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Enhanced mechanical and electrical properties of polyimide film by graphene sheets via in situ polymerization
Polymer 52 (2011) 5237e5242
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Enhanced mechanical and electrical properties of polyimide film by graphene sheets via in situ polymerization Nguyen Dang Luonga, Ulla Hippia, Juuso T. Korhonenb, Antti J. Soininenb, Janne Ruokolainenb, Leena-Sisko Johanssonc, Jae-Do Namd, Le Hoang Sinhd, Jukka Seppäläa, * a
Polymer Technology, Department of Biotechnology and Chemical Technology, Aalto University, School of Chemical Technology, P.O. Box 16100, 00076 Aalto, Finland Molecular Materials, Department of Applied Physics, Aalto University, School of Science, P.O. Box 15100, 00076 Aalto, Finland Forest Products Surface Chemistry Group, Department of Forest Products Technology, Aalto University, School of Chemical Technology, P.O. Box 16300, 00076 Aalto, Finland d Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon 440-746, South Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2011 Received in revised form 9 September 2011 Accepted 20 September 2011 Available online 24 September 2011
In this study, polyimide/graphene nanocomposite films which exhibited significant enhancements in mechanical properties and electrical conductivity were successfully fabricated. Graphene oxide (GO) synthesized by Hummer’s method was chemically modified with ethyl isocyanate to give ethyl isocyanatetreated graphene oxide (iGO), which is readily dispersed in N,N0 -dimethylformamide (DMF). The iGO dispersion in DMF was then used as media for synthesis of polyimide/functionalized graphene composites (PI/FGS) by an in situ polymerization approach. It was shown that addition of only 0.38 wt% of FGS, Young’s modulus of the PI/FGS composite film was dramatically increased from 1.8 GPa to 2.3 GPa, which is approximately 30% of improvement compared to that of pure PI film, and the corresponding tensile strength was increased from 122 MPa to 131 MPa. In addition, the electrical conductivity of the PI/FGS with this graphene content was increased by more than eight orders of magnitude to 1.7 105 S m1. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Graphene Polyimide Polymer nanocomposite
1. Introduction Polyimides (PI) are a class of high-performance polymer owing to various excellent properties, such as high mechanical strength and modulus, high thermal resistance, and good chemical resistance [1e3]. Thus, PIs have been widely used in many applications including aerospace, optics, and microelectronics. However, pure PIs have some limitations such as their insulating nature that causes electrostatic accumulation on the surface of the material [4,5]. As a result, this phenomenon causes local heating and premature degradation of the materials. Besides, with the increasing development of aircraft industries, the properties of PI films should be enhanced to meet the requirements. In this sense, carbon nanotubes (CNTs) have been widely used as effective fillers to enhance electrical conductivity [4e6] and mechanical properties of PIs [7,8]. It was reported that addition of CNTs to PI reduces the surface resistivity and consequently mitigates the build-up electrostatic charges of the PI film surface [4,6]. A significant increase of 40% in tensile strength was demonstrated for the PI/CNTs
* Corresponding author. Tel.: þ358 9 470 22614; fax: þ358 9 451 2622. E-mail address: jukka.seppala@aalto.fi (J. Seppälä). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.09.033
containing 4 wt% of CNTs [4]. However, in this study the corresponding Young’s modulus was not shown. Although various studies have indicated that CNTs are good reinforcements for many polymers including PIs, there are many unresolved problems related to the CNTs materials such as high production cost, chirality related electrical conductivity, and distinct difficulty in dispersion [9,10]. Consequently, the incorporation of CNTs increased the electrical conductivity of the PI/CNTs composite, but it had negligible effects on tensile properties [6]. In another study, only 6% increase in Young’s modulus was observed as the maximum value with addition of CNTs to the PI while the tensile strength was not reported [4]. In contrast, recently discovered graphene-derived materials offer an ideal replacement for CNTs because of their easy production in large scale, easy dispersion in both polar and non-polar matrixes, as well as their excellent electrical properties [11], unique mechanical strength [12], interesting optical contrast [13], and high thermal properties [14]. Therefore, researchers have shown an increasingly growing interest in graphene/polymer nanocomposites. For example, it has been reported that a polystyrene/graphene composite exhibited an electrical percolation threshold at 0.1 vol% graphene and a conductivity of 0.1 S m1 at 1 vol.% graphene, which is sufficient for many electrical applications
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[15]. In another study, an increase in glass transition temperature of around 40 C was observed in polyacrylonitrile/graphene composite with an incorporation of 1 wt% of graphene content. Similarly, upon addition of only 0.05 wt% of graphene content in poly(methyl methacrylate) (PMMA), Tg of the obtained PMMA/ graphene composite increased significantly by 30 C [16]. Very recently, we prepared graphene/cellulose nanocomposite papers that exhibited a significant improvement in tensile strength from 200 to 232 MPa and 273 MPa with addition of 0.3 wt% and 5 wt% of graphene, respectively [17]. Besides, the developed graphene composite paper showed a percolation threshold in electrical conductivity as low as 0.3 wt%, and at this graphene loading the conductivity was measured to be 4.79 104 S m1, which is well above the antistatic value of 106 S m1 for thin films [15]. Accordingly, graphene sheets obviously can serve as reinforcing filler to increase PI film properties including mechanical properties and electrical conductivity. However, so far, too little attention has been paid to polyimide/graphene composites. Recently, it has been found that incorporation of 2 wt% graphene into PI increases the tensile strength from 88 to 118 MPa and the Young’s modulus from 1.3 GPa to 1.7 GPa [18]. In the study, graphene oxide was used and dispersed in the poly(amic acid) (PAA) in DMF. However, it is well known that the graphene oxide prepared by Hummers method is not readily dispersed in organic solvent such as DMF [19], and thus they tend to agglomerate in DMF solvent during the polymerization, and the agglomeration possibly reduces the reinforcement ability of the graphene nanomaterial. To avoid the mentioned agglomeration of the graphene sheets, in this study, graphite oxide, which is insoluble in dipolar aprotic DMF solvent, was successfully converted to a soluble form through a chemical treatment with ethyl isocyanate. Then, we evaluated the effects of the incorporation of the functionalized graphene sheets (FGS) on electrical conductivity and mechanical properties of PI film. We found that the synthesized PI/FGS nanocomposite films showed enhanced tensile properties including tensile strength and Young’s modulus in comparison with pure PI film. For example, with an incorporation of only 0.38 wt% FGS, the Young’s modulus increased significantly from 1.8 GPa to 2.3 GPa, which is nearly 30% of improvement compared to pure PI film. In addition, the tensile strength increased from 122 MPa to 131 MPa. The developed PI/FGS composite films with significant enhancements in mechanical and electrical properties can find potential uses in microelectronics and aerospace industries.
