Facile fabrication of silica–polymer–graphene collaborative nanostructure-based hybrid materials with high conductivity and robust mechanical performance

Abstract

SiO₂@poly(methyl methacrylate)–reduced graphene oxide (SiO₂@PMMA–rGO) composites with outstanding thermal stability, robust mechanical performance and excellent conductivity have been prepared by dispersion polymerization and electrostatic assembly based colloidal blending. The simultaneous construction of well-segregated silica structures and interconnected graphene networks, not only efficiently avoids agglomeration of the incorporated nanofillers, but also ensures enhanced interfacial adhesion between the fillers and the PMMA matrix, endowing the resultant composite with high performance. Specifically, compared to the host polymer, the composite with collaborative structure exhibits high thermal stability, i.e. the decomposition temperature increases by 80 °C and shows robust mechanical properties with a 108% increase in modulus and a 125% improvement in hardness. Besides, an ultra-low percolation threshold of 0.23 vol% is also achieved and the electrical conductivity reached 15.1 S m⁻¹ with only 2.7 vol% graphene loading, which is ∼8 orders of magnitude higher than that for SiO₂–PMMA–rGO (where SiO₂, PMMA and rGO were simply compounded without forming synergetic structures) with the same rGO loading. These results demonstrate that the SiO₂@PMMA–rGO composite has great potential to be applied as mechanical, thermal, and electrical materials.

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Introduction

Silica–polymer hybrid materials have received much scientific and technological attention in recent years,^1–6 because they combine the high mechanical strength and thermal stability of the silica with the flexibility and processability of the polymers. However, such desired properties can be achieved only in situations where the particles are dispersed in a controlled manner without agglomeration.⁷ An elegant solution to avoid agglomeration is to create core–shell nanostructures, chemically encapsulating the primary particles with layers of binder polymer. Various strategies for fabricating perfect core–shell nanostructures have been reported,^8–13 such as emulsion, suspension, miniemulsion and vapor deposition polymerization. However, these methods usually require complicated multistep procedures, special equipment, and harmful organic solvents. To overcome these limitations, a simple and reliable method applied to the high-yield preparation of biphasic supraparticles is dispersion polymerization.^14–16 Based on this method, Bourgeat-Lami et al.¹⁷ have demonstrated an eco-friendly dispersion polymerization to prepare micron-sized polystyrene (PS)/silica clusters with well-defined morphology by using water and ethanol as solvents.

Nevertheless, in addition to high mechanical strength and outstanding thermal stability of silica–polymer colloidal particles based composites, the construction of novel functional materials and devices requires the pristine colloidal particles with not only predictable architectures but also a further optimization over their surface characteristics.¹⁸ Graphene is a unique candidate for surface modification due to its high performance such as high electrical charge mobility, large specific surface area and excellent mechanical flexibility.^19,20 To gain firm interaction between the graphene and colloidal particles, an effective technique is utilizing the chemically derived graphene sheets.^21–23 But disadvantages of this process are that the electrical conjugation of graphene could be damaged by the structural modifications, as demonstrated in the work of Steenackers et al.,²⁴ and the presence of nonconductive functional groups on the surface of graphene will result in the high inter-sheet contact resistance between the graphene sheets. In this respect, electrostatic self-assembly is a promising alternative method because it has already been successfully utilized for the absorption of graphene oxide (GO) on the surfaces of the polymer colloidal particles while allowing for the preservation of the structural and electrical quality of graphene, for example, Wu's group²⁵ reported a highly conductive nanocomposite with 3D compactly interconnected graphene networks self-assembled on the polymer nanospheres. However, it involves the preparation, subsequent functionalization of PS nanospheres and electrostatic self-assembly between PS nanospheres and GO, which is also multi-step. Therefore, it still remains a challenge to develop a facile method to form multifunctional composites with outstanding thermal stability, mechanical strength and excellent conductivity.

Herein, we propose a facile and versatile method to prepare SiO₂@PMMA–rGO hybrids with advanced multifunctionality by dispersion polymerization and electrostatic assembly based colloidal blending. High-yield SiO₂@PMMA core–shell nanoparticles (NPs) with positively activated surfaces are prepared via a green seed-dispersion polymerization process using AIBA as the initiator, which results in synthesis and functionalization in a single step. With the aid of electrostatic interaction, the negatively charged GO is adsorbed onto the surfaces of the positively charged core–shell NPs. SiO₂@PMMA–GO particles are reduced in hydrogen iodide (HI) solution and then pressed to obtain hybrid material with uniform and interconnected graphene networks observed by TEM and SEM. Different from the SiO₂–polymer–rGO composites prepared by the conventional melt processing method, a well-segregated SiO₂ structure and interconnected graphene networks simultaneously formed in SiO₂@PMMA–rGO composites by our approach have obvious advantages: (1) a uniform distribution of both silica and graphene sheets in the polymer host without aggregation; (2) strong adhesion between the fillers and the PMMA matrix; (3) graphene sheets contact directly and tightly with reduced contact resistance. As a consequence, this collaborative structure enables the composites with outstanding thermal stability, robust mechanical strength and excellent conductivity, showing potential applications in functional devices, such as electronic devices and electromagnetic shielding etc.

