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A Facile method to Fabricate Highly Thermally Conductive Graphite/PP Composite with Network Structures Chang Ping Feng, Haiying Ni, Jun Chen, and Wei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03723 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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ACS Applied Materials & Interfaces

A Facile method to Fabricate Highly Thermally Conductive Graphite/PP Composite with Network Structures Changping Feng1, Haiying Ni1, Jun Chen1*, Wei Yang1, 2* 1. College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, Sichuan, China 2. State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, Sichuan, China

ABSTRACT: Thermally conductive polymer composites have aroused significant academic and industrial interest for several decades. Herein, we report a novel fabrication method of graphite/polypropylene (PP) composites with high thermal conductivity, in which graphite flakes construct a continuous thermally conductive network. The thermal conductivity coefficient of the graphite/PP composites is markedly improved to be 5.4 W/mK at a graphite loading of 21.2 vol%. Such a great improvement of the thermal conductivity is ascribed to the occurrence of orientations of crystalline graphite flakes with large particles around PP resin particles and the formation of a perfect thermally conductive network. The model of Hashin-Shtrikman (HS) is adopted to interpret the outstanding thermal conductive property of the graphite/PP composites. This work provides a guideline for the easy fabrication of thermally conductive composites with network structures. KEYWORDS: thermal conductivity, graphite flakes, polymer composites, network structure, orientation, coating resin particles

*

Corresponding authors. Tel.: + 86 28 8546 0130; Fax: + 86 28 8546 0130.

E-mail addresses: [email protected] (J Chen) and [email protected] (W Yang)

ACS Paragon Plus Environment

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INTRODUCTION With the rapid development of polymer industries, polymer materials have expanded to almost all sections of daily life and economic fields. Most polymers are excellent thermal insulating materials.1-2 However, with the continuous decrease in the size of electronic and micromechanical devices, there is an increasing interest in high thermal conductivity materials for the transportation of heat in practical applications.3-6 For practical applications, improved thermal conductivity of polymers is generally achieved by filling polymer matrix with thermal conductive particles.7-8 Therefore, various thermal conductive fillers including graphite,9-10 expended graphite (EG),8,11,12 graphene,4 carbon nanotube (CNT),13 graphite nanoplatelet (GNP)14-16 and boron nitride (BN),17 have been introduced into polymers to improve the thermal conductivity of polymer materials. Generally, solution processing, melt mixing and in situ polymerization methods can be used to fabricate thermally conductive materials and among these technologies, the melt mixing technologies are the most preferable because of their compatibility with current industrial practices.18-20 The conventional high-thermal conductive composites with randomly distributed thermally conductive particles require a high volume fraction (>50vol %) of the fillers to obtain thermal conductivity values of approximately 1-5 W/mK at room temperature. 21 Unfortunately, the high loading of thermally conductive particles leads to great processing difficulty, higher cost and greatly deteriorated mechanical properties of the materials. Such a random distribution of conductive particles also causes difficulty in achieving thermal conductivity values high enough, which is highly required for precise design and manufacturing.22 Moreover, since the thermal energy is transferred mainly in the form of lattice vibration (phonons), poor coupling in vibration modes at the filler-polymer and filler-filler interfaces also leads to significant thermal


