J Polym Res (2014) 21:373 DOI 10.1007/s10965-014-0373-z
ORIGINAL PAPER
High electrical conductive polymethylmethacrylate/graphite composites obtained via a novel pickering emulsion route Bai Xue & Tingting Feng & Shengtai Zhou & Jianjun Bao
Received: 7 November 2013 / Accepted: 23 January 2014 # Springer Science+Business Media Dordrecht 2014
Abstract In this work, high electrically conductive Polymethylmethacrylate/graphite (PMMA/G) composites with a specific core-shell structure were synthesized via Pickering emulsion (solid-stabilized emulsion) route. The electrical conductivity of the core-shell composites was measured by a four-point probe resistivity determiner and a very high value of 9.8×10−3 S/cm (1013 times higher than virgin PMMA) was obtained at 30 wt% graphite. However, the electrical conductivity of the PMMA/G composites gained through traditional blend process was relatively lower and the value only reached 9.4×10−9 S/cm at same graphite loading fraction. Contact angle measurement was applied to determine the surface free energy of the modified graphite which was cladded by Al(OH)3. The morphology of the core-shell composites was observed by SEM and optical microscopy. Dynamic rheology analysis was employed to study the structural change by the interconnection of the graphite flakes and the formation of the networks in the composites. The interconnected networks of the core-shell composites were more easily constructed when compared with the composites obtained by the traditional blending process.
Keywords Electrical conductivity . Graphite . Core-shell structure . Conductive networks . Pickering emulsion
B. Xue : T. Feng : S. Zhou : J. Bao (*) The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China e-mail:
[email protected] B. Xue e-mail:
[email protected]
Introduction Recently, electrically conductive polymeric composites have attracted a considerable research interest in polymer industry due to their superior properties such as high thermal and electrical conductivity, weight advantage, good processing ability, and excellent resistance to corrosive environment compared with metal [1–5]. The addition of highly conductive fillers within a polymer matrix is considered as an effective method to fabricate polymer-based conductive composites which have a ubiquitous industrial application, such as corrosion-resistant materials, battery and fuel cell electrodes, antistatic media, heat sinks as well as heat exchangers [4, 6–8]. Researchers have incorporated a great range of conductive fillers into polymers to improve the electric conduction ability of the matrices. Traditionally, the conductive fillers encompass ceramic slice [9], metal powders such as silver particles [10] and carbonaceous fillers, including carbon black (CB) [11–15], carbon fiber (CF) [16–19], and graphite [20–24]. Carbonaceous fillers appear to be the best promising fillers for their intrinsic highly conductive abilities, light weight and most importantly cost-effective [6]. Miyasaka et al. [25] incorporated CB into a variety of polymer matrices to investigate their electrical conductivity and color stability. Yi et al. [15] introduced graphite into high-density polyethylene (HDPE) to illustrate the current–voltage behavior of HDPE/graphite composites and also studied the resistance as well as impedance properties of the obtained composites via percolation theory. Wirita et al. [26] gained HDPE/CF composite with high conductivity and good mechanical properties by adding a small portion of CF to HDPE matrix. Graphite, a naturally abundant material which consists of a layered structure with a c-axis lattice constant of 0.66 nm [23] is widely utilized in the production of polymeric materials that
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hold very high electrical conductivity, as well as maintain light weight and excellent corrosion resistance. Generally, the fraction of the conductive filler within the polymers needs to be very high to impart the resultant composites relatively high conductive abilities. It is not uncommon that the electrical conductivity of the polymeric composites increases with an incremental content of conductive filler. The formation of conductive networks and high stacking density of conductive fillers within the matrices are responsible for the percolation transition in most composite systems [27, 28]. Navarro-Laboulais et al. [29] have applied the percolation theory to study the election transport mechanism in PE/graphite composites. Nagata et al. [23] investigated the influence of graphite size and shape on the conductivity of the composites by incorporating low-density polyethylene (LDPE) with both spherical and plate-like graphite. Zhou et al. [4] obtained highly conductive composites via in situ exfoliation process of expandable graphite filling polyamide 6 (PA6). The