Fabrication of Spirocyclic Phosphazene Epoxy-Based

Article pubs.acs.org/IECR

Fabrication of Spirocyclic Phosphazene Epoxy-Based Nanocomposites with Graphene via Exfoliation of Graphite Platelets and Thermal Curing for Enhancement of Mechanical and Conductive Properties Hua Feng, Xiaodong Wang,* and Dezhen Wu State Key Laboratory of Organic−Inorganic Composite Materials, School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Nanocomposites of a spirocyclic phosphazene epoxy (SP-epoxy) resin with graphene were prepared through the exfoliation of graphite platelets and thermal curing process. Transmission electric microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy confirmed the chemical exfoliation and following thermal reduction for the graphene. The TEM observation also demonstrated that the reduced graphene oxide, as the single- and/or few-layered stacking sheets, was homogeneously dispersed in the SP-epoxy matrix. The presence of graphene improved both the tensile and flexural properties of the nanocomposites as a result of the great surface area of graphene sheets in contact with the matrix. These nanocomposites also achieved a considerable increase in glass transition temperature, thermal decomposition temperatures, and storage modulus. It is highly emphasized that the SP-epoxy/graphene nanocomposites also presented a low percolation threshold of 0.375 vol % and achieved high electrical conductivity at a volume fraction of graphene higher than 0.75 vol %.

1. INTRODUCTION Graphene, an atomically thick and two-dimensional sheet composed of sp2−bonded carbon atoms arranging in a honeycomb structure, has been considered as a most marvelous material in 21 century.1,2 This thinnest material in the cosmos has many supernatural properties like extremely high strength around 130 GPa and modulus up to 1000 GPa,3 a thermal conductivity of 5000 W/(m3·K) corresponding to the upper bound of the highest values reported for single-wall carbon nanotube bundles,4 and a very high electrical conductivity near 6000 S/cm.5 These properties in addition to extremely high surface area and gas impermeability indicate the tremendous potential applications of graphene for improving mechanical, electrical, thermal, and gas barrier properties of polymers.6 In resent years, polymer-based nanocomposites with carbon black, carbon nanotubes, and layered silicates have been used for improving the mechanical, thermal, electrical, and gas barrier properties of polymers.7−9 With the discovery of graphene with its combination of extraordinary physical properties and ability to be dispersed in various polymer matrices, the graphene has created a new class of polymeric nanocomposites and, thus, has opened a new dimension in the field of materials science. The study on the polymeric nanocomposites with graphene has been attracting a great attention from the worldwide scientists, which is reflected by a large number of research publications. Most of the researches are focused on the effectiveness of graphene as a nanofiller in various polymeric systems, such as polystyrene (PS), polyaniline, polyurethane, poly(vinylidene fluoride), polycarbonate, poly(ethylene terephthalate) (PET), epoxy thermosetting resins, etc.10−16 Mao et al.17 reported the fabrication of PS/poly(methyl methacrylate) nanocomposites with octadecylamine-functionalized graphene and found the nanocomposites exhibit an extremely low © 2013 American Chemical Society

electrical percolation threshold (only 0.5 wt % of graphene) because of the formation of a perfect double percolated structure. Shen et al.18 investigated the effect of melt blending on the interaction between PS and graphene and found that the formation of π−π stacking can enhance the interaction between two phases. Liang et al.11 reported the preparation of poly(vinyl alcohol)/graphene nanocomposites using water as the processing solvent and observed that the mechanical properties of the nanocomposites were superior to that of the pure PVA. Although considerable efforts have been contributed to design and prepare the polymeric nanocomposites with graphene, the graphene is seldom obtained as a commercial product due to its extremely high cost and difficult availability. As a key technique issue, graphene or modified graphene sheets are needed to be produced by exfoliation of graphite or graphite derivatives like graphite oxide and graphite fluoride.19 In general, these methods are suitable for large-scale production required for polymer composite applications. Owing to the abundance of naturally existing graphite as the source material for graphene, starting from graphite or its derivatives offers feasible pathways and significant economic advantages to the preparation of graphene-containing nanocomposites. Schniepp et al.20 described a method to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide and found that this method yielded a wrinkled sheet structure resulting from reaction sites involved in oxidation and reduction processes. Yan et al.21 utilized this thermal splitting method to obtain the single graphene sheets, and then they Received: Revised: Accepted: Published: 10160

February 12, 2013 July 2, 2013 July 5, 2013 July 5, 2013 dx.doi.org/10.1021/ie400483x | Ind. Eng. Chem. Res. 2013, 52, 10160−10171

