[0011] The term "xerogel" is defined herein as a gel whose primary solvent is an ambient gas, such as air, and whose liquid-gas solvent exchange is achieved via evaporation of the liquid under atmospheric conditions near ambient temperature and pressure.

[0012] The term "nanomaterial" refers to the disclosed cellulose materials. The width of these materials is in the nanometer range, but the length of the cellulose materials can vary from nanometer lengths to micrometers. The terms "nanomaterial," "cellulose material," and "cellulose nanomaterial" are used interchangeably throughout this disclosure.

[0013] Disclosed herein are methods for forming gels, such as xerogels and aerogels. Exemplary xerogels include nanocellulose components with liquid crystalline ordering in the polymer backbone structure of the xerogel.

[0014] Further exemplary bacterial cellulose-based flexible gels may include cellulose ribbons, fibers, and other component-particle structures with an aspect ratio of about 1:1000 in one embodiment. These flexible gels may be formed by interlocking cellulose particle networks within the material. The initial cellulose solvent used to form the gel network can be retained or replaced to create various gel types, such as hydrogels, alcogels, aerogels, and liquid crystalline gels. The use of the disclosed cellulose materials to form gel networks allows formulators to tailor various properties of the gel, including its flexibility.

[0015] In addition to flexibility, the optical transparency of the disclosed gels can be tailored to range from opaque to transparent. These results can be achieved by adjusting various properties of the disclosed composite materials, i.e., the density or size distribution of the cellulose nanomaterials. Additionally, the addition of auxiliary components, such as liquid crystal materials, can be used to adjust the optical properties of the disclosed composite materials.

[0016] An additional property that can be tailored is the degree of thermal resistance exhibited by the gel. Several factors allow for tailoring of thermal resistance properties, including (1) the inherently low thermal conductivity of cellulose, (2) tailoring of thermal convection by dilution of the fluid within the cellulose network, and (3) tailoring of the thermal conductivity and convection properties of the fluid comprising the cellulose-gel network.

[0017] Another aspect of the present disclosure is a composition comprising aligned cellulose nanorods, the orientation of which can be controlled by the formulator. The disclosed nanorods can have an aspect ratio of about 1:10 to about 1:100. In one embodiment, the disclosed nanorods can be used to form a composition having a cholesteric phase.

[0018] In one embodiment, the disclosed nanocrystals form ordered films that can be ordered into a cholesteric phase to form periodic structures in the film, the pitch and pitch gradient of which can be tuned for broadband Bragg reflection of incident electromagnetic radiation. In another embodiment, the resulting ordered gel is achieved due to the small relative aspect ratio of cellulose nanorods or similar nanomaterials, including nanocrystals. Nanorods consequently form a distinct phase from other nanomaterials, such as nanofibers. This fact allows for broadband reflection in ordered cellulose gels formed from cellulose structures with aspect ratios of, for example, about 1:10 to about 1:100.

[0019] Thus, mechanical flexibility, optical transparency, and thermal resistance can be set, for example, by adjusting the same parameters described above in relation to nanofibers, except that these parameters now specifically relate to cellulose nanorods or other geometrically anisotropic cellulose structures.

[0020] A further aspect of the present disclosure relates to composite structures comprising lamellae formed from the disclosed aerogels and / or liquid crystalline gels. Composite structures with lamellae may be formed from the disclosed compositions including nanofiber-like cellulose materials (e.g., to form nematic phase materials) or nanorod-like cellulose materials (e.g., to form cholesteric phase materials). These composite structures comprise multiple layers.

[0021] The disclosed gels and / or films can have a thickness of about 1 μm to about 10 cm. In one embodiment, the thickness varies from about 10 μm to about 1 cm. In another embodiment, the thickness varies from about 100 μm to about 10 cm. In a further embodiment, the thickness varies from about 50 μm to about 1 cm. In yet another embodiment, the thickness varies from about 1 cm to about 10 cm. In yet another embodiment, the thickness varies from about 10 μm to about 100 cm. In yet a further embodiment, the thickness varies from about 500 μm to about 10 cm.

[0022] The transmittance of the disclosed gels and / or films relates to the amount of visible electromagnetic radiation that penetrates the gel. 0% transmittance results in an opaque material that allows no transmission. 100% transmittance results in a material that is transparent to electromagnetic radiation. The disclosed gels can have a transmittance of 0% to 100%. In one embodiment, the gel has a transmittance of about 5% to about 15%. In another embodiment, the gel has a transmittance of about 25% to about 50%. In a further embodiment, the gel has a transmittance of about 95% to 100%. In yet another embodiment, the gel has a transmittance of about 15% to about 35%. In yet another embodiment, the gel has a transmittance of about 50% to about 75%. In yet another embodiment, the gel has a transmittance of about 25% to about 75%. As specific examples, the gels and / or films exhibit electromagnetic transmittance of 0% to 100%, or about 25% to about 100%, for light wavelengths of about 400 nm to about 700 nm.

