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Review

The Synthesis and Polymer-Reinforced Mechanical Properties of SiO2 Aerogels: A Review

by
Wang Zhan
1,*,
Le Chen
2,
Qinghong Kong
1,*,
Lixia Li
1,
Mingyi Chen
1,
Juncheng Jiang
3,
Weixi Li
1,
Fan Shi
1 and
Zhiyuan Xu
1
1
School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China
2
Department of Electronic Engineering, School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China
3
College of Safety Science and Engineering, Nanjing Tech University, Nanjing 213000, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(14), 5534; https://doi.org/10.3390/molecules28145534
Submission received: 25 May 2023 / Revised: 4 July 2023 / Accepted: 13 July 2023 / Published: 20 July 2023
(This article belongs to the Special Issue Synthesis, Characterization and Application of Polymer-Based Materials)
Browse Figures
Versions Notes

Abstract

:
Silica aerogels are considered as the distinguished materials of the future due to their extremely low thermal conductivity, low density, and high surface area. They are widely used in construction engineering, aeronautical domains, environmental protection, heat storage, etc. However, their fragile mechanical properties are the bottleneck restricting the engineering application of silica aerogels. This review briefly introduces the synthesis of silica aerogels, including the processes of sol–gel chemistry, aging, and drying. The effects of different silicon sources on the mechanical properties of silica aerogels are summarized. Moreover, the reaction mechanism of the three stages is also described. Then, five types of polymers that are commonly used to enhance the mechanical properties of silica aerogels are listed, and the current research progress is introduced. Finally, the outlook and prospects of the silica aerogels are proposed, and this paper further summarizes the methods of different polymers to enhance silica aerogels.

Graphical Abstract

1. Introduction

Aerogels are the lightest solid materials in the world due to their high porosity and low density [1,2,3,4]. The liquid constituent of these materials are substituted with air and form intact interconnected solid structures. Relying on their unique mesoporous structure, aerogels demonstrate excellent properties and have been favored by researchers [5,6,7,8]. There are many kinds of aerogels, such as silica aerogels [9], carbon aerogels [10,11], polymer aerogels [12,13,14,15], metal oxide aerogels [16,17,18], metal aerogels [18], and bio-based aerogels [19,20]. These aerogels are considered as the “wonder materials” and have broad prospects of application in various fields [21,22,23,24,25].
Silica aerogels with excellent properties of low bulk density (0.003~0.200 g/cm3), high porosity (80~99.8%), large specific surface area (500~1500 m2/g), and low thermal conductivity (0.015~0.030 W/m·K) are the typical representatives of aerogels [26,27,28,29]. It has been more than 90 years since silica aerogel was first invented by the American scientist Kistler in 1931 [30,31]. Although silica aerogels have a long history of development, their commercial production history only dates back to about 20 years. In 2001, Aspen realized the commercial production of silica aerogels for the first time in the United States. As Figure 1 shows, silica aerogels are widely used in construction engineering [32,33], aeronautical domains [34,35,36], environmental protection [37,38], flexible electronics [39], and chemical engineering [40,41,42]. Especially in the aerospace field, silica aerogels have made remarkable contributions to the safety of personnel and equipment [43].
However, the application of silica aerogels on a large scale is still limited due to the poor mechanical properties of silica aerogels [44]. The main reason for the poor mechanical properties of silica aerogels is their pearl-necklace-like three-dimensional network structure. This network structure is connected by the interparticle necks, and the connection strength is very fragile. However, other properties of silica aerogels are excellent in the engineering field. Thus, the problem of the poor mechanical properties of silica aerogels urgently needs to be solved [45,46].
The secondary particles of silica aerogels are connected via point contact with a small contact area and weak bond force between the particles [47]. Once the aerogels have suffered from external stress, the neck region between the secondary particles break, resulting in the connection being disconnected and the gel skeleton collapsing [48]. In order to solve the above problems, scholars around the world have conducted extensive research. Focusing on the problem of the poor mechanical properties of silica aerogels, reinforcing phases have been introduced to improve the mechanical properties of silica aerogels. As shown in Figure 2, the reinforcing phases mainly include carbon, biomaterial, fiber, and polymer. The mechanical properties of silica aerogels have been enhanced to varying degrees through the experiment.
In this paper, the composites of silica aerogels with polymers are summarized, and the properties are discussed in detail.

2. Synthesis of Silica Aerogel

As shown in Figure 3, the preparation of the silica aerogels comprises (a) synthesis, (b) aging, and (c) drying [53]. The synthesis of silica aerogels mainly depends on the method of sol–gel [54]. During the sol–gel process, a three-dimensional network structure is built [55,56]. And the properties of silica aerogels are influenced by the precursors, catalysts, temperature, surface treatments, mass concentration, pH, and drying [57,58,59,60]. Then, the aging process enhances the network structure of the silica sol. Finally, the solvents are removed from the solid via the drying technology.

2.1. Sol–Gel Chemistry

Gels are colloids that are composed of colloidal particles suspended in a solvent. The sol–gel method is a common technique used for the synthesis of wet gels, and then the silica aerogels are produced via the drying process [61,62]. During the sol–gel process, ethanol or methanol is used as a solvent. Using the solvent, the precursors form a soluble gel through hydrolysis and polycondensation reactions [63,64]. The results of the reactions are greatly influenced by the pH of the solution, the concentration of silicon precursor, the reaction time, and other factors. Moreover, the solid network of silica is formed in this process, and the particles are more closely connected [65,66,67,68]. However, the repetitive washing and the tedious water-to-alcohol solvent exchange still need more time [69]. As shown in Figure 4, the primary particles form secondary particles in the solution and then the secondary particles form a continuous solid network connected by the neck regions [70]. With the development of sol–gel chemistry, silica alkoxides are the precursors of silica aerogels, such as tetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS), and so on [71,72,73]. These precursors have a great influence on the morphology and properties of silica aerogels [74]. Moreover, various precursors can also be used together to realize the self-reinforcement of silica aerogel skeletons [75,76]. Bhagat et al. [77] applied nine different co-precursors to prepare TEOS-based silica aerogels and investigated their physical properties. The pre-polymerization of silicon precursors is also a method of silica self-reinforcement that is currently being explored. Zu et al. [78] reported a new method of pre-polymerized silica-based precursors and enhanced the flexibility of aerogels. Table 1 lists some common precursors that are used for the synthesis of silica aerogels.
Table
Table 1. Common silica precursors for silica aerogel synthesis.
Table 1. Common silica precursors for silica aerogel synthesis.
Silica PrecursorChemical FormulaAbbreviationPhysical PropertiesMechanical PropertiesThermal PropertiesRef.
TetraethylorthosilicateSi (OC2H5)4TEOS/G modulus: 10.7 MPa/[79]
TetramethylorthosilicateSi (OCH3)4TMOSSkeletal densities: 2.2 g/cm3//[80]
TrimethylchlorosilaneSi (CH3)3ClTMCSSurface area: 914.4 m2/g; porosity: 96.16%//[81]
MethyltrimethoxysilaneSi (OCH3)3CH3MTMSShrinkage: 3.5% //[82]
MethyltriethoxysilaneSi (OC2H5)3CH3MTESDensity: 0.1 g/cm3; porosity: 95.5%Unrecoverable strain loss: 10%Thermal conductivity: 0.038 W/m·K[83]
AminopropyltrimethoxysilaneH2N (CH2)3Si(OCH3)3APTMS//Young’s modulus: 14 MPa[84]
AminopropyltriethoxysilaneH2N (CH2)3Si(OC2H5)3APTESSurface area: 150.9 m2/gYoung’s modulus: 18 MPaThermal conductivity: 0.037 W/m·K[85]
PropyltriethoxysilaneC9H22O3SiPTESDensity: 0.172 g/cm3; porosity: >90%Elastic module: 0.35 MPa/[86]
VinyltrimethoxysilaneH2C=CHSi(OCH3)3VTMS/Elongation at break: 40~50%Thermal conductivity: 0.06 W/m·K[87]
VinyltriethoxysilaneC8H18O3SiVTESSurface area: 321 m2/gCompressive stress: 0.571 MPaThermal conductivity: 0.024 W/m·K[88]
3-glycidoxypropyltrimethoxysilaneC9H20O5SiGPTMS//Thermal conductivity: 0.032 W/m·K[89]
Bis [3-(triethoxysilyl)propyl]disulfideC18H42O6S2Si2BTSPDDensity: 0.21 g/cm3; porosity: 85.5%Young’s modulus: 2.1 MPa/[90]
1,6-bis(trimethoxysilyl)hexaneC12H30O6Si2BTMSH/Strain: 50%/[91]
Bis(trimethoxysilylpropyl)amineC12H31NO6Si2BTMSPADensity: 0.308 g/cm3; porosity: 78%; surface area: 325 m2/gShrink: 11%, compression Modulus: 15 MPa/[92]
DimethyldiethoxysilaneC6H16O2SiDMDESDensity: 0.082 g/cm3; surface area: 162.1 m2/g; porosity: 94.2%/Maximum degradation rate: 150 °C[93]

2.2. Aging

Aging is an important step in strengthening the mechanical properties of silica aerogels [94]. The aging reaction principle is ascribed to accelerating the movement of sol particles and increases the probability of collision, adding the number of siloxane connections. In the aging process, the number of siloxane linkages between particles can be increased using two different mechanisms simultaneously; thus, the mechanical properties of silica aerogels will be strengthened [95]. These two different mechanisms mainly include dissolution and re-precipitation [96]. Moreover, new monomers are transported to the neck region between the particles and form the network [97]. As an important chemical bond connecting the particles together, the O-Si bonds have a great relationship with the time and temperature of the reaction. The number of the bonds affects the mechanical properties of the silica aerogels. He et al. [98] reported that a controlled temperature and pressure can enhance the mechanical properties during the process of dissolution and reprecipitation.
However, this process requires a lot of time, and the parameters of the aging process are difficult to control. During the aging process, the silica aerogels occur different degrees of shrinkage, leading to an increase in density.

2.3. Drying

Drying is the procedure that transports the wet silica gel to silica aerogel [99]. In this process, gas replaces the liquid in silica wet gel, and ultimately, the solid consisting of the silica network forms the aerogel. Due to differential shrinkage, warping and cracking often occur. However, previous studies have shown that the phenomena can be prevented by controlling the drying process. At present, there are three common drying methods as follows: ambient pressure drying [100,101], freeze drying [102,103], and supercritical drying [104].
Compared with the other two drying methods, the ambient pressure drying method is widely used mainly due to its lower energy consumption and because it does not require high pressure conditions [105,106,107]. However, there are still drawbacks to ambient pressure drying. The most significant disadvantage is that ambient pressure drying may be affected by capillary force, and thus, still result in collapsing or cracking [108]. Freeze drying is the technology that freezes the liquid in the wet gel into a solid and then converts the solid into a gas. In order to reduce the possibility of breakage caused by ambient pressure drying, freeze drying is used to manufacture various types of aerogels. Compared with the above methods, supercritical drying is considered as the most appropriate method to prepare aerogels. It minimizes the aerogel cracking caused by shrinkage. The liquid in the pores is transformed into supercritical fluid via supercritical drying. In this state, the surface tension of the liquid disappears completely and there is no capillary force. Finally, the supercritical fluid can be separated from the solid at a temperature above the critical temperature of the liquid [109]. At present, the more mature supercritical drying technology is low temperature drying using carbon dioxide. However, this method has high requirements for synthesis equipment and the operating environment [110].

