Article pubs.acs.org/Macromolecules
One-Pot Controlled Synthesis of Homopolymers and Diblock Copolymers Grafted Graphene Oxide Using Couplable RAFT Agents Kun Jiang,† Chunnuan Ye,† Peipei Zhang,† Xiaosong Wang,‡ and Youliang Zhao†,* †
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ‡ Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 S Supporting Information *
ABSTRACT: An original strategy is presented to synthesize homopolymers and diblock copolymers grafted graphene oxide by simultaneous coupling reaction and RAFT process. Zfunctionalized S-methoxycarbonylphenylmethyl S′-3(trimethoxysilyl)propyltrithiocarbonate (MPTT) and R-functionalized S-4-(trimethoxysilyl)benzyl S′-propyltrithiocarbonate (TBPT) were used as couplable RAFT agents to prepare the target nanocomposites. Under similar conditions, MPTTmediated grafting reaction was liable to afford grafted chains with shorter chain length, narrower molecular weight distribution and lower grafting density than TBPT-based reaction owing to increased shielding effect and different grafting process. The grafted polymers had nearly controlled molecular weight and polydispersity ranging between 1.11 and 1.38, and the apparent molar grafting ratio was estimated to be 73.6−220 μmol/g as the molecular weights of grafted polymers were in the range of 3980−12500 g/mol. The improved solubility and dispersibility of GO−polymer composites in various solvents comprising hexane and water confirmed their amphiphilicity. The grafting process offers an opportunity to alter GO morphologies, and surface morphologies involving nanosheets, nanoparticles, and nanorods were observed as the composites were dispersed in different solvents with the aid of sonication treatment. This tandem approach is promising for surface modification of solid substrates with hydroxyl surface due to its mild conditions, straightforward synthesis and good controllability.
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INTRODUCTION Graphene is an ideal two-dimensional material with exceptional structural, chemical, and electrical properties which make it promising candidate in various fields such as microelectronics, photonic devices, composite materials and biotechniques.1−24 Both physical and chemical modifications are utilized to increase the solubility and processability and prevent the reaggregation of graphene since it can significantly enhance the electrical, physical, mechanical, and barrier properties of polymer composites. Noncovalent routes such as π−π stacking25−30 and ionic interactions31−34 have the advantage to maintain the structural and electrical quality of graphenebased materials, however, these structures may be unstable due to relatively weak interactions. Although the covalent modification is liable to disrupt basal plane conjugation of solid substrate, it allows for graphene oxide (GO) and graphene grafted with a variety of polymer brushes which enable multiple selective functional sites on each grafted chain and stable environments for further immobilization of functional moieties and biomolecules. A normal route to graphene modification is to synthesize GO−polymer nanocomposites and followed by chemical or thermal reduction to fabricate polymer-tethered graphene. Surface functional groups such as hydroxyls, epoxides, and © 2012 American Chemical Society
carboxylic acids are usually exploited for chemical modification of GO surface. Modification of GO nanosheets plays a vital role in tailoring the structure and properties of GO and improving the solubility and compatibility of GO sheets in polymer systems. Surface functionalization of GO also enables synthesis of novel GO−polymer nanocomposites with enhanced functionalities and properties such as improved thermal and mechanical properties.1−24 Stable polymer brushes with different shapes, dimensions, polymer architectures, and chemical functionalities can be prepared on substrate surface via numerous approaches. Until now, graphene and GO grafted with various functional polymers have been achieved by a variety of methods such as in situ polymerization,35−42 coupling reaction,43−50 ultrasonic irradiation,51 photopolymerization,52 and surface-initiated controlled radical polymerization (SI-CRP) such as atom transfer radical polymerization (ATRP)53−59 and reversible addition− fragmentation chain transfer (RAFT)60−62 polymerization. These methods have advantages and limitations. For instance, in situ polymerization, ultrasonic irradiation and photoReceived: November 17, 2011 Revised: January 3, 2012 Published: January 20, 2012 1346
