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Synthesis of graphene-polystyrene nanocomposites via RAFT polymerization
Polymer xxx (2014) 1e10
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Synthesis of graphene-polystyrene nanocomposites via RAFT polymerization Renpeng Gu, William Z. Xu, Paul A. Charpentier* Department of Chemical and Biochemical Engineering, Western University, London, ON N6A 5B9, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2014 Received in revised form 22 August 2014 Accepted 24 August 2014 Available online xxx
Synthesis of graphene-polymer nanocomposites is of current interest due to their exceptionally physical and chemical properties. However, the ability to produce conductive inks/coatings retaining the properties of the graphene is still a major challenge. In this study, functionalized polydopamine-coated reduced graphene oxide (PDA/RGO) was reacted with a RAFT agent, 2-(dodecylthiocarbonothioylthio)2-methylpropionic acid (DDMAT), to form macro-RAFT agents via an esterification reaction. The chemistry and kinetics of RAFT living/controlled polymerization of styrene was examined from the surface of these macro-RAFT agents to form polystyrene-grafted PDA/RGO (PS-g-PDA/RGO). By examination of the kinetics and GPC traces of the free and grafted polymer, living/controlled polymerization was confirmed. To examine the utility of this approach, the synthesized PS-g-PDA/RGO samples containing different amounts of grafted PS were employed in the preparation of graphene-PS nanocomposites by mixing them with commercial PS. Graphene was found to be well distributed in the PS matrix by this approach, increasing the decomposition temperature range. This research provides an efficient route towards the design and development of value-added functional graphene-PS composites. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Graphene RAFT polymerization Graphene-polystyrene nanocomposites
1. Introduction Recent years have seen growing interest in graphene due to its unique properties such as excellent charge carrier mobility [1], thermal conductivity [2], and superior mechanical strength [3]. Of particular interest is the development of graphene-based advanced materials such as polymer nanocomposites [4e8]. The major challenge in the preparation of graphene-polymer nanocomposites is on how to avoid the aggregation of graphene sheets by overcoming the high energy barriers due to the strong sheet interlayer conjugation, van der Waal forces and p-p stacking between single layers [9]. Covalent bonding of nanofillers to the polymer matrix is often necessary to maximize the thermal and mechanical properties of the resulting nanocomposite [10]. However, covalent bonding may also disrupt the conjugated structure of graphene, with the resulting defects giving compromised properties such as lower electrical conductivity [11]. Among the approaches of chemical modification, including functionalizing the surfaces and edges of graphene via hydrogenation and the diazonium and azide reactions [12], coating
* Corresponding author. Tel.: þ1 519 661 3466; fax: þ1 519 661 3498. E-mail address: [email protected] (P.A. Charpentier).
graphene with a thin layer of polydopamine is promising to protect the structure of graphene, helping to remove the defects. Chemical modification using dopamine demonstrated an unparalleled advantage of simultaneous reduction of graphene oxide into graphene and coating of polydopamine onto the surface of the reduced graphene oxide [13]. Reduced graphene oxide (RGO) has shown high conductivity values of 1.494 103 S m1, whereas oxidized graphene (GO) is insulating with a low conductivity of ~ 105 S m1 [14]. Once graphene has been coated with polydopamine, a variety of polymers can potentially be grown from or attached to the surface of graphene to produce graphene-polymer nanocomposites. Various techniques have been employed in the synthesis of advanced graphene-polymer nanocomposites; including noncovalent [15], grafting-to [7], and grafting-from [16] methods. Living/controlled free radical polymerization has been extensively employed in the synthesis of polymers and polymer composites, resulting in well-defined molecular structure and lower dispersity (Ð) than those obtained from conventional free radical polymerization [17e20]. Graphene-polystyrene nanocomposites were synthesized via atom transfer radical polymerization (ATRP), showing improved thermal conductivity [21]. Due to its tolerance towards polymerizing a wide range of vinyl monomers, reversible addition fragmentation chain transfer (RAFT) polymerization is often employed in the synthesis of polymers and polymer composites.
http://dx.doi.org/10.1016/j.polymer.2014.08.064 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
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Graphene oxide-poly(N-vinylcarbazole) (GO-PVK) was synthesized using RAFT polymerization, leading to a PVK-GO-based memory device with a typical bistable electrical switching and nonvolatile rewritable memory effect [22]. In our previous work [23], graphene oxide was synthesized via oxidative exfoliation of graphite, which was then reduced into graphene and coated with polydopamine by means of self-polymerization of dopamine. This served for keeping the graphene oxide reduced to RGO while the polydopamine surface provided many attachment sites, i.e., hydroxyl groups (Scheme 1), for the RAFT agent, 2-(dodecylthiocarbonothioylthio)-2methylpropionic acid (DDMAT), was attached to the polydopamine-coated graphene via an esterification reaction. Three monomers, i.e., methyl methacrylate (MMA), N-isopropylacrylamide (NIPAM), and tert-butyl acrylate) (tBA) were successfully polymerized on the surface of DDMAT-PDA/RGO by using this macro-RAFT agent. Polystyrene is one of the most widely used thermoplastic materials, having many applications in a variety of areas such as toner inks, protective packaging, automotive, construction, and consumer products, etc. The annual consumption worldwide is even over several billion kg [25]. Design and development of valueadded advanced materials using inexpensive polystyrene is of significant economic interest. Although the kinetics of polymerization of styrene has been well studied including various living/controlled free radical polymerizations, no published work has investigated the kinetics of RAFT polymerization from the surface of polydopamine-coated graphene or graphene oxide. In the present research, styrene was polymerized from the surface of the synthesized DDMAT-PDA/RGO, which has not been examined. The kinetics of polymerization were studied while the formed polystyrene-grafted PDA/RGO (PS-g-PDA/RGO) was applied in the preparation of graphene-polystyrene nanocomposites by mixing with commercial polystyrene to show the potential utility of this approach. 2. Experimental 2.1. Materials Styrene (99%, SigmaeAldrich, containing 4-tert-butylcatechol as stabilizer) was purified by passing through a basic alumina column prior to use. 2,20 -Azobis(2-methylpropionitrile) (AIBN, DuPont) was recrystallized twice from methanol prior to use. 2(Dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT, 98%) and commercial polystyrene (PS, average molecular weight Mw ¼ 350 kDa, dispersity Ɖ ¼ 2.5) were purchased from SigmaeAldrich and used as received. Concentrated sulfuric acid (H2SO4), 1-butanol, N,N-dimethylformamide (DMF), toluene, methanol, and tetrahydrofuran (THF) were purchased from Caledon Laboratories
Ltd. and used as received. Nylon 66 filter membranes (pore size 0.2 mm) were purchased from SigmaeAldrich. 2.2. Characterization Fourier transform infrared (FTIR) spectra were recorded using KBr pellets on a Nicolet 6700 FTIR spectrometer (Thermo Scientific) with a resolution of 2 cm1 and 32 scans for each sample. Energydispersive X-ray (EDX) elemental analysis was performed using a Quartz Xone EDX scattering device attached to a Hitachi S-4500 field emission scanning electron microscope (SEM). The thermal properties were measured by thermal gravimetric analysis (TGA). The samples were heated from room temperature to 800 C at a heating rate of 10 C/min under nitrogen atmosphere on a TA Instruments SDT Q600. TGA measurements for selected samples run 3X showed standard errors to be within 1.5 C. Static light scattering (SLS) experiments to measure the weight average molecule weight Mw were measured with a Zetasizer Nano S. Numberaverage molecular weight (Mn) and dispersity (Ð) of the polymers were measured on a Viscotek TDAmax gel permeation chromatography (GPC) system equipped with a TDA 302 Triple Detector Array, a VE2001 GPC solvent/sample module. THF eluent flow rate was 1 mL/min at 30 C.
2.3. Grafting polystyrene from the surface of PDA/RGO The polydopamine-coated reduced graphene oxide (PDA/RGO) and the RAFT agent-attached graphene sheets (DDMAT-PDA/RGO) were prepared according to the previously reported method [23]. A schematic diagram for the preparation of DDMAT-PDA/RGO, the “graft-from” polymerization of styrene, and the cleavage of grafted PS is shown in Scheme 2. By using the synthesized DDMAT-PDA/ RGO, styrene was polymerized from the surface of PDA/RGO via RAFT polymerization. Typically, the synthesized DDMAT-PDA/RGO (31.5 mg, containing 0.010 mmol trithiocarbonate based on TGA analysis) was suspended in a mixture of styrene (5.0 mL, 44 mmol), AIBN (1.7 mg, 0.010 mmol), and DMF (2 mL) by sonication. The mixture was then introduced into a 10-mL dry Schlenk flask, which had been degassed and refilled with nitrogen three times, using a syringe. After degassing with nitrogen for 30 min, the reaction mixture was heated to allow the reaction to take place at 60 C for 24 h at which point the flask was immersed in an ice bath to quench the polymerization. Subsequently, the reaction mixture was slowly precipitated into methanol. Polystyrene-grafted graphene (PS-gPDA/RGO) was obtained by vacuum filtration with a nylon 66 filter membrane (pore size 0.2 mm), washing with toluene three times to remove unattached (free) polymer, and then drying in vacuo prior to characterization.
