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Preparation and characterization of poly(methyl methacrylate)-intercalated graphite oxide/poly(methyl methacrylate) nanocomposite.

INTRODUCTION

Conducting polymer composites have been extensively studied because of their potential applications in batteries, antistatic, electromagnetic shielding, electrorheological fluids, and other applications (1-8). Since natural graphite (NG) has high electrical conductivity ([approximately][10.sup.4] S/cm at room temperature (9)), it has been considered an ideal candidate for manufacturing conductive polymer composites. When the composite is prepared by blending the polymer and NG, its conductivity depends greatly on the loading of graphite, but poor mechanical properties were obtained at loadings of 15-20 wt% (10).

Like silicates such as montmorillonite, which is widely used in the preparation of nanocomposites (11-13), NG is also of a layered nanostructure. Carbon atoms on each graphite layer plane are tightly held by covalent bonds, while those positioned in adjacent planes are bound by weak van der Waals forces. However, it is difficult to intercalate monomers or polymers into interlayer spaces of graphite because of graphite's high crystal lattice energy. Thus, several methods for intercalating monomers or polymers into graphite have been developed. The first step of these methods is the preparation of graphite oxide (GO) by oxidation of NG (14, 15). Since graphite oxide has larger c-axis spacing and polar groups, such as hydroxyl, ether and carboxylate groups, on its surface, intercalation of water-soluble polymers into GO becomes possible. For example, the GO particles are simply added into the aqueous solution of water-soluble polymers, such as poly(vinyl alcohol) (PVA) (16), poly(ethylene oxide) (PEO) (17, 18), poly(diallyl dimethyl ammonium chloride) (19), and poly(furfuryl alcohol) (PFA) (20), forming the polymer-intercalated GO/polymer composites with different c-axis repeating distances. For hydrophobic polymers, such as poly(vinyl acetate) (PVAc), the PVAc/intercalated GO composite was prepared by in-situ polymerization, in which an n-octanol (or dodecanol)-intercalated GO was first formed; vinyl acetate monomer was then absorbed into interlayer of GO, followed by thermally polymerization. Thermal stability of this nanocomposite was higher than that of pure PVAc, and its electrical conductivity was 0.14 S/cm at room temperature (20).

When expanded graphite (EG) is used to prepare polymer/EG composites, the galleries of EG can be easily filled with suitable monomer molecules and/or catalysts through physical absorption because of the porous feature of EG, and polar interaction between monomers and polar groups on EG sheets (10, 21, 22). A poly(methyl methacrylate) (PMMA)/EG composite was prepared by this method (23). However, the intercalation of MMA into the interlayer spaces of GO and the following in-situ polymerization have not been reported. MMA is a polar monomer, and may be intercalated into GO by the polar interaction between MMA molecules and polar groups on the surface of layered sheets. In this paper, we report the preparation of poly(methyl methacrylate)/intercalated GO composite by emulsion polymerization of MMA in the presence of GO. Various characterization techniques were used to characterize the structure and properties of the composite.

EXPERIMENT

Materials

Natural graphite (NG) powder with average particle diameters of < 30 [micro]m was purchased from Shanghai Colloid Chemical Factory. Methyl methacrylate (MMA) (The First Shanghai Chemical Reagent Plant) was washed with an aqueous solution of 5% sodium hydroxide, dried over anhydrous sodium sulfate, and then distilled under reduced pressure. Potassium persulfate, [H.sub.2][O.sub.2] aqueous solution (30%), anhydrous sodium sulfite, and KMn[O.sub.4] were purchased from The First Shanghai Chemical Reagent Plant; all were of analytical grade and used without further purification.

Preparation of GO

KMn[O.sub.4] (15 g) was gradually added into a suspension solution of NG powder (10 g) in concentrated sulfuric acid (110 mL) at below 20[degrees]C. After the addition was completed, the reaction mixture was stirred at 35[degrees]C [+ or -] 3[degrees]C for 30 min. As the distilled water (230 mL) was slowly added, the temperature of the reaction mixture rose to 90[degrees]C. After holding at 90[degrees]C for an additional 15 min, a large amount of distilled water (700 mL) and 30% [H.sub.2][O.sub.2] solution (50 mL) were added to stop the oxidization. The oxidation product, graphite oxide (GO), was filtered, washed successively with 5% HCl aqueous solution three times, and then with distilled water until neutralization, and dried in a vacuum oven at 50[degrees]C for 24 h.

