Grafting of polystyrene onto reduced graphene oxide by emulsion polymerization for dielectric polymer composites: High dielectric constant and low dielectric loss tuned by varied grafting amount of polystyrene

European Polymer Journal 94 (2017) 196–207

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Grafting of polystyrene onto reduced graphene oxide by emulsion polymerization for dielectric polymer composites: High dielectric constant and low dielectric loss tuned by varied grafting amount of polystyrene

MARK

⁎

Tingting Zhanga, Wenbin Huanga, Nan Zhanga, Ting Huanga, Jinghui Yanga,b, , ⁎ Yong Wanga, a

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Erhuan Road, North I, No 111, Chengdu, Sichuan 610031, China State Key Laboratory of Polymer Materials Engineering, Sichuan University, Yihuan Road, South I, No 24, Chengdu, Sichuan 610065, China

b

AR TI CLE I NF O

AB S T R A CT

Keywords: Dielectric composite Dielectric constant Loss tangent Reduced graphene oxide Polystyrene

In this work, polystyrene-grafted reduced graphene oxides (rGO-PS) were prepared via the emulsion polymerization with reserving the two-dimensional layer-like structure of graphene. TGA, FTIR, Raman results show that the amounts of polystyrene (PS) grafted on rGO from low to medium, high values can be tuned via incorporating different content of styrene in the PS polymerization recipe. Subsequently a series of PS-based composites containing different loading of rGO with variant amount of PS grafted on rGO were prepared. The effects of grafting amount on the resulting dielectric properties were carefully investigated. Compared with the composites containing rGO without any treatment, significantly increased dielectric constant and lower dielectric loss are observed for the composites with rGO-PS. Moreover, with increasing the amounts of grafted PS on rGO, the dielectric constants exhibit an increasing trend whereas the dielectric loss keeps at a stable level. For instance, when the loading of filler is fixed at 5 wt%, the composites containing rGOs with highest amount of grafted PS exhibit a maximum of dielectric constant 394.6@100 Hz, which is over 20 times larger than that of composites containing rGO. However, a relatively low dielectric loss tangent 0.45@100 Hz and conductivity 4.04 × 10−9 S/cm@100 Hz are observed in the composites containing rGOs with highest amount of grafted PS, which is even lower than that of composites containing rGO. The improved interfacial polarization induced by the constructed PS layer between rGO and PS matrix is invoked to be responsible for the improved dielectric constant; on the other hand, the PS layer can restrict the conduction loss and suppress the dielectric loss. This work indicates that the dielectric performance of composites could be effectively ameliorated through the strategy of reasonably designing the interface structure of conductive fillers.

1. Introduction High dielectric constant materials have attracted increasing attentions in recent years due to their great potential in electromechanical actuators [1], high-density electronic packaging technology [2] and capacitors for energy storage [3]. However, the

⁎ Corresponding authors at: Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Erhuan Road, North I, No 111, Chengdu, Sichuan 610031, China (J. Yang). E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.eurpolymj.2017.07.008 Received 19 May 2017; Received in revised form 30 June 2017; Accepted 9 July 2017 Available online 11 July 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.

