Rivet-Inspired Modification of Aramid Fiber by Decorating with Silica Particles to Enhance the Interfacial Interaction and Mechanical Properties of Rubber Composites

A rivet–inspired method of decorating aramid fiber (AF) with silica particles (SiO2) is proposed to produce SiO2@AF hybrid materials that have largely enhanced interfacial interaction with the rubber matrix. AF was firstly surface-modified with polyacrylic acid (PAA) to obtain PAA–AF, and SiO2 was silanized with 3-aminopropyltriethoxysilane to obtain APES–SiO2. Then, SiO2@AF was prepared by chemically bonding APES–SiO2 onto the surface of PAA–AF in the presence of dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP). With the incorporation of SiO2@AF into the rubber matrix, SiO2@AF hybrid materials with high surface roughness can play a role as ‘rivets’ to immobilize large numbers of rubber chains on the surface. The tear strength and tensile strength of rubber composite that filling 4 phr SiO2@AF are dramatically increased by 97.8% and 89.3% compared to pure rubber, respectively. Furthermore, SiO2@AF has superiority in enhancing the cutting resistance of rubber composites, in contrast with unmodified AF and SiO2. SiO2@AF is suitable to be applied as a novel reinforcing filler in rubber composites for high performance.


Introduction
Natural rubber (NR), an important biopolymer, has been presented as one of the most attractive materials owing to its wonderful properties, such as great elasticity and flexibility, antivirus permeation, low cost and corrosion resistance [1][2][3]. However, NR usually needs to be reinforced with various fillers for most practical applications in diverse areas (e.g., tires, sealing material, sport equipment, surgical gloves and adhesives), due to its inherent low strength and modulus [4,5]. For decades, SiO 2 particles have been widely applied as active fillers in rubber composites to enhance their mechanical properties and to deliver multi-functional characteristics [6,7]. Compared to traditional carbon black, SiO 2 can effectively improve wet skid resistance and show higher abrasion resistance for tires, and is an ideal filler for the preparation of green tires [8,9]. At the same time, tires filled with SiO 2 particles have lower rolling resistance, which has the great advantage of reducing vehicle energy consumption. Although SiO 2 particles are extensively assumed as a superior substitute to common carbon black, inhibiting SiO 2 particles from aggregating is extremely challenging because of their high surface energy and large surface area [10]. Heavily hydrophilic SiO 2 particles exhibit a rather poor compatibility with hydrophobic rubber, which leads to weak interfacial adhesion between SiO 2 and the rubber matrix. These devilish puzzles seriously impede the popularization and application of SiO 2 fillers in rubber systems [11].

Grafting PAA onto AF
AF was ultrasonically cleaned in acetone solution and irradiated below a UV lamp for 8 min for pretreatment, with a relative humidity (RH) of 65 RH%. UV radiation can, in a short time, destroy the surface chemical structure of AF and can form new groups without affecting its core structure ( Figures S1 and S2). AF 4.0 g, PAA 5.0 g, DCC 5.0 g and DMAP 0.5 g were dispersed into 500 mL THF under sonication for 1h. Then, the mixture was heated at 60 °C for 24 h to complete the amidation reaction. Finally, the product was filtered and washed with deionized water, and then dried in an oven at 80 °C until a constant weight was obtained. In this reaction, the number of carboxyl groups is excessive in order to remove the amino group from the surface of the AF.

Silanization of SiO2
First, 5.0 g SiO2 was added into 300 mL ethanol/deionized water (5:5, volume ratio) under sonication to achieve a well-dispersed SiO2 suspension. Subsequently, 2 mL APES was added into

Grafting PAA onto AF
AF was ultrasonically cleaned in acetone solution and irradiated below a UV lamp for 8 min for pretreatment, with a relative humidity (RH) of 65 RH%. UV radiation can, in a short time, destroy the surface chemical structure of AF and can form new groups without affecting its core structure ( Figures S1 and S2). AF 4.0 g, PAA 5.0 g, DCC 5.0 g and DMAP 0.5 g were dispersed into 500 mL THF under sonication for 1h. Then, the mixture was heated at 60 • C for 24 h to complete the amidation reaction. Finally, the product was filtered and washed with deionized water, and then dried in an oven at 80 • C until a constant weight was obtained. In this reaction, the number of carboxyl groups is excessive in order to remove the amino group from the surface of the AF.

