Preparation of Polymer Particles Containing Reduced Graphene

Article pubs.acs.org/Macromolecules

Preparation of Polymer Particles Containing Reduced Graphene Oxide Nanosheets Using Ionic Liquid Monomer Masayoshi Tokuda,†,‡ Stuart C. Thickett,†,§ Hideto Minami,*,‡ and Per B. Zetterlund*,† †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia ‡ Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan § School of Physical Sciences (Chemistry), The University of Tasmania, Sandy Bay, TAS 7005, Australia S Supporting Information *

ABSTRACT: The preparation of polymer nanoparticles containing reduced graphene oxide nanosheets (rGO) within their interior has been conducted by radical polymerization in aqueous miniemulsion employing the nonionic surfactant Tween 80. Polymerizations were conducted using the ionic liquid monomer 1-vinyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([Veim][TFSA]) as well as mixtures of this monomer with ethyl methacrylate (EMA). [Veim][TFSA] plays an important role in that it provides stabilization of rGO monomer dispersions, presumably via π−π interactions between rGO and [Veim][TFSA]. If the EMA: [Veim][TFSA] ratio is too high, rGO precipitates during preparation of the monomer phase of the miniemulsion. Interestingly, it was demonstrated that addition of a small amount of the homopolymer of [Veim][TFSA] leads to significantly improved rGO stability (more so than the effect of [Veim][TFSA] monomer), thus enabling preparation of polymer nanoparticles containing higher amounts of EMA.

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INTRODUCTION Graphene is an allotrope of carbon comprising nanosheets with thickness of one atomic layer. It possesses various remarkable properties, including exceptional electrical conductivity, optical, thermal, and mechanical properties.1−6 Graphene has been prepared by various methods such as mechanical exfoliation of graphite utilizing adhesive tape,1,7 liquid-phase exfoliation,8−11 chemical vapor deposition (CVD) on a copper substrate,12−14 and intercalation methods with Bronsted acids.15 A common synthetic method is chemical reduction of exfoliated graphite oxide16,17 because of its high productivity compared to other approaches. Graphene oxide (GO) is the oxidized form of exfoliated sheets from graphite, which is prepared by the oxidation of graphite in the presence of concentrated acids and strong oxidizing agents. GO possesses carboxylic acid groups at the edges with basal plane epoxide and hydroxyl groups, giving the material high dispersibility in water.18−22 However, GO exhibits the properties of an insulator due to the existence of sp3-hybridized carbon atoms. When GO sheets are treated with reducing agents such as hydrazine,17 reduced graphene oxide (rGO) sheets are formed, which possess greater electrical conductivity and similar properties to those of graphene sheets. Graphene and rGO have attracted much attention as functional materials in various fields due to the aforementioned properties.23−27 In order to obtain the maximum benefit from the attractive properties of graphene, it should be dispersed in a © XXXX American Chemical Society

matrix to generate functional materials. Nevertheless, it is difficult to effectively disperse graphene due to the formation of irreversible aggregates via van der Waals interactions between graphene sheets. In order to prepare rGO dispersions, various strategies have been reported. In 2006, the preparation of a stable rGO dispersion in water by the reduction of GO modified by poly(sodium 4-styrenesulfonate) was reported by Ruoff and co-workers.28 The preparation of aqueous dispersions of rGO by reduction of GO functionalized with pyrenebutyric acid utilizing π−π interactions has also been demonstrated.29 Dispersions of rGO have also has been prepared in various organic solvents in the presence of surfactants and polymeric stabilizers.30,31 Ionic liquids, which are composed of organic cations and anions, are salts that are in the liquid state at room temperature.32−34 They have attracted attention as environmentally friendly media in various synthetic fields as well as for functional materials such as electrolytes, membranes for CO2 separation, and dispersants of carbon nanomaterials.35−44 The Liu group reported a stable rGO dispersion in an imidazoliumbased ionic liquid, which features π−π interactions between imidazolium rings and rGO sheets, in the absence of Received: October 7, 2015 Revised: January 20, 2016

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DOI: 10.1021/acs.macromol.5b02216 Macromolecules XXXX, XXX, XXX−XXX

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purged with nitrogen for 5 min. Solution polymerization was carried out at 60 °C for 24 h. The product was precipitated in water/ethanol (w/w = 50/50) to remove residual monomer and subsequently dried under vacuum at room temperature.

