Rubber Nanocomposites with Nano-Scale Phase Structures and

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Materials and Interfaces

Rubber Nanocomposites with Nano-Scale Phase Structures and Kinetically Inhibited Filler Flocculation for Enhanced Integrated Performances via Reactive Multi-Block Copolymer Incorporation Xinping Zhang, Hao Wang, Huicheng Ren, Riguo Wang, and Aihua He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04958 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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Industrial & Engineering Chemistry Research

Rubber Nanocomposites with Nano-Scale Phase Structures and Kinetically Inhibited Filler Flocculation for Enhanced Integrated Performances via Reactive Multi-Block Copolymer Incorporation Xinping Zhang#,†, Hao Wang#,‡, Huicheng Ren†, Riguo Wang*,§, Aihua He*,† † Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ‡ Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Chambroad Chemical Industry Research Institute Co., Ltd, Binzhou 256500, China § Shandong Huaju Polymer materials Co., Ltd, Binzhou 256500, China ABSTRACT: Solution polymerized styrene-butadiene rubber (SSBR) is commonly used as rubber tread stock for high wet-skid resistance, but SSBR suffers from shortcomings including high rolling resistance, unsatisfied abrasion resistance and fatigue resistance, which triggers great research efforts in pursuing tread stock with balanced performances. Herein, a facile strategy was proposed through introducing novel trans-1, 4-poly (butadiene-co-isoprene) rubber (TBIR) into silica filled SSBR composites. The filler networks and rubber phase structures of SSBR/TBIR blends were investigated in details. It was found out that TBIR was responsible for the depression of filler flocculation during annealing and vulcanization processes, which will contribute to the improvements of filler dispersion status in SSBR/TBIR vulcanizates. The co-vulcanization and compatibility of the two domains in SSBR/TBIR blends were also investigated. Consequently, the resultant SSBR/TBIR vulcanizates exhibit excellent integrated properties. Furthermore, the possible mechanism analysis is proposed to illustrate the contribution of TBIR.

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1. INTRODUCTION Elastomer nanocomposites are indispensable in tire applications1 which require low rolling resistance, good abrasion resistance, high wet-skid resistance, and excellent fatigue resistance. Silica-filled solution polymerized styrene-butadiene rubber (SSBR) composite is commonly used as tire tread stock for good wet-skid resistance.2 Unfortunately, SSBR composites also face challenges regarding to high rolling resistance, unsatisfied abrasion resistance and fatigue resistance, which expedites enormous research efforts in developing tire tread stocks with balanced performances. Filler aggregation size and interfacial interaction between filler and polymer correlate closely to the performance of polymer composite.3 Most studies thus focus on resolving problems like poor filler dispersion and weak filler-polymer interaction in highly filled SSBR/silica system considering the incompatibility between hydrophilic silica and hydrophobic polymer matrix.4 The most widely used methodology is surface modification of silica by organosilanes to improve its affinity to rubber matrix and further prevent filler aggregation by decreasing filler surface energy. Bis-(3-triethoxysilylpropyl) disulfide (TESPD)5 is the most commonly used coupling agent for silica modification. The ethoxy groups could bond to silica surface and sulphur groups could participate in vulcanization reaction of rubber matrix, resulting in simultaneous improvements in silica dispersion and silica-rubber interaction. Functionalization of rubber matrix is another attractive approach for enhancing filler dispersion, which involves introduction of functional groups including amide-type,6 nitrile-type,7 etc. directly at the end of or inside rubber chains. These functional groups could interact with groups residing on filler surface to enhance silica-