[0023] The disclosed gels and composite materials have a viscosity of about 10 -3 In another embodiment, the thermal conductivity is from about 10 W / (m·K) to about 10 W / (m·K). -2 In a further embodiment, the thermal conductivity is about 10 -1 In yet a further embodiment, the thermal conductivity is about 10 -3 In yet another embodiment, the thermal conductivity is about 10 -2In yet another embodiment, the thermal conductivity is from about 1 W / (m·K) to about 10 W / (m·K).

[0024] The relative emissivity value of the disclosed gel is approximately 10 -2 The range is ~0.99. The disclosed gels and composite materials have a viscosity of about 1 Pa to about 10 6 In one embodiment, the modulus of elasticity is from about 10 Pa to about 10 5 In another embodiment, the modulus is about 10 Pa. 2 Pa ~ about 10 6 In a further embodiment, the modulus is about 10 Pa. 3 Pa ~ about 10 5 In yet a further embodiment, the modulus of elasticity is from about 10 Pa to about 10 Pa. 10 3 In yet another embodiment, the modulus of elasticity is from about 1 Pa to about 10 Pa. In yet another embodiment, the modulus of elasticity is from about 10 4 Pa ~ about 10 6 It is Pa.

[0025] According to various embodiments of the present disclosure, a method of making a gel includes: a) oxidizing alcohol units of bacterial cellulose to form bacterial cellulose containing a plurality of carboxylate and / or carboxylic acid groups; b) reacting the oxidized bacterial cellulose carboxylate groups with a surface modifier to form surface-modified bacterial cellulose; c) reacting the surface-modified bacterial cellulose with a crosslinker in a solvent to form a bacterial cellulose aerogel; Includes.

[0026] The method may further include exchanging the aqueous solution present in the gel with a solvent and / or removing the volatile solvent by drying to form a xerogel. Bacterial cellulose may be obtained, for example, from one or more of Acetobacter hansenii and Acetobacter xylinum. The crosslinking agent may include a polysiloxane precursor, such as one or more of vinylmethyldimethoxysilane, methyltrimethoxysilane, and methyltriethoxysilane. The surface modifier may include a compound containing an amine functional group and a silicon atom, such as one or more silylamines or one or more aminoalkylsilanes. The gel or film may be formed according to this method or other methods described herein.

[0027] According to a further exemplary embodiment, a method for producing a networked cellulose aerogel comprises: a) contacting a dispersion of bacterial cellulose with an oxidizing system that oxidizes the hydroxyl units of the cellulose to carboxylate and / or carboxylic acid units to form a solution of oxidized cellulose nanofibers; b) reacting the oxidized cellulose nanofibers with a surface modifier to form a solution of surface-modified cellulose nanofibers; c) contacting the surface-modified cellulose nanofibers with a cross-linking agent to form a bacterial cellulose nanofiber matrix; d) hydrolyzing the matrix in the presence of a catalyst to form a networked cellulose hydrogel; e) exchanging the solvent contained in the hydrogel with a volatile solvent to form an organogel; f) removing the solvent to form an aerogel; Includes.

[0028] The bacterial cellulose may be obtained from one or more of Acetobacter hansenii and Acetobacter xylinum. The surface-modified cellulose nanofibers may be modified with a compound selected from C1-C6 linear or branched, saturated or unsaturated alkylamines, low molecular weight compounds containing cationic moieties, oligomers and / or polymers, and / or other modifiers described herein, such as compounds containing allylamine.

[0029] Also provided is a method of making a gel, such as a transparent hydrogel, comprising: a) oxidizing (e.g., primary) alcohol units of bacterial cellulose to form bacterial cellulose containing multiple carboxylate and / or carboxylic acid groups; b) reacting the oxidized bacterial cellulose carboxylate and / or carboxylic acid groups with a surface modifier to form surface-modified bacterial cellulose; c) reacting the surface-modified bacterial cellulose with a (e.g., silyl) crosslinker to form a bacterial cellulose aerogel; A method is also disclosed, including:

[0030] One embodiment relates to oxidizing the bacterial cellulose in step (a) with sodium hypochlorite in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO).

[0031] A further aspect relates to the use of aminopropyltrimethoxysilane or other suitable agent as the surface modifier in step (b). As used throughout this disclosure, a surface modifier may include a compound containing an amine functional group and a silicon atom, such as a silylamine or one or more aminoalkylsilanes.