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traditional dielectric materials, such as ceramics cannot satisfy the rigorous requirements of advanced energy storage materials [4]. Moreover, the ceramics dielectrics are also limited due to their disadvantages such as brittleness, poor processibility and breakdown strength [5,6]. As for polymers, although assemble the good processibility and toughness, they have low dielectric constant, which limits the applications as dielectrics. To overcome these problems and to obtain a high dielectric constant in composites, several approaches were adopted. The common approach is to fabricate the polymer-based nanocomposites with ceramics [7,8] or conductors such as carbon-based fillers [9,10], metal particles [11,12] and other conductive polymers [13]. As for the polymer nanocomposites filled with conductors, the high dielectric constant will be achieved near the percolation threshold based on the threshold theory [14]. According to the threshold theory, high dielectric constant can be achieved when the volume fraction of the electrically conductive filler is very near but cannot reach up to the percolation threshold. For example, Shevchenko et al. fabricated the polypropylene (PP)/graphene nanocomposites with high dielectric constant via in-situ polymerization technology, and only 0.25 vol % graphene nanoplatelets were required to obtain the sharp increase of dielectric constant due to the homogeneous dispersion of graphene nanoplatelets [15]. Therefore, the key issue to prepare nanocomposites with high dielectric constant is to achieve a good dispersion of conductor fillers as well as variation of filler loading. It should be mentioned that another important issue for polymer/conductor composites is that the enhancement of dielectric constant is associated with dielectric loss due to the charge conduction and resultant generation of heat under the exposed electric field, which is a disadvantage for the dielectrics in energy storage field [16]. In order to achieve high dielectric constant as well as low dielectric loss, the most used way is the functionalization of fillers, including covalent functionalization, polymer coating, and inorganic coating. For example, Li et al. reported a compounded epoxy resin with core-shell structured carbon nanotube hybrid filler, representing a high dielectric constant and low dielectric loss [17]. The polymer shells are usually chemically grafted on nanoparticle via click reaction [18] and atom transfer radical polymerization [19]. Obviously, the polymer shells were robust grafted onto the surface of the nanoparticle, either providing strong interchain forces with matrix or suppressing the dielectric loss via disrupting the conductive connection of nanoparticles. However, it is still risky to use percolative dielectrics at their threshold composition because of the abrupt variations of loss tangent, and the conductivity near the threshold [20]. Therefore, the ability to maintain the low dielectric loss in the high dielectric composites over a wide composition range is a great challenge and is still an open question so far. Moreover, to the best of our knowledge, the effect of molecular structure and thickness of the so-called shell grafted to the nanoparticle surface on the dielectric property is still not clear; thus the role of nanoparticle functionalization in determining the dielectric properties of polymer based nanocomposites is needed to be deeply explored. As a member of the carbon family, graphene is more attractive due to its large surface area, high electron mobility, excellent thermal conductivity, excellent mechanical strength, and high thermal stability [21]. Graphene can serve as filler for enhancement of dielectric composites [22–24]. However, homogeneous dispersion of graphene in the polymer matrix is still a key factor as aggregations of graphene sheets usually occur in most cases due to the Van der Waals force among sheets [25]. It is necessary to functionalize the graphene to obtain polymer/graphene nanocomposites with the high dielectric constant as well as low dielectric loss. Polystyrene (PS) is an important engineering plastic exhibiting high mechanical strength, excellent thermal stability, moisture resistance and dimensional stability. However, the dielectric constant of PS is as low as about 2.8@100 Hz, limiting its application in the electronics area [26]. Therefore the dielectric properties of PS should be enhanced, and up to now, there are some reports on the improvement of PS dielectric properties via incorporating dielectric particles such as graphene [27], and BaTiO3 [26]. To our knowledge, for the first time, the effects of the grafting of PS involving different grafting amount on the dielectric properties of PS/ graphene nanocomposites were systematically investigated. In current work, the PS chains have been successfully grafted on the rGOs via surface initiated emulsion polymerization, and the amount of encapsulated PS chains are tailored for a further study on the dielectric properties of PS/PS grafted rGO nanocomposites. This research aims to reveal the relationship between interfacial effect initiated by verifying grafted PS amount and final dielectric properties of nanocomposites, providing a strategy to fabricate polymer/ graphene nanocomposites with high dielectric constant and low dielectric loss, which is benefit for the energy storage application. 2. Experimental section 2.1. Materials PS was supplied by Yangzi Petrochemical - BASF Co., Ltd. The density is 1.05 g/cm3, the melt flow rate (MFR) is 3 g/10 min (200 °C/5 kg). Natural graphite was provided by Black Dragon Graphite Co., Ltd. Qingdao, and the purity is 95%. 2.2. Preparation of PS grafting reduced graphene oxide (rGO-PS) The aqueous solution containing 250 mg GO was placed into a three-neck flask, and then 0.5 g sodium dodecyl sulfate (SDS) and an amount of styrene monomer was added. The mixture was under ultrasonic treatment for 15 min at 80 °C in an oil heating bath. Different amount of potassium persulfate (KPS) (as shown in Table 1) was added into the three-necked flask to initiate polymerization with reflux condensing devices. The mixture was refluxed for 5 h, and the entire reaction was carried out under nitrogen atmosphere. After reaction, 10 ml of hydrazine hydrate was added and then the reaction mixture was again refluxed at 100 °C for 2 h. During the reaction, chemically modified GO was partially reduced by hydrazine. The reaction mixture was cooled to room temperature and was washed with N,N-dimethylformamide (DMF) repeatedly to remove impurities. Finally rGO-PS was dispersed in DMF under ultrasonication, and was dried at 60 °C. 197

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Table 1 Reaction conditions for the preparation of different rGO-PS. Samples

Styrene (g)

KPS (g)

GO (mg)

SDS (g)

