- Email: [email protected]
ultrahigh molecular weight polyethylene composites with a segregated structure
CARBON
5 0 ( 2 0 1 2 ) 4 5 9 6 –4 5 9 9
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Preparation and electrical conductivity of graphene/ultrahigh molecular weight polyethylene composites with a segregated structure Hongliang Hu a, Guo Zhang a, Liguang Xiao b, Hongjie Wang b, Qiushi Zhang c, Zhudi Zhao a,* a b c
Department of Materials Science and Key Lab of Automobile Materials of MOE, Jilin University, Changchun 130012, PR China School of Materials Science and Engineering, Jilin Architecture and Civil Engineering Institute, Changchun 130118, PR China School of Vocational Technology, Jilin Architecture and Civil Engineering Institute, Changchun 130118, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Graphene-coated ultrahigh molecular weight polyethylene (UHMWPE) powders were pre-
Received 5 February 2012
pared by a two-step process. The first step is to coat UHMWPE polymers with graphene
Accepted 21 May 2012
oxide (GO) sheets. The second step is to reduce GO on the powders to graphene. The
Available online 27 May 2012
two-step process can effectively prevent the aggregation of graphene during reduction. The resultant graphene/UHMWPE mixtures were hot pressed at 200 °C to obtain the composites with a segregated structure. The composites exhibit high electrical conductivity at a very low percolation threshold (0.028 vol.%). Our method provides a new route for preparing electrical conductive graphene/polymer composites with low percolation threshold. Ó 2012 Elsevier Ltd. All rights reserved.
Compared to conventional filled polymers, graphene/polymer composites exhibit excellent electrical conductivity at significantly lower graphene loadings [1,2]. Hence, electrically conductive graphene/polymer composites have attracted much attention [3–5]. Recently, some researchers reported graphene/polymer composites with a segregated structure to lower the content of the graphene filler. Pang et al. [6] prepared graphene/ultrahigh molecular weight polyethylene (UHMWPE) composites with a segregated structure by water/ ethanol solvent-assisted dispersion and hot compression. A low percolation threshold was achieved at the graphene concentration of 0.076 vol.%. A similar preparation method has also been reported by Du et al. [7]. In their experiments, stirring and sonication were applied to cover polymer powders with graphene sheets. However, the graphene sheets in solution tend to form agglomerates due to van der Waals interactions during the solution stirring and sonication, which will lead to the formation of large graphene particles and affect the final composite properties.
Graphite oxide can undergo exfoliation completely in water, yielding colloidal suspensions of almost entirely individual graphene oxide (GO) nanosheets [8]. Under such condition, if the GO sheets are homogenously coated on polymer powders and then reduced, we predict that a high electrical conductivity can be achieved at a low percolation threshold. Therefore, in this study, we prepared graphene/UHMWPE composites by coating UHMWPE powders with GO and then reducing the GO. The obtained composites exhibit high electrical conductivity at very low graphene loadings. GO was prepared from natural graphite powder according to Hummers’ method [9]. We prepare graphene/polymer powders in two steps: (1) GO sheets were dispersed in water/ethanol (water:ethanol = 50:1, w/w) solution by sonication. UHMWPE powders were added into the GO solution and stirred at 95 °C. After a large amount of solvent was evaporated, the mixtures of GO and UHMWPE were kept under the vacuum at 80 °C to remove the residual solvent and form GO-coated UHMWPE powders. To convert mass fraction of GO
* Corresponding author: Fax: +86 431 85168246. E-mail address: [email protected] (Z. Zhao). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.05.045
CARBON
5 0 ( 20 1 2 ) 4 5 9 6–45 9 9
on the powders to volume fraction, the density of GO was determined by pycnometer test method, which was 2.16 g/ cm3. (2) The coated powders were added to hydrazine solutions (hydrazine hydrate:GO = 7:10, w/w) and stirred at 95 °C for 4 h to reduce GO to graphene. The graphene/UHMWPE powders prepared in this way are denoted as two-step prepared powders for convenience. One-step prepared powders [6,7] have also been prepared for reference, that is, after GO solutions (the used solvent is a mixed solvent of water/ethanol having a weight ratio of water to ethanol of 50:1) with UHMWPE powders were sonicated and stirred for 2 h, hydrazine hydrate solutions were added to the solvents (hydrazine hydrate:GO = 7:10, w/w) and stirred at 95 °C for 4 h. These reduced products were isolated by filtration, washed with