Core-Shell Nanostructures: Modeling, Fabrication, Properties, and ApplicationsView this Special Issue
Research Article | Open Access
Reinforcement of Natural Rubber with Core-Shell Structure Silica-Poly(Methyl Methacrylate) Nanoparticles
A highly performing natural rubber/silica (NR/SiO2) nanocomposite with a SiO2 loading of 2 wt% was prepared by combining similar dissolve mutually theory with latex compounding techniques. Before polymerization, double bonds were introduced onto the surface of the SiO2 particles with the silane-coupling agent. The core-shell structure silica-poly(methyl methacrylate), SiO2-PMMA, nanoparticles were formed by grafting polymerization of MMA on the surface of the modified SiO2 particles via in situ emulsion, and then NR/SiO2 nanocomposite was prepared by blending SiO2-PMMA and PMMA-modified NR (NR-PMMA). The Fourier transform infrared spectroscopy results show that PMMA has been successfully introduced onto the surface of SiO2, which can be well dispersed in NR matrix and present good interfacial adhesion with NR phase. Compared with those of pure NR, the thermal resistance and tensile properties of NR/SiO2 nanocomposite are significantly improved.
Recently, considerable efforts have been expended on the preparation of organic/inorganic nanocomposite materials [1, 2]. Inorganic nanoparticles possess small particle size, high surface area, as well as the quantum and surface effect, which make it possible for nanocomposites to perform more excellent physical and mechanical properties than conventional composites [3, 4]. Multifarious nanoscale fillers, including calcium carbonate , montmorillonite , carbon black , and aluminum oxide , have been reported in academic journals. Among them, nanosilica (SiO2), an important member of inorganic nanofiller in composites preparation, is widely used in the field of organic/inorganic hybrid composites. Zhu et al.  prepared well-dispersed silica-poly(methyl methacrylate) (SiO2-PMMA) nanoparticles by a suspension-dispersion-polymerization technique in an aqueous system. Subsequently, SiO2-PMMA nanoparticles were induced into PVC matrix, and results showed that the heat resistance, elongation at break, and tensile strength of PVC/SiO2 composites were significantly improved. Also, Peng et al. prepared natural rubber/silica nanocomposites (NR/SiO2)  and natural rubber/multiwalled carbon nanotubes composite  combining self-assembly and latex-compounding techniques. Obtained nanocomposites were perfectly strengthened, with increased thermal resistance and mechanical properties.
But on the other hand, as nanoscaled particles, nanosilica tend to the lowest energy state of bulk aggregation in polymer matrix because of large surface area and high surface energy, which in turn leads to structure flaws in the polymer matrix and interfere with the property of the whole product [12, 13]. In order to enhance interfacial adhesion between nanosilica and NR matrix and improve the dispersion of nanoparticles in NR matrix, it is necessary to reinforce the compatibility between the organic matrix and the inorganic surface of nano-silica. Due to large numbers of hydroxy group on the surface of raw nanosilica [14, 15], its surface is easily compatible with polar polymers, such as PVA and PVC compared to the apolar matrix, natural rubber. According to traditional method, the most commonly used method to disperse nanosilica was the mixing process conducted in two-roll mill [16, 17]. Despite well-distributed polymer composite can be finally obtained, the contamination of fly ash and the side effect on the lung of individuals are difficult to be avoided during the preparation of composite in either the laboratory or factory. Also, this method possesses some other advantages, such as low energy cost and homogeneous dispersion of silica in NR matrix. Latex compounding, therefore, can reduce harmful situation mentioned above, and be more eco-friendly and low-polluted processing. However, in order to obtain homogeneous dispersion of polar nanosilica in apolar natural rubber latex, surface modification of nanoparticles is required. There has been numerous numbers of publications reported in this field, such as chemical coating using coupling agents [18–21] and physical coating using detergents  whereas rare researches are referred to the surface functionalization of nanosilica and dispersing process in natural rubber latex. In this paper, a novel method to prepare NR/SiO2 nanocomposite will be reported. Well-dispersed core-shell SiO2-PMMA nanoparticles were prepared by using MPS as bridge in aqueous solution. In addition, NR latex was modified by PMMA in emulsion system before mixing with core-shell nanoparticles via combining similar dissolve mutually theory and latex-compounding techniques. It is proved that this method is useful and core-shell SiO2-PMMA nanoparticles are rarely reported in NR latex application, and we will systematically discuss the synthesizing process and also investigate the relationship between microstructure and macroproperties.