rGO-PS1 rGO-PS2 rGO-PS3

26 52 78

0.27 0.27 0.27

250 250 250

0.5 0.5 0.5

2.3. Preparation of PS/rGO-PS composites PS/rGO-PS nanocomposites were prepared by master batch method. Before the processing, PS pellets were dried in an oven at 50 °C for 24 h to remove water. PS pellets and rGO-PS were dissolved in DMF, and the mixture was stirred for 0.5 h in which the loading of rGO-PS ranged from 0.2 to 5.0 wt%. DMF solvent was removed by evaporation at 80 °C, and finally the mixture was dried at 60 °C for at least 24 h to obtain the master batch. Next, the master batch and PS were melt blended on a conical micro-twin-screw extruder. The screw speed is 40 rpm, and the temperature was set as 170 °C, 190 °C, 190 °C, 170 °C from the hopper to die. Finally, the product was granulated and stored in an oven at 50 °C. Dumbbell-shaped tensile bars (a width of 4.5 mm, thickness of 2.0 mm) and rectangular impact spline (a width of 10 mm, thickness of 4.0 mm) were prepared through Haake micro-injection molding apparatus (HAAKE MiniJet, German) for subsequent mechanical performance testing. The injection temperature and injection pressure were 200 °C and 700 bar respectively. 2.4. Characterization A Fourier transform infrared spectrophotometer (Nicolet 5700, USA) and Raman spectrometer (LabRAM HR 800 UV, HPRIBA JOBIN YVON) were used to analysis the structure of GO, rGO and rGO-PS. All the FTIR spectra were obtained at a resolution of 4 cm−1 in wave number range of 400–4000 cm−1. The excitation wavelength of the Raman spectra was 633 nm. All samples were dried and tiled on glass slides for observation. Thermal gravimetric analysis was performed using a thermal analyzer (Q5000, TA Instruments, USA) heating from 30 °C to 600 °C in air at a heating rate of 10 °C/min. Prior to the testing, all samples were carefully ground to ensure that the heat was able to spread in time. Wide-angle X-ray diffraction (PANalytical, Netherlands) was used to characterize the layer spacing of different rGO-PS and rGO. The operating voltage and current were 40 kV and 40 mA respectively. Scanning electron microscope (FEI Inspect Quanta 200, FEI, Netherlands) was applied to observe the microstructure and morphology of rGO and composites. The dynamic mechanical properties of the composites were tested by dynamic mechanical analyzer (Q800, TA Instruments, USA). Single-cantilever mode was selected. The sample was a rectangular spline cut directly from the tensile spline with a width of 4.5 mm and a thickness of 2.0 mm. The test conditions were ramped from 30 °C to 120 °C at a rate of 3 °C/min and a frequency of 1 Hz. The mechanical properties were accomplished on dumb-bell shaped specimens using universal testing machine (20 kN) (RGM4020, Shenzhen Reger Instrument Co., LTD, China) with a grip separation of 45 mm at 23 °C and a speed of 10 mm/min. Dielectric properties in the frequency range 102–107 Hz were tested using Broadband Dielectric/Impedance Spectrometer (Novo control Technologies, Germany) at room temperature. 3. Results and discussion Firstly, FTIR spectra are provided to confirm the introduction of PS chains on the rGOs. It should be noted that rGO-PS was prepared through emulsion polymerization and subsequent reduction. As shown in Fig. 1(a), the FTIR spectrum of GO shows several visible peaks at 3400 cm−1 and 1720 cm−1/1650 cm−1, which can be ascribed to the hydroxyl stretching vibration originated from the eOH of GO, eC]O stretching vibration of eCOOH groups, respectively. For three GO-PS samples, several newly emerged peaks are observed in the FTIR spectra. The peaks at 2918 cm−1 and 1604 cm−1 are assigned to the CeH stretching vibration of methylene groups and stretching vibration of non-conjugated CeC double bonds respectively. The peaks at 1580, 1490, 1446, 750, 530 cm−1 correspond to the absorption of the benzene rings of PS segments, while the peak at 2922 cm−1 arises from the attachment of additional methylene groups. On the other hand, as for the GO-PS samples, the disappearance of the peak at 1640 cm−1 represents the disappearance of eC]O group; meanwhile, the peak at 1200 cm−1 shows the new emergence of CeOeC bonding, which indicates the rebuilding of chemically covalent bonding. It should be mentioned that during the preparation of GO-PS, the self-polymerized PS and residual monomer have been carefully washed and removed. Hence, all the characterizations show the chemically grafting of PS on the GO nanosheets. The underlying mechanism for functionalization is assigned to the in situ emulsion polymerization. During the emulsion polymerization, the surfactant micelles were firstly adsorbed onto the GO nanosheets which contain eOH, eCOOH or eC]O groups; styrene monomers were further adsorbed on the hydrophobic end of surfactant, and then initiator was applied to initiate the emulsion polymerization [28]. Fig. 1(b) exhibits the FTIR spectrum of samples after reduction, in which the intensities of polar groups including eOH and eCOOH are weakened and the peaks at 1720 cm−1/1650 cm−1 assigned to eC]O of GO shown in Fig. 1(a) disappear. In this case, the samples are reduced to rGO and partial eOH and eCOOH groups are retained. 198

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PS GO -1

Transmittance

1640 cm

GO-PS1 GO-PS2 GO-PS3

(a)

-1

1200 cm

3500 3000

2500

2000 1500

1000

500

-1

Wavenumber (cm )

(b)

-1

1200 cm

Transmittance

rGO-PS1 rGO-PS2 -1

750 cm

rGO-PS3

-1

-1 1446cm 700cm-1 1580cm -1 1490 cm

rGO

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 1. FTIR spectra of (a) PS, GO, GO-PS1, GO-PS2, GO-PS3 and (b) rGO, rGO -PS1, rGO -PS2 and rGO -PS3.

