Controlled atom transfer radical polymerization of MMA onto the surface of high-density functionalized graphene oxide
© Kumar et al.; licensee Springer. 2014
Received: 13 February 2014
Accepted: 3 March 2014
Published: 10 July 2014
We report on the grafting of poly(methyl methacrylate) (PMMA) onto the surface of high-density functionalized graphene oxides (GO) through controlled radical polymerization (CRP). To increase the density of surface grafting, GO was first diazotized (DGO), followed by esterification with 2-bromoisobutyryl bromide, which resulted in an atom transfer radical polymerization (ATRP) initiator-functionalized DGO-Br. The functionalized DGO-Br was characterized by X-ray photoelectron spectroscopy (XPS), Raman, and XRD patterns. PMMA chains were then grafted onto the DGO-Br surface through a ‘grafting from’ technique using ATRP. Gel permeation chromatography (GPC) results revealed that polymerization of methyl methacrylate (MMA) follows CRP. Thermal studies show that the resulting graphene-PMMA nanocomposites have higher thermal stability and glass transition temperatures (Tg) than those of pristine PMMA.
The discovery of two-dimensional (2D) sp2 hybridized graphene sheets by Novoselov  in 2004 has received much attention due to their extraordinary electrical, thermal, and mechanical properties [1–5]. Due to its high surface-area-to-volume ratio, graphene has been effectively used in the synthesis of polymer nanocomposites which exhibit enhanced physical and chemical properties over individual components . The functionalization of graphene has received much attention in recent years as a way to improve interfacial interactions with other components, including organic and inorganic polymers, as the key to maximizing the end properties of the resulting graphene-polymer nanocomposites is controlling the dispersion of graphene within the matrix of the main components [7–9]. Moreover, the functional groups may not only improve the miscibility of graphene in organic solvents but also may provide nucleation sites for efficient in situ grafting of polymeric chains onto the graphene surface, which results in further improvements in mechanical and thermal properties .
Efforts to enhance the end properties of graphene-polymer nanocomposites using surface polymerization through in situ ‘grafting to’ and ‘grafting from’ techniques have been reported [11, 12]. In situ polymerization offers the ability to control the polymer architecture and final morphology of the resulting composites. Ramanathan et al. reported an extraordinary shift in glass transition temperature (Tg), modulus, ultimate strength, and thermal stability for poly(acrylonitrile) and poly(methyl methacrylate) using very low levels of functionalized graphene sheets . In situ emulsion polymerization of methyl methacrylate (MMA) was carried out by Kuila et al. using graphene as a reinforcing filer, which also enhanced the storage moduli, Tg, and thermal stability of the resulting nanocomposites .
Living ionic polymerization has been widely used to produce homo- and block copolymers with well-defined architectures, controlled molecular weights, and narrow polydispersity index (PDI). However, the industrial applications of ionic polymerization are limited due to the need for rigorous polymerization conditions, such as highly purified monomers and solvents. In addition, living ionic polymerization can only be used to polymerize hydrocarbon monomers and the polar monomer due to unwanted side reactions. Atom transfer radical polymerization (ATRP) is an alternative polymerization technique to improve polymer architectures under simple polymerization conditions in the presence of hydrophilic organic/inorganic fillers such as layered silicates and graphene oxide (GO) [15, 16]. The direct growth of thermoresponsive poly-2-(dimethylamino)ethyl methacrylate polymer brushes with narrow PDI through surface-initiated highly controlled ATRP from initiator functionalized graphene sheets has been reported, where the polymers were grown directly from a graphene surface with covalent functionalization to tune the physical, thermal, and electrical properties . Pyrene-based functionalized graphene has been used for reversible addition fragmentation chain transfer (RAFT) polymerization of dimethyl aminoethyl acrylate, acrylic acid, and styrene in order to avoid graphene aggregation . The efficient functionalization through diazotization of graphene for ATRP of styrene results in high-performance polymer-graphene nanocomposites with increased tensile strength, Tg and Young's modulus . Covalently bounded polystyrene polymer chains have been systematically tuned using ATRP on single-layer graphene nanosheets by Fang et al. . High-density grafted polymer-graphene nanocomposites exhibit an appreciable increase in Tg compared with low-density grafted samples.
In this study, we focused on the functionalization of GO and highs-density grafting of poly(methyl methacrylate) (PMMA) chains onto its surface through an in situ ‘grafting from’ technique using ATRP. Quaternization and esterification after diazotization were carried out to increase the number of anchoring sites for ATRP initiators for increased grafting of polymer chains on the GO surface. ATRP of MMA was carried out using GO with ATRP initiators on the surface, cupric bromide (CuBr, catalyst), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, ligand) at ambient temperature. The resulting graphene-PMMA nanocomposites showed higher thermal stability and higher glass transition temperatures (T g ) than pristine PMMA polymers.
Acid-treated natural expandable graphite (grade 1721) was purchased from Asbury Carbons, Asbury, NJ, USA. Concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), sodium nitrite (NaNO2), sodium carbonate (Na2CO3), hydrochloric acid (HCl, 35%), hydrogen peroxide (H2O2, 30 wt.%), N,N′-dimethylformamide (DMF), MMA, 2-chloroethanol, p-aminobenzoic acid, and 2,2',2"-trihydroxy-triethylamine (triethanolamine) were purchased from Daejung Reagents & Chemicals, Ulsan, Korea. Cuprous bromide (CuBr), N,N,N′,N″,N″-PMDETA and polystyrene standards for gel permeation chromatography (GPC) were purchased from Sigma-Aldrich, St. Louis, MO, USA and were used as received without further purification. The stabilizing agent was removed from commercial MMA by washing three times with sodium hydroxide (NaOH), followed by vacuum distillation; the middle portion was stored at 0°C to 4°C until use. DMF was stirred with anhydrous calcium hydride (CaH2) and then distilled before use.