incorporation of graphite into polymer matrix to obtain conductive composites is not a novel idea. However, the polymer/graphite composites with a core-shell structure are rarely reported. The core-shell composites are obtained via a novel Pickering emulsion (solid-stabilized emulsion) route [30]. The solid powder remains at the surface of the droplets in the emulsion and impedes the amalgamation of two droplets when they approach [31]. The application of Pickering stabilization was mainly limited to cosmetics, food and oilrecovery industries for a long period. Recently, Pickering emulsion has revived and been utilized favorably to create various nanostructures. He et al. [32] prepared silica nanoparticle-armored polyaniline microspheres which based on a silica nanoparticle-stabilized Pickering emulsion. Gu et al. [33] obtained Ag-Fe3O4 nanostructure in an emulsion that is solely stabilized by Fe3O4 nanoparticles. Nadar et al. [34] synthesized core-shell particles with a layer of calcium carbonate in which the core is BMA-coMMA. To the best of our knowledge, the product of solid stabilized Pickering emulsion is almost spherical particles. However, the emulsion stabilized by graphite flakes is rarely reported. In this work, PMMA/graphite conductive composites with a core-shell structure were obtained via a novel Pickering emulsion. The electrical conductivity of the coreshell composites was superior to that of the composites processed by traditional route which was mainly discussed as a function of the weight fraction of graphite. The morphology of the materials was observed by a scanning electron microscope (SEM). The rheological behavior of the composites was investigated by a dynamic rheometer. The products were also characterized by optical microscope and contact angle meter.
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Experimental Materials The colloid graphite, with a particle size of nearly 6 μm, was purchased from Qingdao Zihe Graphite Co., Ltd (China). PMMA, with a density of 1.18 g/cm3 and molecular mass Mw=14 kDa, was obtained from ATOFINA corporation (America). PVA (Synthomer Alcotex B72) was obtained from Synthomer Corporation (Britain). Methyl methacrylate (MMA), benzoyl peroxide (BPO), N, N-dimethylaniline (DMA), sodium hydroxide (NaOH) and aluminum sulfate (Al2(SO4)3) were purchased as analytically pure reagents from Chengdu Kelong Chemical Reagent Co., Ltd (China). MMA was distilled under reduced pressure and BPO was purified by recrystallization. Deionized water was prepared in the lab. Preparation of modified graphite Modified graphite was prepared by cladding Al(OH)3 in the method of heterogeneous deposition. The detailed process can be described as following. A determined amount of (1.2, 0.6, 0.3, 0.22 or 0.15 g) colloidal graphite and 0.2 g 2 wt% PVA solution (dispersant) was blended in 40 mL deionized water which was placed in a 100 mL beaker. The mixtures were stirred at the speed of 40 rpm for 15 min at 30 °C. 0.34 g Al2(SO4)3 and 0.24 g NaOH were respectively dissolved in 10 mL deionized water. Then, the obtained Al2(SO4)3 solution was dropwisely added to the mixture, in the meanwhile the NaOH solution was added to adjust the pH (5–7). The resulting mixtures were then filtered and washed with deionized water for 4–5 times. After that the mixture was freezedried at −5 °C for 5 h. Finally, the modified graphite cladded by 13, 26, 52, 78 and 104 wt% Al(OH)3 was obtained. Preparation of PMMA/graphite composites The core-shell PMMA/graphite composites were synthesized in a modified graphite (with a 13 % Al(OH)3 content) and PVA co-stabilized Pickering emulsion. The detailed process can be described as following. 0.2 (0.4, 0.6 or 0.8) g modified graphite was dispersed in 5 mL deionized water using sonication for 3 min. The blend was labeled as solution A. Then, 0.2 g BPO was dissolved into 4 mL MMA, and this solution was referred as solution B. After that solution B and 0.04 mL PVA 2 wt% aqueous solution were added into solution A under ultrasonic for 3 min, forming an emulsion of MMA/ water complex at room temperature. Then, 0.08 mL DMA was quickly added to the above mixed solutions under violent shake. All the reaction solutions were allowed to proceed for 5 h at room temperature. Finally, the resulting core-shell particles were dried for 6 h at 50 °C. Then the particles were compression-molded under the conditions of 10 MPa and at
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Table 1 Surface free energy parameters of test liquids Text liquid
γl/(mJ/m )
γld/(mJ/m2)
Glycol Deionized water
48.0 72.2
29.0 21.2
2
γlp/(mJ/m2)
Xp =γlp/γl
19.0 50.2
0.4 0.7
dynamic rheometer (Bohlin Gemini 2000, Malvern Instruments Ltd, UK) in the melt state. All the samples were tested at 190 °C in a frequency sweep range from 0.01 to 100 HZ under nitrogen atmosphere.