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rubber interactions and filler dispersion.8 However, functionalization of rubber matrix is often a high cost and complicated synthesis process. Classically, rubber compounds are required to undergo room temperature annealing for a period of time after mixing process, which is essential for promoting bound rubber generation and stress relaxation. However, fillers also tend to flocculate under stress relaxation due to strong affinity with each other.9,10 In addition, during the subsequent hightemperature and high-pressure vulcanization process, the rubber viscosity reduces rapidly, which will further facilitate macromolecular chain motion, resulting in filler flocculation aggravating. Only depending on the above related strategies is thus far from enough. Extra strategies should be involved to further depress filler flocculation dynamically. In our previous works, we have demonstrated that trans-1, 4-poly (butadiene-coisoprene) copolymer (TBIR), as a new generation of synthetic rubber with high regularity and adequate crystallinity,11-13 is highly advantageous to design high performance materials for the following aspects. Firstly, crystalline TBIR plays vital roles in reinforcing conventional amorphous rubber matrix, while quite amounts of amorphous domains in TBIR could guarantee adequate elasticity. Secondly, filler dispersion of composites with TBIR addition improves significantly. Lastly, the possible residual crystalline lamella fibrils from trans-polyisoprene (TPI) blocks in TBIR chains and further being connected by chemical crosslinking could exert positive influence on performance of the vulcanizates. Herein, we propose a facile strategy of incorporating TBIR into SSBR nanocomposites for pursuing improved filler dispersion, achieving good interfacial interaction and ideal phase structure, etc. The effects of TBIR on filler dispersion, the compatibility examinations and co-vulcanizations between TBIR and SSBR domains are



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investigated in details for the first time. This work thus will provide new insights into the design and fabrication of green tire materials with balanced integrated performances. 2. EXPERIMENTAL SECTION 2.1. Materials. Solution polymerized styrene-butadiene rubber (SSBR, BUNA VSL 4526-2HM, Moony viscosity ML3+4100C= 62.0) was purchased from the Lanxess Chemical (China) Co., Ltd. Trans-1, 4-poly (butadiene-co-isoprene) rubber (TBIR, ML3+4100C=60.5) was kindly supplied by Shandong Huaju Polymer Materials Co., Ltd. The chemical structural parameters of TBIR are shown in Table 1. Silica (1165 MP) was supplied by Solvay Fine Chemical Additives Co., Ltd. Other rubber additives were commercial grade and used as received. Table 1. The chemical structural parameters of TBIR.

Mwa, *104 Mw/Mna

54.0

3.1

Microstructure, mol%c

Bd, mol%b

trans-1,4- of Ip unit

18.0

97.1

trans-1,4- of Bd unit 92.6

Dyad distribution, mol%c II 73.1

IB+B I 12.0

BB 14.9

a GPC, b 1H-NMR, c 13C-NMR.

Bd: butadiene, Ip: isoprene, II isoprene-isoprene, BI: butadiene-isoprene, BB: butadienebutadiene. 2.2. Preparation of Unfilled Rubber Gums. Formulation for unfilled rubber gums was as follows: rubber 100, ZnO 3.8, stearic acid 1.0, N-isopropyl-N'-phenyl-pphenylenediamine 1.5, paraffin wax 1.0, accelerator DPG-80 2.5, accelerator CBS-80 1.9, sulfur 1.4.


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Raw rubbers were mixed with a HAPRO torque rheometer (RM-200C, Harbin HAPRO electric Technology Co. LTD, China) at 70 °C for two minutes. Additives except accelerators and sulfur were added at 70 °C and mixed for another 5 min, then rubber gum was taken out of the rheometer. After placing at room temperature for 20 min, the obtained unfilled rubber gum was plasticized by an open two-roll mill at 70 °C followed by addition of accelerators and sulfur and mixed for another 6 minutes. 2.3. Preparation of Filled Rubber Compounds. Formulation for filled rubber compounds was as follows: rubber 100, ZnO 3.8, stearic acid 1.0, silica 1165MP 70, Si-69 7.0, PEG-4000 2.0, silica dispersant 2.0, N-isopropyl-N'-phenyl-p-phenylenediamine 1.5, paraffin wax 1.0, accelerator DPG-80 2.5, accelerator CBS-80 1.9, sulfur 1.4. Raw rubbers were mixed by a HAPRO torque rheometer at 70 °C for two minutes, then half amount of silica and Si-69 and additives except accelerators, sulfur were added at 70 °C and mixed with rubbers for 5 minutes, then another half amount of silica and Si69 were put into the mixer and mixed for another 12 min. The filled rubber compound was taken out at 155 °C and stored at room temperature for 20 min. Then the obtained filled rubber compound was plasticized by an open two-roll mill at 70 °C followed by the addition of accelerators and sulfur and mixed for another 6 minutes. 2.4. Annealing Process of Filled Rubber Compounds. The as-prepared filled rubber compounds were placed at room temperature for 48 h before conducting rheological experiments, which is referred to as “after annealing”. For comparison, samples without annealing process were used to conduct rheological experiments immediately after being plasticized by open two-roll mill, which is referred to as “before annealing”. 2.5. Preparation of Filled Rubber Vulcanizates. The filled rubber compounds after