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polymerization were easily utilized for surface modification, however, the controllability over molecular weight and polydispersity was usually poor if “living”/controlled polymerization was not used for the grafting process. Coupling reactions using as-prepared polymers afforded well-defined polymers grafted solid substrates, however, multistep syntheses were usually necessary, and the grafting density may be relatively low due to serious steric hindrance. SI-CRP techniques were liable to grow grafted polymers on solid surface with controlled functionality and adjustable grafting thickness and density, however, suitable initiators or RAFT agents should be coupled to substrate surface in advance.63 Generally, these methods can be classified into “grafting to” and “grafting from” approaches. The former could afford welldefined homopolymers and block copolymers grafted substrate with relatively low grafting density, while the latter provided hybrid samples with high grafting density and rough control over composition and architecture although the structural defects of the tethered polymeric chains resulting from side reaction such as diradical termination and irreversible chain transfer were usually unavoidable.64 In addition to these methods, one-pot approach such as simultaneous RAFT polymerization and click reaction was also efficiently used to synthesize silica particles grafted with polymers.65−67 The tandem approach has significant advantages over “grafting to” and “grafting from” approaches due to saved labor, time and cost and broad applicability and is thus more promising for surface modification. So far, the examples based on the tandem approach remain limited,65−67 thus it is very important to explore the possibility to generalize such method to surface functionalization of other substrates. In this article, we report on the use of alkoxysilanefunctionalized RAFT agents68−73 for surface modification of GO, and homopolymers and diblock copolymers grafted GO was obtained by tandem approach comprising simultaneous hydroxyl−alkoxysilane coupling reaction and RAFT process (Scheme 1). Two kinds of RAFT agents, namely, Zfunctionalized S-methoxycarbonylphenylmethyl S′-3(trimethoxysilyl)propyltrithiocarbonate (MPTT) and R-functionalized S-4-(trimethoxysilyl)benzyl S′-propyltrithiocarbonate (TBPT), were synthesized and utilized for the synthesis of target nanocomposites. Effects of reaction processes on controllability and grafting density were investigated. The resultant GO−polymer nanocomposites were characterized by IR, Raman, XPS, TGA, SEM, and TEM, and the grafted chains were cleaved from the substrate surface to perform GPC analysis. The purpose and novelty of this study mainly lie in three aspects. First, the tandem reaction via simultaneous RAFT process and coupling reaction was chosen to synthesize GO grafted with homopolymers and block copolymers since it required minimum steps while remained satisfactory controllability,64−67 and a series of composites were prepared to confirm the versatility and generality of this approach. Second, the grafting density of GO−polymer composites could be adjusted by control over reaction conditions and utilization of different types of RAFT agents, and better-defined diblock copolymers with reverse sequences were tethered to the substrate surface using Z- or R-alkoxysilane-functionalized macro chain transfer agents. Last, the resultant GO−polymer composites were dispersed in a wide range of solvents with the aid of sonication treatment, and effects of solvents and grafted polymers on the apparent morphology of composites were
Scheme 1. Synthesis of Homopolymers and Diblock Copolymers Grafted GO by MPTT (a) and TBPT (b) Mediated RAFT Process and Coupling Reaction
investigated. Our preliminary results indicated that the surface modification afforded the opportunity to alter GO morphologies, and some interesting surface morphologies such as nanorods and nanoparticles could be obtained under certain conditions. To the best of our knowledge, synthesis of betterdefined polymers grafted GO via a single step reaction has not been reported in literature thus far although it is of great importance. The success of this research further paves way for surface modification of GO and graphene with more complex macromolecular architectures such as polymer brushes comprising segments of functional block copolymers, V- and Y-shaped copolymers.
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EXPERIMENTAL SECTION
