Letter pubs.acs.org/macroletters
Graphene Oxide as a Radical Initiator: Free Radical and Controlled Radical Polymerization of Sodium 4‑Vinylbenzenesulfonate with Graphene Oxide Dmitry Voylov,*,† Tomonori Saito,*,‡ Bradley Lokitz,§ David Uhrig,§ Yangyang Wang,§ Alexander Agapov,† Adam Holt,∥ Vera Bocharova,‡ Alexander Kisliuk,‡ and Alexei P. Sokolov†,‡ †
Department of Chemistry and ∥Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37916-1600, United States ‡ Chemical Sciences Division and §Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States S Supporting Information *
ABSTRACT: The free radical and controlled radical polymerization of sodium 4-vinylbenzenesulfonate using graphene oxide as a radical initiator was studied. This work demonstrates that graphene oxide can initiate radical polymerization in an aqueous solution without any additional initiator. Poly(sodium 4-vinylbenzenesulfonate) obtained via reversible addition− fragmentation chain transfer polymerization had a controlled molecular weight with a very narrow polydispersity ranging between 1.01 and 1.03. The reduction process of graphene oxide as well as the resulting composite material properties were analyzed in detail. raphene oxide is one of the most promising filling agents for composite applications.1,2 Recently it was shown3 that reduction of graphene oxide occurs even at room temperature. Many authors have proposed a radical mechanism of reactions leading to the reduction.4 It is also known that graphene oxide participates in chemical reactions as a catalyst.5,6 In this regard, the synthesis of polymers and polymer composites using radicals and the catalytic activity of graphene oxide with simultaneously understanding their mechanism are of great importance from both applied and fundamental points of view. Yang et al.7 reported an initiation of polymerization by reduced graphene oxide. The synthesis occurred through a ring-opening polymerization via a grafting mechanism at 170 °C in the presence of catalyst. The grafting of GO sheets with polystyrene (PS) and poly(styrene−isoprene) (PSI) using GO itself as a cationic initiator for homopolymerization of styrene and copolymerization of styrene and isoprene was also studied by Li et al.8 Recently, Beckert et al. grafted polystyrene and styrene copolymers from reduced graphene oxide using self-initiated free radical polymerization in the presence of stearylamine-modified GO.9 In their study, they found that an additional initiator was not required for the self-initiated free radical polymerization, while the reaction occurred with a high loading of GO in the bulk at 130 °C. The initiation of graft polymerization was attributed to the existence of a C-centered radical on a basal plane of the functionalized graphene oxide. At the same time, Muge Acik with coauthors4 suggested that the formation of radicals on the graphene oxide lattices during a temperature reduction may potentially lead to their detachment
G
© XXXX American Chemical Society
from the surface. These radicals may be relatively stable for long enough periods of time for the initiation of polymerization. In Beckert’s study9 the functionalization of graphene oxide by stearylamine was performed at a temperature of 70 °C for a 3 h duration. This step should significantly reduce the number of functional groups in GO4 and therefore reduce the concentration of free radicals formed during the reduction process. Following from the foregoing, such fundamental questions as the existence, reactivity, and ability to participate in a polymerization process of free radicals, which are dispatched from the graphene oxide surface during lowtemperature treatment, remain unclear. Here we report the study of free radical polymerization and reversible addition−fragmentation chain transfer (RAFT) polymerization of sodium 4-vinylbenzenesulfonate at 75 °C using GO as a radical initiator without any additional initiators or catalysts. Graphene oxide was prepared using the Hummers modified method (for more details, see the SI). (1) Free radical polymerization. Sodium 4-vinylbenzenesulfonate (5 g) was placed into a round-bottomed flask equipped with a septa and a magnetic stirrer. (A) Deionized water with GO or (B) deionized water without GO (Scheme 1 (1)) was added to the reaction flask. Each reaction solution was purged with argon for 10 min prior to the reaction. Then, the reaction Received: January 4, 2016 Accepted: January 7, 2016
