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graphite nanocomposites via in situ ring-opening polymerization of macrocyclic oligomers
Polymer 45 (2004) 6713–6718 www.elsevier.com/locate/polymer
Synthesis and properties of poly(4,4 0-oxybis(benzene)disulfide)/graphite nanocomposites via in situ ring-opening polymerization of macrocyclic oligomers X.S. Dub, M. Xiaoa, Y.Z. Menga,b,*, A.S. Hayc a
Institute of Energy and Environmental Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, People’s Republic of China b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c Department of Chemistry, McGill University, 801 Sherbrooke W., Montreal QC, H3A 2K6, Canada Received 4 February 2004; received in revised form 7 June 2004; accepted 15 July 2004 Available online 30 July 2004
Abstract An effective method for the preparation of poly(4,4 0 -oxybis(benzene)disulfide)/graphite nanosheet composites via in situ ring-opening polymerization of macrocyclic oligomers were reported. Completely exfoliated graphite nanosheets were prepared under the microwave irradiation followed by sonication in solution. The nanocomposites were fabricated via in situ melt ring-opening polymerization of macrocyclic oligomers in the presence of graphite nanosheets. The graphite nanosheets and resulted poly(arylene disulfide)/graphite nanocomposites were characterized with field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), tensile tester and electrical conductivity measurements. Compared with pure polymer, the electrical conductivity of the poly(arylene disulfide)/graphite nanocomposites were dramatically increased and had a value of about 10K3 S/cm for the nanocomposite containing 5 wt% graphite. The nanocomposites exhibit as both high performance polymeric material and electrically conductive material. Therefore, they show potential applications as high temperature conducting materials. q 2004 Elsevier Ltd. All rights reserved. Keywords: Conducting polymer; Macrocyclics; Graphite
1. Introduction Polymer composites and nanocomposites with reasonable electrical conductivity are the highlight in the field of functional materials. The conducting polymers and polymeric composites have attracted considerable attention in recent years because of their potential applications in advanced technologies, for example, in antistatic coatings, electromagnetic shielding and in secondary batteries [1,2]. The introduction of electrically conductive fillers such as graphite, carbon black, metal and metal oxide powders into
* Corresponding author. Address: Institute of Energy and Environmental Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, People’s Republic of China. Tel.: C 86-208-523-1343; fax: C86-208-411-4113 E-mail address: [email protected] (Y.Z. Meng). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.07.026
the polymeric matrix is a promising approach to fabricate electrically conductive polymeric materials [2]. Generally, the loading level of such fillers within a conductive polymeric composite is as high as 15 wt% to afford a satisfactory conductivity. However, such highly loading level, can generally lead to the poor mechanical properties and high density of the materials. The recent advancement of nano-scale compounding technique provides an innovative route to prepare highly electrically conductive polymeric nanocomposites with very low loading of conductive fillers. Graphite flakes have a layered structure like layered silicates, such as well-known montmorillonite and vermiculite. Moreover, it is naturally abundant, and therefore, is highly conductive filler (with an electrical conductivity of about 104 S/cm at ambient temperature) for making conducting polymeric composites. It is expected that both
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mechanical properties and electrical conductivity can be improved in case a polymer can be intercalated into the interlayer space of graphite. However, because of the small ˚ ) of the graphite layers and the lack of inter spacings (3.35 A the affinity for either hydrophilic or hydrophobic polymers, it is rather difficult to prepare the conductive graphite/polymer nanocomposites via direct intercalation method [3]. To achieve nanoscale dispersion of graphite in polymer matrix, certain chemical or physical modification to the graphite is generally needed [3,4]. Exfoliated graphite is composed of a large number of delaminated graphite sheets and its electrical conductivity is not obviously affected compared with the original graphite flake [5]. In this sense, monomers and polymers can be intercalated into the pores and galleries of the exfoliated graphite to produce conductive nanocomposite. Using the similar method, polymer/ graphite composites or nanocomposites with low percolation threshold of conductivity and highly thermal storage properties have been disclosed [1,6–11]. For these composites, however, the monomers and polymers might not be able to diffuse into the closed cavities inside the exfoliated graphite. Therefore, the graphite sheets around the cavities tend to overlap each other and form accumulation during processing. The aggregates of graphite sheets may result in the poor mechanical properties of the composites [10,11]. More recently, graphite nanosheets were used to prepare polystyrene/graphite nanocomposite and PMMA/graphite composites via in situ polymerization [10,11]. Poly(arylene disulfide)s were reported to have high resistance to environmental degradation, low watervapor transmission, excellent resistance to acids and bases, and good adhesion properties [12,13]. They might therefore find potential applications in many fields. However, most poly(arylene disulfide)s are insoluble in common solvents, and have very high melting points. These disadvantages have limited its practical application. In previous work, poly(arylene disulfide)s have been synthesized from macrocyclic(arylene disulfide) oligomers via melt ring-opening polymerization [12,13]. These macrocyclic(arylene disulfide) oligomers can instantly polymerize in the melt in the absence of any catalyst or initiator at a temperature as low as 150 8C, without liberation of any volatile by-products [12]. Therefore, they can be processed more easily compared with the corresponding linear polymers with high molecular weight and melt viscosities [12]. These unique properties endow them as a very good matrix candidate for nanocomposites. In this study, we used the macrocyclic oligomers, i.e. cyclo(4,4 0 -oxybis(benzene)disulfide) (COBDS) to fabricate the poly(arylene disulfide)/graphite nanocomposites. The structure, electrical properties, and mechanical properties of the nanocomposites were investigated and discussed.
