Covalent Grafting Approach for Improving the Dispersion of Carbon

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A covalent grafting approach for improving the dispersion of carbon black in styrene-butadiene rubber composites by copolymerizing p-(2,2’diphenylethyl)styrene with a thermally decomposed triphenylethane pendant Minglu Huang, Jianmin Lu, Bingyong Han, Ming Qiu, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02259 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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Industrial & Engineering Chemistry Research

A covalent grafting approach for improving the dispersion of carbon black in styrene-butadiene rubber composites by copolymerizing p-(2,2’-diphenylethyl)styrene with a thermally decomposed triphenylethane pendant Minglu Huang, Jianmin Lu*, Bingyong Han*, Ming Qiu, Liqun Zhang, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China. Correspondence to: J. Lu ([email protected]) and B. Han ([email protected])

Abstract A facile approach for improving the dispersion of carbon black (CB) with few functional groups in a rubber matrix was developed by preparing a new solution-polymerized styrene-butadiene-p-(2,2’-diphenylethyl)styrene (DPES) rubber (SBDR). The SBDR was shown to graft onto the surface of the CB through trapping polymer radical formed by thermally dissociation of triphenylethane pendant. The dispersion of CB in the rubber matrix was markedly improved by increasing the DPES content in SBDR, as demonstrated by transmission electron microscopy, small angle X-ray scattering, and rubber process analysis. Furthermore, SBDR vulcanizates showed improved mechanical properties and good dynamic properties for tread rubber with increasing DPES content. This research provides a universal method for improving the dispersion in polymer matrices of carbon materials containing few functional groups.

1. Introduction Carbon black (CB)-based polymer composite material has been widely used in many fields because of its unique mechanical, electrical, optical, and thermal properties.1-5 The disadvantage of this composite material is its inability to disperse in a polymer matrix, which lowers the properties of the CB composite material.6-8 Conventionally, three principal approaches are used to improve the dispersion of CB in a polymer matrix: (i) the modification of CB9, (ii) the addition of a compatibilizer10-11, and (iii) the functionalization of the polymer matrix12-16. The first two approaches generally require complicated multistep pretreatment and rigorous conditions, whereas the modification of the polymer matrix, which involves the introduction of epoxy12, 13, carboxyl14,amino15,16, and other functional groups directly into the polymer backbone to interact with the functional groups present on the CB surface, is more facile and flexible. However, most previous studies involving the modification of matrix rubber have been unable to improve the dispersion of furnace black and acetylene black because these CB types contain few functional groups. The introduction of thermally decomposed groups into a polymer matrix is a promising 1

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method for addressing these problems. The polycondensed aromatic rings (graphene) of carbon materials, such as CB and carbon nanotubes, can act as strong radical scavenger agents.17,18 Therefore, polymer radicals formed by the decomposition of thermal-decomposition-group-containing polymers are trapped by the polycondensed aromatic rings of CB to form a corresponding polymer-grafted CB connected by high-energy covalent bonds.19-25 Yoshikawa21, Lee22, and Yang23 synthesized a series of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO)-terminated polymers. They found that when the polymers were heated in the presence of CB above the dissociation temperature of the C-ON bond in TEMPO, the polymers were effectively grafted onto the surface of the CB and the modified CB was dispersed uniformly in an organic solvent. However, few studies have focused on introducing thermal decomposition groups into a polymer backbone and in situ improvement of the dispersion of CB in a polymer matrix. We recently reported an unsymmetrical triphenylethane compound ethane-1,1,2-triyltribenzene (ETB) that contains a thermally decomposed C-C bond and can be used as a radical initiator.26-28 ETB-terminated polymers, obtained through living anionic polymerization, can also be used as macroinitiators to prepare block copolymers. On the basis of the aforementioned analysis, we designed a new monomer, p-(2,2’-diphenylethyl)styrene (DPES) with a polymerizable double bond and a pendant of ETB, which can be used in the synthesis and design of various thermally decomposed polymers through copolymerization with other monomers. The anionic copolymerization of DPES with butadiene (Bd) and styrene (St) is particularly suitable for the synthesis of a solution-polymerized St-Bd-DPES rubber (SBDR) matrix with a clearly defined structure and amount of thermal decomposition groups. Moreover, DPES with an ETB pendant can undergo anionic polymerization without side reactions in a hydrocarbon medium at room temperature or higher, which is similar to the conditions required for the industrial production of solution-polymerized Bd-St rubber (SBR). Therefore, DPES-functionalized SBR has prospective industrial applications.31 In the present study, a series of SBDR with different DPES contents was prepared through living anionic polymerization. To demonstrate the covalent interfacial interaction between CB and the SBDR matrix, SBDR was grafted onto a CB surface through the reaction of SBDR radicals formed by the decomposition of ETB with polycondensed aromatic rings on the CB surface. We also prepared CB/SBDR vulcanizates with different DPES contents and the same CB loading to investigate the effect of DPES groups on the dispersion of CB in a rubber matrix. Additionally, the effect of DPES content on the mechanical and dynamic mechanical properties was investigated. With an increase in DPES content, the mechanical, wet skid resistance, and lower-rolling resistance properties of SBDR improved markedly when the material was used as the tread rubber of green tires.