199
DOI: 10.1021/acsmacrolett.6b00003 ACS Macro Lett. 2016, 5, 199−202
Letter
ACS Macro Letters
can be seen that the isolated yield began to increase at GO concentrations higher than 5 × 10−5 mol/L (or ∼10−2 g/L) (Figure 1a). The increase of isolated yield with GO concentration may be caused by either direct production of radicals by GO or its catalytic radical generation in the solution. Thus, there are two main questions regarding the role of GO: (1) whether it is a catalyst or a source of radicals and (2) if it is a source, are the radicals localized or delocalized (in other words, dispatched from the GO structure)? Indeed, even if the polymerization process proceeded through a radical mechanism, GO might play a role of catalyst of radical formation in monomer/water solution. X-ray photoelectron spectroscopy and conductivity measurements showed a significant reduction of neat GO stored at 75 °C within 18 h (Figure 1c,d). Beckert et al.9 reported that the main indicator of GO embedded in the lattice radical-initiated polymerization should be the prevalence of the grafting polymerization mechanism. By taking into account the low loading of GO (10−4 g of GO forms about 4 g of polymer), it is obvious that the majority (>99% of PSSNa) was not grafted to the GO surface. On the other hand, if GO simply catalyzes the formation of radicals in a solution, its surface should not be completely grafted. To address these questions, we employed atomic force microscopy (AFM). The AFM measurements showed that significant area of GO sheets (from 89 to 99%) was covered by globule-like polymer molecules (Figure 2) despite the intensive and long-time
Scheme 1. Synthesis of Poly(sodium 4vinylbenzenesulfonate) via Free Radical Polymerization (1) and RAFT Polymerization (2)
flasks were placed in an oil bath at 75 °C. The thermal polymerization proceeded for 18 h. The isolated yield from the polymerization of sodium 4-vinylbenzenesulfonate with graphene oxide showed a much greater value (21−90%) than that of control solution (8.5−11%, without graphene oxide). Since radical initiators were not present in the reactions, the polymerization of the control (without GO) was due to thermally induced autopolymerization. To confirm that polymerization of sodium 4-vinylbenzenesulfonate proceeds through the radical mechanism, we performed polymerization in the presence of excess inhibitors. All the batches showed no recovery of PSSNa, indicating the reaction was radical based. The dependence of isolated yield and molecular weight of the resulting poly(sodium 4-vinylbenzenesulfonate) (PSSNa) on GO concentration is shown in Figure 1a and b, respectively. There is no clear correlation between molecular weight and GO concentration, suggesting that chemical reactions did not follow a conventional equation for a thermally initiated radical chain polymerization.10 Viscosity measurements are in good agreement with molecular weight results from GPC (Figure 1b). It
Figure 2. AFM images of initial graphene oxide (a, b) and GO extracted from the PSSNa−GO composite (c, d).
purification process (for more details, see SI). The largest concentrations of grafted polymer were observed at the edges of the GO flakes (Figure 2c,d). This observation agrees with the expectation that the edges are mainly responsible for a catalytic activity as well as for intensive reduction processes.5,6 However, the coverage of the GO flake edges should lead to the deactivation of most active catalytic centers by the grafted polymer. Thus, it suggests that production of radicals rather than catalysis is the main driving force behind polymerization. The high grafting density is also in good agreement with the results obtained by Beckert et al.9 If we assume that each
Figure 1. Dependence of isolated yield (a) and molecular weight Mw (b) of the resulting polymer on graphene oxide concentration. XPS results: dependence of the graphitic structure of initial graphene oxide on annealing time at 75 °C (c) and dependence of conductivity of initial graphene oxide on annealing time at 75 °C (d). 200
DOI: 10.1021/acsmacrolett.6b00003 ACS Macro Lett. 2016, 5, 199−202
Letter