2. Experimental 2.1. Materials The graphite used in this study was natural flake graphite supplied by Shandong Pingdu Jiaodong Graphite Company (People’s Republic of China). Concentrated sulfuric acid and hydrogen peroxide (CP) were purchased from commercial suppliers and used as received. Cyclo(4,4 0 -oxybis (benzene)disulfide) (COBDS) (MnZ463 g/mol, M wZ 746 g/mol, dimmer oligomer is the predominant component in the macrocyclics) was synthesized according to previous work [12]. 2.2. Preparation of graphite nanosheet Expandable graphite was prepared with H2O2–H2SO4 system, according to the method reported in the literature [14]. A mixture of concentrated sulfuric acid and hydrogen peroxide (1:0.08, v/v) was mixed with graphite flakes (80 mesh). The mixture was stirred at ambient temperature for about 1 h to afford the H2SO4 intercalated graphite. The graphite was carefully washed and filtrated with deionized water until the pH value of the filtrate reached 6, and then dried at 100 8C for 24 h. The dried expandable graphite was then irradiated in a domestic microwave oven (WP750, GALANZ) for 10 s to obtain exfoliated graphite (or called graphite worm). The graphite worm was dispersed in a 50 wt% alcohol solution and sonicated for 10 h, and then, filtrated and washed with enough distilled water. The obtained graphite powders named graphite nanosheets were dried under vacuum at ambient temperature for 24 h. 2.3. Preparation of the nanocomposite via in situ polymerization Poly(4,4 0 -oxybis(benzene)disulfide) (POBDS) /graphite nanocomposites were prepared as follows: certain amount of graphite nanosheet powders and COBDS were introduced into methylene dichloride solvent followed by keeping the mixture at 50 8C and being sonicated for more than 5 h. Most of the methylene dichloride was removed during the sonicating process. The resulting COBDS/graphite nanosheet mixtures were dried at 50 8C under vacuum for 24 h, and then were melt-molded at 200 8C for 30 min under a pressure of 20 MPa in N2. The obtained nanocomposite plates with dimensions of 30!30!1 mm3 were finally obtained for further characterization. 2.4. Characterization and measurements Wide-angel X-ray diffraction patterns (WAXD) were recorded using a Rigaku RINT X-ray diffractometer (model DMAX 1200) using Ni-filtered Cu Ka radiation. Data were collected at a scanning rate of 58/min in the range of 2qZ2–
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608. Field emission scanning electron microscope (FE– SEM) (Hitachi, Japan, JEOL JSM-6330F) was used to observe the morphologies of the samples. Prior to the examination, the specimens were coated with a very thin layer of gold. Transmission electron microscope (TEM) (JEOL100CX-II model) was run at 100 kV accelerated voltage to observe the morphologies. Thin nanocomposite film (w50 nm thick) was sliced up using IB-V ultrotometer apparatus, and the observations were carried out after retrieving the slices onto Cu grids. By using an universal testing machine (model CMT-4104, SANS) at a speed of 1 mm/min, the flexural properties were determined using a three-point bending load onto a rectangular specimen (30! 3!1 mm3) at 25 8C (relative humidity 50%). Five specimens of each composition were tested and the average values were reported.