annealing at room temperature for 48 h were cured at 150 °C for optimum curing time (tc90) under 10 MPa pressure. 2.6. Characterizations. Samples for AFM observations were prepared as follows. Solutions with concentration of 1 wt% with different blending ratios of SSBR/TBIR (100/0, 90/10, 80/20, 70/30) were prepared after heating at 60 °C for 2 h. The solutions were then spin-coated on silicon wafers at a spinning speed of 1000 rpm for 60 s. The obtained samples were then dried under vacuum at 40 °C for 12 h. After heat treatment at 150 °C for 5 min, samples were placed at 17 °C for 48 h. AFM observations were performed on a Tapping mode with Bruker Multimode 8. Storage modulus (G’) of filled rubber compounds was measured at 100 °C by a rubber process analyzer (Alpha Technologies, United States) at a frequency of 1 Hz and a strain sweep from 0.28% to 200%. Curing characteristics of rubber compounds were observed at 150 °C using RHEOMETER MDR 2000 (Alpha Technologies, United States). Crosslinking density of the vulcanizates was determined by an XLDS-15 crosslink density analyzer and NMR spectrometer (IIC Innovative Imaging Corporation, Blieskastel, Germany) at 80 °C. Silica distributions of filled vulcanizates were examined by JEM-2100 Transmission Electron Microscopy (Hitachi Co. Japan). The thin sections were cut by microtome under -100 °C and collected on cooper grids. Filler dispersion of rubber vulcanizates was also observed by disper GRADERTM view (Alpha Technologies). Fatigue properties of samples were measured on the Dematia fatigue testing machine (GAOTIE Chemical Machinery, China) according to ISO 6943: 2007. Tensile strength and tear strength were tested with a Zwick/Roell Z005 (Germany) electrical tensile tester at ambient temperature, with a constant cross-head speed of 500 mm/min according to GB/T


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528-2009 and GB/T 529-2008, respectively. Shore A hardness observations were measured on an XY-1 rubber hardness apparatus (4th Chemical Industry Machine Factory, Shanghai, China) according to GB/T531-2009. Filled rubber vulcanizates with dumbbell shape were aged at 100 °C for 48 h in a hot air aging box, and the tensile properties after ageing were measured according to GB/T 528-2009. Dynamic mechanical analysis (DMA) was conducted on a dynamic mechanical thermal analyzer (861e Dynamic Mechanical Thermal Analyzer, METTLER Instruments, Switzerland) with a shear deformation mode. DMA observations for tan δ were conducted with temperature ranging from -100 C to 30 C, a heating rate of 3 C/min at 1 Hz and 0.5 % strain amplitude. DMA observations for rolling resistance were operated at 60 °C, with a frequency of 10 Hz and a strain amplitude of 0.05%~15%. 3. RESULTS AND DISCUSSION 3.1. Phase Structures of Unfilled Rubber Gums. Most currently available commercial rubber commodities used are usually immiscible with each other, the blends of which usually display micrometer-scale phase separation and the material performances thus deteriorate. SSBR/TPI blends show the upper critical solution temperature (UCST) type phase diagram, and TPI-rich domains and SSBR-rich domains containing highmolecular-weight TPI components have been observed.14 Considering that TBIR contains almost 80 mol% TPI blocks, SSBR/TBIR blend and SSBR/TPI blend may share some similarity in phase behavior. The detailed phase structures of SSBR/TBIR blends are examined firstly, and the results are shown in Figure 1.