Scheme 1. Suggested structures for polydopamine coating from self-polymerization of dopamine [13,24].
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Scheme 2. Schematic diagram for the preparation of DDMAT-PDA/RGO, the “graft-from” polymerization of styrene, and the cleavage of grafted PS. Reagents and conditions: (a) K2S2O8, P2O5, H2SO4, 80 C, 4.5 h; (b) NaNO3, H2SO4, KMnO4, H2O2; (c) dopamine, PH 8.5 buffer; (d) DDMAT, EDAC, DMAP, DMF; (e) styrene, AIBN, DMF; (f) H2SO4, 1-butanol, toluene.
2.4. Kinetics of polymerization of styrene using DDMAT-PDA/RGO To study the kinetics of the RAFT polymerization process, styrene was polymerized in the presence of DDMAT-PDA/RGO, with and without additional DDMAT. In the case of polymerization without adding free DDMAT, the above polymerization conditions were employed and the reaction was stopped at preset reaction times (12 h, 24 h, 36 h, 47 h, and 67 h, respectively). During the washing process using toluene, the free polystyrene was recovered from the solution via rotary evaporation. In the case of polymerization with additional free DDMAT, DDMAT (0.015 mmol) was added to the reaction mixture before sonication. The polymerization was stopped at preset reaction times (10 h, 23 h, 35 h, 50 h, 66 h, and 80 h, respectively). The formed free polystyrene was also recovered from the solution via rotary evaporation. The conversion of monomer was measured based on the weight ratio between the formed PS and the initially added styrene. In order to measure the molecular weight of the grafted polymer, the isolated PS-g-PDA/RGO was subsequently subjected to an acid-catalyzed transesterfication to cleave the grafted polymer according to the reported method [26,27]. In a typical experiment, to a suspension of PS-g-PDA/RGO (20 mg) in toluene (10 mL), 1butanol (4.5 mL) and concentrated sulfuric acid (1 mL) were added. The mixture was sonicated for 15 min to form a homogeneous black suspension, followed by stirring at 70 C for 10 days. The suspension was then precipitated into methanol (300 mL) with stirring. After vacuum-filtration through a nylon 66 filter membrane, the residual solid was dispersed in toluene. A solution of the
grafted/cleaved polystyrene in toluene was isolated from the solid PDA/RGO by a further filtration using a nylon 66 filter membrane. The grafted PS was obtained after rotary evaporation for the measurement of molecular weight while the solid PDA/RGO was collected for elemental analysis. A conventional free radical polymerization and a solution RAFT polymerization of styrene were conducted under similar conditions for comparison. To a 10-mL dry Schlenk flask was introduced styrene (5.0 mL, 44 mmol), DDMAT (only added for the solution RAFT polymerization, 3.7 mg, 0.010 mmol), AIBN (1.7 mg, 0.010 mmol), and DMF (1 mL). After degassing with nitrogen for 30 min, the reaction mixture was heated to allow the reaction to take place at 60 C for preset times (5 h, 10 h, 20 h, and 48 h, respectively for free radical polymerization and 5 h, 10 h, 20 h, and 30 h, respectively for solution RAFT polymerization) at which point the flask was immersed in an ice bath to quench the polymerization. Subsequently, the reaction mixture was slowly precipitated into methanol. Polystyrene was isolated by filtration, washed with methanol, and then dried in vacuo. 2.5. Preparation of graphene-PS nanocomposites Graphene-PS nanocomposites were prepared by mixing the synthesized PS-g-PDA/RGO with commercial PS. PS-g-PDA/RGO samples comprising different amounts (10.1%, 21.9%, 39.4%, and 46.3%) of RGO were synthesized by employing different reaction times, which were then utilized in the preparation of graphene-PS nanocomposites containing a preset content (0.25%, 0.50%, 1.00%,
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Table 1 Synthesis of PS polymerization.
by
conventional
free
Radical polymerization
radical
polymerization
and
RAFT
RAFT polymerization
Time (h)
Conversion (%)
Mw (kDa)
Time (h)
Conversion (%)
Mw (kDa)
5 20 48
7.8 23.4 46.2
174 175 181
5 10 20 30
6.7 12.4 22.9 35.1
27.3 41.2 66.3 103
Note: Reaction conditions: styrene 44 mmol; DMF 1 mL; AIBN 0.010 mmol; DDMAT 0.010 mmol only for the RAFT polymerization; T ¼ 60 C. Mw was measured by static light scattering (SLS).
and 1.50%) of RGO. In a typical experiment, PS-g-PDA/RGO (15.2 mg, comprising 39.4% of RGO) was dispersed in THF by sonication at room temperature, followed by addition of the commercial PS (584.8 mg). The mixture was further sonicated to achieve a homogeneous suspension, which was subsequently poured onto a clean Teflon dish. A graphene-PS nanocomposite film (600 mg, containing 1.00% of RGO) was then obtained after drying under vacuum. 3. Results and discussion 3.1. Conventional free radical and RAFT polymerization RAFT polymerization of styrene monomer in solution was first examined using the DDMAT RAFT agent. A corresponding free radical polymerization without RAFT agent was also conducted as the control experiment. The molecular weight results measured by static light scattering (SLS) are shown in Table 1, and plotted in Fig. 1. Since RAFT polymerization, like other living/controlled polymerizations, is a much slower process than conventional free radical polymerization, the ratio between RAFT agent and initiator is significant. Too few initiator “active sites” result in long reaction times, while too high of an initiator concentration can impair the ability of the RAFT system, resulting in reduced polymer chain lengths. In most cases, the molar ratio used in the literature between the RAFT agent and initiator (nRAFT/ninitiator) is 10 to 4 [28e30]. Preliminary experiments showed that using a high RAFT: AIBN ratio gave very low conversions, e.g., less than 10% monomer conversion after 10 days. In this work, we found the reaction rate to be acceptable using a ratio of RAFT agent: AIBN ¼ 1:1 (Table 1). To examine whether the RAFT polymerization is well controlled, the following simplified equation is utilized:
Mn;theo ¼
½M 0 $x$mM þ mRAFT ½RAFT 0
(1)
where M n;theo is the theoretical number-average molecular weight, [M]0 and [RAFT]0 are the initial concentrations of the monomer and the RAFT agent, respectively, mM and mRAFT are the molecular weights of the monomer unit and the RAFT agent, respectively, and x is the monomer conversion. The theoretical molecular weight of RAFT polymerization using equation (1) (Fig. 1, right) assumes that the efficiency of initiator is 100% and there is no termination during the polymerization. As Fig. 1-left shows, the conversion of St increases linearly with polymerization time using the DDMAT RAFT agent. As Fig. 1-right shows, the molecular weights of PS made by RAFT polymerization grew nearly linearly as the conversion of monomer increases. As shown in the free-radical control experiments which did not use a RAFT agent, the molecular weight of polymer increases dramatically at low monomer conversion and then increases slowly afterwards. As expected, control of molecular weight using RAFT is much better than from the free radical polymerization. A slight nonzero initial molecular weight is noticed, which is indicative of deviation from ideally living/controlled free radical polymerization at the initial stages of the reaction. This might be attributed to a lag in the activation of the DDMAT RAFT agent and conversion to the polymeric RAFT species. The appearance of high initial molecular weight in RAFT has been termed hybrid behavior [31], attributed to a low transfer constant of the initial RAFT agent. Despite the nonzero initial part, the utilized equal molar ratio between initiator and RAFT agent still maintains the typical character of living/controlled polymerization in solution RAFT polymerization, which was deemed acceptable to measure the corresponding graphene surface “grafting from” RAFT polymerization. 3.2. Synthesis of PS-g-PDA/RGO By using the synthesized macro-RAFT agent, DDMAT-PDA/RGO, which was previously characterized [23], styrene was polymerized on the surface of PDA/RGO via RAFT polymerization as described by Scheme 2. The synthesized PS-g-PDA/RGO nanocomposites were characterized with FTIR. The FTIR spectra of the DDMAT-PDA/RGO and synthesized PS-g-PDA/RGO are compared in Fig. 2. After RAFT polymerization of styrene from the surface of the DDMATPDA/RGO, additional peaks appeared in the spectra of PS-g-PDA/ RGO (Fig. 2b). The peaks at 3103, 3082, 3059, and 3024 cm1 are attributed to the ¼ CeH stretching vibration while the peaks at 1944, 1871, 1774, and 1722 cm1 are ascribed to the overtone and
Fig. 1. Left: Styrene conversion vs. time data for solution RAFT polymerization. Right: Molecular weight vs. styrene conversion data for solution RAFT polymerization (B), free radical polymerization (-) and theoretical Mw of RAFT polymerization (blue line). AIBN initiator, 60 C, in DMF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. FTIR spectra of the synthesized (a) DDMAT-PDA/RGO and (b) PS-g-PDA/RGO.
combination vibration bands of mono-substituted aromatic compounds. The peaks at 1601, 1491, and 1450 cm1 are attributed to aromatic ring-stretching vibrations while the peaks at 1060 and 1032 cm1 are attributable to the ¼ CeH in-plane deformation vibration. The peaks at 754 and 698 cm1 are attributed to the ¼ CeH out-of-plane vibration and ring out-of-plane deformation of mono-substituted aromatic group, respectively. All these peaks indicate successful growth of polystyrene from the surface of PDA/RGO.