3. Results and discussion 3.1. Preparation of graphite nanosheets To avoid the evolution of the poisonous NOX, expandable graphite has been prepared via H2O2–H2SO4 route instead of the usual HNO3–H2SO4 route [14]. Graphite exists in a form of carbon with the carbon atoms bonded together in layers through weak van der Waals forces between the layers. The weak interplanar forces allow intercalation of additional molecules that occupy spaces between the carbon layers. In the presence of an oxidizer, for instance, H2O2 in this work, an intercalating agent or concentrated sulfuric acid can be intercalated into the graphite layers to form expandable graphite. The XRD pattern of the H2SO4 intercalated graphite is shown as trace b in Fig. 1. Comparing with the natural graphite (trace a in Fig. 1), it is evident that the basal peak (0.335 nm) of natural graphite disappeared, indicating that the graphite was expanded completely. The diffraction peaks in trace b of Fig. 1 are (5.58, 001), (10.158, 002), (25.268, 005), (30.318, 006), (51.788, 0010), (57.228, 0011) in turn. All the diffraction peaks are indexed using the c-axis repeat distance Ic of 1.76 nm. The identity period in the c direction is one H2SO4 occupied layer (0.755 nm)Cthree unoccupied layers (3!0.335 nm)Z1.760 nm. These data strongly demonstrate the formation of the stage-4 intercalated graphite with a stacking sequence of /G–H2SO4–G–G– G–G–H2SO4–G–G–G/ (G: graphite layer). When being heated at a certain temperature, the decomposition of the intercalating H2SO4 led to the exfoliation of the expandable graphite. A sudden and dramatically increase in the dimension perpendicular to the carbon layers of the intercalated graphite was resulted, forming loose and porous vermicular or wormlike materials, whose structure is basically layers though collapsed and deformed desultorily as shown in Fig. 2. The corresponding XRD pattern of the obtained exfoliated graphite is shown in
Fig. 1. X-ray diffraction patterns: (a) original graphite; (b) H2SO4 intercalated graphite; and (c) expanded graphite.
Fig. 1c. The diffraction peak appears the same as the natural graphite indicating that the exfoliated graphite was composed of basic elements of a graphite carbon layer of the original graphite [10]. Ultrasonic irradiation has been widely applied in chemical reaction and engineering. When an ultrasonic wave passes through a liquid medium, the effect of ultrasonic cavitation will take place and generate a very strong stirring environment. Therefore, ultrasound has been extensively used in dispersion, emulsifying, crushing, and activation of particles [15]. In this respect, we adopted ultrasonic irradiation technique to break down the graphite worm and then get graphite nanosheets. Fig. 3 shows the SEM micrograph of the as-prepared graphite nanosheets. It can be seen clearly that the graphite worm was torn into
Fig. 2. SEM micrograph of graphite worm.
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Scheme 1. Ring-opening polymerization for macrocyclo(4,4 0 -oxybis(benzene)disulfide) oligomers.
Fig. 3. SEM micrograph of graphite nanosheets.
sheets with thickness in nanoscale. This can also been seen in the TEM micrograph shown in the following part. The sonicated graphite nanosheets appeared as very fine powders that were quite different with un-sonicated one. The result is similar to the reports of other researchers [10, 11,16]. 3.2. Morphological observation As shown in Fig. 4, when mixed with the graphite nanosheets in solution followed by removing the solvent under ultrasonic irradiation, the macrocyclic oligomers were dispersed in and between the graphite nanosheets. Upon heating at 200 8C, an instant ring-opening polymerization of the macrocyclic oligomers occurred as indicated in Scheme 1 [12], and then the graphite nanosheets were fixed and dispersed among the resulted linear polymer molecules. Fig. 5 shows the TEM photograph of the resulting POBDS/
Fig. 4. Schematic illustration of the formation of poly(arylene disulfide)/graphite nanocomposites via in situ ring-opening polymerization of macrocyclic oligomers.
graphite nanocomposite. The gray lines represent the cross section of the graphite layers. It can be seen that the nanoscale patterns of strips of the graphite layers well dispersed in POBDS matrix. Graphite nanosheets as observed had thickness of 10–40 nm, and their length can be more than 500 nm, indicating their large aspect ratio (width-tothickness). The large aspect ratio responded for the improvement in mechanical properties. It is apparent that, in some part of the nanocomposites as indicated in the leftdown corner of Fig. 5, the network consisted of the graphite nanosheets was observed. The conductive network was believed to lead to the greatly improved electrical conductivity for the nanocomposite. Such nanoscale dispersion endows the advantages in formation of the electrical conductive network in the polymer matrix at a very low filler loading. 3.3. Mechanical properties of the nanocomposites The variation of flexural modulus and the flexural strength of the POBDS/graphite nanocomposites versus graphite nanosheet content are shown in Fig. 6. The flexural modulus increases with the increase in graphite content, which is the typical characteristic for inorganic filled composites and nanocomposites. The flexural modulus of the nanocomposite with 10 wt% graphite is 2.32 GPa that is
Fig. 5. TEM micrograph of POBDS/graphite nanocomposites.