(36 wt%) was added into the mixture to reduce the residual permanganate (MnO-4) and manganese dioxide (MnO2) to colorless soluble manganese sulfate (MnSO4), after which the color of the mixture changed to bright yellow. After that, the mixture was diluted in 2000 ml of HCl (10 wt%) and filtered. Subsequently, the mixture was washed several times with DI water to eliminate impurities and to achieve pH 7. Finally, the graphite oxide product was dried under vacuum at 60 C for 48 h.
2. Experimental
2.6. Characterizations
2.1. Materials
Individual sheet thicknesses were measured from drop-cast dispersion of GO and iGO in DMF on silicon substrates using a Dimension 5000 atomic force microscope (AFM, Veeco Inc.) in tapping mode with silicon probes with nominal resonant frequency of 325 kHz and nominal force constant of 46 N/m (Micromasch). X-ray diffraction of the samples was carried on a wide angle X-ray diffraction (Mac Science, Mac-18xhf) with Cu Ka radiation (l ¼ 0.154 nm) and data was collected in the 2q of 5-50 with a scanning speed of 5 min1. Tensile test of the film samples was studied using an Instron 4204 universal testing machine with a test speed of 5 mm min1. At least five specimens were used for each sample in the tensile test. Morphology of the samples was studied by a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500FA). To study the fracturing of the composites, the specimens were cryo-fractured under liquid nitrogen and for high resolution imaging; the exposed cross-sections were sputtered with a thin layer of gold/palladium (Emitech K100X) to promote conductivity before SEM observation. Chemical composition of the sample surfaces was analyzed using a high resolution X-ray
All chemicals including graphite flakes (particle size <150 mm), sulfuric acid (98%), chlohydric acid (36 wt%), hydrogen peroxide (36 wt%), potassium permanganate, sodium nitrate, ethyl isocyanate, pyromellitic dianhydride (PMDA), 4,40 -oxydianiline (ODA), N,N0 -dimethylformamide (DMF), and triethylamine (TEA) were purchased from SigmaeAldrich Co and used as received. 2.2. Synthesis of graphite oxide Graphite oxide was synthesized using a modified Hummers method [19]. Typically, 10 g of graphite flakes, 5 g of sodium nitrate, and 30 g of potassium permanganate were added to a 500-ml round-bottom flask containing 250 ml concentrated sulfuric acid, which cooled down by immersing in an ice bath for 1 h under stirring. The mixture was then slowly heated to 35 C for 2 h to oxidize the graphite. Next, 500 ml of deionized water was slowly added into the mixture. After 30 min, 30 ml of H2O2 solution
2.3. Isocyanate treatment of graphite oxide Graphite oxide (1 g) was added into a 100-ml round-bottom flask. Then, DMF (50 ml) was supplied under nitrogen with magnetic stirring. Subsequently, ethyl isocyanate (0.08 ml) was added and the mixture was kept at this condition overnight. After that, the reaction mixture was washed several times with methylene chloride to eliminate impurities and coagulate the product. Finally, the product was dried under vacuum at 60 C for 24 h [20]. 2.4. Preparation of GO and iGO dispersions Isocyanate-treated graphite oxide (0.5 g) was dispersed in DMF (500 ml) by ultrasonication for 1.5 h, followed by centrifugation at 4000 rpm for 20 min to remove the precipitate. The supernatant was denoted as iGO. In a similar procedure, graphite oxide/DMF was ultrasonicated and the supernatant was denoted as GO; in this case, however, most of GO (w 90 wt%) was precipitated, indicating that most GO is not well dispersed in DMF. 2.5. In situ synthesis of polyimide/graphene composite film Equivalent molar ratios of 2 g ODA and 2.2 g PMDA were dissolved in the iGO dispersion to make two solutions containing 0.38 and 0.75 wt% of iGO (compared to monomers). TEA catalyst (3 wt% to monomers) was added immediately and the reaction was conducted in an ice bath with mechanical stirring for 3 h. After that, viscous polymer solutions of PAA/iGO were obtained. The PAA/iGO solutions were then cast onto glass disks and dried at room temperature overnight. Next, the PAA composite films were heated at 100 C, 200 C and 250 C, each for 1 h under nitrogen, forming the composite films (PI/FGS) with thicknesses of around 30 mm. In a similar procedure, PAA/GO was synthesized using GO dispersion in DMF and neat PI film was fabricated as the reference.
