Polymethylsilsesquioxane Aerogels Research Articles

Biopolymer-Polysiloxane Double Network Aerogels.

A new series of transparent aerogels of biopolymer-polysiloxane double networks is reported. Biopolymer aerogels have attracted much attention from green and sustainable aspects but suffered from strong hydrophilicity and difficulty to make homogeneous structures in nanoscale; these drawbacks are overcome by compositing with a polysiloxane network. Alginate-polymethylsilsesquioxane aerogel has high optical transparency, water repellency, comparable superinsulation property and improved bending flexibility compared to pure polymethylsilsesquioxane aerogel. The nanoscale homogeneity is realized by separating the crosslinking steps for two networks in a sequential protocol: condensation of siloxane bonds and metal-crosslinking of biopolymer. The crosslinking order, biopolymer-first or siloxane-first, and universality/limitation of biopolymer-crosslinker pairs are discussed to construct fundamental chemistry of double network systems for their further application potentials.

Biopolymer‐Polysiloxane Double Network Aerogels

AbstractA new series of transparent aerogels of biopolymer‐polysiloxane double networks is reported. Biopolymer aerogels have attracted much attention from green and sustainable aspects but suffered from strong hydrophilicity and difficulty to make homogeneous structures in nanoscale; these drawbacks are overcome by compositing with a polysiloxane network. Alginate‐polymethylsilsesquioxane aerogel has high optical transparency, water repellency, comparable superinsulation property and improved bending flexibility compared to pure polymethylsilsesquioxane aerogel. The nanoscale homogeneity is realized by separating the crosslinking steps for two networks in a sequential protocol: condensation of siloxane bonds and metal‐crosslinking of biopolymer. The crosslinking order, biopolymer‐first or siloxane‐first, and universality/limitation of biopolymer‐crosslinker pairs are discussed to construct fundamental chemistry of double network systems for their further application potentials.

Synergistic effects of P and Si on the flame retardancy in a polymethylsilsesquioxane aerogel prepared under ambient pressure drying

Synergistic effects of P and Si on the flame retardancy in a polymethylsilsesquioxane aerogel prepared under ambient pressure drying

Comparative Analysis of Superhydrophobic Elastic Polymethylsilsesquioxane Aerogels Prepared at Atmospheric Pressure and under Supercritical Conditions

Comparative Analysis of Superhydrophobic Elastic Polymethylsilsesquioxane Aerogels Prepared at Atmospheric Pressure and under Supercritical Conditions

Hierarchically porous and highly transparent polymethylsilsesquioxane aerogels with an ultrahigh specific surface area using an organic alkali as a catalyst, surfactant, and template

A hierarchically porous aerogel with an ultrahigh specific surface area, high visible light transmittance, excellent mechanical properties, and cubic shell-shaped pores was obtained using a supercritical drying process. Ethanol and methyltriethoxysilane (MTES) were used as a solvent and silicon source, respectively, whereas tetramethylammonium hydroxide (TMAOH) acted as a basic catalyst, surfactant, and template. The phase separation during gelation was finely adjusted by changing the ethanol-to-water ratio to control the gel skeleton structure, the number of crystallization templates, and the light transmittance of the aerogel. The hierarchical porous aerogels exhibited a uniform three-dimensional network. Notably, the specific surface area of the aerogels was 909 m2/g, the visible light transmittance was 58.1%, the hydrophobic angle was 166.5°, the thermal conductivity was 18.2 mW/(m·K), and the stress was 0.301 MPa at 50% compressive strain. The unique structure and excellent performance of the hierarchical porous aerogels make them promising for thermal and sound insulation applications. Moreover, the innovative use of TMAOH as a surfactant and template can be inspiring for the preparation of polymer-reinforced or highly crosslinked aerogels.

Biomimetic Light-Driven Aerogel Passive Pump for Volatile Organic Pollutant Removal.