3. Polymer-Modified Silica Aerogel Composites

Based on existing reports, the polymer matrix is composed of thermoplastics and thermosetting resins [111]. Polymers demonstrate excellent mechanical properties, generally with high elastic deformation and viscoelasticity [112]. In addition to preparing polymer aerogels, polymers can also be combined with silica aerogels to prepare hybrid aerogels [113]. The process of polymerization increases the chemical bonds of O–Si to enhance their mechanical properties [114].
In the past two decades, researchers believed that combining polymer and silica is an effective way to enhance the mechanical properties of aerogels [115,116,117,118]. Meanwhile, the results indicate that the interfaces between silica gel particles and polymers also have a great influence on aerogel properties [119]. In 1994, Novak et al. [120] prepared hybrid aerogels via the method of pre-synthesized polymers or in situ polymerizing species, achieving an enhanced flexibility and compressive strength through the adsorption of energy with silica aerogels compounding the polymers [121,122]. As reported in the previous research, polymer-crosslinked aerogels were studied via the following three technical approaches: (a) modifying the surface of nanoparticles to enhance the aerogel skeleton; (b) applying different types of crosslinking agents; (c) creating the network morphology of the aerogels [123,124].
Using the method of chemical crosslinking, the polymer conformally coats the skeletal framework of the aerogels and maintains the original shape of the mesopores to reinforce their mechanical properties. As shown in Figure 5, with the addition of the polymer, the density of the aerogel will show an increasing trend. Through the growth mechanism, the number of connecting points between the secondary particles will increase, and finally resulting in the polymer-reinforcement.
Although the mechanical properties of aerogels have been improved, their density and thermal conductivity have been increased as well, and the addition of the polymers even reduce their resistance to high temperatures [126]. Using well-controlled polymerization techniques, atom transfer radical polymerization can effectively enhance the performance of aerogels without significantly increasing their density. Moreover, the aggregation and poor interfacial interaction can be solved via the combination of silica aerogels and polymer [127]. The reported polymer-reinforced aerogels are listed in Table 2, and the contents are introduced in further detail.

3.1. Epoxide

The mechanical properties of silica aerogels can be enhanced via epoxide. The functional groups of the epoxide can react with the amino groups on the surface of the gel skeleton. Thus, the addition of epoxide in the matrix of the silica aerogel can change its fragile properties and enhance its mechanical properties [151]. Thus, researchers have focused on applying epoxide to improve the mechanical properties of silica aerogels.
Rezaei et al. [128] have shown a new method to prepare the hybrid silica aerogel with the insulative and flexible properties. As shown in Figure 6, the researchers applied an epoxide ring containing the silica precursor and inserted flexible ether groups into the main chain using the method of ring-opening polymerization. The brittleness properties of the silica aerogels were enhanced due to the non-particulate structure. The results demonstrate that the elastic deformation of the aerogel was increased to 15%, and the mechanical properties were proportional to the density. Moreover, the aerogels have superinsulation properties with a thermal conductivity of only 0.0159 W/m K.
Salimian et al. [129] prepared the silica aerogel–epoxy nanocomposites and investigated the fracture and toughening mechanisms. By analyzing the mechanical and thermal properties, the results suggest that the viscosity of the nanocomposite suspension was increased from 1% to 6% with the silica aerogel addition. In addition, the storage modulus, Tg, Young’s modulus, tensile strength, and toughness were increased by 11%, 5 °C, 35%, 62%, and 126%, respectively. As Figure 7 shows, the epoxy polymers are infiltrated into the mesopores of the silica aerogel. The fracture and toughening mechanisms are explained by the (a) crack pinning and deflection and the (b) plastic deformation.
Salimian et al. [130] prepared the epoxy nanocomposites using two different types (hydrophobic and hydrophilic) of silica aerogels. As shown in Figure 8, the ≡Si−O−C≡ bonds are formed between the silica surface and the epoxy polymer network. Using this method, the storage modulus, viscosity, Tg, fracture toughness, and impact strength were enhanced. Moreover, the fracture toughness (Klc) and impact strength increased with the increase in the hydrophobic aerogel content.
Albooyeh et al. [131] studied the influences of silica aerogel on the mechanical, vibrational, and morphological properties of epoxy. The tensile, bending, compressive, dynamic mechanical thermal analyses, and a series of tests were conducted to verify the Euler–Bernoulli beam theory. The results indicate that silica aerogels can effectively reduce the density of materials. Meanwhile, the tensile, flexural, compressive modulus and hardness of the materials significantly increased when the addition of silica aerogel was 4%.
According to the experimental data, the researchers found that the flexibility and robustness of the pore structure can be enhanced via the combination with polymers. Domènech et al. [132] synthesized a porous organic–inorganic hybrid material composed of silica and epoxy resin via a one-pot sol–gel process and subsequent supercritical drying. These results prove that controlling the bridged alkoxide proportions to enhance the mechanical properties of the silica aerogel is feasible and that the strain at 18 N can achieve 80%.
Selay et al. [133] applied silica aerogel powders with ionic liquid as a nanofiller to prepare the nanocomposite. The silica aerogels with ionic liquid possessed a lower density (0.16 g/cm3), higher porosity (93%), and higher thermal stability (400 °C). Moreover, the composites (silica aerogel with 1wt% ionic liquid) demonstrated better mechanical properties, such as modulus of elasticity (4156.27 MPa) and tensile strength (51.96 MPa).
Epoxide is also a common gelation initiator. He et al. [134] applied epoxides as gelation accelerators to prepare the ZrO2–SiO2 aerogels via aging and supercritical drying. The results demonstrate that the epoxides can accelerate the gelation of sol. Considering that the decomposition of the polymer leads to a decrease in the high temperature resistance of aerogels, mullite fibers were introduced as the skeletons for the aerogels. The compressive strength of the M/ZrO2–SiO2 aerogel reached 0.438 MPa and the thermal conductivity was only 0.027 W/m·K. Selver et al. [135] investigated the influences of epoxy on the mechanical, nondestructive, and thermal properties of silica aerogel composites. The results show that the epoxy composites with a 1% addition of silica aerogel exhibited better flexural strength, impact, and energy absorption. However, the thermal conductivity of the 1% silica aerogel composites increased due to the void inside of the epoxy resin being filled up.

3.2. Polyurea

Polyurea consists of aromatic isocyanate segments and soft polyamine chains, which are synthesized via the reaction of the amino compound with the isocyanate component. This material has excellent anti-corrosion, waterproof, and mechanical properties. Polyurea-crosslinked silica-based aerogels have the characteristics of being nano-porous and mesoporous, so they can exhibit unique thermal management behavior.
Fu et al. [152] used the material point method (MPM) to study the mechanical behavior of the silica aerogels whose skeletal framework was coated by the polyurea at high strain rates. The researchers found that the MPM can model the compression of complex mesoporous structures and that the conformal polymer coating has a reinforcing function. The histograms of the distribution of material points versus the stress level at 19% strains is shown in Figure 9; the data show that the model with a 50% porosity has a wider range and exhibits more material points when under stress.
Churu et al. [136] also investigated the mechanical properties of the polymer-crosslinked templated silica aerogel (CTSA). The results show that the 1,3,5-trimethylbenzene (TMB) and triblock copolymer both have influences on the morphology of the aerogels, resulting in a change in the mechanical properties. The researchers further revealed the intrinsic relationship between the morphology and mechanical properties.
Capadona et al. [137] reinforced the silica skeleton via the polymerization of the di-isocyanate using the amine-modified surface of a sol–gel-derived mesoporous silica network. Through the reaction with amines and urea linkages, the polymer was coated to the surface of the aerogel skeleton, which is shown in Figure 10. The results reveal that the highest density crosslinked aerogel had the highest stress at failure, exhibiting the highest modulus and crosslink.
As Figure 11 shows, Yang et al. [138] prepared the modified silica gels using the method of copolymerizing tetraethylorthosilicate with 3-aminopropyltriethoxy-silane. During the ambient pressure, the researchers successfully controlled the shrinkage of the silica aerogels. The experimental data demonstrate that the elastic modulus of the silica aerogel skeleton increases because of the incorporated polymers.

3.3. Polyurethane

Polyurethane (PU) is a typical foaming material, and its high thermal conductivity is an important issue that restricts sustainable development [153]. Based on the previous reports, the polyurethane-based hybrids prepared using the sol–gel approach showed excellent thermal insulating effectiveness and mechanical properties due to the inorganic and organic co-networks [154]. In general, there are a certain amount of hydroxyl groups remaining on the surface of the solid skeleton of the silica wet gel. Therefore, polyurea can form covalent bonds with silica wet gel and enhance the adhesion on the surface of the solid skeleton.
Cho et al. [140] prepared fabricated foldable silica aerogel/polyurethane composites (APCs), and the properties of the composites were theoretically verified via the proposed model. Figure 12 summarizes the morphological analysis of the PU1000 series, the schematic of the PU synthesis, and the fabrication process of the APCs. In Figure 12, the isocyanate-terminated prepolymers were synthesized by reacting with poly(tetramethylene ether glycol) (PTMG) and 2,4-diphenylmethane diisocyanate (MDI). Then, the prepared isocyanate-terminated PTMGs were chain-extended using 1–4 butanediol (BD). The results show that with an aerogel addition of 30%, the thermal conductivity of the APCs decreased by 72%, and the PU with a longer soft segment length demonstrated no breakage after bending.
Verdolotti et al. [139] synthesized organic–inorganic polyurethane-based hybrids, leading to an enhancement of the mechanical properties and thermal insulation via the isocyanate functional groups of IPTS reacted with OH of polyol under urethane bonds, as shown in Figure 13a. The researchers investigated the influences of mechanical behaviors on the aerogel-like siloxane domains. Figure 13b demonstrates the stress–strain curves of the foams (HPURca1 and HPURca2) compared to the pristine PUR. The Young’s modulus of the foams (HPURca1 and HPURca2) achieved 30.17 MPa and 50 MPa. Moreover, the yield strength of these materials achieved 0.093 MPa and 2.15 MPa, respectively.
The researchers found that the flexibility of the hybrid aerogels are enhanced by the long-chain polymer molecules. Based on this theory, Duan et al. [141] prepared the mechanically reinforced hybrid silica aerogels using silane-end-capped urethane prepolymer and chain-extended polyurethane. The synthesis of prepolymer I, II, and polymer I, II are shown in Figure 14 and Figure 15, respectively. The figures mainly explain the silane end groups participating in the silica network formation and the method that controls the amounts of polyurethane added to the aerogel network. The results show that the mechanical properties were enhanced via chain-extended polyurethane and the aerogels can suffer a 70% compressive strain with the addition of polymers.
The PU foams not only have lightness properties, but also demonstrate a continuous solid network, which can be used as reinforcements. Merillas et al. [142] reinforced the silica aerogel composites using reticulated polyurethane (PU) foams via ambient pressure drying and supercritical drying. Hexamethyldisilazane (HMDZ) was used to modify the surface of silica aerogels and the continuous network hybrid aerogel was formed using polyurethane. The results show that the elastic modulus increased from 130 to 450 kPa and the thermal conductivity was as low as 0.014 W/m·K.