More importantly, the PS chains have been chemically grafted on the rGO for the samples rGO-PS1, rGO -PS2 and rGO -PS3. Thermogravimetric measurements were applied to quantitatively evaluate the variation of grafting amount of PS with variable styrene monomers for rGO-PS1, rGO-PS2 and rGO-PS3. As for pure PS, the decomposition starts at 350 °C [19]. As shown in Fig. 2, the rGO exhibits a mass loss at 200 °C, which is attributed to the pyrolysis of retained oxygen-containing groups on rGO. And the rGO-PS always exhibit two characteristic mass losses, one appears at 200 °C and the other one corresponds to the decomposition of PS at 350 °C. Among the three samples of rGO-PS, the rGO-PS3 exhibits the largest mass loss, and the mass loss of rGO-PS2 is larger than that of rGO-PS1, indicating that the amount of grafted PS can be controlled by adjusting the styrene mass weight during the emulsion polymerization.

100

rGO

Weight (%)

80 rGO-PS1

60

rGO-PS2

40

rGO-PS3

20 0

100

200

300

400

500

600

Temperature (oC) Fig. 2. TGA curves of PS, rGO, rGO-PS1, rGO-PS2 and rGO-PS3 (heating rate of 10 °C/min, in nitrogen atmosphere).

199

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Fig. 3. TEM photos of (a) rGO, (b) rGO-PS1, (c) rGO-PS2 and rGO-PS3.

For a more clear observation, TEM was applied to detect the morphologies of rGO and rGO-PS as shown in Fig. 3. The TEM image (Fig. 3(a)) of rGO shows the two-dimensional layer-like structure and the image of rGO-PS1 (Fig. 3(b)) distinctly reveal that synthesized polystyrene with a width of several couples of nanometers are attached on the surface of rGO shown by yellow arrow. The similar morphology can be found with a similar ellipsoidal structure in PS grafted graphene [29]. As for the rGO-PS2, the morphology shown in Fig. 3(c) is similar to that of rGO-PS1, which is well matched with the TGA results. However, the ellipsoidal structure of PS exhibit a sharply increase for the rGO-PS3. One can find that the rGO is almost covered by PS ellipsoids/spheres, indicating an increasing amount of PS loading on rGO. Raman spectroscopy is a powerful tool for describing the carbonaceous materials with chemical functionalization via distinguishing ordered or disorder crystal structures of carbon. Fig. 4 shows the Raman spectra of rGO and rGO-PS to evaluate the structures of rGO before and after grafting. Two characteristic peaks at 1326 cm−1 (D band) and 1577 cm−1 (G band) are observed in the rGO spectrum. It is well known that the D band arises from defects inherent in the rGO and the edge effect of graphene crystalline [30], while the G band is ascribed to the aromatic domains with ordered sp2 C atoms. After modification via PS emulsion polymerized

1325.7

1576.6

Relative intensity

rGO

I(D/G)=1.27 1317.4

1579.3

rGO-PS1

1315.0

I(D/G)=1.20

1579.1

1314.7

rGO-PS2

1579.6 I(D/G)=1.15

rGO-PS3 I(D/G)=1.18

1000

1200

1400

1600

1800

-1

2000

Wavenumber (cm ) Fig. 4. Raman spectra of rGO and rGO-PS1, rGO-PS2, rGO-PS3.

200

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Intensity [a.u.]

T. Zhang et al.

19.56

O

20.16

O

rGO-PS3

21.42

O

rGO-PS2

24.94

O

rGO-PS1 rGO

10

15

20

25

30

35

40

45

2θ[degree] Fig. 5. WAXD of rGO, rGO-PS1, rGO-PS2 and rGO-PS3.