Industrial & Engineering Chemistry Research

Article

Scheme 1. Molecular Structure of Spirocyclic Phosphazene-Based Epoxy Resin

Scheme 2. Fabrication Route of SP-Epoxy/Graphene Nanocomposites

graphene sheets resulted in significant improvements. It was reported that the thermal conductivity could be increased to as much as 6.44 W·m−3·K−1 by adding 20 wt % of graphene oxide.33 These results show that the polymeric composites with graphene are a kind of promising thermal interface materials for heat dissipation. Meanwhile, a thermal expansion study on these graphene-based composites showed a similar effect to those with carbon nanotubes on the coefficient of thermal expansions.34 Additionally, it was also reported that the graphene oxide could improve the flame retardancy of epoxy resins when functionalized with flame retardant elements like organic phosphate and gave a way of application of graphene in enhancing thermal stabilities of epoxy resins.35 In our previous work, a spirocyclic phosphazene-based epoxy (SP-epoxy) resin was synthesized as an epoxy-functional polymer with halogen-free flame retardancy and high performance.36 This novel epoxy-functional polymer is considered as a potential candidate for fire- and heat-resistant applications in electronic and microelectronic fields with more safety and excellent performance. In current work, a strategy was designed to prepare the SP-epoxy/graphene nanocomposites through the exfoliation of graphite platelets following with thermal curing. The graphite platelets were first oxidized with strong oxidations in an aqueous medium, and then the obtained graphene oxide was reduced in a high temperature. The mechanical enhancement of these nanocomposites was studied throughout, and the effects of graphene on the thermal and electrical properties of SP-epoxy thermosetting composites were investigated. A complementary study on microstructure and morphology of these nanocomposites was also performed and described in this paper.

prepared electrically conductive nanocomposites based on nylon 12 and graphene with a low percolation threshold at 0.3 vol % by melt compounding. Shim et al.22 reported a facile method to prepare the PET/graphene composite through the oxidation and exfoliation of graphite with strong oxidants and then following the functionalization of individual graphene sheets with alkyl and alkyl ether groups. Epoxy resins are one of the most widely used materials in modern industrial areas due to their outstanding properties and great versatility. Although the epoxy-based nanocomposites with a range of nanofillers, such as graphite platelets, clay, carbon nanotubes, and carbon nanofivers, have been widely reported,23−27 the research on epoxy/graphene nanocomposites has still attracted great attention from the scientific community and industrial association. It is expected that the incorporation of graphene can impart much higher performance to the epoxy thermosetting materials and, thus, greatly extent their application in the hi-tech electronic and microelectronic fields. Qiu and Wang28 investigated the fracture toughness of epoxy/graphene oxide nanocomposites through the three-point bending test and found that both toughness and tensile strength achieved a considerable increase. Zhaman et al.29 reported a toughening enhancement of epoxy/graphene nanocomposites through the surface modification for graphene sheets. Yang et al.30 investigated the synergetic effects of graphene sheets and carbon nanotubes on the mechanical and thermal properties of epoxy thermosets. Wang et al.31 found that the graphene could enhance the epoxy thermosets after functionalized with organosilane. Yu et al.32 prepared the epoxy-based nanocomposites by incorporating graphene oxide sheets and examined the level of thermal expansion using a thermo-mechanical analyzer. They noted that the epoxy resins showed very poor thermal conductivity but the inclusion of 10161

dx.doi.org/10.1021/ie400483x | Ind. Eng. Chem. Res. 2013, 52, 10160−10171


Article

to be 2.28 g/cm3,40 and that of the SP-epoxy matrix is 1.16 g/ cm3. The mixture was transferred to a mold and degassed in a vacuum oven at 75 °C until all solvents were evacuated. A twostep curing procedure was carried out in a mold to obtain the thermosetting resins. The epoxy formulations involving various volume fractions of graphene sheets and DDM were first precured at 150 °C for 2 h and, then, were further postcured at 180 °C for 3 h. In the end of the curing procedure, the cured system was cooled gradually to room temperature to avoid stress cracks. 2.3. Characterization. 2.3.1. Raman Spectrum. Raman spectra were obtained with a Renishaw inVia Raman microscope at an excitation laser wavelength of 514.5 nm. All samples were deposited on glass slides in powder form without any solvent. 2.3.2. X-ray Diffraction (XRD). XRD measurements were made directly from the reduced graphene oxide powders. All of the measurements were performed in reflection mode using a Japan Rigaku D/Max 2500 VB2 + P/C X-ray diffractometer using Cu Kα radiation (k = 0.154 nm) and operated at 40 kV and 20 mA with a scan rate of 1°/min in a 2θ range of 0.5°− 30°. 2.3.3. Transmission Electron Microscope (TEM). The reduced graphene oxide was dispersed in acetone by ultrasonicator, and some pieces were collected on carbon-coated 300-mesh copper grids for TEM observation. The morphology was determined by TEM using a Hitachi H-800 transmission electron microscope operating at an accelerating voltage of 200 kV. 2.3.4. High-Resolution Transmission Electron Microscopy (HRTEM). To investigate the laminate structure of exfoliated graphene sheets, HRTEM observation was also performed on a Hitachi JEM-3010 high-resolution transmission electron microscope with the filamentary cathode of lanthanum hexaboride at an accelerating voltage of 300 kV. 2.3.5. X-ray Photoelectron Spectroscopy (XPS). XPS was performed on a ThermoFisher ESCALAB 250 X-ray photoelectron spectrometer with a focused monochromatized Al Kα radiation of 1486.6 eV to determine the changes in atomic ratios of carbon to oxygen and to confirm the existence of different functional groups. The XPS spectra were fitted for the Casa XPS software, in which a Shirley background was assumed. 2.3.6. Scanning Electron Microscopy (SEM). SEM observations of both the exfoliated graphene sheets and the fracture surface of SP-epoxy/graphene nanocomposites were performed on a Hitachi S-4700 scanning electron microscope. The specimens were prepared electrically conductive by sputter coating with a thin layer of gold−palladium alloy. The micrographs were taken in high vacuum mode with 20 kV acceleration voltage and a medium spot size. 2.3.7. Differential Scanning Calorimetry (DSC). The DSC measurement on thermal transition temperatures was carried out on a TA Instruments Q20 differential scanning calorimeter equipped with a thermal analysis data station, operating at scanning rates of 10 °C/min under a nitrogen atmosphere. 2.3.8. Thermogravimetric Analysis (TGA). TGA measurements were performed under a nitrogen atmosphere using a TA Instruments Q50 thermal gravimetric analyzer. The specimens with a mass of about 6 mg were placed in an aluminum crucible, and ramped from room temperature up to about 750 °C at a heating rate of 10 °C/min, while the flow of nitrogen was maintained at 50 mL/min.

