Preparation of Solution-Processable Reduced Graphene Oxide

Jan 13, 2015 - The abilities to enhance the degree of orientational freedom of dipole in polymer dielectrics and strengthen the homogeneous dispersion...
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Preparation of Solution-Processable Reduced Graphene Oxide/ Polybenzoxazole Nanocomposites with Improved Dielectric Properties Yi Chen,† Shuo Zhang,‡ Xiaoyun Liu,*,† Qibing Pei,§ Jun Qian,† Qixin Zhuang,*,† and Zhewen Han† †

The Key Laboratory for Ultrafine Materials of The Ministry of Education, School of Materials Science and Technology, East China University of Science and Technology, Shanghai 200237, China ‡ Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325-3909, United States § Soft Materials Research Laboratory, Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: The abilities to enhance the degree of orientational freedom of dipole in polymer dielectrics and strengthen the homogeneous dispersion of conductive fillers in matrix are of crucial importance for fabricating composite materials with high dielectric constant, low dielectric loss, low density, and good processability. Compared with conventional main-chain polybenzoxazoles, whose processability and dielectric performance are strictly limited by the conjugated benzoxazole groups on the backbone, improved solubility in dimethylformamide and dielectric constant (4.92) were observed for poly(2isopropenylbenzoxazole) (P(2-IBO)), due to the high mobility of the dipole (benzoxazole ring) on the side chains. In addition, improved dispersion of conductive graphene nanosheets was achieved by a surface-initiated atom transfer radical polymerization (ATRP) of the N-(2-hydroxyphenyl)methacrylamide (oHPMAA), the precursor of 2-isopropenylbenzoxazole from reduced graphene oxide (RGO). The nanocomposites of functionalized graphene and P(2-IBO) possess a dielectric constant of 8.35 (approximately 70% higher than that of pure P(2IBO) at 1 kHz) when the weight fraction of functionalized graphene reaches 0.015, the lowest so far among the reports on dielectric property of the graphene/polybenzoxazole system. arranging permanent dipoles as side chains on flexible aliphatic backbone, as demonstrated in this study. Poly(N-(2hydroxyphenyl)methacrylamide) P(o-HPMAA), polymerized from the precursor of 2-IBO, was considered to be well compatible with its dehydrated cyclic counterpart due to the similarity in chemical structure and composition. Recently, conductive fillers/polymer composite was developed as an alternative to high-performance dielectric ceramic/ polymer composite, overcoming the fragility and relatively high dielectric loss induced by high loading fraction of perovskite ceramics.11 Graphene is a two-dimensional sheet of sp2hybridized carbon with excellent electrical, thermal, and mechanical properties12,13 and has great prospect in catalysts, supercapacitors, and energy storage devices.14−16 Among various methods,17−19 chemical reduction of graphene oxide (GO) is considered as the most appealing approach to produce graphene inexpensively on a large scale.20 GO can be well

1. INTRODUCTION Characterized by high energy storage density, charging and discharging rate, resistance to aging, and stable performance in extreme environment, energy storage capacitors have broad potential in both civil and military fields.1−5 To overcome the drawbacks (poor processability, high density, and low breakdown strength) of a traditional perovskite ceramic capacitor, researchers focused on low-density, flexible, and self-healing polymers which can be processed and fabricated from solution at large scale and reduced cost.6−8 Polybenzoxazole (PBO) is a heterocyclic polymer with excellent thermal stability, mechanical strength, and photophysical properties.9,10 However, the dielectric constant of PBO is small due to the poor orientation of the permanent dipoles. In addition, PBO is only soluble in strong acids and therefore cannot be processed in ordinary organic solvents. To improve the mobility or degree of orientational freedom of the dipoles and to enhance the solubility in ordinary organic solvents are of great importance to the development of dielectric polymers capable of storing high-density electricity. Poly(2-isopropenylbenzoxazole) (P(2-IBO)) can overcome these problems by © 2015 American Chemical Society

Received: November 19, 2014 Revised: January 4, 2015 Published: January 13, 2015 365