on the rGO, the D band shifts back to lower frequency, from 1325 cm−1 for rGO to 1317 cm−1 for rGO-PS1, 1315 cm−1 for rGO-PS2, and 1314 cm−1 for rGO-PS3. Such red-shifted D band may be associated with PS chains chemically bonding to rGO. On the other hand, the peak intensity ratios I (D)/I (G) of rGO, rGO-PS1, rGO-PS2, and rGO-PS3 are 1.27, 1.20, 1.15 and 1.18, respectively. With increasing the amount of grafted PS chains, the ratios of rGO-PS2 and rGO-PS3 exhibit further decrease. The decrease on the ratio for all rGO-PS samples can be attributed to new graphitic domains due to the chemically grafting for rGO-PS. On the other hand, the substantial structure of the carbon network in rGO and rGO-PS has been maintained after emulsion polymerization of PS on rGO. It is necessary to note that the slight increase of G band can be found in Raman spectra from 1576 cm−1 for rGO to 1579 cm−1 for rGOPS. According to Rao’s study [31], the G-band position increases with decreasing number of layers in their solid state, which may induced by the intercalation/exfoliation of grafted PS chains. For a further confirmation, the XRD patterns of rGO and rGO-PS were recorded in Fig. 5. The (0 0 2) diffraction peak of rGO is assigned at about 25°, and the interlayer spacing of rGO sheets is about 0.39 nm. Concerning about the rGO-PS, the (0 0 2) diffraction peak shifts forward to 21°, 20°, and 19° for rGO-PS1, rGO-PS2 and rGO-PS3 respectively. And the corresponding spacing of rGO-PS1, rGO-PS2 and rGO-PS3 are 0.41, 0.42, and 0.43 respectively. Such XRD results illustrate that the grafted PS onto rGO can disturb the Van der Waals interaction of stacked rGO nanosheets, exfoliate/intercalate the RGO-PS layers and furthermore enlarge the spacing the rGO nanosheets. Correspondingly, with the increasing of grafting amount of PS, the intercalation of PS chain onto the rGO nanosheets is more significant and the resultant spacing of rGO-PS3 is the largest among three rGO-PS samples. Combined with the FTIR, TGA, Raman and XRD results, it can be concluded that the PS chains have been chemically grafted onto the rGO nanosheets, and the amount of grafted PS can be tailored via varying the styrene mass during the emulsion polymerization. There is no doubt that the varied rGO-PS structure may lead to the versatile properties of rGO based nanocomposites due to possible interfacial effects. Therefore, PS was selected as polymer matrix, and investigation on the dielectric properties of nanocomposites are of great necessity. Generally, the high dielectric constant of electric conductor/polymer composites is based on the appearance of the percolation threshold [32]; although this also tends to bring high dielectric loss due to the appearance of the conductive loss at the insulatorconductor transition. In our work, the dielectric properties of nanocomposites are closely associated with the percolative structure of rGOs, which are decided by the dispersion or distribution of rGOs. Therefore, the microstructure of composite and the dispersion of rGO and rGO-PS in the PS matrix were investigated by OM and SEM as shown in Figs. 6and 7. OM observation was performed via a view of low magnification to detect the dispersion state of nanofillers in the matrix. One can see that some large rGO aggregations of several micrometers appear in Fig. 6(a). With the PS grafting on rGO, the number and size of aggregates are greatly reduced. Besides, OM photos of Fig. 6(b)–(d) show relatively homogeneous morphology, indicating the dispersions of rGO-PS in the PS matrix have been well improved. This observation can be attributed to the easy intercalation of PS chains into the rGO nanosheets due to the enlarged spacing of rGO-PS, leading to exfoliation of rGO nanosheets. Therefore, the more PS grafted onto rGO, the better the dispersion state of rGO in the PS matrix is, which can be found in Fig. 6(b)–(d). In order to get a deep insight into the effect of PS surface polymerization on interfacial interaction, SEM images were also performed to observe the microstructure of composites as shown in Fig. 7. In the PS/rGO nanocomposites, the sparkling spot presents the rGO. In Fig. 7(a), only a small amount of rGO can be found in the PS/rGO nanocomposites; seen from the picture with high magnification as shown in Fig. 7(a1), the rGO was detached from the PS matrix and the isolated rGO can be found revealing poor adhesion between rGO and PS matrix. Combined with the OM picture of PS/rGO composite, such a poor dispersion and interfacial interaction for pristine rGO may be mainly attributed to strong Van der Waals interaction between rGO nanosheets and inert interface. Seen from SEM photos, unlike the pristine rGO, few detached rGO-PS is observed in Fig. 7(b)–(d); more rGO-PS are immersed into the matrix indicating fine interaction between PS matrix and rGO. One can observe rough interface reflecting the encapsulation of PS on the rGO surface. As it is discussed above, the encapsulation of PS originates from the grafting PS on the rGO, which can reduce the Van der Waals between rGO nanosheets and strengthen the chains entanglements with PS matrix. These are main reasons for the improved dispersion and interfacial interaction for PS/rGO-PS composites. 201

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Fig. 6. OM images of (a) PS/rGO, (b) PS/rGO-PS1, (c) PS/rGO-PS2, and (d) PS/rGO-PS3 nanocomposites.

