Recent research progress and advanced applications of silica/polymer nanocomposites

2.1 Preparation of nano-SiO₂

The methods for preparing nano-SiO₂ primarily include the Stöber method, the microemulsion method, and the chemical precipitation method [45]. In the Stöber method, also known as the sol–gel method, silicate is used as a precursor and undergoes a hydrolysis-polymerization reaction with water mixed in a solvent, before adding a suitable surfactant that is stirred to form a uniform sol that is mostly removed by high-speed centrifugation [28]. The organic solvent is converted into a gel, and nano-SiO₂ of a uniform particle size is obtained after post-processing, such as by drying. The advantages of this method are that it can be carried out under mild reaction conditions and results in a product of high purity, uniform particle size, and with a large specific surface area; it is also easier to obtain good dispersion and suspension performance in solution as well as high activity. The main disadvantage is that post-processing is necessary to obtain nanoparticles [46,47].

The microemulsion method involves mixing oil, water, emulsifier, and a co-surfactant (such as alcohol) in an appropriate ratio to prepare a microemulsion, followed by the addition of another reactant that diffuses in the microemulsion [48]. This reactant infiltrates into the reverse micelle and, after demulsification, is washed, separated, dried, and calcined to obtain nano-SiO₂ [49,50]. This method allows convenient particle size regulation in the preparation of nanoparticles with small particle sizes and good monodispersity. However, the process is laborious and time-consuming to carry out due to the long reaction time and complicated post-processing process.

The chemical precipitation method exploits the solubility of metal salts or alkalis to adjust the acidity, temperature, and solvent of the solution when preparing the precipitate, which is then washed, dried, and heated to obtain nano-SiO₂ powder. The advantages of this method are that the process is simple, and the prepared product has a small particle size and a large specific surface area [51,52]. The disadvantages are that the raw materials are expensive, energy consumption is high, complex technology is required, the equipment requirements are high, and it is low yielding.

The Stöber process is the most commonly used approach, in which tetraethyl orthosilicate (TEOS) is often applied as a precursor and undergoes hydrolysis and condensation in the presence of water and low-molecular-weight alcohols, such as ethanol, eventually forming nano-SiO₂ [53,54]. The mechanism of the hydrolysis of the TEOS precursor in the base-catalyzed reaction is shown in Figure 2.

Figure 2

Mechanism of hydrolysis of TEOS precursors in base-catalyzed reactions [55]; Copyright: 2021 with permission from Elsevier Inc.

This process involves the nucleophilic substitution of a silicon-ethoxy group (Si-OR) by a silanol group (–Si–OH) via a pentacoordinate transition state [55]. In ammonia solution, OH⁻ ions are more nucleophilic than H₂O molecules. Ammonia is used as a catalyst to speed up the rate of hydrolysis and condensation. The first hydrolysis step is slower and, thus, is the rate-determining step. As the positive charge of the silicon atom increases, the ethoxy group (–OR) is nucleophilically substituted by OH⁻, the steric hindrance decreases, and the reaction rate increases as more ethoxy groups are converted to silanol groups.

The hydrolysis is followed by condensation, and the TEOS condensation reaction process is shown in Figure 3. The silanol groups (–Si–OH) of SiO₂ monomers or oligomers condense to form siloxane (Si–O–Si) bonds. However, condensation occurs at a much faster rate than hydrolysis because silanol groups deprotonate more easily than H₂O molecules. Increasing the density of positive charges on silicon atoms results in more favorable conditions for nucleophilic attack. This also results in the silanol monomers preferentially attaching to larger siloxane network clusters rather than to other monomers or small oligomers [56].

Figure 3

Mechanism of TEOS precursor condensation in base-catalyzed reactions [55]; Copyright: 2021 with permission from Elsevier Inc.

Heiman-Burstein et al. [57] prepared the hydrophilic nano-SiO₂ based on the Stöber process. In that study, TEOS and alkyltrimethoxysilane (ATMS) were used as precursors and hydrolyze with ethanol in the presence of alkaline solution (NH₄OH) and water to generate silanol (–Si–OH), which then undergoes condensation into silicon oxide. This results in the production of alkane clusters (Si–O–Si) and nucleate growth, finally yielding hydrophilic nano-SiO₂. As shown in Figure 4, TEOS hydrolysis and condensation forms spherical hydrophilic nano-SiO₂, while ATMS hydrolysis and condensation results in the formation of macromolecular networks, which is caused by the alkane via the steric hindrance of the base.

Figure 4

TEOS and ATMS hydrolysis condensation reaction process [57]; Copyright: 2021 with permission from Elsevier Inc.

2.2 Chemical structure of SiO₂/polymer nanocomposites

Fourier transform infrared spectroscopy (FTIR), Fourier transform attenuated total reflection infrared spectroscopy (ATR-IR), and solid-state nuclear magnetic and resonance spectroscopy (ssNMR) are the most common methods used to determine the formation mechanisms and structures of SiO₂ and its nanocomposites [58,59,60].

Figure 5(a) shows the FTIR spectra of SiO₂ and SiO₂ grafted with γ-(methacryloxy)propyltrimethoxysilane (KH570). The strong peaks of nano-SiO₂ at 1,107, 810 and 480 cm⁻¹ correspond to absorption due to Si–O asymmetric stretching vibrations, Si–O–Si symmetric stretching vibrations, and Si–Si–O bending vibrations, respectively. There are three absorption peaks that differ from that of pure SiO₂ in the region of 2,500–3,000 cm⁻¹: the tensile vibration absorption peak of −CH₂– appears near 2,856 cm⁻¹, and the peaks at 2,698 and 2,930 cm⁻¹ are due to the tensile vibration absorption of C–H. The absorption peaks of C═O and C═nucleophilic ally appear at about 1,710 and 1,637 cm⁻¹, respectively, and all are chemical structures existing in KH570. This indicates that KH570 has been grafted onto the SiO₂ surface. In addition, Si–OH absorption occurs at 3,442 and 1,629 cm⁻¹, indicating that KH570 did not completely replace the hydroxyl groups on SiO₂ [61].

Figure 5

(a) FTIR spectra of SiO₂ and KH570 modified SiO₂ [61]; Copyright: 2020 with permission from Elsevier Science SA, (b) Si solid-state NMR spectra for SiO₂ [63]; Copyright: 2016 with permission from Elsevier Science BV, (c) the structure of three kinds of modified SiO₂, and (d) ATR-IR of pure alkyd resin and alkyd resin containing 8% nano SiO₂ [65]; Copyright: 2016 with permission from Elsevier Science Ltd.

In order to further analyze the structure of SiO₂, the formation of the material can be verified by ssNMR. Figure 5(b) shows the Si solid ssNMR diagram of unmodified SiO₂, 3-aminopropyltriethoxysilane (APTES)-modified SiO₂, and n-propyltriethoxysilane (PTES)-modified SiO₂. Absorption peaks in the range of −80 to −40 ppm correspond to the organic peaks of modified SiO₂. The unmodified SiO₂ shows three different structural units, Q₂, Q₃, and Q₄, as shown in Figure 5(c), which appear at −96.9, −103.5, and −112.2 ppm, respectively [62,63]. In Figure 5, we see that chemical shifts have taken place in Q₂, Q₃, and Q₄. In the range of −80 to −120 ppm, the Q₂ peaks of SiO₂-APTES and SiO₂-PTES disappear, but their Q₃ and Q₄ peaks increase, which means that the −OH group on SiO₂ has been replaced, indicating that the silane coupling agent was successfully grafted onto the surface of SiO₂. Therefore, the surface properties of SiO₂ can be inferred by analyzing the strength of Q₂, Q₃, and Q₄, and the formation process of the composite can be further understood by using ssNMR technology to characterize different elements.

ATR-IR is a technique for analyzing the chemical structure of the surface of a sample [64]. Based on the principle of light reflection, ATR-IR addresses the fact that FTIR cannot transmit light to the sample. This method is used to determine the presence of pure alkyd resin and alkyd resin nanocomposites containing 8% nanostructured SiO₂ [64]. Figure 5(d) shows that a new characteristic peak centered at 462 cm⁻¹ appears in the sample due to the successful introduction of SiO₂. Furthermore, the bending vibration absorption peak of Si–O–Si in SiO₂ would affect the spectral intensity and shape in the region of 1,000–1,200 cm⁻¹. Compared with alkyd resin, the position and strength of the absorption peak of the composite has not changed [65].

