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With anionic polymerization, the solutionpolymerized styrenebutadiene rubber (SSBR) and solutionpolymerizedstyrenebutadiene rubber with alkoxysilanefunctionalization at two ends of macromolecularchains (ASSBR) were synthesized by dilithium as initiator. The occurrences of endgroup functionalizationand condensation reaction were confirmed, but also the molecular structure parametersand endfunctionalized efficiency of ASSBR grafted alkoxysilane groups onto the ends of its macromolecularchains were calculated through the characterizations. By this novel structural modification,there were chemical bondings rather than conventional physical adsorption between silica and rubbermatrix. This novel technology was beneficial to not only immobilizing the free chain ends to decrease theamount of macromolecular chains’ free terminals, but also chemically bonding the rubber chains on thesurfaces of silica particles to enhance the fillerpolymer interaction significantly. Furthermore, thecovering layer of endfunctionalized macromolecular chains around the silica particles was conducive toreducing the silica agglomeration and improving the silica dispersion. The structures, morphologies, andproperties of SiO2SSBR and SiOASSBR composites prepared by cocoagulation and mechanicalblending, were investigated. The results showed that SiO2ASSBR composites behaved better comprehensiveperformances including higher wet skid resistance and lower rolling resistance than SiO2SSBRcomposites. Consequently, ASSBR was an ideal material for the green tire treads.
Polymer xxx (2014) 1e13 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites Xiao Liu a, Suhe Zhao b, c, *, Xingying Zhang b, c, Xiaolin Li b, Yu Bai b a b c College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China a r t i c l e i n f o a b s t r a c t Article history: Received 30 October 2013 Received in revised form 10 January 2014 Accepted 28 February 2014 Available online xxx With anionic polymerization, the solution-polymerized styrene-butadiene rubber (SSBR) and solutionpolymerized styrene-butadiene rubber with alkoxysilane-functionalization at two ends of macromolecular chains (A-SSBR) were synthesized by dilithium as initiator The occurrences of end-group functionalization and condensation reaction were confirmed, but also the molecular structure parameters and end-functionalized efficiency of A-SSBR grafted alkoxysilane groups onto the ends of its macromolecular chains were calculated through the characterizations By this novel structural modification, there were chemical bondings rather than conventional physical adsorption between silica and rubber matrix This novel technology was beneficial to not only immobilizing the free chain ends to decrease the amount of macromolecular chains’ free terminals, but also chemically bonding the rubber chains on the surfaces of silica particles to enhance the filler-polymer interaction significantly Furthermore, the covering layer of end-functionalized macromolecular chains around the silica particles was conducive to reducing the silica agglomeration and improving the silica dispersion The structures, morphologies, and properties of SiO2/SSBR and SiO2/A-SSBR composites prepared by co-coagulation and mechanical blending, were investigated The results showed that SiO2/A-SSBR composites behaved better comprehensive performances including higher wet skid resistance and lower rolling resistance than SiO2/SSBR composites Consequently, A-SSBR was an ideal material for the green tire treads Ó 2014 Elsevier Ltd All rights reserved Keywords: Silica Solution-polymerized styrene-butadiene rubber (SSBR) End-group functionalization Introduction Recently, with high attention to environmental protection and saving resources, the reduction of fuel consumption of automobile will play an effective role in protecting environment As an important part of automobile, tire plays an important role in energy-saving and emission-reduction In the process of vehicle driving, the rolling resistance of tire is 20e30% of the total energy consumption of automobile Furthermore, the rolling loss of tread is about 50% of the total energy consumption of tire Accordingly, it is * Corresponding author Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China Tel.: þ86 10 6445 