Inspired by the solar‐light‐driven oxygen transportation in aquatic plants, a biomimetic sustainable light‐driven aerogel pump with a surface layer containing black manganese oxide (MnO2) as an optical absorber is developed. The flow intensity of the pumped air is controlled by the pore structure of nanofilbrillated cellulose, urea‐modified chitosan, or polymethylsilsesquioxane (PMSQ) aerogels. The MnO2‐induced photothermal conversion drives both the passive gas flow and the catalytic degradation of volatile organic pollutants. All investigated aerogels demonstrate superior pumping compared to benchmarked Knudsen pump systems, but the inorganic PMSQ aerogels provide the highest flexibility in terms of the input power and photothermal degradation activity. Aerogel light‐driven multifunctional gas pumps offer a broad future application potential for gas‐sensing devices, air‐quality mapping, and air quality control systems.

Shape-stabilized phase change materials based on superhydrophobic polymethylsilsesquioxane aerogels with extremely high paraffin loading for energy storage

Organic phase change materials (PCMs) play an important role in heat energy storage, but they are also limited by the leakage problem in the process of phase change. Herein, shape-stabilized composite PCMs (ssPCMs) are successfully obtained by impregnating paraffin (PA) into the polymethylsilsesquioxane (PMSQ) aerogels. Due to abundant porosity, light weight, inherent superhydrophobic of PMSQ aerogels, lipophilic PA can be loaded into aerogels with a high PCMs loading rate up to 876% without alkylation process. Under the action of aerogel pores, PA in ssPCMs has high degree of crystallinity. The three-dimensional network structure of aerogels can effectively support the melting PA, and there is almost no leakage problem under the strong capillary force and surface tension. The ssPCMs show high latent heat in the range of 117.2 J/g to 122.9 J/g and excellent thermal stability and recyclability where their latent heat nearly remains 97% after 50 times of melting-crystallization cycles. More interestingly, when the temperature reaches above the melting point of PA, the ssPCMs changes from white to translucent. Thus, the synthesized ssPCMs exhibit considerable potential in energy storage applications, temperature intelligent response, thermal regulation.

Facile synthesis of highly flexible polymethylsilsesquioxane aerogel monoliths with low density, low thermal conductivity and superhydrophobicity

In the present work, a facile method was proposed to synthesize highly flexible polymethylsilsesquioxane aerogel (PMSA) monoliths by adopting methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as co-precursors and ammonia as a base catalyst. The sol-gel process can be completed within 15 min, and gels were directly dried by ambient pressure drying to obtain monolithic PMSAs. The PMSA had low density (as low as 0.087 g cm−3), high porosity (90.6–94.5%), macro-pore structure and low thermal conductivity (0.0321–0.0384 W m−1 K−1). Moreover, excellent compressibility and bending flexibility, together with superhydrophobicity (a contact angle was 162°) were achieved. This work provides an attractive strategy to synthesize highly flexible monolithic PMSAs as promising materials for thermal insulation applications.

Functional nanostructured materials: Aerosol, aerogel, and de novo synthesis to emerging energy and environmental applications

In this review, we introduce advanced synthetic methods for functional nanostructured materials (in powder form) bridging to the development in emerging energy and environmental applications. Three types of synthetic methods (aerosol-based, aerogel-based, and de novo methods) are introduced, all of which have shown to be extensively investigated as novel routes to create nanostructured materials with designed material properties (i.e., controlled size, shape, porosity, and chemical composition are to be achievable). The typical experimental setup and the general experimental procedure for material preparation via the above three synthesis routes are discussed. Complementary characterization approaches are employed to study material properties of the synthesized nanostructured materials via the three synthesis routes. Here we investigate: (1) CuxO-CeO2, Ni-CeO2, and CuxO nanoparticle-encapsulating metal–organic framework (MOF) hybrid nanoparticles synthesized via the aerosol-based method; (2) Cr-encapsulating MOF (Cr-MOF-199), Au-encapsulating MOF (Au@ZIF-8), and MOF-derived nanocomposites (CuO/CuCr2O4) produced via the de novo route; (3) a variety of aerogels (carbon, metal oxide, polymer) with high porosity created by the aerogel-based approach. Finally, several examples of emerging energy and environmental applications are introduced using these functional nanostructured materials, including (1) catalytic transformation to chemicals by using precious metal nanoparticles-embedded MOFs and the MOF-derived nanocomposites as the catalysts; (2) methane combustion using CuxO-CeO2 hybrid nanoparticles as catalyst, (3) methane dry reforming with CO2 using Ni-CeO2 hybrid nanoparticles as catalyst; (4) CO2 capture by fluoroalkyl silane-modified mesoporous silica and polymethylsilsesquioxane (PMSQ) aerogel membranes; (5) adsorption of organic solvent, dye, and oil by cetyltrimethylammonium bromide-modified PMSQ aerogel.