Results and discussion 190 °C, and then cooled instantly under pressure to room temperature. Thus the series core-shell composites containing 5, 10, 20, and 30 wt% were obtained. The traditional PMMA/graphite composites were obtained by simple mechanical blending. A composite system containing different fractions of graphite was fabricated using Brabender (Duisburg, Germany) internal mixer at a screw speed of 50 rpm and at 190 °C for 10 min. Then the samples used for measurements were compression-molded the same conditions as described above. Thus the series traditional composites containing 5, 10, 20, and 30 wt% were obtained. Characterizations Contact angle meter (Krüss K100, Krüss, Germany) was used to characterize the surface free energy of the modified graphite. The test liquids were deionized water and glycol. The morphology of the graphite and composites was observed by a scanning electron microscope (SEM; JSM-5900, JEOL, Japan) with an acceleration voltage of 10 kV. All the fractured surfaces were coated with gold to prevent electrostatic charging. The morphology of the composites was also observed by optical microscope (VHX-1000C, KEYENCE, Japan). The molecular weight of the polymer was determined by gel permeation chromatography (GPC). The electrical conductivity of the PMMA/graphite composites was measured by the four-point probe resistivity determiner (SIGNATONE, America) at room temperature. The hot pressed samples were cut into dimensions of 40×40×0.4 mm3 for testing. Each sample was tested at least 5 times and the average value was obtained. Viscoelastic behavior of the composites was analyzed by a Fig. 1 a Contact angle and b surface free energy parameters of modified graphite as a function of Al(OH)3 content
Contact angle measurement It is generally acknowledged that the contact angle measurement on a given solid surface is one of the most practical ways to obtain surface energies [35]. The contact angle of pure liquids on a solid was evaluated by the Young equation: γ l cosθ ¼ γ s −γ s−l
ð1Þ
where γs is the surface energy of the solid, γl is the surface energy (surface tension) of the liquid, γs−l is the solid/liquid interfacial energy and θ is the contact angle. The surface free energy γ itself was equal to a sum of a dispersion component γd and a polar component γp: γs ¼ γsd þ γsp
and
ð2Þ
γl ¼ γld þ γl p
ð3Þ
where γsd is the dispersion component and γsp is the polar component of the solid, γld is the dispersion component and γlp is the polar component of the liquid. The solid/liquid interfacial energy (γs−l) can be evaluated by the geometric mean equation [36]: γ s−l ¼ γ s þ γ l −2
qffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffi γ s d γ l d −2 γ s p γ l p
ð4Þ
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bond of the cladding Al(OH)3. Both Al-O bond and O-H bond are strong polar bonds. They facilitated the interaction between the modified graphite and polar liquids. The decreasing of the dispersion component of modified graphite was mainly ascribed to the decreasing of the bare c-c non-polar bonds with an incremental weight fraction of Al(OH)3. Morphology of PMMA/graphite composites
Fig. 2 Schematics of the method for preparation of the PMMA/graphite core-shell particles
Relating Eqs. (1) to (4), Eq. (5) can be obtained: γ l ð1 þ cosθÞ ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffi γsdγld þ 2 γs pγlp
ð5Þ
According to Eq. (5), two kinds of different polar test liquids (deionized water and glycol) were used to obtain the surface free energy of the modified graphite. The surface free energy parameters of the two test liquids are given in Table 1. The effect of the loading content of Al(OH)3 on the surface free energy of the modified graphite is shown in Fig. 1. The contact angle with both deionized water and glycol decreased with an increasing weight fraction of Al(OH)3. Therefore, the wettability between the modified graphite and deionized water as well as glycol was improved. The dispersion component of the modified graphite decreased although the polar component increased with an increasing weight fraction of Al(OH)3. Basically, the increasing of the polar component of the modified graphite was mainly ascribed to Al-O bond and O-H Fig. 3 Optical microscope micrographs of core-shell particles (a) and polished particles (b)