X.S. Du et al. / Polymer 45 (2004) 6713–6718
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Fig. 6. Flexural strength and flexural modulus of the POBDS/graphite nanocomosites versus graphite nanosheet content.
68% greater than that of neat POBDS (1.38 GPa). On the other hand, despite of the large aspect ratio of graphite nanosheets, the flexural strength showed very little change with the increase of the graphite content. This is due to the poor mechanical strength of graphite layers. The mechanical strength appeared high when compared with neat POBDS or other macro composites because of the nanoscale dispersion of graphite layers. [7] No tensile strength was reported due to that these nanocomposite were expected to be used as the bipolar plate material for fuel cell. Flexural strength is the main required mechanical property. 3.4. Electrical conductivity of the nanocomposites Fig. 7 shows the volume electrical conductivity of POBDS/graphite nanocomposites versus the graphite nanosheet content. POBDS is electrically nonconductive and has a volume conductivity about 10K16 S/cm in dry state at room temperature. It can be seen that the electrical conductivity of the POBDS/graphite nanocomposite increases with increasing graphite nanosheet content. The incorporation of very small amount of graphite nanosheets
into the POBDS matrix had dramatically increased the conductivity of poly(arylene disulfide) with a sharp transition from an electrical insulator to an electrical semiconductor. After the graphite nanosheet content reached 4 wt%, the electrical conductivity of the nanocomposites tended to level off with further increasing graphite nanosheet content. The similar results were reported on the conductivity of polystyrene/graphite nanocomposites versus the graphite content. This demonstrates the very low percolation threshold value for a conducting nanocomposite prepared with graphite nanosheet, which is much smaller than that of conventional conducting macro composites [11]. The augmentation of the electrical conductivity can be ascribed to the nanoscale dispersion of graphite nanosheets within the polymer matrix and the formation of conducting networks at very low filler loading. With the very low inorganic filler content, the resulted nanocomposites can remain the superior inherent mechanical strength of polymeric matrix. The nanocomposites exhibit as both high performance polymeric material and electrically conductive material. Therefore, they show potential applications in high temperature conducting adhesive, antistatic coatings, electromagnetic shielding, and the bipolar plates of polymer electrolyte membrane fuel cell (PEMFC).
4. Conclusions
Fig. 7. Electrical conductivity of POBDS/graphite nanocomposites versus graphite nanosheet content.
Graphite nanosheet can be prepared via a H2O2–H2SO4 oxidation intercalation of graphite method in solution, followed by exfoliation under microwave and ultrasonic irradiation. Macrocyclic oligomers can then be intercalated into the graphite nanosheets. Poly(arylene disulfide)/ graphite nanocomposites can be fabricated by direct melt ring-opening polymerization from the macrocyclic oligomer/graphite nanosheet precursor. The graphite nanosheets were dispersed as well-exfoliated state within the as-made nanocomposite as evidenced by TEM observation. The synthesized nanocomposites can reserve the superior
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inherent mechanical strength of polymeric matrix together with highly electrical conductivity. The nanocomposite containing 5 wt% graphite nanosheet had a electrical conductivity of about 10K3 S/cm, therefore, it can find many applications as conductive polymeric materials.
Acknowledgements We thank the National High Technology Research and Development 863 Program (Grant No.: 2003AA302410), the Natural Science Foundation of China (Project Grant No. 50203016), China Postdoctoral Science Foundation, the Guangdong Natural Science Foundation of China (Team Project Grant No. 015007) and the Guangdong Province Sci and Tech Bureau (Key Strategic Project Grant No. A1100402) for financial support of this work.
References [1] Zheng W, Wong SC, Sue HJ. Polymer 2002;43:6767.