Figure 1. AFM images of the unfilled rubber gums: a) SSBR/TBIR =100/0, b) SSBR/TBIR =90/10, c) SSBR/TBIR=80/20, d) SSBR/TBIR=70/30; 1, 2: phase images, 1’, 2’: height images. SSBR gum shows homogeneous phase morphology (Figure 1a). However, AFM images of SSBR/TBIR gums present sea-island phase structure with TBIR-rich domains as “island” phase and SSBR-rich domains as “sea” phase (Figure 1b-d), which is similar with that of SSBR/TPI blends. The observed size of TBIR-rich domains is in the range of 50~100 nm which becomes a little larger with increasing amount of TBIR component. Due to the mutual-existence of butadiene units and limited compatibility of SSBR and TBIR 8 ACS Paragon Plus Environment

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components, some low-molecular-weight TBIR are dissolved in SSBR matrix, resulting in inhomogeneous composition of SSBR “sea-like” matrix.14 TBIR components with highmolecular-weight TPI components are incompatible with SSBR matrix, displaying as “island-like” phases distributing in SSBR (with some amounts of TBIR components dissolving) “sea-like” regions. As a result, it’s hard to distinguish all the TBIR components from the SSBR matrix in SSBR/TBIR blends with such complex phase structures. This nano-scale phase separation morphology analogous to the phase structure in integrated rubber, could maintain independent physical and mechanical performance characteristics of each phase so as to combine the advantages of different blending components and achieve the goal of integrating advantageous performances of each component.15 As a consequence, the nano-scale phase separation of TBIR from SSBR is expected to endow the advantages of TBIR such as improved abrasion resistance, low rolling resistance and superb fatigue properties to the blends.13 The crystalline structure of TBIR in SSBR/TBIR gums is observed in Figure 1b. The TBIR lamellar fibrils display as several nanometers in thickness and several-hundred nanometers in length. The lamellar fibrils grow into fibril bundles with increasing TBIR content (Figure 1c). 30 phr TBIR incorporation into SSBR gum leads to the dimensions of TBIR fibril bundles enlarging into several micrometers (Figure 1d). Additionally, it could be observed that TBIR fibrils nucleate preferentially at the phase interface or near some impurities and growing across several TBIR-rich domains and SSBR-rich matrix (as shown in white dotted circle in Figure 1b2-d2). The dynamic origin of concentration fluctuation and the static heterogeneous nucleation could be reasonable to explain the nucleation incidence. Due to the inhomogeneous composition in SSBR-rich domain with the existence



of some TBIR fractions, TBIR lamellar fibrils could grow across SSBR matrix through absorbing TPI blocks of TBIR molecules in SSBR-rich domains to the crystalline growth front. This unique phase structures of SSBR/TBIR gums should influence the performances deeply.

Figure 2. Stress-strain curves of a) unfilled SSBR/TBIR gums, b) silica filled SSBR/TBIR compounds before vulcanization. 3.2. Stress-Strain Curves of Un-Vulcanized Gums and Silica Filled Rubber Compounds. The reinforcing effects of TBIR on unfilled gums and filled compounds before vulcanization are shown in Figure 2. Pure SSBR exhibits a tensile strength of ~0.10 MPa and a strain of ~80%. Filled SSBR compound also shows relatively low tensile strength of 0.28 MPa and a strain of ~160%. With TBIR blending ratio increasing, both the strength and elongation are significantly enhanced in unfilled gums (from 0.10 MPa to 0.46 MPa) and corresponding filled compounds (from 0.28 MPa to 0.89 MPa). Higher strength and modulus of TBIR contribute to enhanced rubber matrix, and SSBR/TBIR rubber matrix presents increased strength with increasing TBIR incorporation. Additionally, for filled SSBR/TBIR compound with a blending ratio of 70/30, greater filler reinforcement effect is observed, which may be attributed to the improved filler dispersion status as demonstrated below.


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3.3. Filler Dispersion in Rubber Compounds and Vulcanizates. Generally, rubber compounds would undergo room-temperature annealing for a period of time after mixing process to promote bound rubber generation and stress relaxation. However, fillers tend to flocculate under stress relaxation resulting in worsened filler dispersion. As analyzed in section 3.2, the incorporation of TBIR into SSBR results in reinforced rubber matrix, which is supposed to be beneficial to kinetically suppress filler flocculation during annealing. We have examined thoroughly the filler network evolution in SSBR/TBIR compounds after room-temperature annealing process by dynamic rheological tests, and the results are shown in Figure 3-4. Figure 3 illustrates the dependence of G’ on strain of different SSBR/TBIR gums and compounds before and after annealing. G1’, G2’ and G3’ are referred to the storage modulus of unfilled rubber gums, filled compounds before annealing and filled compounds after annealing, respectively. G’ increases with the addition of silica for all the compounds compared with unfilled gums, which is indicative of the formation of rigid filler networks.16 In addition, G’ decreases with increasing strain amplitude for all the rubber compounds, which is referred to as Payne effect.17 This phenomenon is connected with the breakdown of filler networks and the release of trapped polymer upon external oscillatory shear. Furthermore, it could be seen in Figure 3 that G’ values of samples after annealing process are larger than those of samples before annealing for all the compounds. The modulus enhancement for samples after annealing is attributed to the formation of filler networks during annealing. However, the degree of modulus enhancement in SSBR/TBIR composite is smaller than that in SSBR composites, implying the depression of filler