[0032] Another embodiment relates to the use of trimethoxymethylsilane or other polysiloxane precursors as the crosslinker in step (c). Additionally, there is provided a method for producing a transparent xerogel, comprising the steps of: a) oxidizing (e.g., primary) alcohol units of bacterial cellulose to form bacterial cellulose containing multiple carboxylate and / or carboxylic acid groups; b) reacting the oxidized bacterial cellulose carboxylate groups and / or carboxylic acids with a surface modifier to form surface-modified bacterial cellulose; c) reacting the surface-modified bacterial cellulose with a (e.g., silyl) crosslinker in a solvent such as water to form a bacterial cellulose aerogel; d) exchanging the solvent present in the aerogel with a solvent; e) removing the volatile solvent by drying to form a xerogel; A method is disclosed, comprising:

[0033] Provided herein is a method for producing a networked cellulose aerogel, comprising the steps of: a) contacting an aqueous dispersion of bacterial cellulose with an oxidation system that oxidizes the cellulose (e.g., C6 hydroxyl units) to carboxylate units and / or carboxylic acids to form an aqueous solution of oxidized cellulose nanofibers; b) reacting the oxidized cellulose nanofibers with a surface modifier to form an aqueous solution of surface-modified cellulose nanofibers; c) contacting the surface-modified cellulose nanofibers with a reagent (e.g., polyvinylmethyl-siloxane (PVMS)) to form a polysiloxane precursor; d) hydrolyzing the polysiloxane precursor in the presence of an acid catalyst to form a PMSQ network cellulose hydrogel; e) exchanging the solvent contained within the hydrogel with a solvent to form an organogel; f) removing the solvent to form an aerogel; Yet further disclosed is a method comprising:

[0034] Methods for preparing xerogels are disclosed herein. An exemplary method is based on a continuous process involving radical polymerization and hydrolytic polycondensation followed by ambient drying directly from alcohol as the drying medium without modification or additional solvent exchange, which is ultra-low cost and highly scalable. Polyvinylpolymethylsiloxane (CH2CH(Si(CH3)O))n flexible polymers may be used as crosslinkers for ambient dried cellulose aerogels. This polymer can be used to crosslink surface-modified bacterial cellulose with vinylmethyldimethoxysilane or or other suitable agent. As used throughout this disclosure, crosslinkers may include polysiloxane precursors such as one or more of vinylmethyldimethoxysilane, methyltrimethoxysilane, and methyltriethoxysilane.

[0035] Radical polymerization of alkene-containing monomers is an effective approach to enhance the mechanical properties of ambient-dried aerogels. Radical polymerization of vinyl groups in a polyvinylsilsesquioxane gel network with silane-modified CNF also yields mechanically reinforced xerogels. Flexible hybrid wet gels and dense gel films can be obtained by radical polymerization of VTMS followed by hydrolytic polycondensation. In PVMDMS, polyethylene chains interconnected by siloxane bonds and CNF-APTMS dispersed within the network provide flexibility to the hybrid gel. In addition, mechanically strong and flexible organic polymer hydrogels with a double network structure have been synthesized via radical polymerization.

[0036] The resulting ambient-dried aerogels exhibit a homogeneous, tunable, highly porous, doubly crosslinked nanostructure with an elastic polymethylsiloxane network crosslinked by flexible hydrocarbon chains and functionalized cellulose nanofibers (CNF-APTMS). The disclosed method results in ultra-low cost, high scalability, uniform pore size, high surface area, high transparency, high hydrophobicity, excellent machinability, ultra-flexibility in compression, ultra-flexibility in bending, and super-insulating properties that can be achieved with a single ambient-dried aerogel.

[0037] In one embodiment disclosed herein, a medium-scale cellulose-polysiloxane aerogel is prepared using the critical point drying method. For example, a 16.51 cm (6.5-inch) diameter aerogel was prepared by crosslinking quaternary amine-capped cellulose nanofibers in a polysiloxane network. Aerogels formed using CNF-APTMS exhibit excellent optical transparency, thermal insulation, and flexibility. The cellulose aerogel has a transmittance of 99% and a haze of 2%. The color rendering index of this aerogel is 0.99. The aerogel has a low thermal conductivity of 11 mW / K / m and a low viscosity of 7.3 W / K / m. 2 has a thermal conductance of less than

[0038] Feedstock Disclosed herein are readily available feedstocks useful for preparing the disclosed gels. In one non-limiting example, bacterial cellulose derived from Acetobacter hansenii in beer wort waste was used. There are many carbohydrates and amino acids in beer wort waste, making it an ideal culture medium for producing bacterial cellulose. 1% glucose was added to the beer wort to provide a suitable medium, which provided a suitable yield of bacterial cellulose as a low-cost alternative to standard media.

[0039] The use of waste beer wort and / or waste beer (WBW) for the large-scale production of bulk bacterial cellulose (BC) and the impact of unwanted contaminants found in the feedstock have been investigated. The resulting BC was used to produce transparent, flexible siloxane aerogels (dried at supercritical pressure with liquid CO2) and xerogels (dried at ambient pressure) useful for window insulation applications.

[0040] Beer production is a significant economic activity in the United States, and therefore thousands of gallons of wasted beer wort are generated annually at each brewery. Because livestock and poultry spent grain is a major by-product of the brewing industry, WBW is discarded in wastewater, creating a huge waste stream and a series of environmental problems. WBW is primarily composed of 48-55% protein, 23-28% carbohydrates, and 6-8% fiber. It consists of 8% RNA, 1% glutathione, and 2% B vitamins. It is also rich in elements such as P, K, Ca, Fe, P, and Mg. Due to this high nutrient content, it may be used as a nutrient source for microorganisms. To directly use these carbohydrates and proteins as a nutrient source by microorganisms, pretreatment may be desired to depolymerize the large polysaccharide molecules, since most proteins and carbohydrates exist in the form of large polymers within the cell wall.