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resistance.23-24 Therefore, a robust thermally conductive pathway and orientation of thermally conductive particles for significantly increasing thermal conductivity is in high demand.25-27 Several studies have been conducted to increase the thermal conductivity of polymer composites by controlling the orientation of thermally conductive particles, and produced robust heat conduction pathways in polymer composites.28-30 Cho and co-workers31 developed a method to fabricate a polysiloxane/hexagonal boron nitride (hBN) nanosheet composite using a high DC electric field, in which hBN nanosheets are oriented perpendicularly to the film plane. Yu32 reported a route to prepare polymeric composites with highly percolated Cu thermal conduction pathway via metallization of polymer beads followed by a compressing molding technique. Yuan and co-worker28 used magnetic aligning method to assemble the hBN platelets and prepared polymer-based composites with high thermal conductivity. In this work, we report a simple rout to prepare high-performance polymer composites with crystalline graphite flakes directionally aligned and by which a continuous thermally conductive network was constructed in a polypropylene (PP) matrix. Our method is based on coating PP resin particles with crystalline graphite flakes followed by a compression molding process. In the prepared PP composites with thermally conductive graphite flake network, the thermally conductive fillers are located at the interface of PP resin particles and oriented around the resin particles instead of being randomly distributed in the polymeric matrix. For the purpose of comparison, the composite with randomly distributed graphite flakes was also prepared by melt mixing. This environmentally friendly technique generates a satisfactory dispersion and orientation of thermally conductive fillers and provides a straightforward, easy and highly industrializable process for high-thermal conductive polymer composites.


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EXPERIMENTAL SECTION Materials. Graphite, with an average particle size of 106 um and an apparent density of 2.25 g/cm3, was from Qingdao Shengda Carbon Machinery co., LTD. (Shandong, China); Polypropylene (PP), F301, was from Sinopec Yangzi petrochemical co., LTD (Nanjing, China). VAE (vinyl acetate–ethylene) resin, VAE707, were purchased from Shanghai Yingjia Industrial Development Co., Ltd. (Shanghai, China). Preparation of Graphite/PP composites. As shown in Figure 1, VAE707 and PP (PP: VAE707=6:1, w/w) were mixed by using a batch kneader (SYH-5, Shanghai Yaochu Machinery co., LTD, China) for 10 min at 25 °C. The VAE707 works as a binder to adhere graphite flakes. Then, graphite flakes was added into the batch kneader. The mixing time was 15 min. The loadings of graphite flakes were 4.3 vol % (10 wt %), 9.6 vol % (20 wt %), 14.8 vol % (30 wt %) and 21.2 vol % (40 wt %). The compounds were then dried under vacuum at 60 °C for at least 24 h before compression molding into sheets (7 mm in thickness and 80 mm in diameter) at 190 °C and 10 MPa for 15 min.

Figure 1. Schematic representation of the preparation process of thermally conductive PP composite

For comparison, the composite with randomly oriented graphite flakes was prepared by melt mixing in a torque rheometer (XSS-300, Shanghai Kechuang rubber Plastics Machinery Set Ltd.,


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China) at 190 °C for 5 min. The loadings of graphite flakes in graphite/PP composites without VAE707 were also 4.3 vol % (10 wt %), 9.6 vol% (20 wt %), 14.8 vol % (30 wt %) and 21.2 vol % (40 wt %). The samples were labeled according to graphite loading volume level and processing method. For example, “NG21.2-Comparison” means that the sample is prepared by melt mixing and contains 21.2 vol % graphite flakes. Preparation of graphite flake block used to measure the thermal conductivity of graphite. The graphite flakes were compressed into graphite flake block (7 mm in thickness and 25 mm in diameter) at 25 °C and 10 MPa for 20 min (see Supporting Information Figure S1). The thermal conductivity coefficient of the graphite used was measured by a Hot Disk instrument (2500-OT, Sweden) at 25 °C and the measured thermal conductivity of graphite flakes is 41.62 W/mK. The measured thermal conductivity of graphite flakes blocks is a result of unordered graphite flakes which includes contribution from both in-plane and through-plane direction. Characterization. X-ray diffraction (XRD) patterns of the graphite/PP composites were obtained by the Rigaku Ultima IV diffractometer (Rigaku, Japan) using a Cu Kα radiation (λ = 0.15406 nm). The thermal conductivity coefficient which is the through-plane thermal conductivity of the composites was measured by a Hot Disk instrument (2500-OT, Sweden) at 25 °C. The sensor supplies a heat pulse of 0.05 W for 10 s to the samples and corresponding change in temperature is recorded. The sample size for the thermal conductivity coefficient measurement was 7×25×25 mm3. Thermo-gravimetric analyses of the samples were performed using a thermogravimetric Analyzer (TGA, TG209F1, Netzsch) in the temperature range of 30600 °C with a heating rate of 10 °C min-1 under air atmosphere. The surface morphology of all the samples was examined by scanning electron microscope (SEM, Quanta 250,


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Quanta 250 FEG, FEI, Oregon, USA). For optical microscope (LEICA DMLP, Leica, Germany) observation, the specimens were polished with emery paper.