Emulsion Polymerization

Into a 250-mL three-neck flask, 1.0 g of dried graphite oxide and 40 mL of MMA were added. When the mixture was stirred at room temperature for 10 h, 150 mL of distilled water and an emulsifier OP-10 (3 mL) were added, and then mechanical stirring was continued for another 10 h. Then, a solution of potassium persulfate (0.1 g) and anhydrous sodium sulfite (0.1 g) in 5 mL of distilled water were added to the emulsion system; the polymerization of MMA was carried out at 63[degrees]C-67[degrees]C for 15 h. The polymerization product, a PMMA/intercalated GO composite, was precipitated, separated by filtration, washed with distilled water, and then dried in a vacuum oven at 50[degrees]C. The conversion was 62%, which was calculated according to Eq 1:

Conversion (%) = [([W.sub.C] - [W.sub.G]/[W.sub.M]] X 100% (1)

where [W.sub.C], [W.sub.G] and [W.sub.M] are the weights of the product, GO, and added MMA, respectively.

Characterizations

Elementary analysis of the samples was performed on a Vario EL III Elementary Analyzer. X-ray diffraction (XRD) patterns of GO and PMMA/GO composites were measured on a Y-4Q (China) X-ray diffractometer (Cu-K[alpha] radiation: [lambda] = 1.54178 [Angstrom]). Fourier transform infrared (FT-IR) spectra of the materials were recorded on a Vector-22 FT-IR Spectrophotometer using a KBr Pellet. Transmission electron microscope (TEM) images of the PMMA/intercalated GO composites and the PMMA/NG composite (a blending of PMMA and NG) were measured on a JEM-100SX microscope. Ultrathin samples, around 20 nm thick, were obtained by a Leica ultra-microtone with a diamond knife.

The testing sheets for less than [10.sup.6] [ohm] resistance were prepared as follows. The composite (1.2 g) dispersed in 2 mL THF was sheeted on a glass slide (100 X 15 m[m.sup.2]) with four copper electrodes, and then dried. Their resistances were measured by a four-probe method on digital multimeter DT9208. When the resistance of a sample was higher than [10.sup.6] [ohm], the measurement of the resistance at 30[degrees]C was carried out by a four-probe method on a Super-High Resistance Instrument ZC-36 ([10.sup.17]). Notched Izod impact strength was measured according to the Chinese testing standard GB-1040-93. The tensile strength was tested according to the Chinese testing standard GB-1843-80.

RESUL TS AND DISCUSSION

It is well known that the mechanical properties of a composite are greatly affected by the properties of each component and the interaction between two phases in the composite. Obviously, the degree of oxidation and the surface structure of GO are important factors for the intercalation of a polymer and improving the compatibility of GO and the polymer. As reported by Lerf, deep oxidation would destroy the crystal structure of NG (15), resulting in a nearly amorphous GO with large reduction of electrical conductivity. Thus, excessive oxidation must be avoided. However, the oxidation of NG is a necessary process because it will modify the properties on the surface of layers in the graphite, which will help monomer molecules insert into graphite interlayers. Obviously, an optimum degree of oxidation plays an important role in the preparation of polymer/graphite composites with good electrical properties. Therefore, it is necessary to study the composition and the structure of GO.

The Structure of GO

The results of the elementary analysis of the dried GO listed in Table 1 demonstrate that the atomic ratio of carbon to oxygen is approximately 15:2. Compared with the ideal GO compound. [C.sub.8][O.sub.2](OH)[.sub.2], its oxidation degree is low (24), but it is much higher than that of EG (the atomic ratio was 155:1) reported by Wang and Pan (25).