network formation. So, TBIR component does play vital roles in inhibiting filler network formation.

Figure 3. Storage modulus vs. strain of different SSBR/TBIR gums or compounds: a) SSBR/TBIR=100/0, b) SSBR/TBIR=90/10, c) SSBR/TBIR=80/20, d) SSBR/TBIR=70/30.

Figure 4. ΔG vs. strain of SSBR/TBIR compounds: a) G3’-G1’, b) G2’-G1’, c) G3’-G2’. We further adopt the G’ difference ΔG’ to investigate the contribution of different factors to the formation of filler networks as shown in Figure 4. ΔG’ (G3’-G1’) referring to the overall contribution to filler network formation decreases with increasing TBIR blending ratio, indicating a gradually breakdown of dense and rigid filler networks with


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TBIR incorporation (Figure 4a). That means Payne effect is attenuated with TBIR addition. This is mainly originated from the inhibition of filler flocculation by TBIR incorporation during annealing process (Figure 4b), and partly from the improved dispersion of silica during mixing process (Figure 4c). Consequently, it could be concluded that TBIR incorporation could improve filler dispersion in rubber composites by depressing filler flocculation kinetically during annealing process due to the enhanced strength and modulus of rubber matrix. SSBR/TBIR compounds with varied TBIR blending ratios, as well as 70 phr silica and 7 phr Si-69 as coupling agent, were vulcanized at 150 °C for filled rubber vulcanizates. As shown in Table 2, the vulcanization time (tc90) of filled SSBR composite is 33.93 min. With increasing TBIR addition, tc90 of filled SSBR/TBIR composite decreases gradually to 23.43 min. This accelerated vulcanization rate is due to that TBIR contains ~80 mol% trans-polyisoprene units with high vulcanization activity originating from short distance between ɑ-H and double bond. During the vulcanization process with such a high temperature, fillers could further undergo flocculation due to the rapid reduction of rubber viscosity. In the case of filled rubber composites with TBIR incorporation, relatively shorter vulcanization time compared with SSBR composite could guarantee weakened degree of filler flocculation. These filler network structures are fixed due to the rapid formation of rubber crosslinking networks during vulcanization process, further contributing to improvement of filler dispersion. In addition, the difference between the highest and the lowest torque (MH-ML) during vulcanization process increases with the addition of TBIR, which implies the increasing crosslinking density with TBIR addition. Indeed, the crosslinking densities of unfilled



gums and filled composites with increasing TBIR incorporation measured using NMR method increase from 0.985 to 1.219 *10-4 mol/cm3 and from 1.253 to 1.575 *10-4 mol/cm3, respectively. The generation of high crosslinking density during vulcanization process of SSBR/TBIR composites would further constrain filler motion during vulcanization, which is also beneficial to filler dispersion improvement. Furthermore, the polysulfide crosslinking contents of SSBR/TBIR vulcanizates are higher compared to those of pure SSBR vulcanizate, and this could contribute to enhanced dynamic properties.