RESULTS AND DISCUSSION Characterization of graphite flake coated PP resin particles. The commercially available thermoplastic PP resin particles were first coated with graphite flakes through the bonding effect of VAE. VAE is white powder resin with vinyl acetate exceeding 70%, having relatively high compatibility with PP and graphite flakes. The graphite flake coated PP resin particles were fabricated according to the following procedures: (i) mixing PP resin particles and VAE using batch kneader at room temperature, to smear VAE resin on the surface of PP resin particles and (ii) bonding crystalline graphite flakes onto the surface of VAE resin coated PP resin particles. The physical appearance of the pure PP resin particles and graphite flake coated PP resin particles are shown in Figure S2. Figure 2a and b show the SEM images of graphite flakes and pure PP resin granule, and Figure 2c-f show the SEM images of PP resin granules coated by different content of graphite flakes, with numerous individual graphite flakes overlapping on the granule surface (Figure 2g). The integrity of coated graphite flakes increases with increasing of content of graphite flakes (Figure 2c-f). Through such a simple coating process, large and unbroken fine graphite can be observed on the PP granule surface, which was absorbed by the binder of VAE707. When the content of graphite was 14.8 vol% (Figure 2e), graphite flakes started to totally cover PP resin granules and the surface of PP granules cannot be observed. Noted that the size of the industrial graphite flakes (10 um-250 um) are inhomogeneous in size, and very large and very small graphite flakes can be observed in the other SEM images (Supporting Information Figure S2).


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Figure 2. SEM images of graphite flakes, pure PP resin granule and PP resin granule decorated with different content of graphite flakes. (a) graphite flakes, (b) pure PP resin granule, (c) 4.3 vol%, (d) 9.6 vol%, (e) 14.8 vol%, (f) 21.2 vol%, (g) graphite flakes overlapped on PP granule surface

Such morphology provides the potential for the construction of a segregated network of graphite flakes in PP composites. Optical micrographs of the composites demonstrating the formation of a segregated thermally conductive network, in which the graphite flake layer predominantly locates at the surfaces of PP granules in the composites, will be shown later. It is believed that higher graphite concentration results in higher degree of filler particle interaction and such an arrangement of graphite flakes is probable to highly increase the thermal conductivity of the composites, as long as suitable connection between graphite flakes exists. XRD characterization of the graphite/PP composites


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Figure 3. (a) Schematic diagram of XRD characterization of graphite flakes, (b) Schematic diagram of XRD characterization of sample NG21.2 and NG21.2-Comparison. (c) XRD patterns of graphite and the graphite/PP composites

As showing in Figure 3a, the XRD patterns obtained from side A and side B of graphite flakes are totally different. The crystal structure of graphite flakes is hexagonal. If the diffraction occurs on side A, we can obtain the diffraction peaks for (002) and (004) crystal planes perpendicular to the c axis which satisfy Bragg’s equation 2d sin θ=nλ at 2θ= 26.5ο and 2θ= 54.7ο. Only when graphite flakes are aligned in the composite, the absence of regular planes that satisfy Bragg’s equation2d sin θ=nλ at 2θ=26.5ο and 2θ=54.7ο can lead to the appearance of (100), (101), (110) and (112) crystal plane diffraction peaks on the X-ray pattern. 33 The relative intensity of XRD peaks between side A and side B is proportional to total area of side A and side B which are monitored by X-ray diffraction. The XRD patterns of graphite and the graphite/PP composite sheets prepared by different processing methods are depicted in Figure 3c. Graphite flakes and NG21.2-Comparison show sharp (002) and (004) diffraction peaks at 2h = 26.5ο and 2θ= 54.7ο but no other peaks of graphite flakes are observed, demonstrating that few side B of graphite flakes were monitored by X-ray diffraction. The crystalline graphite flakes with large particles randomly distributed in the sample of NG21.2-Comparison and pure graphite flakes and just a few side B expose to the outside (Figure 3b), leading to the disappearance of (100), (101), (110)