Fabrication and simulation analysis of flexible polymethylsilsesquioxane (PMSQ) aerogels by using dimethyl sulfoxide (DMSO) as solvent

The flexible bulk polymethylsilsesquioxane (PMSQ) aerogels were synthesized using DMSO as solvent via the sol-gel process and supercritical drying. The porous microstructures and the bonding linkage were investigated by means of Fourier transform infrared (FT-IR) spectra, nitrogen adsorption-desorption method and scanning electron microscopy (SEM). The effects of dimethyl sulfoxide (DMSO) and ammonium hydroxide (AH) in sol-gel process were discussed. The results show that the micro-morphology of PMSQ aerogels is an expression of freezing siloxane system by adjusting the relative ratio of DMSO and AH. The PMSQ aerogels are provided with high specific surface area, low density, favorable flexibility and low thermal conductivity. The multi-scale simulative analysis reveals that DMSO as solvent tends to uniformly disperse M-monomer due to similar solubility parameter (SP) and repulsion parameter (RP). SP and RP can be regarded as important parameters of solvent selection to explain and predict gel compatibility and uniformity.

A Co-Precursor Approach Coupled with a Supercritical Modification Method for Constructing Highly Transparent and Superhydrophobic Polymethylsilsesquioxane Aerogels.

Polymethylsilsesquioxane (PMSQ) aerogels obtained from methyltrimethoxysilane (MTMS) are well-known high-performance porous materials. Highly transparent and hydrophobic PMSQ aerogel would play an important role in transparent vacuum insulation panels. Herein, the co-precursor approach and supercritical modification method were developed to prepare the PMSQ aerogels with high transparency and superhydrophobicity. Firstly, benefiting from the introduction of tetramethoxysilane (TMOS) in the precursor, the pore structure became more uniform and the particle size was decreased. As the TMOS content increased, the light transmittance increased gradually from 54.0% to 81.2%, whereas the contact angle of water droplet decreased from 141° to 99.9°, ascribed to the increase of hydroxyl groups on the skeleton surface. Hence, the supercritical modification method utilizing hexamethyldisilazane was also introduced to enhance the hydrophobic methyl groups on the aerogel’s surface. As a result, the obtained aerogels revealed superhydrophobicity with a contact angle of 155°. Meanwhile, the developed surface modification method did not lead to any significant changes in the pore structure resulting in the superhydrophobic aerogel with a high transparency of 77.2%. The proposed co-precursor approach and supercritical modification method provide a new horizon in the fabrication of highly transparent and superhydrophobic PMSQ aerogels.

Performance of ceramic membrane in vacuum membrane distillation and in vacuum membrane crystallization

Membrane crystallization (MCr) is emerging as an interesting candidate to extract additional freshwater and raw materials from high-concentrated solutions. Traditionally, MCr has been carried out by using polymeric membranes that have limited chemical and mechanical stability. These shortcomings can be overcome by using ceramic membranes. The current study describes the preparation and testing of two hydrophobic ceramic membranes synthesized trough sol-gel process, and combined phase-inversion and sintering method. The first membrane (CM-L) was synthesized by coating hydrophobic polymethylsilsesquioxane aerogels on alumina membrane supports via a sol-gel process. The membrane showed stable hydrophobic character in membrane distillation and crystallization tests but very low flux. To obtain high flux, a second type (CM-S) membrane was prepared by applying Fluoroalkylsilanes (FAS) (1H, 1H, 2H, 2H‑perfluorooctyltriethoxysilane) hydrophobic agent at the relatively thin and more porous as-sintered alumina hollow fibers. The suitability of both membranes for MCr process was analyzed by crystallizing NaCl and LiCl. By using 1 M NaCl and 13 M LiCl aqueous solutions, and under the same operative conditions, CM-S membrane exhibited average flux higher than CM-L membrane. The performance of both the membranes, in terms of hydrophobic character, remained stable throughout the performed tests.