2. Experimental 2.1 Materials All chemicals were purchased from Aldrich Chemical Company (USA). Calcium hydride (CaH2), calcium chloride (CaCl2), methanol, and n-butyllithium (n-BuLi, 2.5 M in cyclohexane) 2


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were used as received. p-Vinylbenzylchloride (p-VBC,≥99%), diphenylmethane (≥99%), toluene (≥98%), St (≥98%), tetramethylethylenediamine (TMEDA,≥98%), and cyclohexane (≥98%) were refluxed over calcium hydride and distilled in dry argon. Tetrahydrofuran (THF,≥98%) was distilled from sodium naphthalene before it was dried over CaCl2 for 48 h. Bd(polymer grade), supplied by Yanshan Petro chemical Company (Beijing, China), was used as received. CB (N330) was purchased from Cabot Corporation (Boston, USA; specific surface area = 78± 5 m2 g−1, DPB value = 121 ± 5 mL 100 g−1, average particle size = 25 nm, pour density = 380 ± 40 g L−1). DPES was synthesized through a nucleophilic substitution of the chlorine atom in p-VBC, a process which is summarized as follows. Diphenylmethane (4.8 mL, 35.0 mmol) and n-BuLi (14.0 mL, 35.0 mmol) were reacted at −78 °C in THF for 2h in dry argon. P-VBC (5.5 mL, 35.0 mmol) was then added at −78 °C. The solution was warmed to room temperature and stirred for an additional 2 h. The crude DPES was purified and recrystallized twice in methanol (−30 °C) to yield colorless needles (4.8 g, 16.9 mmol, 48%).32

2.2 Synthesis of random SBDR Anionic copolymerizations were implemented at 50 °C in a reactor with the protection of a nitrogen atmosphere, in accordance with the recipe summarized in Table S1 in supporting information. First, DPES, Bd, and St were dissolved in cyclohexane with a 10 wt % solution in a 2-L Schlenk tube and then transferred to the reactor. The required amount of TMEDA was added to the reactor by using a hypodermic syringe. After the solution was stirred for 30 min, n-BuLi was then added, also by using a hypodermic syringe, to initiate the reaction. The polymerization was stirred for 6 h and then terminated with methanol. The reaction mixture was poured into excess methanol to precipitate the polymer, which was then separated through filtration and dried in a vacuum at 60 °C in an oven to maintain a constant weight. SBDR was prepared with 0wt%, 2wt%, and 5wt% DPES in the recipe and denoted SBDR-0, SBDR-2, and SBDR-5, respectively.

2.2 Grafting of SBDR onto CB surface through radical trapping SBDR-5(0.1g) and CB (N330, 0.05g) were mixed in toluene (10 mL) and reacted through stirring at 100°C for 24 h. The SBDR (Table S1, sample 3) reacted with CB, which trapped the SBDR radicals on its surface to form SBDR-grafted CB particles. After the reaction, the resultant mixtures were added to excess methanol to separate the CB particles. The separated CB particles were washed with cyclohexane several times to remove the ungrafted SBDR until no remaining precipitated SBDR was detected in the supernatant solution by using the excess methanol. The particles were dried in a vacuum at 60 °C.