Materials. All solvents, monomers, and other chemicals were purchased from Alfa Aesar unless otherwise stated. Graphene oxide (GO) was synthesized from natural graphite powder (40 μm in size, Qingdao Henglide Graphite Co., Ltd.) using a modified Hummers method.74 S-methoxycarbonylphenylmethyl S′-3-(trimethoxysilyl)propyltrithiocarbonate (MPTT) was synthesized and purified according to our previous method. 68 3-(Mercaptopropyl)trimethoxysilane (95%) was purchased from Lancaster. 4(Chloromethyl)phenyltrimethoxysilane (95%) was purchased from ABCR GmbH & Co. KG, Germany. Methyl acrylate (MA, 99%), tertbutyl acrylate (tBA, 98%), N,N-dimethylacrylamide (DMA, 99%), Nacrylomorpholine (NAM, 97%), and styrene (St, 99%) were passed through a basic alumina column to remove the inhibitor before use. NIsopropylacrylamide (NIPAM, 97%) was recrystallized twice from mixtures of hexane and toluene. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized twice from ethanol. N,N-Dimethyl formamide 1347
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where W100,GO−polymer and W100,GO are residual weight of polymer grafted GO and pristine GO at 100 °C, W600,GO−polymer and W600,GO are residual weight of polymer grafted GO and pristine GO at 600 °C, N% was the nitrogen content of GO-g-polymer determined by elemental analysis, Mn(g) was molecular weight of grafted polymer, and MWm was molecular weight of DMA, NIPAM or NAM. Other homopolymers and diblock copolymers grafted GO samples were synthesized and purified according to similar procedures, and MPTT, TBPT, MPTT-PSt, and TBPT-PSt were used as functional RAFT agents. General Procedure to Cleave Grafted Chains from the Substrate Surface Using Large Excess of Thermal Initiators.75−77 This method was used to degraft polymeric chains from GO surface where GO−polymer nanocomposites were prepared using MPTT or MPTT-PSt as a RAFT agent. In a typical run, GO-gpolymer (0.10 g) was dispersed in 10 mL of DMF with the aid of ultrasonic pulse, and then AIBN (328 mg, 2.0 mmol) was added. The mixture was stirred for 10 min, degassed with nitrogen for 20 min, and then heated at 80 °C for 6 h. After cooling, the samples were subjected to centrifugation, decanting, concentration and precipitation. The isolated polymers were used for GPC analysis. General Procedure to Cleave Grafted Chains from the Substrate Surface Using HF. This method was used to degraft polymeric chains from GO surface where GO−polymer nanocomposites were prepared using TBPT or TBPT-PSt as a RAFT agent. In a typical run, GO-g-polymer (0.10 g) was added to a Teflon flask, and 10 mL of THF was added. The mixture was dispersed via ultrasonic pulse for 30 min, and then 0.5 mL of HF was added. The reaction was performed at room temperature in closed system overnight. The mixture was concentrated, redispersed in THF and centrifugated. The sample solution was collected and subjected to GPC analysis. Characterization. The number-average molecular weight (Mn) and polydispersity (PDI) of polymer samples were measured on a Waters 150-C gel permeation chromatography equipped with three Ultrastyragel columns with 10 μm bead size at 35 °C. Their effective molecular weight ranges were 100−10 000 for Styragel HT2, 500−30 000 for Styragel HT3, and 5000−600 000 for Styragel HT4. The pore sizes are 50, 100, and 1000 nm for Styragels HT2, HT3, and HT4, respectively. THF was used as an eluent at a flow rate of 1.0 mL/min, polystyrene samples were calibrated with PSt standard samples; other samples were calibrated using PMMA standard samples. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a Varian spectrometer at 25 °C using CDCl3 as a solvent. Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer 2000 spectrometer using KBr disks. Raman spectra were recorded on a LabRam confocal micro-Raman system. The exciting wavelength was 632.8 nm from an air-cooled He−Ne laser with a power of 16 mW and a spot of ca 3 μm on the surface. Thermal gravimetric analysis (TGA) was carried out using a Perkin-Elmer Pyris 6 TGA instrument with a heating rate of 20 °C/min under nitrogen. Scanning electron microscopy (SEM) images were carried on a Hitachi S-4700 field emission SEM system. Transmission electron microscopy (TEM) images were obtained through a Hitachi H-600 electron microscope. Dynamic light scattering (DLS) measurements were performed at 25 °C using Zetasizer Nano-ZS from Malvern Instruments equipped with a 633 nm He−Ne laser using backscattering detection. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI 5000c ESCA photoelectron spectrometer. C, H, and N were determined by combustion followed by chromatographic separation and thermal conductivity detection using a Carlo Erba 1108 Elemental Analyzer. The electrospray ionization mass spectrometry (ESI-MS) was performed on a Bruker Daltonics micrOTOF mass spectrometer. The sonication treatment was performed on an ultrasonic instrument with a maximum output power of 90 W.