[2] Skotheim TA, Elsenbaumer RL, Reynolds JR. Handbook of conducting polymers. New York: Marcel Dekker; 1998. [3] Xiao M, Luyi S, Jinging L, Yun L, Gong KC. Polymer 2002;43(8): 2245. [4] Shioyama H, Tatsumi K, Iwashita N. Synth Met 1998;96:229. [5] Celzard A, Mareche JF, Furdin G, Puricelli S. J Phys D: Appl Phys 2000;33:3094. [6] Chen GH, Wu DJ, Weng WG, He B, Yan WL. Polym Int 2001;50: 980. [7] Xiao P, Xiao M, Gong KC. Polymer 2001;42:4813. [8] Xiao M, Feng B, Gong KC. Energy Conversion Manage 2002;43:103. [9] Xiao M, Feng B, Gong KC. Solar Energy Mater Solar Cell 2001;69: 293. [10] Chen GH, Wu DJ, Weng WG, He B, Wu CL. Carbon 2003;41:579. [11] Chen GH, Wu CL, Weng WG, Wu DJ, Yan WL. Polymer 2003;44: 1781. [12] Meng YZ, Tjong SC, Hay AS. Polymer 2001;42:5215. [13] Meng YZ, Hay AS. J Appl Polym Sci 1999;74:3069. [14] Yang DX, Kang FY, Zheng YP. Carbon Tech 2000;107:6. [15] Xia HS, Wang Q. Chem Mater 2002;14:2158. [16] Fukuda K, Kikuya K, Isono K, Yoshio M. J Power Source 1997;69: 165.
Synthesis and properties of poly(4,4 0-oxybis(benzene)disulfide)/graphite nanocomposites via in situ ring-opening polymerization of macrocyclic oligomers X.S. Dub, M. Xiaoa, Y.Z. Menga,b,*, A.S. Hayc a
Institute of Energy and Environmental Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, People’s Republic of China b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c Department of Chemistry, McGill University, 801 Sherbrooke W., Montreal QC, H3A 2K6, Canada Received 4 February 2004; received in revised form 7 June 2004; accepted 15 July 2004 Available online 30 July 2004
Abstract An effective method for the preparation of poly(4,4 0 -oxybis(benzene)disulfide)/graphite nanosheet composites via in situ ring-opening polymerization of macrocyclic oligomers were reported. Completely exfoliated graphite nanosheets were prepared under the microwave irradiation followed by sonication in solution. The nanocomposites were fabricated via in situ melt ring-opening polymerization of macrocyclic oligomers in the presence of graphite nanosheets. The graphite nanosheets and resulted poly(arylene disulfide)/graphite nanocomposites were characterized with field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), tensile tester and electrical conductivity measurements. Compared with pure polymer, the electrical conductivity of the poly(arylene disulfide)/graphite nanocomposites were dramatically increased and had a value of about 10K3 S/cm for the nanocomposite containing 5 wt% graphite. The nanocomposites exhibit as both high performance polymeric material and electrically conductive material. Therefore, they show potential applications as high temperature conducting materials. q 2004 Elsevier Ltd. All rights reserved. Keywords: Conducting polymer; Macrocyclics; Graphite
1. Introduction Polymer composites and nanocomposites with reasonable electrical conductivity are the highlight in the field of functional materials. The conducting polymers and polymeric composites have attracted considerable attention in recent years because of their potential applications in advanced technologies, for example, in antistatic coatings, electromagnetic shielding and in secondary batteries [1,2]. The introduction of electrically conductive fillers such as graphite, carbon black, metal and metal oxide powders into
* Corresponding author. Address: Institute of Energy and Environmental Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, People’s Republic of China. Tel.: C 86-208-523-1343; fax: C86-208-411-4113 E-mail address: [email protected] (Y.Z. Meng). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.07.026
the polymeric matrix is a promising approach to fabricate electrically conductive polymeric materials [2]. Generally, the loading level of such fillers within a conductive polymeric composite is as high as 15 wt% to afford a satisfactory conductivity. However, such highly loading level, can generally lead to the poor mechanical properties and high density of the materials. The recent advancement of nano-scale compounding technique provides an innovative route to prepare highly electrically conductive polymeric nanocomposites with very low loading of conductive fillers. Graphite flakes have a layered structure like layered silicates, such as well-known montmorillonite and vermiculite. Moreover, it is naturally abundant, and therefore, is highly conductive filler (with an electrical conductivity of about 104 S/cm at ambient temperature) for making conducting polymeric composites. It is expected that both
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mechanical properties and electrical conductivity can be improved in case a polymer can be intercalated into the interlayer space of graphite. However, because of the small ˚ ) of the graphite layers and the lack of inter spacings (3.35 A the affinity for either hydrophilic or hydrophobic polymers, it is rather difficult to prepare the conductive graphite/polymer nanocomposites via direct intercalation method [3]. To achieve nanoscale dispersion of graphite in polymer matrix, certain chemical or physical modification to the graphite is generally needed [3,4]. Exfoliated graphite is composed of a large number of delaminated graphite sheets and its electrical conductivity is not obviously affected compared with the original graphite flake [5]. In this sense, monomers and polymers can be intercalated into the