dispersants, and prepared polymer/rGO composites using this dispersion.45 The obtained composite polymeric materials exhibited enhanced mechanical properties compared to the original polymer. Recently, the utilization of poly(ionic liquid)s as dispersants of rGO and graphene sheets has also been reported by some groups.46 Suh and co-workers prepared graphene sheets modified by a poly(ionic liquid) and subsequently demonstrated the graphene sheets can be transferred between the water phase and oil phase by anion exchange of the poly(ionic liquid) with the dispersion retaining its stability at all times.47 Moreover, some groups have reported that nanocomposite materials containing rGO have shown an enhancement of conductivity.2,44,48 Although poly(ionic liquid)/rGO composite materials have been prepared in the bulk state, there are to date no reports on poly(ionic liquid)/ rGO composites in the (nano)particle state. GO sheets further possess the ability to act as “surfactants” in mixtures of hydrophobic liquids and water due to their amphiphilic nature (carboxylic acid groups at the sheet periphery and a hydrophobic basal plane (epoxy and hydroxyl groups)).49,50 This characteristic has led to the development of emulsion-based approaches toward composite polymeric materials based on GO.51−61 For example, Etmimi et al.51,52 reported the preparation of nanocomposite particles containing GO by miniemulsion polymerization using GO modified with reactive surfactant and reversible addition−fragmentation chain transfer (RAFT) agent. Our group recently pioneered the use of nanosized and uniform GO as sole surfactant (no use of conventional surfactants as in previous reports) in miniemulsion polymerization systems for synthesis of polymeric nanoparticles “armoured” with GO nanosheets.55,56,58,59,61 Nanocomposite materials containing GO can be considered precursors to rGO/polymer composite materials. However, given that GO fulfills the role of surfactant in GO-plated nanoparticles, reduction of GO to rGO at a nanoparticle interface in an emulsion may lead to compromised colloidal stability.55,61 In the present work, poly(ionic liquid)/rGO composite particles have been prepared by miniemulsion polymerization of monomer droplets containing rGO. rGO was prepared by reduction of GO in an initial separate step, and an ionic liquid monomer possessing an imidazolium ring was used because of effective rGO stabilization by π−π interactions as mentioned above. Overall, this approach offers an attractive synthetic route to conductive nanocomposite particles with potential applications in the field of electrochemistry.

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Scheme 1. Structure of Ionic Liquid Monomer

Preparation of Polymer Particles Containing RGO by Miniemulsion Polymerization. rGO was added to a mixture of [Veim][TFSA] and EMA (1.0 g; [Veim[TFSA]/EMA = 100/0, 90/ 10, 75/25, 50/50, 25/75, and 10/90, w/w] followed by ultrasonication for 5 min to prepare the rGO-dispersed monomer. AIBN (40 mg) was subsequently dissolved in the oil phase, which was then mixed with 1 wt % Tween 80 aqueous solution (10 g), and emulsified using ultrasonication at 50% amplitude for 10 min. The obtained emulsion was transferred to a round-bottom Schlenk flask, sealed off with a silicon rubber septum, and purged with nitrogen for 5 min. Miniemulsion polymerization was carried out at 60 °C for 24 h with magnetic stirring at 240 rpm. Preparation of Polymer Particles Containing RGO by Miniemulsion Polymerization Using [Veim][TFSA]/EMA Mixtures Containing Dissolved Poly([Veim][TFSA]). Poly([Veim][TFSA]) homopolymer (0.5 and 2.5 wt % relative to monomer) was dissolved in [Veim][TFSA]/EMA mixtures (w/w = 50/50, 25/75, 10/ 90) at room temperature for 24 h. rGO (1.0 mg) was added to the mixture (1.0 g) and ultrasonicated for 10 min to prepare rGOdispersed monomer. AIBN (40 mg) was subsequently dissolved in the oil phase, which was then mixed with 1 wt % Tween 80 aqueous solution (10 g), and emulsified using ultrasonication at 50% amplitude for 10 min. The obtained emulsion was then transferred to a roundbottom Schlenk flask, sealed off with a silicon rubber septum, and purged with nitrogen for 5 min. Miniemulsion polymerization was carried out at 60 °C for 24 h at 240 rpm. Characterization. Hydrodynamic diameters were measured using a Malvern ZetaSizer Nanoseries instrument with DTS software, operating a 4 mW He−Ne laser at 633 nm at an angle of 173° at 25 °C. Reported values are based on the average of five separate measurements. FTIR spectra were recorded using a Bruker IFS66/S instrument in attenuated total reflectance (ATR) mode, using an average of 64 scans recorded over the wavenumber range 500−4000 cm−1 at a resolution of 4 cm−1. TEM images were obtained using a JEOL1400 transmission electron microscope at an accelerating voltage of 100 kV. The specimens were prepared by casting a drop of diluted aqueous polymerized miniemulsion onto a Formvar-coated copper grid followed by drying at room temperature. X-ray photoelectron spectra (XPS) were recorded using a Kratos Axis ULTRA XPS using monochromatic Al X-rays (1486.6 eV) at 225 W (15 kV, 15 mA). Survey scans were carried out over 1360−0 eV binding energy range with 1 eV steps and a dwell time of 100 ms; high-resolution scans were run with 0.2 eV steps and a dwell time of 250 ms. The 1H NMR measurements were carried out with a NMReady-60e (60 MHz, Nanalysis Corporation) spectrometer at room temperature in acetoned 6.

