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Poly(methyl methacrylate)/Graphene Oxide Nanocomposites by a Precipitation Polymerization Process and Their Dielectric and Rheological Characterization Jean-Michel Thomassin,*,† Milana Trifkovic,‡ Walid Alkarmo,† Christophe Detrembleur,† Christine Jérôme,† and Christopher Macosko§ †

Center for Education and Research on Macromolecules (CERM), University of Liege, Sart-Tilman, B6, 4000 Liege, Belgium Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada § Chemical Engineering and Material Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡

ABSTRACT: We report a method for achieving controlled dispersion of graphene oxide (GO) in poly(methyl methacrylate) (PMMA) via the precipitation polymerization process in a water/ methanol mixture. GO acts as a surfactant and adsorbs on the interface between polymerized PMMA particles and solvent mixture. Scanning electron and transmission electron microscopy confirmed that the precipitate consists of polymer particles (99%, Aldrich), α,α′azoisobutyronitrile (AIBN) (Fluka), hydrazine monohydrate (Aldrich), and methanol (Aldrich) were used as received. Graphene oxide solution (GO; N002-PS: 0.5 wt % in water, thickness 1.0−1.2 nm, x−y dimension ∼100 nm) was purchased from Angstron Materials. Nanocomposite Preparation. In a typical experiment, 14 mL of methanol was mixed with 6 mL of an aqueous solution of GO (0.1−0.5 wt %). 4 mL of MMA and AIBN (0.132 g) were then added to the solution. The solution was bubbled with nitrogen for 2 min and then placed in an oil bath at 60 °C. After 1−1.5 h, the coloration turned from black to brown with the appearance of the polymer particles. The powder was then recovered by filtration and dried under vacuum. A nearly complete conversion of the MMA was achieved in all cases. For the preparation of chemically reduced samples, 2 mL of hydrazine was added at the end of the polymerization, and the solution was kept at 60 °C for 3 h. The powder was then compression molded for TEM, electrical, and rheological characterization. Characterization. The morphology of the polymer particles was characterized by scanning electron microscopy (SEM; JEOL JSM 840A) after metallization with Pt (30 nm). The particle size was determined by manually measuring the diameter of at least 50 particles on five different SEM images. The compression-molded samples were characterized with a transmission electron microscope (TEM; PHILIPS M100) at an accelerating voltage of 100 kV. Thin sections (90 nm) were prepared by ultramicrotomy (ULTRACUT E from REICHERTJUNG) at room temperature. Micrographs were analyzed by using the Megaview GII (Olympus) software. Electrical conductivities of PMMA/GO samples were measured by the method of volume resistivity with a Keithley 617 programmable electrometer. The resistivity is calculated from the geometry of the electrodes and the thickness of the sample.46 To ensure the good contact between electrodes and samples, two copper electrodes (purchased from FILSSYNFLEX Thernko 300H DrADEN, d = 0.3 mm, nominal resistance 0 ohm at 20 °C) were attached to sample surfaces by silver paint (AGAR Scientific G3649 Electrodag 1415). Differential scanning calorimetry (DSC) measurements (Q100, TA Instruments) were recorded at 10 °C/min using a heating/cooling/ heating cycle from −80 to 210 °C. Thermogravimetric analysis (TGA) (Q500, TA Instruments) was performed at 10 °C/min heating rate.

Figure 1. (a) MMA/GO solution before polymerization, (b) PMMA/ GO at the end of polymerization, and (c) PMMA/CNT at the end of polymerization.

the PMMA was polymerized using the same experimental procedure, only in the presence of CNTs instead of GO, resulted in a completely different precipitation mechanism. The resulting precipitate was in the form of big black particles (Figure 1c) instead of uniform PMMA particles coated with GO.47 In the present study, GO sheets are able to stabilize the polymer particles during their formation and prevent macroscopic phase separation. Eventhough CNTs and GO have different shape, the difference in precipitation mechanism can be mainly attributed to amphiphilic properties of the GO sheets which drives them to the interface of the polar solvent mixture and PMMA. Liao et al. have 2150