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Macromolecules dispersed in neutral and basic aqueous solutions21 and stabilized by electrostatic interactions between negatively charged basal planes. However, reduced GO (RGO) nanosheets in dispersions tend to irreversibly aggregate due to π−π interaction. Therefore, grafting functionalities from RGO was proposed to weaken such interlayer interactions and promote compatibility between RGO and solvents or polymers, and this strategy was demonstrated in numerous examples by modifying RGO with either polymers22,23 or small molecules24,25 to gain the composite materials with improved dispersity, optical, mechanical, and thermal properties. To best of our knowledge, for the first time o-HPMAA was polymerized via atom transfer radical polymerization (ATRP) and grafted from RGO macroinitiator via the same strategy. The P(o-HPMAA)-grafted RGO can be readily blended with P(2-IBO) via a solution process, and a dielectric constant of 8.35 was observed in the resulting nanocomposite at filler (functionalized RGO) weight fraction (Mc) of 0.015, much lower than the previously reported data at the ε′ maxima (12 wt % for multiwalled nanotubes;26 4 wt %27 and 8 wt %28 for graphene, etc.). This is also the first report on P(o-HPMAA)grafted RGO via ATRP and on the dielectric property of P(2IBO) and its composites with graphene nanosheets.

graphene. Samples were prepared by dropping colorless dilute solution in ethanol on silicon wafers for FESEM and AFM and on amorphous carbon-coated copper grids for TEM. They were dried thoroughly at ambient conditions prior to characterization. Thermal gravimetric analysis (TGA) was carried out on a DuPont 1090B thermal gravimetric analyzer at heating rate of 10 °C/min under nitrogen flow (20 mL/min). 2.3. Preparation of the P(2-IBO)/Graphene Composite Material. 2.3.1. Free Radical Polymerization of o-HPMAA and 2IBO. o-HPMAA and 2-isopropenylbenzoxazole were synthesized according to the reported methods,30,31 and the structure characterization could be found in Figures S1−Figure S8. 1.2144 g of oHPMAA (6.9 mmol) and 16 mg of AIBN (0.1 mmol) were dissolved in 8 mL of acetone in a 25 mL flask. The polymerization proceeded at 70 °C for 20 h under an argon atmosphere. The polymer was recovered by pouring the solution into 50 mL of Et2O and collected with Buchner funnel followed by copious washing with deionized water and ethanol, respectively. The white solid (P(o-HPMAA)) was dried at 40 °C for 10 h in a vacuum oven. In the IR spectrum (Figure S4), successful polymerization of o-HPMAA was confirmed by disappearance of absorption bands at νCC (1619 cm−1), γ=C−H (948 cm−1), and its doubled frequency (1896 cm−1). Polymerization of 2-IBO was carried out via the similar procedures. In the IR spectrum (Figure S8), successful polymerization of 2-IBO was confirmed by the absence of absorption bands at 1870 cm−1 (γ=C−H) and reduced absorption at 932 cm−1 (γ=C−H), and the characteristic absorption bands of the oxazole group at 1637 cm−1 (νCN), 1095 cm−1 (νsC−O−C), and 1122 cm−1 (νasC−O−C) were preserved. The number-average molecular weight (M n ) and polydispersity index (PDI) of P(2-IBO) were determined to be 3.8 × 104 and 2.12, respectively, by GPC (Figure S9). 2.3.2. ATRP of o-HPMAA. 2.2150 g of o-HPMAA (12.5 mmol), 0.0250 g of EBIB (0.128 mmol), 0.0500 g of PMDETA (0.288 mmol), and 15 mL of DMF were degassed in a Schlenk flask via three freeze− pump−thaw cycles, and 0.0180 g of CuBr (0.125 mmol) was added under a nitrogen flow followed by another freeze−pump−thaw cycle. The solution was stirred magnetically at 70 °C for 24 h in an oil bath, and the polymerization was ceased by quenching the flask into liquid nitrogen. The raw product was collected by Buchner funnel after the solution was precipitated into methanol. The copper complex was removed from the polymer/ethyl acetate solution through column chromatography with neutral alumina repeatedly. The colorless P(oHPMAA) was obtained via rotary evaporation of solvent at 35 °C under reduced pressure. The disappearance of chemical shift at δ = 5.54 and δ = 5.94 in the 1H NMR spectrum (Figure S10) suggests the polymerization of o-HPMAA monomer. The number-average molecular weight (Mn) and polydispersity index (PDI) of P(oHPMAA) were determined to be 1.2 × 105 and 1.53, respectively, by GPC (Figure S11). 2.3.3. Synthesis of Hydroxyl-Functionalized Graphene Nanosheets (RGO-OH). RGO nanosheets were produced by chemically reducing GO sheets in aqueous solution32 and further functionalized with hydroxyl groups via diazonium addition reaction.25 A typical practice was described as follows. 500 mg of GO and 500 mg of sodium dodecylbenzenesulfonate (SDBS, 1.8 mmol) were added into 250 mL of deionized water, and the mixture was ultrasonicated for 30 min and stirred overnight. 2 mL of hydrazine hydrate (80%) was added slowly to the suspension, and the mixture was refluxed at 100 °C for 4 h. Subsequently, 2 g of 2-(4-aminophenyl)ethanol (14.6 mmol) and 1.5 mL of isoamyl nitrite (11.2 mmol) were added to the mixture followed by reaction at 80 °C in an oil bath under magnetic stirring overnight. After being cooled to room temperature, the suspension was filtered with a Teflon filter (0.2 μm in pore size) and washed copiously with deionized water and N,N-dimethylformamide (DMF) until the filtrate was transparent. The black solid (RGO-OH) obtained was dried at 60 °C under vacuum. 2.3.4. ATRP of o-HPMAA Grafted from Graphene.33−35 Graphene macroinitiator (RGO-Br) used in ATRP was synthesized according to reported methods.34,36,37 41 mg of RGO-Br (dispersed in 2 mL of NMP with ultrasonication), 8.86 g of o-HPMAA (50.0 mmol), 0.02