[0041] Because Acetobacter hansenii can use monosaccharides to produce bacterial cellulose, it is desirable to cleave large polysaccharide molecules into smaller ones for use as a nutrient source. A one-step pretreatment, i.e., thermochemical high-temperature, high-pressure autoclave treatment in a weakly acidic atmosphere, could be effective for this purpose. This method should be effective not only in destroying cells and dispersing large polymer aggregates, but also in improving hydrolysis. In a previous report, waste brewer's yeast cells were used for bioethanol production through the release of nutrients by chemical pretreatment, including acid hydrolysis, alkaline hydrolysis, and enzymatic hydrolysis. For the production of BC, WBW collected from a local brewery was heated at 120°C and 6.4516 cm. 2The WBW was autoclaved (thermochemically) for 45 minutes at 50 pounds per square inch (22.6796 kg) of pressure. After the thermochemical treatment, it was subjected to a high-speed homogenizer followed by a chemical treatment with 1 M NaOH to bring the pH of the WBW to 5.5, which is necessary for bacterial growth.

[0042] After autoclave pretreatment under weakly acidic conditions, the WBW was homogenized and centrifuged at 4000 g for 15 min to remove precipitates, and the supernatant was collected and added with sterilized glucose solution (50% w / v) to reach a final concentration of 1% (w / v).

[0043] The WBW hydrolysate prepared as described above was then treated with 1 M NaOH to adjust its pH to 5.5. Finally, the prepared Acetobacter hansenii culture inoculum was transferred (5% w / v) to a glass dish (2000 mL) containing 1500 mL of WBW culture and statically incubated at 26°C for 14–21 days.

[0044] After hydrolysis, Acetobacter hansenii was directly fed into the WBW hydrolysate as a carbon and nutrient source for BC production. Some researchers have investigated various cellulosic wastes derived from renewable forest residues or industrial by-products to produce BC, but some additional nutrients are added to the medium to improve BC yield. This may be due to the fact that these pre-treated, non-centrifuged samples had high sugar concentrations (showing the highest sugar yield), which may cause inhibition of BC production and reduce the oxygen supply by the liquid medium. Meanwhile, the decrease in sugar concentration in the centrifuged samples could be reduced by diluting the supernatant with water, which may result in a better concentration for BC production. Perhaps, in this case, Acetobacter Cellulose production by S. hansenii was inhibited due to the low sugar concentrations present in the centrifuged samples, which further diluted the already low sugar yields of WBW obtained from these pretreatments. Therefore, we fed 1% sugar to the WBW cultures.

[0045] BC thin films with a thickness of 10-14 mm were successfully produced in the pretreated WBW medium as shown in Figures 3A-3C. Figure 3A shows the WBW after autoclaving in the culture chamber, Figure 3B shows it after 2 weeks, and Figure 3C shows the thin film removed for purification. After cultivation, the BC membrane was rinsed overnight with running water and immersed in 1 M NaOH at 80 °C for 2 hours to remove bacteria, and then washed several times with deionized water to completely remove the alkali. Figures 4A-4C show the thin film at different stages of purification. Figure 4A shows the material treated with 1% NaOH at 80 °C, Figure 4B shows the material treated with DI water, and Figure 4C shows the final purified BC. The thin film was then mixed with DI water as a solvent for further analysis, application, and TEMPO oxidation. The WBW medium was stored in a sealed container for 24 hours. BC production using WBW showed a yield equivalent to that of conventionally used chemical media.

[0046] Oxidation of bacterial cellulose To provide the desired carboxylate and / or carboxylic acid groups for crosslinking, bacterial cellulose was oxidized in water at pH 10 using, for example, sodium hypochlorite and a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical. For example, 2 g of bacterial cellulose was suspended in 150 mL of water containing 0.025 g of TEMPO and 0.25 g of NaBr. 4 mL of 1.8 M NaClO solution was added, and the pH of the suspension was maintained at 10 by adding 0.5 M NaOH. The reaction was terminated when no further decrease in pH was observed. The pH was then adjusted to 7 by adding 0.5 M HCl. The TEMPO-oxidized product was cellulose nanorods with diameters of 4–10 nm and lengths of 1,000–3,000 nm, which were then thoroughly washed with water by filtration and stored at 4 °C. The resulting CNF-COOH was a transparent, highly viscous material in aqueous dispersion.