The fabrication process of the PMMA particles with a shell of modified graphite is illustrated in Fig. 2. Emulsion was stabilized by the modified graphite and PVA. The graphite flakes were fixed at the surface of the particle with MMA polymerization because the flakes with the cladding of Al(OH)3 own partial hydrophily. The weight-average molecular weight (Mw) of the synthesized PMMA was about 12 kDa–14 Da. Figure 3(a) shows a typical image of the PMMA particles with a shell of modified graphite. The particles with a size range of 100–120 μm were observed using an optical microscopy at dry state. In order to determine whether the graphite flakes are only existed on the surface of the particles or dispersed into the core of PMMA, the particles were polished to get rid of the shell of graphite (see Fig. 3(b)). The polished particles were obtained by putting core-shell particles between two pieces of abrasive papers and then repeatedly rubbing the two abrasive papers. The core without the shell of graphite was much lighter than the core-shell particles, which confirms that few graphite flakes dispersed into the core of PMMA. It is clearly shown in Fig. 4 that the presence of the graphite shells appears on the outside of the particle, giving spherical coreshell morphology. The graphite flakes covered all the surface of the core of PMMA. Electrical conductivity The effect of the loading content of Al(OH)3 on the electrical conductivity of the modified graphite is shown in Fig. 5. It was apparent that the electrical conductivity value of the coated graphite decreased with the increasing Al(OH)3 coating content. The electrical conductivity decreased drastically at the loading concentrations of 100 wt% Al(OH)3. This was
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Fig. 4 SEM micrographs of core-shell particles at different magnification
mainly due to the insulativity of the solid Al(OH)3. With the increasing content, the solid Al(OH)3 separated the graphite flakes from each other and obstructed the conductive route in the graphite. The electrical conductivity values of PMMA/graphite composites are plotted against the loading fractions of graphite in Fig. 6. It was clear that the conductivity value of the traditional composites was lower and increased slowly with the increasing graphite loading content. Basically, the conductivity increased slowly was mainly ascribed to the inadequate formation of interconnected networks by graphite flakes and the
high resistance of polymer matrix [9]. The conductivity of the core-shell composites was superior to the traditionalfabricated composites, and had a drastic increasing between 5 and 10 wt% of graphite content. This was mainly due to the formation of conductive networks by graphite flakes which was confirmed by Optical Microscope (see Fig. 7). The brilliant networks in Fig. 7 interlaced all the fractured surfaces of the core-shell composite system. By contrast, the brilliant networks were not detected in the fractured surfaces of the traditional composites. The brilliant networks of the core-shell composites broadened with the increasing weight fraction of graphite. It was exactly why the electrical conductivity of the
Fig. 5 Electrical conductivity of graphite coated with different Al(OH)3 contents
Fig. 6 Electrical conductivity of core-shell composites and traditional composites as a function of graphite content
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Fig. 7 Optical microscope images of fractured surfaces of the core-shell composite system
core-shell composites increased quickly with the increasing graphite content. Fig. 8 a G′ and c G″ as a function of ω for different graphite content for the core-shell composites; b G ′ and d G″ as a function of ω for different graphite content for the traditional composites
A high conductivity value of 9.8×10−3 S/cm was obtained for the PMMA/graphite composites with a core-shell structure
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Fig. 9 Dependence of G′ value at the ω of 0.01 Hz (G′0.01) on the weight fraction of graphite for estimating the critical weight fraction defined as percolation threshold
at the graphite loading of 30 % by weight. However, the conductivity value was only 9.4×10−9 S/cm for the traditional composites blend at the same graphite loading fraction. The core-shell composites using Pickering emulsion could exhibit high conductivity with a low weight fraction of graphite. It is indicated that graphite flakes could easily form the connected networks in the PMMA/graphite composites with a core-shell structure.