Gao et al. [66] prepared rod-shaped SiO₂/polyacrylate (PAA) hybrid nanocomposite latex and spherical SiO₂/PAA hybrid nanocomposite latex. With the same amount of TEOS as in the silicon source, the addition of sodium dodecyl sulfate as the structure inducer resulted in both spherical and rod-like micelles. At the same time, the effects of the two different structures of SiO₂ on the properties of the composites were discussed. The rod-shaped SiO₂/PAA composite film exhibited higher mechanical strength (Figure 6(a)). This is because spherical SiO₂ has a small particle size, high specific surface area and specific surface energy, and is easy to agglomerate, resulting in aggregation and distribution in the polypropylene (PP) matrix, which preclude it from being considered and thus functioning as a nanoparticle. The elongation at break results show that the increase in the rigidity of the rod-like SiO₂ restricts movement of the segments and, thus, the elongation at break of the film decreases (Figure 6(b) and (c)). The rod-shaped SiO₂/PAA composite film has lower water absorption capacity and, therefore, higher water resistance (Figure 6(d)). Therefore, the different structures of SiO₂ are determinative of differences in the properties of the overall composite materials. The rod-shaped SiO₂/PAA composite film with excellent properties can be used in coating applications, such as printing adhesives, as well as in papermaking and other fields.

Figure 6

(a) Schematic illustration of the preparation of spherical and rod-like SiO₂/PAA hybrid nanocomposite latexes, (b) mechanical strength, (c) elongation at break, and (d) water absorption. Note: 1^# and 2^# are spherical and rod-shaped SiO₂/PAA composite films, respectively [66]; copyright 2019 with permission from Elsevier Science BV.

2.3 Morphology of SiO₂/polymer nanocomposites

Observing the surface morphology of polymer nanocomposites allows understanding the influence of SiO₂ on the nanocomposites, which is helpful in determining the optimal preparation conditions. Figure 7 shows the scanning electron microscopy (SEM) and atomic-force microscopy (AFM) images of nanocomposites with different SiO₂ loadings [67]. SEM images provide a strong stereoscopic sense and wide field of vision and show that there are obvious changes occurring on the material surface as the SiO₂ content is increased. The diameter of the fibers can also be directly obtained from the image. It is worth noting that SiO₂ adheres to the fibers and roughens the surface. In AFM images, some data (Root mean square of roughness, Rq) can be applied to characterize the surface roughness of materials, which may offer more detailed information.

Figure 7

FE-SEM images of PU film under different treatments: (a) untreated, (b) grafted with -NCO group, (c) treated with APTES, (d) clustered with SiO₂ [67]; Copyright: 2019 with permission from Elsevier; AFM images of (e) raw polyurethane (PU) membrane, (f) raw PU membrane single fiber, (g) SiO₂/PU membrane, and (h) SiO₂/PU membrane single fiber [67]; Copyright: 2019 with permission from Elsevier Science BV.

TEM images are also routinely used to observe the morphology of SiO₂ and its dispersion in the polymer matrix. Compared with SEM, TEM enables greater magnification, and its images can clearly show the size and surface structure of nanoparticles, making it very suitable for studying the effect of filler morphology on the properties of composites. TEM images of several different core–shell nano-SiO₂ are shown in Figure 8. First of all, it can be seen that these nanoparticles are well dispersed, without agglomeration. Compared with the rough and loose nanospheres shown in Figure 8(a), the composite microspheres in Figure 8(b) have smooth surfaces, larger sizes, and clear core–shell structures. In Figure 8(c)–(f), the size of the SiO₂ nanospheres further increases, and a more uniform pore size distribution can be observed [68].

Figure 8

TEM micrograph of core–shell composite: (a) synthesis of hydrophilic magnetite particles, (b) ferric tetroxide @nSiO₂ core–shell microspheres, (c) Fe₃O₄@nSiO₂@polydopamine (PDA) and dopamine interface co-assembled, self-oxidized composite microspheres, (d–f) extraction of acetone from the composite microspheres, core–shell magnetic mesoporous PDA microspheres [68]; Copyright: 2020 with permission from Wiley.

Zhang et al. [69] fabricated a unique yolk–shell SiO₂/Ni₂P/rGO (reduced graphene oxide)/Cd_0.5Zn_0.5S nanocomposite via a simple hard templating method involving efficient photocatalytic water splitting to generate H₂, the preparation process of which is shown in Figure 9(a). First, SiO₂ nanospheres with an average diameter of 250 nm were synthesized. Then, the ultrafine Ni₂P catalyst was uniformly deposited on the surface of the SiO₂ nanospheres via in situ phosphorylation-assisted hydrothermal treatment. Governed by electrostatic forces, the negatively charged rGO nanosheets become tightly wrapped around the surface of the positively charged APTES-modified SiO₂/Ni₂P nanospheres and uniformly nucleate. Finally, thioacetamide (TAA) was added via a hydrothermal process for hierarchical assembly into the yolk–shell-like SiO₂/Ni₂P/rGO/Cd_0.5Zn_0.5S nanocomposites. As shown in Figure 9(b) and (c), the composite can achieve the highest hydrogen evolution rate (11.62%) under an Ni₂P loading of 3 mmoL and still has high catalytic activity after 5 cycles of photocatalytic reaction. The photocatalytic principle is outlined in Figure 9(d). Taking advantage of this unique yolk–shell structure, the inner SiO₂ core can completely reflect all incident light, which causes the incident light to travel between the SiO₂ inner layer scattering core and the outermost outer layer. This causes multiple scattering, thereby improving the utilization efficiency of light and generating more charge carriers. The intercalated rGO conductive network and Ni₂P cocatalyst within the shell can provide a faster charge separation speed and more photocatalytically active sites, thereby enhancing the light-harvesting efficiency and achieving high hydrogen evolution performance. Compared with the SiO₂ single-layer microsphere and nanosheet structure, this unique multi-layer yolk–shell structure, with SiO₂ as the core, endows the composite with a multi-layer light scattering effect and a large number of catalyst attachment sites, thereby significantly improving the photocatalytic hydrogen production performance of the composites.

Figure 9

(a) Preparation process of SiO₂/Ni₂P/rGO/Cd_0.5Zn_0.5S nanocomposite, (b) H₂ yield of S-1, S-2, and S-3 composites (the composites are named S-1, S-2, and S-3 composites with Ni₂P loadings of 1 mmoL, 3 mmoL, and 5 mmoL, respectively), (c) S-2 composite has 5 cycles of photocatalytic reaction under visible light, and (d) schematic diagram of the photocatalytic hydrogen evolution reaction of the composite [69]; Copyright: 2021 with permission from Elsevier Science BV.

Qiao et al. [70] developed a novel microwave-absorbing nanocomposite, namely, one-dimensionally oriented yolk–shell Fe₃O₄@void@SiO₂@PPy (polypyrrole) nanochains (named FVSP). The preparation process is shown in Figure 10(a). First, Fe₃O₄@P(DVB-MAA) nanochains were prepared by magnetic field-induced precipitation polymerization with Fe₃O₄ magnetic particles. Then, core-dual shell Fe₃O₄@P(MAA-DVB)@SiO₂ nanochains were prepared using the sol–gel method, and high-temperature sintering was done at 600°C to obtain egg yolk–shell Fe₃O₄@void@SiO₂ nanochains. Then, in the presence of polyvinyl alcohol (PVA) and p-toluenesulfonic acid, chemical oxidative polymerization occurred to successfully grow PPy on the surface of the FVS nanochains, and a yolk–shelled FVSP nanochain was finally obtained. It can be seen from the SEM image in the Figure 10a that the product maintains a one-dimensional oriented structure, due to support and protection of the core material by the stable SiO₂ shell layer. This one-dimensional structural chain is expected to show improved performance compared with the bulk and particle forms in terms of microwave absorption. As shown in Figure 10(b)–(e), the FVSP nanochains have a rough shell structure, which is due to the stacking of PPy nanoparticles. The introduction of the PPy shell had no effect on the chain length of the precursor, and the nanochain length primarily fell in the range of 8–13 μm. The TEM image also reveals the distinct core–shell structure of the nanochains; moreover, it can be seen in the magnified TEM image that the PPy shell has a thickness of about 20 nm. A microwave absorption performance study showed that the microwave reflection loss absorption peak of FVSP nanochains is −54.20 dB (17.7 GHz), and the effective absorption bandwidth ranges from 11.49 to 17.39 GHz (Figure 10(f)). FVSP nanochains have a strong microwave absorption ability and effective absorption bandwidth. This is due to the synergistic effect of the PPy shell and the wave-transmitting SiO₂ shell, which enhances the microwave multiple reflection and absorption of the composite material such that its microwave absorption performance is significantly improved.