3.3. Kinetics of polymerization of styrene using DDMAT-PDA/RGO With polystyrene successfully grown from the surface of PDA/ RGO, it is necessary to examine whether or not the polymerization of styrene was in a controlled manner in the presence of the synthesized DDMAT-PDA/RGO macro-RAFT agent. After polymerization, the grafted PS was cleaved by acid-catalyzed transesterification in 1-butanol with the results shown in Table 2. The GPC analysis of the cleaved PS shows that the Mn increases directly with the consumption of monomer (i.e., conversion). While growing PS from the PDA/RGO surface during RAFT polymerization, some free PS would also be produced in solution. This free PS was removed from the PS-g-PDA/RGO by extensive Table 2 RAFT polymerization of styrene using DDMAT-PDA/RGO without additional free DDMAT (g: cleaved PS from PS-g-PDA/RGO, f: free PS in the reaction mixture). Reaction time (h)
Conversion (%)
Mn (g) (kDa)
Mw (g) (kDa)
Ð (g)
Mn (f) (kDa)
Mw (f) (kDa)
Ð (f)
a
24 36 47 67
5.7 14.0 24.0 34.0
12.4 36.6 53.4 63.1
15.0 46.7 68.9 85.1
1.21 1.28 1.29 1.36
34.4 61.7 80.4 80.4
54.3 95.5 116.6 121.3
1.58 1.55 1.50 1.51
26.3 64.0 109.4 154.9
Mntheo (kDa)
Note: Reaction conditions: DDMAT-PDA/RGO 31.5 mg (containing 0.010 mmol of trithiocarbonate); styrene 44 mmol; DMF 2 mL; AIBN 0.010 mmol; T ¼ 60 C. a Mntheo is theoretical molecular weight calculated according to equation (1).
washing with toluene by filtration and also measured by GPC. As shown in Fig. 3, compared with the cleaved PS (using acid hydrolysis), the free PS produced at different monomer conversions shows increased non-linear behavior with Mn, indicating weaker living/control of the molecular weights, compared with the grafted PS chains. The Mn value of the free polymer increases with conversion at the early stages of polymerization, but levels off at about 25% conversion. The Mn deviates from the theoretical molecular weight from the beginning of the polymerization. According to the plot of Mn of graft polymer, although the value of Mn does not level off before 35% conversion, the curve tends to even out at higher conversion with the free polymer having slightly higher Mn values. To explain these trends, we must consider that during the surface-initiated RAFT polymerization, a grafted-polymer radical may undergo the additionefragmentation equilibrium process with a neighboring grafted polymer, or with a free polymer chain (Scheme 3) [32]. The primary radicals produced via decomposition of AIBN predominantly attacked styrene to generate PS radicals. Some of these polymer radicals could undergo a RAFT process with one of the grafted RAFT agents, converting them to a free PS chain capped with a RAFT moiety (PSeY) [32]. The concentration of free PSeY would increase with time and conversion, although it would have little influence on controlling the polymerization. On the other hand, the grafted RAFT agent activated by the free radical would undergo propagation until it undergoes another RAFT process with a neighboring grafted chain or a termination reaction with either grafted or polymer free radicals. In this way, the molecular weight of grafted chains would slowly increase in a controlled manner. Hence, the characteristics of the grafted and free polymer can be different at the early stages of polymerization [32]. Therefore, the plot of Mn of grafted polymer does not level off as early as the plot of Mn of free polymer. As the number of dormant grafted chains decreases with conversion, the exchange reaction with neighboring dormant grafted chains would become less likely to occur. Hence, the control of molecular weight of grafted PS would become weaker as the conversion of styrene increased.
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Fig. 3. Dependence of Mn of (a) grafted and (b) free polymer on conversion. Condition: without free RAFT agents in solution. Blue line: theoretical Mn (Eq. (1)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 3. Key processes in RAFT-mediated graft polymerizations.
3.4. Kinetics of adding extra RAFT agent For the “grafting from” using living/controlled polymerization, it is common to add extra RAFT agent to the solution to help increase the livingness of polymerization [32e34]. Some research groups do not measure the molecular weight of the grafted polymer chains directly, but add free RAFT agent to the polymerization system and then measure the molecular weight of the polymer produced in solution [35,36]. To examine both approaches, free RAFT agent was added to the reaction system to investigate the polymerizations, with the molecular weight of the free polymer and grafted polymer measured by GPC (Table 3). In order to understand the effect of DDMAT-PDA/RGO and additional DDMAT on the kinetics of the polymerization of styrene, the RAFT polymerization with and without additional DDMAT was compared with the solution RAFT polymerization. As shown in Fig. 4 and Table 4, the RAFT polymerization with DDMAT-PDA/RGO demonstrated pseudo first-order kinetics with a more apparent induction period compared to the solution RAFT polymerization.
Fig. 4. First-order kinetic plots for the graft polymerization of styrene with functionalized PDA/RGO; solution RAFT polymerization, surface RAFT polymerization with and without free RAFT agent. See Tables 1, 2 and 3 for reaction conditions.
According to the results, adding free DDMAT to the reaction mixture, further strengthens the induction effect and decreases the polymerization rate. Such an induction period is commonly observed in RAFT polymerization [37]. In RAFT polymerization, the initially generated small amount of free radicals prefer to chain transfer rather than propagate, resulting in an induction period. This induction period may also partially result from possible impurities in the synthesized DDMAT-PDA/RGO, which could be from either un-reduced graphene functionalities or from the PDA chemistry. These impurities could consume some initially generated free radicals. Hence this rate retardation is dependent on the initial concentration of RAFT agent, with higher concentration of RAFT agent leading to slower polymerization rate. [38,39].
Table 3 RAFT polymerization of styrene using DDMAT-PDA/RGO with additional free DDMAT (g: cleaved PS from PS-g-PDA/RGO, f: free PS in the reaction mixture). Reaction time (h)
Conversion (%)
Mn (g) (kDa)
Mw (g) (kDa)
Ð (g)
Mn (f) (kDa)
Mw (f) (kDa)
Ð (f)
a
35 50 66 80
6.4 15.0 25.0 28.0
12.6 21.9 28.2 33.1
15.8 27.5 42.2 49.6
1.25 1.26 1.50 1.50
16.6 29.0 40.9 45.2
26.6 43.5 56.4 63.6
1.60 1.50 1.38 1.41
12.0 27.6 45.8 51.3
Mntheo (kDa)
Note: Reaction conditions: DDMAT-PDA/RGO 31.5 mg (containing 0.010 mmol of trithiocarbonate); styrene 44 mmol; DMF 2 mL; AIBN 0.010 mmol; DDMAT 0.015 mmol; T ¼ 60 C. a Mntheo is theoretical molecular weight calculated according to equation (1).
Table 4 Induction time (ti) and the apparent reaction coefficient (kapp) for different polymerization systems. Sample Identity
RAFT polymerization
“Graft from” RAFT polymerization without additional RAFT free agent
“Graft from” RAFT polymerization with additional RAFT free agent
ti (h) kapp (h1)
~ 0.73 1.44 102
~ 16.86 8.44 103
~ 20.76 5.76 103
Note: ti is the cross-over time, and kapp is the slope of ln([M]0/[M]) curve versus time.
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3.5. GPC traces
Fig. 5. A comparison of number-average molecular weights of free (red) and grafted (blue) PS synthesized with (* or þ) and without (, or B) addition of free DDMAT versus conversion of monomer. See Tables 2 and 3 for reaction conditions. A smooth line is drawn through the data points to aid visualization. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5 shows the plot of Mn of grafted (cleaved) and free PS as a function of styrene conversion. Compared with polymerizations of styrene without adding free DDMAT in the presence of DDMATPDA/RGO, polymerization with additional RAFT agent DDMAT demonstrated more pronounced character of living/controlled polymerization, with nonzero initial molecular weights, which is similar to solution RAFT polymerization. Meanwhile, the molecular weights of the formed PS were significantly affected by the additional free DDMAT. As shown in Fig. 5, the number-average molecular weights of both free and grafted PS samples obtained at the same conversion decreased substantially after addition of free DDMAT to the reaction mixture. According to equation (1), an increase in overall initial concentration of the RAFT agents would eventually result in a decrease in molecular weight of the formed polymers.