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and (112) crystal plane diffraction peaks. In comparison to the diffraction pattern of NG21.2Comparison and pure graphite flakes, diffraction pattern of NG21.2 shows four obvious peaks corresponding to the (100), (101), (110) and (112) crystal planes of graphite, indicating that many sides B of graphite flakes were monitored by X-ray diffraction (Figure 3b). NG21.2 also show sharp (002) and (004) diffraction peaks at 2h = 26.5ο and 2θ= 54.7ο since the graphite flakes are of large sizes and a certain amount of flakes have not been aligned perfectly (Figure 3b). The significant difference between the XRD pattern of NG21.2-Comparison and pattern of NG21.2 proves that the occurrence of orientation of graphite flakes around PP resin particles in the graphite/PP composite

33-34

prepared using the method developed in this work. The

orientation direction was parallel to the (002) crystal plane of crystalline graphite flakes.35 Thermal conductivities of graphite/PP Composites The thermal conductivity of graphite/PP composites depends on the integrality of the thermally conductive network and the thermally conductive network consisting of graphite flakes was studied by optical microscopy, owing to that the large size of coated PP resin granules. In Figure 4a, for graphite/PP composites with the content of graphite flakes of 4.3 vol%, some conductive channels were not connected, indicating that such a low graphite loading was not sufficient to construct a well-developed network. When the content of graphite increase to 9.6 vol% and 14.8 vol%, the thermally conductive network is not continuous but broken at some points (Figure 4b, c). However, at a high content of graphite flakes (21.2 vol.%) shown in Figure 4d, the graphite flakes formed a legible thermal conductivity network and the thermally conductive path on the surface of PP granules became wider compared with the low content graphite composites (Figure 4d). In other words, the thermally conductive pathways were well established. For comparison,


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the optical images of NG21.2-comparison shown in Figure 4e clearly showed that the graphite flakes random distributed in PP matrix. The formation of such a segregated thermally conductive network can be easily understood from its fabrication process. During the hot press processing, PP granules coated with graphite flakes were almost not broken up, because of absence of large shear effect on the melted PP domains. SEM photos were also taken to observe the detailed microstructure of the graphite/PP composites (Figure 4f-g). Obviously, graphite flakes located at the surfaces of PP resin granules and regularly stacked to each other parallel to direction of the interface between PP granules (Figure 4f). The results were consistent with XRD patterns of graphite/PP composites. In


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Figure 4. Optical images of compression-molded PP/graphite composites with different content of graphite: NG4.3 (a), NG9.6 (b), NG14.8 (c) and NG21.2 (d) NG21.2-comparison (e). SEM micrograph of NG21.2 (f) and NG21.2-comparison (g)

contrast, in the composites prepared by conventional melt mixing, the graphite platelets oriented randomly and both through-plan and in-plane alignments can be found in the SEM images (Figure 4g).


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Figure 5. Thermal conductivity of PP/graphite composites filled different content of graphite via melt mixing (Random distribution) and via PP resin granule decorating method (Ordered distribution)