[0047] Chemical modification of CNFs with silanes offers a versatile route for designing structures and properties suitable for polysiloxane coupling reactions. BC, successfully produced by a low-cost method, was oxidized using TEMPO-mediated oxidation, ultimately yielding a 0.2 wt.% transparent carboxylated CNF aqueous dispersion. Due to the chemical functionality of CNFs, which possess hydroxyl and carboxylic acid groups, amidation with amine silanes would be a suitable method. Using this method, carboxylate groups were selectively activated on each cellulose molecule with the water-soluble carbodiimide EDC·HCl [N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride] and N-hydroxysuccinimide. This procedure is illustrated in the scheme shown in Figure 11.

[0048] Next, the compound aminopropyltrimethoxysilane (APTMS), which has terminal amine functionality, was grafted onto the surface-activated, oxidized CNF molecules by amidation to obtain functionalized CNFs. The reaction was carried out at room temperature for 24 h under stirring in a N2 atmosphere. The CNFs and catalyst must be well dispersed in a nonaqueous solvent (DMSO), and any traces of moisture must be removed before the siloxane pendants of APTMS can be stored for reaction with the studied polysiloxane precursors, MTMS, MTES, and vinylsilane. This is a prerequisite for forming covalent bonds with the siloxane precursors (MTMS / MTES / PVMDMS) in the final step of the method. Chemical characterization by FTIR analysis of modified and unmodified CNFs is shown in Figure 5. In the case of APTMS-modified CNFs, the carbonyl band at 1658 cm-1 relative to TEMPO-oxidized bacterial cellulose is reduced and split into two peaks. The new carbonyl peak at 1710 cm-1 is a direct indication of the formation of amide and is evidence of the successful functionalization of CNFs with APTMS.

[0049] To fabricate polysiloxane-crosslinked cellulose aerogels, a two-step sol-gel process consisting of hydrolysis under acidic conditions and polycondensation under basic conditions in a liquid surfactant generates a homogeneous pore structure based on crosslinked nanosized colloidal particles. Large cellulose aerogels were produced using APTES-functionalized cellulose nanofibers crosslinked by the polycondensation reaction of MTMS and APTMS silanes.

[0050] One example of the disclosed xerogel may be prepared by the procedure outlined in the scheme shown in Figure 12 and described in Example 1 below. Example 1 Vinylmethyldimethoxysilane (VMDMS) and di-tert-butyl peroxide (DTBP) (1 mol%) were placed in a hydrothermal reactor. After flushing the space above the precursor solution with nitrogen, the reactor was sealed. The entire reactor was then heated at 120 °C for 48 h and then allowed to cool to room temperature, yielding a clear, viscous liquid containing primarily polyvinylmethyldimethoxysilane. To this solution, benzyl alcohol (4.3 mol / mol Si), HO (2 mol / mol Si), cellulose nanofibers grafted with aminopropyltrimethoxysilane (CNF-APTMS) (1 mol / mol Si), and a base catalyst (trimethylammonium hydroxide) (0.03 mol / mol Si) were added in specific molar ratios under stirring. After stirring for 5 min, the resulting sol was transferred into a mold container, which was then sealed and placed in an oven at 80 °C. A gel formed within 1 h. The gel was aged at 100 °C for 4 days and subjected to solvent exchange with IPA three times (8 h each) at 60 °C to remove residual chemicals. The gel was slowly dried by evaporation at room temperature for 2–5 days for ambient drying from IPA and then dried at 80 °C for 4 h to obtain the desired xerogel.

[0051] Ambient-dried cellulose aerogels were prepared by varying the crosslinking technique to improve their flexibility, optical transmission, and thermal conductivity. One method is to use ethylene-bridged polysiloxanes, which increase the molecular flexibility of the crosslinks. Another method is based on a continuous process involving radical polymerization and hydrolytic polycondensation followed by ambient pressure drying directly from alcohol as the drying medium, which is ultra-low-cost and highly scalable, without modification or additional solvent exchange. Ambient-dried cellulose aerogels exhibit high visible light transmittance of 90% in the visible wavelength range of 400-700 nm, an average haze value of 3%, and a thermal conductivity of less than 0.01 W / K / m.

[0052] 1A-1E show transparency characteristics during the process of xerogel formation described in Example 1. FIG. 1A shows the gel before solvent exchange with IPA, FIG. 1B shows the gel after the first 8-hour alcogel exchange with IPA, FIG. 1C shows the alcogel shrinkage behavior during the second 8-hour exchange with IPA, FIG. 1D shows the alcogel regaining transparency during the third 8-hour exchange with IPA, and FIG. 1E shows the final PVMDMS ambient pressure dried aerogel.

[0053] Upon drying at ambient pressure, the IPA-wet gel undergoes a large, temporary linear shrinkage of approximately 21% due to capillary forces acting throughout the gel skeleton, before recovering to nearly its original size as a result of its elastic molecular structure, which contains abundant methyl and aliphatic hydrocarbon chains and a small number of OH groups. Without wishing to be bound by theory, this recovery phenomenon is believed to be due to the flexible skeleton folding inward toward the pores during compression. This skeleton remains folded within the pores immediately after the force is removed, then gradually and partially recovers at room temperature, and continues to recover during thermal treatment due to the resilience and relaxation of the methyl- and aliphatic hydrocarbon chain-rich network. Large, crack-free xerogels have been obtained via the CNF-APTMS / PVMDMS combination. Because they are obtained by ambient drying without additional solvents and processing time, time, energy, and cost savings are significantly reduced for formulators seeking the disclosed ambient-dried aerogels. Figure 2 shows that alcogels a and b shrink and turn white during ambient drying. Once dried, the gel recovers to its original size and becomes transparent (c and d).