3.4. Polyimide

Compared with other organic constituents, the imide ring demonstrates a higher initial decomposition temperature [155]. Moreover, the chain structure constructed by the imide rings also create a polyimide (PI) with superior mechanical properties and a favorable chemical resistance [156]. Therefore, it is feasible to use polyimide to enhance the mechanical properties of silica aerogels.
Tian et al. [143] designed polyimide/silica composite aerogels using an integrated binary network via the in-situ synthesis. As shown in Figure 16a, the polyimide nanofiber aerogel (PINA) is used for the growth of the polymethylsilsesquioxane (PMSQ) network to synthesize the polyimide/silica (PSi) composite aerogel. The composite aerogels demonstrate excellent compressibility and flexibility, recovering from large compression (ε = 60%) and showing no collapsing. Meanwhile, the stress–strain curves (Figure 16b) demonstrate excellent compression recovery properties from 15%, 30%, 45% and 60% strain, respectively. In addition, the thermal conductivity of the polyimide/silica aerogel is as low as 0.0212 W/m·K and shows excellent resistance under 1200 °C.
Kantor et al. [144] synthesized heterogeneous polyimide–silica aerogels with low shrinkage by adding silica aerogel particles into a polyimide sol. The polyimide–silica aerogels exhibited heterogeneous structures and have properties of a high surface area over 609 m2/g and a low thermal conductivity of 0.0175 W/m·K. Compared with general polyimide materials, the composite aerogels demonstrated potential commercial value. As shown in Figure 17, the BTC solution and silica aerogel powders occurred gelation in the polypropylene container, and the samples were obtained through aging and supercritical drying.
Fei et al. [145] prepared the polyimide-crosslinked silica aerogels using different weight percentages of polyimide via the condensation reaction. The thermal conductivity of the aerogel is as low as 0.0306~0.0347 W/m·K, and the relatively high compressive strength can achieve a 1.03~3.82 MPa. Besides using polyimide to crosslink with silica aerogels to enhance their mechanical properties, using polyimide as a reinforcing phase is an efficient technical approach to improve the mechanical properties of silica aerogels. Fei et al. [146] used glass fiber and polyimide (PI) to reinforce the silica aerogel.
Zhang et al. [147] applied the “co-gel” strategy to fabricate the novel silica/polyimide (SiO2/PI) nanocomposites. The SiO2/PI nanocomposite aerogels exhibited excellent mechanical properties due to the hierarchically porous structure, such as a compressive modulus (1.96 MPa) and specific modulus (52.7 m2/s2). Moreover, the materials exhibited excellent flame resistance and low thermal conductivities between 25 °C and 300 °C.

3.5. Polystyrene

Polystyrene (PS) is a non-polar material that can improve the hydrophobic properties of silica aerogels. The silica precursors modified via amine, vinyl, and AIBN were also used to crosslink with PS to prepare the PS-reinforced silica aerogels.
Ilhan et al. [148] designed a new three-dimensional core–shell structure in which the PS was applied as the shell via the method of the free-radical polymerization process. Compared to the composite material prepared via polyurea and epoxy, the PS-crosslinked silica aerogels demonstrate a better hydrophobicity. Moreover, the silica aerogels showed excellent mechanical properties and maintained their integrity, while the thermal conductivity was as low as 0.041 W/m·K.
Maleki et al. [149] applied the growth of grafted polymers from the surface of silica gel to prepare the mechanically reinforced polymer–silica aerogels. The method of surface-initiated reversible addition–fragmentation chain transfer polymerization can significantly improve the compression strength of silica aerogels. Matias et al. [87] used polybutylacrylate (PBA) and polystyrene (PS) to prepare crosslinked flexible, monolithic, and superhydrophobic silica aerogels. Compared with the non-reinforced aerogel, the PBA-reinforced aerogel, MTMS-derived aerogel, and PS-reinforced aerogel demonstrated excellent Young’s modulus values and compression strength, which can reach 91 kPa and 68 kPa, respectively. DeFriend et al. [150] used polystyrene beads to prepare mesoporous silica aerogel to investigate the influences of the surface area and pore volume on mechanical compression. These results demonstrate that the templating agents had a great effect on the compressive strength of the aerogels and that the concentrations were a great factor.

4. Conclusions

In this review, the synthesis chemistry and three main stages were introduced. Then, the process of sol–gel chemistry and the role of aging were also described in detail. The advantages and disadvantages of the three drying methods listed were also carefully analyzed and compared.
The five common polymers used to enhance the mechanical properties of silica aerogels were summarized. In the process of modification, the crosslinking agent has an important function. More importantly, the improvement in the mechanical properties is significantly influenced by process parameters such as time, temperature, and ratio. The linear density and shrinkage of the material increases significantly when the process parameters are not well controlled. This inevitably makes the aerogel lose the original advantages and causes a reduction in the mechanical properties. The silica aerogels’ crosslinked polymers demonstrated good mechanical properties. In particular, polystyrene demonstrated a better performance of hydrophobicity due to its characteristics.

5. Outlook and Prospects

Compared with physical strengthening, the bonding tightness of silica aerogel particles strengthened via polymer crosslinking has the advantages of firmness and reliability. However, some problems still need to be further studied in the field of polymer reinforcement. Researchers should not only focus on the influence of polymers on the thermo-mechanical properties of aerogels during the research process, but should also carefully consider the effects of crosslinked polymers on other aspects of silica aerogels, such as the increase in density due to the addition of polymers, the poor flame retardancy of some polymers, and the difficult aging resistance of hybrid aerogels. The technology of eliminating the negative effects of polymers should be carefully studied. The functional role of silica in various fields should also be given more attention. Polymer crosslinked silica aerogels fully demonstrate the lightweight porous properties of aerogels and the related properties of polymers. Thus, this work will lay a solid and reliable foundation for the future development of multifunctional hybrid aerogels.