2.3 Preparation of CB/SBDR vulcanizates The preparation of CB/SBDR vulcanizates involved the following steps, as specified in the 3



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recipe shown in Table S2. First, a given amount of CB (N330) was mixed with SBDR in an internal mixer (HAAKE Rheomix 600 OS, Thermo Fisher Scientific, Waltham, MA, USA) at 120 °C and 50 rpm for 30 min. After this process, the final batch temperature reached 150 °C. The blended CB/SBDR was then removed and cooled at room temperature. The other rubber ingredients were blended with the CB/SBDR compound in a 6-inch open two-roll mill according to the conventional blending method. Finally, the compounds were vulcanized in the form of 2-mm thick sheets at 150 °C and 15 MPa for the optimum cure time (t90), which was determined using a moving die rheometer. Various CB/SBDR vulcanizates were obtained from different SBDR samples, namely SBDR-0, SBDR-2, and SBDR-5.

2.4 Measurements 1

H NMR (5 wt%, CDCl3) spectra were recorded on anARX400 (400MHz) spectrometer (Bruker Corporation, USA) with CDCl3 and tetramethylsilane as the solvent and internal reference, respectively. Gel-permeation chromatography (GPC) was performed using a Waters gel permeation chromatograph 515 HPLC component system (2140 refractive index detector). All samples were run in THF at 30°C at a rate of 1.0 mL/min and linear polystyrene standards were used for calibration. Fourier transform infrared spectroscopy (FTIR) was performed on a Vertex Fourier transform infrared spectrometer (Bruker Corporation, USA) operated in attenuated total reflectance or transmission modes. Differential scanning calorimetry (DSC) was conducted with a Q2000 differential scanning calorimeter (TA Instruments, USA) under nitrogen purging at a heating rate of 10 °C min−1from −100 °C to 100 °C. Thermo gravimetric analysis (TGA) was conducted with Q5000-TA Instruments under nitrogen atmosphere from 30 °C to 750 °C with a heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250 X-ray photoelectron spectroscope (Thermo Fisher Scientific, UK) with Al Kα radiation of 1486.6 eV. Transmission electron microscopy (TEM) for ultramicrotomed samples was conducted on a Tecnai G2 20 S-Twin electron microscope (FEICompany, USA) operated at an accelerating voltage of 200 kV. Small-angle X-ray scattering (SAXS) measurement was performed at various distances by using a Xeuss 2.0 and employing Cu Kα radiation from a PW 1830 X-ray generator (Philips Healthcare, USA). Contents of bound rubber were determined by extraction of unbound free rubbers with toluene for 5 days and drying in a vacuum at 60 °C in an oven to maintain a constant weight. Weights of the samples before and after the extraction were measured and the bound rubber contents were calculated.33 Strain-sweep experiments were performed with a Rubber Process Analyzer 2000 rheometer (Alpha Technologies, USA) at 60 °C and1 Hz in a strain range from 0.28% to 400%. The cure characteristics of the CB/SBDR compounds were studied by a MR-C3 rotorless rheometer (Beijing Ruida Chenyu Instrument Co., China) at 150 °C and a test frequency of 1.67 Hz. Tensile and tear properties were determined using a CMT 4104 electrical tensile instrument (Shenzhen SANS Test Machine Co., Ltd., Shenzhen, China) by following the ISO 37-2005 and ISO 34/1-1994 protocols, respectively. Dynamic mechanical thermal analysis was conducted using a VA 3000 dynamic mechanical thermal analyzer (01 dB-Metravib Co., France)by using a rectangular specimen (12mm×12mm×1mm) and operating the analyzer in tension mode. The test temperatures ranged from −80 °C to 80 °C and the heating rate was 3 °C/min−1at a frequency of 1Hz.

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3. Results and discussion 3.1 Synthesis of random SBDR The anionic copolymerization of Bd, St, and DPES was carried out in cyclohexane by using the n-BuLi/TMEDA system as an initiator, as shown in Scheme 1.A series of random SBDR with different content of DPES was synthesized as detailed in Table S1. The results obtained are listed in Table 1.