Figure 10

(a) Schematic diagram of the preparation of Fe₃O₄@void@SiO₂@PPy nanochains, SEM (b and c) and TEM images (d and e) of Fe₃O₄@void@SiO₂@PPy nanochains, and (f) microwave reflection loss curves of the composites [70]; Copyright: 2020 with permission from Springer.

2.4 Crystallization behavior

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) can be used to investigate the crystallization behavior of polymer nanocomposites in different ways [71,72,73]. The XRD pattern of mesoporous SiO₂ (SBA-15) is shown in Figure 11, with three diffraction peaks in the range of 1–3°. After the introduction of a bromine initiator, the first diffraction peak shifts to the left and its intensity decreases. This indicates that the structure of SBA-15 is changed by the bromine atoms, which may be due to the bromine functional groups decreasing the specific surface area and pore size of SBA-15. In addition to the first diffraction peak, the strength of the other peaks of SBA-15/PGMA (poly(glycidyl methacrylate)) composites decreased significantly, and some even disappeared. This is due to the fact that the pores of SBA-15 are filled with polymer, destroying the mesoporous structure.

Figure 11

XRD patterns of SBA-15 and its composites [74]; Copyright: 2018 with permission from Elsevier Science BV.

The melting behavior, especially in terms of crystallinity and crystal structure, has a significant effect on the cell morphology of foamed polymers. Yuan et al. [75] used modified SiO₂ to improve the melt strength of PP. Figure 12 shows the melting and crystalline properties of PP samples characterized by DSC. As shown in Figure 12, the addition of PP-g-SiO₂ slightly increased the melting point of the pure PP, resulting in a significant increase in the crystallization temperature. The results show that the grafting of PP endows SiO₂ with remarkable nucleation ability and promotes the crystallization of PP. Furthermore, the crystallinity (X _c) of the PP/PP-g-SiO₂ composites decreased with the increase in SiO₂ content, which may be due to the well-dispersed PP-g-SiO₂ restricting the movement of PP chains. In contrast, PP/A-5 had a broadened melting peak due to the addition of lower-molecular-weight PP-MAH (maleic anhydride). The crystallization behavior of PP/A-5 is similar to that of PP because there is no interaction between unmodified SiO₂ and the PP matrix, and the motion of the PP chains is thus barely affected.

Figure 12

(a) Melting curves, and (b) crystallization curves of PP and PP nanocomposites. PP nanocomposites are called PP/SX or PP/AX, S and A stand for PP-g-SiO₂ and PP-MAH/A200 masterbatch, both masterbatches contain 15 wt% nano-SiO₂. X is the final particle loading in the PP nanocomposite [75]; Copyright: 2020 with permission from Wiley.

Li et al. [76] grafted PP particles onto PSM and then prepared PP-based nanocomposites with SiO₂ via the sol–gel method. Figure 13 shows the nanocomposites with SiO₂ mass fractions of 1.94% (PS-1) and 4.39% (PS-2), pure PP, and PSM. It can be seen that the PSM particles are the largest, while the crystal size of the composite containing SiO₂ is smaller. SiO₂ has a heterogeneous nucleation effect on PP, which promotes the crystallization of PP, so the spherulite size is relatively small. With the increase in SiO₂ content, the crystallization rate of PP becomes faster and the crystal becomes smaller.

Figure 13

TEM diagrams of PP and SiO₂/PP composites: (a) PP, (b) styrene-maleic anhydride copolymer (PSM) (PSM), (c) PS-1, and (d) PS-4 [76]; Copyright: 2015 with permission from Springer.

3.1 Mechanical properties

Tensile strength, impact strength, elongation at break, and bending properties are commonly used to characterize the mechanical properties of materials [77,78,79,80]. Jiao et al. [81] prepared SiO₂/epoxy resin (EP) nanocomposites using the sol–gel method, and they found that the tensile strength, elongation at break, impact strength, and flexural strength of EP composites increased with the increase in SiO₂ content. With the addition of 1.41 wt% SiO₂, these properties of the composites increase by 67.6, 190, 82.1, and 15.7%, respectively, compared with pure EP. The strengthening mechanism is related to the fact that the presence of SiO₂ makes it easier for EP to experience stress concentration. On the one hand, where there is stress concentration, more deformation energy can be absorbed through the formation of cracks in the matrix, and the toughness of EP increases accordingly. On the other hand, the rigid nano-SiO₂ prevent the continuous crack growth in the matrix. However, when the SiO₂ content increases to a certain extent, agglomeration occurs, which degrades the mechanical properties of EP.

He et al. [82] added a low content of phenyl-functionalized SiO₂ (P-SiO₂) to the polydicyclopentadiene (PDCPD) matrix through the melt compounding/solution mixing method. The tensile strength and impact strength of PDCPD significantly improved due to the strong covalent bonds that formed between the SiO₂ and the polymer matrix. Prajapati et al. [83] demonstrated that the introduction of SiO₂ could effectively enhance the mechanical properties of the epoxy glass composite, especially with regard to the bending strength. This may be due to the presence of SiO₂, which served as a compatibilizer to strengthen the adhesion between the glass fiber and EP. Moreover, Ji et al. [84] studied the influence of SiO₂ particle size on the mechanical properties of the obtained polybutadiene (PB)/SiO₂ nanocomposites. Figure 14 shows the relationship between the storage modulus and strain of PB for different SiO₂ particle sizes (90/250/350 nm) under loading, which is indicated as PB–SP. Among these, the storage modulus of PB-SP90 with the smallest particle size reached 3.20 MPa at 2% strain, which is the largest of the three composites. As the particle size of the loaded SiO₂ increased, the storage modulus decreased with the increase in strain. This is because smaller particles have a larger filling-polymer interface area, and chemical bonds are formed between filler and polymer at the interface. When the interface area increases, more chemical bonds are formed, and the strength of the material is increased, so the material’s tensile strength also increases.

Figure 14

The dynamic storage modulus of PB-SP (90/250/350 nm) with different size SiO₂ [84]; Copyright: 2018 with permission from Wiley.

In a similar study, it was also found that smaller nanoparticles have a greater impact on the macroscopic mechanical properties of nanocomposites. For nanoparticles with the same mass fraction, the Young’s modulus increases by about 4–5% when the diameter of nanoparticles is decreased from 5 to 2 nm. This is related to the large cross-phase region when the particles are small, and the increase in the interphase area leads to an increase in the interaction between the matrix and the nanoparticles. Therefore, more polymer chains present in the nanocomposites become strongly adsorbed on the surface of the nanoparticles, which leads to an increase in the Young’s modulus [85].

Sometimes, tensile strength is also used to assess the mechanical properties, which determine the deformation and failure characteristics of an object against external forces [86,87]. Table 1 lists and compares the improvements in tensile strength of different polymers resulting from doping with SiO₂. In order to assess the reinforcement and toughening of high-density polyethylene (HDPE) by SiO₂, Wang et al. [88] modified SiO₂ with hydrophobic lignin, thereby improving the interfacial compatibility between SiO₂ and HDPE. Hydrophobic lignin/nano-SiO₂ were prepared by the quaternization and alkylation of hydrophobically modified lignin, followed by in situ one-pot coprecipitation from aqueous solution. When incorporating hydrophobic lignin/nano-SiO₂ into HDPE using 10 wt% nanoparticle doping, the strength of HDPE is increased by 24.5 MPa and the elongation is increased by 1,096%, which are improvements of 10.4 and 14.3% compared with undoped HDPE, respectively.

Guangmei et al. [89] prepared polyurethane acrylate (PUA)/SiO₂ composite emulsions with different SiO₂ contents by the sol–gel method, and the tensile strength of nanocomposites was increased with the increase in SiO₂ loading. The addition of nano-SiO₂ led to the formation of an interpenetrating network inside the PUA/SiO₂ composite film. This crosslinked structure can effectively disperse the stress, thereby improving the mechanical properties of the composite material.

Wang et al. [90] measured the strength of EP/SiO₂ nanocomposites by microindentation testing. The results show that the strength of the nanocomposites increases with the increase in SiO₂ content. When the SiO₂ filling rate reaches 15 wt%, the strength increases by 33%, and the mechanical properties of the composites are significantly improved. Therefore, based on the strong binding force between nanoparticles and the matrix, an important way to improve the strength is to enhance the load-bearing capacity of the composite material.