Silanization of SiO 2
First, 5.0 g SiO 2 was added into 300 mL ethanol/deionized water (5:5, volume ratio) under sonication to achieve a well-dispersed SiO 2 suspension. Subsequently, 2 mL APES was added into the SiO 2 suspension. The mixture was stirred at 60 • C for 12 h to accomplish the silanization. The product (APES-SiO 2 ) was filtrated, washed with ethanol and dried overnight at 60 • C in a vacuum oven.

Preparation of SiO 2 @AF
First, 4.0 g PAA-AF and 6.0 g APES-SiO 2 were dispersed in 400 mL THF solution under sonication for 0.5 h. Then, the mixture was stirred mildly for another 0.5 h to perform the electrostatic assembly between the negatively charged PAA-AF and the positively charged APES-SiO 2 . Finally, 2.0 g DCC and 0.5 g DMAP were added, and the reaction was maintained at 60 • C for 48 h. In order to deposit as much SiO 2 on the AF surface as possible, the amount of APES-SiO 2 was excessive. Thus, SiO 2 @AF hybrid materials with 48 wt% SiO 2 particles were obtained by drying at 60 • C overnight, after filtering the remaining APES-SiO 2 and washing with deionized water.

Preparation of the Natural Rubber Composites
NR was compounded with the ingredients and SiO 2 @AF on a two-roll mixer at room temperature. The basic formulation of the NR composites is shown in Table 1. SiO 2 @AF/NR composites were vulcanized at 143 • C up to the optimum time, as determined by a Rubber Process Analyzer (RPA). Furthermore, SiO 2 +AF/NR composites (SiO 2 +AF represents the mixture filler of untreated SiO 2 and AF with a weight ratio of 48:52) were prepared following a similar process.

Characterization
Fourier transform infrared spectroscopy (FTIR) was recorded by a Thermo Nicolet 6700 FTIR spectrometer (Waltham, MA, USA) in the 4000-600 cm −1 region at room temperature. A resolution of 4 cm −1 and a total accumulation of 24 scans were applied to obtain the high signal-to-noise spectra. In order to analyze the chemical elements of PAA-AF, APES-SiO 2 and SiO 2 @AF, X-ray photoelectron spectroscopic (XPS) analysis was conducted on an XPS analyzer (K-Alpha, Thermo, Waltham, MD, USA) with Al Kα radiation under an X-ray power of 150 W. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TA Q50, Newcastle, DE, USA) at a heating rate of 20 • C/min from 30 to 800 • C. Then, samples of 15 mg were isolated in a dry nitrogen atmosphere with a purge rate of 80 mL/min. The morphology of unmodified AF and SiO 2 @AF was observed by scanning electron microscope (SEM; JSM-7500F, JEOL, Tokyo, Japan) at an acceleration voltage of 10 kV. All of the fiber surfaces were sprayed with gold to prevent electrostatic charging. Differential scanning calorimetry (DSC) measurements were carried out on a DSC instrument (Q20, TA, Newcastle, DE, USA) with a nitrogen flow of 40 mL/min. Samples with a weight of 7-8 mg were scanned from 70 to 100 • C at a heating rate of 20 • C/min. The mechanical properties of the NR composites and pure NR were investigated using a universal testing machine (Hegewald & Peschke, Berlin, Germany). Specimens with a dumbbell shape were used for tensile characterizations according to GB/T 528-1998 at a crosshead speed of 500 mm/min. Tear tests were carried out with right-angle samples according to GB/T 529-1999. A cutting tester (RCC-1, Beijing Wanhui Yifang Science and Technology Development Company, Beijing, China) was utilized to analyze the cut resistance of the NR composites and the pure NR. The strip specimens were rolled at a speed of 720 cpm under a cutting frequency of 120 Hz for 20 min, and the loss of mass was recorded every 4 min. The weight loss was recorded as a function of cutting time in the measurement process.