Natural rubber latex (NRL) with a total solid content of 60% was obtained from Qianjin state farm (Guangdong, China). Silica nanoparticles (average diameter: 7 nm) were purchased from Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO). Methyl methacrylate (MMA) and tert-butyl hydroperoxide were purchased from Sinopharm, Ltd. (Shanghai, China). 3-(trimethoxysilyl) propyl methacrylate (MPS) was brought from Guangzhou Chemical Reagent Co. (Guangdong, China). Potassium persulfate (KPS) and Tetraethylenepentamine were offered by Xilong Chemical Co. (Guangdong, China). All experiments were carried out with distilled water.
2.2. Preparation of the NR/SiO2 Nanocomposite
2.2.1. Coupling Modification of Nanosilica
We modified the surface of SiO2 particles with MPS. Firstly, MPS was hydrolyzed in 95% ethanol solution for 2 hours. Then a fixed amount of SiO2 with 95% ethanol as solvent was treated with an ultrasonic vibrator for 15 min. Finally, SiO2 dispersion was dropped into hydrolyzed MPS with mechanical stir at 25°C for 5 hours, and its pH was adjusted to 10 with triethylamine . The SiO2/MPS dispersion was centrifuged followed by rinsing with ethanol for 3 times and then with distilled with water for 2 times.
2.2.2. Copolymerization of MMA on the Surface of Nanosilica
The surface-functionalized SiO2 was dispersed in an aqueous system with MMA and sodium dodecyl sulfate (SDS) in order to obtain nano-silica colloid  which was mixed with an aqueous solution of initiator potassium persulfate (KPS), and then charged into a flask with mechanical stirring at 55°C for 5 hours. PMMA, as the shell, was grafted onto the surface of SiO2/MPS due to the reaction of the vinyl groups between SiO2/MPS and MMA.
2.2.3. Modification of NRL
NRL was treated in an aqueous solution where MMA, oleic acid, and tert-butyl hydroperoxide were mixed accompanying gentle mechanical stir at room temperature for 2 hours. The colloid system was heated to 60°C, before dropping an aqueous solution of activating agent (tetraethylenepentamine), and then cooled to room temperature, finally NR-PMMA latex was obtained .
2.2.4. Synthesis of Nanocomposite
The SiO2-PMMA aqueous dispersion was dropped into a certain amount of NR-PMMA latex with a total solid content of 30% and the mixture was stirred for 24 hours, which was then dried to obtain NR/SiO2 nanocomposite films. The loading of silica is 2 wt% in NR matrix.
A Perkin-Elmer Spectra GX-I FTIR spectroscopy (Perkin-Elmer, Fremont, CA) was used to investigate surface chemistry of SiO2 after modification with a resolution of 4 cm−1 in the transmission mode. The liquid SiO2 were dragged up by copper network for transmission electron microscopy (TEM), and then were observed on a JEM-100CXII instrument (JEOL, Peabody, MA) with an accelerating voltage of 100 kV. The morphology of the nanocomposites was taken with a scanning electron micrographs (SEM), Philips XL30-EDAX instrument (Philips, Eindhoven, Netherlands), at an acceleration voltage of 10 kV. The cross-section of the samples was obtained by splitting bulk sample with liquid nitrogen treatment.
The dynamic-mechanical thermal analysis (DMTA) spectra were taken on rectangular specimens (20 × 4 × 0.1 mm) in tensile mode at a frequency of 5 Hz using a NETZSCH DMA 242C instrument (Germany), ranged from the temperature of −120°C to 50°C. Thermal decomposition of samples was performed with a Perkin-Elmer TGA-7 thermogravimetric analyzer (TGA); (Perkin-Elmer, Fremont, CA). The measurement of the samples (ca. 10 mg) was carried out in nitrogen from 50°C to 600°C at a heating rate of 10°C/min. Tensile test experiments were conducted on the instrument in compression model with a cross-head speed of 500 mm/min, and the sample length was 75 mm, the minimum width was 4 mm, and the thickness was 2 mm.
3. Results and Discussion
3.1. Mechanism of Preparing NR/SiO2 Nanocomposite Process
Figure 1 describes the scheme of the manufacturing process of NR/SiO2 nanocomposite. There are three main steps in this process. The first step is to form core-shell SiO2-PMMA nanoparticles. The silane-coupling agent MPS are hydrolyzed to form silanol groups which condense with hydroxyl groups on the surface of silica to form a covalent bond. Because of the grafted MPS, unsaturated double bond (C=C) is introduced onto the surface of silica nanoparticles . When the silane-coupling agent grafted on the surface of SiO2 counters an unsaturated functional group in MMA, the copolymerization between SiO2/MPS and the copolymer PMMA will take place if there is an appropriate initiator. Eventually the core-shell SiO2-PMMA nanoparticles are successfully obtained.
The second process is to prepare NR-PMMA latex. The modification mechanism is as follows .