Author Contributions

Investigation, W.Z., Q.K., J.J., W.L., F.S. and Z.X.; resources, L.C., Q.K. and L.L.; writing—original draft, W.Z., L.C., Q.K., J.J. and F.S.; writing—review and editing, W.Z., L.C., Q.K., L.L., M.C., W.L., F.S. and Z.X.; supervision, W.Z., L.C., J.J. and M.C.; validation, W.Z., L.C., Q.K., L.L., M.C., J.J., W.L., F.S. and Z.X.; funding acquisition, L.L. and M.C.; project administration, L.L., M.C., J.J., W.L. and Z.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 52004131), the National Natural Science Foundation of China (Grant No. 52204213), the Students’ Scientific Research Training Program of the College of Emergency Management of Jiangsu University (JG-03-07), and the Education Reform Research and Talent Training Project of the College of Emergency Management of Jiangsu University (JG-01-18).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Li, Z.; Cheng, X.; He, S.; Huang, D.; Bi, H.; Yang, H. Preparation of ambient pressure dried MTMS/TEOS co-precursor silica aerogel by adjusting NH4OH concentration. Mater. Lett. 2014, 129, 12–15. [Google Scholar] [CrossRef]
  2. Du, A.; Zhou, B.; Zhang, Z.; Shen, J. A special material or a new state of matter: A review and reconsideration of the aerogel. Materials. 2013, 6, 941–968. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  3. Kim, C.H.J.; Zhao, D.; Lee, G.; Liu, J. Strong, machinable carbon aerogels for high performance supercapacitors. Adv. Funct. Mater. 2016, 26, 4976–4983. [Google Scholar] [CrossRef]
  4. Hrubesh, L.W.; Coronado, P.R.; Satcher, J.H. Solvent removal from water with hydrophobic aerogels. J. Non-Cryst. Solids 2001, 285, 328–332. [Google Scholar] [CrossRef]
  5. Su, L.; Niu, M.; Lu, D.; Cai, Z.; Li, M.; Wang, H. A review on the emerging resilient and multifunctional ceramic aerogels. J. Mater. Sci. Technol. 2021, 75, 1–13. [Google Scholar] [CrossRef]
  6. Shen, J.; Zhang, P.; Song, L.; Li, J.; Ji, B.; Li, J.; Chen, L. Polyethylene glycol supported by phosphorylated polyvinyl alcohol/graphene aerogel as a high thermal stability phase change material. Compos. Part B Eng. 2019, 179, 107545. [Google Scholar] [CrossRef]
  7. Yan, M.; Pan, Y.; Cheng, X.; Zhang, Z.; Deng, Y.; Lun, Z.; Gong, L.; Gao, M.; Zhang, H. “Robust–Soft” anisotropic nanofibrillated cellulose aerogels with superior mechanical, flame-retardant, and thermal insulating properties. ACS Appl. Mater. Interfaces 2021, 13, 27458–27470. [Google Scholar] [CrossRef]
  8. Cuce, E.; Cuce, P.M.; Wood, C.J.; Riffat, S.B. Toward aerogel based thermal superinsulation in buildings: A comprehensive review. Renew. Sustain. Energy Rev. 2014, 34, 273–299. [Google Scholar] [CrossRef]
  9. Wa, L.; Fengyun, L.; Fanlu, Z.; Mengjing, C.; Qiang, C.; Jue, H.; Weijun, Z.; Mingwei, M. Preparation of silica aerogels using CTAB/SDS as template and their efficient adsorption. Appl. Surf. Sci. 2015, 353, 1031–1036. [Google Scholar] [CrossRef]
  10. Sun, H.; Xu, Z.; Gao, C. Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels. Adv. Mater. 2013, 25, 2554–2560. [Google Scholar] [CrossRef]
  11. Long, D.; Chen, Q.; Qiao, W.; Zhan, L.; Liang, X.; Ling, L. Three-dimensional mesoporous carbon aerogels: Ideal catalyst supports for enhanced H2S oxidation. Chem. Commun. 2009, 3898–3900. [Google Scholar] [CrossRef]
  12. Lee, J.K.; Gould, G.L.; Rhine, W. Polyurea based aerogel for a high performance thermal insulation material. J. Sol-Gel Sci. Technol. 2009, 49, 209–220. [Google Scholar] [CrossRef]
  13. Hou, X.; Mao, Y.; Zhang, R.; Fang, D. Super-flexible polyimide nanofiber cross-linked polyimide aerogel membranes for high efficient flexible thermal protection. Chem. Eng. J. 2021, 417, 129341. [Google Scholar] [CrossRef]
  14. Merillas, B.; Villafañe, F.; Rodríguez-Pérez, M.Á. Improving the insulating capacity of polyurethane foams through polyurethane aerogel inclusion: From insulation to superinsulation. Nanomaterials 2022, 12, 2232. [Google Scholar] [CrossRef]
  15. Kulkarni, A.; Jana, S.C. Surfactant-free syndiotactic polystyrene aerogel foams via pickering emulsion. Polymer 2021, 212, 123125. [Google Scholar] [CrossRef]
  16. Wu, Y.; Wang, X.; Shen, J. Metal oxide aerogels for high-temperature applications. J. Sol-Gel Sci. Technol. 2023, 106, 360–380. [Google Scholar] [CrossRef]
  17. Liao, J.; Liu, P.; Xie, Y.; Zhang, Y. Metal oxide aerogels: Preparation and application for the uranium removal from aqueous solution. Sci. Total Environ. 2021, 768, 144212. [Google Scholar] [CrossRef] [PubMed]
  18. Gao, W.; Wen, D. Recent advances of noble metal aerogels in biosensing. View 2021, 2, 20200124. [Google Scholar] [CrossRef]
  19. Nita, L.E.; Ghilan, A.; Rusu, A.G.; Neamtu, I.; Chiriac, A.P. New trends in bio-based aerogels. Pharmaceutics 2020, 12, 449. [Google Scholar] [CrossRef]
  20. Paninho, A.B.; Mustapa, A.N.; Mahmudov, K.T.; Pombeiro, A.J.L.; Guedes Da Silva, M.F.C.; Bermejo, M.D.; Martín, Á.; Cocero, M.J.; Nunes, A.V.M. A bio-based alginate aerogel as an ionic liquid support for the efficient synthesis of cyclic carbonates from CO2 and epoxides. Catalysts 2021, 11, 872. [Google Scholar] [CrossRef]
  21. Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L. Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide. Nat. Nanotechnol. 2015, 10, 277–283. [Google Scholar] [CrossRef] [PubMed]
  22. Kettunen, M.; Silvennoinen, R.J.; Houbenov, N.; Nykänen, A.; Ruokolainen, J.; Sainio, J.; Pore, V.; Kemell, M.; Ankerfors, M.; Lindström, T.; et al. Photoswitchable superabsorbency based on nanocellulose aerogels. Adv. Funct. Mater. 2011, 21, 510–517. [Google Scholar] [CrossRef]
  23. Sajab, M.S.; Chia, C.H.; Chan, C.H.; Zakaria, S.; Kaco, H.; Chook, S.W.; Chin, S.X.; Noor, A.A.M. Bifunctional graphene oxide–cellulose nanofibril aerogel loaded with Fe(iii) for the removal of cationic dye via simultaneous adsorption and fenton oxidation. Rsc Adv. 2016, 6, 19819–19825. [Google Scholar] [CrossRef]
  24. Maleki, H.; Durães, L.; García-González, C.A.; Del Gaudio, P.; Portugal, A.; Mahmoudi, M. Synthesis and biomedical applications of aerogels: Possibilities and challenges. Adv. Colloid Interface Sci. 2016, 236, 1–27. [Google Scholar] [CrossRef]
  25. Maleki, H. Recent advances in aerogels for environmental remediation applications: A review. Chem. Eng. J. 2016, 300, 98–118. [Google Scholar] [CrossRef]
  26. Ren, J.; Feng, J.; Wang, L.; Chen, G.; Zhou, Z.; Li, Q. High specific surface area hybrid silica aerogel containing POSS. Microporous Mesoporous Mater. 2021, 310, 110456. [Google Scholar] [CrossRef]
  27. Schultz, J.M.; Jensen, K.I.; Kristiansen, F.H. Super insulating aerogel glazing. Sol. Energy Mater. Sol. Cells 2005, 89, 275–285. [Google Scholar] [CrossRef]
  28. Gurav, J.L.; Jung, I.-K.; Park, H.-H.; Kang, E.S.; Nadargi, D.Y. Silica aerogel: Synthesis and applications. J. Nanomater. 2010, 2010, 409310. [Google Scholar] [CrossRef] [Green Version]
  29. Long, H.; Harley-Trochimczyk, A.; Pham, T.; Tang, Z.; Shi, T.; Zettl, A.; Carraro, C.; Worsley, M.A.; Maboudian, R. High surface area MoS2/graphene hybrid aerogel for ultrasensitive NO2 detection. Adv. Funct. Mater. 2016, 26, 5158–5165. [Google Scholar] [CrossRef] [Green Version]
  30. Kistler, S.S. Coherent expanded aerogels and jellies. Nature 1931, 127, 741. [Google Scholar] [CrossRef]
  31. Kistler, S.S. Coherent Expanded-Aerogels. J. Phys. Chem. 1932, 36, 52–64. [Google Scholar] [CrossRef]
  32. Zhao, C.; Yan, Y.; Hu, Z.; Li, L.; Fan, X. Preparation and characterization of granular silica aerogel/polyisocyanurate rigid foam composites. Constr. Build. Mater. 2015, 93, 309–316. [Google Scholar] [CrossRef]
  33. Baetens, R.; Jelle, B.P.; Gustavsen, A. Aerogel insulation for building applications: A state-of-the-art review. Energy Build. 2011, 43, 761–769. [Google Scholar] [CrossRef] [Green Version]
  34. Bheekhun, N.; Abu Talib, A.R.; Hassan, M.R. Aerogels in aerospace: An overview. Adv. Mater. Sci. Eng. 2013, 2013, 406065. [Google Scholar] [CrossRef] [Green Version]
  35. Jones, S.M. Aerogel: Space exploration applications. J. Sol-Gel Sci. Technol. 2006, 40, 351–357. [Google Scholar] [CrossRef]
  36. Ochoa, M.; Durães, L.; Beja, A.M.; Portugal, A. Study of the suitability of silica based xerogels synthesized using ethyltrimethoxysilane and/or methyltrimethoxysilane precursors for aerospace applications. J. Sol-Gel Sci. Technol. 2012, 61, 151–160. [Google Scholar] [CrossRef]
  37. Mohammadi, A.; Moghaddas, J. Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. 2015, 94, 475–484. [Google Scholar] [CrossRef]
  38. Perdigoto, M.L.N.; Martins, R.C.; Rocha, N.; Quina, M.J.; Gando-Ferreira, L.; Patrício, R.; Durães, L. Application of hydrophobic silica based aerogels and xerogels for removal of toxic organic compounds from aqueous solutions. J. Colloid Interface Sci. 2012, 380, 134–140. [Google Scholar] [CrossRef]
  39. He, S.; Du, C.; Sheng, H.; He, C.; Liu, X.; Jin, X.; Chen, Q.; Tian, F. Ultrasensitive and self-powered multiparameter pressure–temperature–humidity sensor based on ultra-flexible conductive silica aerogel. Gels 2023, 9, 162. [Google Scholar] [CrossRef]
  40. Feng, X.; Wang, L.; Yao, X.; Dong, H.; Wang, X.; Wang, Y. Trace water/amino-modified silica aerogel catalytic system in the one-pot sequential reaction of benzaldehyde dimethyl acetal and nitromethane. Catal. Commun. 2017, 90, 106–110. [Google Scholar] [CrossRef]
  41. Ward, D.A.; Ko, E.I. Cheminform abstract: Preparing catalytic materials by the sol-gel method. Cheminform 1995, 26, 421–433. [Google Scholar] [CrossRef]