Transparent, elastic and crack-free polymethylsilsesquioxane aerogels prepared by controllable shrinkage of the hydrogels in the aging process

Polymethylsilsesquioxane (PMSQ) aerogels have gained extensive attention owing to their improved mechanical properties, good optical transparency and low thermal conductivity. However, the aerogels with high densities prepared by the conventional sol-gel method are fragile and opaque due to the increase in the particles size and the decrease in the uniformity of pore structure. In this research, transparent PMSQ aerogels over a wide range of densities from 0.15 g/cm3 to 0.25 g/cm3 are prepared by controllable shrinkage of the hydrogels in the aging process. Resulted PMSQ aerogels have smaller particle and pore size than that prepared by the conventional method. Surprisingly, the light transmittance of the aerogels with the density of 0.25 g/cm3 at 550 nm wavelength is up to 70%. Meanwhile, the Young's modulus of the aerogel is up to 4.33 MPa. The strength and resilience compressed to 50% can be 3.57 MPa and 99%, respectively. The thermal conductivities of the aerogels prepared by the novel method increase from 18.7 mW m−1 K−1 to 32.9 mW m−1 K−1, with the density increasing from 0.15 g/cm3 to 0.25 g/cm3. These results would extend the range of practical applications for the high density PMSQ aerogels such as the transparent insulators.

Efficient preparation of crack-free, low-density and transparent polymethylsilsesquioxane aerogels via ambient pressure drying and surface modification.

Polymethylsilsesquioxane (PMSQ) aerogels have gained considerable attention due to their high transparency, good mechanical properties and low thermal conductivity. However, low-density PMSQ aerogels are difficult to obtain by ambient pressure drying due to irreversible shrinkage. Inspired by previous research, we speculate that reducing surface silanol groups could reduce irreversible shrinkage along with the skeleton-strengthening effect. In addition, extending the ageing process is expected to lead to increased density. Thus, in this paper, we applied a mature technique to modify the surfaces of PMSQ gels with terminal silane groups to reduce hydrophilic surface silanol groups without strengthening the skeletons. This surface modification process greatly reduced irreversible shrinkage and allowed the PMSQ gels to return to their original sizes, in accompany with the decrease of silanol group (NMR results as the direct evidence). This method exhibits extremely high efficiency in the preparation of crack-free, low-density and transparent PMSQ aerogels. The PMSQ aerogels dried at ambient pressure had a low density of 48 mg cm−3, low thermal conductivity (21.1 mW m−1 K−1), high transparency (81.3% at 550 nm), super-hydrophobicity (contact angle of 155°) and excellent mechanical properties. The proposed method will be useful for the industrial production of transparent insulating materials and has potential applications in space exploration.

Aerogel Photocatalyst Composed of Transparent Mesoporous Polymethylsilsesquioxane Softly Post‐Modified with a Visible‐Light‐Absorbing Metal Complex

AbstractPhysical adsorption of various metal complexes including multinuclear heterometallic species onto polymethylsilsesquioxane (PMSQ) aerogel afforded their highly transparent mesoporous hybrids. One of the fabricated materials containing Ru(II) tris‐phenanthroline complex RuPhen3−OH (RuPhen3−OH@PMSQ) exhibited a photocatalytic activity for the dehydrogenation of the Hantzsch 1,4‐dihydropyridine 1H2, which was close to that observed for the corresponding homogeneous system. The photocatalytic activity of RuPhen3−OH@PMSQ was superior than that of RuPhen3−OH on other silica gels having larger or smaller pores, suggesting that the pore size of the PMSQ aerogel plays an important role for the high photocatalytic activity by allowing both the light and substrates to easily access the catalysts located at the inner space of the material.

Fabrication of hydrophobic polymethylsilsesquioxane aerogels by a surfactant-free method using alkoxysilane with ionic group

Phase separation control is an important factor to prepare a porous monolith by an aqueous sol–gel reaction. Here, we report a surfactant-free synthesis method to obtain hydrophobic polymethylsilsesquioxane aerogels by copolymerizing a cationic-functionalized alkoxysilane N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride. The resultant materials have low-density, high visible-light transmittance, and high thermal insulating equivalent to those of prepared under the presence of surfactant.