(DMF) was dried over anhydrous MgSO4 and distilled under reduced pressure. Other solvents with analytical grade were obtained from Sinopharm Chemical Reagent Co., Ltd., and used as received. Synthesis of TBPT. To a stirred solution of 1-propanethiol (98%, 2.33 g, 30 mmol) in 50 mL of anhydrous methanol was added dropwise a solution of sodium methoxide in methanol (25 wt %, 6.48 g, 30 mmol) under nitrogen. CS2 (3.05 g, 40 mmol) was added dropwise to the solution after 30 min, and the mixture was further stirred at ambient temperature for 5 h. To the yellow solution was added 4-(chloromethyl)phenyltrimethoxysilane (95%, 7.80 g, 30 mmol), and the mixture was stirred overnight under nitrogen. The mixture was concentrated, diluted with dichloromethane, filtered off, and concentrated under reduced pressure until constant weight. S-(4Trimethoxysilyl)benzyl S′-propyltrithiocarbonate (TBPT, 10.9 g, 30 mmol) was obtained as an orange oil and used without further purification. 1H NMR (CDCl3): δ 7.59 and 7.38 (dd, J 8.0, 4H, ArH), 4.61 (s, 2H, CH2), 3.62 (s, 9H, CH3O), 3.35 (t, J 7.2, 2H, CH2), 1.74 (m, 2H, CH2), 1.02 (t, J 7.4, 3H, CH3). 13C NMR (CDCl3): δ 223.5 (CS), 137.7, 135.0, 128.7, 128.6 (PhC), 50.8, 41.2, 38.7, 21.6, 13.5. ESI-MS: m/z = 363.0564 [M + H]+; theoretical value = 363.0573 (calcd for C14H23O3S3Si). Synthesis of Couplable PSt Macro RAFT Agents. St (31.2 g, 300 mmol), MPTT (0.842 g, 2.0 mmol), and AIBN (33.0 mg, 0.20 mmol) were added to a glass tube with a magnetic stirring bar. The tube was sealed with a rubber septum, and the contents cooled with ice−water bath were degassed with bubbled nitrogen for 20 min. The polymerization was performed at 60 °C for 18 h, and then Zfunctionalized MPTT-PSt (9.36 g, 27.3% of conversion) was obtained by concentration and precipitation into cold methanol. The molecular weight and polydispersity of PSt obtained by GPC were Mn(GPC) = 4820, PDI = 1.10 (theoretical value Mn(th) = 4690). R-functionalized TBPT-PSt (Mn(GPC) = 5040, PDI = 1.27) was synthesized and purified according to a similar procedure using TBPT as a mediator. Synthesis of GO Grafted with Homopolymers and Diblock Copolymers by Tandem Approach. In a typical run, 100 mg of GO and 12.0 mL of DMF in a glass tube was subjected to ultrasonic pulse for 30 min, and NIPAM (2.26 g, 20 mmol), MPTT (84.3 mg, 0.20 mmol), and AIBN (6.6 mg, 0.020 mmol) were then added and stirred until the monomer and initiator were completely dissolved. The tube was sealed with a rubber septum, and the contents cooled with ice−water bath were degassed with bubbled nitrogen for 20 min. The tube was subsequently immersed into an oil bath preheated to 60 °C and subjected to reaction for 30 h. The mixture was cooled down and subjected to reduced pressure distillation to remove most of DMF. About 5 mL of THF was added to dissolve free PNIPAM, the mixtures were then precipitated into diethyl ether twice, and monomer conversion (79.8%) was determined by gravimetry. The mixture was dispersed in DMF and filtrated, free PNIPAM was recovered by precipitation, and GO-g-PNIPAM crude product was subjected to redispersion, centrifugation and thoroughly washed with toluene and THF for at least five times. The composite was carefully collected and dried under vacuum at 60 °C until constant weight, and about 150 mg of GO-g-PNIPAM was obtained as black powders. Grafted PNIPAM was recovered by radical-induced addition−fragmentation process:75−77 Mn(g) = 6070, PDI(g) = 1.18; free PNIPAM formed in solution: Mn(g) = 9300, PDI(g) = 1.15. The weight grafting ratio (Gr = Wgraft polymer:WGO, defined as weight ratio of grafted polymer to GO substrate in GO−polymer nanocomposite) and apparent molar grafting ratio (Gp) were determined by TGA or elemental analyses using the following equations to be Gr = 62.1% and Gp = 102 μmol/g.
Gr = (W100,GO − polymer × W600,GO) /(W600,GO − polymer × W100,GO) − 1
(1)
Gr = N% × MWm /(14 − N% × MWm)
(2)
Gp = Gr /M n(g)
(3)
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RESULTS AND DISCUSSION For surface modification of graphene and GO, one-step reaction such as in situ polymerization or radical copolymeriza1348
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Table 1. Synthesis of GO Grafted with Homopolymers by Tandem Reactiona run
M
% Cb
Mn(th)c
Mn(f)d
PDI(f)d
Mn(g)e
PDI(g)e
Gr (%)f
Gp (μmol/g)g
1 2 3 4 5 6 7 8 9 10 11 12
St NIPAM DMA NAM MA tBA St NIPAM DMA NAM MA tBA
28.5 79.8 78.7 78.8 82.9 86.3 29.8 82.4 80.6 81.2 84.5 88.6
6360 9440 8220 11500 7560 11500 6570 9670 8350 11800 7640 11700
7120 9300 8500 11900 8600 11600 6650 10300 8560 10900 8970 12200
1.17 1.15 1.17 1.12 1.16 1.15 1.30 1.25 1.24 1.21 1.22 1.24
3980 6070 5660 7550 6120 7240 5860 9680 8020 10500 8630 11200
1.14 1.18 1.16 1.12 1.15 1.19 1.33 1.24 1.26 1.24 1.26 1.25
58.6 62.1 59.4 69.9 66.1 68.6 129 158 139 148 157 145
147 102 105 92.6 108 94.8 220 163 173 141 182 129
a
Reaction conditions: [M]0:[CTA]0:[AIBN]0 = 200:1:0.2 (runs 1 and 7) or 100:1:0.2 (other runs), [M]0 = 1.5 mol/L, [CTA]0:WGO = 2.0 mmol/g, CTA = MPTT (runs 1−6) or TBPT (runs 7−12), in DMF at 60 °C for 30 h. bMonomer conversion determined by gravimetry. cTheoretically calculated molecular weight. dMolecular weight and polydispersity of free polymers. eMolecular weight and polydispersity of grafted polymers obtained by aminolysis (runs 1−6) or HF-based decoupling reaction (runs 7−12). fWeight grafting ratio. gApparent molar grafting ratio.