After the polymerization, the free polymer and the synthesized PS-g-PDA/RGO composite were separated by membrane filtration, with the synthesized PS-g-PDA/RGO composite acid cleaved to obtain cleaved PS. Complete cleavage of PS from the surface of PDA/ RGO was confirmed by the absence of any sulfur signal from EDX elemental analysis of the residual solid (data not shown). The obtained free and grafted polymers were then characterized by GPC. As shown in Fig. 6aeb, with the polymerization proceeding from 24 h to 67 h, the retention times of the grafted and free PS decreased from 16.66 min to 14.06 min (Fig. 6a) and from 15.33 min to 13.67 min (Fig. 6b), respectively. The dispersities of the grafted PS were between 1.21 and 1.36, lower than those of the free PS between 1.50 and 1.58, again being consistent with the RAFT polymerization mechanism. As shown in Fig. 6ced when adding in extra RAFT agent, the dispersities of the grafted PS were between 1.25 and 1.50, close to those of the free PS between 1.38 and 1.60. Similar to the RAFT polymerization without adding free DDMAT, the molecular weights of the free PS were higher than those of the grafted PS, which can be visualized by comparing the chromatograms of the grafted and free PS, as the retention times of the free PS (Fig. 6d) are shorter than those of grafted PS (Fig. 6c) at the same reaction time. The slower polymerization may in part be attributed to limited diffusivity of monomer to the free radicals on the layered PDA/RGO surface. The detailed surface chemistry in which PDA is coated onto reduced graphene oxide is not completely understood [40,41], and the PDA chains may hinder access to the RAFT active sites. This lower localized concentration of monomer on the surface of PDA/ RGO would lead to slower growth of the grafted polymer than the free polymer chains, as illustrated in Scheme 4. It was also noticed that the chromatograms of the grafted PS (Fig. 6a and c) show high-molecular weight shoulders, particularly at shorter reaction times, e.g., less than 36 h. These shoulders become insignificant after long reaction times such as 66 h. Moreover, there is no such shoulder appearing in the chromatograms of the free PS (Fig. 6b and d). These shoulders may be from either dead polymer or PDA-attached grafted PS. As most clearly noted in Fig. 6c, as the shoulder disappears in the longer
Fig. 6. GPC traces of the (top) grafted and (bottom) free polystyrene synthesized (left) without and (right) with additional DDMAT. See Tables 2 and 3 for reaction conditions.
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Scheme 4. Main equilibrium of the RAFT polymerization showing the growth of the free (Pf) and the grafted (Pg) PS chains.
polymerization times, this phenomenon is mainly attributed to PDA-attached grafted PS. In explanation, during the transesterification using strong acid, a small amount of PDA may be cleaved from the surface of RGO, resulting in the PDA-attached PS, which would have higher molecular weight than the cleaved PS without PDA attached. At longer reaction times with the PS chains growing much longer, the difference between the molecular weights of grafted PS and PDA-attached grafted PS became smaller, leading to less significant shoulders. The free PS was isolated directly without the acid-catalyzed trans esterification, so there was no such shoulder observed in the chromatograms of free PS.
3.6. Preparation of graphene-PS nanocomposites using the synthesized PS-g-PDA/RGO The “grafting-from” polymerization of styrene on the surface of DDMAT-PDA/RGO was also evidenced by solubility testing of the obtained PS-g-PDA/RGO in various commonly used organic solvents and water. As shown in Fig. 7, PS-g-PDA/RGO was well dispersed in toluene, chloroform, acetone, THF, and DMF but completely precipitated in hexane, methanol, ethanol, and water. With polystyrene grown on the surface of DDMAT-PDA/RGO, the synthesized PS-g-PDA/RGO demonstrated the characteristic solubility of polystyrene in these solvents. For potential use in a real system, it was also examined on whether after mixing with commercial PS, if the synthesized PS-gPDA/RGO could be dispersed in the PS matrix. Fig. 8 displays the images of the blended graphene-PS nanocomposites containing different types and amounts of PS-g-PDA/RGO samples. Four different PS-g-PDA/RGO samples comprising 10.1%, 21.9%, 39.4%, and 46.3% of RGO, respectively, were synthesized by RAFT polymerization and subsequently employed in the preparation of the blended graphene-PS nanocomposites containing three different contents, 0.50%, 1.00%, and 1.50%. As shown in Fig. 8 Row a, RGO of 0.50%e1.50% was homogeneously dispersed in the PS matrix when using the PS-g-PDA/RGO sample containing 10.1% of RGO. Slight aggregation of RGO was observed in the graphene-PS nanocomposites when using the PS-g-PDA/RGO samples containing 21.9% and 39.4% of RGO (Fig. 8 Row b and c). Employment of the PSg-PDA/RGO sample containing 46.3% of RGO resulted in significant phase separation (Fig. 8 Row d). Hence, the higher amount of PS
Fig. 7. Solubility tests of the synthesized PS-g-PDA/RGO sample dispersed in various solvents (1 mg/mL). From left to right: hexane, toluene, chloroform, methanol, ethanol, acetone, THF, DMF, and water.
grafted to the surface of RGO, the better dispersity of the surfacefunctionalized PDA-RGO in the blended graphene-PS nanocomposites.
3.7. Thermal properties of the synthesized PS-g-PDA/RGO nanocomposites TGA was used for both analyzing: i) the growing PS chains from the surface of the PDA/RGO with increased RAFT polymerization time, and ii) the graphene-PS composites prepared by mixing the PS-g-PDA/RGO with commercial PS. Fig. 9 compares the TGA plots of the synthesized DDMAT-PDA/RGO with the PS-g-PDA/RGO samples obtained at different reaction times. Since PS decomposes completely at 600 C (Fig. 9 insert), the amount of PS grown from the surface of PDA/RGO can be measured, which increased continuously from 8.6% to 49.6% at the reaction time from 10 h to 80 h. The gradually increased amount of PS on the surface of PDA/RGO agreed well with the increased molecular weight with conversion (as described above), providing additional proof of
Fig. 8. Images of the blended graphene-PS nanocomposites prepared by mixing the commercial PS and the synthesized PS-g-PDA/RGO containing different amount of RGO: (a) 10.1%, (b) 21.9%, (c) 39.4%, and (d) 46.3% (w/w). Total RGO content after blending into PS: (I) 1.5%, (II) 1.0%, and (III) 0.5%. The diameter of each sample is 7 cm.
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Fig. 9. TGA plots of the synthesized (a) DDMAT-PDA/RGO and PS-g-PDA/RGO samples obtained at different reaction time: (b) 10 h, (c) 23 h, (d) 35 h, (e) 50 h, (f) 66 h, and (g) 80 h. See Table 3 for reaction conditions. The insert is the thermogram of the commercial PS, used as a reference.
living/controlled polymerization of styrene in the presence of DDMAT-PDA/RGO. The thermal stability of the blended graphene-PS nanocomposites was also measured by TGA. Fig. 10 compares the TGA results of the commercial PS before and after mixing with different types and amounts of the synthesized PS-g-PDA/RGO. All the samples including the commercial PS showed one major weight loss corresponding to the decomposition of PS above 400 C. In order to study the effect of the different PS-g-PDA/RGO samples on the thermal stability of the final blended graphene-PS nanocomposites, the loading of RGO in the blended graphene-PS nanocomposites was maintained at a constant value of 0.25%. This was done by adding different amounts of the synthesized PS-g-
PDA/RGO samples to the commercial PS matrix, i.e., 0.54%, 0.63%, and 1.14% of PS-g-PDA/RGO comprising 46.3%, 39.4%, and 21.9% of RGO, respectively. While the commercial PS decomposed around a relatively low temperature of 418 C, the blended graphene-PS nanocomposites prepared from the PS-g-PDA/RGO samples comprising 46.3%, 39.4%, and 21.9% of RGO decomposed around 424 C, 422 C, and 426 C, respectively (Fig. 10a). The blended graphene-PS nanocomposite prepared from the PS-g-PDA/RGO samples comprising 21.9% of RGO showed the best thermal stability among the three blended samples, agreeing well with the best dispersion of the PS-g-PDA/RGO sample consisting of the highest amount of PS, as discussed above. Further study was conducted by incorporating different amounts of PS-g-PDA/RGO comprising
Fig. 10. A comparison of thermal stability of the commercial polystyrene before and after mixing with different (a) types and (b) amounts of the synthesized PS-g-PDA/RGO.
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21.9% of RGO into the commercial PS matrix. The decomposition temperature increased from 418 C to 426 C, 432 C, 434 C, and 437 C with increasing RGO content from 0% to 0.25%, 0.50%, 1.00%, and 1.50%, respectively (Fig. 10b), suggesting that the more PS-gPDA/RGO being added, the higher thermal stability being achieved by using the well-grafted PS-g-PDA/RGO sample.
[14]
4. Conclusions
[15]
[8] [9] [10] [11] [12] [13]
[16]
Polystyrene-grafted PDA/RGO was synthesized via RAFT polymerization of styrene on the surface of DDMAT-PDA/RGO, showing the characteristic FTIR peaks of polystyrene and solubility of polystyrene in several solvents. Graphene-PS nanocomposites were prepared by mixing the synthesized PS-g-PDA/RGO with commercial PS. The PS-g-PDA/RGO containing higher amounts of grafted PS demonstrated better dispersion in the matrix of the commercial PS. The thermal stability of the prepared graphene-PS nanocomposites increased with increasing content of RGO up to 1.50% by using the well-grafted PS-g-PDA/RGO sample. The kinetics study of the polymerization of styrene in the presence of DDMATPDA/RGO confirmed a characteristic mechanism of living/ controlled polymerization with and without adding DDMAT. Higher molecular weight of polymer was formed in the solution than on the surface of the macro-RAFT agent DDMAT-PDA/RGO. Pseudo first-order polymerization mechanism was observed and not affected by the additional RAFT agent DDMAT but the polymerization rate decreased significantly after adding free DDMAT. Acknowledgments
[17] [18] [19] [20] [21] [22] [23] [24] [25]
[26] [27] [28] [29] [30] [31]
The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Foundation for Innovation (CFI) for financial support.