SCHEME 1. Synthesis of random SBDR. TABLE 1 Monomer feed ratio, microstructure, and composition data for SBDRa

a

Compositionsc (wt %)

Samples

Yield (%)

Bd

St

SBDR－0 SBDR－2 SBDR－5

100 100 100

74.8 75.1 74.8

25.2 24 20.1

b

DPES

(Mn)d× 104

(Mw/ Mn)d

0 1.9 5.1

18.8 18.8 19.6

1.08 1.14 1.23

Polymerization was executed in a dry nitrogen atmosphere at 50 °C for 6 h.

b

SBDR with different DPES content: 0, 1.9, and 5.1 wt % for SBDR-0, SBDR-2, and SBDR-5,respectively. c The compositions of the three monomer units in the SBDR calculated from the 1H spectra by using eqs (1)–(5). d Mn and Mw/Mn were estimated using a GPC by designating Polystyrene as the standard.

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FIGURE 1. Typical 1H NMR spectra of SBDR-0, SBDR-2, SBDR-5. Figure 1 shows the 1H spectrum of SBDR-5 obtained through polymerization. The peaks at approximately 4.0 ppm and 3.3 ppm are assigned to methylidyne and methylene protons in DPES, demonstrating that DPES groups were successfully introduced to the backbones of rubber chains.32 The peaks from 4.6 to 5.0 are attributed to the methylene protons of the double bond of the 1,2-Bd and the peaks from 5.0 to 5.6 are attributed to the methylene protons of the double bonds of the 1,2-Bd and 1,4-Bd. The peaks from 6.7 to 7.3 ppm are assigned to aromatic protons in DPES and St. On the basis of these results, we conclude that the three monomer units were successfully introduced to the backbone of the polymer chain.

FIGURE 2. Compositions of Bd and DPES versus total conversion for SBDR-0, SBDR-2 and SBDR-5.

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FIGURE 3. Differential scanning calorimetry curves of SBDR-0,SBDR-2, and SBDR-5. The composition of SBDR can be estimated from the 1H NMR spectra by using eqs (1)–(5):

2 NBd 1, 4 + NBd 1, 2 A5 − 5.6 = 2 NBd 1, 2 A4.9 − 5

(1)

NBd 1, 4 + NBd 1, 2 =1

(2)

NDPES A4 − 4.3 = 2 NBd 1, 2 A4.9 − 5

(3)

NDPES A 4 − 4 .3 = 14 NDPES + 5 NSt A6.5 − 7.26

(4)

MBd × NBd 1, 4 + MBd × NBd 1, 2 + MSt × NSt + MDPES × NDPES = Mn

(5)

where NBd1,2, NBd1,4, NSt, and NDPES represent the molar numbers of 1,2-Bd, 1,4-Bd, St, and DPES in SBDR, respectively, and A5–5.6, A4.9–5, A4–4.3, and A6.5–7.26 correspond to the ratios of the areas of peaks in the 1H NMR spectra in ranges of δ=5–5.6, 4.9–5, 4–4.3, and 6.5–7.26 ppm, respectively. The contents of monomers in SBDR are calculated using eqs (1)–(5) and summarized in Table 1. The contents of monomers in SBDR appear to be consistent with the monomer feed. Combining the conversion data of monomers and the results of this analysis, we conclude that the three monomer units were quantitatively introduced to the SBDR chain. By contrast, the peaks of the block St sequences at 6.6 ppm disappeared in the 1H NMR spectra, indicating the random distribution of St and DPES in SBDR. Additionally, the composition of SBDR-5 shown in Figure 2 remained unchanged throughout the entire reaction process and the DSC curves of SBDR displayed in Figure 3 show only one glass transition temperature, providing further evidence of the random distribution of monomers along the SBDR chain.

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3.2 Grafting of SBDR onto CB surface through radical trapping Unsaturated hydrogen atoms and double bonds on the surface of CB can trap polymer radicals, and the presence of delocalized radicals has also been confirmed through electron spin resonance spectroscopy.21,23 In our previous papers, we reported that polymers containing triphenylethane structures produce benzyl and diphenyl-methyl radicals after thermal treatment.27,28 Because DPES-functionalized polymers may be dissociated thermally and polycondensed aromatic rings can trap the produced polymer radicals, SBDR can be grafted onto the CB. Our investigation of the grafting mechanism of SBDR is shown in Scheme 2.

SCHEME 2. A possible mechanism for the trapping of SBDR radicals on a carbon black surface.