MPTT-mediated reaction was similar to Z-supported graft polymerization (grafting to process),68,69,78−80 in which the chain propagation was conducted in solution other than on surface, all the side reactions remained in reaction media, and the grafting reaction could be performed only if polymeric chain radicals participated in addition−fragmentation process on substrate surface. As a comparison, typical GPC traces of free and grafted PSt samples are shown in Figure 1a. In GPC
tion reported to date generally lacks good controllability although it is facile,35−42 and CRP techniques require multistep syntheses although they usually have satisfactory controllability over molecular weight, polydispersity and grafting density. The latter usually requires step by step reactions in which functional initiators or RAFT agents are tethered to the substrate surface via linkage chemistry.53−62 Thus, the development of facile methods to fabricate the target composites with minimal reaction steps and similar controllability is very urgent. In this study, alkoxysilane-functionalized RAFT agents MPTT (Zfunctionalized) and TBPT (R-functionalized) were used to synthesize homopolymers and diblock copolymers grafted GO via one-pot reaction. The tandem approach based on simultaneous hydroxyl-alkoxysilane coupling reaction and RAFT polymerization was a straightforward synthetic route with minimum steps for surface modification, in which the functional RAFT agents acted as both coupling and RAFT agents during graft reaction. Since free RAFT agent was necessary to enhance the controllability on chain length and polydispersity, they also played a role of “sacrificial” free chain transfer agent. Synthesis of GO Grafted with Homopolymers via Tandem Approach. MPTT- and TBPT-mediated grafting reaction was performed in DMF at 60 °C for 30 h, and GO− polymer nanocomposites such as GO grafted with polystyrene (PSt), poly(N-isopropyl acrylamide) (PNIPAM), poly(N,Ndimethyl acrylamide) (PDMA), poly(N-acrylomorpholine) (PNAM), poly(methyl acrylate) (PMA), and poly(tert-butyl acrylate) (PtBA) were obtained after thorough washing and centrifugation. Monomer conversion as listed in Table 1 was determined by gravimetry, and then theoretically calculated molecular weight (Mn(th)) was obtained. Free polymers produced in solution were isolated by concentration and precipitation, and surface-tethered polymers were degrafted from the substrate surface by radical-induced addition− fragmentation reaction (for MPTT-mediated reaction)75−77 or HF-based cleavage reaction of Si−O bond (for TBPTmediated reaction). As can be seen, the molecular weights of free polymers were close to the expected values, both of them were different from those of grafted polymers, and the polydispersity indices (PDI) of free and grafted polymers were always comparable. These results could be ascribed to distinct reaction processes described as follows.
Figure 1. GPC traces of free (dashed line) and grafted (solid line) PSt samples obtained by MPTT (a) or TBPT (b) mediated one-pot grafting reaction.