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limit of 100 MU. A plasma etching (O2 1500 Sccm, power 500 W, time 15 min) was applied to the specimens before electrical conductivity measurement to remove thin polymer layers on the film surfaces. The plasma etching was used to increase the contact between the film surface and machine probes. 3. Results and discussion We first chemically modified GO with ethyl isocyanate and thereby the resulting iGO sheets can be readily dispersed in DMF solvent by sonication to form a stable dispersion. The iGO dispersion was subsequently used as the media for the synthesis of PAA. In the next step, the PAA/iGO solutions were cast on glass substrates and heat-treated at high temperature under nitrogen, forming the PI/FGS composite films with thickness of around 30 mm. For comparison, GO/PAA film with 0.1 wt% GO was prepared by
Scheme 1. In-situ polymerization pathway of polyimide/graphene composites films.
photoelectron spectroscopy (XPS, AXIS 165, Kratos Analytical) with monochromated Al Ka irradiation at 100 W. Electrical conductivity of the samples was measured by a four-point probe method from Jandel Engineering Ltd. connected to a Keithley 2400 source meter using a standard four-probe technique with a threshold detection
Fig. 1. Typical AFM images of GO sheets (a) and iGO (b) Sheets with height profiles taken along the lines. The samples were prepared by drop-casting dilute GO in water and iGO in DMF dispersions onto silicon wafer. XRD spectra of graphite and graphene oxide (c).
Fig. 2. (a) Photographs of stable dispersion of iGO in DMF, PAA/GO solution, and two PAA/iGO solutions containing 0.38 wt% and 0.75 wt% of iGO loadings; it is notice that all the pictures were taken after two days of preparation. (b) Photographs of PAA, PAA/ GO, and PAA/iGO composite films (top), and PI, PI/FGS composite film containing 0.38 wt% and 0.75 wt% of graphene loadings (bottom). All the films have thickness of around 30 mm.
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dispersion of GO in DMF. Regarding the GO dispersion in DMF, it was confirmed in this study and another study that GO synthesized by Hummer’s method is not well dispersed in DMF and thus only a very small content of GO is dispersed after sonication [20]. Scheme 1 demonstrates these steps to produce PI/FGS composite films in which PAA was formed from the reaction of two precursors, a diamine and a dianhydride. Briefly, at low temperature in a dipolar aprotic DMF solvent, nucleophilic attack of the amino group on the carboxyl carbon of the anhydride group led to the opening of the anhydride ring, generating an amic acid group. Water molecules were then eliminated during cycloimidization upon heat treatment of the PAA and the final polyimide was formed. AFM images of GO (Fig. 1a) and iGO (Fig. 1b) show clearly single graphene oxide and isocyante-treated graphene oxide sheets, which means that the iGO sheets were dispersed uniformly in DMF solvent. The thickness of the sheets was between 1 and 2 nm which is close to the data reported previously [15,16]. While the thickness of a flat graphene sheet is w0.34 nm [20], the higher thickness values of GO and iGO sheet are originated from the presence of the functional groups on the graphene plane that were introduced upon chemical oxidation/modification steps. Notably, the thickness of iGO sheets did not seem to differ measurably from GO sheets, indicating that the isocyanate layer must be very thin. Additionally, the XRD spectra of pristine graphite flakes and graphene oxide are shown in Fig. 1c. As seen clearly, a strong and sharp peak at 26.5 is regarded as the characteristic peak of graphite while graphene oxide shows only a very small peak at around 11. The disappearance of the characteristic peak at 26.5 is due to the presence of oxygen-containing groups, such as carboxyl, hydroxyl, and epoxide groups on the basal plane of graphene [21]. Fig. S1 (see Electronic Supplementary Material) shows energycorrected high resolution XPS spectra of the O 1s, N 1s, and C 1s regions for the GO and iGO samples (Fig. S1a) and full length XPS for the GO sample (Fig. S1b). It is clear that the iGO sample shows the peak for the N 1s and this is not the case for the GO sample (Fig. S1a and b), confirming a presence of nitrogen bonds in the iGO. It thus can be an evidence for the successful modification of graphene oxide by ethyl isocyanate. As seen in the photographs (Fig. 2) of the PAA/GO solution with 0.1 wt% of GO loading (Fig. 2a), after 2 days of storage, most GO sheets were aggregated and settled down at the vial bottom and these agglomerates of GO could be observed by naked eye from the PAA/GO film (Fig. 2b). On the contrary, in Fig. 2a, the PAA/iGO
solutions with 0.38 wt% and 0.75 wt% of iGO loadings were still clear, which means that the ethyl isocyanate treatment of GO was effective for its stabilization in DMF and PAA solution. These stable PAA/iGO solutions were used for the formation of PI/FGS nanocomposite films with uniform and shiny color surfaces (Fig. 2b), which get darker with the increased incorporation of graphene. We therefore assume that iGO sheets bound strongly to PAA chains, which prevented them from agglomerating. The successful synthesis of PAA and PI was distinctly confirmed by IR analysis (Fig. 3a and b). Fig. 3a shows characteristic peaks of PAA. The peaks from 2500 to 3500 cm1 indicate the presence of carboxylic acid and amine groups. The peaks at 1538 cm1 and 1648 cm1 can be assigned to CeNH stretching (amide II) and C]O in CONH (amide I) stretching bands. The peak at 1719 cm1 confirms the C]O in COOH groups in the PAA macromolecules. For the PI characteristic peaks in both pure PI and PI/FGS composites, the peaks related to the functional groups of PAA such as carboxylic acid and amine groups at 2500e3500 cm1 were disappeared. Two peaks at 1776 cm1 and 1719 cm1 are due to C]O asymmetric stretching vibration and C]O symmetric stretching vibration of the imide absorption band, respectively, with the former overlapping with the strong carboxylic acid band of the PAA molecules. Finally, new peaks at 1371 cm1 and 723 cm1 are explained by CeN stretching and C]O bending modes, respectively, of the imide groups through hydrative cycloimidization [22]. During the high temperature treatment in the cycloimidization step, the iGO could be partially thermally reduced to functionalized graphene sheets (FGS) and the resulting PI/FGS composite films may conduct electricity. Thus, electrical conductivity measurement was performed. The conductivity of neat polyimide was measured to be 1.2 1013 S m1and conductivities of the PI/FGS composite films were 1.7 105 and 8.9 105 S m1, which correspond to 0.38 wt% and 0.75 wt% of graphene loadings, respectively. These values are above the antistatic criterion of 106 S m1 [15], indicating the effective connection of high aspect ratio of partiallyreduced graphene sheets in the films promotes the electrical conductivity. Although the degree of thermal reduction of iGO in PI/ FGS composite films should be further investigated, it is notably to address that the conductivity of the PI/FGS composite films prepared in this study compared well with that of a reported PI/ SWCNT composite [23]. In addition, our PI/FGS composite films’ conductivities were even higher than that of polyimide reinforced with multi-walled CNT (PI/MWCNT) [7]. Therefore, we strongly believe that the good conductivity of the PI films reinforced by FGS
Fig. 3. FT-IR spectra of PAA, PI (a), and PI/FGS with 0.38 wt% and 0.75 wt% (b).