water, and dried to obtain graphene-coated polymer powders. The used filters were filter papers, which were placed in a Buchner funnel to isolate the coated polymer powder from the solutions. After the solutions were filtrated, the colors of the filtrated liquids were still colorless, indicating that the graphenes were adsorbed on UHMWPE particles. All coated powders were compressively molded at room temperature under 30 MPa, and then hot pressed at 200 °C under 1 MPa for 5 min to form composite sheets of 1.5 mm in thickness. Optical microscopic observation indicated that a segregated structure was successfully achieved for these composites derived from one-step and two-step prepared powders. Fig. 1a shows the dispersibility of GO in water/ethanol medium. After sonication, a brown solution was clear and stable without any visible aggregation. A typical atomic force microscope (AFM) image shows that GO sheets with thick less than 1 nm are achieved (Fig. 1b), indicating that GO sheets are exfoliated to monolayer or few-layer state. After UHMWPE powders are coated with the GO, the color of the polymer powders changes gradually from white to yellow–brown with increasing GO volume fraction (Fig. 1c–f). Fig. 2a–d shows the color changes in solutions with GOcoated (a and b) and uncoated (c and d) polymer powders
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before and after reduction. The solutions were kept at room temperature several hours and then photographed after they were prepared. Before reduction, the liquid with GO-coated polymer powders was stirred for 2 h, however, the color of the liquid is still colorless (Fig. 2a), suggesting that GO sheets can still be adsorbed onto the surfaces of polymer powders. After reduction, the color of GO-coated polymer powders changes from yellow–brown to dark-gray (see the top of the solutions in Fig. 2a and b). For the solution with GO-uncoated polymer powders, however, the brown solution turns colorless after reduction with hydrazine (Fig. 2c and d), indicating that GO in the solution is reduced to graphene. In order to examine whether there are graphene aggregates after reduction, both the reduced solutions are filtrated and obtained powders are shown in Fig. 2e and f. A lot of black spots are observed on the gray powders for the one-step prepared powders. It is clear that the GO sheets in brown solution form graphene aggregates during reduction. However, the two-step prepared powders have a relatively homogenous gray color, indicating graphene sheets are relatively homogenously dispersed on polymer powders. Fig. 3 shows field emission scanning electron microcopy (FESEM) images of 0.1 vol.% graphene-coated polymer powders. The images of the two-step prepared powders demonstrate that the graphene sheets are well exfoliated and dispersed on polymer powders (Fig. 3a). The surfaces of some polymer particles are almost entirely covered with graphene sheets (Fig. 3b). No distinct graphene aggregates are observed. However, the dispersion of graphene sheets on the one-step prepared powders is not homogenous. No graphene sheets can be observed on the surfaces of some polymer particles (Fig. 3c). Moreover, some graphene aggregates can easily be seen in the powders (Fig. 3d). These results are consistent with observations in Fig. 2e and f. Hence, the dispersivity of graphene sheets is higher on the surfaces of the two-step prepared powders than on that of the one-step prepared powders.
Fig. 1 – (a) GO solution. (b) AFM image of exfoliated GO sheets on a mica substrate with height profile from the solution in (a). (c–f) The color changes in GO-coated UHMWPE powders at different GO loadings.
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Fig. 2 – The color changes in the solutions with GO-coated (a and b) and GO-uncoated (c and d) UHMWPE powders before (a and c) and after (b and d) reduction. The digital photographs of one-step (e) and two-step (f) prepared powders containing 0.1 vol.% graphene after filtration.
The electrical conductivity of the composites is shown in Fig. 4. The electrical conductivity of the composite derived from two-step prepared powders is much better than that of
the composite derived from one-step prepared powders. The percolation threshold of the composite derived from two-step prepared powders is about 0.028 vol.%, which is lower than
Fig. 3 – FESEM images of two-step (a and b) and one-step (c and d) prepared graphene/UHMWPE powders. The selected regions in (a) are graphene sheets.
Electrical conductivity (S/cm)
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GO on UHMWPE powders is still unclear, the two-step preparation process provides a new route for preparing electrical conductive graphene/polymer composites with a low percolation threshold.