traces, free MPTT-PSt exhibited significant shoulder and tailing corresponding to side reactions such as radical termination and irreversible transfer, while grafted PSt obtained by degrafting process was of symmetric distribution. The molecular weight of MPTT-PSt (Mn(f)) was significantly higher than that of grafted PSt (Mn(g)), and PDI(f) was slightly higher than PDI(g). The different molecular weights of free and grafted polymers could be ascribed to shielding effect and distinct rate of addition− fragmentation chain transfer in solution and on surface. During the tandem reaction, the shielding effect can not only slow down the polymerization rate at the surface by hindering the diffusion of monomer and polymeric chain radicals to the reactive sites, but exclude the polymeric chains with higher molecular weight from the surface to participate in the RAFT process and coupling reaction. The different rate was also liable to result in grafted chains with shorter chain length since the 1349
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RAFT process was usually slowed down at the surface due to heterogeneous reaction environment. The molecular weight ratio of Mn(g)/Mn(f) was in the range of 0.56−0.71, and the polydispersity was typically lower than 1.2, suggesting MPTTmediated tandem approach could afford well-defined polymers grafted GO. All the grafted chains were of reactive thiocarbonyl thio functionalities at GO surface, thus they were “living” and potentially used for the synthesis of GO grafted with diblock copolymers. On the other hand, TBPT-mediated reaction was close to Rsupported graft polymerization (grafting from process),81−83 in which the chain propagation was conducted in solution and on surface as well, side reactions such as irreversible termination and chain transfer on substrate surface was liable to produce dead chains such as linear chains and looplike chains produced by coupling termination between surface-tethered polymeric chain radicals. Due to the presence of such dead chains, Rsupported graft reaction was difficult to be further used to produce well-defined block copolymers via straightforward process. The rates of chain propagation in solution and on surface were usually close, therefore free and grafted polymers obtained by grafting from approach in the presence of sacrificial initiator or RAFT agent always possessed similar molecular weights and polydispersity indices, evident from the results as listed in runs 7−12 of Table 1. Typical GPC traces of free and grafted PSt samples are listed in Figure 1b. The molecular weight of grafted PSt was slightly lower than that of free polymer, noticeable tailing was observed in GPC traces, and grafted PSt showed broader molecular weight distribution in GPC traces, suggesting the surface graft reaction may be more complex than homogeneous polymerization in solution. The molecular weight ratio of Mn(g)/Mn(f) was in the range of 0.88−0.96, indicating the chain length of free polymers was only slightly longer than that of grafted polymers. For surfacetethered polymers obtained by TBPT-mediated reaction, their molecular weights were close to theoretical values, and the polydispersity indices of grafted polymers were in the range of 1.24−1.33, suggesting TBPT-mediated tandem reaction could be also efficiently used for the synthesis of GO−polymer nanocomposites. In Table 1, the weight grafting ratio (Gr, defined as weight ratio of grafted polymer to GO) was determined by TGA, and the apparent molar grafting ratio was estimated by equation Gp = Gr/Mn(g). TGA curves of GO and polymers grafted GO obtained by TBPT-mediated grafting reaction are listed in Figure 2. For nanocomposites comprising grafted chains such as PNIPAM, PDMA and PNAM, elemental analysis results based on nitrogen content also gave Gr values similar to those calculated by TGA, and the deviation was estimated to be within ±8%, suggesting both of them could be efficiently used to determine the Gr values. By comparing results in Table 1, it can be seen that TBPT-mediated reaction could afford GO− polymer nanocomposites with higher molecular weight, broadened molecular weight distribution and increased grafting density, which was typical characteristic of graft reaction on the basis of grafting from approach. TBPT-mediated reaction was more similar to grafting from approach, in which the steric hindrance during grafting reaction was usually neglectable, and the rates of chain propagation and transfer on substrate surface were similar to those performed in solution; thus, the grafting density was relatively high. For MPTT-mediated reaction, however, the steric hindrance was usually unavoidable, the chain propagation was only conducted in solution, and the
Figure 2. TGA curves of GO and polymers grafted GO obtained by TBPT-mediated grafting reaction.