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FIGURE 4. Solubility photo of CB and SBDR-5 grafted CB in a mixed cyclohexane/dimethylformamide solvent: CB (a), SBDR-5 grafted CB (b). In our experiment, SBDR-5 was heated at 100 °C in the presence of CB in toluene and the resultant composite particles were washed with cyclohexane to remove the ungrafted polymers. To demonstrate that the SBDR-5was grafted onto the CB, the initial CB composite particles and SBDR-5-grafted CB composite particles were dispersed in an incompatible two-phase solution of dimethylformamide (DMF) and cyclohexane. The colloidal dispersion of the initial CB composite particles in the DMF phase was stable, and this process did not occur in the cyclohexane phase, as shown in Figure 4 (a). By contrast, the CB composite particles were transferred to the cyclohexane phase after the grafting reaction, indicating that SBDR-5 may have grafted onto the surface of the CB.

FIGURE 5. Fourier transform infrared spectrograms of CB, SBDR-5,and SBDR-5 grafted CB. Figure 5 shows the FTIR spectra of CB, SBDR-5, and SBDR-5-grafted CB composite 9



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particles. The absorptions at 2920 and 2851 cm−1 shown on the spectrum of SBDR-5 are attributed to the stretching vibration of the methylene bond.34 The peaks at 1450, 1401, and 701 cm−1 were assigned to benzene ring absorption and the peaks at 909 and 759 cm−1 were attributed to the double bond of SBDR-5.35 The characteristics of SBDR-5-grafted CB composite particles were notably different from those of CB, namely in the presence of new peaks at 2920, 2851, 997, 969, 911, 759, and 701 cm−1.

FIGURE 6. Thermogravimetric analysis curves of CB, SBDR-5,and SBDR-5 grafted CB. TGA was conducted to evaluate the grafting content of SBDR. As shown in Figure 6, pure CB remained essentially stable below 491 °C, but exhibited approximately 4.7% weight loss between 491and 750 °C. SBDR-5 was mostly decomposed between 395 and 460 °C. The TGA curve of the SBDR-5-grafted CB composite particles contained two main regions. In the first region, ranging from 395 to 491 °C, the weight loss was measured as the content of the SBDR grafted onto the CB. In the second region, ranging from 491 to 750 °C, weight loss was ascribed to the decomposition of functional groups on the CB surface. The SBDR weight content in the SBDR-grafted CB composite particles was approximately 16.9%.

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FIGURE 7. C1s X-ray photoelectron spectra of CB and SDBR-5 grafted CB. XPS was conducted to further evaluate the grafting of SBDR onto CB. The C1s spectra for CB and SBDR-grafted CB composite particles are shown in Figure 7.The spectrum of the SBDR-grafted CB composite particles was clearly different from that of the CB in figure 7 (a). In figure 7 (b) and (c), the C1s spectra of CB and SBDR-grafted CB composite particles were decomposed into two peaks at 284.6 eV and 285.5 eV, corresponding to C=C and C–C, respectively. After the reaction, the peak intensity of C–C was increased from 24.1% to 26.3%, providing further evidence of the grafting of SBDR onto the CB surface through covalent bonds. On the basis of these results, we concluded that the SBDR was successfully grafted onto the surface of CB with covalent bonds. Additionally, the bound rubber contents in CB/SBDR-0, CB/SBDR-2 and CB/SBDR-5 compounds were 13.5 wt %, 19.1 wt % and 26.9%, respectively. The bound rubber content increased dramatically with increasing DPES content in SBDR. This was in accordance with expectation based on the strong covalent bond mechanism presented above. These results revealed that the DPES group in SBDR could improve the interfacial adhesion between the CB and SBDR matrix.