N.D. Luong et al. / Polymer 52 (2011) 5237e5242
Fig. 4. (a) Typical stress-strain curves of neat PI (1) and PI/FGS nanocomposite films containing 0.38 wt% (2) and 0.75 wt% of graphene loadings (3). (b) Summary of mechanical properties of the PI and PI/FGS composite films with 0.38 wt% and 0.75 wt% graphene loadings.
in this study could be attributed to the good particleeparticle connections of well-distributed FGS sheets, which may be partially reduced by the thermal treatment during cycloimidization, in PI matrix. Typical stress-strain curves of the pure PI and PI/FGS nanocomposite films are shown in Fig. 4a and the tensile properties are summarized in Fig. 4b. As expected, with the addition of only 0.38 wt% of FGS, the tensile strength was clearly improved from 122 to 131 and Young’s modulus was significantly improved from 1.8 MPa to 2.3 MPa, which is approximately 30% improvement compared to that of pure PI film. Meanwhile, the elongation at break decreased from 69% to 42%. When graphene content was further increased to 0.75 wt%, however, the tensile strength was gradually decreased in comparison with that of PI/FGS composite film containing 0.38 wt% of FGS. Nevertheless, this tensile strength
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of 127 MPa was still higher than that of the pure PI film. It should be further addressed that the Young modulus of the PI/FGS composite film continued to increase to 2.4 MPa at 0.75 wt% of graphene content, which is around 33% higher than the value of the pure PI film. Considering the mechanical enhancement of graphene to polyimide in this study, it is worthwhile to address that the efficiency of graphene is better than that of CNTs [4,6,8]. The effects of graphene on polymer properties were also demonstrated in case of cellulose/graphene nanocomposite paper exhibiting the enhanced mechanical properties and electrical conductivity that was not reported previously for any cellulose/ carbon-based materials nanocomposite papers [17]. The enhancement in both tensile strength and modulus of the PI/FGS composites could be attributed to the uniform dispersion of FGS and its anisotropic orientation in PI matrix, and thus allowing an efficient stress transfer from polymer matrix to the filler. Yet, another important factor concerning the reinforcement effect of the FGS filler is the good interfacial interactions between FGS sheets and PI macromolecules. To better understand the enhanced tensile properties of the PI/ FGS composites, we performed SEM study at the fractured surfaces of the PI and PI/FGS composite films. As can be seen in Fig. 5a, PI film shows a smooth fractured surface. In contrast, the PI/FGS composite films containing 0.38 wt% and 0.75 wt% of graphene possess rough fractured surfaces that could be attributed to the strong interfacial adhesion and good compatibility between the PI matrix and FGS sheets (Fig. 5b and c). Such strong interactions are favorable to the stress transfer from the polymer matrix to the graphene sheets, leading to the improvement in the tensile strength and modulus of the composite films compared to those of the pure PI films. XPS was further used to study the interaction between the filler and matrix, and the result was shown in Fig. 6. In the case of pure PI, high resolution carbon region was resolvable into four compounds, identified in ref [23,24] as carbon atoms without O or N neighbors (at 285.0 eV), carbons with either one O or N neighbor (286.1 eV), carbonyl carbon (three bonds to O, at 288.8 eV) and a shallow plasmon feature, due to aromatic structures at higher BE. In the case of FGS addition, the two carbon compounds with lowest BE merged into a broader, unresolvable compound. There are two plausible explanations for that phenomenon. Firstly, incorporation of FGS onto PI surface would broaden the signal due to slightly different major compound positions in FGS (see Fig. S1) with 284.6 eV for graphene, and 286.6 eV for hydroxylated carbon (as in cellulose, see ref [25]). In addition to incorporated graphene, chemical linkages between PI and GO might further broaden the
Fig. 5. SEM for PI (a) and PI/FGS composite films containing 0.38 wt% (b) and 0.75 wt% of graphene loadings (c).
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Fig. 6. N 1s, C1s, and O 1s XPS results for PI (a) and PI/FGS composite films with 0.38 wt% (b) and 0.75 wt% of graphene loadings (c).
XPS features slightly, which indeed seems to be the case for C, O and N regions. It is also noteworthy to mention that pure PI exhibits a sharp N 1s peak originated from the imide nitrogen [23]; however, in the PI/FGS composites, the N 1s peaks are broader. The broader N 1s peaks of the composites are possibly due to the present of another nitrogen species of FGS which was introduced by the modification of graphene oxide with ethyl isocyanate (Fig. S1), and again, confirming the successful modification of graphene oxide by ethyl isocyanate. 4. Conclusion In conclusion, we have demonstrated herein a facile approach for the preparation of PI films reinforced with functionalized graphene sheets which exhibited the enhancements in both mechanical properties and electrical conductivity with the addition of small graphene content. We strongly believe that the developed PI/FGS composite film can find uses in microelectronics and aircraft industries. Acknowledgments Dr. Joseph Campbell at Aalto University is acknowledged for the XPS measurements. Mr. Matti Lehtimäki is appreciated with XRD characterization. Appendix. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.polymer.2011.09.033.