Experimental

Materials

Tetraethoxysilane (TEOS) and ammonia hydroxide (NH₄OH, 28–32% in water) were obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd, and used without further purification. Methyl methacrylate (MMA, 99%, distilled under reduced pressure), the cationic initiator, 2,2′-azobis(isobutyramidine) dihydrochloride (AIBA), γ-methacryloyloxypropyltrimethoxysilane (MPS) and poly(N-vinylpyrrolidone) (PVP, K29-30, 58 000 g mol⁻¹) were purchased from Aladdin Chemistry Co., Ltd, Shanghai, China. Deionized water was applied for all polymerization and treatment processes.

Synthesis of MPS-modified SiO₂ NPs

The monodisperse SiO₂ NPs with mean diameters of ∼300 nm were prepared by hydrolysis and condensation of TEOS at stabilized temperature originally described by StÖber.²⁶ In a typical synthesis of 300 nm SiO₂ particles, 14 ml of H₂O, 40 ml of absolute ethanol, 2 ml of ammonia hydroxide were introduced into a three-neck round flask of 250 ml equipped with a reflux condenser and homogenized through stirring at 400 rpm at 25 °C. The mixture of 8 ml of TEOS and 22 ml of absolute ethanol prepared separately was then added, and the reaction lasted for 24 h under continuously stirring. Unreacted precursors were removed via solvent exchange: samples were centrifuged at 3000 rpm and redispersed in fresh ethanol by ultrasonication in a bath for 30 min. SiO₂ NPs pretreated in vacuum at 200 °C for 2 h were added into ethanol containing calculated amount of MPS, ultrasonically dispersed for 1 h. Then the reaction proceeded for 24 h at ambient temperature under vigorous stirring. After that, MPS modified SiO₂ NPs was obtained by centrifugation and redispersion with deionized water and subsequently dried at 80 °C for 24 h in vacuum.

Synthesis of SiO₂@PMMA NPs

In a three-neck round flask of 100 ml fitted with a nitrogen inlet and a reflux condenser, 0.5 g of the SiO₂ NPs and 1 g of the PVP were previously dispersed in an aqueous ethanol solution followed by 30 min of ultrasonication. Then, the appropriate amount of initiator AIBA (0.5 wt% relative to monomer) was added into the suspension. The reaction medium was degassed by several cycles of evacuation-nitrogen purge and MMA of varying amount, depending on the different desired final particle morphology, were subsequently introduced quickly to begin the reaction after the system was heated to 70 °C in an oil bath. After the polymerization was completed, the resulting white-milky products were collected by centrifugation (5000 rpm, 15 min) to carefully remove the remaining reactants before drying in the vacuum freeze dryer.

Fabrication of the SiO₂@PMMA–rGO composite film

According to the Hummers method,²⁷ graphite oxide was synthesized from powdered graphite, washed by several times of centrifugation/washing and stably dispersed in water by ultrasonic for 1 h. Then 10 ml of aqueous graphene oxide suspension (desired concentration) were added dropwise into 30 ml positive charged SiO₂@PMMA NPs dispersion (4 mg ml⁻¹) under the magnetic stirring. After 4 h, the coagulation of the polymer composite was obtained via filtration, washed with deionized water, and then immersed into 50 ml HI solution (50 wt%) at 60 °C for 12 h to reduce graphene oxide. The reduced coagulation was washed with distilled water until the supernatant was clear and dried in the vacuum freeze dryer. Finally, the obtained SiO₂@PMMA–rGO hybrids were hotly pressed to platelets at 160 °C for 2 min. As for the comparison, the composite without synergetic structure, SiO₂–PMMA–rGO, has also been made. Dry PMMA powder, SiO₂ and graphite oxide were blended simultaneously in a Haake mixer (Typ 557-2200, Thermo Electron) at 230 °C. Then the received compound was crushed into powder and hotly pressed to platelets at 160 °C for 2 min after the HI solution treatment.