Characterizations of SiO 2 @AF
The grafting degree of PAA-AF and APES-SiO 2 can be quantitatively evaluated by TGA. Figure 2a illustrates the TGA curves of neat SiO 2 , APES-SiO 2 , AF and PAA-AF. For all of the specimens, the weight loss at a temperature lower than 100 • C is mainly due to the evaporation of physically adsorbed water. The mass loss of pristine SiO 2 exhibits another decomposition stage at 200-600 • C, which is attributed to the dehydration of the silanol groups [30]. In the TGA trace of AF, the sharp weight loss from approximately 500-600 • C represents the partial dihydroxylation and alkoxide decomposition of AF [31]. In addition, the PAA-AF fibers began to decompose at 180 • C due to the grafting of PAA (Figure 2a), and the weight loss between 500 and 800 • C further decreased to 41.4%. As shown in the TGA curves of APES-SiO 2 and PAA-AF, it can be inferred that the grafting modification has little influence on the decomposition of the SiO 2 or AF matrices. Thus, the weight loss percentage of the matrix (SiO 2 or AF) at the temperature of 100-800 • C in both the untreated and treated samples is identical.
Taking AF as an example, the weight loss ratio (ε) of the aramid fiber matrix between 100 and 800 • C in PAA-AF is equal to that in pristine AF, which can be expressed as [32] where M A100 (99.8%) and M A800 (43.6%) represent the residual weight percentage of AF at 100 • C and 800 • C, respectively. Considering that the grafting PAA is completely degraded during the heating process, the grafting content (χ) can be figured out with the help of Equation (2): where M P100 (99.3 %) and M P800 (41.4%) stand for the residual weight percentage of PAA-AF at 100 • C and 800 • C, respectively. After calculation, the grafting content (χ) of PAA-AF is 4.56%. In addition, the grafting content of APES-SiO 2 is calculated to be 3.07% by the same equations. Figure 2b presents the FTIR spectra of AF, PAA-AF, SiO 2 , APES-SiO 2 and SiO 2 @AF in the region of 600-4000 cm −1 . As shown in Figure 2b, the C-N stretching peak at 1392 cm −1 , N-H bending peak at 1548 cm −1 and C=O stretching peak at 1630 cm −1 can be found in the AF and PAA-AF spectra [33]. By contrast with common AF, the new absorption band assigned to -OH in the carboxyl groups is clearly observed at 3318 cm −1 in the FTIR spectrum of PAA-AF [34], which can be attributed to the carboxyl group from PAA. In addition, the peaks at 2970 cm −1 for the PAA-AF can be ascribed to the stretching vibrations of the methylene groups from PAA. Compared to AF, PAA-AF showed a broader and stronger band around 1548 cm −1 , 1630 cm −1 and 1392 cm −1 (aromatic rings, amide I and amide Π) due to the introduction of PAA by the acylamino bond [35]. Figure 3 shows the SEM image of AF and PAA-AF, and it clear that the surface AF is very smooth (Figure 3a), whereas the PAA-AF is obviously covered by a rough grafted layer ( Figure 3b). Therefore, it is reasonable to conclude that PAA was successfully deposited on the AF surface.
In the SiO 2 FTIR spectrum, the peaks observed at 3450 cm −1 and 1630 cm −1 are related to the stretching and bending vibrations of the hydroxyl groups from the hydrate water, respectively [36,37]. The absorption bands around 1100 cm −1 and 796 cm −1 are attributed to the asymmetric and symmetric stretching of Si-O-Si, respectively [38]. As shown in the APES-SiO 2 FTIR graph, the bands at 2980 and 1345 cm −1 correspond to the stretching and bending vibrations of the methylene groups in siloxane, respectively [39]. This feature demonstrates that APES is indeed combined with SiO 2 . From the FTIR spectrum of SiO 2 @AF, the O-H stretching band disappears, which indicates that the-COOH grafted on PAA-AF is exhausted and the modified SiO 2 is dehydrated during the hybridization process. However, the peak still observed at 1629 cm −1 is due to the stretching vibration of carbonyl in amide [40]. Another peak, which exists at 1547 cm −1 , is ascribed to the N-H bending vibration of amide. This confirms that SiO 2 @AF was successfully prepared by the chemical bonding between APES-SiO 2 and PAA-AF.
Materials 2020, 13, x FOR PEER REVIEW 6 of 16 In the SiO2 FTIR spectrum, the peaks observed at 3450 cm −1 and 1630 cm −1 are related to the stretching and bending vibrations of the hydroxyl groups from the hydrate water, respectively [36,37]. The absorption bands around 1100 cm −1 and 796 cm −1 are attributed to the asymmetric and symmetric stretching of Si-O-Si, respectively [38]. As shown in the APES-SiO2 FTIR graph, the bands at 2980 and 1345 cm −1 correspond to the stretching and bending vibrations of the methylene groups in siloxane, respectively [39]. This feature demonstrates that APES is indeed combined with SiO2. From the FTIR spectrum of SiO2@AF, the O-H stretching band disappears, which indicates that the-COOH grafted on PAA-AF is exhausted and the modified SiO2 is dehydrated during the hybridization process. However, the peak still observed at 1629 cm −1 is due to the stretching vibration of carbonyl in amide [40]. Another peak, which exists at 1547 cm −1 , is ascribed to the N-H bending vibration of amide. This confirms that SiO2@AF was successfully prepared by the chemical bonding between APES-SiO2 and PAA-AF. In order to further explore the combination of PAA-AF and APES-SiO2, XPS was applied to detect detailed any structural and compositional changes of the SiO2@AF samples. The XPS peaks fitted by the multipeak Lorentzian fitting program are presented in Figure 4. The Si2p core level spectrum of SO2 (Figure 4a) exhibits an obvious peak corresponding to O-Si-O at 103.5 eV. However, a new binding energy of the silicon atoms for APES-SiO2 (Figure 4b) is observed at 102.5 eV, which is assigned to the O-Si-C bond [41]. Figure 4c shows the N1s peak at 399.5 eV for APES-SiO2, which is attributed to nitrogen in amine. As displayed in Figure 4d, the N1s spectrum of SiO2@AF presents two new peaks at -NHOC-(400.7 eV) and -NH3 +-OOC-(401.6 eV), which illustrates that SiO2 was combined with AF by covalent and ionic bonds [42]. The O1s peaks for PAA-AF observed at 529.7 eV, 531.0 eV and 534.2 eV in Figure 4e are ascribed to -OH, C-O, and C=O, respectively. Nevertheless, the -OH (529.7 eV) and C-O (531.0 eV) peaks disappear in the O1s curve of SiO2@AF (Figure 4f), which indicates that the -COOH grafted onto the AF was consumed during the preparation process.
Another new peak at 532.3 eV, assigned to the oxygen atom (Si-O-Si), appears with the introduction of SiO2. All of these features confirm that the SiO2@AF hybrid materials were successfully obtained, which is consistent with the results of the FTIR analysis. In order to further explore the combination of PAA-AF and APES-SiO 2 , XPS was applied to detect detailed any structural and compositional changes of the SiO 2 @AF samples. The XPS peaks fitted by the multipeak Lorentzian fitting program are presented in Figure 4. The Si2p core level spectrum of SO 2 (Figure 4a) exhibits an obvious peak corresponding to O-Si-O at 103.5 eV. However, a new binding energy of the silicon atoms for APES-SiO 2 (Figure 4b) is observed at 102.5 eV, which is assigned to the O-Si-C bond [41]. Figure 4c shows the N1s peak at 399.5 eV for APES-SiO 2 , which is attributed to nitrogen in amine. As displayed in Figure 4d, the N1s spectrum of SiO 2 @AF presents two new peaks at -NHOC-(400.7 eV) and -NH 3 +-OOC-(401.6 eV), which illustrates that SiO 2 was combined with AF by covalent and ionic bonds [42]. The O1s peaks for PAA-AF observed at 529.7 eV, 531.0 eV and 534.2 eV in Figure 4e are ascribed to -OH, C-O, and C=O, respectively. Nevertheless, the -OH (529.7 eV) and C-O (531.0 eV) peaks disappear in the O1s curve of SiO 2 @AF (Figure 4f), which indicates that the -COOH grafted onto the AF was consumed during the preparation process. Another new peak at 532.3 eV, assigned to the oxygen atom (Si-O-Si), appears with the introduction of SiO 2 .
All of these features confirm that the SiO 2 @AF hybrid materials were successfully obtained, which is consistent with the results of the FTIR analysis.    SEM was performed to investigate the surface morphology of AF and SiO2@AF ( Figure 5). As shown in Figure 5a,b, the surface of the unmodified AF is smooth. Nevertheless, it is clearly seen in Figure 5c,d that a layer of modified SiO2 particles with different sizes and shapes is strongly linked to the surface of SiO2@AF. Furthermore, no aggregation of the SiO2 particles occurs on the AF surface due to the enhanced dispersibility by the APES grafting. A small amount of the AF is connected together by the SiO2 particles due to the excessive APES-SiO2 in the preparation of SiO2@AF. The SEM was performed to investigate the surface morphology of AF and SiO 2 @AF ( Figure 5). As shown in Figure 5a,b, the surface of the unmodified AF is smooth. Nevertheless, it is clearly seen in Figure 5c,d that a layer of modified SiO 2 particles with different sizes and shapes is strongly linked to the surface of SiO 2 @AF. Furthermore, no aggregation of the SiO 2 particles occurs on the AF surface due to the enhanced dispersibility by the APES grafting. A small amount of the AF is connected together by the SiO 2 particles due to the excessive APES-SiO 2 in the preparation of SiO 2 @AF. The combination of SiO 2 on the AF surface obviously alters its morphology and highly enhances its surface roughness, which is beneficial to the interfacial interaction between SiO 2 @AF and the NR matrix.