R E F E R E N C E S
b a
0.0
0.1
0.2
0.3
0.4
Graphene content (vol.%) Fig. 4 – Variation of electrical conductivity with graphene loading of the composites derived from one-step (a) and two-step (b) prepared powders.
that of the composite derived from one-step prepared powders (0.085 vol.%). Pang et al. [10] used a water/ethanol solvent-assisted dispersion and hot-pressing process (i.e. onestep process) to fabricate graphene/UHMWPE composites with a segregated structure, a percolation threshold as low as 0.07 vol.% was achieved because of the formation of a two dimensional conductive network. Du et al. [7] prepared graphene/high density polyethylene (HDPE) composites with a segregated network structure by alcohol-assisted dispersion and hot-pressing process (one-step process), it was found that the percolation threshold of the graphene/HDPE composites was 1 vol.%. There results show the electrical conductivity of the composites is remarkably influenced by the preparation process of graphene-coated polymer powders. In summary, graphene-coated UHMWPE powders were prepared by a two-step process. Compared to one-step preparation process, the two-step process can effectively prevent the aggregation of graphene during reduction. The resulting composites exhibit a very low percolation threshold and high electrical conductivity. Though the adsorption mechanism of
[1] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282–6. [2] Steurer P, Wissert R, Thomann R, Muelhaupt R. Functionalized graphenes and thermoplastic nanocomposites based upon expanded graphite oxide. Macromol Rapid Commun 2009;30:316–27. [3] Kuila T, Bose S, Hong CE, Uddin ME, Khanra P, Kim NH, et al. Preparation of functionalized graphene/linear low density polyethylene composites by a solution mixing method. Carbon 2011;49:1033–7. [4] Spitalsky Z, Tasis D, Papagelis K, Galiotis C. Carbon nanotube–polymer composites: chemistry, processing, mechanical and electrical properties. Prog Polym Sci 2010;35:357–401. [5] He F, Lau S, Chan HL, Fan J. High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates. Adv Mater 2009;21:710–5. [6] Pang H, Zhang YC, Chen T, Zeng BQ, Li ZM. Tunable positive temperature coefficient of resistivity in an electrically conducting polymer/graphene composite. Appl Phys Lett 2010;96:19071–3. [7] Du JH, Zhao L, Zeng Y, Zhang LL, Li F, Liu PF. Comparison of electrical properties between multi-walled carbon nanotube and graphene nanosheet/high density polyethylene composites with a segregated network structure. Carbon 2011;49:1094–100. [8] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nat Nanotechnol 2009;4:217–24. [9] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [10] Pang H, Chen T, Zhang GM, Zeng BQ, Li ZM. An electrically conducting polymer/graphene composite with a very low percolation threshold. Mater Lett 2010;64:2226–9.
5 0 ( 2 0 1 2 ) 4 5 9 6 –4 5 9 9
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Preparation and electrical conductivity of graphene/ultrahigh molecular weight polyethylene composites with a segregated structure Hongliang Hu a, Guo Zhang a, Liguang Xiao b, Hongjie Wang b, Qiushi Zhang c, Zhudi Zhao a,* a b c
Department of Materials Science and Key Lab of Automobile Materials of MOE, Jilin University, Changchun 130012, PR China School of Materials Science and Engineering, Jilin Architecture and Civil Engineering Institute, Changchun 130118, PR China School of Vocational Technology, Jilin Architecture and Civil Engineering Institute, Changchun 130118, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Graphene-coated ultrahigh molecular weight polyethylene (UHMWPE) powders were pre-
Received 5 February 2012
pared by a two-step process. The first step is to coat UHMWPE polymers with graphene
Accepted 21 May 2012
oxide (GO) sheets. The second step is to reduce GO on the powders to graphene. The
Available online 27 May 2012
two-step process can effectively prevent the aggregation of graphene during reduction. The resultant graphene/UHMWPE mixtures were hot pressed at 200 °C to obtain the composites with a segregated structure. The composites exhibit high electrical conductivity at a very low percolation threshold (0.028 vol.%). Our method provides a new route for preparing electrical conductive graphene/polymer composites with low percolation threshold. Ó 2012 Elsevier Ltd. All rights reserved.
Compared to conventional filled polymers, graphene/polymer composites exhibit excellent electrical conductivity at significantly lower graphene loadings [1,2]. Hence, electrically conductive graphene/polymer composites have attracted much attention [3–5]. Recently, some researchers reported graphene/polymer composites with a segregated structure to lower the content of the graphene filler. Pang et al. [6] prepared graphene/ultrahigh molecular weight polyethylene (UHMWPE) composites with a segregated structure by water/ ethanol solvent-assisted dispersion and hot compression. A low percolation threshold was achieved at the graphene concentration of 0.076 vol.%. A similar preparation method has also been reported by Du et al. [7]. In their experiments, stirring and sonication were applied to cover polymer powders with graphene sheets. However, the graphene sheets in solution tend to form agglomerates due to van der Waals interactions during the solution stirring and sonication, which will lead to the formation of large graphene particles and affect the final composite properties.