EXPERIMENTAL SECTION

Materials. Ethyl methacrylate (EMA, 99%, Aldrich) was purified by passing through a column of activated basic aluminum oxide (Ajax) to remove the inhibitor. 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich) was purified by recrystallization in methanol. Hydrazine (SigmaAldrich), ammonia solution (28 wt %), graphene nanofibers (Catalytic Materials Ltd., >98%), HCl (Ajax, 32 wt %), H2SO4 (Ajax, 98%), H3PO4 (BDH Chemical), KMnO4 (Ajax), H2O2 (Ajax, 30 wt %), 1ethyl bromide, 1-vinylimidazole (Nakalai Tesque Inc., Kyoto, Japan), and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) (99.7%, Kanto Chemical Co., Inc., Japan) were used as received. Deionized water was used in all experiments. Preparation of Poly([Veim][TFSA]) by Solution Polymerization. 1-Vinyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([Veim][TFSA]) (5.0 g) and AIBN (0.5 g, 10 wt % based on monomer) were dissolved in acetone (10 g), transferred to a roundbottom Schlenk flask, sealed off with a silicon rubber septum, and

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RESULTS AND DISCUSSION Preparation of GO and RGO. GO sheets were prepared from graphite nanofibers of diameter ∼100 nm using a modified Hummer’s method as described previously.62 The size of the obtained GO sheets was approximately 100 nm based on DLS (DLS theory is based on spherical objects and as such this estimate is semiquantitative). XPS analysis resulted in a C:O atomic ratio of 1.85 (Figure 1a), consistent with previous work.55 After reduction of GO to reduced GO (rGO) using B

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Figure 3. (a) Visual appearances of rGO dispersion in [Veim][TFSA] and (b) TEM photograph of rGO dispersion after acetone treatment on TEM grid.

estimate for GO above. The ultrasonication step involved in the rGO synthesis as well as the ultrasonication performed to disperse rGO in [Veim][TFSA] are likely to have led to a reduction in sheet size. Figure 4 shows various stages of the synthetic route to poly([Veim][TFSA])/rGO composite particles via miniemulsion polymerization. When the dispersion of rGO in the ionic liquid monomer was added to a 1 wt % Tween 80 aqueous solution, the monomer phase formed the lower layer due to the high density of the ionic liquid, and rGO dispersed in the monomer phase remained stable. After ultrasonication, a milkygray emulsion was obtained without rGO precipitation (Figure 4b). Miniemulsion polymerization using AIBN as initiator was conducted at 60 °C for 24 h, resulting in a stable emulsion without any coagulation. The color of the emulsion after polymerization was light gray, in contrast to the typical white color of an emulsion prepared by emulsifier-free emulsion polymerization of styrene (as comparison, Figure 4e). TEM imaging revealed that the obtained particles were spherical (Figure 4d), although the location of rGO could not be confirmed, which would be due to well-dispersed rGO in the particles. In order to confirm that rGO was indeed located within the interior of the particles, the emulsion was treated with acetone. This would result in dissolution of the poly(ionic liquid) and disintegration of the particles, and dispersion of the “free” rGO in the resulting solution due to π−π interactions between rGO and poly(ionic liquid), the latter thus acting as stabilizer (note that rGO cannot be dispersed in water/acetone). Before acetone treatment, a monomodal particle size distribution was observed by DLS, consisting of submicron (Dn = 665 nm) particles, which is consistent with the TEM imaging (Dn = 540 nm) (Figure 4d). On the other hand, a monomodal distribution consisting of much smaller nanometer size entities (Dn = 30 nm) was detected after acetone treatment. The average number of rGO sheets per particle was calculated from the initial stoichiometry (assuming all rGO sheets located within particles) based on an average particle diameter of 665 nm and circular rGO sheets of diameter 30 nm and thickness 1 nm, resulting in 140 rGO sheets/particle. It can be deduced that the nanosized material is rGO, and these DLS data thus demonstrate that poly(ionic liquid)/rGO composite particles were successfully prepared. Effect of [Veim][TFSA] on RGO Dispersibility. In order to investigate the effect of ionic liquid monomer on rGO dispersibility in the hydrophobic methacrylate monomer EMA, which has the same ethyl side chain as [Veim][TFSA], rGO was added to [Veim][TFSA] and EMA mixtures of different ratios ([Veim][TFSA]/EMA (w/w) = 90/10, 75/25, 50/50,