Characterizations

Electron microscopy measurements were carried out with a transmission electron microscope (TEM, Oxford JEM-2100) and scanning electron microscopy (SEM, Hitachi S-4800, operated at 10 kV) for the morphologies characterization of as-produced hybrids. Zeta potential measurements were carried out using a Zetasizer (Nano ZS, Malvern Instruments) with water as the solvent. Thermogravimetric analysis (TGA, TA Instruments Q50) were also performed under N₂ atmosphere with a heating rate of 5 °C min⁻¹. The corresponding mechanical properties of nanocomposite films have been analysed by nanoindentation tests (Hysitron TRIBOINDENTER) with Berkovich tips.

Results and discussion

The synthetic process, as demonstrated in Scheme 1, is composed of (1) the addition of MMA to form SiO₂@PMMA core–shell NPs in a seed-dispersion polymerization reaction, (2) the introducing of GO via electrostatic assembly based colloidal blending to further improve the properties of pristine NPs, thereby facilitating their functionalization for a myriad of applications, and (3) SiO₂@PMMA–rGO hybrids were hotly pressed to platelets.

Scheme 1 Schematic illustration of general fabrication procedures of the SiO₂@PMMA–rGO nanocomposite.

Fabrication of the SiO₂@PMMA core–shell NPs

High silica encapsulation efficiency is a significant requirement because the uncoated silica particles have weak interaction with the polymer matrix for the absence of chemical binding and thus to be unstable for many applications. There are some determined parameters in the synthesis of SiO₂@PMMA core–shell NPs are determined, especially the reaction medium and monomer concentration. In order to investigate the influence of the medium on the size and monodispersity of particles, polymerization reactions were carried out in various ratios of ethanol–water medium using 2.5 ml MMA, 1 g PVP, and 0.08 g AIBA.

At first, the reaction was carried out when the volume ratio of ethanol and water was set as 3 : 1. It was found that the reaction system did not yield SiO₂@PMMA particles at the ratio mentioned above, while other monomers, such as styrene,²⁸ were usually able to precipitate from the solvent in such a situation.

Considering that the PMMA was much more solvable than polystyrene (PS) in ethanol and thus the poorer solvent, water, was needed to shorten the critical chain length at which the PMMA nuclei are formed, we adjusted the ratio of ethanol–water to 1/1 and thus, the concentric SiO₂@PMMA core–shell NPs with small polydispersity were obtained, which implies that increasing the water portion would be efficient to decrease the solubility of polymer chains. However, on further increasing the portion of water (1 : 3), the trend of forming individual pure PMMA particles is obvious and uncoated silica particle appeared. In conclusion, these experiments indicated the chemical composition of the reaction medium strongly affects the morphology of the SiO₂@PMMA NPs. When the water portion in medium was below or above 50% (0.3 wt%), silica spherical particles with a low coating efficiency were obtained. Hence, we conclude that the appropriate coating was achieved at an ethanol : water volume ratio of 1 (Fig. 1b) and this ratio was applied for the following research.

Fig. 1 TEM and SEM images of SiO₂@PMMA NPs obtained in an ethanol (100 − x)–water (x) % medium in the presence of 300 nm SiO₂ seeds (8.3 g l⁻¹): (a and d) x = 25, (b and e) x = 50, and (c and f) x = 75. In all cases, [methyl methacrylate] = 33 g l⁻¹, [PVP] = 17 g l⁻¹, [AIBA] = 1.3 g l⁻¹, and T = 70 °C.

When the initial concentration of MMA was 40 g l⁻¹, as shown in Fig. 2a, the SiO₂@PMMA contained well dispersed SiO₂ NPs, but they extensively coagulated into a dense matrix, indicating the self-agglomeration of PMMA shell was favored at the high MMA concentration. For purpose of limiting the coalescence phenomenon, it is meaningful to deliberate the effect of MMA concentration. The MMA concentration was varied from 33 g l⁻¹ to 20 g l⁻¹ in a train of experiments. It can be observed that the tendency of particles agglomeration was weakened and particle-size distribution could be precisely tuned as lowering the MMA concentration. When the MMA concentration was lowered to 20 g l⁻¹, well-defined core–shell NPs were achieved (Fig. 2d).

Fig. 2 Typical TEM images of the SiO₂@PMMA NPs morphology as a function of MMA concentration: (a) 40 g l⁻¹, (b) 33 g l⁻¹, (c) 27 g l⁻¹, and (d) 20 g l⁻¹.