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Graphene oxide (GO) was prepared via the Hummers method.29 2-Bromoisobutyryl bromide (BIBB), hydrazine hydrate (N2H4·H2O), cuprous bromide (CuBr), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), o-aminophenol, lithium chloride (LiCl), and methacryloyl chloride were purchased from Aladdin Reagents (Shanghai) Co., Ltd. 2-(4Aminophenyl)ethanol from Alfa Aesar, ethyl α-bromoisobutyrate (EBIB), and isoamyl nitrite from Tokyo Chemical Industry Co., Ltd. (TCI), were used as received. CuBr was washed copiously with glacial acetic acid and ethanol, respectively, and dried under vacuum at room temperature before sealed in a brown vial. N-Methyl-2-pyrrolidone (NMP) and tetrahydrofuran (THF) were distilled over calcium hydride (CaH2) immediately prior to use. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized from absolute ethanol, filtrated, and dried under vacuum at room temperature over P2O5. All other reagents were used as received. 2.2. Instruments and Measurements. Fourier-transform infrared spectroscopy (FTIR) was performed at room temperature with a Nicolet Magna-IR 550 FTIR spectrometer. The samples were pressed into plates with spectroscopic grade KBr. 1H NMR spectra were recorded on a Bruker AVANCE III NMR spectrometer operating at 500 MHz. The molar mass of o-HPMAA was determined via electrospray ionization−liquid chromatography/time-of-flight mass spectrometry (ESI-LC/TOF MS) on a Micromass LCT (Micromass UK Ltd., Altrincham, UK). Molar mass of 2-IBO was collected on an Agilent 6120 LC/MS instrument equipped with an ESI-TOF mass detector (Agilent Technologies, Santa Clara, CA). Molecular weights of polymers were determined by multidetector gel permeation chromatography (GPC) on a Waters liquid chromatograph equipped with a Waters 515 HPLC pump, Optilab rEX, DAWN HELEOS (laser light scattering detector, λ = 690 nm) and ViscoStar (Wyatt Technology Corporation) with DMF as an eluent (flow rate: 1 mL/ min). Number-average molecular weight Mn and polydispersity index (PDI, Mw/Mn) were determined by gel permeation chromatography (Waters 1515 GPC, THF) with polystyrene standard. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250Xi (Thermo Fisher, USA) equipped with a monochromatic Al Kα X-ray source. The morphology was probed by Hitachi S-4800 fieldemission scanning electron microscope (FESEM) and JEOL JEM2010 transmission electron microscope (TEM). Height and phase images were collected on Digital Instruments MultiMode atomic force microscope (AFM) to measure the thickness of functionalized 366