The influence of the volume fraction of graphite flakes on the thermal conductivities of graphite/PP composites is shown in Figure 5. It can be seen that the thermal conductivity of the composites increases with increasing filler loading. It is also revealed that the composites with the constructed segregated thermally conductive network show higher thermal conductivity and electrical conductivity (Supporting Information Figure S3) than those prepared by conventional melt mixing. The thermal conductivity coefficient of the graphite/PP composites with the constructed segregated thermally conductive network and a content of graphite flakes of 21.2 vol% graphite flakes is greatly improved to 5.4 W/mK, 3.27 times higher than that of the composite with the same content of filler but prepared by conventional melt mixing. The higher thermal conductivity can be ascribed to the orientation of crystalline graphite flakes with a large particle size and the formation of perfect thermally conductive networks. It has been reported that natural crystalline graphite flakes have a thermal conductivity of 2200 W/mK in the (002)


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crystal plane.36 Thus, one of the key elements for improving the thermal conductivity is by controlling the orientation of graphite platelets in composites.35 Considering the morphology of the composites, its thermal conductivity can be easy understood. Because the thermally conductive network was not well established (Figure 4a, Figure 4b), the thermal conductivity was only 0.64 W/mK and 1.08 W/mK. When the thermally conductive pathways have been integrally structured (Figure 4d), the thermal conductivity increased to 5.4 W/mK at 21.2 vol % of graphite. The results were consistent with the microstructures observed in Figure 4d and Figure 4f. Theoretical considerations The thermal conductivity of graphite/PP composites can be theoretically described according to Hashin-Shtrikman (HS) model. In this model, the thermal conductivity of a composite consisting of a matrix M with a thermal conductivity σp (0.25 W/mK), particles P with a thermal conductivity of σf (experimentally measured to be 41.62 W/mK), and a volume fraction of spheres χf can be described as 38

σHS+ =σf

_

σHS =σp

2σf +σp -2χp ൫σf -σp ൯ 2σf +σp +χp ൫σf -σp ൯

2σp +σf -2χf ൫σp -σf ൯ 2σp +σf +χf ൫σp -σf ൯

(1)

(2)

Xinterconnected is a relative parameter to estimate the interconnectivity of the high thermally conducting path according to Shilling, Partzsch39 and Bernd Weidenfeller40. The Xinterconnected can be calculated using equation (3). Noted that this equation does not give a linear relation


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between Xinterconnected and the measured thermal conductivity because the term (

1 + σHS -σHS

) is not a

constant. -

Xinterconnected =

σmeasured -σHS +

-

σHS -σHS

(3)

Figure 6. Thermal conductivity of graphite/ PP composites filled with different content of graphite flakes and for comparison the upper (HS+) and lower bounds (HS-) of graphite/ PP composites filled with different content of graphite flakes are plotted.

As shown in Figure 6, the measured thermal conductivity values of graphite/PP composites with a random distribution of graphite flakes are close to Hashin-Shtrikman lower bound (HS-), indicating that a polymer matrix surrounds the graphite flakes. As graphite orderly distributed in polymer matrix, the thermal conductivity of graphite/PP composites with 21.2 vol% content of graphite flakes gets closer to Hashin-Shtrikman upper bound (HS+), due to that the high thermally conducting graphite flakes surround the polymer particles as shown in Figure 4d. Furthermore, as can be seen in Figure 7, the X

interconnected

of graphite/PP composites with an

ordered distribution of graphite flakes increases significantly at a filler content around 21.2


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vol%. It is the result of ordered graphite flakes forming a thermally conductive network (Figure 4d). The lower X

interconnected

of graphite/PP composites with a random distribution of graphite

flakes is owing to that an ideally interconnected network of the high thermally conducting path was not constructed.

Figure 7. Interconnectivity of graphite flakes in the composites at different volume content of graphite flakes

Thermal Properties of the Graphite/PP Composites Thermal stability is an essential property of electronic packaging materials, being the limiting factor in both processing and applications. 9 The thermal stability of original PP and graphite/PP composites was measured by TGA and the results are shown in Figure 8. The corresponding thermal data of the original PP and graphite/PP composites are listed in Table 1. The initial decomposition temperatures (corresponding to the decomposition of 5 wt %) of pure PP and the composites of NG4.3, NG9.6, NG14.8 and NG21.2 are 252.5 °C, 259.3 °C, 262.0 °C, 262.5 °C and 263.8 °C, respectively. The initial decomposition temperatures increased with increasing content of graphite flakes and the values of T10% and T50% are also improved when filling graphite flakes in PP matrix. These results show that the thermal stabilities of the graphite/PP