Dynamic mechanical analysis (DMA) and Raman spectrum of all nanocomposites were investigated to give a deep survey on the interfacial behaviors in nanocomposites. Fig. 8 shows typical loss tangent (tan δ) curves of nanocomposites as a function of temperature. All the curves exhibit a relaxation peak in the measured temperature range as a result of the movement of PS segments, which are applied to determine the glass transition temperature (Tg). As shown in Fig. 8, results show that Tg is increased by the addition of rGO, more importantly, the increasing grafting amount of PS on rGO further induce the shifting of Tg to higher temperature. As we know, Tg is commonly increased by presence of confinements for relaxation of polymer chains, thus more grafting amounts of PS is an effective parameter on the increasing Tg due to the enhanced entanglement between grafted PS and PS matrix. The Raman spectra can also be applied to evaluate the interfacial interactions between PS and rGO -PS. Fig. 9 shows that with the increasing amount of grafted PS chains, G band shifts to higher wavenumber whereas the D band shifts to lower wavenumber. The increase of G band can be found in Raman spectra from 1586 cm−1 for PS/rGO to 1588 cm−1 for PS/rGO-PS1 and PS/rGO-PS2, 1591 cm−1 for PS/rGO-PS3. As it was mentioned above, the increase of G band is relative to the decreasing number of layers in their solid state, which may induced by the intercalation of grafted PS chains. It is obvious that the grafting of PS can result in a better interfacial interaction between rGO and PS, and with increasing the grafting amount of PS, increasing interaction can be found in the composites. In this case, the plots of frequency-dependent dielectric constants at room temperature are shown in Fig. 10. The dielectric constant and dielectric loss values are found to decrease with an increase in frequency. As for the PS/rGO composites, an increase of dielectric constant to about 20.1 can be found when the loading of rGO reaches up to 5 wt%. As for the PS/rGO-PS composites, the enhancement of dielectric constants is more sensitive to the loading of rGOs, which means the increased amount of grafted PS can result in a more significant improvement of dielectric constant. For example, when the loading of rGO is fixed at 5 wt%, a sharp increase was observed in dielectric constants for the PS/rGO-PS3, the maximum constant can be achieved at 394.6@100 Hz, which is more than 20 times than that of PS/rGO. As for the PS/rGO-PS1 and PS/rGO-PS2, the dielectric constants also increase to 121.1@ 100 Hz and 101.0@100 Hz respectively. On the other hand, the dielectric constants in these composites depend on the loading of rGO, whereas the percolation threshold at 2 wt% for PS/rGO composites and the percolation threshold at 1.5 wt% for PS/rGO-PS can be found. On the other hand, the dielectric constant curves of PS/rGO-PS exhibits frequency independence at low and medium frequency regions, which indicates the interfacial polarization is dominant in this frequency region [33]. The plots of dielectric loss tangent vs. frequency can be found in Fig. 10(b). One can see that (i) with increasing the loading of rGO, dielectric loss tangents of all composites show a relatively slight increase in the low frequency regions although the dielectric constants show a sharp increase when a high loading of rGO (2.5 or 5 wt%) were incorporated into PS matrix; (ii) as for the PS/rGO composites, the loss tangent has reached up to above 1@100 Hz when the loading of rGO is more than the percolation threshold (> 2 wt%); whereas the loss tangent of composites remains as low as 0.3–0.5@100 Hz when the PS grafted rGO has been applied; (iii) it should be noted unlike the trends of dielectric constants, the increase of the dielectric loss at low frequency is rather limited, and remains at a relatively stable value. For examples, the dielectric constant of PS/rGO -PS3 have achieved a maximum value of 394.6@ 100 Hz whereas the loss tangent remain at 0.45@100 Hz, indicating composites with high constant and low loss tangent can be

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(a)

(a1)

5 μm

50 μm

(b)

(b1)

50 μm

5 μm

50 μm

5 μm

(c)

(d)

(d1)

50 μm

5 μm

Fig. 7. SEM of PS/rGO and PS/rGO-PS composites. (a) PS/rGO, (b) PS/rGO-PS1, (c) PS/rGO-PS2 and (d) PS/rGO-PS3, (a1)-(d1) are the corresponding magnified pictures.

obtained. In addition, loss tangent is not as sensitive to the variations of frequency at low frequency region (102 Hz–106 Hz). According to Fig. 10, the parameters of dielectric properties @100 Hz have been summarized in Fig. 11. It is clearly shown that the surface grafted PS on rGO can effectively improve the dielectric constants and suppress the dielectric loss; and with increasing the amount of grafted PS, the positive effects on dielectric properties are more significant, such as for the PS/rGO–PS3 composites. It is worthy to mention that the dielectric loss tangents always remain at low value. Especially, the values of dielectric loss of PS/rGO-PS1 and PS/rGO-PS2 are as low as 0.01–0.02. To our best of knowledge, the dielectric property in this study is better than others’ work 203

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1.0

o PS 90.5 C PS/rGO PS/rGO-PS1 PS/rGO-PS2 PS/rGO-PS3

Tan δ

0.8 0.6

o

93.3 C o 92.5 C o 90.4 C o 88 C

0.4 0.2 0.0 50

60

70

80

90

o

100

110

120

Temperature ( C) Fig. 8. Loss factor of the PS, PS/rGO and PS/ rGO-PS composites obtained through DMA measurements.

Relative intensity

1316.0

1590.9

PS/rGO-PS3

I(D/G)=1.02

1318.7

1588.3

1324.1

1588.3

PS/rGO-PS2

I(D/G)=1.05

PS/rGO-PS1

I(D/G)=1.04

1326.9

1585.7

PS/rGO

1000

I(D/G)=1.10

1200

1400

1600

1800

2000

Wavenumber (cm-1) Fig. 9. Raman spectra of PS/rGO and PS/rGO-PS nanocomposites. The loading of fillers is 5 wt%.