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Scheme 1. Synthesis Routes of RGO-P(o-HPMAA) and Preparation routes of P(2-IBO)/RGO-P(o-HPMAA) Nanocomposite

mL of ethyl-2-bromoisobutanoate (0.1 mmol), 0.05 mL of PMEDTA (0.2 mmol), and 15 mL of THF were degassed in a Schlenk flask via three freeze−pump−thaw cycles. 30 mg of CuBr (0.2 mmol) was added to the flask under nitrogen flow, and the mixture was further sealed under vacuum. Grafting o-HPMAA from RGO-Br via ATRP was achieved by heating the Schlenk flask at 60 °C for 12 h under magnetic stirring in an oil bath, and the polymerization was ceased by quenching the flask into liquid nitrogen. RGO-P(o-HPMAA) was collected with a Teflon filter (0.2 μm in pore size) and washed copiously with THF. In addition, the homo-P(o-HPMAA) was recovered by dropping the filtrate into 40 mL of methanol and collected with a Buchner funnel followed by thorough washing with deionized water and methanol. The copper complex in both RGOP(o-HPMAA) and homo-P(o-HPMAA) was removed through column chromotography with neutral alumina repeatedly. The molecular weight of the P(o-HPMAA) on graphene nanosheets was estimated via GPC over the homo-P(o-HPMAA) (Mn = 17 600, PDI = 1.7, Figure S12) initiated by the ethyl α-bromoisobutyrate during the ATRP. RGO-P(o-HPMAA) and homo-P(o-HPMAA) were dried at 60 °C under vacuum. 2.3.5. Fabrication of P(2-IBO)/RGO-P(o-HPMAA) Nanocomposite. A solution method was used to fabricate the P(2-IBO)/RGO-P(oHPMAA) nanocomposite. A typical practice to fabricate the sample containing 1.0 wt % RGO-P(o-HPMAA) was described as follows. 30.5 mg of RGO-P(o-HPMAA) powder was dispersed in 10 mL of DMF with 10 min ultrasonication at room temperature. The suspension was then mixed with P(2-IBO)/DMF solution (3 g/20 mL) followed by 15 min ultrasonication and 2 h mechanical stirring. The nanocomposite (P(2-IBO)/RGO-P(o-HPMAA)) was obtained by precipitating the mixture into 100 mL of deionized water and collected with a Buchner funnel. The gray precipitate was washed copiously with deionized water and ethanol and dried at 60 °C in a vacuum oven.

3. RESULTS AND DISCUSSION o-HPMAA was synthesized by reaction of o-aminophenol and methacryloyl chloride (Scheme 1) as confirmed by UV−vis spectra in Figure 1. The absorption bands at around 226 nm, derived from the ethylenic band in phenyl groups (E1 band, λmax = 184 nm), and a bathochromic shift can be indexed to the ortho-substituent auxochrome groups (−OH and −NH). A broad absorption band at around 261 nm in Figure 1a,b was

Figure 1. UV−vis absorption spectra of o-HPMAA, P(o-HPMAA), 2IBO, and P(2-IBO). 367

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Figure 2. XPS survey spectrum (a), high-resolution Br 3d spectrum (inset of (a)), and C 1s spectrum (b) of RGO-Br.