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Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Enhanced mechanical and electrical properties of polyimide film by graphene sheets via in situ polymerization Nguyen Dang Luonga, Ulla Hippia, Juuso T. Korhonenb, Antti J. Soininenb, Janne Ruokolainenb, Leena-Sisko Johanssonc, Jae-Do Namd, Le Hoang Sinhd, Jukka Seppäläa, * a
Polymer Technology, Department of Biotechnology and Chemical Technology, Aalto University, School of Chemical Technology, P.O. Box 16100, 00076 Aalto, Finland Molecular Materials, Department of Applied Physics, Aalto University, School of Science, P.O. Box 15100, 00076 Aalto, Finland Forest Products Surface Chemistry Group, Department of Forest Products Technology, Aalto University, School of Chemical Technology, P.O. Box 16300, 00076 Aalto, Finland d Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon 440-746, South Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2011 Received in revised form 9 September 2011 Accepted 20 September 2011 Available online 24 September 2011
In this study, polyimide/graphene nanocomposite films which exhibited significant enhancements in mechanical properties and electrical conductivity were successfully fabricated. Graphene oxide (GO) synthesized by Hummer’s method was chemically modified with ethyl isocyanate to give ethyl isocyanatetreated graphene oxide (iGO), which is readily dispersed in N,N0 -dimethylformamide (DMF). The iGO dispersion in DMF was then used as media for synthesis of polyimide/functionalized graphene composites (PI/FGS) by an in situ polymerization approach. It was shown that addition of only 0.38 wt% of FGS, Young’s modulus of the PI/FGS composite film was dramatically increased from 1.8 GPa to 2.3 GPa, which is approximately 30% of improvement compared to that of pure PI film, and the corresponding tensile strength was increased from 122 MPa to 131 MPa. In addition, the electrical conductivity of the PI/FGS with this graphene content was increased by more than eight orders of magnitude to 1.7 105 S m1. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Graphene Polyimide Polymer nanocomposite
1. Introduction Polyimides (PI) are a class of high-performance polymer owing to various excellent properties, such as high mechanical strength and modulus, high thermal resistance, and good chemical resistance [1e3]. Thus, PIs have been widely used in many applications including aerospace, optics, and microelectronics. However, pure PIs have some limitations such as their insulating nature that causes electrostatic accumulation on the surface of the material [4,5]. As a result, this phenomenon causes local heating and premature degradation of the materials. Besides, with the increasing development of aircraft industries, the properties of PI films should be enhanced to meet the requirements. In this sense, carbon nanotubes (CNTs) have been widely used as effective fillers to enhance electrical conductivity [4e6] and mechanical properties of PIs [7,8]. It was reported that addition of CNTs to PI reduces the surface resistivity and consequently mitigates the build-up electrostatic charges of the PI film surface [4,6]. A significant increase of 40% in tensile strength was demonstrated for the PI/CNTs
* Corresponding author. Tel.: þ358 9 470 22614; fax: þ358 9 451 2622. E-mail address: jukka.seppala@aalto.fi (J. Seppälä). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.09.033
containing 4 wt% of CNTs [4]. However, in this study the corresponding Young’s modulus was not shown. Although various studies have indicated that CNTs are good reinforcements for many polymers including PIs, there are many unresolved problems related to the CNTs materials such as high production cost, chirality related electrical conductivity, and distinct difficulty in dispersion [9,10]. Consequently, the incorporation of CNTs increased the electrical conductivity of the PI/CNTs composite, but it had negligible effects on tensile properties [6]. In another study, only 6% increase in Young’s modulus was observed as the maximum value with addition of CNTs to the PI while the tensile strength was not reported [4]. In contrast, recently discovered graphene-derived materials offer an ideal replacement for CNTs because of their easy production in large scale, easy dispersion in both polar and non-polar matrixes, as well as their excellent electrical properties [11], unique mechanical strength [12], interesting optical contrast [13], and high thermal properties [14]. Therefore, researchers have shown an increasingly growing interest in graphene/polymer nanocomposites. For example, it has been reported that a polystyrene/graphene composite exhibited an electrical percolation threshold at 0.1 vol% graphene and a conductivity of 0.1 S m1 at 1 vol.% graphene, which is sufficient for many electrical applications
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[15]. In another study, an increase in glass transition temperature of around 40 C was observed in polyacrylonitrile/graphene composite with an incorporation of 1 wt% of graphene content. Similarly, upon addition of only 0.05 wt% of graphene content in poly(methyl methacrylate) (PMMA), Tg of the obtained PMMA/ graphene composite increased significantly by 30 C [16]. Very recently, we prepared graphene/cellulose nanocomposite papers that exhibited a significant improvement in tensile strength from 200 to 232 MPa and 273 MPa with addition of 0.3 wt% and 5 wt% of graphene, respectively [17]. Besides, the developed graphene composite paper showed a percolation threshold in electrical conductivity as low as 0.3 wt%, and at this graphene loading the conductivity was measured to be 4.79 104 S m1, which is well above the antistatic value of 106 S m1 for thin films [15]. Accordingly, graphene sheets obviously can serve as reinforcing filler to increase PI film properties including mechanical properties and electrical conductivity. However, so far, too little attention has been paid to polyimide/graphene composites. Recently, it has been found that incorporation of 2 wt% graphene into PI increases the tensile strength from 88 to 118 MPa and the Young’s modulus from 1.3 GPa to 1.7 GPa [18]. In the study, graphene oxide was used and dispersed in the poly(amic acid) (PAA) in DMF. However, it is well known that the graphene oxide prepared by Hummers method is not readily dispersed in organic solvent such as DMF [19], and thus they tend to agglomerate in DMF solvent during the polymerization, and the agglomeration possibly reduces the reinforcement ability of the graphene nanomaterial. To avoid the mentioned agglomeration of the graphene sheets, in this study, graphite oxide, which is insoluble in dipolar aprotic DMF solvent, was successfully converted to a soluble form through a chemical treatment with ethyl isocyanate. Then, we evaluated the effects of the incorporation of the functionalized graphene sheets (FGS) on electrical conductivity and mechanical properties of PI film. We found that the synthesized PI/FGS nanocomposite films showed enhanced tensile properties including tensile strength and Young’s modulus in comparison with pure PI film. For example, with an incorporation of only 0.38 wt% FGS, the Young’s modulus increased significantly from 1.8 GPa to 2.3 GPa, which is nearly 30% of improvement compared to pure PI film. In addition, the tensile strength increased from 122 MPa to 131 MPa. The developed PI/FGS composite films with significant enhancements in mechanical and electrical properties can find potential uses in microelectronics and aerospace industries.