Analysis of Immobilized Rubber Approaching the Filler Surface
For rubber composites, the chains immobilized on the fillers play an important role in enhancing their mechanical properties. In detail, the immobilized chains are able to promote the interfacial interactions, decrease the filler mobility and improve the compatibility between fillers and the matrix [43,44]. Thus, polymer chains immobilized on the filler, which indicate the interfacial interaction, are of great significance for precise evaluation in rubber composites. Figure 6a shows the DSC curves of the neat NR and SiO2+AF/NR, and the SiO2@AF/NR composites, and the typical results such as the glass transition temperature (Tg) and the heat capacity step (∆Cp) are listed in Table 2. It can obviously be seen that the Tg of SiO2@AF/NR is higher than that of SiO2+AF/NR, and the Tg of the neat NR is the minimum. These features illustrate that the introduction of SiO2@AF can improve network structures. Furthermore, the ∆Cp of SiO2@AF/NR is the lowest among the three samples, which represents the decreasing chain mobility owing to the strong interaction between NR and SiO2@AF. This result is consistent with the value of the Tg. According to Leterrier's [45] and Liu's [46] previous reports, the weight fraction of immobilized chains (χi) can be quantitatively determined using Equations (4) and (5).
where ∆Cp stands for the heat capacity step of the NR composites at Tg, ∆Cpn is the heat capacity step of the NR composites at Tg normalized to the rubber content, ∆Cp0 denotes the heat capacity step of