ACS Macro Letters polymer molecule was produced by one new radical from GO, the estimated amount of radical is in a similar range to the number of all atoms in GO (for more details, see SI). Considering that not all atoms can participate in polymerization, the termination of a polymer molecule has to be accompanied by transfer of radicals to solvent or monomers.7 It has also been reported that the active radical species of the propagation chains are reactive toward the CC groups of GO9 and possibly involved with the chain-transfer mechanism proposed in this study. (2) RAFT polymerization. Our finding of a suggested chaintransfer mechanism via GO led us to pursue RAFT polymerization of sodium 4-vinylbenzenesulfonate via GO by taking advantage of the chain-transfer mechanism of RAFT polymerization. RAFT polymerization requires a RAFT agent with a radical initiator. To minimize dead chain formation during the polymerization, the active radical concentration should be as low as possible.11 In the case of a thermal initiator, a RAFT agent/free-radical initiator ratio of 1:1 to 10:1 is typically used to produce polymers with narrow molecular weight distributions in a reasonable reaction time.12 On the basis of an almost linear dependence of reduction on time (Figure 1c), we hypothesized that use of GO as a radical source in RAFT polymerization would be ideal since GO possibly may generate a slow but steady radical production. Thus, RAFT polymerization of sodium 4-vinylbenzenesulfonate with GO as a radical initiator (without a thermal initiator) was performed in this study. The reaction was performed with sodium 4-vinylbenzenesulfonate (2.5 g) in deionized water (9.5 mL), GO 0.95 × 10−4 g, and RAFT agent (4-cyano-4(phenylcarbonothioylthio)pentanoic acid) (A−D) or without the RAFT agent (E). The reaction time was varied (0−23 h) to access the ability of the controlled polymerization. The series of PSSNa prepared via RAFT polymerization with GO showed a linear correlation of Mn as a function of conversion (Figure 3a), and the resulting Mw/Mn was quite narrow (1.01−1.03) (Table 1). While the linear regression of Mn vs conversion was slightly off from a theoretical molecular weight, the obtained molecular weights with a narrow Mw/Mn indicate a successful controlled RAFT polymerization of sodium 4-vinylbenzenesulfonate using GO as an initiator. The kinetic plot of conversion and ln([M]0/ [M]) as a function of reaction time (Figure 3b) further confirms that the RAFT polymerization with GO occurred via a controlled polymerization. The kinetic plot was similar to the reported kinetic plot of RAFT polymerization of sodium 4vinylbenzenesulfonate using a regular thermal initiator.13 Moreover, the control experiment without the RAFT agent resulted in much higher molecular weight (∼1 order of magnitude higher), broader Mw/Mn, and lower conversion. The high molecular weight obtained from the control further indicates that the concentration of radicals generated via GO was low. The conventional free radical polymerization of sodium 4-vinylbenzenesulfonate with GO as an initiator was an ill-controlled polymerization as shown in Figure 1, while it is significant to demonstrate that radical polymerization can occur using GO as an initiator. To the best of our knowledge, this is the first report of a successful controlled polymerization using as-prepared (untreated) GO as a radical initiator. Considering that RAFT polymerization with GO showed excellent control and the conventional free radical polymerization with GO was not well-controlled, the polymerization with GO as a radical initiator might be better suited for RAFT polymerization, rather than used for a conventional free radical polymerization. As
Figure 3. (a) RAFT polymerization of sodium 4-vinylbenzenesulfonate with GO Mn and Mw/Mn as a function of conversion. Dashed line represents theoretical Mn. (b) Kinetic plot of RAFT polymerization of sodium 4-vinylbenzenesulfonate with GO. The conversion and ln([M]0/[M]) as a function of reaction time.
Table 1. Properties of PSSNa Synthesized via RAFT Polymerization with GO
A B C D E (no RAFT) a
reaction time, min
Mn (g/mol)
Mw (g/mol)
Mw/Mn
conversiona (%)
160 795 1080 1380 1080
30100 42900 55100 58300 377000
30200 43500 56700 58700 474000
1.01 1.01 1.03 1.01 1.26
33.5 57.1 71.3 66.2 16.5
Conversion was determined gravimetrically.