Figure 5. TEM images of filled rubber vulcanizates: a) SSBR/TBIR=100/0, b) SSBR/TBIR=90/10, c) SSBR/TBIR=80/20, d) SSBR/TBIR=70/30. In order to evaluate the combined effects of the above-mentioned factors on filler dispersion of the resultant rubber vulcanizates, we have examined the microscopic silica network structures of the vulcanizates by TEM observations (Figure 5). In filled SSBR vulcanizate, silica agglomerates are in size of several microns. With the incorporation of only 10 phr TBIR component, the dimensions of large filler agglomerates reduce to several hundred nanometers (Figure 5b). With 30 phr TBIR incorporation, much smaller filler aggregates with dimensions below 100 nm are dispersed very well in the matrix and connect to form uniform filler networks (Figure 5d). Additionally, as could also be seen in Figure S1, the number of large aggregations with dimensions exceeding 20 μm decreases with increasing TBIR blending ratio, which is consistent with the results obtained by TEM. The improved filler dispersion could result in more efficient filler volume fraction and


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further favor 3D connection of filler particles, leading to the formation of effective reinforcement phase percolating networks.18 3.4. Compatibility and Co-Vulcanization between SSBR-Rich and TBIR-Rich Domains. In integrated rubber, the separated phases are linked by chemical bonds, and the phase interfaces are closely arranged. When integrated rubber is subjected to an external stress, the stress is transmitted through the molecular chain with high speed. So, the response of integrated rubber is relatively fast, resulting in small hysteresis loss.

Figure 6. DMA observations of a) un-vulcanized rubber gums, b) vulcanized rubber gums. In the case of TBIR incorporated SSBR gums, enhanced compatibility and interfacial interaction between SSBR and TBIR components analogous to those in integrated rubber are observed with increasing TBIR incorporation as shown in Figure 6. The two glass transition peaks are corresponding to SSBR (-14.3~-19.2 ºC, -10.0~-15.0 ºC) and TBIR (57.7 ºC, -50.8 ºC), respectively. These peaks move closer and the Tg difference (ΔT=Tg SSBR - Tg

TBIR)

become smaller with increasing TBIR incorporation amount in both un-

vulcanized and vulcanized gums, implying enhanced compatibility. In addition, the increase in Tg values after vulcanization reaction for both TBIR-rich domains and SSBRrich domains is observed in Figure 6b indicating the chemical crosslinking-network



formation in both TBIR-rich domains and SSBR-rich domains. Furthermore, ΔT decreases from 39.5 ºC for un-vulcanized gum to 37.8 ºC for vulcanized gum with only 10 phr TBIR incorporating, which indicates the occurrence of covalent connection (crosslinking) between SSBR and TBIR chains through co-vulcanization reactions. The enhanced covulcanization between SSBR-rich domains and TBIR-rich domains is observed with increasing blending ratio of TBIR into SSBR, with ΔT decreasing from 39.9 ºC, 38.5 ºC for un-vulcanized gum to 36.6 ºC, 35.8 ºC for vulcanized gum when 20 phr and 30 phr TBIR incorporating into SSBR, respectively. The interfacial strength of SSBR/TBIR vulcanizates is thus enhanced due to the formation of covalent connection between the two rubber phases through the occurrence of co-vulcanization reaction. The nano-scale TBIRrich domains resulted from partial compatibility between TBIR and SSBR components and the interfacial strength enhancement ascribed to the co-vulcanization between SSBR and TBIR chains will definitely lead to integrated performance enhancements of SSBR/TBIR vulcanizates. 3.5. Mechanical Performances of Rubber Vulcanizates. The overall properties of rubber vulcanizates are summarized in Table 2, Figure S2 and Figure 7. Modulus at 300% increases from 11.7 to 14.8 MPa when 30 phr TBIR is incorporated into SSBR matrix, which is ascribing to the reinforcement effect of TBIR and silica (Figure 7a). Tear strength of vulcanizates with TBIR blended remains in the same level of pure filled SSBR vulcanizates. Rolling resistance represents fuel consumption and CO2 emission of vehicles, which could be implied by tan δ at 60 ºC under a strain of 5%. The corresponding results are shown in Table 2, Figure 7b and Figure S2. Tan δ at 60 ºC under a strain of 5% decreases


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gradually with increasing TBIR content in the cases of both unfilled and filled rubber vulcanizates, indicating reduced rolling resistance of SSBR/TBIR vulcanizates. Tan δ is mainly affected by the internal friction in polymer-filler interphase.19 Improved filler dispersion, enhanced interfacial connection and increased crosslinking density, resulting in depressed frictions between filler aggregations and different domains, may be responsible for the decrease in rolling resistance of SSBR/TBIR vulcanizates. Furthermore, SSBR/TBIR vulcanizates also exhibit superb tensile fatigue properties, with fatigue life increasing from 96*104 to 124*104 cycles when 10 phr TBIR replacing SSBR. Further increasing TBIR contents, SSBR/TBIR vulcanizates exhibit much longer tensile fatigue life beyond testing range (Figure 7c). The outstanding fatigue properties can be explained by the synergistic effects of uniform filler dispersion, enhanced interfacial connections, reinforced rubber matrix and existence of TBIR crystalline fibrils.20 Good filler dispersion and strong interfacial connection could prolong crack initiation time. Crack propagation could further be deviated or even halted by crystalline lamella fibrils or bundles and reinforced rubber matrix.