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Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Synthesis of graphene-polystyrene nanocomposites via RAFT polymerization Renpeng Gu, William Z. Xu, Paul A. Charpentier* Department of Chemical and Biochemical Engineering, Western University, London, ON N6A 5B9, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2014 Received in revised form 22 August 2014 Accepted 24 August 2014 Available online xxx
Synthesis of graphene-polymer nanocomposites is of current interest due to their exceptionally physical and chemical properties. However, the ability to produce conductive inks/coatings retaining the properties of the graphene is still a major challenge. In this study, functionalized polydopamine-coated reduced graphene oxide (PDA/RGO) was reacted with a RAFT agent, 2-(dodecylthiocarbonothioylthio)2-methylpropionic acid (DDMAT), to form macro-RAFT agents via an esterification reaction. The chemistry and kinetics of RAFT living/controlled polymerization of styrene was examined from the surface of these macro-RAFT agents to form polystyrene-grafted PDA/RGO (PS-g-PDA/RGO). By examination of the kinetics and GPC traces of the free and grafted polymer, living/controlled polymerization was confirmed. To examine the utility of this approach, the synthesized PS-g-PDA/RGO samples containing different amounts of grafted PS were employed in the preparation of graphene-PS nanocomposites by mixing them with commercial PS. Graphene was found to be well distributed in the PS matrix by this approach, increasing the decomposition temperature range. This research provides an efficient route towards the design and development of value-added functional graphene-PS composites. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Graphene RAFT polymerization Graphene-polystyrene nanocomposites
1. Introduction Recent years have seen growing interest in graphene due to its unique properties such as excellent charge carrier mobility [1], thermal conductivity [2], and superior mechanical strength [3]. Of particular interest is the development of graphene-based advanced materials such as polymer nanocomposites [4e8]. The major challenge in the preparation of graphene-polymer nanocomposites is on how to avoid the aggregation of graphene sheets by overcoming the high energy barriers due to the strong sheet interlayer conjugation, van der Waal forces and p-p stacking between single layers [9]. Covalent bonding of nanofillers to the polymer matrix is often necessary to maximize the thermal and mechanical properties of the resulting nanocomposite [10]. However, covalent bonding may also disrupt the conjugated structure of graphene, with the resulting defects giving compromised properties such as lower electrical conductivity [11]. Among the approaches of chemical modification, including functionalizing the surfaces and edges of graphene via hydrogenation and the diazonium and azide reactions [12], coating
* Corresponding author. Tel.: þ1 519 661 3466; fax: þ1 519 661 3498. E-mail address: [email protected] (P.A. Charpentier).
graphene with a thin layer of polydopamine is promising to protect the structure of graphene, helping to remove the defects. Chemical modification using dopamine demonstrated an unparalleled advantage of simultaneous reduction of graphene oxide into graphene and coating of polydopamine onto the surface of the reduced graphene oxide [13]. Reduced graphene oxide (RGO) has shown high conductivity values of 1.494 103 S m1, whereas oxidized graphene (GO) is insulating with a low conductivity of ~ 105 S m1 [14]. Once graphene has been coated with polydopamine, a variety of polymers can potentially be grown from or attached to the surface of graphene to produce graphene-polymer nanocomposites. Various techniques have been employed in the synthesis of advanced graphene-polymer nanocomposites; including noncovalent [15], grafting-to [7], and grafting-from [16] methods. Living/controlled free radical polymerization has been extensively employed in the synthesis of polymers and polymer composites, resulting in well-defined molecular structure and lower dispersity (Ð) than those obtained from conventional free radical polymerization [17e20]. Graphene-polystyrene nanocomposites were synthesized via atom transfer radical polymerization (ATRP), showing improved thermal conductivity [21]. Due to its tolerance towards polymerizing a wide range of vinyl monomers, reversible addition fragmentation chain transfer (RAFT) polymerization is often employed in the synthesis of polymers and polymer composites.
http://dx.doi.org/10.1016/j.polymer.2014.08.064 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
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Graphene oxide-poly(N-vinylcarbazole) (GO-PVK) was synthesized using RAFT polymerization, leading to a PVK-GO-based memory device with a typical bistable electrical switching and nonvolatile rewritable memory effect [22]. In our previous work [23], graphene oxide was synthesized via oxidative exfoliation of graphite, which was then reduced into graphene and coated with polydopamine by means of self-polymerization of dopamine. This served for keeping the graphene oxide reduced to RGO while the polydopamine surface provided many attachment sites, i.e., hydroxyl groups (Scheme 1), for the RAFT agent, 2-(dodecylthiocarbonothioylthio)-2methylpropionic acid (DDMAT), was attached to the polydopamine-coated graphene via an esterification reaction. Three monomers, i.e., methyl methacrylate (MMA), N-isopropylacrylamide (NIPAM), and tert-butyl acrylate) (tBA) were successfully polymerized on the surface of DDMAT-PDA/RGO by using this macro-RAFT agent. Polystyrene is one of the most widely used thermoplastic materials, having many applications in a variety of areas such as toner inks, protective packaging, automotive, construction, and consumer products, etc. The annual consumption worldwide is even over several billion kg [25]. Design and development of valueadded advanced materials using inexpensive polystyrene is of significant economic interest. Although the kinetics of polymerization of styrene has been well studied including various living/controlled free radical polymerizations, no published work has investigated the kinetics of RAFT polymerization from the surface of polydopamine-coated graphene or graphene oxide. In the present research, styrene was polymerized from the surface of the synthesized DDMAT-PDA/RGO, which has not been examined. The kinetics of polymerization were studied while the formed polystyrene-grafted PDA/RGO (PS-g-PDA/RGO) was applied in the preparation of graphene-polystyrene nanocomposites by mixing with commercial polystyrene to show the potential utility of this approach. 2. Experimental 2.1. Materials Styrene (99%, SigmaeAldrich, containing 4-tert-butylcatechol as stabilizer) was purified by passing through a basic alumina column prior to use. 2,20 -Azobis(2-methylpropionitrile) (AIBN, DuPont) was recrystallized twice from methanol prior to use. 2(Dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT, 98%) and commercial polystyrene (PS, average molecular weight Mw ¼ 350 kDa, dispersity Ɖ ¼ 2.5) were purchased from SigmaeAldrich and used as received. Concentrated sulfuric acid (H2SO4), 1-butanol, N,N-dimethylformamide (DMF), toluene, methanol, and tetrahydrofuran (THF) were purchased from Caledon Laboratories
Ltd. and used as received. Nylon 66 filter membranes (pore size 0.2 mm) were purchased from SigmaeAldrich. 2.2. Characterization Fourier transform infrared (FTIR) spectra were recorded using KBr pellets on a Nicolet 6700 FTIR spectrometer (Thermo Scientific) with a resolution of 2 cm1 and 32 scans for each sample. Energydispersive X-ray (EDX) elemental analysis was performed using a Quartz Xone EDX scattering device attached to a Hitachi S-4500 field emission scanning electron microscope (SEM). The thermal properties were measured by thermal gravimetric analysis (TGA). The samples were heated from room temperature to 800 C at a heating rate of 10 C/min under nitrogen atmosphere on a TA Instruments SDT Q600. TGA measurements for selected samples run 3X showed standard errors to be within 1.5 C. Static light scattering (SLS) experiments to measure the weight average molecule weight Mw were measured with a Zetasizer Nano S. Numberaverage molecular weight (Mn) and dispersity (Ð) of the polymers were measured on a Viscotek TDAmax gel permeation chromatography (GPC) system equipped with a TDA 302 Triple Detector Array, a VE2001 GPC solvent/sample module. THF eluent flow rate was 1 mL/min at 30 C.
2.3. Grafting polystyrene from the surface of PDA/RGO The polydopamine-coated reduced graphene oxide (PDA/RGO) and the RAFT agent-attached graphene sheets (DDMAT-PDA/RGO) were prepared according to the previously reported method [23]. A schematic diagram for the preparation of DDMAT-PDA/RGO, the “graft-from” polymerization of styrene, and the cleavage of grafted PS is shown in Scheme 2. By using the synthesized DDMAT-PDA/ RGO, styrene was polymerized from the surface of PDA/RGO via RAFT polymerization. Typically, the synthesized DDMAT-PDA/RGO (31.5 mg, containing 0.010 mmol trithiocarbonate based on TGA analysis) was suspended in a mixture of styrene (5.0 mL, 44 mmol), AIBN (1.7 mg, 0.010 mmol), and DMF (2 mL) by sonication. The mixture was then introduced into a 10-mL dry Schlenk flask, which had been degassed and refilled with nitrogen three times, using a syringe. After degassing with nitrogen for 30 min, the reaction mixture was heated to allow the reaction to take place at 60 C for 24 h at which point the flask was immersed in an ice bath to quench the polymerization. Subsequently, the reaction mixture was slowly precipitated into methanol. Polystyrene-grafted graphene (PS-gPDA/RGO) was obtained by vacuum filtration with a nylon 66 filter membrane (pore size 0.2 mm), washing with toluene three times to remove unattached (free) polymer, and then drying in vacuo prior to characterization.
Scheme 1. Suggested structures for polydopamine coating from self-polymerization of dopamine [13,24].