Zhang et al. [91] synthesized a mesoporous SiO₂-coated SiO₂ (SiO₂@mSiO₂) core–shell structure for use as a nanofiller, which was inserted into polyethylene (PE) to prepare a high-strength SiO₂@mSiO₂/PE nanocomposite. The SiO₂@mSiO₂/PE composite exhibited significantly enhanced breakdown strength for SiO₂@mSiO₂ filling levels as low as 1 wt%, which is 108% higher than that of pure PE. This is mainly because uniform dispersion of the SiO₂@mSiO₂ core–shell structure in the PE matrix more effectively restricts the movement of molecular chains of the PE matrix, which is beneficial to the transfer of stress, thereby optimizing the mechanical strength and toughness of the composite. Wu et al. [92] used three silane coupling agents, KH550, KH560, and KH570, to modify the surface of SiO₂ and then to fill polymethyl acrylate (PMA) to prepare nanocomposites. The results regarding the mechanical properties show that at the same filler content (2 wt%), the tensile strength of PMA/KH570-SiO₂ was the highest, which was 57% higher than that of pure PMA. This indicates that there is a strong interfacial interaction force between KH570-SiO₂ microspheres and PMA.

He et al. [93] added P-SiO₂ at low loading levels to improve the tensile strength and impact toughness of PDCPD. The mechanical property results show that when only 0.3 wt% P-SiO₂ is added, the tensile strength of P-SiO₂/PDCPD nanocomposites can be significantly enhanced from 37.51 to 61.14 MPa, which is an increase of about 63% relative to the unmodified SiO₂/PDCPD. This is mainly attributed to the excellent dispersion of P-SiO₂ in PDCPD and strong interfacial bonding.

3.2 Friction and wear properties

Friction is tangential resistance caused by the relative movement or relative movement trend of two objects in contact, which leads to energy loss. Wear is the phenomenon of surface loss after friction and is a form of material failure. The friction coefficient and wear rate are commonly used to characterize the friction and wear properties of materials. The friction coefficient can be obtained using a specialized tester or machine, and the wear rate is often expressed by mass or volume loss [94,95,96].

Cao et al. [97] used APTES and bis-(triethoxysilyl)-tetrasulfide to modify GPE and SiO₂, respectively. After the modified GPE (rGO) and modified SiO₂ were hybridized, their hybrid particles were introduced into nanorod (NR) to study the friction and wear properties of the nanocomposites. The results show that the composite exhibited excellent wear resistance. The mechanism involved is as follows: the rGO layer and modified SiO₂ are bridged by silane and embedded into the tire matrix with a spring-like structure, which can stimulate additional wear resistance and prolong the service life of the composite.

Rahsepar and Mohebbi [98] used mesoporous SiO₂ loaded with 2-mercaptobenzothiazole (MBT) as a filler to investigate the wear resistance of epoxy-based coating. The results show that the wear resistance of the coating improved while the friction coefficient decreased. This can be explained via two aspects: first, the surface hardness increases with the increase in SiO₂ content, resulting in a decrease in coating surface loss; second, the coating has a self-lubricating effect since MBTs are released from the SiO₂ container during friction with an object, which results in a lubrication effect on the frictional surface that reduces the friction coefficient. However, when SiO₂ was used as a filler, the coating surface became rougher, and more contact points were produced. As a result, the contact points were subjected to large forces and were easily dislodged, which results in removal of the coating surface. This also explains why the friction coefficient decreased when the SiO₂ content increased to 2 wt%. In addition, the smaller the particle size and the larger the specific surface area, the higher the efficiency of loading MBT, meaning a more uniform distribution in the EP matrix, and a more pronounced lubrication effect; in other words, the corresponding coating has enhanced wear resistance.

3.3 Thermal properties

Thermogravimetric analysis (TGA) can be used to investigate thermal stability, and it has been used to determine the decomposition temperature and mass fraction of SiO₂ in SiO₂/polymer nanocomposites [99,100,101]. Saoud et al. [102] added different amounts of SiO₂ to a methacrylate polymer and studied the effect of SiO₂ content on its thermal properties. As shown in Figure 15, among the five sample curves, the abscissa corresponding to the inflection point of the curve is the temperature at which the sample begins to decompose. Therefore, with the increase in SiO₂ content, the decomposition temperature of the composite is increased, while the slope of the corresponding curve decreases during the decomposition process, which means that inclusion of SiO₂ in composites reduces their decomposition rate and improves their heat resistance and thermal stability. The reason for this is that SiO₂ has high thermal stability and forms a crosslinking network with the polymer matrix through chemical bonds. When heated, the movement of the polymer molecular chain is hindered by the crosslinking network structure.

Figure 15

TGA diagram of latex film with different components [102]; Copyright: 2017 with permission from Springer.

SiO₂ is also loaded with flame-retardant materials in a variety of ways to improve the flame-retardant properties of composites. These properties can be evaluated using limiting oxygen index (LOI) or cone calorimetry tests. Farid et al. [103] used sunflower oil as a raw material to prepare isocyanate and then introduced SiO₂/zirconia (ZrO₂) nanocomposite particles to synthesize a nontoxic, environmentally friendly, flame-retardant coating. In LOI testing, when the content of SiO₂/ZrO₂ nanoparticles reached 3 wt%, the LOI value increased by 2%, smoke suppression was satisfactory, and the heat release rate and total heat release rate were the lowest. The nanocomposites showed the best flame retardancy and met the requirements for disaster prevention. Vertical combustion tests are also commonly used, wherein the material is vertically burned by the flame; the ratings are divided into V0, V1, and V2 levels depending on the time at which the composite automatically extinguishes and whether a dripping phenomenon is observed. Esmaeili-Bafghi-Karimabad et al. [104] prepared polycarbonate (PC)/SiO₂ and polystyrene (PS)/SiO₂ nanocomposites for which the flame-retardant grades were all V0, with SiO₂ acting as a barrier layer.

In general, SiO₂ itself has excellent thermal properties, and its incorporation can improve the temperature tolerance of polymers. The above examples are compared, and the results are listed in Table 2. When increasing the ambient temperature, SiO₂ forms a protective barrier between the material and the surrounding environment, resulting in improvements in the decomposition temperature and temperature stability of the material. At this time, increasing the mass fraction of SiO₂ as much as possible is conducive to this phenomenon. This explains why the SiO₂ load required to ensure the optimal thermal stability of the material is much higher than that required for other properties. Because of the strong interface between SiO₂ and the matrix, the material has improved viscoelasticity, which is even more pronounced at increased temperatures. On the other hand, the greater the number of crystal regions, the higher the temperature required for the migration of polymer chain segments. In other words, heating promotes polymer crystallization because SiO₂ acts as the crystallization nucleus, and this manifests an increase in the melting temperature.

3.4 Optical properties

Transparency and refractive index are the most important optical properties in composite materials. Ultraviolet-visible spectroscopy is often used to determine the transparency of materials. Soliman et al. [107] prepared SiO₂/PVA composites using ultrasonic dispersion technology and the solution casting method. As shown in Figure 16(a), the increase in the concentration of nano-SiO₂ in the PVA matrix contributes to an increase in the light absorption of the composite film. The absorbance increases with the increase in SiO₂ concentration, which may be due to strong hydrogen bonding between nano-SiO₂ and the hydroxyl groups of PVA, which enhanced the binding effect between PVA and SiO₂. Figure 16(b) shows the transmission spectrum of the PVA/SiO₂ nanocomposite film. The results show that the transmittance of all polymer films can reach about 85% and remains constant in the visible and near-infrared (NIR) regions (400–1,000 nm), which is due to the low energy of incident photons. Furthermore, in the UV region (200–400 nm), the transmittance decreases with increasing SiO₂ nanoparticle concentration and shifts to lower wavelengths in the UV region. This is due to the fact that the incident photon energy can sufficiently be absorbed by the electrons and transferred from a lower to a higher state. Therefore, the light transmission of the PVA/SiO₂ nanocomposite film decreases, while the absorption increases.

Figure 16

UV-Vis (a) absorption spectrum and (b) transmission spectrum of PVA/SiO₂ nanocomposite films [107]; Copyright: 2020 with permission from Wiley.

SiO₂ is often used as a matting agent because of its porous structure and large pore size. It can be combined with polymers to produce polymer films or coatings with low gloss, high haze, and diffuse reflection matting effects [108,109]. Mousa et al. [110] prepared transparent films loaded with PVA/polyethylene glycol (PEG) blends with different nano-SiO₂ contents via solution casting. The effect of SiO₂ nanoparticle loading on the optical properties of selected PVA/PEG mixing ratios was investigated in the spectral range of 200–700 nm. The polymer surface becomes rough due to the presence of SiO₂, and diffuse reflection can occur, which assists in achieving extinction. The results show that the refractive index and extinction coefficient of the composite increase with the increase in SiO₂ content, and the requirements for SiO₂ are not high, making it very suitable for use in a photoelectric field.