grafting reaction could happen only if addition−fragmentation chain transfer was efficiently performed on the substrate surface, therefore the grafting density was relatively low. For grafting reaction of a same monomer performed under similar conditions, the ratios of apparent molar grafting ratio of composites obtained by TBPT-mediated reaction to that originated from MPTT-mediated reaction were in the range of 1.36−1.69 even if the former could afford longer grafted chains. Synthesis of GO Grafted with Diblock Copolymers with Reverse Sequences via Tandem Approach. The simultaneous RAFT process and coupling reaction were potentially applicable to synthesize solid substrate grafted with block copolymers with reverse sequences.65−67 To confirm the ability to synthesize block copolymers grafted GO via onepot method, Z- and R-functionalized macro CTAs were synthesized by RAFT polymerization mediated by MPTT or TBPT, and the resultant MPTT-PSt and TBPT-PSt were then used to mediate graft polymerization of NIPAM, MA, and tBA. When the reaction was performed in DMF at 60 °C for 30 h, the results are listed in Table 2. As expected, the molecular weights of free diblock copolymers were close to the theoretical values, and grafted copolymers obtained by MPTT-PSt mediated grafting reaction were usually lower than those prepared by reaction using TBPT-PSt mediator under similar conditions. The GPC traces of the resultant grafted block copolymers were completely shifted toward higher molecular weight values as compared with those of original macro CTAs, corresponding to efficient chain extension polymerization. Graft reaction mediated by MPTT-PSt afforded grafted copolymers with low polydispersity (PDI = 1.11−1.16), and no notable shoulder and tailing were observed in GPC traces (Figure 3a), suggesting well-defined diblock copolymers grafted GO were obtained by tandem approach. During MPTT-PSt mediated reaction, the resultant free diblock copolymers had enhanced polydispersity (PDI = 1.28−1.36) due to the presence of coupling termination and condensation reaction between trimethoxysilane moieties, evident from significant shoulders in GPC traces (Figure 3a). TBPT-PSt mediated reaction was liable to afford free and grafted diblock copolymers with similar polydispersity ranging between 1.32 and 1.41, and obvious shoulder and tailing were observed in GPC traces (Figure 3b), corresponding to the presence of impurities containing triblock copolymers formed by coupling termination. Meanwhile, the grafting density obtained by 1350
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Table 2. Synthesis of Diblock Copolymers Grafted GO by Tandem Approach Using MPTT-PSt (Runs 1−3) and TBPT-PSt (Runs 4−6) as Macro CTAsa run
M
% Cb
Mn(th)c
Mn(f)d
PDI(f)d
Mn(g)e
PDI(g)e
Gr (%)f
Gp (μmol/g)g
1 2 3 4 5 6
NIPAM MA tBA NIPAM MA tBA
33.8 36.2 34.3 35.8 38.3 36.8
12500 11800 13600 13100 12400 14500
12400 13000 13300 13500 13900 14200
1.31 1.36 1.28 1.41 1.35 1.33
9830 10300 9820 11900 12500 12200
1.16 1.11 1.15 1.38 1.36 1.32
74.9 79.6 72.3 142 155 140
76.2 77.3 73.6 119 124 115
a
Reaction conditions: [M]0:[macro CTA]0:[AIBN]0 = 200:1:0.2, [M]0 = 1.2 mol/L, [CTA]0:WGO = 2.0 mmol/g, macro CTAs were MPTT-PSt (runs 1−3, Mn = 4820, PDI = 1.10) or TBPT-PSt (runs 4−6, Mn = 5040, PDI = 1.27), in DMF at 60 °C for 30 h. bMonomer conversion. c Theoretical molecular weight. dMolecular weight and polydispersity of free polymers. eMolecular weight and polydispersity of grafted polymers cleaved from the substrate surface. fWeight grafting ratio. gApparent molar grafting ratio.
Figure 3. GPC traces of macro CTA PSt (solid line), free (dashed line), and grafted (dotted line) PSt-b-PMA diblock copolymers obtained by one-pot grafting reaction. PSt samples were synthesized by RAFT polymerization mediated by MPTT (a) and TBPT (b), and diblock copolymers were obtained by runs 1 and 4 of Table 2.
Figure 4. IR spectra of GO−polymer nanocomposites obtained by TBPT-mediated grafting reaction.
TBPT-PSt mediated reaction was significantly higher than that using MPTT-PSt mediator even if the grafted copolymers had longer chain length. These results demonstrated both R- and Zfunctionalized macro CTAs could be efficiently used to synthesize diblock copolymers grafted GO using the versatile tandem approach, and MPTT-PSt mediated grafting reaction was liable to afford better-defined grafted copolymers at the cost of lower grafting density. These AB and BA-type diblock copolymers grafted onto GO surface are expected to exhibit different physicochemical properties due to their distinct sequence structures. In addition, functional tri- and tetrablock copolymers grafted onto GO were potentially prepared by tandem approach mediated by alkoxysilane-functionalized diand triblock copolymers.66,67 Characterization of GO−Polymer Nanocomposites. Chemical structures of GO−polymer nanocomposites were confirmed by FT-IR, Raman, and XPS. In IR spectra (Figure 4), typical absorptions corresponding to characteristic groups of GO and grafted chains appeared at about 1726 (νCO, tBA unit, and GO sheet), 1736 (νCO, MA unit, and GO sheet), 1732 (νCO, GO sheet), 1650 (νCC of GO sheet and νCO of NIPAM unit), 1635 (νCC of GO sheet and νCO of NAM unit), 1628 (νCC of GO sheet and νCO of DMA unit), 1625 (νCC, St unit, and GO sheet), and 697 cm−1 (C−H out of plane bending, St unit). In Raman spectra (Figure 5), two representative peaks were noted at around 1332 (D band) and 1592 cm−1 (G band). The D/G intensity ratio increased from
Figure 5. Raman spectra of GO (a) and various GO-g-polymer samples (b−g).