  42. Ulker, Z.; Erkey, C. An emerging platform for drug delivery: Aerogel based systems. J. Control. Release 2014, 177, 51–63. [Google Scholar] [CrossRef]
  43. Shang, L.; Lyu, Y.; Han, W. Microstructure and thermal insulation property of silica composite aerogel. Materials 2019, 12, 993. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  44. Madyan, O.A.; Fan, M.; Feo, L.; Hui, D. Physical properties of clay aerogel composites: An overview. Compos. Part B Eng. 2016, 102, 29–37. [Google Scholar] [CrossRef]
  45. Leventis, N.; Sotiriou-Leventis, C.; Zhang, G.; Rawashdeh, A.-M.M. Nanoengineering strong silica aerogels. Nano Lett. 2002, 2, 957–960. [Google Scholar] [CrossRef]
  46. Bertino, M.F.; Hund, J.F.; Zhang, G.; Sotiriou-Leventis, C.; Tokuhiro, A.T.; Leventis, N. Room temperature synthesis of noble metal clusters in the mesopores of mechanically strong silica-polymer aerogel composites. J. Sol-Gel Sci. Technol. 2004, 30, 43–48. [Google Scholar] [CrossRef]
  47. Ma, H.-S.; Roberts, A.P.; Prévost, J.-H.; Jullien, R.; Scherer, G.W. Mechanicalstructure-propertyrelationshipofaerogels. J. Non-Cryst. Solids 2000, 277, 127–141. [Google Scholar] [CrossRef]
  48. Meti, P.; Mahadik, D.B.; Lee, K.-Y.; Wang, Q.; Kanamori, K.; Gong, Y.-D.; Park, H.-H. Overview of organic–inorganic hybrid silica aerogels: Progress and perspectives. Mater. Des. 2022, 222, 111091. [Google Scholar] [CrossRef]
  49. Parmar, K.R.; Dora, D.T.K.; Pant, K.K.; Roy, S. An ultra-light flexible aerogel-based on methane derived CNTs as a reinforcing agent in silica-CMC matrix for efficient oil adsorption. J. Hazard. Mater. 2019, 375, 206–215. [Google Scholar] [CrossRef]
  50. Yi, Z.; Zhang, X.; Yan, L.; Huyan, X.; Zhang, T.; Liu, S.; Guo, A.; Liu, J.; Hou, F. Super-insulated, flexible, and high resilient mullite fiber reinforced silica aerogel composites by interfacial modification with nanoscale mullite whisker. Compos. Part B Eng. 2022, 230, 109549. [Google Scholar] [CrossRef]
  51. Guo, H.; Meador, M.A.B.; Mccorkle, L.; Quade, D.J.; Guo, J.; Hamilton, B.; Cakmak, M.; Sprowl, G. Polyimide aerogels cross-linked through amine functionalized polyoligomeric silsesquioxane. ACS Appl. Mater. Interfaces 2011, 3, 546–552. [Google Scholar] [CrossRef] [PubMed]
  52. Fan, Q.; Ou, R.; Hao, X.; Deng, Q.; Liu, Z.; Sun, L.; Zhang, C.; Guo, C.; Bai, X.; Wang, Q. Water-induced self-assembly and in situ mineralization within plant phenolic glycol-gel toward ultrastrong and multifunctional thermal insulating aerogels. Acs Nano 2022, 16, 9062–9076. [Google Scholar] [CrossRef] [PubMed]
  53. Brinker, C.J.; Keefer, K.D.; Schaefer, D.W.; Ashley, C.S. Sol-gel transition in simple silicates. J. Non-Cryst. Solids 1982, 48, 47–64. [Google Scholar] [CrossRef]
  54. Maleki, H.; Durães, L.; Portugal, A. An overview on silica aerogels synthesis and different mechanical reinforcing strategies. J. Non-Cryst. Solids 2014, 385, 55–74. [Google Scholar] [CrossRef] [Green Version]
  55. Yang, M.; Li, J.; Man, Y.; Peng, Z.; Zhang, X.; Luo, X. A three-dimensional mullite-whisker network ceramic with ultra-light weight and high-strength prepared by the foam-gelcasting method. J. Asian Ceram. Soc. 2020, 8, 387–395. [Google Scholar] [CrossRef]
  56. Borzęcka, N.H.; Nowak, B.; Gac, J.M.; Głaz, T.; Bojarska, M. Kinetics of mtms-based aerogel formation by the sol-gel method—experimental results and theoretical description. J. Non-Cryst. Solids 2020, 547, 120310. [Google Scholar] [CrossRef]
  57. Rao, A.P.; Rao, A.V.; Gurav, J.L. Effect of protic solvents on the physical properties of the ambient pressure dried hydrophobic silica aerogels using sodium silicate precursor. J. Porous Mater. 2008, 15, 507–512. [Google Scholar] [CrossRef]
  58. Xia, B.; Wang, Z.; Gou, L.; Zhang, M.; Guo, M. Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis, structure, and performance. Ceram. Int. 2022, 48, 10472–10479. [Google Scholar] [CrossRef]
  59. Rao, A.V.; Kulkarni, M.M.; Amalnerkar, D.P.; Seth, T. Superhydrophobic silica aerogels based on methyltrimethoxysilane precursor. J. Non-Cryst. Solids 2003, 330, 187–195. [Google Scholar] [CrossRef]
  60. García-González, C.A.; Alnaief, M.; Smirnova, I. Polysaccharide-based aerogels-promising biodegradable carriers for drug delivery systems. Carbohydr. Polym. 2011, 86, 1425–1438. [Google Scholar] [CrossRef]
  61. Pierre, A.C.; Pajonk, G.M. Chemistry of aerogels and their applications. Chem. Rev. 2002, 102, 4243–4266. [Google Scholar] [CrossRef]
  62. Zarzycki, J. Past and present of sol-gel science and technology. J. Sol-Gel Sci. Technol. 1997, 8, 17–22. [Google Scholar] [CrossRef]
  63. Alié, C.; Ferauche, F.; Pirard, R.; Lecloux, A.J.; Pirard, J.-P. Preparation of low-density xerogels by incorporation of additives during synthesis. J. Non-Cryst. Solids 2001, 289, 88–96. [Google Scholar] [CrossRef]
  64. Wu, G.; Wang, J.; Shen, J.; Yang, T.; Zhang, Q.; Zhou, B.; Deng, Z.; Bin, F.; Zhou, D.; Zhang, F. Properties of sol-gel derived scratch-resistant nano-porous silica films by a mixed atmosphere treatment. J. Non-Cryst. Solids 2000, 275, 169–174. [Google Scholar] [CrossRef]
  65. Venkateswara Rao, A.; Bhagat, S.D. Synthesis and physical properties of TEOS-based silica aerogels prepared by two step (acid–base) sol–gel process. Solid State Sci. 2004, 6, 945–952. [Google Scholar] [CrossRef]
  66. Kirkbir, F.; Murata, H.; Meyers, D.; Chaudhuri, S.R.; Sarkar, A. Drying and sintering of sol-gel derived large SiO2 monoliths. J. Sol-Gel Sci. Technol. 1996, 6, 203–217. [Google Scholar] [CrossRef]
  67. Harlick, P.J.E.; Sayari, A. Applications of pore-expanded mesoporous silicas. 3. Triamine silane grafting for enhanced co2 adsorption. Ind. Eng. Chem. Res. 2006, 45, 3248–3255. [Google Scholar] [CrossRef]
  68. Araki, S.; Doi, H.; Sano, Y.; Tanaka, S.; Miyake, Y. Preparation and CO2 adsorption properties of aminopropyl-functionalized mesoporous silica microspheres. J. Colloid Interface Sci. 2009, 339, 382–389. [Google Scholar] [CrossRef]
  69. Lee, C.J.; Kim, G.S.; Hyun, S.H. Synthesis of silica aerogels from waterglass via new modified ambient drying. J. Mater. Sci. 2002, 37, 2237–2241. [Google Scholar] [CrossRef]
  70. Siouffi, A.-M. Silica gel-based monoliths prepared by the sol-gel method: Facts and figures. J. Chromatogr. A 2003, 1000, 801–818. [Google Scholar] [CrossRef]
  71. Li, Q.; Afeworki, M.; Callen, N.M.; Colby, R.J.; Gopinadhan, M.; Nines Kochersperger, M.L.; Peterson, B.K.; Sansone, M.; Weston, S.C.; Calabro, D.C. Template-free self-assembly of mesoporous organosilicas. Chem. Mater. 2018, 30, 2218–2228. [Google Scholar] [CrossRef]
  72. Nakanishi, K.; Minakuchi, H.; Soga, N.; Tanaka, N. Double pore silica gel monolith applied to liquid chromatography. J. Sol-Gel Sci. Technol. 1997, 8, 547–552. [Google Scholar] [CrossRef]
  73. Tamon, H.; Kitamura, T.; Okazaki, M. Preparation of silica aerogel from TEOS. J. Colloid Interface Sci. 1998, 197, 353–359. [Google Scholar] [CrossRef]
  74. Wong, J.C.H.; Kaymak, H.; Brunner, S.; Koebel, M.M. Mechanical properties of monolithic silica aerogels made from polyethoxydisiloxanes. Microporous Mesoporous Mater. 2014, 183, 23–29. [Google Scholar] [CrossRef]
  75. Nguyen, B.N.; Meador, M.A.B.; Tousley, M.E.; Shonkwiler, B.; Mccorkle, L.; Scheiman, D.A.; Palczer, A. Tailoring elastic properties of silica aerogels cross-linked with polystyrene. ACS Appl. Mater. Interfaces 2009, 1, 621–630. [Google Scholar] [CrossRef]
  76. Nadargi, D.Y.; Latthe, S.S.; Hirashima, H.; Rao, A.V. Studies on rheological properties of methyltriethoxysilane (MTES) based flexible superhydrophobic silica aerogels. Microporous Mesoporous Mater. 2009, 117, 617–626. [Google Scholar] [CrossRef]
  77. Bhagat, S.D.; Rao, A.V. Surface chemical modification of TEOS based silica aerogels synthesized by two step (acid–base) sol–gel process. Appl. Surf. Sci. 2006, 252, 4289–4297. [Google Scholar] [CrossRef]
  78. Zu, G.; Kanamori, K.; Shimizu, T.; Zhu, Y.; Maeno, A.; Kaji, H.; Nakanishi, K.; Shen, J. Versatile double-cross-linking approach to transparent, machinable, supercompressible, highly bendable aerogel thermal superinsulators. Chem. Mater. 2018, 30, 2759–2770. [Google Scholar] [CrossRef] [Green Version]
  79. Hæreid, S.; Dahle, M.; Lima, S.; Einarsrud, M.-A. Preparation and properties of monolithic silica xerogels from TEOS–based alcogels aged in silane solutions. J. Non-Cryst. Solids 1995, 186, 96–103. [Google Scholar] [CrossRef]
  80. Tajiri, K.; Igarashi, K.; Nishio, T. Effects of supercritical drying media on structure and properties of silica aerogel. J. Non-Cryst. Solids 1995, 186, 83–87. [Google Scholar] [CrossRef]
  81. Nguyen, T.H.; Mai, N.T.; Reddy, V.R.M.; Jung, J.H.; Truong, N.T.N. Synthesis of silica aerogel particles and its application to thermal insulation paint. Korean J. Chem. Eng. 2020, 37, 1803–1809. [Google Scholar] [CrossRef]
  82. Yuan, X.; Xu, L.; Pan, H.; Shen, Y.; Wang, L.; Xie, M. Eco-friendly approach for preparation of water-based superhydrophobic silica aerogels via ambient pressure drying. Mater. Res. Express 2021, 8, 015021. [Google Scholar] [CrossRef]
  83. Shao, Z.; He, X.; Cheng, X.; Zhang, Y. A simple facile preparation of methyltriethoxysilane based flexible silica aerogel monoliths. Mater. Lett. 2017, 204, 93–96. [Google Scholar] [CrossRef]
  84. Lamy-Mendes, A.; Malfait, W.J.; Sadeghpour, A.; Girão, A.V.; Silva, R.F.; Durães, L. Influence of 1D and 2D carbon nanostructures in silica-based aerogels. Carbon 2021, 180, 146–162. [Google Scholar] [CrossRef]