Graphite oxide can undergo exfoliation completely in water, yielding colloidal suspensions of almost entirely individual graphene oxide (GO) nanosheets [8]. Under such condition, if the GO sheets are homogenously coated on polymer powders and then reduced, we predict that a high electrical conductivity can be achieved at a low percolation threshold. Therefore, in this study, we prepared graphene/UHMWPE composites by coating UHMWPE powders with GO and then reducing the GO. The obtained composites exhibit high electrical conductivity at very low graphene loadings. GO was prepared from natural graphite powder according to Hummers’ method [9]. We prepare graphene/polymer powders in two steps: (1) GO sheets were dispersed in water/ethanol (water:ethanol = 50:1, w/w) solution by sonication. UHMWPE powders were added into the GO solution and stirred at 95 °C. After a large amount of solvent was evaporated, the mixtures of GO and UHMWPE were kept under the vacuum at 80 °C to remove the residual solvent and form GO-coated UHMWPE powders. To convert mass fraction of GO
* Corresponding author: Fax: +86 431 85168246. E-mail address: [email protected] (Z. Zhao). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.05.045
CARBON
5 0 ( 20 1 2 ) 4 5 9 6–45 9 9
on the powders to volume fraction, the density of GO was determined by pycnometer test method, which was 2.16 g/ cm3. (2) The coated powders were added to hydrazine solutions (hydrazine hydrate:GO = 7:10, w/w) and stirred at 95 °C for 4 h to reduce GO to graphene. The graphene/UHMWPE powders prepared in this way are denoted as two-step prepared powders for convenience. One-step prepared powders [6,7] have also been prepared for reference, that is, after GO solutions (the used solvent is a mixed solvent of water/ethanol having a weight ratio of water to ethanol of 50:1) with UHMWPE powders were sonicated and stirred for 2 h, hydrazine hydrate solutions were added to the solvents (hydrazine hydrate:GO = 7:10, w/w) and stirred at 95 °C for 4 h. These reduced products were isolated by filtration, washed with water, and dried to obtain graphene-coated polymer powders. The used filters were filter papers, which were placed in a Buchner funnel to isolate the coated polymer powder from the solutions. After the solutions were filtrated, the colors of the filtrated liquids were still colorless, indicating that the graphenes were adsorbed on UHMWPE particles. All coated powders were compressively molded at room temperature under 30 MPa, and then hot pressed at 200 °C under 1 MPa for 5 min to form composite sheets of 1.5 mm in thickness. Optical microscopic observation indicated that a segregated structure was successfully achieved for these composites derived from one-step and two-step prepared powders. Fig. 1a shows the dispersibility of GO in water/ethanol medium. After sonication, a brown solution was clear and stable without any visible aggregation. A typical atomic force microscope (AFM) image shows that GO sheets with thick less than 1 nm are achieved (Fig. 1b), indicating that GO sheets are exfoliated to monolayer or few-layer state. After UHMWPE powders are coated with the GO, the color of the polymer powders changes gradually from white to yellow–brown with increasing GO volume fraction (Fig. 1c–f). Fig. 2a–d shows the color changes in solutions with GOcoated (a and b) and uncoated (c and d) polymer powders
4597
before and after reduction. The solutions were kept at room temperature several hours and then photographed after they were prepared. Before reduction, the liquid with GO-coated polymer powders was stirred for 2 h, however, the color of the liquid is still colorless (Fig. 2a), suggesting that GO sheets can still be adsorbed onto the surfaces of polymer powders. After reduction, the color of GO-coated polymer powders changes from yellow–brown to dark-gray (see the top of the solutions in Fig. 2a and b). For the solution with GO-uncoated polymer powders, however, the brown solution turns colorless after reduction with hydrazine (Fig. 2c and d), indicating that GO in the solution is reduced to graphene. In order to examine whether there are graphene aggregates after reduction, both the reduced solutions are filtrated and obtained powders are shown in Fig. 2e and f. A lot of black spots are observed on the gray powders for the one-step prepared powders. It is clear that the GO sheets in brown solution form graphene aggregates during reduction. However, the two-step prepared powders have a relatively homogenous gray color, indicating graphene sheets are relatively homogenously dispersed on polymer powders. Fig. 3 shows field emission scanning electron microcopy (FESEM) images of 0.1 vol.% graphene-coated polymer powders. The images of the two-step prepared powders demonstrate that the graphene sheets are well exfoliated and dispersed on polymer powders (Fig. 3a). The surfaces of some polymer particles are almost entirely covered with graphene sheets (Fig. 3b). No distinct graphene aggregates are observed. However, the dispersion of graphene sheets on the one-step prepared powders is not homogenous. No graphene sheets can be observed on the surfaces of some polymer particles (Fig. 3c). Moreover, some graphene aggregates can easily be seen in the powders (Fig. 3d). These results are consistent with observations in Fig. 2e and f. Hence, the dispersivity of graphene sheets is higher on the surfaces of the two-step prepared powders than on that of the one-step prepared powders.