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Figure 7. Performances of filled SSBR/TBIR vulcanizates: a) modulus at 300%, b) tan delta at 60 °C under a strain of 5%, c) tensile fatigue, and d) DIN abrasion. Table 2. Curing characterizations, crosslinking density and mechanical properties of the rubber blends. Ratios of SSBR/TBIR ML, dN∙m MH, dN∙m MH-ML, dN∙m tc10, min tc90, min Crosslinking density, *10-4 mol/cm3 Poly-sulfide crosslinking density, *10-4 mol/cm3 Tensile strength, MPa Modulus at 100%, MPa Modulus at 300%, MPa

100/0 0.87 3.93 3.06 2.73 9.22

Unfilled 90/10 80/20 70/30 Curing characterizations 0.96 1.04 1.14 4.45 4.98 5.59 3.49 3.94 4.45 2.54 2.23 1.97 7.42 6.57 5.85 Crosslinking density

100/0

Filled 90/10

80/20

70/30

2.52 18.86 16.34 2.95 33.93

2.53 19.40 16.87 2.62 29.87

2.62 20.49 17.87 2.40 26.77

2.74 21.72 18.98 2.08 23.43

0.985

1.080

1.107

1.219

1.255

1.434

1.515

1.575

0.245

0.415

0.341

0.335

0.283

0.436

1.404

0.743

15.0 2.5 11.7

15.7 2.7 12.7

16.0 2.8 13.3

17.4 2.9 14.8

1.5 0.4 0.9

Mechanical properties 1.6 1.6 1.5 0.5 0.5 0.6 1.0 1.1 1.3 18 ACS Paragon Plus Environment

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Modulus at 300%/Modulus at 100% Elongation at break, % Tear strength, KN/m Hardness, Shore A DIN abrasion, cm3/40 m Rebounce, % Tensile fatigue life, *104 cycles Tan δ at 0 °C, Tan δ at 60 °C, 5% strain Tensile strength, MPa Modulus at 100%, MPa Modulus at 300%, MPa Elongation at break, %

--

--

--

--

4.68

4.70

4.75

5.10

406 5.5 27 -56

393 5.6 30 -60

382 6.4 32 -63

340 6.5 35 -66

358 31.7 64 0.15 32

350 30.3 65 0.13 35

342 30.0 66 0.12 39

336 30.9 67 0.11 42

103

>124

>124

>124

96

124

>124

>124

0.482 0.157

0.410 0.146

0.368 0.143

0.319 0.137

15.4 2.9 13.3 336

12.3 2.8 -284

14.1 3.1 -295

11.2 2.9 -271

1.111 0.735 0.601 0.505 0.054 0.052 0.051 0.050 After ageing at 100 °C for 48h 1.3 1.2 1.3 1.2 0.4 0.5 0.5 0.6 0.9 1.0 0.9 -368 370 404 294

Abrasion resistance is another key parameter considering the serving life of tire materials. Vulcanized SSBR/TBIR composites exhibit excellent abrasion performance, with DIN abrasion decreasing almost 30% (from 0.15 to 0.11 cm3/40 m) when 30 phr TBIR is incorporated into SSBR composite (Figure 7d). As analyzed above, the increased crosslinking density and improved filler dispersion level in vulcanizates contribute to better abrasion resistance of SSBR/TBIR vulcanizates. Additionally, the excellent abrasion resistance also benefits from the nano-scale phase structure, enhanced domains interfacial interactions and hindering effects of TBIR fibrils on crack initiation and propagation.13 It could also be seen in Table 2 that, with increasing the amount of TBIR, the rebounce increases from 56 to 66 for unfilled vulcanizates and from 32 to 42 for filled vulcanizates, indicating improved comfort of car tires. In addition, tan delta at 0 °C which represents the wet-skid resistance, slightly decreases with increasing TBIR addition. However, the wetskid resistance of SSBR/TBIR vulcanizates is still almost 60% and 25% higher in the case of unfilled gum and filled composite compared with the commonly used tire tread recipe of SSBR/BR vulcanizates.13 19 ACS Paragon Plus Environment