3.5 Gas transport properties

Diffusivity, permeability, solubility, and selectivity are important gas transport properties of composites. The process of gas transport in a membrane generally occurs as follows: the gas first dissolves on the surface of the composite, then diffuses in the material, and finally moves to the downstream side of the composite material for desorption. Therefore, the microstructure of the composite membrane plays a key role in gas transport performance [111,112]. Ibrahim [113] prepared an ultra-thin PS/SiO₂ hydrophobic coating to improve the water vapor barrier of packaging paper. The results show that the resulting wrapping paper has obvious water repellency, which is affected by the coating thickness. On the one hand, as part of the coating was deposited in the paper gap, water molecules struggled to pass through. On the other hand, the coating itself is hydrophobic and forms a protective layer, and the solubility of water molecules on the paper surface is reduced. When the content of SiO₂ was increased, there were fewer active sites available to combine with the water molecules in the paper, and the moisture content thus decreased accordingly. However, when the content of SiO₂ was more than 4%, the agglomeration phenomenon led to an increase in the free volume of the coating and reduced the effect on water molecules, which manifested as an increase in the paper moisture content.

Ahmadizadegan et al. [114] prepared composite membranes for gas separation by introducing cellulose-modified SiO₂ into PE using ultrasonic radiation. Taking CH₄, O₂, CO₂, and N₂ as the research objects, the gas transport characteristics of the composite membrane were studied. The results show that the permeability of the membrane was improved as SiO₂ and cellulose increased the free space volume of PE, enabling small gas molecules to diffuse to the other side of the membrane through these gaps. Additionally, the hydroxyl groups on the surfaces of the SiO₂ and cellulose formed polar space, and the solubility of the polar CO₂ with a double bond was enhanced compared with pure PE, meaning the membrane containing SiO₂ is suitable for separating CO₂ from CH₄ and N₂.

4.1 The influence of SiO₂ concentration and particle size

Yuan et al. [120] prepared amino-functionalized nano-SiO₂ (SN-NH₂) by co-condensation of TEOS and APTES. Then, PP-MAH was grafted onto SN-NH₂ by melt blending to obtain PP-g-SiO₂. PP/PP-g-SiO₂ nanocomposites with different SiO₂ contents were obtained by blending PP-g-SiO₂ with the original PP matrix. As shown in Figure 17, the storage modulus (G′), loss modulus (G″), and complex viscosity (|η ^*|) of the nanocomposites were significantly improved with an increase in the content of PP-g-SiO₂. This is because there is a strong interaction between SiO₂ and the PP matrix, such that PP-g-SiO₂ is entangled on the molecular chain of the PP matrix, and the crosslinked network structure that forms greatly hinders the movement of the polymer molecular chain, thus significantly improving the melt strength of the PP matrix resulting in the nanocomposites having high viscosity and satisfactory elasticity as well as the polymer having improving processability.

Figure 17

Frequency responses of (a) storage modulus (G′), (b) loss modulus (G″), and (c) complex viscosity (|η ^*|) for PP and PP composites [120]; Copyright: 2018 with permission from Elsevier Science Ltd.

Yu et al. [121] prepared a series of shear thickeners based on the suspension of nano-SiO₂ in PEG. The viscosity versus shear rate for SiO₂ fluids of different particle sizes and concentrations is given in Figure 18. It can be seen in Figure 18(a) and (b) that the nano-SiO₂ have particle sizes of 15 and 30 nm, and there is significant shear thickening behavior when the fluid concentration is 10%. The viscosity of the fluid increases significantly from 1.18 to 19.52 Pa s⁻¹ at a concentration of 15% SiO₂ of 15 nm size. In Figure 18(c)–(e), the particle size of SiO₂ is on the micron scale (2, 5, and 10 μm), and here the fluid with a concentration of 70% exhibits the clearest shear thickening behavior, with the fluid viscosity increasing from 1.13 to 18.98 Pa s⁻¹. In both cases, whether using nano-SiO₂ or microparticles, the viscosity of the fluid increases by an order of magnitude after shear thickening. The higher the SiO₂ concentration, the higher the initial viscosity and peak viscosity of the fluid and, thus, the higher the critical shear rate and peak shear rate, meaning the shear thickening effect is more significant [122]. When the concentration is low, there is little change in the fluid viscosity. At low concentrations, the reduction in solid particles leads to weakening of the interaction between SiO₂ and PEG, and the fluid viscosity decreases accordingly. It can be seen that both nano- and microscale SiO₂ have significant viscosity enhancement effects, though nanoparticle fluids are more sensitive to external shear fields.

Figure 18

Plots of viscosity versus shear rate for the SiO₂ fluids with different particle sizes and concentrations: (a) 15 nm, (b) 30 nm, (c) 2 μm, (d) 5 μm, and (e) 10 μm [121]; Copyright: 2018 with permission from Springer Nature.

Particle size is a key factor in changing the fluidity and interphase interactions of hybrid-filled nanocomposites. Wang et al. [123] performed surface modification using Gold nanoparticles-grafted nano-SiO₂ (JGS), which was blended with polyvinylidene fluoride/poly-l-lactide (PVDF/PLLA) to prepare the nanocomposites. Here two JGSs of different particle sizes (20 and 110 nm) were introduced for comparison, and the results regarding their rheological properties show that (Figure 19(a)–(d)) the G′, G″, and |η ^*| of the 20 nm JGS compatibilized blend were much higher than those of the 110 nm system, but the tan δ was lower. This indicates that the addition of JGS of a smaller size has a greater impact on the rheological properties of the system. This is because, as the size of the nanoparticles decreases, the number of nanoparticles per unit volume fraction increases, their relative surface area increases, and the number of polymer chains adsorbed by the nanoparticles also increases, so the |η ^*|, G′, and G″ increase.

Figure 19

Rheological properties of PVDF/PLLA blends filled with two nano-sized JGS (20 nm/110 nm): (a) G′, (b) G″, (c) tan δ, and (d) |η*| [123]; Copyright: 2017 with permission from ACS.

4.2 The influence of temperature

Hu et al. [124] used maleic anhydride modified β-cyclodextrin (MAH-β-CD) copolymerized with acrylamide (AM) and 2-acrylamido-2-methyl propane sulfonic acid to form a copolymer (denoted as AAMC) through redox free-radical polymerization. And mixed nano-SiO₂ to prepare an AAMC/SiO₂ hybrid agent. It can be seen from Figure 20 that the viscosity of both the AAMC solution and the AAMC/SiO₂ mixture strongly depends on the temperature, whereby the system viscosity decreases with increase in the temperature. The viscosity of the AAMC/SiO₂ mixture was two times higher than that of pure AAMC in water at all temperatures. This is due to hydrogen bonding between the hydroxyl groups on the surface of SiO₂ and AAMC, forming a three-dimensional network structure of AAMC/SiO₂. However, the hydrogen bonding of AAMC/SiO₂ hybrids is temperature-sensitive, and the network connections and bridges between AAMC chains dissociate at higher temperatures, resulting in a sizeable reduction in viscosity. For AAMC, increasing temperature reduces the intermolecular interactions and the curling of polymer macromolecules, which results in hydrolysis of amide groups in AAMC into carboxyl groups, resulting in decreased viscosity. It can thus be seen that temperature has a great influence on the rheological properties of SiO₂/polymer nanocomposites.

Figure 20

The effects of temperature on viscosity of composite materials [124]; Copyright: 2018 with permission from Elsevier Science BV.

4.3 The influence of pH

Chen et al. [125] prepared three colloidal suspensions by mixing SiO₂, SiO₂–NH₂, and SiO₂–COOH microspheres with a PEG fluid medium and carefully analyzed their rheological behavior. They found that the rheological behavior of the three suspensions varied differently in response to changing pH values. The shear rate and viscosity curves of suspensions with different pH values are shown in Figure 21.

Figure 21

Viscosity curves of three suspensions at different pH values and critical shear rates: (a) SiO₂ suspension, (b) SiO₂–H₂ suspension, (c) SiO₂–COOH suspension, and (d) critical shear rate of the suspension [125]; Copyright: 2019 with permission from RSC Advances.

From Figure 21(a) and (b), it can be seen that the curves of the SiO₂ and SiO₂–NH₂ suspensions have similar trends, that is, as the pH value increases, the critical shear rate (r) and viscosity increase significantly, and the initial thinning and subsequent thickening effects become dominant. For the SiO₂–COOH suspension in Figure 21(c) and (d), the effect is the opposite. In the SiO₂ and SiO₂–NH₂ suspensions, SiO₂ and PEG are weakly acidic due to the −OH group, and SiO₂–NH₂ particles are weakly alkaline due to the dissociation of −NH₂.