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite platelets were kindly provided by Angstron Materials, LLC, Ohio, USA. These graphite platelets were derived from natural graphite and were mechanically crushed to the thin sheets with an average z-dimension of around 100 nm and the average x- and y-dimensions of approximately 5−6 μm. Concentrated sulfuric acid (H2SO4, 98 wt %), concentrated nitric acid (HNO3, 86 wt %), and potassium chlorate (KClO3) were purchased from Beijing Chemical Reagents Co., Ltd., China. The SP-epoxy resin with the molecular structure shown by Scheme 1 was synthesized according to the method as previously reported.35 This epoxy resin has an epoxide equivalent weight around 682.5 g/equiv and a number-average molecular weight of 1629. A conventional epoxy resin, diglycidyl ether of bisphenol A (DGEBA), was kindly supplied by BlueStar New Chemical Materials Co., Ltd., China. 4,4′-Diamino-diphenylmethane (DDM) was selected as hardener and was also purchased from Beijing Chemical Reagent Co., Ltd., China. 2,4,6-Tris(dimethylaminomethyl)phenol (DMP-30), used as a curing accelerator, was commercially obtained from Sinophurm Chemical Reagents Co., Ltd., China. 2.2. Preparation of SP-Epoxy/Graphene Nanocomposites. The fabrication strategy of SP-epoxy/graphene nanocomposites is shown in Scheme 2. The graphite platelets were first exfoliated to form graphene oxide using a modified Staudenmaier’s method.37−39 In a typical process, 67.0 mL of concentrated nitric acid and 131.0 mL of concentrated sulfuric acid was mixed in a round-bottom flask for 10 min, and then 10.0 g of graphite platelets was added to this homogeneous mixture under vigorous stirring to obtain a dark-colored suspension with the graphite platelets dispersed uniformly. Subsequently, 21.0 g of potassium chlorate was added slowly into the above suspension in an ice bath over 4 h. The flask was capped loosely to allow the escape of ClO2 gas until the potassium chlorate was dissolved and left to stir vigorously for 96 h at room temperature. A reddish brown-colored viscous mixture was obtained, indicating the completion of oxidation reaction. This mixture was poured into 500 mL of deionized water and then was sonicated in an ultrasonic bath for 24 h to obtain a uniform suspension. During the sonication, the graphite platelets could be split to graphene sheets. Afterward, the suspension was centrifuged, washed, and filtered to give graphene oxide. The graphene oxide obtained was ultimately extracted from the original solution and then completely dried at 100 °C in a vacuum oven for 12 h. The dried graphene oxide was quickly inserted into a muffle furnace preheated to 1050 °C and then was held in the furnace for 30 s. Finally, the graphene sheets were obtained from this thermal reduction process. The graphene sheets were finely dispersed in acetone by an ultrasonicator for 6 h at room temperature. Then SP-epoxy resin, hardener, and curing accelerator were added, and the slurry was stirred for 1 h to obtain good homogeneity. It should be mentioned that the content of graphene was transformed from weight fraction (wt %) to volume fraction (vol %) by the following equation: ϕ=

w/ρg w/ρg + (1 − w)/ρe

(1)

where ϕ is the volume fraction of graphene, w is the weight fraction of graphene, ρg is the density of graphite, and ρe is the density of SP-epoxy matrix. The density of graphene is reported 10162



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2.3.9. Dynamic Mechanical Analysis (DMA). The dynamic mechanical behaviors of SP-epoxy/graphene nanocomposites were measured on a TA Q800 dynamic mechanical analyzer under a dual cantilever mode. The test was run during the temperature range from 23 to 220 °C with a heating rate of 5 °C/min and a strain amplitude of 10 mm at a frequency of 1 Hz. 2.3.10. Mechanical Property Tests. The tensile and flexible properties of SP-epoxy/graphene nanocomposites were measured with a SANS CMT-4104 universal test machine using a 10 000 Newton load transducer according to ASTM D-638 and D-790 standards, respectively. The critical stress intensity factor (K1C) was used to evaluate the fracture toughness of SP-epoxy/ graphene nanocomposites. This fracture toughness parameter was measured and calculated by three-point bending method using the SANS CMT-4104 universal test machine according to ASTM E-399 standard. The single edge-notch bending specimens were molded in accord with the dimension as described by the ASTM E-399 standard, and the depth of crack was set in the range of 40−50% of the specimen width. All of the tests were carried out at room temperature, and the values reported reflected an average from five tests. 2.3.11. Electrical Conductivity Test. The volume electrical conductivity of SP-epoxy/graphene nanocomposites was measured on a PC68 digital teraohmmeter in the case of the conductivity lower than 10−6 S/m. For the nanocomposites with the conductivity higher than 10−6 S/m, the electrical conductivity was measured with a SB100/21A four-probe electrical resistivity instrument through a four-probe method. All of the measurements were carried out according to ASTM D-257 standard. 2.3.12. Limiting Oxygen Index (LOI) Test. The LOI test was performed using a HD-2 oxygen index apparatus with a magneto-dynamic oxygen analyzer, according to the ASTM D2863 standard. The average values of LOI were obtained from the results of five tests. 2.3.13. UL−94 Vertical Burning Experiment. The UL-94 vertical burning experiment was carried out based on the testing method proposed by Underwriter Laboratory according to ASTM D-1356 standard.