Analysis of Immobilized Rubber Approaching the Filler Surface
For rubber composites, the chains immobilized on the fillers play an important role in enhancing their mechanical properties. In detail, the immobilized chains are able to promote the interfacial interactions, decrease the filler mobility and improve the compatibility between fillers and the matrix [43,44]. Thus, polymer chains immobilized on the filler, which indicate the interfacial interaction, are of great significance for precise evaluation in rubber composites. Figure 6a shows the DSC curves of the neat NR and SiO 2 +AF/NR, and the SiO 2 @AF/NR composites, and the typical results such as the glass transition temperature (T g ) and the heat capacity step (∆C p ) are listed in Table 2. It can obviously be seen that the T g of SiO 2 @AF/NR is higher than that of SiO 2 +AF/NR, and the T g of the neat NR is the minimum. These features illustrate that the introduction of SiO 2 @AF can improve network structures. Furthermore, the ∆C p of SiO 2 @AF/NR is the lowest among the three samples, which represents the decreasing chain mobility owing to the strong interaction between NR and SiO 2 @AF. This result is consistent with the value of the T g . According to Leterrier's [45] and Liu's [46] previous reports, the weight fraction of immobilized chains (χ i ) can be quantitatively determined using Equations (4) and (5).
where ∆C p stands for the heat capacity step of the NR composites at T g , ∆C pn is the heat capacity step of the NR composites at T g normalized to the rubber content, ∆C p0 denotes the heat capacity step of pure NR at T g and φ is the weight fraction of the fillers in the NR composites.  The obtained χi is displayed in Figure 6b. Evidently, the value of χi for SiO2@AF/NR is much higher than that for SiO2+AF/NR. As the amount of constrained rubber is not due to a diffuse hardening of the rubber, the amount of constrained rubber does not change before and after vulcanization [46]. The incremental fraction of the immobilized chains reflects a stronger interfacial interaction, because the fixed chains can play a role as a bridge between the fillers and the matrix. The possible reason for these results is the filler aspect. SiO2 particles loaded on the AF surface can avoid severe aggregation in NR matrix, and thus greatly increase the contact area. Ultimately, the mobility of the SiO2 particles combined on the AF surface is largely weakened, which can more highly confine the movement of the NR chains linked to the SiO2 particles. These results imply that SiO2@AF acts as a 'rivet' to immobilize large numbers of polymer chains on its surface, which greatly enhances the interfacial adhesion between SiO2@AF and NR.
To further evaluate the 'rivet' function of SiO2@AF in the rubber composites, SEM photographs of the fracture in the tensile rupture of the SiO2+AF/NR and SiO2@AF/NR composites are shown in Figure 7. From Figure 7a,b, the fibers on the failure surface of the SiO2@AF/NR composite exhibit a fracture state (the blue circles), and it can be seen that the fractured fibers are still tightly bound to the rubber matrix. Furthermore, many peeled microfibrils are observed on the failure surface of SiO2@AF/NR due to the weak shin/core structure of AF that results from wet spinning. For comparison, there are many holes in the failure surface of the SiO2+AF/NR composite (the red circles in Figure 7c), which are caused when the fiber pulls away from the NR matrix during the tensile process [47]. As shown in Figure 7d, a relatively complete fiber can be seen on the surfaces of the SiO2+AF/NR composites (the blue circles), which means that the interface between the AF and the NR matrix is weak, and that AF is pulled out. The results illustrate that SiO2@AF has a better interface adhesion ability with the rubber matrix than ordinary AF, and can bond with the NR matrix tightly without being pulled away. The obtained χ i is displayed in Figure 6b. Evidently, the value of χ i for SiO 2 @AF/NR is much higher than that for SiO 2 +AF/NR. As the amount of constrained rubber is not due to a diffuse hardening of the rubber, the amount of constrained rubber does not change before and after vulcanization [46]. The incremental fraction of the immobilized chains reflects a stronger interfacial interaction, because the fixed chains can play a role as a bridge between the fillers and the matrix. The possible reason for these results is the filler aspect. SiO 2 particles loaded on the AF surface can avoid severe aggregation in NR matrix, and thus greatly increase the contact area. Ultimately, the mobility of the SiO 2 particles combined on the AF surface is largely weakened, which can more highly confine the movement of the NR chains linked to the SiO 2 particles. These results imply that SiO 2 @AF acts as a 'rivet' to immobilize large numbers of polymer chains on its surface, which greatly enhances the interfacial adhesion between SiO 2 @AF and NR.
To further evaluate the 'rivet' function of SiO 2 @AF in the rubber composites, SEM photographs of the fracture in the tensile rupture of the SiO 2 +AF/NR and SiO 2 @AF/NR composites are shown in Figure 7. From Figure 7a,b, the fibers on the failure surface of the SiO 2 @AF/NR composite exhibit a fracture state (the blue circles), and it can be seen that the fractured fibers are still tightly bound to the rubber matrix. Furthermore, many peeled microfibrils are observed on the failure surface of SiO 2 @AF/NR due to the weak shin/core structure of AF that results from wet spinning. For comparison, there are many holes in the failure surface of the SiO 2 +AF/NR composite (the red circles in Figure 7c), which are caused when the fiber pulls away from the NR matrix during the tensile process [47]. As shown in Figure 7d, a relatively complete fiber can be seen on the surfaces of the SiO 2 +AF/NR composites (the blue circles), which means that the interface between the AF and the NR matrix is weak, and that AF is pulled out. The results illustrate that SiO 2 @AF has a better interface adhesion ability with the rubber matrix than ordinary AF, and can bond with the NR matrix tightly without being pulled away.