(36 wt%) was added into the mixture to reduce the residual permanganate (MnO-4) and manganese dioxide (MnO2) to colorless soluble manganese sulfate (MnSO4), after which the color of the mixture changed to bright yellow. After that, the mixture was diluted in 2000 ml of HCl (10 wt%) and filtered. Subsequently, the mixture was washed several times with DI water to eliminate impurities and to achieve pH 7. Finally, the graphite oxide product was dried under vacuum at 60 C for 48 h.
2. Experimental
2.6. Characterizations
2.1. Materials
Individual sheet thicknesses were measured from drop-cast dispersion of GO and iGO in DMF on silicon substrates using a Dimension 5000 atomic force microscope (AFM, Veeco Inc.) in tapping mode with silicon probes with nominal resonant frequency of 325 kHz and nominal force constant of 46 N/m (Micromasch). X-ray diffraction of the samples was carried on a wide angle X-ray diffraction (Mac Science, Mac-18xhf) with Cu Ka radiation (l ¼ 0.154 nm) and data was collected in the 2q of 5-50 with a scanning speed of 5 min1. Tensile test of the film samples was studied using an Instron 4204 universal testing machine with a test speed of 5 mm min1. At least five specimens were used for each sample in the tensile test. Morphology of the samples was studied by a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500FA). To study the fracturing of the composites, the specimens were cryo-fractured under liquid nitrogen and for high resolution imaging; the exposed cross-sections were sputtered with a thin layer of gold/palladium (Emitech K100X) to promote conductivity before SEM observation. Chemical composition of the sample surfaces was analyzed using a high resolution X-ray
All chemicals including graphite flakes (particle size <150 mm), sulfuric acid (98%), chlohydric acid (36 wt%), hydrogen peroxide (36 wt%), potassium permanganate, sodium nitrate, ethyl isocyanate, pyromellitic dianhydride (PMDA), 4,40 -oxydianiline (ODA), N,N0 -dimethylformamide (DMF), and triethylamine (TEA) were purchased from SigmaeAldrich Co and used as received. 2.2. Synthesis of graphite oxide Graphite oxide was synthesized using a modified Hummers method [19]. Typically, 10 g of graphite flakes, 5 g of sodium nitrate, and 30 g of potassium permanganate were added to a 500-ml round-bottom flask containing 250 ml concentrated sulfuric acid, which cooled down by immersing in an ice bath for 1 h under stirring. The mixture was then slowly heated to 35 C for 2 h to oxidize the graphite. Next, 500 ml of deionized water was slowly added into the mixture. After 30 min, 30 ml of H2O2 solution
2.3. Isocyanate treatment of graphite oxide Graphite oxide (1 g) was added into a 100-ml round-bottom flask. Then, DMF (50 ml) was supplied under nitrogen with magnetic stirring. Subsequently, ethyl isocyanate (0.08 ml) was added and the mixture was kept at this condition overnight. After that, the reaction mixture was washed several times with methylene chloride to eliminate impurities and coagulate the product. Finally, the product was dried under vacuum at 60 C for 24 h [20]. 2.4. Preparation of GO and iGO dispersions Isocyanate-treated graphite oxide (0.5 g) was dispersed in DMF (500 ml) by ultrasonication for 1.5 h, followed by centrifugation at 4000 rpm for 20 min to remove the precipitate. The supernatant was denoted as iGO. In a similar procedure, graphite oxide/DMF was ultrasonicated and the supernatant was denoted as GO; in this case, however, most of GO (w 90 wt%) was precipitated, indicating that most GO is not well dispersed in DMF. 2.5. In situ synthesis of polyimide/graphene composite film Equivalent molar ratios of 2 g ODA and 2.2 g PMDA were dissolved in the iGO dispersion to make two solutions containing 0.38 and 0.75 wt% of iGO (compared to monomers). TEA catalyst (3 wt% to monomers) was added immediately and the reaction was conducted in an ice bath with mechanical stirring for 3 h. After that, viscous polymer solutions of PAA/iGO were obtained. The PAA/iGO solutions were then cast onto glass disks and dried at room temperature overnight. Next, the PAA composite films were heated at 100 C, 200 C and 250 C, each for 1 h under nitrogen, forming the composite films (PI/FGS) with thicknesses of around 30 mm. In a similar procedure, PAA/GO was synthesized using GO dispersion in DMF and neat PI film was fabricated as the reference.