3.6. Mechanism Analysis for SSBR/TBIR Vulcanizates with Excellent Integrated Performances. We further summarize the contribution of TBIR to the integrated performances of SSBR-based rubber nanocomposites, as illustrated in Figure 8. The strong polarity of silica nanoparticles makes them easy to flocculate during annealing process for filled SSBR compound. During the vulcanization process, pure SSBR composite also features with slower vulcanization rate and lower crosslinking density. These factors could further intensify filler flocculation degree during high-temperature vulcanization process and result in the formation of large silica aggregates with dimensions around several tens of microns in the final SSBR vulcanizate. The poor filler distribution and the existence of large filler aggregations are responsible for the high hysteresis loss at 60 °C which implies unsatisfied rolling resistance and also deteriorate fatigue resistance and abrasion resistance of pure SSBR vulcanizate.

Figure 8. Illustrations of the structures of different vulcanized rubber composites. When TBIR is blended into SSBR, the modulus and strength of rubber matrix enhance, which plays vital roles in depressing filler flocculation during annealing process. During the high-temperature vulcanization process, fillers flocculation could further be weakened by shortened vulcanization time and constrained by the generation of high crosslinking 20 ACS Paragon Plus Environment

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density. As a result, the filler dispersion of SSBR/TBIR vulcanizates is improved. In addition, the limited compatibility between TBIR and SSBR results in generation of nanoscale phase structures. The crystalline nature of TBIR renders the generation of lamellar fibrils which is beneficial to the inhibition of crack generation and propagation. Once SSBR/TBIR gums undergo vulcanization, the covalent connection between TBIR-rich domains and SSBR-rich domains occurs resulting in enhanced interfacial interactions. SSBR/TBIR vulcanizates have the features like higher crosslinking density and polysulfide crosslinking content, and possible existence of TBIR lamellar fibrils. These features synergize with each other and result in significantly improved integrated properties of SSBR/TBIR vulcanizates, i.e. low rolling resistance, improved abrasion resistance, greatly prolonged fatigue life, and enhanced mechanical properties, etc. Consequently, TBIR, as a special multi-functional rubber material, is highly advantageous to the design and development of green tires, which will effectively tackle the pressing issues faced with tire industry. 4. CONCLUSIONS In summary, we have proposed a facile strategy considering rational incorporation of moderately crystalline TBIR into conventional amorphous SSBR matrix. SSBR/TBIR vulcanizates with TBIR incorporation feature with improved filler dispersion, high mechanical strength and modulus, nano-scale phase separation structures and enhanced interfacial interactions between different domains, enhanced chemical network structures, and possible existence of lamellar fibrils. All these factors synergize with each other and result in significantly improved comprehensive properties like low rolling resistance, improved abrasion resistance, greatly prolonged fatigue life, and enhanced mechanical



properties, etc. The novel strategy proposed here is beneficial to the development of highperformance green tires. It is demonstrated that TBIR as a special multi-functional rubber material is highly advantageous to effectively tackle the pressing issues in tire industry without changing the present processing technology and formulations. ASSOCIATED CONTENT Supporting Information. Macroscopic filler dispersion in the filled rubber vulcanizates; Tan δ vs. strain of unfilled and filled SSBR/TBIR vulcanizates. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Author Contributions #These

authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China [grant numbers 2015CB654700 (2015CB654706)]; Major Program of Shandong Province Natural Science Foundation [grant number ZR2017ZA0304]; the National Natural Science Foundation of China [grant number 51473083] and Taishan Scholar Program. REFERENCES (1) Guo, B.; Tang, Z.; Zhang, L. Transport Performance in Novel Elastomer Nanocomposites: Mechanism, Design and Control. Prog. Polym. Sci. 2016, 61, 29.


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Rubber Nanocomposites with Nano-Scale Phase Structures and

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