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composites are increased and the reason is that the crystalline graphite flakes on the surface of PP granules possess barrier effect which improves the resistance to thermal degradation and prevents the diffusion of decomposition products from the polymer into the gas phase. In the temperature range between approximately 250 °C and 400 °C, the weight loss of the samples can be mostly allocated to the degradation of PP resin. In the temperature range between 400 °C and 500 °C the slowed weight loss of the samples can be contributed to barrier effect of the graphite network structure which will been destroyed at higher temperature. Hence, the weight of composites at 600 °C when all PP component has been thermally and oxidatively decomposed and only graphite flakes are left can been used to characterize the content of graphite flakes. The thermal stability of original PP, NG21.2 and NG21.2-comparison were also compared as shown in Figure S4 and related parameters are listed in Table S1.

Figure 8. TGA curves of the original PP and graphite/PP composites


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Table 1. Thermal data of the original PP and graphite/PP composites

T5% Samples

T10%

T50%

Graphite flakes content (wt%)

Residual mass

(°C)

(°C)

(°C)

600 °C (wt%)

PP

252.5

266.2

316.3

0

0.0

NG4.3

259.3

277.0

332.7

10

9.9

NG9.6

262.0

277.5

328.4

20

18.8

NG14.8

262.5

279.2

336.5

30

30.2

NG21.2

263.8

279.2

395.5

40

41.3

T5%, T10%, and T50%, the temperatures where 5 wt%, 10 wt%, and 50 wt% weight loss occurred, respectively.

CONCLUSIONS Graphite/PP composites with a well-established segregated network structures were fabricated through a novel fabrication method. The method incudes two steps, coating PP resin particles with graphite flakes to construct a good thermal conductive shell structure and compression molding. Compared with graphite/PP composites prepared by melt mixing, the thermal conductivity of the composites with a well-established segregated network structure showed dramatic improvements. The thermal conductivity coefficient of graphite/PP composites is markedly improved to be 5.4 W/mK with 21.2 vol% graphite flakes, owing to the oriented distribution of crystalline graphite flakes around the large PP resin particles and the formation of a perfect thermally conductive network. Furthermore, the thermal stabilities of the graphite/PP composites are increased with increasing content of graphite flakes. The Hashin-Shtrikman (HS) model gives a nice explanation of the outstanding thermally conductive property of the


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graphite/PP composites with a well-established segregated network structure. However, further work still needs to be done in the future to optimize this facile method concerning the matrix, the filler and the binder to further enhance the thermal conductivity and improve the mechanical properties (Supporting Information Table S2) of polymer composites. ASSOCIATED CONTENT Supporting Information. The optical image of graphite flakes block used to measure the thermal conductivity of graphite (Figure S1), optical images of pure PP resin particles and graphite flake coated PP resin particles with 9.6 vol% graphite flake and SEM images of graphite flake coated PP resin particles with 9.6 vol% graphite flake (Figure S2), electric resistivity of graphite / PP composites (Figure S3), the comparison of TGA curves between the NG21.2 and NG21.2comparison (Figure S4), Thermal data of original PP, NG21.2 and NG21.2-comparison from TGA analysis (Table S1) and mechanical properties of PP and graphite/PP composites (Table S2) are provided. AUTHOR INFORMATION Corresponding Author *Tel.: + 86 28 8546 0130; Fax: + 86 28 8546 0130. *E-mail addresses: [email protected] (J Chen) and [email protected] (W Yang) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was funded by the National Natural Science Foundation of China (NNSFC Grant Nos. 51422305 and 51421061), Major State Basic Research Development Program of China (973 program) (Grant No. 2012CB025902), the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant No. 2014TD0002) and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2014-2-02). ABBREVIATIONS


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