attached to Figure 2a as an inset. The peak at 66−69 eV corresponding to BE of Br 3d further confirmed the presence of isobutyrate on the graphene nanosheets and successful modification of the RGO-OH with BIBB. RGO-P(o-HPMAA) was obtained via ATRP of o-HPMAA using RGO-Br macroinitiator and CuBr/PMDETA catalyst in THF. In Figure 2b, the high-resolution spectrum of C 1s could be fitted into a four-component model with binding energies (BEs) at 283.8, 284.6, 286.4, and 288.4 eV, attributed to the sp2-hybridized carbon, sp3-hybridized carbon, and C−O/C−Br and O−CO species, respectively.47−50 In addition, due to the residual C−O on the RGO nanosheets, the integration ratio [C−O/C−Br]/[O−CO] (determined from the sensitivityfactor-corrected C 1s high-resolution spectrum) was higher than the theoretical value of 2 (according to the structure of the bromide substituted ester), as expected. The FT-IR spectra of RGO-OH, RGO-Br, and RGO-P(oHPMAA) are shown in Figure 3. For RGO-OH, the broad absorption band at 3430 cm−1 is ascribed to the stretching vibration of hydroxyl groups on the RGO-OH and the one at 1176 cm−1 to the stretching vibration of hydroxyl groups (νC−O). For RGO-Br, the prominent vibration peak of carbonyl group located at 1720 cm−1 was ascribed to stretching vibration

related to the benzenoid band (B band). The absence of the fine structure on this B band absorption was also ascribed to the substituents on the benzene ring. Absorption around 293 nm (R band), corresponding to the n → π* transition in the p−π conjugation structure in carbonyl groups (−CO), was the result of the conjugation effect from the adjacent unsaturated carbon−carbon double bond (CC) and the auxochrome −NH. After polymerization, the conjugation effect was weakened to some extent, and thus the absorption had a slight hypochromatic shift to 285 nm in Figure 1b. In addition, the reduction of the whole molecular system restricted the formation of the monomer−excimer after the polymerization, and every basic chromophore was separated by the repeating units on the backbone of the polymer. Therefore, a strong absorption around 245 nm (E2 band, equivalent to the K-band) emerged, corresponding to another characteristic π → π* transition in the phenyl group.38,39 A through-conjugation structure formed after cyclization of o-HPMAA. Based on Passerini’s study, the aromatic ring, was regarded as the basic chromophore instead of the heterocyclic ring.40 Therefore, the absorption (around 288 nm) deriving from the B2u benzene system was a corollary of weak conjugation from heteroatoms and the carbon−carbon double bond with the benzo ring. Similar to the results from o-HPMAA and P(o-HPMAA), monomer−excimer interaction was inhibited, splitting the broad band into finer dual-band systems. The reduction of the conjugation with the benzoxazole ring resulted in hypochromatic shifts to 272 and 279 nm.41 A broad absorption around 263 nm in Figure 1c was related to the B band. This could also be found on the shoulders of the shortwave sides on the main bands in Figure 1d. Furthermore, according to the Passerini’s hypothesis, the absorption system of benzene rings (231 nm in Figure 1c and 235 nm in Figure 1d) derived from either the B1u or E2g species. Based on comparison of the UV−vis spectra between monomer and its corresponding polymer both before and after the cyclization, it can be concluded that the heterocyclic ring and the unsaturated carbon−carbon double bonds had a weak conjugation effect on the benzo ring. 2-Bromoisobutyryl bromide was covalently bonded to RGO−OH via condensation between acyl bromide groups (−CO−Br) and hydroxyl groups (−OH).42 The Br 3d (at BE of about 69.1 eV), Br 3p (at BE of about 181.8 eV), and Br 3s (at BE of about 256.9 eV) signals, characteristic of covalently bonded bromine, were observed on the graphene macroinitiator.43−46 The high-resolution spectrum of Br 3d was

Figure 3. FTIR spectra of RGO-OH, RGO-Br, and RGO-P(oHPMAA). 368

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Figure 4. SEM images of (a) RGO-OH, (b) RGO-Br, and (c) RGO-P(o-HPMAA). AFM height images of (d) GO, (e) RGO-Br, and (f) RGO-P(oHPMAA). (g) Photographs of dispersion of (1) GO in water and (2) RGO-OH, (3) RGO-Br, and (4) RGO-P(o-HPMAA) in DMF taken a week after ultrasonication. (h) Bright-field TEM image of RGO-P(o-HPMAA).