Tailoring structural and physical properties of polymethylsilsesquioxane aerogels by adjusting NH3·H2O concentration

The polymethylsilsesquioxane (PMSQ) aerogels obtained from the precursor of methyltrimethoxysilane (MTMS) have been a kind of high-performance thermal insulating materials due to its ultra-low thermal conductivity and improved mechanical property. In this paper, a facile way was introduced to control the microstructure of the PMSQ aerogels using a sol-gel system with supercritical ethanol drying. It was found that the skeleton structures changed from the tenuous branched chains to a stronger network of particle aggregates with the increase of NH3·H2O concentration. The pore sizes in the aerogels were controllable and gradually changing from concentrating at ∼10 nm to 40 nm. The effect of different assembly structures on the physical properties were explored in detail. The aerogels with smaller particles and pore sizes are transparent and have the specific surface area as high as 615.0 m2/g, while the particle aggregates and larger pore sizes make the skeleton sturdier, which endow the aerogels with improved resilience ability of 80%. All the aerogels show low thermal conductivity in the range of 20.7–21.4 mW/m·K and hydrophobic property with a contact angle higher than 140°. More importantly, with high ammonia concentration, aerogels obtained by ambient pressure drying have a resilience of 98% and exhibit low thermal conductivity of 22.3 mW/m·K. This extends the range of practical applications for the PMSQ aerogels.

Solvent-resistant CTAB-modified polymethylsilsesquioxane aerogels for organic solvent and oil adsorption

In this work, we describe the successful synthesis of super-hydrophobic polymethylsilsesquioxane (PMSQ) aerogels with a porous network by a sol-gel polymerization process using methyltrimethoxysilane (MTMS) reactants with the addition of cetyltrimethylammonium bromide (CTAB) surfactants. The specific surface area, pore size and hydrophobicity of the CTAB-modified PMSQ aerogels were investigated for different amounts of CTAB doping. The specific surface areas of the pristine and CTAB-modified PMSQ aerogels with C/M molar ratios of 0.001, 0.002 and 0.004 were 491, 301, 342 and 298m2/g, respectively. The water contact angles of the CTAB-modified PMSQ aerogels with C/M molar ratios of 0.001, 0.002 and 0.004 were 166.38°, 165.62° and 166.43°, confirming the super-hydrophobicity of the CTAB-modified PMSQ aerogels. Compared to pristine PMSQ aerogels, the as-prepared CTAB-modified PMSQ aerogels exhibit more solvent-resistant properties in ethanol (EtOH) and n-hexane solvents. The as-prepared CTAB-modified PMSQ aerogels were further used for organic solvent and oil adsorption applications. Consequently, the as-prepared CTAB-modified PMSQ aerogels have great potential for organic solvent and oil adsorption in industrial applications.

Sol–gel preparation of polymethylsilsesquioxane aerogel membranes for CO2 absorption fluxes in membrane contactors

The hydrophobic membrane in a membrane contactor serves as the interface between gaseous mixtures that contain CO2 and an aqueous amine solution. This hydrophobic membrane prevents the amine solution from passing through and wetting the membrane; however, gaseous CO2 can enter the membrane pores and be absorbed by the amine. In this study, hydrophobic polymethylsilsesquioxane (PMSQ) aerogels were coated successfully via a sol–gel process onto an Al2O3 membrane support using methyltrimethoxysilane (MTMS) precursors. We examined the effects of the NH4OH concentration and the molar ratio of EtOH2nd (ethanol adding in second step of sol–gel processes) and MTMS on this sol–gel process for membrane preparation. In addition, we investigated the CO2 absorption fluxes obtained using these aerogel membranes. The PMSQ aerogel membranes prepared with 17% NH4OH and a 1:1M ratio of EtOH2nd to MTMS exhibited the highest CO2 absorption flux of 1.2mmole/m2s. Furthermore, the as-prepared PMSQ aerogel membranes can be used continuously to absorb CO2 for at least 4days, and they can be reused in three consecutive cycles of CO2 absorption. The results of this study demonstrated that these as-prepared PMSQ aerogel membranes are not only durable but also reusable. Thus, these as-prepared PMSQ aerogel membranes show promise for use in membrane contactors for CO2 absorption.

The thermal conductivity of polymethylsilsesquioxane aerogels and xerogels with varied pore sizes for practical application as thermal superinsulators

The relationship between the thermal conductivity, gas pressure and pore size of polymethylsilsesquioxane aerogels and xerogels has been investigated for practical applications.