Accordingly, in this study, we can make a conclusion that both the chemical composition of the reaction medium and the MMA concentration are key parameters for controlling the morphology. When the water portion in medium was 50% and monomer content was 20 g l⁻¹, SiO₂@PMMA with high coating efficiency and well distribution can be achieved.

Fabrication of the SiO₂@PMMA–rGO nanocomposites

To verify the adsorption of rGO on the PMMA shell surface, zeta-potential analyses of the nanoparticles at different stages of modification were performed at pH = 7 (Fig. 3a). Because of the negatively charged –OH groups on the silica surface, the zeta-potential of SiO₂ NPs (pure white) showed to be negative value (ca. 40.3 mV). Then, for the prepared SiO₂@PMMA core–shell NPs (white), the zeta-potential value was 24.7 mV, demonstrating that the existence of the cationic ionizable surface groups on PMMA shell. After adding the GO solution, the zeta-potential value of SiO₂@PMMA–GO (brown) appeared to be negative again, which indicated the successful GO modification. What's more, the high reduction level of surface GO was proved by the value turning up to positive again after HI solution treatment with the colour of NPs changing to black.

Fig. 3 (a) Zeta potentials of SiO₂, SiO₂@PMMA, SiO₂@PMMA–GO and SiO₂@PMMA–rGO NPs at pH = 7. The inset photos show the colour change of nanoparticles at different stages of modification. (b) Typical TEM image of the SiO₂@PMMA–rGO NPs with a graphene loading of 0.5 vol%. (c–e) Typical TEM image of the SiO₂@PMMA–rGO NPs with a graphene loading of 2.7 vol%.

The microstructure of SiO₂@PMMA–rGO has been elaborately designed, as clearly displayed in Fig. 3b–e. The graphene uniformly assembled on the SiO₂@PMMA surface even at a low graphene content of 0.5 vol% (Fig. 3b), which is mainly owing to the strong electrostatic interaction between the highly activated PMMA shells and graphene. When increasing the graphene content to 2.7 vol%, NPs are encapsulated by graphene with almost saturation density, exhibiting more obviously crinkled and rough surface textures. Remarkably, the graphene sheet not only was adsorbed onto the SiO₂@PMMA surface tightly, but also contacted or overlapped between the adjacent nanospheres (Fig. 3d), building interconnected conductive pathways of the composite.

Thermal and mechanical properties of SiO₂@PMMA–rGO composite

As novel fillers, silica and graphene are divinable to have significant effects on the thermal and mechanical properties of PMMA matrix. Therefore, TG analysis and nanoindentation tests were performed to assess silica and graphene as thermal stabilizers and mechanical reinforcement.

As shown in Fig. 4, for the four samples, slight weight loss below 200 °C is assigned to the volatilization of the physisorbed moisture. SiO₂–MPS is quite stable with a minor mass loss in the whole test temperature range. The obvious decomposition of pure PMMA appears near 280 °C, while the temperature shifted to 350 °C for SiO₂@PMMA–rGO composite. The enhanced thermal stability of SiO₂@PMMA–rGO composites is mainly due to the addition of silica and graphene, which acted as efficient barriers to delay the thermal decomposition.^29,30

Fig. 4 TGA curves of SiO₂–MPS, SiO₂@PMMA–rGO, SiO₂–PMMA–rGO and pure-PMMA composites with a SiO₂ loading of 31 wt% and a graphene loading of 2.7 vol%.

The corresponding mechanical properties of SiO₂@PMMA–rGO nanocomposite film have been analyzed during the nanoindentation tests, and the outcomes are shown in Fig. 5a and b along with those for SiO₂–PMMA–rGO film (made by conventional melt compounding of the fillers and the polymer) and pure PMMA film (p-PMMA) for comparison.

Fig. 5 (a) Typical indentation load–displacement curves of the films. (b) The corresponding elastic modulus (E_r) and hardness values of the films. (c and d) The surface and cross-sectional SEM images of SiO₂–PMMA–rGO composite. (e and f) The surface and cross-sectional SEM images of SiO₂@PMMA–rGO composite. The inset digital photograph of the SiO₂@PMMA–rGO composite film (Fig. 5e) with 2.7 vol% graphene shows excellent flexibility as it can be bent as shown in the inset of (Fig. 5f).