  85. Wu, X.; Man, J.; Liu, S.; Huang, S.; Lu, J.; Tai, J.; Zhong, Y.; Shen, X.; Cui, S.; Chen, X. Isocyanate-crosslinked silica aerogel monolith with low thermal conductivity and much enhanced mechanical properties: Fabrication and analysis of forming mechanisms. Ceram. Int. 2021, 47, 26668–26677. [Google Scholar] [CrossRef]
  86. Li, S.; Zhang, L.; Li, J.; Wu, Z.; Yang, C. Silica nanowires reinforced self-hydrophobic silica aerogel derived from crosslinking of propyltriethoxysilane and tetraethoxysilane. J. Sol-Gel Sci. Technol. 2017, 83, 545–554. [Google Scholar] [CrossRef]
  87. Matias, T.; Varino, C.; de Sousa, H.C.; Braga, M.E.M.; Portugal, A.; Coelho, J.F.J.; Durães, L. Novel flexible, hybrid aerogels with vinyl- and methyltrimethoxysilane in the underlying silica structure. J. Mater. Sci. 2016, 51, 6781–6792. [Google Scholar] [CrossRef]
  88. Yang, Z.; Yu, H.; Li, X.; Ding, H.; Ji, H. Hyperelastic and hydrophobic silica aerogels with enhanced compressive strength by using VTES/MTMS as precursors. J. Non-Cryst. Solids 2019, 525, 119677. [Google Scholar] [CrossRef]
  89. He, S.; Sun, G.; Cheng, X.; Dai, H.; Chen, X. Nanoporous SiO2 grafted aramid fibers with low thermal conductivity. Compos. Sci. Technol. 2017, 146, 91–98. [Google Scholar] [CrossRef]
  90. Guo, H.; Nguyen, B.N.; Mccorkle, L.S.; Shonkwiler, B.; Meador, M.A.B. Elastic low density aerogels derived from bis[3-(triethoxysilyl)propyl]disulfide, tetramethylorthosilicate and vinyltrimethoxysilane via a two-step process. J. Mater. Chem. 2009, 19, 9054–9062. [Google Scholar] [CrossRef]
  91. Meador, M.A.B.; Weber, A.S.; Hindi, A.; Naumenko, M.; Mccorkle, L.; Quade, D.; Vivod, S.L.; Gould, G.L.; White, S.; Deshpande, K. Structure-property relationships in porous 3d nanostructures: Epoxy-cross-linked silica aerogels produced using ethanol as the solvent. ACS Appl. Mater. Interfaces 2009, 1, 894–906. [Google Scholar] [CrossRef] [PubMed]
  92. Randall, J.P.; Meador, M.A.B.; Jana, S.C. Polymer reinforced silica aerogels: Effects of dimethyldiethoxysilane and bis(trimethoxysilylpropyl) amine as silane precursors. J. Mater. Chem. A 2013, 1, 6642–6652. [Google Scholar] [CrossRef]
  93. Zhang, Y.; Shen, Q.; Li, X.; Wang, L.; Nie, C. Facile preparation of a phenyl-reinforced flexible silica aerogel with excellent thermal stability and fire resistance. Mater. Chem. Front. 2021, 5, 4214–4224. [Google Scholar] [CrossRef]
  94. Estella, J.; Echeverría, J.C.; Laguna, M.; Garrido, J.J. Effects of aging and drying conditions on the structural and textural properties of silica gels. Microporous Mesoporous Mater. 2007, 102, 274–282. [Google Scholar] [CrossRef]
  95. Hong, J.-Y.; Yun, S.; Wie, J.J.; Zhang, X.; Dresselhaus, M.S.; Kong, J.; Park, H.S. Cartilage-inspired superelastic ultradurable graphene aerogels prepared by the selective gluing of intersheet joints. Nanoscale 2016, 8, 12900–12909. [Google Scholar] [CrossRef]
  96. Einarsrud, M.-A.; Haereid, S. Preparation of transparent, monolithic silica xerogels with low density. J. Sol-Gel Sci. Technol. 1994, 2, 903–906. [Google Scholar] [CrossRef]
  97. Omranpour, H.; Motahari, S. Effects of processing conditions on silica aerogel during aging: Role of solvent, time and temperature. J. Non-Cryst. Solids 2013, 379, 7–11. [Google Scholar] [CrossRef]
  98. He, F.; Zhao, H.; Qu, X.; Zhang, C.; Qiu, W. Modified aging process for silica aerogel. J. Mater. Process. Technol. 2009, 209, 1621–1626. [Google Scholar] [CrossRef]
  99. Rashid, A.B.; Shishir, S.I.; Mahfuz, M.A.; Hossain, M.T.; Hoque, M.E. Silica aerogel: Synthesis, characterization, applications, and recent advancements. Part. Part. Syst. Charact. 2023, 40, 2200186. [Google Scholar] [CrossRef]
  100. Wu, G.-P.; Yang, J.; Wang, D.; Xu, R.; Amine, K.; Lu, C.-X. A novel route for preparing mesoporous carbon aerogels using inorganic templates under ambient drying. Mater. Lett. 2014, 115, 1–4. [Google Scholar] [CrossRef]
  101. Leventis, N.; Palczer, A.; Mccorkle, L.; Zhang, G.; Sotiriou-Leventis, C. Nanoengineered silica-polymer composite aerogels with no need for supercritical fluid drying. J. Sol-Gel Sci. Technol. 2005, 35, 99–105. [Google Scholar] [CrossRef]
  102. Chen, H.-B.; Hollinger, E.; Wang, Y.-Z.; Schiraldi, D.A. Facile fabrication of poly(vinyl alcohol) gels and derivative aerogels. Polymer 2014, 55, 380–384. [Google Scholar] [CrossRef]
  103. Chen, H.-B.; Liu, B.; Huang, W.; Wang, J.-S.; Zeng, G.; Wu, W.-H.; Schiraldi, D.A. Fabrication and properties of irradiation-cross-linked poly(vinyl alcohol)/clay aerogel composites. ACS Appl. Mater. Interfaces 2014, 6, 16227–16236. [Google Scholar] [CrossRef] [PubMed]
  104. Tang, Q.; Wang, T. Preparation of silica aerogel from rice hull ash by supercritical carbon dioxide drying. J. Supercrit. Fluids 2005, 35, 91–94. [Google Scholar] [CrossRef]
  105. Maleki, H.; Durães, L.; Portugal, A. Development of mechanically strong ambient pressure dried silica aerogels with optimized properties. J. Phys. Chem. C 2015, 119, 7689–7703. [Google Scholar] [CrossRef]
  106. Zuo, L.; Zhang, Y.; Zhang, L.; Miao, Y.-E.; Fan, W.; Liu, T. Polymer/carbon-based hybrid aerogels: Preparation, properties and applications. Materials 2015, 8, 6806–6848. [Google Scholar] [CrossRef] [Green Version]
  107. Zhu, J.; Xie, J.; Lü, X.; Jiang, D. Synthesis and characterization of superhydrophobic silica and silica/titania aerogels by sol–gel method at ambient pressure. Colloids Surf. A Physicochem. Eng. Asp. 2009, 342, 97–101. [Google Scholar] [CrossRef]
  108. Ślosarczyk, A.; Barełkowski, M.; Niemier, S.; Jakubowska, P. Synthesis and characterisation of silica aerogel/carbon microfibers nanocomposites dried in supercritical and ambient pressure conditions. J. Sol-Gel Sci. Technol. 2015, 76, 227–232. [Google Scholar] [CrossRef] [Green Version]
  109. Błaszczyński, T.; Ślosarczyk, A.; Morawski, M. Synthesis of silica aerogel by supercritical drying method. Procedia Eng. 2013, 57, 200–206. [Google Scholar] [CrossRef] [Green Version]
  110. Meador, M.A.B.; Alemán, C.R.; Hanson, K.; Ramirez, N.; Vivod, S.L.; Wilmoth, N.; Mccorkle, L. Polyimide aerogels with amide cross-links: A low cost alternative for mechanically strong polymer aerogels. ACS Appl. Mater. Interfaces 2015, 7, 1240–1249. [Google Scholar] [CrossRef] [Green Version]
  111. Ku, H.; Wang, H.; Pattarachaiyakoop, N.; Trada, M. A review on the tensile properties of natural fiber reinforced polymer composites. Compos. Part B Eng. 2011, 42, 856–873. [Google Scholar] [CrossRef] [Green Version]
  112. Boday, D.J.; Stover, R.J.; Muriithi, B.; Keller, M.W.; Wertz, J.T.; Defriend Obrey, K.A.; Loy, D.A. Strong, low-density nanocomposites by chemical vapor deposition and polymerization of cyanoacrylates on aminated silica aerogels. ACS Appl. Mater. Interfaces 2009, 1, 1364–1369. [Google Scholar] [CrossRef]
  113. Li, X.; Wang, J.; Zhao, Y.; Zhang, X. Superhydrophobic polyimide aerogels via conformal coating strategy with excellent underwater performances. J. Appl. Polym. Sci. 2020, 137, 48849. [Google Scholar] [CrossRef]
  114. Kim, H.S.; Jang, J.-U.; Lee, H.; Kim, S.Y.; Kim, S.H.; Kim, J.; Jung, Y.C.; Yang, B.J. Thermal management in polymer composites: A review of physical and structural parameters. Adv. Eng. Mater. 2018, 20, 1800204. [Google Scholar] [CrossRef]
  115. Mahadik, D.B.; Jung, H.-N.; Han, W.; Cho, H.H.; Park, H.-H. Flexible, elastic, and superhydrophobic silica-polymer composite aerogels by high internal phase emulsion process. Compos. Sci. Technol. 2017, 147, 45–51. [Google Scholar] [CrossRef]
  116. Maleki, H.; Durães, L.; Portugal, A. A new trend for development of mechanically robust hybrid silica aerogels. Mater. Lett. 2016, 179, 206–209. [Google Scholar] [CrossRef]
  117. Wei, T.-Y.; Lu, S.-Y.; Chang, Y.-C. Transparent, hydrophobic composite aerogels with high mechanical strength and low high-temperature thermal conductivities. J. Phys. Chem. B 2008, 112, 11881–11886. [Google Scholar] [CrossRef]
  118. Meador, M.A.B.; Vivod, S.L.; Mccorkle, L.; Quade, D.; Sullivan, R.M.; Ghosn, L.J.; Clark, N.; Capadona, L.A. Reinforcing polymer cross-linked aerogels with carbon nanofibers. J. Mater. Chem. 2008, 18, 1843–1852. [Google Scholar] [CrossRef]
  119. Kim, M.; Eo, K.; Lim, H.J.; Kwon, Y.K. Low shrinkage, mechanically strong polyimide hybrid aerogels containing hollow mesoporous silica nanospheres. Compos. Sci. Technol. 2018, 165, 355–361. [Google Scholar] [CrossRef]
  120. Novak, B.M.; Auerbach, D.; Verrier, C. Low-density, mutually interpenetrating organic-inorganic composite materials via supercritical drying techniques. Chem. Mater. 1994, 6, 282–286. [Google Scholar] [CrossRef]
  121. Leventis, N.; Sadekar, A.; Chandrasekaran, N.; Sotiriou-Leventis, C. Click synthesis of monolithic silicon carbide aerogels from polyacrylonitrile-coated 3d silica networks. Chem. Mater. 2010, 22, 2790–2803. [Google Scholar] [CrossRef]
  122. Guise, M.T.; Hosticka, B.; Earp, B.C.; Norris, P.M. An experimental investigation of aerosol collection utilizing packed beds of silica aerogel microspheres. J. Non-Cryst. Solids 2001, 285, 317–322. [Google Scholar] [CrossRef]
  123. Boday, D.J.; Keng, P.Y.; Muriithi, B.; Pyun, J.; Loy, D.A. Mechanically reinforced silica aerogel nanocomposites via surface initiated atom transfer radical polymerizations. J. Mater. Chem. 2010, 20, 6863–6865. [Google Scholar] [CrossRef]
  124. Meador, M.A.B.; Capadona, L.A.; Mccorkle, L.; Papadopoulos, D.S.; Leventis, N. Structure-property relationships in porous 3d nanostructures as a function of preparation conditions:  isocyanate cross-linked silica aerogels. Chem. Mater. 2007, 19, 2247–2260. [Google Scholar] [CrossRef] [Green Version]