of ester bond and the one at 1161 cm−1 to the asymmetrical stretching vibration (νasC‑O) of the ester bond, indicating the successful condensation between RGO-OH and 2-bromoisobutyryl bromide. The weak absorption shoulder around 2952 cm−1 was attributed to the asymmetrical stretching vibration (νasC−H) of methyl on the acyl bromide. The o-HPMAA was grafted from RGO-Br macroinitiator via ATRP with ethyl α-bromoisobutyrate/CuBr/PMEDTA system. The successful polymerization was evidenced by significant intensity increase of the absorption band at around 2959 cm−1 due to the presence of methyl groups on o-HPMAA. This was also supported by the intensified absorption at 1722 and 1527 cm−1, corresponding to stretching vibration of carbonyl group (νCO) (1651 cm−1) and in-plane bending vibration of N−H bond (νN−H) (1525 cm−1) in P-(o-HPMAA), respectively. GO was prepared in our lab via the Hummer’s method. The morphology of GO nanosheets was probed by AFM as shown in Figure 4d, where the thickness and lateral dimension of GO nanosheets are 0.963 and ∼800 nm, respectively. An increase in thickness was observed for RGO-Br and RGO-P(o-HPMAA) due to the presence of functionality and grafted P(o-HPMAA), respectively, as evidenced by AFM height images (Figure 4e,f). For RGO-Br, the lateral size was measured to be 3.828 μm, and the thickness of 3 nm was almost identical to previously reported value of 2.9 nm.34 The gauze-like morphology of RGO-OH and RGO-Br was probed via SEM in Figures 4a and 4b, respectively, and the smooth feature on the edge of RGO in Figure 4c was considered to be the grafted P(o-HPMAA). The grafted P(oHPMAA) can be discerned from graphene by gray scale due to the reduced electron transmission and was squared in TEM bright field image as additional evidence as shown in Figure 4h. The grafted P(o-HPMAA) was observed both in the center and

on the edge of RGO, suggesting random distribution of the initiating moieties. The domain difference of grafted P(oHPMAA) between Figures 4c and 4h indicated the heterogeneous distribution of the initiator groups. Both GO and functionalized RGO could be dispersed well in either water or DMF and remained stable and homogeneous even a week after ultrasonication, as evidenced by the absence of precipitates in Figure 4g. The thermal stability of GO was assessed from its stepwise TGA curve (Figure 5) and considered to be inferior to its derivatives. The first step took place slightly below 100 °C,

Figure 5. TGA curves of RGO-Br, RGO-P(o-HPMAA), RGO-OH, P(o-HPMAA), and GO at heating rate of 10 °C/min under nitrogen flow. 369