Therefore, the SiO₂ microspheres more easily bound to PEG when alkaline solution was added to the suspension. When an acidic solution was added, PEG more readily bound to the SiO₂–NH₂ microspheres, resulting in weak shear thickening and an almost constant shear thinning behavior of the system [126]. In the SiO₂–COOH suspension, the acidity of particles in the aqueous solution was stronger than that of the SiO₂ microspheres, and the law of selective binding was opposite to that in the SiO₂ and SiO₂–NH₂ suspensions, resulting in different rheological behaviors. In a word, the pH value also has a significant effect on the rheological behavior of the SiO₂-reinforced polymer system.

5.1 Coatings

SiO₂, a rigid inorganic nanoparticle, possesses unique chemical and interfacial properties. It has an acceptable polymer film-forming performance, and SiO₂ coatings usually exhibit excellent thermal stability and mechanical properties. The performance can be markedly improved by addition of only small amounts of SiO₂. Thus, SiO₂/polymer nanocomposites offer great potential for use as high-performance and multifunctional coatings [127,128,129]. Table 3 lists the applications of different SiO₂ and other nanofillers/polymer composites in coatings.

Li et al. [130] reported a UV-triggered self-healing SiO₂/PDA microcapsule, which they combined with EP to prepare a polymer coating material with UV-shielding and self-healing properties, the process is shown in Figure 22(a). UV-curable EP (E-51 and A1815) and UV catalysts (PI6992) were encapsulated into SiO₂ shells using interfacial and in situ polymerization strategies. Then, by virtue of the strong hydrogen bonding between the PDA chain and the surface of SiO₂, an ultra-thin PDA layer for UV protection was deposited in the SiO₂ mesopores to obtain SiO₂/PDA microcapsules. At the same time, the π–π stacking effect and hydrogen bonding between SiO₂/PDA microcapsules and EP results in microcapsules that have higher compatibility with EP, which greatly promotes the dispersion of SiO₂. The UV shielding principle and compatibility of SiO₂/PDA microcapsule-enhanced coatings are shown in Figure 22(b). A good self-healing capacity was maintained after 192 h of aging under UV light irradiation (Figure 22(c)–(f)), which is attributed to enhancements in the UV shielding ability and compatibility. Therefore, this novel SiO₂/PDA microcapsule with UV shielding and self-healing functions has great potential applicability in polymer coatings on spacecraft.

Figure 22

(a) Preparation process of SiO₂/PDA microcapsules, (b) principle of UV shielding and compatibility of SiO₂/PDA microcapsule-reinforced coatings, (c and d) self-healing effect of SiO₂/PDA microcapsule coating before UV irradiation aging, (e and f) self-healing effect after UV irradiation aging [130]; Copyright: 2022 with permission from Elsevier Science BV.

In view of the excellent physical properties of the coating, Lyu et al. [131] prepared a kind of SiO₂ polymer hybrid shell-coated fluorescent material, which was used in luminescent waterborne road marking coatings. This hybrid coating has better compatibility and thermal stability with luminescent materials than coatings with a single polymer shell and excellent durability, and it does not interfere with the fluorescent materials. When combined with the physical properties of porous SiO₂, this nanocomposite coating has great potential use in the field of release and control. Research has shown that combination with SiO₂ significantly improves the hardness of PAA, providing the coating good wear resistance. Nguyen et al. [132] prepared a polyaniline (PANi)/SiO₂ nanocomposite for improving the antistatic and mechanical properties of EP coatings. When the SiO₂ loading was 17.2 wt%, the coating exhibited good mechanical properties and allowable surface and volume resistivities. Le et al. [133] reported a composite coating of PAA/SiO₂ grafted silver nanoparticles (SiO₂–Ag). The most excellent adhesion and wear resistance (2.61 N/mm² and 115.78 L/mil) were obtained when the SiO₂–Ag loading amount was 2 wt%. At the same time, the introduction of Ag nanoparticles also endows the composite with good antibacterial properties. This composite coating has broad prospects as architectural coatings.

Bahramnia et al. [134] introduced a coating with excellent properties, prepared using multi-walled carbon nanotubes (MWCNTs)/SiO₂ as hybrid nanofillers and EP/PU as the polymer matrix to enhance the wear resistance of steel pipes. MWCNTs and nano-SiO₂ are responsible for strengthening and toughening the hybrid EP/PU matrix, respectively. Since nano-SiO₂ forms strong hydrogen bonds with the polar groups of EP and PU chains, the compatibility between them is increased, which significantly improves the abrasion resistance of coatings and the adhesion between carbon steel substrates. At the same time, nano-SiO₂ have self-lubricating properties, and appropriately increasing the content of nano-SiO₂ will lead to a higher coverage of nanoparticles, thereby improving wear resistance. An EP/PU matrix containing 0.75 wt% MWCNTs and 2.5 wt% nano-SiO₂, compared with other ratios, was found to have optimal anti-wear performance and hardness and elastic modulus.

SiO₂ can be used to prepare superhydrophobic coating with a high roughness level. However, the increase in SiO₂ content usually leads to a decrease in the composite transparency. Therefore, it is necessary to control the amount of SiO₂ to balance the transparency and superhydrophobicity level. Ke et al. [135] prepared a PU/fluorinated acrylic copolymer/SiO₂ (PU/FAP/SiO₂) superhydrophobic wear-resistant composite coating with a water contact angle (WCA) of 159° and a rolling angle of 3°. Meanwhile, the optical transmittance of the coating is 88%, indicating that the coating has good transparency. Soo et al. [136] prepared a superhydrophobic durable coating by introducing fluorinated SiO₂ (F-SiO₂) into polycaprolactone (PCL). When the F-SiO₂ content is 2 wt%, the WCA is 163°, which is mainly attributed to the close physical bond between F-SiO₂ and the polymer matrix, which further increases the surface roughness.

SiO₂ has high thermal stability and insulating properties, and can be used to prepare flame retardant and insulating coatings. Jiang et al. [137] prepared an environmentally friendly flame retardant by introducing hollow mesoporous SiO₂ (HM-SiO₂), chitosan (CS), and cellulose phosphate (PCL) into EP via layer-by-layer self-assembly. Besides Si, HM-SiO₂ contains P, S, and other flame-retardant elements. Meanwhile, the synergistic effect between HM-SiO₂ and CS/PCL endows EP with excellent flame retardancy. Chen et al. [138] synthesized a SiO₂-encapsulated Ag nanowire (AgNWs@SiO₂) by a sol–gel method, then incorporated into EP. AgNWs has high aspect ratio and thermal conductivity but have poor compatibility with EPs. The interfacial interaction with EP was significantly improved by wrapping core–shell nanostructured SiO₂ on AgNWs, thereby enhancing the thermal conductivity of EP matrix. Meanwhile, the insulating SiO₂ nanolayer effectively avoids the formation of the conductive network of AgNWs in the EP, resulting in the composite with high electrical insulation.

Therefore, it is precisely because of the unique structure and easy-to-modify specific surface of SiO₂, as well as the excellent synergistic effect of SiO₂ and other nanoparticles, the SiO₂/polymer nanocomposites has a wide range of applications in wear-resistant, UV-resistant, heat-resistant, and insulating coatings. In addition, not only in the abovementioned conventional applications, but more attention should be paid to the development of some intelligent and multifunctional nano-coating materials, such as temperature and light-responsive self-healing, superhydrophobicity, and antibacterial coatings and so on.

5.2 Smart devices

SiO₂ has been increasingly used in smart devices due to its large specific surface area, unique pore structure, easy-to-modify surface, and excellent conductivity, barrier, wave absorption, and shielding properties. It has found uses in flexible electronic devices as well as in self-assembly, self-healing, and self-cleaning in combination with other smart materials [144,145,146,147]. Table 4 lists the applications of different SiO₂ and other nanofillers/polymer composites in smart devices.

Li et al. [148] fabricated a lightweight, flexible, and superhydrophobic polyacrylonitrile (PAN)@SiO₂–Ag nanocomposite fiber film with electromagnetic interference (EMI) shielding by electrospinning. Due to the strong polar groups of PAN-CN and the large number of hydroxyl groups on the surface of SiO₂, which provide a large number of attachment sites for Ag precursors, Ag nanoparticles (AgNPs) are uniformly attached to the surface of PAN fibers. Then, AgNPs were deposited on the surface of the PAN@SiO₂ electrospun nanofibers via wet chemical deposition such that the resulting nanofibers had a core–shell structure; the preparation scheme is shown in Figure 23(a). The conductive PAN@SiO₂–Ag films gain superhydrophobic properties after being treated with 1H, 1H, 2H, and 2H-perfluorodecanethiol (PFDT), resulting in a change in the WCA of up to 156.99° (Figure 23(b) and (c)). In Figure 23(d), we see that the composite membrane also exhibits high electrical conductivity (about 177.88 S/m). The shielding performance also gradually increases with time. Here the average shielding effectiveness (SE), specific shielding effectiveness (SSE), and unit volume shielding effectiveness (SSE/t) reach 82 dB, 367 dB cm³ g⁻¹, and 73,478 dB cm²g⁻¹, respectively, (Figure 23(e)–(g)). This study provides a simple method to fabricate biomimetic composite films with high electrical conductivity and electromagnetic shielding properties and wide potential applicability, such as in wearable and flexible sensors.