Figure 1. XPS wide scan spectra of GO (a) and material obtained after reduction to rGO (b).

hydrazine, the XPS spectrum exhibited a strong peak at 284.5 eV (C 1s) and a weak peak at 530 eV (O 1s) (Figure 1b), and the C:O atomic ratio increased to 8.7. Moreover, the two peaks attributed to ketone (CO) groups at 287.3 eV and carboxylic (O−CO) groups at 289.1 eV were significantly reduced, and the peak attributed to the graphitic regions at 285 eV increased (Figure 2). These results indicate the formation of rGO.17

Figure 2. C 1s XPS spectra of GO (a) and material obtained after reduction to rGO (b), where A = carboxyl groups, B = ketone groups, C = hydroxyl and epoxide groups, and D = graphitic region (C−H, C−C, CC).

Preparation of Polymer Particles Containing RGO Using Ionic Liquid Monomer. The dispersibility of rGO in the ionic liquid monomer [Veim][TFSA] was first investigated. As shown in Figure 3a, a black dispersion was obtained after ultrasonication of a mixture of rGO/monomer. The obtained dispersion exhibited good stability without phase separation over 24 h. In order to confirm the shape and size of rGO, TEM observation was carried out. A TEM sample was prepared by placing the dispersion on a TEM grid followed by acetone treatment to remove [Veim][TFSA] (which is soluble in acetone). The lateral dimensions of the rGO were significantly less than 100 nm, i.e., smaller than the semiquantitative DLS C

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Figure 4. Synthetic route to polymer particles containing rGO by miniemulsion polymerization: (a) rGO dispersed in [Veim][TFSA] in an aqueous solution of Tween 80 before and (b) after ultrasonication; (c) emulsion obtained after miniemulsion polymerization and (d) TEM image of obtained particles; (e) emulsion prepared by emulsifier-free emulsion polymerization of styrene.

Figure 5. Number-based DLS distributions of diameter of poly([Veim][TFSA]) particles containing rGO prepared by miniemulsion polymerization (solid line) and after acetone treatment (dashed line).

25/75, and 10/90), followed by dispersibility tests. As shown in Figure 6a, black dispersions were obtained after ultrasonication in all systems. However, after 10 min, rGO precipitation was observed in the systems containing lower amounts of [Veim][TFSA] (below 50 wt %)the stability of the rGO dispersions increases with increasing amount of [Veim][TFSA]. The increased stability/compatibility of rGO with a monomer mixture comprising [Veim][TFSA] is attributed to π−π interactions between rGO and the IL monomer.47 rGO dispersions in [Veim][TFSA]/EMA mixtures were subsequently added to aqueous solutions of Tween 80 and ultrasonicated for 5 min. Kinetically stable emulsions were obtained without rGO precipitation when the system contained more than 75 wt % [Veim][TFSA] in the dispersed phase. In the systems containing less than 75 wt % [Veim][TFSA], a small amount of rGO precipitate was visible at the bottom of the emulsion after 5 min (Figure 7). Miniemulsion polymerizations were conducted at 60 °C using AIBN as initiator for 24 hthe obtained emulsions were grayish in the case of high ionic liquid monomer content and deep blue when less ionic liquid monomer was used. The darker deep blue color observed

Figure 6. Visual appearances of rGO dispersions in [Veim][TFSA]/ EMA mixtures (above) just after ultrasonication and (below) after 10 min. [Veim][TFSA]/EMA (w/w): (a, a′) 90/10; (b, b′) 75/25; (c, c′) 50/50; (d, d′) 25/75; (e, e′) 10/90.