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limit of 100 MU. A plasma etching (O2 1500 Sccm, power 500 W, time 15 min) was applied to the specimens before electrical conductivity measurement to remove thin polymer layers on the film surfaces. The plasma etching was used to increase the contact between the film surface and machine probes. 3. Results and discussion We first chemically modified GO with ethyl isocyanate and thereby the resulting iGO sheets can be readily dispersed in DMF solvent by sonication to form a stable dispersion. The iGO dispersion was subsequently used as the media for the synthesis of PAA. In the next step, the PAA/iGO solutions were cast on glass substrates and heat-treated at high temperature under nitrogen, forming the PI/FGS composite films with thickness of around 30 mm. For comparison, GO/PAA film with 0.1 wt% GO was prepared by
Scheme 1. In-situ polymerization pathway of polyimide/graphene composites films.
photoelectron spectroscopy (XPS, AXIS 165, Kratos Analytical) with monochromated Al Ka irradiation at 100 W. Electrical conductivity of the samples was measured by a four-point probe method from Jandel Engineering Ltd. connected to a Keithley 2400 source meter using a standard four-probe technique with a threshold detection
Fig. 1. Typical AFM images of GO sheets (a) and iGO (b) Sheets with height profiles taken along the lines. The samples were prepared by drop-casting dilute GO in water and iGO in DMF dispersions onto silicon wafer. XRD spectra of graphite and graphene oxide (c).
Fig. 2. (a) Photographs of stable dispersion of iGO in DMF, PAA/GO solution, and two PAA/iGO solutions containing 0.38 wt% and 0.75 wt% of iGO loadings; it is notice that all the pictures were taken after two days of preparation. (b) Photographs of PAA, PAA/ GO, and PAA/iGO composite films (top), and PI, PI/FGS composite film containing 0.38 wt% and 0.75 wt% of graphene loadings (bottom). All the films have thickness of around 30 mm.
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dispersion of GO in DMF. Regarding the GO dispersion in DMF, it was confirmed in this study and another study that GO synthesized by Hummer’s method is not well dispersed in DMF and thus only a very small content of GO is dispersed after sonication [20]. Scheme 1 demonstrates these steps to produce PI/FGS composite films in which PAA was formed from the reaction of two precursors, a diamine and a dianhydride. Briefly, at low temperature in a dipolar aprotic DMF solvent, nucleophilic attack of the amino group on the carboxyl carbon of the anhydride group led to the opening of the anhydride ring, generating an amic acid group. Water molecules were then eliminated during cycloimidization upon heat treatment of the PAA and the final polyimide was formed. AFM images of GO (Fig. 1a) and iGO (Fig. 1b) show clearly single graphene oxide and isocyante-treated graphene oxide sheets, which means that the iGO sheets were dispersed uniformly in DMF solvent. The thickness of the sheets was between 1 and 2 nm which is close to the data reported previously [15,16]. While the thickness of a flat graphene sheet is w0.34 nm [20], the higher thickness values of GO and iGO sheet are originated from the presence of the functional groups on the graphene plane that were introduced upon chemical oxidation/modification steps. Notably, the thickness of iGO sheets did not seem to differ measurably from GO sheets, indicating that the isocyanate layer must be very thin. Additionally, the XRD spectra of pristine graphite flakes and graphene oxide are shown in Fig. 1c. As seen clearly, a strong and sharp peak at 26.5 is regarded as the characteristic peak of graphite while graphene oxide shows only a very small peak at around 11. The disappearance of the characteristic peak at 26.5 is due to the presence of oxygen-containing groups, such as carboxyl, hydroxyl, and epoxide groups on the basal plane of graphene [21]. Fig. S1 (see Electronic Supplementary Material) shows energycorrected high resolution XPS spectra of the O 1s, N 1s, and C 1s regions for the GO and iGO samples (Fig. S1a) and full length XPS for the GO sample (Fig. S1b). It is clear that the iGO sample shows the peak for the N 1s and this is not the case for the GO sample (Fig. S1a and b), confirming a presence of nitrogen bonds in the iGO. It thus can be an evidence for the successful modification of graphene oxide by ethyl isocyanate. As seen in the photographs (Fig. 2) of the PAA/GO solution with 0.1 wt% of GO loading (Fig. 2a), after 2 days of storage, most GO sheets were aggregated and settled down at the vial bottom and these agglomerates of GO could be observed by naked eye from the PAA/GO film (Fig. 2b). On the contrary, in Fig. 2a, the PAA/iGO
solutions with 0.38 wt% and 0.75 wt% of iGO loadings were still clear, which means that the ethyl isocyanate treatment of GO was effective for its stabilization in DMF and PAA solution. These stable PAA/iGO solutions were used for the formation of PI/FGS nanocomposite films with uniform and shiny color surfaces (Fig. 2b), which get darker with the increased incorporation of graphene. We therefore assume that iGO sheets bound strongly to PAA chains, which prevented them from agglomerating. The successful synthesis of PAA and PI was distinctly confirmed by IR analysis (Fig. 3a and b). Fig. 3a shows characteristic peaks of PAA. The peaks from 2500 to 3500 cm1 indicate the presence of carboxylic acid and amine groups. The peaks at 1538 cm1 and 1648 cm1 can be assigned to CeNH stretching (amide II) and C]O in CONH (amide I) stretching bands. The peak at 1719 cm1 confirms the C]O in COOH groups in the PAA macromolecules. For the PI characteristic peaks in both pure PI and PI/FGS composites, the peaks related to the functional groups of PAA such as carboxylic acid and amine groups at 2500e3500 cm1 were disappeared. Two peaks at 1776 cm1 and 1719 cm1 are due to C]O asymmetric stretching vibration and C]O symmetric stretching vibration of the imide absorption band, respectively, with the former overlapping with the strong carboxylic acid band of the PAA molecules. Finally, new peaks at 1371 cm1 and 723 cm1 are explained by CeN stretching and C]O bending modes, respectively, of the imide groups through hydrative cycloimidization [22]. During the high temperature treatment in the cycloimidization step, the iGO could be partially thermally reduced to functionalized graphene sheets (FGS) and the resulting PI/FGS composite films may conduct electricity. Thus, electrical conductivity measurement was performed. The conductivity of neat polyimide was measured to be 1.2 1013 S m1and conductivities of the PI/FGS composite films were 1.7 105 and 8.9 105 S m1, which correspond to 0.38 wt% and 0.75 wt% of graphene loadings, respectively. These values are above the antistatic criterion of 106 S m1 [15], indicating the effective connection of high aspect ratio of partiallyreduced graphene sheets in the films promotes the electrical conductivity. Although the degree of thermal reduction of iGO in PI/ FGS composite films should be further investigated, it is notably to address that the conductivity of the PI/FGS composite films prepared in this study compared well with that of a reported PI/ SWCNT composite [23]. In addition, our PI/FGS composite films’ conductivities were even higher than that of polyimide reinforced with multi-walled CNT (PI/MWCNT) [7]. Therefore, we strongly believe that the good conductivity of the PI films reinforced by FGS
Fig. 3. FT-IR spectra of PAA, PI (a), and PI/FGS with 0.38 wt% and 0.75 wt% (b).