Mechanical Properties
The mechanical properties of the neat NR, SiO2+AF/NR and SiO2@AF/NR composites were investigated, as shown in Figure 8 (the sample with a filler content of 0 represents pure rubber) and Figure S3 (the stress-strain curve of the composites). Obviously, the tear strength, stress at 100% strain, stress at 300% strain and tensile strength of the NR composites are all promoted by the introduction of SiO2@AF and SiO2+AF (physically mixing SiO2 and the AF filler with the ratio of 48:52). However, SiO2@AF performs better in improving the mechanical properties of the rubber composites than SiO2+AF, especially in the case of a high filling content. As shown in Figure 8e, the elongation at the break of SiO2@AF/NR is higher than that of the SiO2+AF/NR composites. When the filling content is higher, the elongation at the break of SiO2+AF/NR decreases faster, and the gap between SiO2+AF/NR and SiO2+AF/NR is more obvious. The probable reason is that physically mixing SiO2 particles undergo aggregation due to their physicochemical surface properties [48], and numerous stress concentration regions that can propagate the cracks at lower stress appear. What is more, the interfacial adhesion between the smooth AF and the NR matrix is weak, which makes it easy for the AF to be pulled away during rubber composite stretching, and thus results in the increased stress concentration of the composites.
The tear strength, stress at 100% strain, stress at 300% strain and tensile strength of the 4 phr SiO2@AF/NR composite are dramatically increased by 97.8%, 167.4%, 144.8% and 89.3% that of the pure rubber, respectively. Furthermore, in contrast to the 4 phr SiO2+AF/NR composite, the tear strength, stress at 100% strain, stress at 300% strain and tensile strength for the 4 phr SiO2@AF/NR composite are increased by 26.1%, 16.1%, 18.3% and 29.1%, respectively. Obviously, SiO2@AF has a good reinforcing effect on rubber, especially in terms of the characteristics of the tear strength and the tensile strength of NR. This can be attributed to the ability of SiO2@AF to immobilize large numbers of rubber chains, which plays the role of a rivet in the tensile process of composites, and enhances the interface interaction of rubber. According to the performance of the mechanical