Figure 1. SEM micrographs of (a) graphite platelets, (b) graphene oxide, and (c, d) reduced graphene oxide.

nanometric scale as shown by Figure 2. Such a huge aspect ratio is very favorable for enhancing the contact areas with the epoxy

3. RESULTS AND DISCUSSION 3.1. Exfoliation and Structural Characterization of Graphene. To prepare the epoxy/graphene nanocomposites, graphite platelets must first be exfoliated to obtain single and few-layered graphene sheets via an oxidation reaction following with a thermal reduction. The epoxy/graphene nanocomposites were prepared through an in situ thermal curing process. The processing pathway and the structural features of the graphite platelets, graphene oxide, and graphene sheets are illustrated in Scheme 2. Figure 1 shows the SEM micrographs of graphite platelets and reduced graphene oxide, which clearly reflect the morphologies of graphite platelets before and after exfoliation. The graphite platelets presented a typical multilayer structure, whereas their stacking graphitic sheets were separated from one another after oxidation with the modified Staudenmaier’s method. It is highly notable in Figure 1(c) that the reduced graphene oxide exhibits a very thin thickness. It is observed from magnified SEM micrograph of Figure 1d that the graphene obtained in this work reveals some wrinkled nanosheets with an irregular shape. TEM and HR-TEM micrographs indicate that these graphene sheets have x- and y-dimensions around 1.5 μm and a very thin z-dimension in the

Figure 2. TEM micrograph (a) and HR-TEM micrograph (b) of reduced graphene oxide.

resins. These morphological results also suggest that the oxidation and reduction processes caused less damage to the aspect and graphitic structure of the graphene. Raman spectroscopy and XRD patterns were employed to characterize the graphitic structures of the graphite platelets, graphene oxide, and graphene sheets. From the Raman spectra shown in Figure 3, the significant changes in graphitic structure can be clearly distinguished from the graphite platelets to the graphene oxide and to the graphene. The graphite platelets exhibit an intensive G-band at 1575 cm−1 and a 2D-band at 2720 cm−1 in the Raman spectrum, corresponding to the firstorder scattering of the E2g vibration mode and the second-order two phonon mode, respectively. Moreover, a weak D-band is also observed at 1352 cm−1, which is attributed to the disorderinduced mode due to the presence of less structural defects. 10163



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been successfully exfoliated into the single- or few-layered stacking sheets as the graphene. XPS spectroscopy was further conducted to investigate the extent of reduction toward the graphene oxide, and the corresponding spectra are shown in Figure 5. The XPS

Figure 3. Raman spectra of graphite platelets, graphene oxide, and reduced graphene oxide.

However, the graphene oxide demonstrates a strong and broad D-band with a downward shift to 1357 cm−1. The intensity ratio of D- and G-bands also increases up to 0.671 from 0.233 of the graphite platelets, indicating that the distortion of bonds and the destruction of graphitic structural symmetry due to the reduction in the in-plane sp2 carbon atoms caused by oxidation. The graphene presents a similar profile in its Raman spectrum; however, its intensity ratio of D- and G-bands increases to 1.177. This may be ascribed to the fact that the carbon lattice in graphite oxide has developed some degree of amorphous character due to oxidation process itself. On the other hand, the XRD pattern of graphite platelets shows an intensive diffraction peak at 2θ = 26.5° (see Figure 4), reflecting an interlay spacing

Figure 5. XPS C1s spectra of (a) graphite platelets, (b) graphene oxide, and (c) reduced graphene oxide.

Figure 4. XRD patterns of graphite platelets, graphene oxide, and reduced graphene oxide.

spectrum of graphite platelets shows a typical C−C peak at 284.49 eV as well as a weak peak at 285.74 eV assigned to the carbon atoms in C−O bonds. Meanwhile, in the spectrum of graphene oxide, intensive C−O and CO peaks are observed at 286.10 and 287.48 eV, respectively, while a weak O−CO peak appeared at 288.03 eV. This suggests that the graphene oxide obtained a considerable degree of oxidation with several different oxygen-containing groups like hydroxyl and carboxyl.

of 0.34 nm between the platelets. In the case of the graphene oxide, a well-defined diffraction peak is observed at 2θ = 12.3° instead of at 26.5° in the pattern of Figure 4, indicating that the interlay spacing extended into 0.72 nm. Nevertheless, the XRD pattern of graphene does not demonstrate any diffraction peaks due to the disappearance of the long-term ordering graphitic structure. These results confirm that the graphite platelets have 10164