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Scheme 2. Schematic diagram for the preparation of DDMAT-PDA/RGO, the “graft-from” polymerization of styrene, and the cleavage of grafted PS. Reagents and conditions: (a) K2S2O8, P2O5, H2SO4, 80 C, 4.5 h; (b) NaNO3, H2SO4, KMnO4, H2O2; (c) dopamine, PH 8.5 buffer; (d) DDMAT, EDAC, DMAP, DMF; (e) styrene, AIBN, DMF; (f) H2SO4, 1-butanol, toluene.
2.4. Kinetics of polymerization of styrene using DDMAT-PDA/RGO To study the kinetics of the RAFT polymerization process, styrene was polymerized in the presence of DDMAT-PDA/RGO, with and without additional DDMAT. In the case of polymerization without adding free DDMAT, the above polymerization conditions were employed and the reaction was stopped at preset reaction times (12 h, 24 h, 36 h, 47 h, and 67 h, respectively). During the washing process using toluene, the free polystyrene was recovered from the solution via rotary evaporation. In the case of polymerization with additional free DDMAT, DDMAT (0.015 mmol) was added to the reaction mixture before sonication. The polymerization was stopped at preset reaction times (10 h, 23 h, 35 h, 50 h, 66 h, and 80 h, respectively). The formed free polystyrene was also recovered from the solution via rotary evaporation. The conversion of monomer was measured based on the weight ratio between the formed PS and the initially added styrene. In order to measure the molecular weight of the grafted polymer, the isolated PS-g-PDA/RGO was subsequently subjected to an acid-catalyzed transesterfication to cleave the grafted polymer according to the reported method [26,27]. In a typical experiment, to a suspension of PS-g-PDA/RGO (20 mg) in toluene (10 mL), 1butanol (4.5 mL) and concentrated sulfuric acid (1 mL) were added. The mixture was sonicated for 15 min to form a homogeneous black suspension, followed by stirring at 70 C for 10 days. The suspension was then precipitated into methanol (300 mL) with stirring. After vacuum-filtration through a nylon 66 filter membrane, the residual solid was dispersed in toluene. A solution of the
grafted/cleaved polystyrene in toluene was isolated from the solid PDA/RGO by a further filtration using a nylon 66 filter membrane. The grafted PS was obtained after rotary evaporation for the measurement of molecular weight while the solid PDA/RGO was collected for elemental analysis. A conventional free radical polymerization and a solution RAFT polymerization of styrene were conducted under similar conditions for comparison. To a 10-mL dry Schlenk flask was introduced styrene (5.0 mL, 44 mmol), DDMAT (only added for the solution RAFT polymerization, 3.7 mg, 0.010 mmol), AIBN (1.7 mg, 0.010 mmol), and DMF (1 mL). After degassing with nitrogen for 30 min, the reaction mixture was heated to allow the reaction to take place at 60 C for preset times (5 h, 10 h, 20 h, and 48 h, respectively for free radical polymerization and 5 h, 10 h, 20 h, and 30 h, respectively for solution RAFT polymerization) at which point the flask was immersed in an ice bath to quench the polymerization. Subsequently, the reaction mixture was slowly precipitated into methanol. Polystyrene was isolated by filtration, washed with methanol, and then dried in vacuo. 2.5. Preparation of graphene-PS nanocomposites Graphene-PS nanocomposites were prepared by mixing the synthesized PS-g-PDA/RGO with commercial PS. PS-g-PDA/RGO samples comprising different amounts (10.1%, 21.9%, 39.4%, and 46.3%) of RGO were synthesized by employing different reaction times, which were then utilized in the preparation of graphene-PS nanocomposites containing a preset content (0.25%, 0.50%, 1.00%,
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Table 1 Synthesis of PS polymerization.
by
conventional
free
Radical polymerization
radical
polymerization
and
RAFT
RAFT polymerization
Time (h)
Conversion (%)
Mw (kDa)
Time (h)
Conversion (%)
Mw (kDa)
5 20 48
7.8 23.4 46.2
174 175 181
5 10 20 30
6.7 12.4 22.9 35.1
27.3 41.2 66.3 103
Note: Reaction conditions: styrene 44 mmol; DMF 1 mL; AIBN 0.010 mmol; DDMAT 0.010 mmol only for the RAFT polymerization; T ¼ 60 C. Mw was measured by static light scattering (SLS).
and 1.50%) of RGO. In a typical experiment, PS-g-PDA/RGO (15.2 mg, comprising 39.4% of RGO) was dispersed in THF by sonication at room temperature, followed by addition of the commercial PS (584.8 mg). The mixture was further sonicated to achieve a homogeneous suspension, which was subsequently poured onto a clean Teflon dish. A graphene-PS nanocomposite film (600 mg, containing 1.00% of RGO) was then obtained after drying under vacuum. 3. Results and discussion 3.1. Conventional free radical and RAFT polymerization RAFT polymerization of styrene monomer in solution was first examined using the DDMAT RAFT agent. A corresponding free radical polymerization without RAFT agent was also conducted as the control experiment. The molecular weight results measured by static light scattering (SLS) are shown in Table 1, and plotted in Fig. 1. Since RAFT polymerization, like other living/controlled polymerizations, is a much slower process than conventional free radical polymerization, the ratio between RAFT agent and initiator is significant. Too few initiator “active sites” result in long reaction times, while too high of an initiator concentration can impair the ability of the RAFT system, resulting in reduced polymer chain lengths. In most cases, the molar ratio used in the literature between the RAFT agent and initiator (nRAFT/ninitiator) is 10 to 4 [28e30]. Preliminary experiments showed that using a high RAFT: AIBN ratio gave very low conversions, e.g., less than 10% monomer conversion after 10 days. In this work, we found the reaction rate to be acceptable using a ratio of RAFT agent: AIBN ¼ 1:1 (Table 1). To examine whether the RAFT polymerization is well controlled, the following simplified equation is utilized:
Mn;theo ¼
½M 0 $x$mM þ mRAFT ½RAFT 0
(1)
where M n;theo is the theoretical number-average molecular weight, [M]0 and [RAFT]0 are the initial concentrations of the monomer and the RAFT agent, respectively, mM and mRAFT are the molecular weights of the monomer unit and the RAFT agent, respectively, and x is the monomer conversion. The theoretical molecular weight of RAFT polymerization using equation (1) (Fig. 1, right) assumes that the efficiency of initiator is 100% and there is no termination during the polymerization. As Fig. 1-left shows, the conversion of St increases linearly with polymerization time using the DDMAT RAFT agent. As Fig. 1-right shows, the molecular weights of PS made by RAFT polymerization grew nearly linearly as the conversion of monomer increases. As shown in the free-radical control experiments which did not use a RAFT agent, the molecular weight of polymer increases dramatically at low monomer conversion and then increases slowly afterwards. As expected, control of molecular weight using RAFT is much better than from the free radical polymerization. A slight nonzero initial molecular weight is noticed, which is indicative of deviation from ideally living/controlled free radical polymerization at the initial stages of the reaction. This might be attributed to a lag in the activation of the DDMAT RAFT agent and conversion to the polymeric RAFT species. The appearance of high initial molecular weight in RAFT has been termed hybrid behavior [31], attributed to a low transfer constant of the initial RAFT agent. Despite the nonzero initial part, the utilized equal molar ratio between initiator and RAFT agent still maintains the typical character of living/controlled polymerization in solution RAFT polymerization, which was deemed acceptable to measure the corresponding graphene surface “grafting from” RAFT polymerization. 3.2. Synthesis of PS-g-PDA/RGO By using the synthesized macro-RAFT agent, DDMAT-PDA/RGO, which was previously characterized [23], styrene was polymerized on the surface of PDA/RGO via RAFT polymerization as described by Scheme 2. The synthesized PS-g-PDA/RGO nanocomposites were characterized with FTIR. The FTIR spectra of the DDMAT-PDA/RGO and synthesized PS-g-PDA/RGO are compared in Fig. 2. After RAFT polymerization of styrene from the surface of the DDMATPDA/RGO, additional peaks appeared in the spectra of PS-g-PDA/ RGO (Fig. 2b). The peaks at 3103, 3082, 3059, and 3024 cm1 are attributed to the ¼ CeH stretching vibration while the peaks at 1944, 1871, 1774, and 1722 cm1 are ascribed to the overtone and
Fig. 1. Left: Styrene conversion vs. time data for solution RAFT polymerization. Right: Molecular weight vs. styrene conversion data for solution RAFT polymerization (B), free radical polymerization (-) and theoretical Mw of RAFT polymerization (blue line). AIBN initiator, 60 C, in DMF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. FTIR spectra of the synthesized (a) DDMAT-PDA/RGO and (b) PS-g-PDA/RGO.
combination vibration bands of mono-substituted aromatic compounds. The peaks at 1601, 1491, and 1450 cm1 are attributed to aromatic ring-stretching vibrations while the peaks at 1060 and 1032 cm1 are attributable to the ¼ CeH in-plane deformation vibration. The peaks at 754 and 698 cm1 are attributed to the ¼ CeH out-of-plane vibration and ring out-of-plane deformation of mono-substituted aromatic group, respectively. All these peaks indicate successful growth of polystyrene from the surface of PDA/RGO.