Figure 23

(a) Schematic diagram of the preparation of PAN@SiO₂-4 wt% Ag-X-PFDT composite film, (b and c) PFDT concentration and immersion time on the contact angle (CAs) of PAN@SiO₂-4 wt% Ag film, (d) PAN@SiO₂-4 wt% Ag film conductivity of SiO₂-4  wt% Ag-X composite film, (e) SE_A, SE_R, and SE_T of PAN@SiO₂-4 wt% Ag-X film, SE_A, SE_R, and SE_T represent absorption, reflection, and total EMI shielding values, (f and g) comparison of EMI SE and SSE of various composite fabrics [148]; Copyright: 2020 with permission from Elsevier Science Ltd.

Kar et al. [149] reported the fabrication of a versatile, simple, and flexible hybrid piezoelectric electronic skin (HPES) with tactile mechanosensing, energy harvesting, self-cleaning, ultraviolet (UV) protection, and microwave shielding properties (Figure 24(a)). The HPES is mainly composed of SnO₂ nanosheets@SiO₂ (SiO₂-encapsulated tin oxide nanosheets)/PVDF. SiO₂ encapsulated on SnO₂ nanosheets (NS) can delay the agglomeration of 2D SnO₂NS in the PVDF matrix. The presence of well-dispersed SnO₂NS@SiO₂ in the PVDF matrix endows the nanocomposite with excellent piezoelectric, dielectric, hydrophobic, EMI shielding, and UV absorption properties. The SS-15.0 (15 wt% SnO₂NS@SiO₂/PVDF) nanocomposite film can absorb about 99.97% of the incident electromagnetic radiation. Figure 24(b) shows that at 12 GHz, the values of SE_A and SE_R are 32.9 and 2.03 dB, respectively. Figure 24(c) shows that a PES flexible device (consisting of an SS-15.0 film sandwiched between two flexible copper electrodes) was successfully used to monitor motions in different body parts, including heel motion, toe motion, and wrist flexion mechanical energy. Figure 24(d) shows that HPES is hydrophobic (WCA = 85.6°), endowing it with the property of self-cleaning. Figure 24(e) shows the monitoring of human directional finger joint movement using a health data glove. This novel composite material could open up new avenues for the development of flexible multifunctional electronic skins (e-Skin) in green energy harvesting, smart robotic hands, and healthcare monitoring.

Figure 24

(a) Schematic diagram of HPES application, (b) total shielding effectiveness of pure PVDF and SS films as a function of frequency, (c) PES-generated voltages under (i) heel movement, (ii) toe movement, and (iii) wrist flexion, (d) self-cleaning process of PES membrane, (e) monitoring of human directional finger joint movements through a health data glove [149]; Copyright: 2019 with permission from ACS.

Xiang et al. [150] reported a novel three-dimensional porous cellulose acetate (3DPCA)/SiO₂-based hybrid membrane for use in an all-day radiative self-cooling system. The hybrid membranes were designed with micropores of about 5 µm for sunlight reflection, while single-sided autodeposited SiO₂ microspheres were used for infrared radiation. The 3DPCA/SiO₂ thin films exhibited ultra-high average solar reflectance (R _solar) and average infrared emissivity (ε _IR) values of 96, 97, and 95%, respectively, establishing them as state-of-the-art. In addition, the achievable cooling temperature was around 8.6°C at night and around 6.2°C during the day, both of which are the best that have been achieved anywhere in the world. This enhanced performance is mainly attributed to the effective synergy established within the unique three-dimensional porous architecture, whereby SiO₂ microspheres are utilized as infrared heat radiators to maximize radiation output, while 3DPCA provides high solar reflectivity to minimize solar heat gain. This unique and facile self-cooling material is expected to find wide applicability in building cooling, cold chain transportation, and radiant cooling fabrics for personal thermal management.

Chen et al. [151] prepared a flexible superhydrophobic PU/SiO₂ modified graphene (MS-GPE) composite film with self-healing ability. The disulfide bond of hydroxyethyl disulfide was introduced into the PU backbone, and the hydrogen bonds formed between MS-GPE and multiple amine groups of PU were introduced to achieve dual self-healing functions on the surface and the interior. A 100% healing rate can be achieved after heating at 60°C for 40 min. The obtained composite film also exhibits excellent electrical conductivity and superhydrophobicity (WCA = 154.5°), which can be used in wearable electronic devices. Dutta et al. [152] reported a NiO@SiO₂/PVDF nanocomposite. NiO can enhance the piezoelectric properties of PVDF, but it severely limits the formation of conductive paths due to its easy agglomeration. In order to solve the problem, uniformly coating a layer of non-conductive nano-SiO₂ on the surface of NiO is an effective method. This enables NiO@SiO₂ to be dispersed discretely and uniformly in the PVDF matrix with high loading rate. The PVDF-based triboelectric nanogenerator fabricated under a 15 wt% NiO@SiO₂ load has a maximum output voltage of ∼53 V, a current density of ∼0.3 μA/cm², and an instantaneous power density of about 685 W/m³. At the same time, a gentle pressing of a human finger on the nanogenerator can generate an output voltage sufficient to illuminate 85 LEDs. The nanocomposite has great potential applications in human motion-based energy harvesting and tactile electronic skin mechanosensing.

In short, SiO₂/polymer nanocomposites are gaining popularity in smart device applications. A series of smart devices such as self-healing fabrics, wearable triboelectric nanogenerators, and flexible electronic skins with temperature, pH, and humidity responses have been developed. It is worth noting that in order to fully exploit the excellent properties of SiO₂, the synergistic effect of size and structure between the composite systems should be considered. Finally, we will further develop refined, diversified and lightweight intelligent devices to meet people’s needs.

5.3 Biomedicine

SiO₂ is nontoxic to cells and has good biocompatibility. Furthermore, its nanocomposites can be widely used in drug release control, targeted therapy, detection, medical imaging, and other biomedical fields, giving it great potential in biomedicine generally. Table 5 lists the applications of different SiO₂ and other nanofillers/polymer composites in biomedicine.

Ma et al. [162] prepared a gold nanorod (AuNRs)/mesoporous SiO₂/hydroxyapatite (HAP) nanocarrier with a thermoresponsive polymer cap for remote-controlled drug delivery; the preparation process is shown in Figure 25. By introducing pH-responsive HAP into mesoporous SiO₂, the hybrid nanocarriers were able to achieve HAP degradation in acidic media. Heat/pH-responsive poly(N-isopropylacrylamide-co-acrylic acid) (PNA, P(NIPAM–co–AAc)) was used as a smart polymer cap to control the drug delivery of hybrid nanoparticles, relying on the fact that AuNRs/nano-SiO₂ are responsive to NIR light, and long-range NIR irradiation was employed to induce phase transition in the hybrid nanoparticles and subsequent smart drug delivery, as shown in the Figure. The P(NIPAM–co–AAc)-capped AuNRs/SiO₂/HAP nanoparticles also exhibited excellent biocompatibility. This scheme provides a facile method for the preparation of multi-responsive nanocarriers with excellent biocompatibility and degradability by combining smart polymers with hybrid inorganic frameworks, which has great potential applicability for use in remote-controlled drug release.

Figure 25

Preparation of AuNRs/SiO₂/HAP/PNA nanoparticles [162]; Copyright: 2019 with permission from Springer Nature.

The larger the specific surface area of nanocontainers, the higher the drug loading efficiency. Afrashi et al. [163] used SiO₂ aerogels with highly mesoporous structures as containers. As the structure of the aerogel is amorphous, the energy required for drug release is reduced, and the pore size of the porous aerogel and PVA can reach the nano scale, meaning the corresponding drug release efficiency is improved. Compared with other materials with a pore structure, the most significant features of SiO₂ are that the pore size is adjustable and the corresponding operation is very simple, which provide a larger space for the design of a drug delivery carrier structure. Figure 26 shows the principle of the preparation and release control of drug-loaded mesoporous SiO₂.

Figure 26

Preparation of polymer@mesoporous SiO₂-DOX (doxycycline) and principle of pH induced release [164]; Copyright: 2019 with permission from Elsevier Science BV.