in the latter cases is caused by rGO being located throughout the system (i.e., including in the continuous phase), given that it cannot be stably dispersed in the monomer phase as demonstrated above (Figure 6). For the high ionic liquid monomer content systems, the obtained particles were spherical and the particles were sufficiently large for incorporation of rGO inside the particles (Figure 8). In the other systems with lower [Veim][TFSA] content, the particle diameter was less than 50 nm (although it should be noted that D

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Figure 7. Visual appearances of rGO miniemulsions using [Veim][TFSA]/EMA mixtures before (top row) and after polymerization (bottom row). [Veim][TFSA]/EMA (w/w): (a, a′) 90/10; (b, b′) 75/25; (c, c′) 50/50; (d, d′) 25/75; (e, e′) 10/90.

deformation and destruction of particles via the electron beam during TEM observation took place), thus severely restricting the ability for these particles to incorporate rGO. It is apparent from the above results that the dispersibility of rGO in the monomer phase is improved if there are specific interactions, such as π−π interactions, between rGO and the mixture. To this end, rGO dispersibility tests (0.5 wt % relative to monomer) were carried out using monomer mixtures ([Veim][TFSA]/EMA (w/w): 50/50, 25/75, 10/90) containing a small amount of dissolved poly([Veim][TFSA]) homopolymer (0.5 and 2.5 wt % relative to monomer). The dispersions obtained after ultrasonication showed higher stability (over 24 h) compared to that in the absence of PIL homopolymer (visible rGO precipitation in less than 1 h as shown in Figure 6). After miniemulsion polymerization using these dispersions (using the same recipe/conditions as above), a small amount of rGO precipitate was observed, and the diameter of the obtained particles was 70 nm (0.5 wt % polymer) and 60 nm (2.5 wt % polymer) from DLS results. These results are consistent with TEM observation (Figure 9), and rGO is anticipated to be incorporated within the particles (the rGO is smaller than the particles). The above results thus

Figure 8. TEM images of particles obtained by miniemulsion polymerization of [Veim][TFSA]/EMA mixtures containing rGO. [Veim][TFSA]/EMA (w/w): (a) 90/10, (b) 75/25, (c) 50/50, (d) 25/75, (e) 10/90.

Figure 9. Visual appearances (a, b) of obtained emulsion and TEM photographs (a′, b′) and intensity-based DLS distributions (a″, b″) of obtained particles by miniemulsion polymerization of [Veim][TFSA]/EMA mixtures containing rGO in the presence of poly([Veim][TFSA]) homopolymer. [Veim][TFSA]/EMA/poly([Veim][TFSA]) (w/w/w): (a, a′, a″) 1/1/0.01, (b, b′, b″) 1/1/0.05. E

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demonstrate that PIL provides markedly improved stability of rGO in monomer dispersions than the IL monomer itself.

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CONCLUSIONS From the perspective of high performance nanocomposite polymeric materials, it is desirable to prepare polymeric nanoparticles containing well-dispersed graphene. In this work, we have taken steps toward this goal by exploiting miniemulsion polymerization of the ionic liquid monomer 1vinyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([Veim][TFSA]), also including mixtures of this monomer with ethyl methacrylate (EMA), with rGO dispersed in the monomer phase. Successful dispersion of rGO in vinyl monomer is not trivialwe have found that the use of [Veim][TFSA] is advantageous as it enables formation of rGO dispersions without rGO precipitation (i.e., referring to the monomer phase of the miniemulsion). This presumably occurs as a result of π−π interactions between rGO and [Veim][TFSA]. In the case of monomer mixtures containing EMA, the rGO stability is compromised if the EMA:[Veim][TFSA] ratio is too high. Miniemulsion polymerization under conditions without rGO precipitation yields submicron-size spherical polymeric nanoparticles with relatively narrow particle size distributions with rGO nanosheets contained within their interior. Finally, it has been demonstrated that the presence of a small amount of the homopolymer of [Veim][TFSA] results in markedly improved rGO stability, thereby enabling preparation of polymer nanoparticles containing higher amounts of EMA.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02216. Preparation of ionic liquid monomer ([Veim][TFSA]), GO from graphite nanofibers, reduced graphene oxide (rGO), and calculation of number of rGO sheets per particle (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.B.Z.). *E-mail: [email protected] (H.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (grant no. 26288103) from the Japan Society for the Promotion of Science (JSPS) and by a Research Fellowship of JSPS for Young Scientists (given to M.T.).

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02216 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02216 Macromolecules XXXX, XXX, XXX−XXX