3.3. Kinetics of polymerization of styrene using DDMAT-PDA/RGO With polystyrene successfully grown from the surface of PDA/ RGO, it is necessary to examine whether or not the polymerization of styrene was in a controlled manner in the presence of the synthesized DDMAT-PDA/RGO macro-RAFT agent. After polymerization, the grafted PS was cleaved by acid-catalyzed transesterification in 1-butanol with the results shown in Table 2. The GPC analysis of the cleaved PS shows that the Mn increases directly with the consumption of monomer (i.e., conversion). While growing PS from the PDA/RGO surface during RAFT polymerization, some free PS would also be produced in solution. This free PS was removed from the PS-g-PDA/RGO by extensive Table 2 RAFT polymerization of styrene using DDMAT-PDA/RGO without additional free DDMAT (g: cleaved PS from PS-g-PDA/RGO, f: free PS in the reaction mixture). Reaction time (h)
Conversion (%)
Mn (g) (kDa)
Mw (g) (kDa)
Ð (g)
Mn (f) (kDa)
Mw (f) (kDa)
Ð (f)
a
24 36 47 67
5.7 14.0 24.0 34.0
12.4 36.6 53.4 63.1
15.0 46.7 68.9 85.1
1.21 1.28 1.29 1.36
34.4 61.7 80.4 80.4
54.3 95.5 116.6 121.3
1.58 1.55 1.50 1.51
26.3 64.0 109.4 154.9
Mntheo (kDa)
Note: Reaction conditions: DDMAT-PDA/RGO 31.5 mg (containing 0.010 mmol of trithiocarbonate); styrene 44 mmol; DMF 2 mL; AIBN 0.010 mmol; T ¼ 60 C. a Mntheo is theoretical molecular weight calculated according to equation (1).
washing with toluene by filtration and also measured by GPC. As shown in Fig. 3, compared with the cleaved PS (using acid hydrolysis), the free PS produced at different monomer conversions shows increased non-linear behavior with Mn, indicating weaker living/control of the molecular weights, compared with the grafted PS chains. The Mn value of the free polymer increases with conversion at the early stages of polymerization, but levels off at about 25% conversion. The Mn deviates from the theoretical molecular weight from the beginning of the polymerization. According to the plot of Mn of graft polymer, although the value of Mn does not level off before 35% conversion, the curve tends to even out at higher conversion with the free polymer having slightly higher Mn values. To explain these trends, we must consider that during the surface-initiated RAFT polymerization, a grafted-polymer radical may undergo the additionefragmentation equilibrium process with a neighboring grafted polymer, or with a free polymer chain (Scheme 3) [32]. The primary radicals produced via decomposition of AIBN predominantly attacked styrene to generate PS radicals. Some of these polymer radicals could undergo a RAFT process with one of the grafted RAFT agents, converting them to a free PS chain capped with a RAFT moiety (PSeY) [32]. The concentration of free PSeY would increase with time and conversion, although it would have little influence on controlling the polymerization. On the other hand, the grafted RAFT agent activated by the free radical would undergo propagation until it undergoes another RAFT process with a neighboring grafted chain or a termination reaction with either grafted or polymer free radicals. In this way, the molecular weight of grafted chains would slowly increase in a controlled manner. Hence, the characteristics of the grafted and free polymer can be different at the early stages of polymerization [32]. Therefore, the plot of Mn of grafted polymer does not level off as early as the plot of Mn of free polymer. As the number of dormant grafted chains decreases with conversion, the exchange reaction with neighboring dormant grafted chains would become less likely to occur. Hence, the control of molecular weight of grafted PS would become weaker as the conversion of styrene increased.
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Fig. 3. Dependence of Mn of (a) grafted and (b) free polymer on conversion. Condition: without free RAFT agents in solution. Blue line: theoretical Mn (Eq. (1)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 3. Key processes in RAFT-mediated graft polymerizations.
3.4. Kinetics of adding extra RAFT agent For the “grafting from” using living/controlled polymerization, it is common to add extra RAFT agent to the solution to help increase the livingness of polymerization [32e34]. Some research groups do not measure the molecular weight of the grafted polymer chains directly, but add free RAFT agent to the polymerization system and then measure the molecular weight of the polymer produced in solution [35,36]. To examine both approaches, free RAFT agent was added to the reaction system to investigate the polymerizations, with the molecular weight of the free polymer and grafted polymer measured by GPC (Table 3). In order to understand the effect of DDMAT-PDA/RGO and additional DDMAT on the kinetics of the polymerization of styrene, the RAFT polymerization with and without additional DDMAT was compared with the solution RAFT polymerization. As shown in Fig. 4 and Table 4, the RAFT polymerization with DDMAT-PDA/RGO demonstrated pseudo first-order kinetics with a more apparent induction period compared to the solution RAFT polymerization.
Fig. 4. First-order kinetic plots for the graft polymerization of styrene with functionalized PDA/RGO; solution RAFT polymerization, surface RAFT polymerization with and without free RAFT agent. See Tables 1, 2 and 3 for reaction conditions.
According to the results, adding free DDMAT to the reaction mixture, further strengthens the induction effect and decreases the polymerization rate. Such an induction period is commonly observed in RAFT polymerization [37]. In RAFT polymerization, the initially generated small amount of free radicals prefer to chain transfer rather than propagate, resulting in an induction period. This induction period may also partially result from possible impurities in the synthesized DDMAT-PDA/RGO, which could be from either un-reduced graphene functionalities or from the PDA chemistry. These impurities could consume some initially generated free radicals. Hence this rate retardation is dependent on the initial concentration of RAFT agent, with higher concentration of RAFT agent leading to slower polymerization rate. [38,39].
Table 3 RAFT polymerization of styrene using DDMAT-PDA/RGO with additional free DDMAT (g: cleaved PS from PS-g-PDA/RGO, f: free PS in the reaction mixture). Reaction time (h)
Conversion (%)
Mn (g) (kDa)
Mw (g) (kDa)
Ð (g)
Mn (f) (kDa)
Mw (f) (kDa)
Ð (f)
a
35 50 66 80
6.4 15.0 25.0 28.0
12.6 21.9 28.2 33.1
15.8 27.5 42.2 49.6
1.25 1.26 1.50 1.50
16.6 29.0 40.9 45.2
26.6 43.5 56.4 63.6
1.60 1.50 1.38 1.41
12.0 27.6 45.8 51.3
Mntheo (kDa)
Note: Reaction conditions: DDMAT-PDA/RGO 31.5 mg (containing 0.010 mmol of trithiocarbonate); styrene 44 mmol; DMF 2 mL; AIBN 0.010 mmol; DDMAT 0.015 mmol; T ¼ 60 C. a Mntheo is theoretical molecular weight calculated according to equation (1).
Table 4 Induction time (ti) and the apparent reaction coefficient (kapp) for different polymerization systems. Sample Identity
RAFT polymerization
“Graft from” RAFT polymerization without additional RAFT free agent
“Graft from” RAFT polymerization with additional RAFT free agent
ti (h) kapp (h1)
~ 0.73 1.44 102
~ 16.86 8.44 103
~ 20.76 5.76 103
Note: ti is the cross-over time, and kapp is the slope of ln([M]0/[M]) curve versus time.
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3.5. GPC traces
Fig. 5. A comparison of number-average molecular weights of free (red) and grafted (blue) PS synthesized with (* or þ) and without (, or B) addition of free DDMAT versus conversion of monomer. See Tables 2 and 3 for reaction conditions. A smooth line is drawn through the data points to aid visualization. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5 shows the plot of Mn of grafted (cleaved) and free PS as a function of styrene conversion. Compared with polymerizations of styrene without adding free DDMAT in the presence of DDMATPDA/RGO, polymerization with additional RAFT agent DDMAT demonstrated more pronounced character of living/controlled polymerization, with nonzero initial molecular weights, which is similar to solution RAFT polymerization. Meanwhile, the molecular weights of the formed PS were significantly affected by the additional free DDMAT. As shown in Fig. 5, the number-average molecular weights of both free and grafted PS samples obtained at the same conversion decreased substantially after addition of free DDMAT to the reaction mixture. According to equation (1), an increase in overall initial concentration of the RAFT agents would eventually result in a decrease in molecular weight of the formed polymers.