  125. Randall, J.P.; Meador, M.A.B.; Jana, S.C. Tailoring mechanical properties of aerogels for aerospace applications. ACS Appl. Mater. Interfaces 2011, 3, 613–626. [Google Scholar] [CrossRef]
  126. Rezaei, S.; Jalali, A.; Zolali, A.M.; Alshrah, M.; Karamikamkar, S.; Park, C.B. Robust, ultra-insulative and transparent polyethylene-based hybrid silica aerogel with a novel non-particulate structure. J. Colloid Interface Sci. 2019, 548, 206–216. [Google Scholar] [CrossRef]
  127. Lenarda, M.; Chessa, G.; Moretti, E.; Polizzi, S.; Storaro, L.; Talon, A. Toward the preparation of a nanocomposite material through surface initiated controlled/“living” radical polymerization of styrene inside the channels of mcm-41 silica. J. Mater. Sci. 2006, 41, 6305–6312. [Google Scholar] [CrossRef]
  128. Rezaei, S.; Zolali, A.M.; Jalali, A.; Park, C.B. Novel and simple design of nanostructured, super-insulative and flexible hybrid silica aerogel with a new macromolecular polyether-based precursor. J. Colloid Interface Sci. 2020, 561, 890–901. [Google Scholar] [CrossRef]
  129. Salimian, S.; Malfait, W.J.; Zadhoush, A.; Talebi, Z.; Naeimirad, M. Fabrication and evaluation of silica aerogel-epoxy nanocomposites: Fracture and toughening mechanisms. Theor. Appl. Fract. Mech. 2018, 97, 156–164. [Google Scholar] [CrossRef]
  130. Salimian, S.; Zadhoush, A.; Talebi, Z.; Fischer, B.; Winiger, P.; Winnefeld, F.; Zhao, S.; Barbezat, M.; Koebel, M.M.; Malfait, W.J. Silica aerogel-epoxy nanocomposites: Understanding epoxy reinforcement in terms of aerogel surface chemistry and epoxy–silica interface compatibility. ACS Appl. Nano Mater. 2018, 1, 4179–4189. [Google Scholar] [CrossRef]
  131. Albooyeh, A.; Bayat, M.; Rafieian, P.; Dadrasi, A.; Khatibi, M.M. Silica aerogel/epoxy nanocomposites: Mechanical, vibrational, and morphological properties. J. Appl. Polym. Sci. 2020, 137, 49338. [Google Scholar] [CrossRef]
  132. Domènech, B.; Mata, I.; Molins, E. Tuning the structure and the mechanical properties of epoxy-silica sol-gel hybrid materials. RSC Adv. 2016, 6, 10736–10742. [Google Scholar] [CrossRef] [Green Version]
  133. Çok, S.S.; Ünal, H.Y.; Koç, F.; Pekbey, Y.; Gizli, N. Ionic liquid functionalized silica aerogels as reinforcing agents for epoxy nanocomposites. J. Inorg. Organomet. Polym. Mater. 2021, 31, 2445–2458. [Google Scholar] [CrossRef]
  134. He, J.; Li, X.; Su, D.; Ji, H.; Qiao, Y. High-strength mullite fibers reinforced ZrO2–SiO2 aerogels fabricated by rapid gel method. J. Mater. Sci. 2015, 50, 7488–7494. [Google Scholar] [CrossRef]
  135. Selver, E.; Öztaş, B.; Uçar, M.; Uçar, N.; Baydoğan, M.; Altay, P.; Geygel, B. Mechanical and thermal properties of glass/epoxy composites filled with silica aerogels. Plast. Rubber Compos. 2021, 50, 371–383. [Google Scholar] [CrossRef]
  136. Churu, G.; Zupančič, B.; Mohite, D.; Wisner, C.; Luo, H.; Emri, I.; Sotiriou-Leventis, C.; Leventis, N.; Lu, H. Synthesis and mechanical characterization of mechanically strong, polyurea-crosslinked, ordered mesoporous silica aerogels. J. Sol-Gel Sci. Technol. 2015, 75, 98–123. [Google Scholar] [CrossRef]
  137. Capadona, L.A.; Meador, M.A.B.; Alunni, A.; Fabrizio, E.F.; Vassilaras, P.; Leventis, N. Flexible, low-density polymer crosslinked silica aerogels. Polymer 2006, 47, 5754–5761. [Google Scholar] [CrossRef]
  138. Yang, H.; Kong, X.; Zhang, Y.; Wu, C.; Cao, E. Mechanical properties of polymer-modified silica aerogels dried under ambient pressure. J. Non-Cryst. Solids 2011, 357, 3447–3453. [Google Scholar] [CrossRef]
  139. Verdolotti, L.; Oliviero, M.; Lavorgna, M.; Santillo, C.; Tallia, F.; Iannace, S.; Chen, S.; Jones, J.R. “Aerogel-like” polysiloxane-polyurethane hybrid foams with enhanced mechanical and thermal-insulating properties. Compos. Sci. Technol. 2021, 213, 108917. [Google Scholar] [CrossRef]
  140. Cho, J.; Jang, H.G.; Kim, S.Y.; Yang, B. Flexible and coatable insulating silica aerogel/polyurethane composites via soft segment control. Compos. Sci. Technol. 2019, 171, 244–251. [Google Scholar] [CrossRef]
  141. Duan, Y.; Jana, S.C.; Lama, B.; Espe, M.P. Reinforcement of silica aerogels using silane-end-capped polyurethanes. Langmuir 2013, 29, 6156–6165. [Google Scholar] [CrossRef] [PubMed]
  142. Merillas, B.; Lamy-Mendes, A.; Villafañe, F.; Durães, L.; Rodríguez-Pérez, M.Á. Silica-based aerogel composites reinforced with reticulated polyurethane foams: Thermal and mechanical properties. Gels 2022, 8, 392. [Google Scholar] [CrossRef] [PubMed]
  143. Tian, J.; Yang, Y.; Xue, T.; Chao, G.; Fan, W.; Liu, T. Highly flexible and compressible polyimide/silica aerogels with integrated double network for thermal insulation and fire-retardancy. J. Mater. Sci. Technol. 2022, 105, 194–202. [Google Scholar] [CrossRef]
  144. Kantor, Z.; Wu, T.; Zeng, Z.; Gaan, S.; Lehner, S.; Jovic, M.; Bonnin, A.; Pan, Z.; Mazrouei-Sebdani, Z.; Opris, D.M.; et al. Heterogeneous silica-polyimide aerogel-in-aerogel nanocomposites. Chem. Eng. J. 2022, 443, 136401. [Google Scholar] [CrossRef]
  145. Fei, Z.; Yang, Z.; Chen, G.; Li, K. Preparation of tetraethoxysilane-based silica aerogels with polyimide cross-linking from 3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride and 4, 4′-oxydianiline. J. Sol-Gel Sci. Technol. 2018, 85, 506–513. [Google Scholar] [CrossRef]
  146. Fei, Z.; Yang, Z.; Chen, G.; Li, K.; Zhao, S.; Su, G. Preparation and characterization of glass fiber/polyimide/SiO2 composite aerogels with high specific surface area. J. Mater. Sci. 2018, 53, 12885–12893. [Google Scholar] [CrossRef]
  147. Zhang, X.; Ni, X.; Li, C.; You, B.; Sun, G. Co-gel strategy for preparing hierarchically porous silica/polyimide nanocomposite aerogel with thermal insulation and flame retardancy. J. Mater. Chem. A 2020, 8, 9701–9712. [Google Scholar] [CrossRef]
  148. Faysal Ilhan, U.; Fabrizio, E.F.; Mccorkle, L.; Scheiman, D.A.; Dass, A.; Palczer, A.; Meador, M.B.; Johnston, J.C.; Leventis, N. Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica. J. Mater. Chem. 2006, 16, 3046–3054. [Google Scholar] [CrossRef]
  149. Maleki, H.; Durães, L.; Portugal, A. Synthesis of mechanically reinforced silica aerogels via surface-initiated reversible addition-fragmentation chain transfer (raft) polymerization. J. Mater. Chem. A 2015, 3, 1594–1600. [Google Scholar] [CrossRef] [Green Version]
  150. Defriend, K.A.; Espinoza, B.; Patterson, B. Templating silica aerogel with polystyrene to improve their mechanical properties. Fusion Sci. Technol. 2007, 51, 693–700. [Google Scholar] [CrossRef]
  151. Meador, M.A.B.; Fabrizio, E.F.; Ilhan, F.; Dass, A.; Zhang, G.; Vassilaras, P.; Johnston, J.C.; Leventis, N. Cross-linking amine-modified silica aerogels with epoxies: Mechanically strong lightweight porous materials. Chem. Mater. 2005, 17, 1085–1098. [Google Scholar] [CrossRef] [Green Version]
  152. Fu, B.; Luo, H.; Wang, F.; Churu, G.; Chu, K.T.; Hanan, J.C.; Sotiriou-Leventis, C.; Leventis, N.; Lu, H. Simulation of the microstructural evolution of a polymer crosslinked templated silica aerogel under high-strain-rate compression. J. Non-Cryst. Solids 2011, 357, 2063–2074. [Google Scholar] [CrossRef]
  153. Li, M.-E.; Wang, S.-X.; Han, L.-X.; Yuan, W.-J.; Cheng, J.-B.; Zhang, A.-N.; Zhao, H.-B.; Wang, Y.-Z. Hierarchically porous sio2/polyurethane foam composites towards excellent thermal insulating, flame-retardant and smoke-suppressant performances. J. Hazard. Mater. 2019, 375, 61–69. [Google Scholar] [CrossRef] [PubMed]
  154. Cimavilla-Román, P.; Pérez-Tamarit, S.; Santiago-Calvo, M.; Rodríguez-Pérez, M.Á. Influence of silica aerogel particles on the foaming process and cellular structure of rigid polyurethane foams. Eur. Polym. J. 2020, 135, 109884. [Google Scholar] [CrossRef]
  155. Liaw, D.-J.; Wang, K.-L.; Huang, Y.-C.; Lee, K.-R.; Lai, J.-Y.; Ha, C.-S. Advanced polyimide materials: Syntheses, physical properties and applications. Prog. Polym. Sci. 2012, 37, 907–974. [Google Scholar] [CrossRef]
  156. Li, X.; Wang, J.; Zhao, Y.; Zhang, X. Template-free self-assembly of fluorine-free hydrophobic polyimide aerogels with lotus or petal effect. ACS Appl. Mater. Interfaces 2018, 10, 16901–16910. [Google Scholar] [CrossRef]
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Figure 1. The applications of silica aerogels in various fields.
Figure 1. The applications of silica aerogels in various fields.
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Figure 2. Materials for enhancing mechanical properties of silica aerogel. Reprinted with permission from Refs. [49,50,51,52]: Copyright 2019, Elsevier; Copyright 2022, Elsevier; Copyright 2011, American Chemical Society; Copyright 2022, American Chemical Society.
Figure 2. Materials for enhancing mechanical properties of silica aerogel. Reprinted with permission from Refs. [49,50,51,52]: Copyright 2019, Elsevier; Copyright 2022, Elsevier; Copyright 2011, American Chemical Society; Copyright 2022, American Chemical Society.
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Figure 3. The preparation of aerogel using sol–gel process. Reprinted with permission from Ref. [54]: Copyright 2014, Elsevier.
Figure 3. The preparation of aerogel using sol–gel process. Reprinted with permission from Ref. [54]: Copyright 2014, Elsevier.
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Figure 4. Primary and secondary silica particles of the silica aerogel. Reprinted with permission from Ref. [54]: Copyright 2014, Elsevier.