[0054] The disclosed method can be scaled up to produce large quantities of aerogel as described in Example 2.

[0055] Example 2 200 mg of CTAB and 1.5 g of urea were dissolved in 5 mL of water until a clear solution was obtained. Then, 1 mL of MTMS and 28.7 μL of 100x diluted glacial acetic acid were added to the mixture and stirred for 30 minutes. Finally, the mixture was added with APTMS-functionalized CNF. (0.02 mg / g Si) was added and stirred for 30 minutes at room temperature. The solution was poured into a mold and tightly sealed. The entire mold was kept in an oven at 60°C until a gel was formed, which usually took 12 to 24 hours. Occasional shaking during gelation is recommended to remove air bubbles that may have formed within the gel. After the gel was formed, the mold was transferred to a 60°C water bath for an additional 72 hours. After the gelation and aging process, the gel was gently and carefully removed from the mold, taking great care not to break the hydrogel. A chemical reaction and polycondensation occurred between the siloxane precursor and the added CNF-APTMS. This chemical reaction and polycondensation are clearly depicted in the scheme shown in Figure 13, along with its proposed reaction mechanism.

[0056] CNFs act as cross-linking molecules that induce strength and flexibility to the resulting polysiloxanes, making them particularly advantageous for preparing rollable and foldable aerogels and xerogels for window applications.

[0057] Figures 6A-6F show the fabrication of large MTMS / CNF-APTMS aerogels using glass molds. Figure 6A shows the mold for the polycondensation reaction chamber made of glass, Figure 6B shows the hydrogel fabrication setup in a heated water bath, Figure 6C shows the hydrogel in DI water, Figure 6D shows the alcogel in water / isopropanol, Figure 6E shows the alcogel in isopropanol, and Figure 6F shows the final aerogel.

[0058] A custom mold was fabricated using soda-lime glass and a 3.2 mm thick silicone rubber spacer. The mold was tightly sealed with epoxy rubber and further held in place with metal clips. The mold was then placed in a water bath at 60 °C for 72 h for final aging (Figure 6B). After aging, the removed hydrogel was first treated with DI water (Figure 6C), followed by a water / isopropanol (IPA) mixture (Figure 6D), and finally with IPA (Figure 6E). Figure 6F shows a transparent aerogel prepared with 0.02 wt% CNF-APTMS (visible transmittance >99%, haze <3%).

[0059] The next step in the disclosed method is the solvent exchange of the gel into isopropanol. To this end, the base and all other components (unreacted chemicals, urea, CTAB, and water) are removed by successive washes with water, a 50:50 water:IPA mixture, and finally, the gel is stored in pure IPA. Typically, the entire solvent exchange and washing of excess components takes several days to complete. Room temperature is maintained throughout this entire solvent exchange process. Finally, the gel in IPA is dried in a critical point dryer (CPD) powered by liquid CO2. The 6-inch alcogels in IPA are carefully transferred to a CPD chamber containing thin 6-inch glass slides. A spacer is placed between each alcogel. The sample is immersed in ethanol in the CPD chamber. The spacer should be thicker than the aerogel to ensure that the upper glass does not touch the aerogel. A 5 mm spacer was used to allow an additional 2 mm of free space between each sample (3 mm for each sample) to allow for the placement of additional glass on top of the final alcogel sample. The spacer prevents ethanol from being trapped between the CPD and the glass and prevents the easy flow of liquid CO2 during the final drying step. The entire CPD process typically takes 6-10 hours to complete, depending on the thickness of the alcogel and the number of samples in the chamber.

[0060] Figures 7A-7C show large-scale production samples of CNF-APTMS / MTMS aerogels with a diameter of 16.51 cm (6.5 in). As shown in Figures 7A and 7B, they are transparent, with thicknesses of 1.5 mm and 3 mm, 99% visible light transmittance, and the aforementioned haze values ​​of 2% and 3%, respectively. The CNF-APTMS content of these samples is (<0.02 wt%), which reduces their flexibility.

[0061] Aerogels made with a higher percentage of CNF-APTMS (0.2 wt%) offer greater flexibility (Figure 7C). Transmittance and haze measurements for this aerogel are shown in Figure 8.

[0062] The CNF aerogel exhibits a high visible light transmittance of 99% in the visible wavelength range of 400–700 nm and an average haze value of 2% (Figure 8). These values ​​are commensurate with the fact that we applied its highest concentration, exceeding 0.2 wt% of its dry weight. To obtain transparent aerogels, we employed and optimized an acid-base sol-gel reaction in a liquid surfactant-based solution to suppress phase separation between the hydrophobic MTMS condensate and the polar solvents of water and DMSO. We also reduced the size of the CNFs to 2 nm to reduce traces of scattering due to the bulky fibers and ultimately to a much more flexible aerogel.