pores and galleries of the exfoliated graphite to produce conductive nanocomposite. Using the similar method, polymer/ graphite composites or nanocomposites with low percolation threshold of conductivity and highly thermal storage properties have been disclosed [1,6–11]. For these composites, however, the monomers and polymers might not be able to diffuse into the closed cavities inside the exfoliated graphite. Therefore, the graphite sheets around the cavities tend to overlap each other and form accumulation during processing. The aggregates of graphite sheets may result in the poor mechanical properties of the composites [10,11]. More recently, graphite nanosheets were used to prepare polystyrene/graphite nanocomposite and PMMA/graphite composites via in situ polymerization [10,11]. Poly(arylene disulfide)s were reported to have high resistance to environmental degradation, low watervapor transmission, excellent resistance to acids and bases, and good adhesion properties [12,13]. They might therefore find potential applications in many fields. However, most poly(arylene disulfide)s are insoluble in common solvents, and have very high melting points. These disadvantages have limited its practical application. In previous work, poly(arylene disulfide)s have been synthesized from macrocyclic(arylene disulfide) oligomers via melt ring-opening polymerization [12,13]. These macrocyclic(arylene disulfide) oligomers can instantly polymerize in the melt in the absence of any catalyst or initiator at a temperature as low as 150 8C, without liberation of any volatile by-products [12]. Therefore, they can be processed more easily compared with the corresponding linear polymers with high molecular weight and melt viscosities [12]. These unique properties endow them as a very good matrix candidate for nanocomposites. In this study, we used the macrocyclic oligomers, i.e. cyclo(4,4 0 -oxybis(benzene)disulfide) (COBDS) to fabricate the poly(arylene disulfide)/graphite nanocomposites. The structure, electrical properties, and mechanical properties of the nanocomposites were investigated and discussed.
2. Experimental 2.1. Materials The graphite used in this study was natural flake graphite supplied by Shandong Pingdu Jiaodong Graphite Company (People’s Republic of China). Concentrated sulfuric acid and hydrogen peroxide (CP) were purchased from commercial suppliers and used as received. Cyclo(4,4 0 -oxybis (benzene)disulfide) (COBDS) (MnZ463 g/mol, M wZ 746 g/mol, dimmer oligomer is the predominant component in the macrocyclics) was synthesized according to previous work [12]. 2.2. Preparation of graphite nanosheet Expandable graphite was prepared with H2O2–H2SO4 system, according to the method reported in the literature [14]. A mixture of concentrated sulfuric acid and hydrogen peroxide (1:0.08, v/v) was mixed with graphite flakes (80 mesh). The mixture was stirred at ambient temperature for about 1 h to afford the H2SO4 intercalated graphite. The graphite was carefully washed and filtrated with deionized water until the pH value of the filtrate reached 6, and then dried at 100 8C for 24 h. The dried expandable graphite was then irradiated in a domestic microwave oven (WP750, GALANZ) for 10 s to obtain exfoliated graphite (or called graphite worm). The graphite worm was dispersed in a 50 wt% alcohol solution and sonicated for 10 h, and then, filtrated and washed with enough distilled water. The obtained graphite powders named graphite nanosheets were dried under vacuum at ambient temperature for 24 h. 2.3. Preparation of the nanocomposite via in situ polymerization Poly(4,4 0 -oxybis(benzene)disulfide) (POBDS) /graphite nanocomposites were prepared as follows: certain amount of graphite nanosheet powders and COBDS were introduced into methylene dichloride solvent followed by keeping the mixture at 50 8C and being sonicated for more than 5 h. Most of the methylene dichloride was removed during the sonicating process. The resulting COBDS/graphite nanosheet mixtures were dried at 50 8C under vacuum for 24 h, and then were melt-molded at 200 8C for 30 min under a pressure of 20 MPa in N2. The obtained nanocomposite plates with dimensions of 30!30!1 mm3 were finally obtained for further characterization. 2.4. Characterization and measurements Wide-angel X-ray diffraction patterns (WAXD) were recorded using a Rigaku RINT X-ray diffractometer (model DMAX 1200) using Ni-filtered Cu Ka radiation. Data were collected at a scanning rate of 58/min in the range of 2qZ2–
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608. Field emission scanning electron microscope (FE– SEM) (Hitachi, Japan, JEOL JSM-6330F) was used to observe the morphologies of the samples. Prior to the examination, the specimens were coated with a very thin layer of gold. Transmission electron microscope (TEM) (JEOL100CX-II model) was run at 100 kV accelerated voltage to observe the morphologies. Thin nanocomposite film (w50 nm thick) was sliced up using IB-V ultrotometer apparatus, and the observations were carried out after retrieving the slices onto Cu grids. By using an universal testing machine (model CMT-4104, SANS) at a speed of 1 mm/min, the flexural properties were determined using a three-point bending load onto a rectangular specimen (30! 3!1 mm3) at 25 8C (relative humidity 50%). Five specimens of each composition were tested and the average values were reported.