As demonstrated in the typical nanoindentation load–displacement curves of the films (Fig. 5a), under a fixed maximum load of 4 mN, the displacement of the film decrease from 543 nm for the p-PMMA film, to 469 nm for SiO₂–PMMA–rGO film, and 292 nm for SiO₂@PMMA–rGO film, indicating SiO₂@PMMA–rGO film is superior for withstanding the external force and exhibits the minimum plastic deformation. Moreover, compared to the p-PMMA film, it is observed that the properties of SiO₂@PMMA–rGO nanocomposite film are improved dramatically with the well-designed incorporation of fillers, showing an enhancement of over 2-fold both in modulus and hardness. However, for the SiO₂–PMMA–rGO film, only a relatively low increase trend in the properties can be seen. Such a disagreement in mechanical performances between the two composite film mentioned above can be ascribed to nanostructural differences in the morphology of composite films.³¹ In order to investigate the structure–property relationship, we make further morphological analysis. As clearly shown in Fig. 5c and e, SiO₂–PMMA–rGO displays worse film uniformity with a great rough surface than SiO₂@PMMA–rGO, which can be attributed to the weak interaction between the fillers and the matrix and severe nanoparticle aggregate formation (Fig. 5d).

The applied seed dispersion polymerization technique and electrostatic assembly based colloidal blending can effectively avoid the fillers aggregation by uniformly encapsulation of the silica and absorption of the graphene (Fig. 5f and 7a) as well as afford strong interaction at the interfacial region linked by chemical bonds, avoiding the formation of stress concentration area and effectively accelerating load transfer from the matrix to silica and graphene.

Fig. 6 Electrical conductivity of the SiO₂@PMMA–rGO composites as a function of graphene volume fraction.

Fig. 7 Typical cross-sectional SEM image of the remained graphene skeleton of SiO₂@PMMA–rGO nanocomposites with 2.7 vol% graphene after removal of the PMMA matrix at high temperature under nitrogen atmosphere and SiO₂ through HF etching.

Electrical properties of the SiO₂@PMMA–rGO composite

In additional to the outstanding mechanical properties, the superior synergistic SiO₂–PMMA–rGO structure also endows the composite with excellent electrical performance. Fig. 6 showed the variation of electrical conductivity (σ) of the SiO₂@PMMA–rGO composites as a function of graphene volume fraction (ρ) and the change trend can be theoretically described using the well-known power law equation:

σ = σ₀ [(ρ − ρ_c)(1 − ρ_c)]^t

where σ₀ is the electrical conductivity of the graphene, ρ_c is the percolation threshold and t is a critical exponent related to the electrical network.³² Percolation in the composites didn't reached until the graphene content is near 0.23 vol%. This electrical percolation threshold is relatively low, mainly attributing to a good dispersion of graphene in the polymer host. The conductivity of our composites abruptly increased and was up to the maximum conductivity of about 15.1 S m⁻¹ at a loading of 2.7 vol% finally, which is ∼8 orders of magnitude higher than the values for the SiO₂–PMMA–rGO with the same rGO loading.

The low percolation threshold and high electrical conductivity of SiO₂@PMMA composites can be attributed to the formation of a compactly interconnected conducting network (Fig. 7). The well-designed binary-shelled nanostructures and the firmly interaction among SiO₂, highly activated particle surface and the graphene have played an important role in assuring the fantastic 3D graphene network remained intactly even after the hot-pressing process. Furthermore, preparation of SiO₂@PMMA–rGO composites via one-step electrostatic assembly without damaging the electrical conjugation caused by the presence of nonconductive functional groups and the change of graphene structure is also a key reason, render the composite superior electrical performance.

Conclusions

In a word, we have successfully realized the preparation of SiO₂@PMMA–rGO composite via a green, facile and reproducible seed dispersion polymerization and self-assembly process. The morphology of the formed SiO₂@PMMA NPs could be precisely controlled by changing the synthesis conditions. Further attempt to use the surface activated core–shell supraparticles as flexible precursors to fabricate advanced multifunctional nanocomposite was made by investigating the comprehensive properties of SiO₂@PMMA–rGO nanocomposite. The resultant film possesses outstanding thermal stability, hardness and electrical conductivity, which closely relates to the construction of well-segregated SiO₂ structure and compactly interconnected graphene networks as well as due to the covalent molecular binding and strongly electrical interaction. It should be noted that SiO₂@PMMA–rGO composites are promising to be used as multifunctional nanocomposites with versatile electrical and thermal properties for electronic devices, reinforced materials and flame retardant.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51173043, 21136006, 21236003, 21322607), the Special Projects for Nanotechnology of Shanghai (12nm0502700), the Basic Research Program of Shanghai (13JC1408100), the Key Scientific and Technological Program of Shanghai (14521100800), the Fundamental Research Funds for the Central Universities.

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