6158; fax: þ86 10 6443 3964 E-mail addresses: liux@bjut.edu.cn, liulaugh@126.com (X Liu), zhaosh@mail buct.edu.cn (S Zhao) urgent for researchers to develop and produce the highperformance and energy-saving “green tire tread material” with low rolling resistance and high wet skid resistance [1] At the end of twentieth century, researchers found that the hysteresis loss of rubber mainly originated from unconstrained “free terminals” of macromolecular chains in the three-dimensional crosslinked network of vulcanizate Although they contribute to tire tread with excellent wet skid resistance, lots of friction heat produced by their random motion greatly increases the rolling resistance In order to lower the hysteresis loss, introducing the functional groups which can either “passivate” free terminals or react with reinforcing filler into the ends of macromolecular chains becomes a research focus in recent years [2] For instance, due to the restriction of SneC bond on a part of free terminals of macromolecular chains, the properties of SSBR with end groups coupled by SnCl4 such as high wet skid resistance, high wear resistance, and low rolling resistance are significantly improved [3,4] http://dx.doi.org/10.1016/j.polymer.2014.02.067 0032-3861/Ó 2014 Elsevier Ltd All rights reserved Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 With the excellent performances such as low hysteresis loss and high reinforcement, nanosilica is a widely used filler for preparing green tire treads in rubber industry [5e9] However, due to a large number of hydroxyl groups on the surfaces of silica particles, they usually show high surface energy, easy self-aggregation, and poor affinity with non-polar rubber macromolecules [10e13] Therefore, the improvements of silica dispersion and filler-polymer interfacial interaction, as well as the reduction of filler-polymer interfacial friction are the problems which many researchers have made great efforts to resolve The common method to improve filler-polymer affinity is the silica particles’ organic modification by a silane coupling agent [14e 18] However, it is difficult to ensure each silica particle is organically modified by the molecules of silane coupling agent, and thus the dispersion of silica particles in polymer matrix cannot achieve the primary particle scale, i.e 15e30 nm Besides, the small-size effect and quantum size effect of silica particle cannot be exhibited at all To overcome these difficulties, the method of grafting functional groups [19] which are able to react with silica particles onto the free terminals of macromolecular chains can be considered According to this assumption, silica particles will be adsorbed or bonded on the terminals of rubber macromolecular chains This leads to the strengthened filler-polymer interfaces and “passivated” free terminals of macromolecular chains, which can reduce the contribution of random thermal motion to hysteresis loss There are many reported literatures about the end-group functionalization such as ethylene-propylene-diene terpolymer (EPDM), polybutadiene, polyisoprene, polydimethylsiloxane, and polystyrene terminated by glycidyl methacrylate [20], chlorophosphine [21], 1[2-(4-chlorobutoxy)ethyl] aziridine [22], (aminopropyl) dimethylsiloxy [23], and (tridecafluouo-1,1,2,2-tetrahydrooctyl) dimethylchlorosilane [24], respectively As for the SSBR used for tread material, the published achievements about its end-group functionalization are divided into several categories according to endfunctionalized reagent, including amide-type (N-phenyl-2pyrrolidone) [25] to improve storage stability, aminobenzophenone-type (4,40 -bis(diethylamino)-benzophenone) [26,27] to improve rebound resilience or physical properties, nitrile-type (chloroacetonitrile [28] or benzonitrile [29]) to improve its affinity for carbon black or lower rolling resistance, fused-ring polynuclear aromatic compound (benzanthracenes) [30] and Schiff bases (dimethylaminobenzylidenemethylamine) [31] to reduce hysteresis properties, and carbodiimide-type (dialkylcarbodiimides) [32] to improve impact resilience All these endgroup functionalizations involve the carbon black-filled vulcanizates, in which the end-groups physically or chemically react with carbon black to improve their properties, but for silica-filled system, it is a