Wang et al. [165] prepared a core–shell hybrid of AuNRs/PPy/mesoporous SiO₂ (MSNs) for photothermal therapy of cancer. The results show that the photothermal effect induced by NIR light is beneficial to promote the release of doxorubicin (DOX) anticancer drug, and the drug release rate is over 65%. This is mainly attributed to the coordinated effect of MSNs with high surface area and tunable mesoporosity on photothermally responsive AuNRs to prevent their aggregation, thereby enhancing their stability and drug-loading capacity.

SiO₂ with good biocompatibility can be used as a container for fluorescent substances, and to label biological proteins. Huang et al. [166] introduced the dyes PHE–OH and poly(2-ethyl-2-oxazoline) into mesoporous SiO₂ to prepare a composite with strong fluorescence emission. The composite showed good compatibility and hydrophilicity, enabling it to emit strong yellow fluorescence. This effect was mainly introduced by the concentration of the solution and can be exploited in drug loading, protein labeling, etc. Similarly, the polymer can be used to further functionalize luminescent composites. Timin et al. [167] used a magnetic polymer to magnetize SiO₂, polymerized the modified SiO₂ with polymer guanidine, and then functionalized the prepared composite materials with the novel bilirubin-induced fluorescent protein UnaG. When bilirubin was detected in solution, the composite particles were able to emit fluorescence, and the presence of magnetized SiO₂ could also separate bilirubin via the effect of a magnetic field, which can be used for liver function clinical diagnosis. Xia et al. [168] prepared a bioactive PVA/SiO₂ hybrid fibers by sol–gel combined electrospinning technique. Compared with pure PVA fibers, PVA/SiO₂ hybrid fibers exhibited significantly improved ability to induce bone-like apatite formation in vitro and biocompatibility. It has potential applications in bone tissue engineering. Guo et al. [169] reported a biocompatible L-polylactic acid (PLLA)/halloysite nanotube (HNTs) @SiO₂ composite scaffold. Nano-SiO₂ were grown in situ on HNTs by TEOS hydrolysis. The results show that the tensile strength and modulus of the composite are 18.12 MPa and 637.46 MPa, which are 72.08 and 51.54% higher than PLLA, respectively. This is mainly attributed to the improved interfacial adhesion and dispersion between HNTs and PLLA by nano-SiO₂. In addition, the PLLA/HNTs@SiO₂ scaffold exhibited excellent apatite formation ability and osteoblast proliferation and differentiation ability. It has great application potential in bone tissue engineering.

In conclusion, SiO₂/polymer nanocomposites play a pivotal role in biomedicine. Especially in drug-loaded nanocapsules, human tissue engineering and medical device protection. This is mainly attributed to the nontoxic, tunable size and pore size, and excellent mechanical properties of SiO₂. Its synergistic effect with biocompatible polymers can further improve the cytocompatibility and value-added differentiation ability of composite materials, which makes outstanding contributions to key research directions such as multi-responsive sustained-release drug nanocarriers and human tissue scaffolds.

5.4 Environment

SiO₂/polymer nanocomposites are also widely used in gas adsorption and separation. Table 6 lists the applications of different SiO₂ and other nanofillers/polymer composites in environment. Jeong et al. [177] prepared a polymer-grafted nanoparticle (PGN) composite film made of polybutyl methacrylate (PBMA)-grafted nano-SiO₂; the preparation process is shown in Figure 27(a). Specifically, PBMA was prepared by using PBMA and 3-(trimethoxysilyl)propyl 2-bromo-2-methylpropionate (ATRP) to initiate the grafting reaction on the surface of the functionalized nano-SiO₂. The selective permeability of CO₂, He, and CH₄ gas was controlled by adjusting the graft density (GD) of the PBMA and nano-SiO₂ (Figure 27(b)). GD and SiO₂ volume fractions have huge effects on gas permeability and selectivity. Compared with pure polymer, when the volume fraction of nano-SiO₂ was 4%, the gas permeability increased significantly with the increase in GD (Figure 27(b)). The He/CH₄ and CO₂/CH₄ selectivities were improved by 38 and 50%, respectively (Figure 27(c)). Therefore, rationally adjusting the synergistic effects of nano-SiO₂ volume fraction and GD can improve the gas permeability and selectivity of polymer membranes, thereby enabling the transport, collection, and purification of complex gases, ultimately facilitating environmental protection.

Figure 27

(a) PBMA grafting process of nano-SiO₂, (b) relationship between SiO₂ graft density and gas permeability, and (c) gas selectivity of PBMA PGN films as a function of SiO₂ volume fraction [177]; Copyright: 2021 with permission from ACS.

SiO₂/polymer nanocomposites can not only be used to enhance gas separation performance but also have significant advantages regarding the separation and treatment of water. By using SiO₂ with hydrophilic and oleophobic properties, the ability of the composite membrane to adsorb and dissolve water (or oil) molecules is improved, and oil (or water) molecules are prevented from passing through the membrane, thus facilitating oil/water separation. The separation efficiency is related to the pore size of the membrane material. Cao et al. [178] sprayed modified PU and hydrophobic SiO₂ onto the copper mesh to prepare a simple oil–water separation membrane that could separate oil substances in high-temperature environments (nearly 100°C). The results show that the copper mesh has a high separation efficiency for kerosene, and a satisfactory oil/water separation effect is maintained even after being recycled dozens of times. Li et al. [179] prepared a poly(dimethylsiloxane)-block-poly(4-vinylpyridine) (PDMS-b-P4VP)/SiO₂ fiber for oil/water separation using electrospinning technique pad (Figure 28(a)). The results show that the composite exhibits good pH-switchable oil/water wettability, and effectively separates oil (hexane)/water mixtures under gravity drive by adjusting the pH. The mat exhibits superhydrophilicity (pH = 4) for acidic water and superoleophilic for water at pH 7 (Figure 28(b)). The fibermat with 1.0 wt% SiO₂ can significantly improve thermal stability and pH-switchable wettability. Benefiting from the improved surface wetting properties of SiO₂, the composite fiber mats exhibited better separation performance with a water flux of about 32,000 L h⁻¹ m⁻². The composite material has broad prospects for making smart oil/water separation materials.

Figure 28

(a) (PDMS-b-P4VP)/SiO₂ fiber mat preparation and pH-controllable oil/water separation process; (b) oil wettability of pure polymer and polymer/silica composite fiber mats in aqueous media at different pH values [179]; Copyright: 2016 with permission from ACS.

SiO₂/polymer nanocomposites also make important contributions to the treatment of heavy metal ions and dye wastewater [180,181,182]. Alzahrani et al. [183] prepared magnetic polymer-based nanocomposites by co-precipitation polymerization from PPy/Fe₃O₄/SiO₂. The surface modification of magnetic Fe₃O₄ nanoparticles by SiO₂ improves the compatibility with PPy and adsorption capacity. The maximum adsorption capacity of the composite for the removal of hexavalent chromium ions (Cr(vi)) and Congo red in water are 298.22 and 361.43 mg g⁻¹, and the composites were recyclable for 6 times. This nanocomposite is a promising heavy metal/dye water treatment material. Plohl et al. [184] prepared a core–shell magnetic SiO₂ particles (MNPs), and its surface was coated with a polymer. The results show that the maximum adsorption capacity of Cu²⁺ reached 143 mg g⁻¹, and the adsorbent could be recycled. This kind of adsorbent is very suitable for sludge, which has the characteristics of high efficiency and environmental protection. Different polymers can be flexibly used to modify SiO₂ and greatly improve its adsorption performance in relation to certain heavy metals. Hu et al. [185] prepared a cheap and efficient SiO₂/polyacrylamide adsorbent (SP-C). It was found that SP-C exhibited an excellent adsorption capacity for Cd²⁺ in wastewater, and the optimal adsorption period lasted for around 1 h. Bucatariu et al. [186] grafted polycation, polyethyleneimine, polylysine, polyvinylamine, polyaniline hydrochloride, and polyanion onto SiO₂ by layer deposition. The composite particles prepared by this method were used to capture Cu²⁺, Pb²⁺, Ni²⁺, and Fe²⁺. It was found that the main factors affecting the adsorption capacity of the composites were contact time, pH value, ionic strength, and the ratio of the functional group number.

To sum up, SiO₂/polymer nanocomposites are mainly used to separate pollutants in gas transport separation, oil/water separation, heavy metal ions and dye wastewater. Its main use is to treat industrial waste gas, wastewater, and sludge, and it is often recyclable for sustainable development. In addition, the costs of SiO₂ and the adsorbent preparation process are not high, making it very suitable for environmental applications.