3. Results and discussion 3.1. Preparation of graphite nanosheets To avoid the evolution of the poisonous NOX, expandable graphite has been prepared via H2O2–H2SO4 route instead of the usual HNO3–H2SO4 route [14]. Graphite exists in a form of carbon with the carbon atoms bonded together in layers through weak van der Waals forces between the layers. The weak interplanar forces allow intercalation of additional molecules that occupy spaces between the carbon layers. In the presence of an oxidizer, for instance, H2O2 in this work, an intercalating agent or concentrated sulfuric acid can be intercalated into the graphite layers to form expandable graphite. The XRD pattern of the H2SO4 intercalated graphite is shown as trace b in Fig. 1. Comparing with the natural graphite (trace a in Fig. 1), it is evident that the basal peak (0.335 nm) of natural graphite disappeared, indicating that the graphite was expanded completely. The diffraction peaks in trace b of Fig. 1 are (5.58, 001), (10.158, 002), (25.268, 005), (30.318, 006), (51.788, 0010), (57.228, 0011) in turn. All the diffraction peaks are indexed using the c-axis repeat distance Ic of 1.76 nm. The identity period in the c direction is one H2SO4 occupied layer (0.755 nm)Cthree unoccupied layers (3!0.335 nm)Z1.760 nm. These data strongly demonstrate the formation of the stage-4 intercalated graphite with a stacking sequence of /G–H2SO4–G–G– G–G–H2SO4–G–G–G/ (G: graphite layer). When being heated at a certain temperature, the decomposition of the intercalating H2SO4 led to the exfoliation of the expandable graphite. A sudden and dramatically increase in the dimension perpendicular to the carbon layers of the intercalated graphite was resulted, forming loose and porous vermicular or wormlike materials, whose structure is basically layers though collapsed and deformed desultorily as shown in Fig. 2. The corresponding XRD pattern of the obtained exfoliated graphite is shown in
Fig. 1. X-ray diffraction patterns: (a) original graphite; (b) H2SO4 intercalated graphite; and (c) expanded graphite.
Fig. 1c. The diffraction peak appears the same as the natural graphite indicating that the exfoliated graphite was composed of basic elements of a graphite carbon layer of the original graphite [10]. Ultrasonic irradiation has been widely applied in chemical reaction and engineering. When an ultrasonic wave passes through a liquid medium, the effect of ultrasonic cavitation will take place and generate a very strong stirring environment. Therefore, ultrasound has been extensively used in dispersion, emulsifying, crushing, and activation of particles [15]. In this respect, we adopted ultrasonic irradiation technique to break down the graphite worm and then get graphite nanosheets. Fig. 3 shows the SEM micrograph of the as-prepared graphite nanosheets. It can be seen clearly that the graphite worm was torn into
Fig. 2. SEM micrograph of graphite worm.
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Scheme 1. Ring-opening polymerization for macrocyclo(4,4 0 -oxybis(benzene)disulfide) oligomers.
Fig. 3. SEM micrograph of graphite nanosheets.
sheets with thickness in nanoscale. This can also been seen in the TEM micrograph shown in the following part. The sonicated graphite nanosheets appeared as very fine powders that were quite different with un-sonicated one. The result is similar to the reports of other researchers [10, 11,16]. 3.2. Morphological observation As shown in Fig. 4, when mixed with the graphite nanosheets in solution followed by removing the solvent under ultrasonic irradiation, the macrocyclic oligomers were dispersed in and between the graphite nanosheets. Upon heating at 200 8C, an instant ring-opening polymerization of the macrocyclic oligomers occurred as indicated in Scheme 1 [12], and then the graphite nanosheets were fixed and dispersed among the resulted linear polymer molecules. Fig. 5 shows the TEM photograph of the resulting POBDS/
Fig. 4. Schematic illustration of the formation of poly(arylene disulfide)/graphite nanocomposites via in situ ring-opening polymerization of macrocyclic oligomers.