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3.3 Dispersion of CB in SBDR vulcanizates In polymer composites, the dispersion of the filler and interfacial interactions between the filler and rubber matrix are critical for determining the properties of the composites. To demonstrate the effect of the thermal decomposition group DPES on the dispersion of CB and interfacial interaction, three CB/SBDR vulcanizates with different DPES contents and the same CB loading were prepared. Information on these vulcanizates is presented in Tables 1 and S2. The dispersion states of CB were first evaluated through TEM observations, as shown in Figure 8. When CB particles were dispersed in the SBDR-0 matrix, clear CB aggregations were observed (Figure 8 (a)), as a result of the weak interfacial interaction between CB and the rubber matrix and strong interaction among CB particles, which created agglomerates of CB particles in the rubber matrix. When the DPES content was increased from 1.9 wt% to 5.1wt%, a great improvement was observed in the dispersion states of CB particles in the SBDR matrix (Figure 8 (b) and 8 (c)). Because these vulcanizates were prepared under the same conditions, the improved dispersion of CB particles in the rubber matrix was mainly attributed to the introduction of DPES. The covalent bonding interfaces formed afterward were much stronger than those of conventional vander Waals forces. Moreover, when DPES content was increased, the numbers of covalent bonds at the interface increased correspondingly, leading to enhanced interfacial interaction that could further prevent the aggregation of CB particles.

FIGURE 8. Transmission electron micrographs of CB/SBDR vulcanizates with different DPES contents: (a) CB/SBDR-0, (b) CB/SBDR-2, (c) CB/SBDR-5. The SAXS technique enables investigating and quantifying the size and morphology of CB dispersion in a rubber matrix by measuring the radius of gyration Rg of CB aggregation through the Guinier equation36,37:

lnI (q) = 2lnI (∆ρV) − q 2 Rg2 / 3

(6)

where V is the scattering volume and ∆ρ is the difference in electron density between CB and the rubber matrix. The scattering intensity I (q) is characterized by the scattered wave vector or momentum transfer q, which for elastic scattering is proportional to sin(θ/2), where θ is the angle of scattering intensity relative to the incident beam.

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FIGURE 9. Small-angle X-ray scattering profile of CB/SBDR vulcanizates with different DPES contents. TABLE 2. Rg of carbon black (CB) in different solution-polymerized styrene-butadiene-p-(2,2’-diphenylethyl)styrene (DPES) rubber (SBDR) vulcanizates Samples CB/SBDR–0 CB/SBDR–2 CB/SBDR–5

DPES content (wt %) 0 1.9 5.1

Rg (nm) 24.15 19.55 18.16

Figure 9 shows the Guinier analysis of SAXS data (logI VS, q2) for different SBDR vulcanizates. The Porod slope for the three samples is linear and overlapped. However, analysis of the Guinier region of the samples showed a difference in average particle size. The Rg of the CB dispersed aggregations is shown in Table 2. The Rg of CB in the rubber sample decreased from 24.15 to 18.16 nm as the DPES content increased, indicating that the size of CB aggregation in the rubber matrix decreased when DPES was introduced into the rubber matrix. This agrees with the results derived through TEM.

FIGURE 10. Strain amplitude dependence of the storage modulus (G’) of CB/SBDR vulcanizates with different DPES contents.

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The strain amplitude dependence of the storage modulus (G’) of CB/SBDR vulcanizates with different DPES contents is shown in Figure 10. G’ was nonlinear and decreased with strain. A high plateau value G0’was observed at lower strain amplitudes and a low plateau value G∞’ was observed at higher strain amplitudes in every sample. (G∞’–G0’) is generally used to characterize the contributions of filler networks to the elastic modulus, commonly known as the Payne effect.38 The values of (G∞’–G0’), as shown in Table S3, decreased with increasing DPES content, indicating that a wider dispersion of CB in the SBDR matrix was obtained by increasing the content of DPES in the rubber matrix.38 We offer an explanation for this phenomenon: a higher DPES content leads to a higher number of covalent bonds at the interface, which causes the breakage of CB aggregates. The distance between CB particles is then increased and the interaction between CB particles is reduced, leading to a lower Payne effect. By analyzing the results of TEM, SAXS, and rubber process analysis, we concluded that the enhanced CB–rubber interaction and improved CB dispersion in SBDR vulcanized composites can be attributed to the DPES groups along the polymer backbones.