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Fig. 4. (a) Typical stress-strain curves of neat PI (1) and PI/FGS nanocomposite films containing 0.38 wt% (2) and 0.75 wt% of graphene loadings (3). (b) Summary of mechanical properties of the PI and PI/FGS composite films with 0.38 wt% and 0.75 wt% graphene loadings.
in this study could be attributed to the good particleeparticle connections of well-distributed FGS sheets, which may be partially reduced by the thermal treatment during cycloimidization, in PI matrix. Typical stress-strain curves of the pure PI and PI/FGS nanocomposite films are shown in Fig. 4a and the tensile properties are summarized in Fig. 4b. As expected, with the addition of only 0.38 wt% of FGS, the tensile strength was clearly improved from 122 to 131 and Young’s modulus was significantly improved from 1.8 MPa to 2.3 MPa, which is approximately 30% improvement compared to that of pure PI film. Meanwhile, the elongation at break decreased from 69% to 42%. When graphene content was further increased to 0.75 wt%, however, the tensile strength was gradually decreased in comparison with that of PI/FGS composite film containing 0.38 wt% of FGS. Nevertheless, this tensile strength
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of 127 MPa was still higher than that of the pure PI film. It should be further addressed that the Young modulus of the PI/FGS composite film continued to increase to 2.4 MPa at 0.75 wt% of graphene content, which is around 33% higher than the value of the pure PI film. Considering the mechanical enhancement of graphene to polyimide in this study, it is worthwhile to address that the efficiency of graphene is better than that of CNTs [4,6,8]. The effects of graphene on polymer properties were also demonstrated in case of cellulose/graphene nanocomposite paper exhibiting the enhanced mechanical properties and electrical conductivity that was not reported previously for any cellulose/ carbon-based materials nanocomposite papers [17]. The enhancement in both tensile strength and modulus of the PI/FGS composites could be attributed to the uniform dispersion of FGS and its anisotropic orientation in PI matrix, and thus allowing an efficient stress transfer from polymer matrix to the filler. Yet, another important factor concerning the reinforcement effect of the FGS filler is the good interfacial interactions between FGS sheets and PI macromolecules. To better understand the enhanced tensile properties of the PI/ FGS composites, we performed SEM study at the fractured surfaces of the PI and PI/FGS composite films. As can be seen in Fig. 5a, PI film shows a smooth fractured surface. In contrast, the PI/FGS composite films containing 0.38 wt% and 0.75 wt% of graphene possess rough fractured surfaces that could be attributed to the strong interfacial adhesion and good compatibility between the PI matrix and FGS sheets (Fig. 5b and c). Such strong interactions are favorable to the stress transfer from the polymer matrix to the graphene sheets, leading to the improvement in the tensile strength and modulus of the composite films compared to those of the pure PI films. XPS was further used to study the interaction between the filler and matrix, and the result was shown in Fig. 6. In the case of pure PI, high resolution carbon region was resolvable into four compounds, identified in ref [23,24] as carbon atoms without O or N neighbors (at 285.0 eV), carbons with either one O or N neighbor (286.1 eV), carbonyl carbon (three bonds to O, at 288.8 eV) and a shallow plasmon feature, due to aromatic structures at higher BE. In the case of FGS addition, the two carbon compounds with lowest BE merged into a broader, unresolvable compound. There are two plausible explanations for that phenomenon. Firstly, incorporation of FGS onto PI surface would broaden the signal due to slightly different major compound positions in FGS (see Fig. S1) with 284.6 eV for graphene, and 286.6 eV for hydroxylated carbon (as in cellulose, see ref [25]). In addition to incorporated graphene, chemical linkages between PI and GO might further broaden the
Fig. 5. SEM for PI (a) and PI/FGS composite films containing 0.38 wt% (b) and 0.75 wt% of graphene loadings (c).
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Fig. 6. N 1s, C1s, and O 1s XPS results for PI (a) and PI/FGS composite films with 0.38 wt% (b) and 0.75 wt% of graphene loadings (c).
XPS features slightly, which indeed seems to be the case for C, O and N regions. It is also noteworthy to mention that pure PI exhibits a sharp N 1s peak originated from the imide nitrogen [23]; however, in the PI/FGS composites, the N 1s peaks are broader. The broader N 1s peaks of the composites are possibly due to the present of another nitrogen species of FGS which was introduced by the modification of graphene oxide with ethyl isocyanate (Fig. S1), and again, confirming the successful modification of graphene oxide by ethyl isocyanate. 4. Conclusion In conclusion, we have demonstrated herein a facile approach for the preparation of PI films reinforced with functionalized graphene sheets which exhibited the enhancements in both mechanical properties and electrical conductivity with the addition of small graphene content. We strongly believe that the developed PI/FGS composite film can find uses in microelectronics and aircraft industries. Acknowledgments Dr. Joseph Campbell at Aalto University is acknowledged for the XPS measurements. Mr. Matti Lehtimäki is appreciated with XRD characterization. Appendix. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.polymer.2011.09.033.
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