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TEM was used to examine the GO sheet dispersion. The polymer powder was melt-compressed at 210 °C for 30 s into disks which were then ultramicrotomed to prepare thin sections suitable for TEM imaging. Figure 4 shows that the GO sheets are

shown that during polymerization some PMMA chains appear to covalently bond to GO.48,49 These PMMA grafts are favorable in preventing removal of the GO sheets from the surface of the PMMA particles during subsequent rinsing and compression molding. CNTs do not have any hydrophilic groups at their surface which results in their localization in PMMA phase during polymerization and subsequent macrophase separation during the precipitation process. The presence of GO-coated PMMA particles was confirmed by TEM and SEM observations at different GO content. After filtration and drying of the powder, small polymer particles can clearly be observed at GO contents larger than 0.3 wt % (Figure 2b−d). At lower concentration of GO, polymer particles were

Figure 2. SEM micrographs of PMMA/GO: (a) 0.2, (b) 0.35, (c) 0.7, and (d) 2.4 wt % GO.

Figure 4. TEM micrographs of PMMA/GO samples after 30 s compression molding: (a, b) 0.7 wt % GO and (c, d) 2.4 wt %. (e) TEM micrographs of PMMA/CNT samples, 1 wt % CNT, prepared by the same method.

much bigger with much broader size distribution (Figure 2a). Figure 3 shows that the average particle size decreases with the GO content which confirms that the sheets stabilize the formation of small particles. At content lower than 0.3 wt %, the particle size goes to infinity.

dispersed in a regular pattern within the polymer matrix. The average size of the polymer region surrounded by the GO sheets closely matches the diameter of the particles observed by SEM, which confirms that polymer particles were coated by GO sheets

Figure 3. Average size of PMMA particles (measured from SEM micrographs) and calculated GO coating thickness (from eq 3) as a function of GO content. 2151

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Figure 5. (a) DSC thermograms for the first heating cycle of PMMA/GO at different GO content. (b) Comparison of the first and second heating cycle of PMMA/0.97 wt % GO.

polymer. Reaggregation of the graphene sheets during the reduction process should then be limited due to the high viscosity of the polymer matrix. Ye et al. have shown that the optimal temperature for the main reduction step depends on the polymer’s polarity.9 Because of its ability to interact with GO sheets via its CO bonds, PMMA has a much lower reduction temperature (around 140 °C) in comparison to apolar polymers (180 °C). These observations were confirmed by DSC measurements on our samples, which showed an exothermic peak around 130 °C, proportional to the amount of GO dispersed in the polymer matrix (Figure 5a). This peak can be clearly assigned to the partial reduction of the GO sheets. When a second heating cycle is performed on the PMMA/GO nanocomposite, the reduction peak almost completely disappeared (Figure 5b). The first heating cycle (10 °C/min to 210 °C) is then already enough to almost completely reduce the GO sheets. In order to determine the efficiency of the thermal reduction, the GO sheets were also chemically reduced by addition of hydrazine at the end of the polymerization step. PMMA precipitated into particles very similar to those shown in Figure 2 and after compression molding very similar to Figure 4b,d, demonstrating that the addition of hydrazine does not alter the dispersion of the GO sheets within PMMA. Figure 6 shows the comparison of the electrical conductivity of the PMMA/GO nanocomposites thermally and chemically treated to reduce the GO sheets to graphene. Note that when the PMMA/GO nanocomposites were prepared by compression of the powder at room temperature, very low electrical conductivities were observed (1.20 × 10−3 S/ m was achieved at a GO content of only 0.2 wt %. Parallel dielectric and rheological characterization showed that the main increase in electrical conductivity occurred during the first minutes of the thermal treatment but continued for about 30 min. The absence of the dramatic change in the storage confirmed that the increase in conductivity was not due to change in particle dispersion.

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Figure 9. Glass transition temperature as a function of GO content. The value at 0 wt % corresponds to the average value of the PMMA extracted from the different PMMA/GO samples (error ±1 °C).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-M.T.). Notes

appears to be determined by the amount of interfacial area that can be covered by 2−4 GO sheets. The persistence of this GO

The authors declare no competing financial interest. 2154

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ACKNOWLEDGMENTS The authors thank the Fonds National de la Recherche Scientifique (F.R.S.-F.N.R.S., Belgium) and the University of Liege for financial support. The authors also thank David Giles and Lucas McIntosh for technical support.

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