technical problem in the field of organiceinorganic interaction which needs to be investigated There have few related studies on this In this study, two kinds of rubbers were synthesized by anionic polymerization with initiator of dilithium The one was normal SSBR, the other was A-SSBR prepared by adding an endfunctionalized reagent, which can graft onto the end of macromolecular chain at one end and can react with silica particle at the other end, to SSBR solution in the last stage of polymerization One part of A-SSBR solution was coagulated directly through removing solvent to get the solid rubber, and the other part of A-SSBR solution was treated through the successive steps which include adding a small amount of silica particles, condensation reaction, and cocoagulation to obtain an A-SSBR/SiO2 co-coagulated rubber Their molecular structure parameters were characterized and the endfunctionalized efficiency values of A-SSBR were calculated The morphological structure, bound rubber content, crosslink density, glass-transition characteristics, rheological properties, mechanical properties and other dynamic properties of three rubbers (including one SSBR and two A-SSBRs) filled with silica were investigated respectively Besides, the mechanism and physics of the structure formation and the relationship with the properties were analyzed in detail It is expected that these experimental results can provide the theoretical basis and novel design for preparing a new nanocomposite with excellent performances and potential superiorities as green tire tread material Experimental 2.1 Materials Styrene (analytical reagent) was from Beijing Chemical Reagents Company (Beijing, China) Butadiene (industrial grade) and cyclohexane (analytical reagent) were supplied by Beijing Yanshan Petrochemical Co., Ltd (Beijing, China) Tetrahydrofuran (THF, analytical reagent) and ethanol (analytical reagent) were purchased from Beijing Chemical Works (Beijing, China) Butyl dilithium initiator was self-made in laboratory g-chloropropyl trimethoxy silane (CPTMO, industrial grade) and nitrogen (!99.999%) were provided by Qufu Wanda Chemical Co., Ltd (Shandong, China) and Beijing Shunanqite Gas Company (Beijing, China), respectively Precipitated silica (Tixosil 383) with an average particle diameter of 20e40 nm and specific surface area of 100e200 m2/g came from Qingdao Rhodia Co., Ltd (Shandong, China) The other rubber additives, such as zinc oxide, stearic acid, and sulfur, were commercial grades 2.2 Formula The formula of all the vulcanizates was as follows: 100 parts SSBR, 30 parts precipitated silica, parts zinc oxide, part stearic acid, 1.5 parts polymerized 2,2,4-trimethyl-1,2-dihydroquinoline, 1.2 parts benzothiazyl disulfide, 1.2 parts diphenyl guanidine, part triethanolamine, and 1.8 parts sulfur 2.3 Synthesis and preparation 2.3.1 Purification The reaction conditions of anionic polymerizations were so severe that only a few of impurities such as hydrogen and water in the system can terminate the reaction; thus it was necessary to purify the monomers, solvents, and other chemicals In this experiment, the styrene as a monomer was bathed in calcium hydride for 24 h followed by reduced pressure distillation, and then was stored with seal to avoid light under the environment of high purity nitrogen at À15 C The THF, a structure regulator, was purified by using the same methods as the styrene except for the atmospheric distillation The cyclohexane as a solvent was bathed in calcium hydride for 24 h followed by atmospheric distillation to collect the fraction at 65e70 C, and then the sodium wire was put into it to remove the micro-water before storing with seal under the environment of high purity nitrogen, which was bubbled into the solvent for 15 before experiment to remove a small amount of hydrogen The CPTMO as an end-functionalized reagent was bathed in calcium hydride for an hour followed by atmospheric distillation to collect the fraction at 78e80 C, and then was stored with seal to avoid light under the environment of high purity nitrogen 2.3.2 Synthesis and numbering First, the entire polymerization plant was cleaned by both high purity nitrogen and reactive polymer to ensure the reaction conditions of anionic polymerization Second, the butyl dilithium initiator was synthesized through a small