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from the matrix, indicating that the fracture did not occur preferentially at the interfaces between the graphene sheets and the epoxy matrix. Such a good dispersion of the graphene can be due to the good interaction between the carboxyl and hydroxyl groups on the edge of graphene and the polar groups of SP-epoxy resin. It is also observed from Figure 6g that the incorporation of 1.5 vol % graphene sheets dramatically changes the morphology of SP-epoxy/graphene nanocomposites. Evidently, a large amount of graphene sheets could generate a 2-dimensional hindrance effect on the matrix, which would result in a much coarser fracture surface. 3.3. Thermal Properties. The thermal resistance is one of the most important properties of epoxy-based thermosetting materials because it establishes the service environment for these materials. Usually, glass transition temperature (Tg) is an important parameter for the thermal property of epoxy-based materials, which are only used well, in most cases, at a temperatures below Tg. Furthermore, Tg is also a parameter which gives the information about the structure of cross-linked epoxy-based materials. Therefore, a higher Tg is expected when designing the SP-epoxy/graphene nanocomposites. Figure 7

However, after the thermal reduction were performed for the graphene oxide, the intensity of C−O and CO peaks decreased significantly as shown by Figure 5c, indicating that a considerable deoxygenation occurred during the thermal reduction process, and oxygenous groups were largely removed. The conversion of graphene oxide to graphene can lead to a transformation of sp3-bonded carbon atoms back to the sp2bonded ones. Such a transformation will restore the electrical conductivity for graphene and further imparts valuable conductivity to the SP-epoxy/graphene nanocomposites. 3.2. Microstructure of SP-Epoxy/Graphene Nanocomposites. The cryofractured surfaces of SP-epoxy/graphene nanocomposites were inspected by SEM, and the corresponding micrographs are shown in Figure 6. These cross-sectional

Figure 7. DSC thermograms of SP-epoxy thermoset and its nanocomposites with graphene.

shows the cross-sectional DSC thermograms of SP-epoxy/ graphene nanocomposites, and the obtained results are summarized in Table 1. The SP-epoxy thermoset exhibits a high Tg of more than 150 °C, indicating an excellent thermal resistance of the thermoset based on the epoxy resin synthesized in this work. It is interesting to note that the incorporation of the graphene results in an improvement in Tg, and the higher the volume fraction of graphene sheets, the higher the Tg values of SP-epoxy/graphene nanocomposites. It is understandable that the well-dispersed graphene sheets have a great surface area in contact with the SP-epoxy matrix, which may confine the chain mobility. This reduces the degree of freedom for the thermal motion of SP-epoxy segments and, thus, increases the Tg.41 However, the Tg begins to decline when the graphene content exceeds 1.25 vol %. The higher volume fraction of graphene may decrease the cross-linking degree of SP-epoxy matrix and, thus, reduce the packing density for polymeric networks. Consequently, a lower Tg was achieved. The thermal degradation behaviors of SP-epoxy/graphene nanocomposites were investigated by TGA under a nitrogen

Figure 6. SEM micrographs of (a) SP-epoxy thermoset and its nanocomposites with (b) 0.25 vol %, (c) 0.5 vol %, (d) 0.75 vol %, (e) 1.0 vol %, (f) 1.25 vol %, and (g) 1.5 vol % of graphene.

SEM micrographs somewhat reflect the dispersion of graphene in the SP-epoxy matrix. The SP-epoxy thermoset reveals a smooth and fragile fracture surface [see Figure 6a], suggesting the brittle failure of a thermosetting material. It is notable that the graphene sheets are crumpled and finely distributed in the matrix, confirming their presence in the SP-epoxy matrix. Even if the content of graphene is increased up to 1.0 vol %, the graphene still shows a homogeneous dispersion, and few agglomerates are observed. This not only decreases the incidence of defects, but also increases the contact area of graphene with the SP-epoxy matrix, resulting in a synergetic improvement in the properties of nanocomposites. The SEM micrographs also demonstrate that the graphene sheets are embedded in the matrix and are held tightly by SP-epoxy resin. These graphene sheets are seldom observed to be pulled out 10165



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372.5 °C in nitrogen, and meanwhile, its main degradation starts at a maximum decomposition temperature (Tmax) of 392.7 °C, at which the weight loss occurs at a maximum rate. These results indicate an excellent thermal stability of the epoxy resin synthesized in this work. It is also suggested by Table 1 that the presence of graphene does not influence the thermal stability of SP-epoxy matrix significantly. In some cases, the nanocomposites show a slight decrease in the Tonset, which may be ascribed to the defects in polymeric networks formed during thermal curing. On the other hand, the Tmax exhibits a slight increase with increasing the content of graphene. It was reported that the incorporation of carbon materials in a polymeric matrix could reduce the heat release rate, which played a key role to retard the decomposition temperature.42 In addition, the introduction of the graphene also leads to an improvement in char yield of the nanocomposites as shown by Table 1. It is believable that the well-dispersed graphene sheets can act as physical barriers to hinder the transport of volatile decomposed products out of the SP-epoxy matrix during pyrolysis and enhance the carbonization of the polymeric matrix.43 This results in the retardation of the weight loss of thermal degradation products as well as the thermal insulation of SP-epoxy matrix. Consequently, a high char yield was achieved at the end of the thermal decomposition of SP-epoxy/ graphene nanocomposites. 3.4. Mechanical Properties. The influence of graphene on the mechanical performance of SP-epoxy/graphene nanocomposites was evaluated in terms of the tensile, flexural, and notched impact measurements. Figures 9 and 10 show the

Table 1. The Thermal Analysis Results Obtained from DSC and TGA Measurements for SP-Epoxy/Graphene Nanocomposites TGA data nanocomposite samples SP-epoxy thermoset SP-epoxy/0.125 vol % graphene SP-epoxy/0.25 vol % graphene SP-epoxy/0.5 wol % graphene SP-epoxy/0.75 vol % graphene SP-epoxy/1.0 vol % graphene SP-epoxy/1.25 vol % graphene SP-epoxy/1.5 vol % graphene SP-epoxy/1.75 vol % graphene