[0063] Aerogel has a low thermal conductivity of 11 mW / K / m and a thermal conductance of 7.3 W / K / m 2 The infrared images show the excellent thermal insulation of the aerogel on the surface of a hot plate and on ice water. Figures 9A and 9B are IR images of cellulose aerogel on a hot (Figure 9A) and cold (Figure 9B) surface, demonstrating the excellent thermal insulation properties of cellulose aerogel. Cellulose aerogel is also inherently very flexible; it could easily recover after the load is removed.

[0064] The disclosed aerogels are suitable for use as glass or attachments to glass in traditional windows. Figure 10A is a photograph taken through the aerogel slab (dotted red square). It measures 16.51 cm (6.5 inches) diagonally and 2.5 mm thick. Figure 10B is an IR image of the same photograph showing the insulating properties of the aerogel.

[0065] Freestanding aerogels must be handled with care and may not be suitable for pilot-scale production. Furthermore, storing aerogels can be tricky, as they stick together when stacked.

[0066] In some cases, it may be desirable to form an aerogel (such as one of the aerogels described herein) directly on a substrate, such as a glass plate. In some cases, maximizing the reaction of the hydrolyzed MTMS with the silanols on the glass surface is desirable to obtain strong adhesion of the hydrogel. The idea is to keep the gel bonded to the glass throughout the entire process and then sell it with that same substrate.

[0067] There are various ways to modify the density of silanols on glass. One method is to treat the substrate with piranha solution. Piranha is a dangerous chemical obtained by mixing sulfuric acid with hydrogen peroxide. Although it can be unstable, it is used to clean wafers and resins in microelectronics. After this treatment, we expect a higher density of silanols on the surface of our substrate, as shown in Figure 14. One great thing about this modification is that it does not affect further steps. The gel is made in exactly the same way as in the lab, whether the glass is modified or not.

[0068] After the hydrogel hardens, it can be removed from the mold. At this step, it can be seen that the gel is bonded to the modified glass substrate (Figure 15). The gel is then solvent exchanged against IPA and dried in a CPD chamber. To allow for stacking of several samples in the CPD chamber, the alcogel on the glass substrate can be protected by an aluminum casing. Finally, an air film is dried on the glass.

[0069] The use of piranha is quite common in chemistry laboratories, but it is relatively unstable and prone to explosion, so it may not be the best choice in some situations. Commercially available stable piranha solutions may be used. Alternatively, concentrated solutions of KOH or other chemical treatments may be used.

[0070] Plasma treatment is used to treat the surface of various materials prior to any coating, printing, or bonding. Treatment with plasma removes any foreign matter present on the surface of the material, making the surface more suitable for further processing. When used under certain conditions, it can also implant some foreign ions into the substrate to modify the surface functionality. This treatment not only works for glass, but can also be used for polymer films as a retrofit application. Plasma treatment can be used to make glass substrates and cellophane film surfaces more hydrophilic, ultimately resulting in better adhesion to hydrogels.

[0071] Finally, a primer layer containing a siloxane or silica precursor may be deposited onto the substrate via physical or chemical vapor deposition, which can be done using typical low-E coaters present on industrial glass production lines.

[0072] Growing the gel directly on the substrate not only helps streamline production using existing equipment, but the direct bonding at the glass / gel interface prevents the gel from shrinking and cracking (during gelation and drying), and it also allows the gel to be cut using standard glass-cutting techniques.

[0073] The present invention has been described above with reference to several exemplary embodiments and examples. It should be understood that the specific embodiments shown and described herein are illustrative of preferred embodiments and best modes of the invention and are not intended to limit the scope of the invention. For example, unless otherwise specified, steps may be performed in any order, and some steps may be performed simultaneously. It will be recognized that changes and modifications may be made to the embodiments described herein without departing from the scope of the invention. These and other changes or modifications are intended to be included within the scope of the invention. The present invention includes the following aspects. [1] 1. A method for preparing a gel, comprising: a) oxidizing alcohol units of bacterial cellulose to form bacterial cellulose containing a plurality of carboxylate and / or carboxylic acid groups; b) reacting the oxidized bacterial cellulose carboxylate groups with a surface modifier to form surface-modified bacterial cellulose; c) reacting the surface-modified bacterial cellulose with a crosslinker in a solvent to form a bacterial cellulose aerogel; A method comprising: [2] d) Replacing the aqueous solution present in the aerogel with a solvent The method according to [1], further comprising: [3] e) removing the volatile solvent by drying to form a xerogel; The method according to [2], further comprising: [4] The method according to [1], wherein the bacterial cellulose is obtained from one or more of Acetobacter hansenii and Acetobacter xylinum. [5] The method of [1], wherein the crosslinking agent comprises a polysiloxane precursor. [6] [5] The method of [5], wherein the polysiloxane precursor comprises one or more of vinylmethyldimethoxysilane, methyltrimethoxysilane, and methyltriethoxysilane. [7] The method of [1], wherein the surface modifier comprises a compound containing an amine functional group and one or more silicon atoms. [8] The method of [1], wherein the surface modifier comprises one or more of silylamine and aminopropyltrimethoxysilane. [9] The method according to [1], wherein the surface modifier comprises an aminoalkylsilane.