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Fig. 8(a), the elastic modulus became insensitive to frequency when the fillers content reached 10 wt%. This indicated that the rheological behavior changed from a liquid-like state to a solid-like state, which accompanied with the well construction of conductive networks by the graphite flakes. That could be referred to the percolation threshold. The transition was observed exactly at the same composition in Fig. 8(c). In addition, the transition was not observed in the traditional composites, which indicated that the conductive networks in traditional composites were not formed. This explained that the conductivity of the core-shell composites was superior to the traditional composites at the same concentration of graphite. If the G′ at the lowest frequency applied (G′0.01) is plotted against the weight fractions of graphite (Fig. 9), two distinct regions can be seen on the curve of core-shell composites. The turning point observed at which concentration of graphite was defined as the percolation threshold for the PMMA/G composites. However, there is not a turning point on the curve of traditional composites. In combination with the electrical conductivity results, it can be concluded that the percolation threshold of the core-shell PMMA/G blends is near the concentration of 5 wt% graphite, or more specifically, between 5 and 10 wt%. However, the percolation threshold of the traditional composites is higher than the concentration of 30 wt% graphite.
Dynamic rheological tests
Conclusion
The dependence of elastic modulus (G′) and viscous modulus (G″) on angular frequency (ω) for the PMMA/graphite composites (a, c for core-shell composites and b, d for traditional composites) is shown in log–log plots in Fig. 8, respectively. It was obvious that the modulus values of the core-shell composites increased faster than the traditional composites with an increasing weight fraction of graphite in the range of frequency employed. The slopes of the curves decreased at the low frequency regime with the increasing graphite content. Particularly, starting from 10 wt% graphite content in the core-shell composites, a plateau was observed at the low frequencies, which indicated the formation of a mechanically connected network by the graphite flakes. The connected network formation was mainly responsible for enhancing the conductivity of the composites. However, the plateau was not observed in the traditional composites. This demonstrated that the connected network in the core-shell composites was more easily constructed than the composites obtained by traditional blend process. The percolation concentration (θc) of the composites can be performed using the rheological data [3]. As shown in
In this study, high electrical conductive PMMA/graphite composites with a core-shell structure were achieved via Pickering emulsion route. Pickering emulsion was stabilized by modified graphite flakes. An electrical conductivity value of 9.8× 10−3 S/cm was obtained for the core-shell PMMA/graphite composite at its maximum filler loading fraction of 30 wt%, which was approximately 1013 times higher than that of virgin PMMA and 106 times higher than the value achieved for the composites by traditional melt blending process at the same graphite content. Graphite flakes could well form interconnected networks in the core-shell composites, which were accepted to be the main factor to the enhancement of conductivity properties of the core-shell composites. The connected network of the core-shell composites was more easily formed than the composites by traditional blend process. The network formation by graphite flakes in the core-shell composites was confirmed by dynamic rheology measurements. Acknowledgments The authors would like to express their thanks to the Analytical and Testing Center of Sichuan University for providing dynamic rheological tests and SEM observations.
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