Tg (°C)

a

Tonset (°C)

Tmaxb (°C)

char yield at 750 °C (wt %)

157.9 159.2

372.5 372.7

392.7 391.2

29.0 30.1

161.7

371.9

392.9

31.6

162.9

371.4

393.5

35.2

164.3

371.8

393.9

36.5

165.5

372.9

390.7

37.9

166.1

373.8

394.6

40.6

165.2

372.7

394.2

42.5

164.8

372.3

393.4

45.2

a

The onset decomposition temperature, at which the thermoset undergoes 3 wt % of weight loss. bThe characteristic temperature, at which maximum rate of weight loss occurs.

atmosphere. Figure 8 shows the cross-sectional TGA thermograms of these nanocomposites, and the analysis results are also

Figure 8. TGA thermograms of SP-epoxy thermoset and its nanocomposites with graphene.

Figure 9. Tensile strength and Young’s modulus of SP-epoxy/ graphene nanocomposites as a function of the volume fraction of graphene.

summarized in Table 1. These thermograms indicate that moisture and solvent have been successfully removed from the SP-epoxy resin and its nanocomposites because there is almost no weight loss below 100 °C. It is noteworthy that the TGA traces present a typical one-stage degradation for all of the specimens, indicating that both the SP-epoxy thermoset and its nanocomposites with graphene underwent one-stage decomposition for the major components of the polymeric networks. It is also suggested that the SP-epoxy/graphene nanocomposites have a good phase interconnection between the graphene sheets and the matrix. The temperature corresponding to 1.5 vol % weight loss is defined as a onset decomposition temperature (Tonset) and, furthermore, is taken as an index of thermal stability. The SP-epoxy thermoset presents a Tonset of

tensile and flexural data of the nanocomposites as a function of the content of graphene, respectively, in which the enhancement effect of the graphene is clearly distinguished. As is expected, these tensile and flexural properties present a considerable improvement when the graphene is incorporated into the SP-epoxy thermoset. Such an increasing trend remains with the increase of graphene content till 0.75 vol %. In this case, the nanocomposite achieved about 35% and 44% increment in tensile and flexural strength, respectively, compared to the SP-epoxy thermoset. Meanwhile, the SPepoxy/graphene nanocomposites also exhibit a similar variation trend in tensile and flexural moduli and obtained more than a 25% increment over the SP-epoxy thermoset in these two 10166



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fraction of graphene. As shown by Figure 11, the K1C value of SP-epoxy thermoset is only 1.26 MPa·m1/2, indicating the brittleness of this epoxy thermoset in nature. It is observed that incorporating 0.125 vol % of graphene sheets leads to an increase in K1C by about 25%, and the K1C value reaches a maximum of 2.21 MPa·m1/2 when 0.75 vol % of graphene is introduced. However, the K1C values of the nanocomposites are found to collapse with the increasing addition of the volume fraction of graphene. These results imply that the graphene has a considerable toughening effect on SP-epoxy thermoset only at an appropriate concentration. Beyond simple reinforcing effects, the enhancement of fracture toughness has been noted in graphene-based nanocomposites.44,47,48 Generally, for nanofiller-reinforced composites, the fracture toughness is dominated by two factors.47,48 The first one is that new stress concentrations are formed around the filler edges, area of poor adhesion, and region of filler aggregation. The second one is that the nanofillers can enhance fracture toughness through reducing the crack propagation rate by forcing cracks around the fillers. The practical effects of nanofillers on the fracture toughness of nanocomposites depend on the competition of these two factors. In the case of the SP-epoxy/graphene nanocomposites, the high specific surface areas of graphene sheets can increase the contacting area between the matrix and the filler, which favors the new fracture surfaces, thus increasing the required strain energy for the continuation of fracture. The energy required to produce a new fracture surface increases the impact strength. However, the sheet-like nanofillers like graphene sheets easily lead to the interfacial debonding between the matrix and the fillers and, thus, locally destroy the continuity of matrix. This directly results in the decrease of crack propagation capability and a severe matrix deformation. Therefore, the sheet-like structure is a disadvantage for the graphene to achieve a good toughening effect on the SP-epoxy thermoset. 3.5. Dynamical Mechanical Properties. DMA was employed to investigate the dynamic mechanical properties of SP-epoxy/graphene nanocomposites, and the temperature dependences of storage modulus and loss factor (tan δ) for the nanocomposites with different contents of graphene are illustrated in Figure 12. As shown by Figure 12a, with the test temperature rising up, both SP-epoxy thermoset and its nanocomposites with graphene show a gradual reduction in storage modulus, followed by a sharp drop at the glass transition temperature (Tg). This is attributed to the transition of a polymer-based material from a glassy state to a rubbery state. Nevertheless, the SP-epoxy/graphene nanocomposites are found to have fairly higher dynamic storage moduli than SPepoxy thermoset in the whole range of test temperature, especially below Tg, indicating a considerably higher rigidity. This result is consistent with the tensile and flexure moduli of the nanocomposites and can be attributed to the improvement of resilience of the composites resulting from the stiffening effect of graphene and the intensive interfacial contacting effect of the matrix with graphene. Therefore, the nanocomposites achieved a continual improvement in storage modulus with increasing the graphene content. The DMA thermograms also identified the Tgs of SP-epoxy thermoset and its nanocomposites when the specimens were heated through the glass transition region as shown by Figure 12b. This characteristic temperature is located at the peak corresponding to the maximum tan δ, where sufficient vibration energy has been accumulated in molecules to rearrange the