graphite nanocomposite. The gray lines represent the cross section of the graphite layers. It can be seen that the nanoscale patterns of strips of the graphite layers well dispersed in POBDS matrix. Graphite nanosheets as observed had thickness of 10–40 nm, and their length can be more than 500 nm, indicating their large aspect ratio (width-tothickness). The large aspect ratio responded for the improvement in mechanical properties. It is apparent that, in some part of the nanocomposites as indicated in the leftdown corner of Fig. 5, the network consisted of the graphite nanosheets was observed. The conductive network was believed to lead to the greatly improved electrical conductivity for the nanocomposite. Such nanoscale dispersion endows the advantages in formation of the electrical conductive network in the polymer matrix at a very low filler loading. 3.3. Mechanical properties of the nanocomposites The variation of flexural modulus and the flexural strength of the POBDS/graphite nanocomposites versus graphite nanosheet content are shown in Fig. 6. The flexural modulus increases with the increase in graphite content, which is the typical characteristic for inorganic filled composites and nanocomposites. The flexural modulus of the nanocomposite with 10 wt% graphite is 2.32 GPa that is
Fig. 5. TEM micrograph of POBDS/graphite nanocomposites.
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Fig. 6. Flexural strength and flexural modulus of the POBDS/graphite nanocomosites versus graphite nanosheet content.
68% greater than that of neat POBDS (1.38 GPa). On the other hand, despite of the large aspect ratio of graphite nanosheets, the flexural strength showed very little change with the increase of the graphite content. This is due to the poor mechanical strength of graphite layers. The mechanical strength appeared high when compared with neat POBDS or other macro composites because of the nanoscale dispersion of graphite layers. [7] No tensile strength was reported due to that these nanocomposite were expected to be used as the bipolar plate material for fuel cell. Flexural strength is the main required mechanical property. 3.4. Electrical conductivity of the nanocomposites Fig. 7 shows the volume electrical conductivity of POBDS/graphite nanocomposites versus the graphite nanosheet content. POBDS is electrically nonconductive and has a volume conductivity about 10K16 S/cm in dry state at room temperature. It can be seen that the electrical conductivity of the POBDS/graphite nanocomposite increases with increasing graphite nanosheet content. The incorporation of very small amount of graphite nanosheets
into the POBDS matrix had dramatically increased the conductivity of poly(arylene disulfide) with a sharp transition from an electrical insulator to an electrical semiconductor. After the graphite nanosheet content reached 4 wt%, the electrical conductivity of the nanocomposites tended to level off with further increasing graphite nanosheet content. The similar results were reported on the conductivity of polystyrene/graphite nanocomposites versus the graphite content. This demonstrates the very low percolation threshold value for a conducting nanocomposite prepared with graphite nanosheet, which is much smaller than that of conventional conducting macro composites [11]. The augmentation of the electrical conductivity can be ascribed to the nanoscale dispersion of graphite nanosheets within the polymer matrix and the formation of conducting networks at very low filler loading. With the very low inorganic filler content, the resulted nanocomposites can remain the superior inherent mechanical strength of polymeric matrix. The nanocomposites exhibit as both high performance polymeric material and electrically conductive material. Therefore, they show potential applications in high temperature conducting adhesive, antistatic coatings, electromagnetic shielding, and the bipolar plates of polymer electrolyte membrane fuel cell (PEMFC).
4. Conclusions
Fig. 7. Electrical conductivity of POBDS/graphite nanocomposites versus graphite nanosheet content.
Graphite nanosheet can be prepared via a H2O2–H2SO4 oxidation intercalation of graphite method in solution, followed by exfoliation under microwave and ultrasonic irradiation. Macrocyclic oligomers can then be intercalated into the graphite nanosheets. Poly(arylene disulfide)/ graphite nanocomposites can be fabricated by direct melt ring-opening polymerization from the macrocyclic oligomer/graphite nanosheet precursor. The graphite nanosheets were dispersed as well-exfoliated state within the as-made nanocomposite as evidenced by TEM observation. The synthesized nanocomposites can reserve the superior
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inherent mechanical strength of polymeric matrix together with highly electrical conductivity. The nanocomposite containing 5 wt% graphite nanosheet had a electrical conductivity of about 10K3 S/cm, therefore, it can find many applications as conductive polymeric materials.
Acknowledgements We thank the National High Technology Research and Development 863 Program (Grant No.: 2003AA302410), the Natural Science Foundation of China (Project Grant No. 50203016), China Postdoctoral Science Foundation, the Guangdong Natural Science Foundation of China (Team Project Grant No. 015007) and the Guangdong Province Sci and Tech Bureau (Key Strategic Project Grant No. A1100402) for financial support of this work.
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