Mechanical Properties
The mechanical properties of the neat NR, SiO 2 +AF/NR and SiO 2 @AF/NR composites were investigated, as shown in Figure 8 (the sample with a filler content of 0 represents pure rubber) and Figure S3 (the stress-strain curve of the composites). Obviously, the tear strength, stress at 100% strain, stress at 300% strain and tensile strength of the NR composites are all promoted by the introduction of SiO 2 @AF and SiO 2 +AF (physically mixing SiO 2 and the AF filler with the ratio of 48:52). However, SiO 2 @AF performs better in improving the mechanical properties of the rubber composites than SiO 2 +AF, especially in the case of a high filling content. As shown in Figure 8e, the elongation at the break of SiO 2 @AF/NR is higher than that of the SiO 2 +AF/NR composites. When the filling content is higher, the elongation at the break of SiO 2 +AF/NR decreases faster, and the gap between SiO 2 +AF/NR and SiO 2 +AF/NR is more obvious. The probable reason is that physically mixing SiO 2 particles undergo aggregation due to their physicochemical surface properties [48], and numerous stress concentration regions that can propagate the cracks at lower stress appear. What is more, the interfacial adhesion between the smooth AF and the NR matrix is weak, which makes it easy for the AF to be pulled away during rubber composite stretching, and thus results in the increased stress concentration of the composites. the NR matrix: (1) when SiO2 is filled with a high filling content, agglomeration will appear in the matrix; and (2) AF forms less immobilized chains with the rubber matrix, which are easily pulled out from the matrix. It can be reasonably concluded that the greater the fraction of the immobilized chains, the higher the mechanical properties of the composites; SiO2@AF hybrid materials with the 'rivet' effect have greater superiority in immobilizing the polymer chains and enhancing the mechanical properties.  The tear strength, stress at 100% strain, stress at 300% strain and tensile strength of the 4 phr SiO 2 @AF/NR composite are dramatically increased by 97.8%, 167.4%, 144.8% and 89.3% that of the pure rubber, respectively. Furthermore, in contrast to the 4 phr SiO 2 +AF/NR composite, the tear strength, stress at 100% strain, stress at 300% strain and tensile strength for the 4 phr SiO 2 @AF/NR composite are increased by 26.1%, 16.1%, 18.3% and 29.1%, respectively. Obviously, SiO 2 @AF has a good reinforcing effect on rubber, especially in terms of the characteristics of the tear strength and the tensile strength of NR. This can be attributed to the ability of SiO 2 @AF to immobilize large numbers of rubber chains, which plays the role of a rivet in the tensile process of composites, and enhances the interface interaction of rubber. According to the performance of the mechanical properties of the rubber composites, combined with the DSC and SEM testing results, the potential reinforcing mechanism of SiO 2 @AF was analyzed. As shown in Figure 9, SiO 2 @AF can be regarded as a 'rivet' embedded into the NR matrix, and can immobilize a mass of polymer chains on its surfaces, which highly promotes the interfacial interaction between NR and SiO 2 @AF, and efficiently delivers external stress. There are two situations when SiO 2 and AF are physically incorporated into the NR matrix: (1) when SiO 2 is filled with a high filling content, agglomeration will appear in the matrix; and (2) AF forms less immobilized chains with the rubber matrix, which are easily pulled out from the matrix. It can be reasonably concluded that the greater the fraction of the immobilized chains, the higher the mechanical properties of the composites; SiO 2 @AF hybrid materials with the 'rivet' effect have greater superiority in immobilizing the polymer chains and enhancing the mechanical properties.