(1) The initiators decompose to generate free radical:(1)
(2) MMA monomers are initiated to polymerize by free radical R•.(2)
(3) PMMA chain growth process is accompanied with the radical transferring to NR macromolecule.(3)
(4) The combination of two radicals terminates the entire reaction.(4)
The last stage is to blend core-shell SiO2-PMMA nanoparticles and NR-PMMA via similar dissolve mutually theory. Compared with the other techniques [28, 29], the above-mentioned process is more convenient and effective in preparing NR/SiO2 nanocomposite.
3.2. Characterization of Core-Shell SiO2-PMMA Nanoparticles
3.2.1. FTIR Study
After the formation of core-shell SiO2-PMMA nanoparticles, either chemical reaction or physical adsorption would be characterized in the FTIR spectra. Therefore, before the measurement of FTIR, all the SiO2 nanoparticles modified with MMA have been extracted in acetone solution for 72 hours in order to eliminate un-grafted PMMA.
Figure 2 displays the infrared spectra of the eluate and pure acetone solution. It can be seen that the peak at 1730 cm−1 (Figure 2(A)) is assigned to C=O stretching vibration of PMMA while after 72 hours extracting, this peak disappears and only C=O stretching vibration of acetone at 1715 cm−1 could be seen (Figure 2(B)). It can be proved that after 72 hours extracting, there is no redundant PMMA absorbed physically on SiO2-PMMA nanoparticles. Subsequently, rinsed solid SiO2-PMMA and unmodified SiO2 were measured through FTIR. It can be seen from Figure 3 that the adsorption peak at 1697 cm−1 assigned to the C=O functional groups (Figure 3(B)) does not appear in Figure 3(A), which indicates that MPS has been successfully grafted on the surface of SiO2  and SiO2 and MPS are linked with covalent bonds. Therefore, it is possible for MMA to graft onto the modified SiO2 nanoparticles. As can be seen from Figure 3(C) that the absorption peak at 1730 cm−1 assigned to the vibration absorption of C=O is obvious, accompanying with one less pronounced peak at 2975 cm−1 corresponding to CH3 groups of PMMA. All the results obtained from FTIR spectra confirm that PMMA successfully grafted onto the SiO2 modified by MPS through chemical bonds rather than simply physical adsorption.
3.2.2. TEM Observation
It is obvious that unmodified SiO2 nanoparticles are aggregated together and the diameter ranges from 10 nm to 30 nm (Figure 4(a)) whilst because of the introduction of PMMA, nanoparticles (SiO2/MMA = 1 : 0.3) indicate well-dispersed morphology (Figure 4(b)). However, after being grafted by PMMA, dispersibility of SiO2 is improved (Figure 4(b)). It can be also seen from Figure 4(b) that single nanoparticle with a diameter around 40–60 nm can be clearly observed, which is 30 nm thicker than unmodified SiO2 particles. This illuminates that the core, SiO2 nanoparticles, is successfully coated by the shell, PMMA, and the thickness of PMMA shell is about 25–35 nm. It can also be observed in the SEM images (Figure 5) where the core-shell SiO2-PMMA nanoparticles are evenly dispersed throughout NR matrix. However, when the ratio of SiO2/MMA reaches to 1 : 3, the sphere morphology is out of order (Figure 4(c)) due to the increasing prevalence of PMMA homopolymerization, and PMMA will physically be absorbed on the surface of nanosilica. Meanwhile, the aggregation of nano-SiO2 deteriorates, and the diameter of the SiO2 clusters becomes much bigger compared with Figure 4(b). Therefore, from the results above the suitable SiO2/MMA ratio is 1 : 0.3, which is proved to be the proper grafting rate through large amounts of experimental work.
3.3. Characterization of NR/SiO2 Nanocomposites
3.3.1. SEM Observation
In recent work, NR/SiO2 nanocomposite was prepared with a constant mixture rate (NR/SiO2 = 98/2). Because SiO2 has been surface-functioned with MPS and MMA, the compatibility is improved between SiO2 nanoparticles and NR matrix, and, moreover, SiO2-PMMA nanoparticles with an average size ranged from 60 nm to 100 nm are uniformly dispersed in the matrix as individual spherical core-shell structure (Figure 5(c)). Whereas unmodified SiO2 in NR matrix (Figures 5(a) and 5(b)) aggregate significantly and the size of the SiO2 clusters reaches to more than 200 nm. These results illustrate that the excellent compatibility between the SiO2-PMMA and NR-PMMA enhances the interaction between SiO2 and NR, and thus improves the adhesion and morphological structure of NR/SiO2 nanocomposites. The preparation method of polymer/SiO2 composites has been widely reported, such as melt compounding  and other physical blending , however, the process presented in this paper possesses significant advantages, for instance, the compatibility with NR is excellent and the size of SiO2 is approximately 80 nm.