Fig. 1 – (a) GO solution. (b) AFM image of exfoliated GO sheets on a mica substrate with height profile from the solution in (a). (c–f) The color changes in GO-coated UHMWPE powders at different GO loadings.
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Fig. 2 – The color changes in the solutions with GO-coated (a and b) and GO-uncoated (c and d) UHMWPE powders before (a and c) and after (b and d) reduction. The digital photographs of one-step (e) and two-step (f) prepared powders containing 0.1 vol.% graphene after filtration.
The electrical conductivity of the composites is shown in Fig. 4. The electrical conductivity of the composite derived from two-step prepared powders is much better than that of
the composite derived from one-step prepared powders. The percolation threshold of the composite derived from two-step prepared powders is about 0.028 vol.%, which is lower than
Fig. 3 – FESEM images of two-step (a and b) and one-step (c and d) prepared graphene/UHMWPE powders. The selected regions in (a) are graphene sheets.
Electrical conductivity (S/cm)
CARBON
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10
-3
10
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10
-5
10
-6
10
-7
10
-8
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-9
5 0 ( 20 1 2 ) 4 5 9 6–45 9 9
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GO on UHMWPE powders is still unclear, the two-step preparation process provides a new route for preparing electrical conductive graphene/polymer composites with a low percolation threshold.
R E F E R E N C E S
b a
0.0
0.1
0.2
0.3
0.4
Graphene content (vol.%) Fig. 4 – Variation of electrical conductivity with graphene loading of the composites derived from one-step (a) and two-step (b) prepared powders.
that of the composite derived from one-step prepared powders (0.085 vol.%). Pang et al. [10] used a water/ethanol solvent-assisted dispersion and hot-pressing process (i.e. onestep process) to fabricate graphene/UHMWPE composites with a segregated structure, a percolation threshold as low as 0.07 vol.% was achieved because of the formation of a two dimensional conductive network. Du et al. [7] prepared graphene/high density polyethylene (HDPE) composites with a segregated network structure by alcohol-assisted dispersion and hot-pressing process (one-step process), it was found that the percolation threshold of the graphene/HDPE composites was 1 vol.%. There results show the electrical conductivity of the composites is remarkably influenced by the preparation process of graphene-coated polymer powders. In summary, graphene-coated UHMWPE powders were prepared by a two-step process. Compared to one-step preparation process, the two-step process can effectively prevent the aggregation of graphene during reduction. The resulting composites exhibit a very low percolation threshold and high electrical conductivity. Though the adsorption mechanism of
[1] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282–6. [2] Steurer P, Wissert R, Thomann R, Muelhaupt R. Functionalized graphenes and thermoplastic nanocomposites based upon expanded graphite oxide. Macromol Rapid Commun 2009;30:316–27. [3] Kuila T, Bose S, Hong CE, Uddin ME, Khanra P, Kim NH, et al. Preparation of functionalized graphene/linear low density polyethylene composites by a solution mixing method. Carbon 2011;49:1033–7. [4] Spitalsky Z, Tasis D, Papagelis K, Galiotis C. Carbon nanotube–polymer composites: chemistry, processing, mechanical and electrical properties. Prog Polym Sci 2010;35:357–401. [5] He F, Lau S, Chan HL, Fan J. High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates. Adv Mater 2009;21:710–5. [6] Pang H, Zhang YC, Chen T, Zeng BQ, Li ZM. Tunable positive temperature coefficient of resistivity in an electrically conducting polymer/graphene composite. Appl Phys Lett 2010;96:19071–3. [7] Du JH, Zhao L, Zeng Y, Zhang LL, Li F, Liu PF. Comparison of electrical properties between multi-walled carbon nanotube and graphene nanosheet/high density polyethylene composites with a segregated network structure. Carbon 2011;49:1094–100. [8] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nat Nanotechnol 2009;4:217–24. [9] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [10] Pang H, Chen T, Zhang GM, Zeng BQ, Li ZM. An electrically conducting polymer/graphene composite with a very low percolation threshold. Mater Lett 2010;64:2226–9.