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Macromolecules when absorbed water was removed via thermal evaporation. The other step emerged at around 200 °C, when the liable oxygen-containing functional groups decomposed. The mass loss of RGO-Br was depressed compared to RGO-OH below 200 °C, but it was accelerated dramatically thereafter. Based on Laoutid’s perspective, halogenated flame retardants could react with free radical species released in the thermally induced polymer decomposition to prevent further chain decomposition.51 This retarded mass loss of RGO-Br below 200 °C could be explained as a result of HBr, an effective flame retardant. Above 200 °C, the decomposition of organic substances was promoted due to depletion of HBr. Based on the curves shown in Figure 5, the grafting density of phenethyl alcohol groups on RGO was estimated to be around 1 in every 153 carbon atoms (calculation method of the grafting density could be found in the Supporting Information, Figure S13). Similar to RGO-Br, the mass loss of RGO-P(o-HPMAA) was insignificant before it accelerated above 250 °C. The mass loss (18.1 wt %, step 1) between 250 and 330 °C could be ascribed to decomposition of side groups on the grafted P(o-HPMAA). The fraction of the grafted P(o-HPMAA) could be therefore estimated no more than 41.1 wt % based on the feed, since the mass loss of RGO-P(o-HPMAA) was 7.44 wt %. A dramatic mass loss above 330 °C could be ascribed to the decomposition of P(o-HPMAA) backbone, and similar behavior was observed for P(o-HPMAA). In addition, the number-average molecular weight (Mn) and polydispersity (PDI) of P(o-HPMAA) were determined to be 17.6K and 1.7, respectively, via GPC. Although model compounds and kinetic studies discovered a much lower equilibrium constant in ATRP of (meth)acrylamide and their derivatives under typical ATRP conditions than that in ATRP of acrylates or styrene, a few attempts were made on ATRP of (meth)acrylamide and their derivatives.52−54 Nevertheless, block copolymers of poly(methyl acrylate-b-N,N-dimethylacrylamide) (Mn = 4800, Mw/Mn = 1.33) and poly[n-butyl acrylateb-N-(2-hydroxypropyl)methacrylamide] (Mn = 34 000, Mw/Mn = 1.69) were successfully synthesized with tailor-made ligand (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, Me4Cyclam) and well-defined macroinitiators via ATRP.54,55 The Cu(I)Br/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) system was widely used in ATRP of methyl acrylates, methyl methacrylate, n-butyl acrylate, and styrene56−58 and was not intended for ATRP of (meth)acrylamide. Although detailed kinetic and mechanistic studies are still ongoing, the reactivity of o-HPMAA could be possibly explained by the pentagon or heptagon heterocyclic rings stabilized by intramolecular hydrogen bonding adjacent to the carbon radicals (Figure S14), which produce sufficiently large atom transfer equilibrium constant but do not interfere with the propagating radicals and catalytic system. Based on interfacial polarization or Maxwell−Wagner−Sillars theories, composite materials with high dielectric constants can be prepared by incorporating conductive fillers into insulating polymers.59 The electric conductivity and dielectric constant (ε) of the composite increase dramatically when the conductive filler fraction is close to the percolation threshold (fc, an important parameter describing electrical properties of a material). The dielectric constant of the nanocomposites was plotted against frequency at varied RGO-P(o-HPMAA) fractions at room temperature, as shown in Figure 6. Usually, the rigid segments on the backbone of PBO restrict the vibration and

Figure 6. Dependence of the dielectric constant of the P(2-IBO)/ RGO-P(o-HPMAA) nanocomposites on the frequency at varied RGOP(o-HPMAA) weight fractions, mRGO‑P(o‑HPMAA) = 0−0.018 at room temperature.

orientation of permanent dipoles (benzoxazole groups) (Figure 7a), and thus result in relatively low dielectric constant under external electric field (ε′ = 2.78, fluorine-containing PBO;26 ε′ = 2.90, PBO;27 ε′ = 3.6, PBO28). Improved dielectric performance (ε′ = 4.92) and solubility in dimethylformamide discussed above were achieved via promoted mobility of the benzoxazole dipoles (Figure 7b). Furthermore, large enhancement in the dielectric constant of the composite near the percolation threshold, mc = 0.0154, was observed, as shown in Figures 6 and 8. When the weight fraction of RGO-P(oHPMAA) was 0.015, the maximum dielectric constant increased to 8.35 at 1 kHz, 70% higher than that of the pure P(2-IBO) (4.92 at 1 kHz). The dielectric constant decreased by 17.1% (0.1 Hz−1 kHz) and further reduced to 80.5% (0.1 Hz− 1000 kHz), compared to the highest value (10.1) at 0.1 Hz, suggesting the formation of the percolation network. In addition, all these samples showed a stable dielectric property in the high-frequency range (1−1000 kHz); for example, no more than 3% deviation was observed in the composite with 1.5 wt % of RGO-P(o-HPMAA). Conventionally, a power law was used to describe the relationship between the dielectric constant and the volume fraction of the conductive filler in the composites when the weight fraction of RGO-P(o-HPMAA) was approaching the percolation threshold.60 ε ∝ (fc − fRGO‐P(o‐HPMAA) )−S

for fRGO‐P(o‐HPMAA) < fc (1)

where ε is the dielectric constant of the composite, fc is the critical volume fraction at the percolation threshold, f RGO‑P(o‑HPMAA) is the volume fraction of the RGO-P(oHPMAA) filler in the composite, and s is the critical exponent. For simplification, the weight fraction of RGO-P(o-HPMAA) m is replaced by the volume fraction f, and the equation is rewritten as ε ∝ (mc − mRGO‐P(o‐HPMAA))−S

for mRGO‐P(o‐HPMAA) < mc (2)