3.4 Vulcanization characteristics of CB/SBDR compounds The vulcanization characteristics of CB/SBDR compounds are shown in Table 3 and Figure S 1. The torque difference showed that the extent of the cross-linking density was increased with increasing DPES content. Considering the same operations and the same loading of CB were used in all the compounds, these increases were ascribed to the DPES group. We speculated that the C-C bond in DPES group may be dissociated thermally and involved in the curing process. This would provide the extra cross-linking density. Moreover, with increasing DPES content, the optimum cure time of CB/SBDR compounds decreased dramatically, indicating that DPES groups accelerated the vulcanization of CB/SBDR compounds. TABLE 3. Vulcanization characteristics of CB in different SBDR compounds Samples CB/SBDR–0 CB/SBDR–2 CB/SBDR–5

Minimum torque (Nm)

Maximum torque (Nm)

Torque difference (Nm)

Optimum cure time t90 (min)

20.4 21.1 22.1

45.7 54.5 57.3

25.3 33.4 35.2

13.1 9.1 7.1

3.5 Mechanical properties of CB/SBDR vulcanizates The mechanical properties of CB/SBDR vulcanizates with different DPES contents are summarized in Table S4, and the representative stress–strain curves are illustrated in Figure 11. We observed that the tensile strength and elongation at break of the vulcanizates increased simultaneously with increasing DPES content. For example, 5.1 wt % DPES in the CB/SBDR-5 vulcanizate resulted in significant increases both in tensile strength and elongation at break of 43.8% and 11.6%, respectively, compared with the SBDR-0 vulcanizate. The enhancements in tensile strength and elongation at break can be ascribed to the strong covalent interactions between CB and the SBDR matrix and the wide dispersion of CB particles, leading to excellent stress transfer 14


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through the filler–matrix interface.39 Meanwhile, the increased crosslink density was also attributed to the enhanced mechanical properties.

FIGURE 11. Stress-strain curves of CB/SBDR vulcanizates with different DPES contents. The dynamic mechanical thermal properties of the SBDR vulcanizates with different DPES contents are shown in Figure 12. The loss factor, tan δ , at 0 °C and 60 °C is particularly relevant to the design of the tread rubber of green tires from the standpoints of safety and fuel consumption.40-43 A higher loss factor at 0 °C and lower loss factor at 60 °C impart higher wet skid resistance and lower rolling resistance, respectively. Figure 12 shows that the value of the loss factor at 0 °C for SBDR vulcanizates increased with an increase in DPES content, indicating that the functionalization of a rubber matrix by introducing DPES groups along its backbone improves wet skid resistance. The values of the loss factor at 60 °C for SBDR-2 and SBDR-5 vulcanizates were lower than that of SBDR-0, suggesting lowered rolling resistance. For example, 1.9 wt % and 5.1 wt % DPES in SBDR led to 18.3% and 30.8% decreases in the value of tanδ at 60 °C, respectively (Table S5). This observation can be attributed to the strengthened interaction between CB and the DPES-functionalized rubber matrix as a result of the formation of covalent bonds. The strong interaction between CB and the rubber matrix lowered the mobility of the rubber chain and improved CB dispersion.

FIGURE 12. Variation of dynamic loss coefficient (tanδ) with temperature at 0.02% strain and 1Hz for CB/SBDR vulcanizates with different DPES contents. 15



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4. Conclusions Chain-functionalized SBDR with different DPES contents were prepared through anionic polymerization. A SBDR chain can efficiently graft onto a CB surface through covalent bonds through the trapping of rubber radicals formed by the thermal decomposition of DPES. DPES functional groups greatly improve the interfacial interaction between CB and the rubber matrix and the dispersion morphology of CB in the rubber matrix. Such improvements affect the mechanical properties of rubber, such as its tensile strength and elongation at break. In particular, the introduction of DPES into a rubber matrix will be favorable for higher wet skid resistance and lower rolling resistance, indicating great potential for application in the tread rubber of green tires. This DPES-functionalized rubber matrix can also interact covalently with other carbon materials, such as carbon nanotubes and graphene, a phenomenon that we will investigate in future studies.

5. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant nos. 51373009 and 51473010) and National Basic Research Program of China (Grant no. 2015CB654701).

6. Supporting Information Recipes for the anionic polymerization of SBDR. Recipes for preparing the CB filled rubber composites. G0’, G∞’, and G∞’–G0’ of CB/SBDR vulcanizates with different DPES contents. Mechanical properties of CB/SBDR vulcanizates with different DPES contents. Variation of dynamic loss coefficient (tanδ) with temperature at 0.02% strain and 1Hz for CB/SBDR vulcanizates with different DPES contents. Vulcanization curve of CB/SBDR compounds.

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Covalent Grafting Approach for Improving the Dispersion of Carbon

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