The existence of oxygen in GO must result from the oxidation of graphite, which might be present as C-O-C and C-OH groups. It was reported that carboxyl groups may exist on the surface of GO particles (25). In order to confirm whether these functional groups existed in the GOs, their FT-IR spectra were measured, and Fig. 1b represents a typical IR spectrum of GO. The stretching vibrations of hydroxyl (-OH) are clearly seen at 3242 and 1395 c[m.sup.-1]. The peaks at 1226 and 1150 c[m.sup.-1] indicate the existence of oxygen-containing functional groups (C-O-C and/or C-OH). We can see an absorption peak at v = 1716 c[m.sup.-1], verifying the presence of a carboxylic group in GO.

As was mentioned, the oxidation reaction of graphite will affect its crystal structure. Thus, its XRD spectrum was measured, and Fig. 2 represents a typical XRD pattern. Two strong peaks at 2 [theta] = 8.89[degrees] and 26.6[degrees] were observed. A peak at 2 [theta] = 26.6[degrees] is a peculiar peak of NG, indicating that there are regions with an intact crystal lattice in GO particles. Another peak at 2 [theta] = 8.89[degrees] corresponds to a c-axis spacing ([I.sub.c]) of 9.1 [Angstrom], due to the intercalations of sulfuric acid and water into carbon interlayers of GO. During the oxidation of graphite by KMn[O.sub.4] in concentrated sulfuric acid, the reaction between graphite and sulfuric acid formed graphite-intercalated compound C * nHS[O.sub.4.sup.-]. After treatment with a large amount of water, it became C * n[H.sub.2]O as shown in Scheme 1.

Structure of PMMA/Intercalated GO Composite

For preparation of PMMA-intercalated GO, the first step is the intercalation of MMA into GO by adding GO into MMA while stirring fully. Then in-situ emulsion polymerization was carried out at 63[degrees]C-67[degrees]C in water using OP-10 as emulsifier and [K.sub.2][S.sub.2][O.sub.8]-[Na.sub.2]S[O.sub.3] as initiator. A black solid polymer-intercalated GO was obtained.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

C + n[H.sub.2]S[O.sub.4] [right arrow] C * nHS[O.sub.4.sup.-] + nH

C * nHS[O.sub.4.sup.-] [right arrow] C * m[H.sub.2]O + nHS[O.sub.4.sup.-]

Scheme 1.

For characterizing their structures, their IR spectra were measured. A typical IR spectrum is shown in Fig. 1a. Upon comparison with Fig. 1b of GO, we find the characteristic peaks of PMMA at v = 2997; 2949 (C-H); 1731 (C = O); 1445 (C[H.sub.3]-O); 1388 ([alpha]-C[H.sub.3]); 1149 [cm.sup.-1] (C-C), which indicates that the PMMA-intercalated GO (PMMA/GO) was really formed.

To confirm whether grafting polymerization on GO took place. 0.500 g of PMMA/GO containing 0.024 g of GO was extracted until a constant weight was reached. The 0.026 g of residual product obtained is heavier than the GO used, indicating that grafting polymerization of MMA on GO occurred. This can be further verified by their IR spectra. Figure 3 shows the IR spectra of the PMMA/GO before and after extraction. When comparing the IR spectrum of PMMA/GO before extraction in Fig. 3a, the characteristic peaks of PMMA at v = 2997; 2949 (C-H); 1731 (C=O); 1445 (C[H.sub.3]-O); 1388 ([alpha]-C[H.sub.3]) in PMMA/GO extracted are greatly weakened, but they do not disappear completely in Fig. 3b. Thus, we can conclude that some of MMA underwent grafting polymerization.

When the intercalation of MMA and in-situ polymerization occur, the structure of GO must be changed. The XRD patterns of PMMA/GO were measured, and Fig. 4 represents a typical XRD pattern. Compared with the XRD pattern of GO in Fig. 4a, the two strong peaks at 2 [theta] = 8.89[degrees] and 2 [theta] = 26.6[degrees] almost disappear in Fig. 4b, indicating that the crystal lattice of GO was destroyed during the polymerization of MMA. This must result from the intercalation of MMA and further in-situ polymerization. Therefore, PMMA-intercalated GO was produced.