[34]. For example, polypyrrole-coated MWNT incorporated PS composites exhibit dielectric constant of 45@100 Hz and loss at 0.06@100 Hz [35] when the loading of MWCNT was fixed at 10.0 wt%; Poly(vinylidene fluoride) (PVDF)/xGnP composites exhibit dielectric constant of 200 and loss of 0.48@100 Hz when the loading of xGnP is 1 vol% [36]. The improvements of dielectric properties can be mainly ascribed to interfacial polarization between rGO and polymer matrix [37]. The interfacial polarization, so called Maxwell-Wagner-Sillars (MWS) polarization, which is associated with the entrapment of free charges at the interfaces of heterogeneous systems, can significantly enhance the dielectric constant in the low-frequency range [38]. In the present work, as shown in the Scheme 1, PS insulation layers from emulsion polymerization on rGO that serve as electrical barriers between the rGO and PS matrix, would store charges as dielectric in the microcapacitor, and finally realize the significant increase in dielectric constant of PS/rGO composites, based on the MWS polarization mechanism. This effect is more significant for PS/rGO-PS3 composites with the thick layer of PS. Therefore, the trends for the enhancement of dielectric constant with increasing the amount of grafted PS can be mainly ascribed to the enhanced interfacial polarization as well as the exfoliation of rGO and its optimized dispersion in PS; On the other hand, the dielectric loss in the physically blended composites could be mainly attributed to the conduction loss arising from high leakage current caused by the direct connection between the conductive fillers based on conductive percolation structure. Therefore, the decreased dielectric loss tangent can be understood by the following explanation: i) PS insulation layer plays a positive role on hindering the conduction network of rGO and impairing the formation of large leakage current, resulting in the suppression of dielectric loss; ii) the surface modification by PS improves the dispersion of rGO and enhances the interfacial adhesion of the nanocomposites, which may further restrict the movement of the molecular dipoles [39]. It should be noted that with increasing the amount of grafted PS, the insulating PS layer could be more perfect which can entirely encapsulate the rGO, thus a core-shell-like structure can be found. In others work, the core-shell structure was regarded as an ideal model for the interfacial polarization with restricting the charge loss at interface. However, the lowest loss tangents 0.01–0.02@100 Hz is for the composites containing rGO with lower amount of grafted PS (rGOPS1 and rGO-PS2), not for the rGO-PS3. This possibly attributes to the reason that it is easier for rGO-PS3 to achieve percolation structure and the conduction connection could more nearly occur. This can be evidenced by the conductivity curves as shown in Supporting Information (Fig. S1), the composites PS/rGO -PS3 shows to be more conductive than other PS/rGO-PS composites, whereas much lower than that of PS/ rGO. In conclusion, compared with the PS/rGO, due to the existence of PS layer the composites with rGO-PS were endowed with both the high dielectric constant, as well as low loss tangent and frequency independence, is attractive and crucial for high energy capacitors used within the relatively wide frequency.

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Fig. 10. Frequency dependence of (a) dielectric constant and (b) loss tangent of samples of PS/rGO, PS/rGO-PS1, PS/rGO-PS2 and PS/rGO-PS3 with different loading of fillers.

394.6

1.74

Dielectric constant

Dielectric constant

1.6 1.4

Loss tangent

300

1.8

1.2

@100Hz

1.0 200

0.8 121.1

101.0

100 20.1

0

0.45

0.01

0.02

PS/rGO PS/rGO-PS1 PS/rGO-PS2 PS/rGO-PS3

0.6 0.4

Loss tangent

400

0.2 -0.2

Samples Fig. 11. Summary of the dielectric properties PS/rGO and PS/rGO-PS nanocomposites when the loading of fillers is fixed at 5 wt% at 100 Hz.

4. Conclusion In present study, PS was covalently grafted onto reduced graphene oxides by miniemulsion polymerization with reserving the two-dimensional layer-like structure of graphene. We prepare a series of PS/rGO composites, in which the rGOs were chemically grafted with different amounts of PS chains. After grafting PS on rGO, Tg of composites gradually increased and enhanced interfacial adhesions were attained; meanwhile the grafting of PS improve the exfoliation of rGO and optimize the dispersion in PS matrix. More importantly, compared with PS/rGO composites, dielectric constants of the composites containing PS grafted rGO exhibit sharp increase and the trends for dielectric constant enhancement is associated with the increasing of PS grafting amount. On the other hand, unlike the dielectric constants, the dielectric losses remain at a low level, and lower than that of PS/rGO composites. The high dielectric constant (396.4@100 Hz), as well as low dielectric loss tangent (0.45@100 Hz) and frequency independence can be found in the composites with largest amount of PS chains on rGO. The increasing interfacial polarization with increasing the chain length of 205

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Scheme 1. Schematic diagram dielectric properties PS/rGO and PS/rGO-PS nanocomposites shows that the formation of the core-shell-like structure.