Figure 10. Flexure strength and modulus of SP-epoxy/graphene nanocomposites as a function of the volume fraction of graphene.

mechanical parameters when 1.0 vol % of graphene was incorporated. These remarkable results clearly highlight the important improvement in mechanical performance induced by the graphene for SP-epoxy thermoset. The observed mechanical reinforcement may be related to the high specific surface area of the graphene sheets as well as their two-dimensional planar geometry.44 These features can enhance the mechanical interlocking between the SP-epoxy matrix and graphene sheets, which allows an effective stress transfer from fibres to the matrix and, thus, may enable the nanocomposites to withstand more test stress. However, the overloading of graphene is also disadvantageous for the mechanical performance of the nanocomposites, because the huge contacting interface between two phases and the aggregation of graphene make a great impact on the reinforcement effect and, thus, diminish the reinforcing capability of graphene.45,46 The effect of graphene on the fracture toughness of SPepoxy/graphene nanocomposites was also investigated in terms of the three-point bending tests. The critical stress intensity factor, K1C, was obtained from the test results and used to evaluate the fracture toughness of the nanocomposites. Figure 11 demonstrates the plots of K1C as a function of the volume

Figure 11. Critical stress intensity factor, K1C, of SP-epoxy/graphene nanocomposites as a function of the volume fraction of graphene. 10167



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chains is reduced. In general, the damping of the polymer is much greater than that of the rigid fillers. The incorporation of inorganic fillers into a polymer matrix will increase its elasticity and reduce its viscosity, and less energy will be consumed to overcome the friction forces between molecular chains.53 As a result, the loss factor of the SP-epoxy matrix decreased in the presence of graphene. 3.6. Electrical Properties. The volume electrical conductivity of SP-epoxy/graphene nanocomposites as a function of graphene content is shown in Figure 13. It is clearly observed

Figure 13. Electrical conductivity of SP-epoxy/graphene nanocomposites as a function of the volume fraction of graphene. Inset: the double-logarithmic plot of σ versus (ϕ − ϕc) with a least-squares fitting line to the experimental data of SP-epoxy/graphene nanocomposites.

that the nanocomposites have much higher electrical conductivity than the SP-epoxy thermoset, and the higher the graphene content is, the higher is the electrical conductivity. This result indicates that the graphene effectively induces a significant improvement in the electrical conductivity of the SPepoxy thermoset. It is highly noteworthy in Figure 13 that there is a an electrical percolation threshold in the range of 0.2−0.5 vol % of graphene, in which the nanocomposites exhibit a rapid transition from an electrically insulating nature to a conducting state, implying the formation of an interconnected graphene network for electrons transport throughout the polymeric matrix. It is well-known that the conductivity of a conductor− insulator composite follows the critical phenomena around the percolation threshold. According to the framework of the percolation theory,54,55 the electrical conductivity for a composite consisting of two materials can be modeled by a universal power-law expressed as follows:

Figure 12. Temperature dependence of (a) storage modulus and (b) tan δ obtained from DMA measurement for SP-epoxy thermoset and its nanocomposites with graphene.

cross-linked chains, known as a relaxation behavior. The Tg of SP-epoxy/graphene nanocomposites seems to slightly shift to higher temperatures with the addition of the graphene. In general, the increase in Tg in any polymeric system is associated with a restriction in molecular motion, a reduction in free volume, and a higher degree of cross-linking. Many references have reported the improvement of Tg when incorporating the sheet-like nanofillers into epoxy resins,49−51 so it is understandable that the graphene as a typical sheet-like filler can easily restrict the chain mobility of the SP-epoxy matrix. However, it is interesting to observe a reduction of Tg for the nanocomposites with the high contents of graphene. In this case, the high content of graphene may cause a decrease in the cross-linking density of the SP-epoxy thermosetting system and, thus, leads to a decrease of Tg. It is also noteworthy that the peak height of the SP-epoxy/graphene nanocomposites decreases considerably, indicating that the loss factor of the SP-epoxy thermoset is dramatically reduced by incorporating the graphene. It is well-known that the damping in the glass transition zone measures the imperfection in the elasticity and that much of the energy expended for the deformation of a material during DMA testing is dissipated directly into heat.52 This indicates that the molecular mobility of the SP-epoxy matrix is hindered by the graphene sheets, and thus, the mechanical loss to overcome interfriction among the molecular

σ = σf (ϕ − ϕc)β

for ϕ > ϕc

(2)

where σ is the conductivity of composite, σf is the conductivity of filler, ϕ is the volume fraction of graphene, ϕc is the critical volume fraction at the percolation threshold, and β is a scaling critical exponent. The percolation threshold ϕc can usually be separated into two states: (1) for ϕ < ϕc, only finite clusters of a conductor exist, and (2) for ϕ > ϕc, there are conducting paths between opposite edges of the lattice (infinite cluster). The critical exponent β is assumed to be universal and depends only on the dimension of the percolation system and not on the details of structures or interactions. The electrical percolation thresholds achieved with graphene-based nanocomposites have 10168