For understanding the distribution of GO particles in a PMMA/GO composite, its TEM photo was taken (Fig. 5a). For comparison, the PMMA/NG composite is also shown in Fig. 5b. From these two figures, we can find that NG in flake shape exists in the composite (Fig. 5b), but spherical GO particles distribute homogeneously in the composite, which is attributed to the surface modification of GO. The shape difference between GO and NG particles is due to oxidation, intercalation of MMA and in-situ emulsion polymerization.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Mechanical Properties

Figure 6 shows the variation of notched Izod impact strength with GO content of the PMMA/PMMA-intercalated GO composites; the notched Izod impact strength of the composite decreases as the content of GO increases from 1 to 8 wt%. A sharp decrease is observed for the increase of GO content from 1 to 2 wt%. When the content is more than 2 wt%, the decrease slows down. It is different from the phenomenon observed in the EG-g-PSt/EG composites, where the notched Izod impact strength increased with the content increase of EG (26). This difference is attributed to the compatibility difference of the EG-g-PSt and EG with the PMMA and GO. In PMMA-intercalated GO, PMMA grafted on GO is small, and the surface properties of GO are less improved.

[FIGURE 5 OMITTED]

Figure 7 shows the effect of the GO content on the tensile strength of PMMA/GO composite. The same phenomenon observed in the measurement of the notched Izod impact strength was observed. The reason must be related to less compatibility of GO with PMMA.

Conductivity

The structure and properties of PMMA/intercalated GO composites are greatly different from NG or EG/polymer composites. Thus, their conductivities were studied. The variation of volume electrical conductivity with GO content is shown in Fig. 8.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The addition of GO into PMMA will enhance the volume electrical conductivity of PMMA greatly. As shown in Fig. 8, the conductivity increases from [10.sup.-16] S [cm.sup.-1] of pure PMMA to [10.sup.-4] S [cm.sup.-1] of PMMA/intercalated GO composite at GO content of 8.0 wt%. The composite with less than 2 wt% GO content is an insulator. When the content of GO increases, the conductivity of the composite increases rapidly, and then levels off gradually above 6 wt% of GO content. The percolation threshold value is about 2 wt% of GO.

The addition of GO induced the conductivity increase of the composite because GO has better conductivity than PMMA. The grape-cluster-like structure of GO particles is easier to form the conducting net as shown in Fig. 5a, which can be applied to explain the higher volume conductivity of PMMA/intercalated GO composites.

CONCLUSION

The oxidation of natural graphite by KMn[O.sub.4] in concentrated sulfuric acid solution produced graphite oxide (GO) with hydroxyl, ether and carboxyl groups, and the interlayer spacing of GO in c-axis ([I.sub.c]) increased to 9.1 [Angstrom] because of the intercalation of [H.sub.2]O.

The intercalation of MMA and in-situ polymerization occurred during the emulsion polymerization of MMA in the presence of GO; PMMA-intercalated GO/PMMA composites were produced. During the emulsion polymerization, the grafting polymerization of a small amount of MMA took place. TEM showed that the spherical particles of GO are homogeneously distributed in the composites.

Both the notched Izod impact strength and the tensile strength of PMMA/intercalated GO composites decreased, and the conductivity of the composite increased with the increase of the GO content in the composite. The percolation threshold value is about 2 wt% of GO.

[FIGURE 8 OMITTED]
Table 1. The Content of Carbon and Oxygen in GO. (a)

Element Weight% Atomic% Atomic Ratio

 C 84.68 88.17 15:2
 O 15.32 11.83

(a) Preparation conditions: See Experimental section.


ACKNOWLEDGMENT

This work was supported by a special grant of nanomaterial of Anhui province.

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WANG WEN-PING (1,2) and PAN CAI-YUAN (1*)

(1) Department of Polymer Science and Engineering

University of Science and Technology of China

Hefei, 230026, Anhui, P.R. China

(2) Department of Chemical Engineering

Hefei University of Technology

Hefei, 230009, Anhui, P.R. China

*To whom correspondence should be addressed. E-mail: pcy@ustc.edu.cn
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Date:Dec 1, 2004
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