[10] A gel formed according to the method described in [1].

[11] The gel according to

[10] , comprising a xerogel.

[12] A film comprising the gel according to

[10] .

[13] The film according to

[12] , comprising cellulose nanorods.

[14]

[12] The film according to

[12] , comprising one or more of cellulose nanoribbons, nanofibers and nanowires.

[15] The film according to

[12] , having a thickness of about 1 μm to about 10 cm.

[16] The film according to

[12] , having an electromagnetic transmittance of 0% to 100%, or about 25% to about 100%, for light wavelengths of about 400 nm to about 700 nm.

[17] about 10 -3 The film according to

[12] , having a thermal conductivity of W / (m·K) to about 10 W / (m·K).

[18] Approximately 1 Pa to approximately 10 6

[12] The film according to

[12] , having a bulk modulus of 10 Pa.

[19] 1. A method for preparing a networked cellulose aerogel, comprising: a) contacting a dispersion of bacterial cellulose with an oxidizing system that oxidizes the hydroxyl units of the cellulose to carboxylate and / or carboxylic acid units to form a solution of oxidized cellulose nanofibers; b) reacting the oxidized cellulose nanofibers with a surface modifier to form a solution of surface-modified cellulose nanofibers; c) contacting the surface-modified cellulose nanofibers with a cross-linking agent to form a bacterial cellulose nanofiber matrix; d) hydrolyzing the matrix in the presence of a catalyst to form a networked cellulose hydrogel; e) exchanging the solvent contained in the hydrogel with a solvent to form an organogel; f) removing the solvent to form an aerogel; A method comprising:

[20] The method according to

[19] , wherein the bacterial cellulose is obtained from one or more of Acetobacter hansenii and Acetobacter xylinum.

[21] The surface-modified cellulose nanofibers are C 1 ~C 6 The method according to

[19] , wherein the modified polymer is modified with a compound selected from linear or branched, saturated or unsaturated alkylamines, low molecular weight compounds containing cationic moieties, oligomers and / or polymers.

[22] The method according to

[19] , wherein the surface of the nanofiber is modified with a compound containing allylamine.

Claims

1. 1. A method for preparing a gel, comprising: a) oxidizing alcohol units of bacterial cellulose to form bacterial cellulose containing multiple carboxylate and / or carboxylic acid groups; b) reacting the oxidized bacterial cellulose carboxylate groups with a surface modifier comprising one or more silicon atoms and an amine functional group to form a surface-modified bacterial cellulose; c) reacting the surface-modified bacterial cellulose with a cross-linking agent in a solvent to form a matrix; A method comprising:

2. d) hydrolyzing the matrix in the presence of a catalyst to form a networked cellulose hydrogel.

3. 3. The method of claim 2, further comprising the step of: e) exchanging the aqueous solution present within the hydrogel with a solvent.

4. 4. The method of claim 3, further comprising the step of: f) removing the solvent to form a xerogel.

5. The method of claim 1 , wherein the crosslinker comprises a polysiloxane precursor.

6. 6. The method of claim 5, wherein the polysiloxane precursor comprises one or more of vinylmethyldimethoxysilane, methyltrimethoxysilane, and methyltriethoxysilane.

7. 2. The method of claim 1, wherein the bacterial cellulose is obtained from one or more of Acetobacter hansenii and Acetobacter xylinum.

8. Surface modifier is C １ ~C ６ 10. The method of claim 1, comprising linear or branched, saturated or unsaturated alkylamines, low molecular weight compounds, oligomers and / or polymers containing cationic moieties.

9. 10. The method of claim 1, wherein the surface modifier comprises one or more of a silylamine and an aminoalkylsilane.

10. A transparent and flexible cellulose aerogel comprising cellulose nanorods having a diameter of about 4 to about 10 nm, The transparent and flexible cellulose aerogel is formed using a crosslinker comprising a polysiloxane precursor and a surface modifier comprising a compound comprising one or more silicon atoms and an amine functional group, wherein the surface modifier modifies the surface of the cellulose nanorods.

11. A film comprising the aerogel of claim 10.

12. The film of claim 11 having a thickness of from about 1 μm to about 10 cm.

13. 13. The film of claim 12, having an electromagnetic transmittance of from about 25% to about 100% for light wavelengths from about 400 nm to about 700 nm. 【Request Item 14】 about 10 －３ 14. The film of claim 13, having a thermal conductivity of from W / (m·K) to about 10 W / (m·K).

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