quantity of butadiene Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 initiated by the ready-made naphthalene-lithium at 25 C for h in cyclohexane solution Next, the cyclohexane, styrene, butadiene, and THF were added successively in a 2L reaction vessel which was purified by butyl dilithium as a purifying agent at room temperature, followed by adding the initiator (butyl dilithium) The polymerization lasted h at 50 C with a stirring speed of 250r/min Finally, SSBR was prepared by adding alcohol to SSBR solution to terminate the reaction The two solutionpolymerized styrene-butadiene rubbers with alkoxysilane functionalized end-groups, i.e A-SSBR-1 and A-SSBR-2, were prepared by adding CPTMO to SSBR solution in the last stage of polymerization and then reacting for half an hour at 65 C The amounts or concentration of all the used chemicals are listed in Table To examine the kinetics of anionic polymerization of SSBR, ASSBR-1, and A-SSBR-2 in detail, polymerizations were monitored at 50 C The conversionetime relationships (the total conversion, styrene conversion, and butadiene conversion as a function of time with an interval of 30 min) were obtained by determining the amount of unconsumed monomers and polymerization product For SSBR, the total, styrene, butadiene conversion respectively were 66.3%, 63.6%, 67.2% (t ¼ 30 min), 81.2%, 79.4%, 81.8% (t ¼ 60 min), 91.7%, 90.5%, 92.1% (t ¼ 90 min), 95.4%, 95.1%, 95.5% (t ¼ 120 min), and 98.9%, 98.6%, 99% (t ¼ 150 min) For A-SSBR-1, the total, styrene, butadiene conversion respectively were 63.7%, 60.4%, 64.8% (t ¼ 30 min), 79.7%, 77%, 80.6% (t ¼ 60 min), 90.9%, 90.3%, 91.1% (t ¼ 90 min), 95.2%, 94.9%, 95.3% (t ¼ 120 min), and 98.8%, 98.5%, 98.9% (t ¼ 150 min) For A-SSBR-2, the total, styrene, butadiene conversion respectively were 65.8%, 62.5%, 66.9% (t ¼ 30 min), 82.5%, 80.1%, 83.3% (t ¼ 60 min), 93.1%, 92.5%, 93.3% (t ¼ 90 min), 96.2%, 95.9%, 96.3% (t ¼ 120 min), and 98.9%, 98.9%, 98.9% (t ¼ 150 min) H NMR (CDCl3): d ¼ 6.85e7.40 (aromatic proton in each random-copolymerized styrene unit), 6.20e6.85 (aromatic proton in each block-copolymerized styrene unit), 5.50e5.60 (aCHe proton in each 1, 2-butadiene structural unit), 5.37e5.50 (eCHa and aCHe proton in each 1, 4-butadiene structural unit), 4.79e4.99 (aCH2 proton in each 1, 2-butadiene structural unit), and 3.40e 3.60 ppm (proton in eSie(OCH3)3) [33e36] FTIR (KBr): 3080e3020 (CeH stretching vibration peak of benzene), 1495e1453 (skeleton vibration peak of benzene ring), 968 (CeH bending vibration peaks of polybutadiene’s trans-1,4 structures), 910 (CeH bending vibration peaks of polybutadiene’s vinyl structures), 728 (CeH bending vibration peaks of polybutadiene’s cis-1,4 structures), 1178 and 1090e1020 cmÀ1 (eSieOeC stretching vibration peaks) [37] Thereafter, co-coagulated SiO2/A-SSBR-1 was prepared through a successive process of adding hr (parts per hundred of rubber) silica powder to rubber solution, stirring and reflux at 85 C for h, Table Amounts of all the used chemicals Chemical Cyclohexane (g) Styrene (g) Butadiene (g) THF (ml) Butyl dilithium (purifying agent) (ml) Butyl dilithium (initiator) (ml) CPTMO (end-functionalized reagent) (ml) Monomer concentration (%) Molar ratio of THF to active center Molar ratio of and CPTMO to active center Compound no SSBR A-SSBR-1 A-SSBR-2 1161.5 23 89.3 1.12 3.45 9.67 68.18:1 1183.9 31.1 122.8 1.87 4.85 0.33 13 48.5:1 1:1 1816.4 37.3 149.7 8.5 1.49 0.41 10.3 47.6:1 1:1 and removing solvent A-SSBR-2 solid rubber was obtained by a direct co-coagulation and then removing solvent The samples are identified as follows: 1#-Adding 30 phr silica powder to SSBR by mechanical blending; 2#- Adding the rest silica powder to SiO2/A-SSBR-1 cocoagulated rubber by mechanical blending (the total amount of silica was kept constant at 30 phr); 3#- Adding 30 phr silica powder to A-SSBR-2 by mechanical blending 2.3.3 Mixing and vulcanization The mixing of silica with rubber was carried out on a KGSA11 Haake internal mixer (Xiamen Rectifier Co., Ltd, Fujian, China) with a volume of 55 ml and a rotating speed of 10r/min The torque value as a function of time was recorded to investigate the condensation reaction The other rubber additives were added to rubber in a 6inch open mill (Zhanjiang Machinery Plant, Guangdong, China) by the conventional mixing