PS onto graphene was invoked to be responsible for the improved dielectric constant and suppressed dielectric loss. This work conceptually provides a method to control the interfacial polarization between graphene and polymer via chemically tuning the grafted polymer layer onto graphene. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51473137 and 51203129) and the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. SKPME2016-4-27). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj. 2017.07.008. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

M. Rahaman, S.P. Thomas, I.A. Hussein, S.K. De, Polym. Compos. 34 (2013) 494–499. C. Brosseau, Surf. Coat Tech. 206 (2011) 753–758. S.A. Wilson, R.P. Jourdain, Q. Zhang, R.A. Dorey, C.R. Bowen, M. Willander, C. Johansson, Mater. Sci. Eng: R: Report 56 (2007) 1–129. R. Pelrine, P. Sommer-Larsen, R.D. Kornbluh, R. Heydt, G. Kofod, Q. Pei, P. Gravesen, Int. Soc. Opti. Pho. (2001) 335–349. L. Xie, X. Huang, Y. Huang, K. Yang, P. Jiang, J. Phys. Chem. C 117 (2013) 22525–22537. W. Xia, Z. Xu, F. Wen, Z. Zhang, Ceram. Int. 38 (2012) 1071–1075. R.P. Ortiz, A. Facchetti, T.J. Marks, Chem. Rev. 110 (2010) 205–239. I. Balberg, D. Azulay, D. Toker, O. Millo, Int. J. Mod. Phys. B 18 (2004) 2091–2121. Z.M. Dang, L. Wang, Y.I. Yin, Q. Zhang, Q.Q. Lei, Adv. Mater. 19 (2007) 852–857. L. Wang, Z.M. Dang, Appl. Phys. Lett. 87 (2005) 042903. L. Qi, B.I. Lee, S. Chen, W.D. Samuels, G.J. Exarhos, Adv. Mater. 17 (2005) 1777–1781. J. Lu, K.S. Moon, J. Xu, C.P. Wong, J. Mater. Chem. 16 (2006) 1543–1548. G. Sui, S. Jana, W.H. Zhong, M.A. Fuqua, C.A. Ulven, Acta Mater. 56 (2008) 2381–2388. A. Göldel, G. Kasaliwal, P. Pötschke, Macromol. Rapid Commun. 30 (2009) 423–429. V.G. Shevchenko, S.V. Polschikov, P.M. Nedorezova, A.N. Klyamkina, A.N. Shchegolikhin, A.M. Aladyshev, V.E. Muradyan, Polymer 53 (2012) 5330–5335. L. Zhu, J. Phys. Chem. Lett. 5 (2014) 3677–3687. H. Tang, G.X. Chen, Q. Li, Mater. Lett. 184 (2016) 143–147. K. Yang, X. Huang, M. Zhu, L. Xie, T. Tanaka, P. Jiang, ACS Appl. Mater. Inter. 6 (2014) 1812–1822. Z. Chen, L. Xie, X. Huang, S. Li, P. Jiang, Carbon 95 (2015) 895–903. F. Wen, Z. Xu, S. Tan, W. Xia, X. Wei, Z. Zhang, ACS Appl Mater. Inter. 5 (2013) 9411–9420. Y. Sun, Q. Wu, G. Shi, Energy Environ. Sci. 4 (2011) 1113–1132. N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, J.K. Kim, Adv. Mater. 26 (2014) 5480–5487. T. Chen, J. Qiu, K. Zhu, J. Li, J. Wang, S. Li, X. Wang, J. Phys. Chem. B 119 (2015) 4521–4530. Y.J. Wan, W.H. Yang, S.H. Yu, R. Sun, C.P. Wong, W.H. Liao, Compos. Sci. Technol. 122 (2016) 27–35. D. Cai, M. Song, J. Mater. Chem. 20 (2010) 7906–7915. F.A. He, K.H. Lam, J.T. Fan, L.W. Chan, Polym. Test. 32 (2013) 927–931. S. Basu, M. Singhi, B.K. Statapathy, M. Fahim, Polym. Compos. 34 (2013) 2082–2093. H. Hu, X. Wang, J. Wang, L. Wan, F. Liu, H. Zheng, R. Chen, C. Xu, Chem. Phys. Lett. 484 (2010) 247–253. Y.T. Liu, J.M. Yang, X.M. Xie, X.Y. Ye, Mater. Chem. Phys. 130 (2011) 782–794. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dressehaus, J. Kong, Nano Lett. 9 (2008) 30–35. C.N.R. Rao, K. Biswas, K.S. Subrahmanyam, A. Govindaraj, J. Mater. Chem. 19 (2009) 2457–2469. B. Wang, D. Qin, G. Liang, A. Gu, L. Liu, L. Yuan, J. Phys. Chem. C 117 (2013) 15487–15495. V.K. Thakur, R.K. Gupta, Chem. Rev. 116 (2016) 4260–4317. G.S. Kumar, D. Vishnupriya, K.S. Chary, T.U. Patro, Nanotechnology 27 (2016) 439501. C. Yang, Y. Lin, C.W. Nan, Carbon 47 (2009) 1096–1101.

206

European Polymer Journal 94 (2017) 196–207

T. Zhang et al. [36] [37] [38] [39]

F. He, S. Lau, H.L. Chan, J. Fan, Adv. Mater. 21 (2009) 710–715. J. Chang, G. Liang, A. Gu, S. Cai, L. Yuan, Carbon 50 (2012) 689–698. D. Wang, T. Zhou, J.W. Zha, J. Zhao, C.Y. Shi, Z.M. Dang, J. Mater. Chem. A 1 (2013) 6162–6168. K. Yang, X. Huang, Y. Huang, L. Xie, P. Jiang, Chem. Mater. 25 (2013) 2327–2338.

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