After the polymerization, the free polymer and the synthesized PS-g-PDA/RGO composite were separated by membrane filtration, with the synthesized PS-g-PDA/RGO composite acid cleaved to obtain cleaved PS. Complete cleavage of PS from the surface of PDA/ RGO was confirmed by the absence of any sulfur signal from EDX elemental analysis of the residual solid (data not shown). The obtained free and grafted polymers were then characterized by GPC. As shown in Fig. 6aeb, with the polymerization proceeding from 24 h to 67 h, the retention times of the grafted and free PS decreased from 16.66 min to 14.06 min (Fig. 6a) and from 15.33 min to 13.67 min (Fig. 6b), respectively. The dispersities of the grafted PS were between 1.21 and 1.36, lower than those of the free PS between 1.50 and 1.58, again being consistent with the RAFT polymerization mechanism. As shown in Fig. 6ced when adding in extra RAFT agent, the dispersities of the grafted PS were between 1.25 and 1.50, close to those of the free PS between 1.38 and 1.60. Similar to the RAFT polymerization without adding free DDMAT, the molecular weights of the free PS were higher than those of the grafted PS, which can be visualized by comparing the chromatograms of the grafted and free PS, as the retention times of the free PS (Fig. 6d) are shorter than those of grafted PS (Fig. 6c) at the same reaction time. The slower polymerization may in part be attributed to limited diffusivity of monomer to the free radicals on the layered PDA/RGO surface. The detailed surface chemistry in which PDA is coated onto reduced graphene oxide is not completely understood [40,41], and the PDA chains may hinder access to the RAFT active sites. This lower localized concentration of monomer on the surface of PDA/ RGO would lead to slower growth of the grafted polymer than the free polymer chains, as illustrated in Scheme 4. It was also noticed that the chromatograms of the grafted PS (Fig. 6a and c) show high-molecular weight shoulders, particularly at shorter reaction times, e.g., less than 36 h. These shoulders become insignificant after long reaction times such as 66 h. Moreover, there is no such shoulder appearing in the chromatograms of the free PS (Fig. 6b and d). These shoulders may be from either dead polymer or PDA-attached grafted PS. As most clearly noted in Fig. 6c, as the shoulder disappears in the longer
Fig. 6. GPC traces of the (top) grafted and (bottom) free polystyrene synthesized (left) without and (right) with additional DDMAT. See Tables 2 and 3 for reaction conditions.
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Scheme 4. Main equilibrium of the RAFT polymerization showing the growth of the free (Pf) and the grafted (Pg) PS chains.
polymerization times, this phenomenon is mainly attributed to PDA-attached grafted PS. In explanation, during the transesterification using strong acid, a small amount of PDA may be cleaved from the surface of RGO, resulting in the PDA-attached PS, which would have higher molecular weight than the cleaved PS without PDA attached. At longer reaction times with the PS chains growing much longer, the difference between the molecular weights of grafted PS and PDA-attached grafted PS became smaller, leading to less significant shoulders. The free PS was isolated directly without the acid-catalyzed trans esterification, so there was no such shoulder observed in the chromatograms of free PS.
3.6. Preparation of graphene-PS nanocomposites using the synthesized PS-g-PDA/RGO The “grafting-from” polymerization of styrene on the surface of DDMAT-PDA/RGO was also evidenced by solubility testing of the obtained PS-g-PDA/RGO in various commonly used organic solvents and water. As shown in Fig. 7, PS-g-PDA/RGO was well dispersed in toluene, chloroform, acetone, THF, and DMF but completely precipitated in hexane, methanol, ethanol, and water. With polystyrene grown on the surface of DDMAT-PDA/RGO, the synthesized PS-g-PDA/RGO demonstrated the characteristic solubility of polystyrene in these solvents. For potential use in a real system, it was also examined on whether after mixing with commercial PS, if the synthesized PS-gPDA/RGO could be dispersed in the PS matrix. Fig. 8 displays the images of the blended graphene-PS nanocomposites containing different types and amounts of PS-g-PDA/RGO samples. Four different PS-g-PDA/RGO samples comprising 10.1%, 21.9%, 39.4%, and 46.3% of RGO, respectively, were synthesized by RAFT polymerization and subsequently employed in the preparation of the blended graphene-PS nanocomposites containing three different contents, 0.50%, 1.00%, and 1.50%. As shown in Fig. 8 Row a, RGO of 0.50%e1.50% was homogeneously dispersed in the PS matrix when using the PS-g-PDA/RGO sample containing 10.1% of RGO. Slight aggregation of RGO was observed in the graphene-PS nanocomposites when using the PS-g-PDA/RGO samples containing 21.9% and 39.4% of RGO (Fig. 8 Row b and c). Employment of the PSg-PDA/RGO sample containing 46.3% of RGO resulted in significant phase separation (Fig. 8 Row d). Hence, the higher amount of PS
Fig. 7. Solubility tests of the synthesized PS-g-PDA/RGO sample dispersed in various solvents (1 mg/mL). From left to right: hexane, toluene, chloroform, methanol, ethanol, acetone, THF, DMF, and water.
grafted to the surface of RGO, the better dispersity of the surfacefunctionalized PDA-RGO in the blended graphene-PS nanocomposites.
3.7. Thermal properties of the synthesized PS-g-PDA/RGO nanocomposites TGA was used for both analyzing: i) the growing PS chains from the surface of the PDA/RGO with increased RAFT polymerization time, and ii) the graphene-PS composites prepared by mixing the PS-g-PDA/RGO with commercial PS. Fig. 9 compares the TGA plots of the synthesized DDMAT-PDA/RGO with the PS-g-PDA/RGO samples obtained at different reaction times. Since PS decomposes completely at 600 C (Fig. 9 insert), the amount of PS grown from the surface of PDA/RGO can be measured, which increased continuously from 8.6% to 49.6% at the reaction time from 10 h to 80 h. The gradually increased amount of PS on the surface of PDA/RGO agreed well with the increased molecular weight with conversion (as described above), providing additional proof of
Fig. 8. Images of the blended graphene-PS nanocomposites prepared by mixing the commercial PS and the synthesized PS-g-PDA/RGO containing different amount of RGO: (a) 10.1%, (b) 21.9%, (c) 39.4%, and (d) 46.3% (w/w). Total RGO content after blending into PS: (I) 1.5%, (II) 1.0%, and (III) 0.5%. The diameter of each sample is 7 cm.
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Fig. 9. TGA plots of the synthesized (a) DDMAT-PDA/RGO and PS-g-PDA/RGO samples obtained at different reaction time: (b) 10 h, (c) 23 h, (d) 35 h, (e) 50 h, (f) 66 h, and (g) 80 h. See Table 3 for reaction conditions. The insert is the thermogram of the commercial PS, used as a reference.
living/controlled polymerization of styrene in the presence of DDMAT-PDA/RGO. The thermal stability of the blended graphene-PS nanocomposites was also measured by TGA. Fig. 10 compares the TGA results of the commercial PS before and after mixing with different types and amounts of the synthesized PS-g-PDA/RGO. All the samples including the commercial PS showed one major weight loss corresponding to the decomposition of PS above 400 C. In order to study the effect of the different PS-g-PDA/RGO samples on the thermal stability of the final blended graphene-PS nanocomposites, the loading of RGO in the blended graphene-PS nanocomposites was maintained at a constant value of 0.25%. This was done by adding different amounts of the synthesized PS-g-
PDA/RGO samples to the commercial PS matrix, i.e., 0.54%, 0.63%, and 1.14% of PS-g-PDA/RGO comprising 46.3%, 39.4%, and 21.9% of RGO, respectively. While the commercial PS decomposed around a relatively low temperature of 418 C, the blended graphene-PS nanocomposites prepared from the PS-g-PDA/RGO samples comprising 46.3%, 39.4%, and 21.9% of RGO decomposed around 424 C, 422 C, and 426 C, respectively (Fig. 10a). The blended graphene-PS nanocomposite prepared from the PS-g-PDA/RGO samples comprising 21.9% of RGO showed the best thermal stability among the three blended samples, agreeing well with the best dispersion of the PS-g-PDA/RGO sample consisting of the highest amount of PS, as discussed above. Further study was conducted by incorporating different amounts of PS-g-PDA/RGO comprising
Fig. 10. A comparison of thermal stability of the commercial polystyrene before and after mixing with different (a) types and (b) amounts of the synthesized PS-g-PDA/RGO.
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21.9% of RGO into the commercial PS matrix. The decomposition temperature increased from 418 C to 426 C, 432 C, 434 C, and 437 C with increasing RGO content from 0% to 0.25%, 0.50%, 1.00%, and 1.50%, respectively (Fig. 10b), suggesting that the more PS-gPDA/RGO being added, the higher thermal stability being achieved by using the well-grafted PS-g-PDA/RGO sample.
[14]
4. Conclusions
[15]
[8] [9] [10] [11] [12] [13]
[16]
Polystyrene-grafted PDA/RGO was synthesized via RAFT polymerization of styrene on the surface of DDMAT-PDA/RGO, showing the characteristic FTIR peaks of polystyrene and solubility of polystyrene in several solvents. Graphene-PS nanocomposites were prepared by mixing the synthesized PS-g-PDA/RGO with commercial PS. The PS-g-PDA/RGO containing higher amounts of grafted PS demonstrated better dispersion in the matrix of the commercial PS. The thermal stability of the prepared graphene-PS nanocomposites increased with increasing content of RGO up to 1.50% by using the well-grafted PS-g-PDA/RGO sample. The kinetics study of the polymerization of styrene in the presence of DDMATPDA/RGO confirmed a characteristic mechanism of living/ controlled polymerization with and without adding DDMAT. Higher molecular weight of polymer was formed in the solution than on the surface of the macro-RAFT agent DDMAT-PDA/RGO. Pseudo first-order polymerization mechanism was observed and not affected by the additional RAFT agent DDMAT but the polymerization rate decreased significantly after adding free DDMAT. Acknowledgments
[17] [18] [19] [20] [21] [22] [23] [24] [25]
[26] [27] [28] [29] [30] [31]
The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Foundation for Innovation (CFI) for financial support.
[33]
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