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epoxy resin due to the unique combination of phosphorus and nitrogen followed by a synergistic effect on fire resistance. It is interestingly noted that both SP-epoxy/graphene nanocomposites and DGEBA/graphene ones exhibit a gradual increase in LOI with the addition of graphene sheets. The graphene sheets dispersed in the matrix seem to act as a barrier and effectively slow the heat released and hinder the transfer of combustion gases to the flame zone and energy feedback. This effect is similar to the function of layered fillers like montmorillonite and hydrotalcite.59,60 However, the introduction of the graphene sheets did not upgrade the nonflammability classification of UL94 experiments. Differing from the cyclotriphosphazene moiety, which directly induces reactive flame retarding, the graphene sheets are eventually a kind of inert inorganic filler and cannot essentially improve the nonflammability of the epoxy matrix. In addition, it is noteworthy that the DGEBA nanocomposite containing 1.5 vol % of graphene sheets shows no flaming drips during the vertical burning experiment. Such a high loading of graphene sheets seems to increase the viscosity of char during combustion of the nanocomposite, thus preventing the molten epoxy matrix from flowing.

been widely reported. In this work, the theoretical percolation threshold and the critical exponent could be achieved by using a least-squares fit through eq 2. On the basis of the experimental data shown in Figure 13, the double-logarithmic plot of σ versus (ϕ − ϕc) and the fitting line are presented in the inset of Figure 13. It is important to note from the inset of Figure 13 that the straight line fits well with the experimental data, giving the theoretical percolation threshold ϕc = 0.39 vol % and β = 2.17. This result indicates that the 2-dimensional graphene sheets can form a conducting network at such a low content due to its homogeneous dispersion in the SP-epoxy matrix. It is highly agreeable that the wrinkled and overlapped graphene sheets can effectively link individual graphene sheets and carry a high density of current, resulting in high electrical conductivity. As proposed by Alig et al.,56 the network was composed of abundant single-layered graphene sheets as well as the thin stacks of few-layered sheets, which were bridged by the crumpled or overlapped graphene sheets. Therefore, such a conductive network can be considered as a network built by conductive graphene sheets, which are separated by local contact regions with polymer chains in-between. In the current work, because of the large specific surface area of the graphene, the area of interface between the graphene sheets and the SPepoxy matrix is huge, providing numerous tunneling sites for electron transport.57,58 3.7. Flame Retardancy. The flammability characteristics of SP-epoxy/graphene nanocomposites were investigated in light of the LOI test and UL94 vertical burning experiment, and the corresponding results were collected in Table 2, where the

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CONCLUSIONS SP-epoxy/graphene nanocomposites were prepared through thermal curing based on the successful exfoliation of graphite platelets. SEM, HR-TEM, Raman spectroscopy, and XPS spectroscopy confirmed this chemical exfoliation and following thermal reduction for the graphene. These graphene sheets with nanoscale thickness also achieved a homogeneous dispersion in the SP-epoxy matrix. The presence of graphene increased both the tensile and flexural properties of the nanocomposites as a result of the great surface area of graphene sheets in contact with the matrix; however, the graphene showed little toughening effect on SP-epoxy resin. The studies of thermal and dynamic mechanical properties demonstrated that the incorporation of graphene led to a considerable increase in Tg values, thermal decomposition temperatures, and storage moduli of the nanocomposites. It is highly emphasized that the SP-epoxy/graphene nanocomposites also showed a low percolation threshold of 0.375 vol % and achieved high electrical conductivity at a volume fraction of graphene higher than 0.75 vol %. These results highlight the possibility to achieve advanced epoxy/graphene nanocomposites with outstanding mechanical and electrical performance.

Table 2. The LOI Values and UL94 Vertical Burning Results of Graphene-Based Nanocomposites with SP-Epoxy Resin and DGEBA UL94 vertical burning result nanocomposite sample DGEBA thermoset SP-epoxy thermoset DGEBA/0.25 vol % graphene SP-epoxy/0.25 vol % graphene DGEBA/0.5 vol % graphene SP-epoxy/0.5 vol % graphene DGEBA/1.0 vol % graphene SP-epoxy/1.0 vol % graphene DGEBA/1.5 vol % graphene SP-epoxy/1.5 vol % graphene

LOI (vol %) classification 21.64 31.58 22.47 33.82 22.69 34.26 24.05 34.94 25.17 34.61

HB V-0 HB V-0 HB V-0 HB V-0 HB V-0

flaming drips yes none yes none yes none yes none none none

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 6441 0145. Fax: +86 10 6442 1693. E-mail: [email protected].

flame-retardant testing results of conventional DGEBA/ graphene nanocomposites were also attached as references. It is evidently observed that the SP-epoxy thermoset shows a very high LOI of 31.58 vol %, which means that it should be nonflammable in air. Furthermore, the SP-epoxy thermoset achieved a V-0 classification in UL-94 vertical experiments, and its burned residue did not fall off during vertical burning as a result of the good structural stability. This is in good agreement with the results we reported previously.36 However, the DGEBA thermoset as a control only obtained a low LOI of 21.64 vol % and just achieved a HB classification, indicating its highly flammable characteristic in nature. These results confirm that the incorporation of spiro-cyclotriphosphazene units into backbone can impart an excellent flame-retardant property to

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (Project Grant No.: 50973005 and 51173010) is gratefully acknowledged.

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REFERENCES

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Fabrication of Spirocyclic Phosphazene Epoxy-Based

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