Figure 4. Primary and secondary silica particles of the silica aerogel. Reprinted with permission from Ref. [54]: Copyright 2014, Elsevier.
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Figure 5. Technical approaches to improve the mechanical properties of aerogels. Reprinted with permission from Ref. [125]: Copyright 2011, American Chemical Society.
Figure 5. Technical approaches to improve the mechanical properties of aerogels. Reprinted with permission from Ref. [125]: Copyright 2011, American Chemical Society.
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Figure 6. Polymerization reaction of GPTMS and molecular structure of PGPTMS. Reprinted with permission from Ref. [128]: Copyright 2020, Elsevier.
Figure 6. Polymerization reaction of GPTMS and molecular structure of PGPTMS. Reprinted with permission from Ref. [128]: Copyright 2020, Elsevier.
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Figure 7. The structural model of the “interpenetrating organic-inorganic network”. Reprinted with permission from Ref. [129]: Copyright 2018, Elsevier.
Figure 7. The structural model of the “interpenetrating organic-inorganic network”. Reprinted with permission from Ref. [129]: Copyright 2018, Elsevier.
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Figure 8. Schematic illustration of the silica aerogel particle network structure and possible covalent interactions between the epoxy and silica surface. Reprinted with permission from Ref. [130]: Copyright 2018, American Chemical Society.
Figure 8. Schematic illustration of the silica aerogel particle network structure and possible covalent interactions between the epoxy and silica surface. Reprinted with permission from Ref. [130]: Copyright 2018, American Chemical Society.
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Figure 9. Histograms of the distribution of material points with different stress levels at 19% compressive strain deformation. Reprinted with permission from Ref. [152]: Copyright 2011, Elsevier.
Figure 9. Histograms of the distribution of material points with different stress levels at 19% compressive strain deformation. Reprinted with permission from Ref. [152]: Copyright 2011, Elsevier.
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Figure 10. Synthetic route for amine-modified aerogel crosslinked with diisocyanates. Reprinted with permission from Ref. [137]: Copyright 2006, Elsevier.
Figure 10. Synthetic route for amine-modified aerogel crosslinked with diisocyanates. Reprinted with permission from Ref. [137]: Copyright 2006, Elsevier.
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Figure 11. Reaction–modification of amine-modified aerogel using toluene diisocyanate. Reprinted with permission from Ref. [138]: Copyright 2011, Elsevier.
Figure 11. Reaction–modification of amine-modified aerogel using toluene diisocyanate. Reprinted with permission from Ref. [138]: Copyright 2011, Elsevier.
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Figure 12. The photographs of (a) APC; (b) mechanical properties test; (c) m-CT image; (d) SEM and EDS image; (e) schematic of PU synthesis and fabrication process of APC. Reprinted with permission from Ref. [140]: Copyright 2019, Elsevier.
Figure 12. The photographs of (a) APC; (b) mechanical properties test; (c) m-CT image; (d) SEM and EDS image; (e) schematic of PU synthesis and fabrication process of APC. Reprinted with permission from Ref. [140]: Copyright 2019, Elsevier.
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Figure 13. (a) Functionalization reaction between polyol-polyether and IPTS; (b) stress–strain curves of the foams. Reprinted with permission from Ref. [139]: Copyright 2021, Elsevier.
Figure 13. (a) Functionalization reaction between polyol-polyether and IPTS; (b) stress–strain curves of the foams. Reprinted with permission from Ref. [139]: Copyright 2021, Elsevier.
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Figure 14. Synthesis of urethane prepolymer (prepolymer I) and APTES-end-capped prepolymer (prepolymer II). Reprinted with permission from Ref. [141]: Copyright 2013, American Chemical Society.
Figure 14. Synthesis of urethane prepolymer (prepolymer I) and APTES-end-capped prepolymer (prepolymer II). Reprinted with permission from Ref. [141]: Copyright 2013, American Chemical Society.
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Figure 15. Synthesis of MDI-end-capped polyurethane (polymer I) and APTES-end-capped chain-extended polyurethane (polymer II). Reprinted with permission from Ref. [141]: Copyright 2013, American Chemical Society.
Figure 15. Synthesis of MDI-end-capped polyurethane (polymer I) and APTES-end-capped chain-extended polyurethane (polymer II). Reprinted with permission from Ref. [141]: Copyright 2013, American Chemical Society.
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Figure 16. Preparation and composition of PSi aerogels: (a) schematic illustration of the preparation of PSi aerogels; (b) compressive stress–strain curves of PSi aerogel during loading–unloading cycles in the radial direction; (c) images of folded PSi-6 aerogel. Reprinted with permission from Ref. [143]: Copyright 2022, Elsevier.
Figure 16. Preparation and composition of PSi aerogels: (a) schematic illustration of the preparation of PSi aerogels; (b) compressive stress–strain curves of PSi aerogel during loading–unloading cycles in the radial direction; (c) images of folded PSi-6 aerogel. Reprinted with permission from Ref. [143]: Copyright 2022, Elsevier.
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Figure 17. The preparation of PI-silica composites. Reprinted with permission from Ref. [144]: Copyright 2022, Elsevier.
Figure 17. The preparation of PI-silica composites. Reprinted with permission from Ref. [144]: Copyright 2022, Elsevier.
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Table
Table 2. Overview of reported mechanical properties of polymer-reinforced aerogels.
Table 2. Overview of reported mechanical properties of polymer-reinforced aerogels.
Precursor
Formulation		Polymer MatrixEnhanced PropertiesRef
GPTMS/VTMSEpoxide➢ Elastic deformation: 3~5%[128]
/Epoxide➢ Elastic modulus: 35%, tensile strength: 62%, toughness: 126%[129]
/Epoxide➢ (Hydrophobic aerogel) contact angles: 107°, fracture toughness: improved by up to ∼70%, impact strength: improved by up to ∼120%[130]
/Epoxide➢ Elastic modulus: 3770 ± 71 MPa, stress at yield point: 43.2 ± 1.8 MPa, strain at yield point: 1.24 ± 0.03 MPa, ultimate tensile strength: 51.0 ± 2.1 MPa, strain at break point: 3.3 ± 0.3%, toughness: 1.29 ± 0.08 J/m3[131]
TEOS/APTESEpoxide➢ Strain: 80% (18 N)[132]
TEOS/APTESEpoxide➢ Tensile strength: 45.05± 4.56 MPa, modulus of elasticity: 4363.88± 209.57 MPa, break strain: 1.19 ± 0.17%[133]
TEOSEpoxide➢ Density: 0.419 g/cm3, porosity: 89%, compressive strength: 0.438 MPa, thermal conductivity: 0.0273 W/m·K[134]
/Epoxide➢ (Warp direction) strength: 464.3 MPa, modulus: 1.76 GPa, (weft direction) strength: 410.2 MPa, modulus: 1.68 GPa[135]
TMOSPolyurea➢ Shrinkage: 14.6 ± 0.7%, bulk density: 0.594 ± 0.026 g/cm3, skeletal density: 1.290 ± 0.003 g/cm3, porosity: 54%[136]
TEOS/APTESPolyurea➢ Bulk density: 0.046 g/cm3, flexural modulus: 0.14 MPa[137]
TEOS/APTESPolyurea➢ Linear shrinkage: 15.73%, bulk density: 0.392 g/m3, average elastic modulus: 14.57 MPa[138]
TEOS /MTEOSPolyurethane➢ Density: 0.190 ± 0.006 g/m3, yield strength: 2.15 ± 0.04 MPa, Young’s modulus: 50 ± 0.09 MPa[139]
/Polyurethane➢ Heat resistance index: 193.6%, char yield: 31.6%, bulk density: 0.580 g/mL[140]
TEOS/APTESPolyurethane➢ BET surface area: 242.9 m2/g, BJH desorption average pore diameter: 10.8 nm[141]
TEOSPolyurethane➢ Density: 117.68 kg/m3, porosity: 92.3%, linear shrinkage: −8.38%, thermal conductivity: 0.014 ± 0.00033 W/m·K[142]
MTMSPolyimide➢ Compressive strain: 50%, thermal conductivity: 0.0212 W/m·K[143]
/Polyimide➢ Surface area: 609 m2/g, thermal conductivity: 0.017.5 W/m·K[144]
TEOS/APTESPolyimide➢ Compressive strength: 3.82 MPa, Young’s modulus: 44.16 MPa[145]
TEOS/APTESPolyimide➢ Density: 0.145 g/cm3, strain: 9%, strength: 0.29 MPa, Young’s modulus: 3.22 MPa[146]
TEOSPolyimide➢ Compressive modulus:1.96 MPa, thermal conductivity: 0.0311~0.0585 W/m·K[147]
TMOS/APTESPolystyrene➢ Density: 0.41~0.77 g/cm3, surface area: 213~393 m2/g, thermal conductivity: 0.041 W/m·K, contact angles: 120°[148]
TMOSPolystyrene➢ Density: 0.13~0.17 g/cm3, surface area: 350~780 m2/g, thermal conductivity: 0.03~0.04 W/m·K[149]
MTMS/VTMS/TMOSPolystyrene➢ Bulk density: 163.1 ± 11.7 kg/cm3, porosity: 88%, surface area: 227 m2/g, thermal conductivity: 0.072 ± 0.001 W/m·K, Young’s modulus: 91 kPa, compression strength: 68 kPa[87]
TMOSPolystyrene➢ Modulus: 3 MPa[150]
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Zhan, W.; Chen, L.; Kong, Q.; Li, L.; Chen, M.; Jiang, J.; Li, W.; Shi, F.; Xu, Z. The Synthesis and Polymer-Reinforced Mechanical Properties of SiO2 Aerogels: A Review. Molecules 2023, 28, 5534. https://doi.org/10.3390/molecules28145534

AMA Style

Zhan W, Chen L, Kong Q, Li L, Chen M, Jiang J, Li W, Shi F, Xu Z. The Synthesis and Polymer-Reinforced Mechanical Properties of SiO2 Aerogels: A Review. Molecules. 2023; 28(14):5534. https://doi.org/10.3390/molecules28145534

Chicago/Turabian Style

Zhan, Wang, Le Chen, Qinghong Kong, Lixia Li, Mingyi Chen, Juncheng Jiang, Weixi Li, Fan Shi, and Zhiyuan Xu. 2023. "The Synthesis and Polymer-Reinforced Mechanical Properties of SiO2 Aerogels: A Review" Molecules 28, no. 14: 5534. https://doi.org/10.3390/molecules28145534

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