Preparation of DGO-Br
The preparation steps of GO, diazotized GO (DGO-COOH), tetrakis(2-hydroxyethyl) ammonium chloride (THAC), DGO-COO−Na+, and DGO-OH have been reported in our previous paper . DGO-Br was prepared by the esterification of α-bromoisobutyryl bromide with DGO-OH. In brief, 2 g of DGO-OH powder was dispersed in distilled dry DMF using sonication, and 1 mL of triethyl amine was added to the suspension under a nitrogen atmosphere. The α-bromoisobutyryl bromide was added slowly to the above suspension at 0°C using a gas-tight syringe. The reaction mixture was stirred at the same temperature for 6 h and then increased to 25°C and stirred for 12 h. The resulting suspension (DGO-Br) was centrifuged and washed repeatedly with acetone and methanol and dried at 65°C in a vacuum oven.
Polymerization of MMA on the surface of DGO-Br
The ATRP of MMA was carried out using the prepared DGO-Br. In a typical procedure, 30 mg of DGO-Br was dissolved in 5 mL of distilled dry DMF and was homogenously dispersed by ultrasonication for 30 min before starting polymerization. Next, 15 mg of CuBr, 15 μL of PMDETA catalyst, and 5 mL of MMA were added successively. The reaction mixture was then degassed three times and vacuum-sealed with a septum, followed by nitrogen purging for 30 min to evacuate the residual oxygen. The mixture was then placed in a thermo-stated oil bath at 80°C for the designated period of time. Polymerization was stopped by quenching the polymerization tube in ice cold water. The resulting solution was poured into a petri dish to evaporate the excess solvent. The polymerization yields were calculated gravimetrically.
Detachment of the polymer chains from the GO surface
To determine the molecular weight by GPC, polymeric chains were detached from the surface of DGO-Br through a reverse cation exchange process. In brief, the resulting graphene-PMMA nanocomposite (0.5 g) was dissolved in 50 mL of tetrahydrofuran (THF), and lithium chloride (0.05 g) was added to the reaction mixture. The solution was refluxed for 24 h and filtered through Celite (Sigma-Aldrich). The free polymer was recovered by adding the filtrate into methanol and was then filtered and dried in a vacuum oven.
Raman spectra were recorded using a confocal Raman spectrometer (Alpha300S, WITec, Ulm, Germany) with a 633-nm wavelength incident laser light. The crystallographic structures of the materials were determined by a wide-angle X-ray diffraction (WAXRD) system (Rigaku RU-200 diffractometer, Shibuya-ku, Japan) equipped with a Ni-filtered Cu Kα source (40 kV, 100 mA, λ = 0.15418 nm). Bragg's equation (nλ = 2d sinθ) was used to calculate the d spacing between the layers. X-ray photoelectron spectroscopy (XPS) was performed to determine the oxidation status of carbon using a Thermo Fisher X-ray photoelectron spectrophotometer (Waltham, MA, USA) employing an Al Kα X-ray source (1,486.6 eV). Thermogravimetric analysis (TGA) was performed to analyze the thermal behavior of the samples using a TGA analyzer (Q50, TA Instruments, New Castle, DE, USA) with a 10°C min−1 heating rate in a nitrogen atmosphere. Number of average molecular weight (), weight average molecular weight (), and molecular weight distribution (MWD) were determined using a Waters GPC instrument (Milford, MA, USA) equipped with a 2414 differential refractive index detector and two (HR 2 and HR 4) Waters μ-Styragel columns. Spectral grade THF was used as an eluent at a flow rate of 1.0 ml min−1, and the molecular weight calibrations were carried out using polystyrene standards.
Results and discussion
Raman spectra of DGO-OH and DGO-Br are shown in Figure 2c. The G and D bands in the Raman spectra originate from the first-order scattering of E2g phonons of sp2-bonded carbon atoms and with a breathing mode of j-point photons of A1g symmetry of sp3-bonded carbon atoms of disordered graphene. The Raman spectrum of DGO-OH shows sp2-bonded carbon stretching related to the G band at 1,594 cm−1 and disordered, D band, sp3-bonded carbon atoms at 1,330 cm−1. The intensity ratio of the D and G bands (ID/IG) for DGO-OH and DGO-Br were 1.3 and 1.35, respectively. The slightly increased ID/IG ratio may be due to increased functionalization after esterification. WAXRD patterns of DGO-OH and DGO-Br are shown in Figure 2d. The DGO-OH and DGO-Br exhibit peaks at 8.6° and 8.45°, indicating d spacings of 1.01 nm and 1.04 nm, respectively (based on Bragg's equation). The slightly increased d spacing of DGO-Br over DGO-OH can be also attributed to the esterification of DGO-OH with α-bromoisobutyryl bromide.
Controlled study of radical polymerization
Polymerization of MMA using TPEBMP at 80°C in DMF using DGO-Br
ATRP initiator-attached high-density functionalized graphene oxide (DGO-Br) was prepared and used for MMA polymerization, resulting in graphene-PMMA nanocomposites through controlled radical polymerization. DSC and TGA studies show that the graphene-PMMA nanocomposites exhibited higher T g and higher thermal stability compared to pristine PMMA polymers. GPC results confirmed the presence of a controlled radical polymerization mechanism using functionalized DGO-Br. We believe that high-density functionalized GO can be used to develop graphene-polymer nanocomposites with enhanced properties.
This research was supported by the Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2A10004468).
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