1.7 (GO) to 2.0−2.5 (nanocomposite) due to increasingly disordered structures resulting from the formation of covalent bonds between GO and polymer chains.1−20 In addition, characteristic signals for N 1s, O 1s, C 1s, S 2p, Si 2s, and Si 2p in XPS spectra also confirmed the grafted polymers were covalently tethered to GO surface. In XPS spectra of typical GO−polymer nanocomposites (Figure 6), C1s peaks appeared at about 285.0 (C−C), 286.9 (C−O, C−N and C−S) and 289.0 eV (O−CO), and other peaks corresponding to surface-tethered RAFT functional moieties were observed at about 161.8 (S 2p), 150.8 (Si 2s), and 99.8/101.5 eV (Si 2p). The silicon weight contents were determined to be 0.72% (GO1351
dx.doi.org/10.1021/ma2024655 | Macromolecules 2012, 45, 1346−1355
Macromolecules
Article
the molar grafting ratio of homopolymer and diblock copolymer grafted onto GO surface was varied between 73.6 and 220 μmol/g, suggesting about 3.7−11% of trimethoxysilane functionalities had efficiently participate in the coupling reaction to form covalently bonded grafted chains. Our study also indicated that the chemical compositions and grafting density of GO−polymer nanocomposites could be potentially adjusted by control of reaction conditions such as feed ratio, temperature, chain length and types of functionalized RAFT agents. The resultant GO−polymer nanocomposites obtained by surface modification exhibited considerable solubility and dispersibility in a wide range of solvents, which also demonstrated the success of grafting. After sonication treatment, all the homopolymers and diblock copolymers grafted GO could be partly or completely dispersed in various solvents including hexane, toluene and water. These results indicated the composites were of more or less amphiphilicity, however, their solubility and dispersibility were usually distinct from both GO and the corresponding polymers. With the aid of sonication treatment, GO were efficiently dispersed in methanol, ethanol, THF, DMF, and water and partly dispersible in acetone, however, it could not be dispersed in hexane, toluene, and chloroform at all. Homopolymers and diblock copolymers were usually precipitated in poor solvents such as hexane and/or methanol. Because of their considerable dispersibility in a series of polar and apolar solvents, the amphiphilicity behaviors of GO−polymer composites in solvents were more like surfactants. Table 3 lists the rough sonication time to obtain pseudo homogeneous solutions with dispersion degree close to a maximum value, after that only limited enhancement in solubility and dispersibility was observed even if much longer time was used. All the GO−polymer composites showed limited solubility lower than 50 μg/mL in hexane and had variable solubility up to 1.0 mg/mL or more in most of other solvents. For instance, with the aid of sonication treatment, GO-g-PSt could be completely dispersed in water within 1 h and other media in a few minutes except that it had poor dispersibility in hexane and methanol, and hydrophilic segments such as PNIPAM, PDMA, PNAM, and PAA grafted GO could be partly or completely dispersed in various solvents typically within 5−60 min. From these results, we could deduce that stable solutions were liable to form by the utilization of solvents that could dissolve the grafted chains and/or possess strong interactions with composites, in which it only took short time to form “homogeneous” solution. The resultant solutions could be stably stored at room temperature for a period of time
Figure 6. XPS wide-scan spectra (A) and Si 2p core-level spectra (B) of pristine GO (a) and GO grafted polymers (b−d). GO−polymer composites were synthesized by runs 7, 8, and 11 of Table 1.
g-PSt), 0.57% (GO-g-PNIPAM), and 0.64% (GO-g-PMA) by combination of XPS and elemental analyses. If all the surfacetethered RAFT agents had participated in the graft polymerization to form well-defined grafted chains, the corresponding silicon contents were estimated to be 0.27% (GO-g-PSt), 0.18% (GO-g-PNIPAM), and 0.20% (GO-g-PMA). The difference in silicon contents indicated that part of surface-tethered RAFT agents had not efficiently participated in the graft polymerization or only formed short grafted chains on GO surface. The weight ratio of grafted polymeric chains to GO was estimated to be in the range of 0.586−1.55 by TGA (Tables 1 and 2), and
Table 3. Solubility and Rough Ultrasonic Time To Attain an Efficiently Dispersed Solution of GO−Polymer Nanocomposites in Various Solventsa solvent sample
hexane
toluene
chloroform
methanol
ethanol
acetone
THF
DMF
water
GO GO-g-PSt GO-g-PNIPAM GO-g-PDMA GO-g-PNAM GO-g-PAA
>60, − >60, ± >60, ± >60, ± 15, ± 60, ±
>60, −