3.3.2. Dynamic Thermal Mechanical Analysis
Dynamic thermal mechanical analysis (DTMA) can characterize the reaction between polymer molecular chains and inorganic particles and the transformation from glass state to viscoelastic state. It can be seen from Figure 6 that peaks of curves move to the direction of high temperature with the enhancement of compatibility which can be seen from SEM images (Figure 5). The glass-transition temperature can be calculated from the peak of tand-T curve. of NR-PMMA/SiO2-PMMA is obviously higher than that of the others (Figure 6), which reaches to −48°C. It could be attributed to the well-dispersed core-shell SiO2-PMMA nanoparticles in NR-PMMA matrix. PMMA play an important role in the molecular interaction between SiO2 and NR chains, declining the thermal motion of the NR molecule . Therefore, the adapting temperature range of rubber products has been expanded with the loading of 2 wt% core-shell SiO2-PMMA nanoparticles.
In addition, NR-PMMA composite filled with core-shell SiO2-PMMA nanoparticles has a higher storage modulus than the others at the temperature ranged from −100°C to −50°C (Figure 7). When the temperature is below , the core-shell SiO2-PMMA nanocomposites possess a high storage modulus, which is caused by introducing rigid nanosilica particles . The increasing storage modulus attributes to the friction between the core-shell SiO2-PMMA nanoparticles and the NR-PMMA molecules when the nanosilica particles are thoroughly dispersed in the NR matrix.
3.3.3. The Thermal Gravimetric Analysis
All samples are measured in nitrogen environment, which means no oxygenolysis of polymer molecules and that it well proves the effect of the bridge molecule, PMMA. Adding SiO2-PMMA nanoparticles can enhance the thermal stability of composites. The main thermal decomposition of NR matrix is at about 360°C (Figure 8) where C–C chain bonds ruptured and hydrogen transferred . The degradation curve of NR-PMMA/SiO2-PMMA is slightly shifted to a higher temperature, compared with the other samples, due to the intertwining between one PMMA chain on the surface of nanosilica and another PMMA chain grafted on NR molecules.
During thermal decomposition, there are three main degraded temperatures, onset temperature , peak temperature , and flow temperature . Those of the NR-PMMA/SiO2-PMMA nanocomposites increase by 5.4°C, 4.9°C and 5.7°C, respectively, in comparison with those of the pure NR. Incorporating with the distribution observed from SEM, the thermal stability depends on the distribution of SiO2 nanoparticles. The more homogenously the SiO2 nanoparticles disperse in NR matrix, the stronger the reaction between the molecules is . Because the decomposition has been slowed down, the ageing resistance of nanocomposite is improved compared to the pure NR. This from another aspect supports the feasibility and efficiency of similar dissolve mutually theory in the preparation of NR/silica composite.
3.3.4. Tensile Property
The tensile performance has been investigated in detail among different samples, namely, NR, NR/PMMA, NR-PMMA/SiO2, and NR-PMMA/SiO2-PMMA. Obviously, with the improvement of modification and the introduction of nanosilica, the tensile strength as well as tensile modulus at different elongations is increasingly high. It can be seen from Table 1 that the tensile strength increases by 3.66 MPa from 6.99 MPa (NR) to 10.65 MPa (NR-PMMA/SiO2-PMMA). Since PMMA is grafted both on SiO2 and NR, its molecule chains twist together via similar dissolve mutually theory, reinforcing the interaction between nano-SiO2 and NR matrix and hindering the movement of NR macromolecule chains . That is to say that PMMA as a bridge between the inorganic nanoparticles and polymer matrix improve the mutual compatibility which in turn results in excellent mechanical property. In addition, comparing the tensile strength between NR-PMMA and NR-PMMA/SiO2, nanosilica without grafting PMMA on its surface can also reinforce the tensile property, from 7.23 to 8.95 MPa.
A novel methodology, similar dissolve mutually theory, is introduced to prepare NR/SiO2 nanocomposite in this paper. The core-shell SiO2-PMMA particles ranged from 60 nm to 100 nm are well dispersed in latex at a suitable SiO2/MMA ratio of 1 : 0.3, and the thickness of PMMA shell is about 25–35 nm. Moreover, the dispersion of core-shell SiO2-PMMA nanoparticles has been improved by grafting PMMA on both nano-SiO2 and NR molecule chains. Based on the excellent dispersion of SiO2-PMMA nanopaticles, and thermal ageing resistances of nanocomposite are considerably enhanced. Meanwhile, the tensile strength increases by 3.66 MPa compared with the pure NR. The recent research is merely focusing on 2 wt% loading of nanosilica, and further work will be extended into different addition of nanosilica.
The financial support from the Natural Science Foundation of China (Contract Grant no. 50763006), Ministry of Science and Technology R & D research institutes special fund (Contract Grant no. 2008EG134285) and 973 Program special fund (Contract Grant no. 2010CB635109) are gratefully acknowledged.
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