hypothesized, the low concentration of radicals generated via GO minimizes dead chain formation and thus resulted in a very low Mw/Mn. Unfortunately, it was not possible to detect the type of radicals formed during our experiments. However, according to the literature4 and our previous study,14 one of the most probable candidates for this role is the OH• radical. Our previous results showed intensive chemical redox reactions on the GO surface, where OH• radicals played a key role. This radical may be produced directly during thermal reduction of GO4 or decomposition of water. In the composite water molecules are still present in a significant amount even for the annealed PSSNa−GO composite (∼10 wt %) (for more details, see SI). The latter follows from the fact that relaxation properties of the PSSNa−GO composite were mainly caused by absorbed water, even though the preparation procedure included annealing at 150 °C for 3 days (Figure S7). To analyze the interaction between the polymer and GO, we employed 201
DOI: 10.1021/acsmacrolett.6b00003 ACS Macro Lett. 2016, 5, 199−202
ACS Macro Letters
■
Raman spectroscopy. We compared the Raman spectra measured in two locations (Figure 4): on a graphene oxide
Letter
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences ORNL, which is a DOE Office of Science User Facility. D.V. thanks Dr. S. Kurochkin for fruitful discussions. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy. gov/downloads/doe-public-access-plan).
Figure 4. Raman spectra and optical microscopy image taken at Raman measurement (inset). Raman spectra of initial GO (red curve), of PSSNa (blue curve) measured in the point shown with a blue circle on the inset, and PSSNa-GO (black curve) measured in the point shown with a black circle.
flake in the composite and in the region without GO. The measurements showed detectable shifts of the bands corresponding to the benzene ring and sulfonic acid group stretching (Table S3). The position of the G-band peak was found at 1595 cm−1, while for initial GO it was located at 1604 cm−1, which is related to the reduction process. In conclusion, we demonstrated the first example of radical polymerization in aqueous media induced by as-prepared GO without any additional thermal initiators or catalysts. The results show that GO not only produces C-centered radicals but also free radicals, which cause the initiation and propagation of radical polymerization. The result suggests that a chain-transfer mechanism dominates and governs the polymerization process, even though the GO surface was grafted by polymer chains. Moreover, we have demonstrated a successful controlled polymerization using GO as a radical initiator. RAFT polymerization with GO showed excellent control with a narrow polydispersity, suggesting that using GO as a radical initiator might be better suited for the RAFT polymerization. Raman spectroscopy measurements showed a shift of characteristic bands of polymer molecules located nearby graphene oxide flakes, indicating interaction through the benzene ring and sulfonic acid group. These results can lead to a new approach of radical polymerization of various monomers and in preparation of polymer composite materials, where GO is directly used as a filler agent.
■
■
REFERENCES
(1) Kim, H.; Abdala, A. A.; Macosko, C. W. Macromolecules 2010, 43, 6515−6530. (2) Tjong, S. C. J. Nanosci. Nanotechnol. 2014, 14, 1154−1168. (3) Kim, S.; et al. Nat. Mater. 2012, 11, 544−9. (4) Acik, M.; et al. J. Phys. Chem. C 2011, 115, 19761−19781. (5) Schwartz, V.; et al. ChemSusChem 2013, 6, 840−6. (6) Dathar, G. K. P.; et al. ChemSusChem 2014, 7, 483−91. (7) Yang, J.-H.; Lin, S.-H.; Lee, Y.-D. J. Mater. Chem. 2012, 22, 10805. (8) Li, B.; et al. Macromolecules 2015, 48, 994−1001. (9) Beckert, F.; et al. Macromolecules 2013, 46, 5488−5496. (10) Odian, G. Principles of Polymerization; John Wiley & Sons, Inc.: New York, 2004. DOI: 10.1002/047147875X. (11) Favier, A.; Charreyre, M.-T. Macromol. Rapid Commun. 2006, 27, 653−692. (12) Perrier, S.; Takolpuckdee, P. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5347−5393. (13) Mitsukami, Y.; Donovan, M. S.; Lowe, A. B.; McCormick, C. L. Macromolecules 2001, 34, 2248−2256. (14) Voylov, D. N.; Agapov, A. L.; Sokolov, A. P.; Shulga, Y. M.; Arbuzov, A. A. Carbon 2014, 69, 563−570.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00003. Chemicals, details of synthesis of GO and polymer, experimental techniques and sample preparation description, gel permeation chromatography analysis, elemental analysis, X-ray photoelectron spectroscopy results, conductivity behavior of GO during reduction, dielectric properties of PSSNa-GO, Raman spectroscopy results, thermogravimetry analysis (PDF) 202
DOI: 10.1021/acsmacrolett.6b00003 ACS Macro Lett. 2016, 5, 199−202