Cutting Resistance
The influence of SiO2@AF on the cutting resistance of the NR composites was evaluated by a cut tester. Cutting resistance is an important performance index of engineering truck tires, which is usually characterized by the mass loss of composite materials before and after the experiment ( Figure  S4). As shown in Figure 10, the weight loss of the pure NR, SiO2+AF/NR and SiO2@AF/NR composites is gradually increased with incremental cutting times. In addition, the weight loss of the SiO2@AF/NR composite is clearly lower than that of the neat NR and the SiO2+AF/NR composite after an identical cutting time. For example, a weight loss of 19.14% is reached for the SiO2@AF/NR composite at a cutting time of 20 min, which is relatively lower than the 22.43% for the SiO2+AF/NR composite and the 24.73% for the neat NR. These features demonstrate that the introduction of SiO2@AF can preferably promote the cutting resistance of NR, in comparison to untreated AF and SiO2. SiO2@AF with higher surface roughness can play a role as a 'rivet' to immobilize more polymer chains on the surface. The interfacial interaction between SiO2@AF and the NR matrix is significantly enhanced, which results in the difficult exfoliation of the NR chains from the filler. Hence, similarly to the mechanical properties, the cutting resistance of the NR composite is greatly increased with the introduction of SiO2@AF.

Cutting Resistance
The influence of SiO 2 @AF on the cutting resistance of the NR composites was evaluated by a cut tester. Cutting resistance is an important performance index of engineering truck tires, which is usually characterized by the mass loss of composite materials before and after the experiment ( Figure S4). As shown in Figure 10, the weight loss of the pure NR, SiO 2 +AF/NR and SiO 2 @AF/NR composites is gradually increased with incremental cutting times. In addition, the weight loss of the SiO 2 @AF/NR composite is clearly lower than that of the neat NR and the SiO 2 +AF/NR composite after an identical cutting time. For example, a weight loss of 19.14% is reached for the SiO 2 @AF/NR composite at a cutting time of 20 min, which is relatively lower than the 22.43% for the SiO 2 +AF/NR composite and the 24.73% for the neat NR. These features demonstrate that the introduction of SiO 2 @AF can preferably promote the cutting resistance of NR, in comparison to untreated AF and SiO 2 . SiO 2 @AF with higher surface roughness can play a role as a 'rivet' to immobilize more polymer chains on the surface. The interfacial interaction between SiO 2 @AF and the NR matrix is significantly enhanced, which results in the difficult exfoliation of the NR chains from the filler. Hence, similarly to the mechanical properties, the cutting resistance of the NR composite is greatly increased with the introduction of SiO 2 @AF.
preferably promote the cutting resistance of NR, in comparison to untreated AF and SiO2. SiO2@AF with higher surface roughness can play a role as a 'rivet' to immobilize more polymer chains on the surface. The interfacial interaction between SiO2@AF and the NR matrix is significantly enhanced, which results in the difficult exfoliation of the NR chains from the filler. Hence, similarly to the mechanical properties, the cutting resistance of the NR composite is greatly increased with the introduction of SiO2@AF.

Conclusions
In this paper, SiO 2 @AF hybrid materials were successfully prepared by chemically bonding APES-SiO 2 onto the surface of PAA-AF, with a SiO 2 macroparticle mass of 48 wt%. With the incorporation of SiO 2 @AF into the rubber matrix, the immobilized chain fraction of the SiO 2 @AF/NR composite was 14.7 wt%, which was much higher than that of the SiO 2 +AF/NR composite. The interfacial interaction between the filler and the matrix was improved with incremental weight fractions of the immobilized chains. Hence, hierarchical SiO 2 @AF materials with the 'rivet' effect have greater superiority in enhancing the mechanical properties of natural rubber. Compared to pure NR, the tear strength, stress at 100% strain, stress at 300% strain and tensile strength of the SiO 2 @AF/NR composite were dramatically increased by 97.8%, 167.4%, 144.8% and 89.3%, respectively. Moreover, the cutting resistance of the SiO 2 +AF/NR composite was promoted to a greater extent, due to the difficult exfoliation of the NR chains from the SiO 2 @AF filler. Therefore, hierarchical SiO 2 @AF materials have great potential for the application of fabricating high-performance rubber composites.