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Figure 7. Schematic diagrams of dipoles on the backbone of PBO (a) and on the side chains of P(2-IBO) (b).

overcoming the processing difficulty brought by the highly conjugated structure in conventional PBOs. Additionally, tailored morphology and improved dispersion of RGO were achieved via ATRP of o-HPMAA. The well-defined morphologies were probed by AFM, SEM, and TEM, and the fraction of the grafted polymer was 44 wt % as determined by TGA. The uniform dispersion of RGO-P(o-HPMAA), together with the compatible interfacial interaction between the graphene nanosheets and the matrix, resulted in high dielectric constant of RGO-P(o-HPMAA)/P(2-IBO) composites (8.35 at mRGO‑P(o‑HPMAA) = 1.5 wt %), approximately 70% higher than that of pure P(2-IBO) at 1 kHz. Compared to previously reported data, a much lower percolation threshold (mc = 0.0154) in this system suggested that the good dispersion of functionalized graphene nanoseets with low conductive fillers loading can be achieved during fabrication of low-density highdielectric-constant materials. This composite could be potentially applied in ultraviolet absorption and energy storage devices, through rational design of the polymer matrix and selection in the wide range of varieties of functionalized graphene nanosheets.

Figure 8. Dependence of the dielectric constant of the P(2-IBO)/ RGO-P(o-HPMAA) nanocomposites on the weight fraction of RGOP(o-HPMAA) f RGO‑P(o‑HPMAA) = 0−0.018 at 1 kHz at room temperature. The inset showed the fitted curve to eq 2.

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where mRGO‑P(o‑HPMAA) is the weight fraction of the RGO-P(oHPMAA) filler and mc is the critical weight fraction at the percolation threshold. In Figure 8, the experimental data of the dielectric constant agreed well with eq 2, where mc = 0.0154 and s = 0.15. At the percolation threshold, RGO-P(o-HPMAA) nanosheets were closely arranged yet remained isolated and electrically insulated due to the thin P(2-IBO) layer between the neighboring graphene nanosheets. When an electric field was applied, charge separation was achieved within conjugated graphene nanosheets on a long-range order, and this heterogeneous system could be treated as many parallel or a series of microcapacitors connected to each other. Therefore, the low percolation threshold here suggest a well dispersion of RGO-P(o-HPMAA) in the matrix and good interfacial interactions between the graphene nanosheets and the matrix. However, when the RGOP(o-HPMAA) weight fraction exceeded 0.018, the percolation network broke down by forming a conductive network with a reduction in dielectric constant and a considerable increase in dielectric loss due to a circuit shortcut. It should be emphasized that the maximum dielectric constant (8.35) is larger than the previously reported data (5.8 in silicon elastomer composite with a graphene nanoribbon fraction of 0.5 wt %). Besides, benefiting from the viscosity change induced by more flexible backbone of P(2-IBO), this composite possess a lower critical weight fraction (mc = 0.0154) compared with other systems (PBO/graphene composite, mc = 0.037).27

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S14. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(X.L.): Fax +86 21 64253163, e-mail [email protected]. *(Q.Z.): Fax +86 21 64253163, e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to the Basic Innovation Research Program of Science, Technology Commission of Shanghai Municipality (13JC1402002) for their financial support. This project is also supported by the Innovation Program of Shanghai Municipal Education Commission (12ZZ049), National Natural Science Foundation of China (50973028), Shanghai Natural Science Foundation (12ZR1407900), and China Scholarship Council.

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4. CONCLUSION In this study, a substitute to PBO, P(2-IBO), was synthesized via free radical polymerization of heterocyclic 2-isopropenylbenzoxazole. The high dielectric constant (4.92 at 1 kHz) of P(2-IBO) was observed as expected due to the improved mobility of the dipoles on the side chains, and the flexible backbone led to good solubility in ordinary organic solvent, 371

DOI: 10.1021/ma502326v Macromolecules 2015, 48, 365−372

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