technique A XLB-D350  350 plate vulcanization machine (Huzhou Dongfang Machinery Co., Ltd, Zhejiang, China) was used to prepare vulcanizates, and the curing condition was 150 C  t90(cure time) The hydraulic pressure was 15 MPa on the mould and each vulcanizate had a thickness of about mm The cure times of rubber compounds were determined at 150 C with a P3555B2 oscillating disk rheometer (Huanfeng Chemical Technology and Experiment Machine Plant, Beijing, China) About g of rubber compound was used for each test and a 1 arc oscillating angle was applied 2.4 Characterization of structure and properties 2.4.1 Gel permeation chromatography (GPC) The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (Mw/Mn) of the synthesized copolymers were measured by using a Waters150C gel permeation chromatograph (Waters Corporation, United States) with three Waters Styragel columns (pore size 102, 103, and 104 Å, respectively) in series calibrated by narrow polystyrene standard with molecular weight ranging from 2.2  103 to 5.15  105 g/mol THF was used as the eluent at a flow rate of 1.0 mL/ at 40 C 2.4.2 1H nuclear magnetic resonance (1H NMR) The characteristic groups including functionalized end-groups of polymers were tested by 1H NMR measurement carried out on a Bruker AV600 high-resolution NMR spectrometer (Bruker Corporation, Bremen, Germany) with a frequency of 600 MHz at room temperature (25 C) The polymer samples were dissolved in CDCl3 in a mm NMR tube Chemical shifts were reported in ppm and referenced to tetramethylsilane (TMS) as an internal standard and calculated by using the residual isotopic impurities of the deuterated solvents 2.4.3 Fourier transform infrared (FTIR) spectrometry FTIR spectra were recorded on a Tensor-37 FTIR spectrometer (Bruker Optik Gmbh, Germany) at room temperature The rubber samples were extracted by boiling ethanol for 72 h, and then the extraction products were dried to a constant weight in a vacuum drying oven followed by dissolving in organic solvent at a concentration of about 10% The sample films were prepared by spreading a small amount of rubber solution on a KBr pellet uniformly after the evaporation of solvent In all cases, 64 scans in a Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 wavenumber range of 400e4000 cmÀ1 at a resolution of 0.6 cmÀ1 were used to record the spectra 2.4.4 Energy dispersive X-ray spectroscopy (EDS) The elemental distributions on the surfaces of sample residues were analyzed by a Hitachi S-4300 field emission scanning electron microscope (FE-SEM) equipped with a Genesis-60 energy dispersive spectrometer (EDAX Inc., United States) 2.4.5 Transmission electron microscopy (TEM) observation The micrographs of vulcanizates were observed by a Hitachi H800-1 transmission electron microscope (Hitachi Corporation, Tokyo, Japan) with an acceleration voltage of 200 kV and a magnification of  104 The samples were ultramicrotomed at À100 C under liquid nitrogen cooling to give the ultrathin section with a thickness of 70e90 nm, and then were placed onto a 200 mesh cooper grid coated with carbon film 2.4.6 Mechanical properties The tensile strength, tear strength, Shore A hardness, and dynamic compression properties of vulcanizates were measured according to ASTM D412 (dumbbell shaped), ASTM D624 (right-angle shaped), ASTM D2240, and ASTM D395, respectively The tensile and tear strengths of the samples prepared from hot pressed sheets were clamped at the both ends and pulled in uniaxial elongation with a CMT4104 electrical tensile tester (Shenzhen SANS, Guangdong, China) at 25 Æ C, with a constant crosshead speed of 500 mm/min and an initial gauge length of 25 mm The Shore A hardness and dynamic compression properties of vulcanizate were measured by an XY-1 rubber hardness apparatus (4th Chemical Industry Machine Factory, Shanghai, China) and a YS-25 compression fatigue testing machine (Shanghai Chemical Machinery No.4 Factory, Shanghai, China), respectively The dynamic compression measurement lasted 25 at 55 C with a load of 1.01 MPa, a compression stroke of 4.45 mm, and a compression frequency of 1800 minÀ1 During tensile, tear, and dynamic compression test, five, five, and three specimens were tested to give the average value, respectively, and during the hardness test, the hardness values of three different sample (over mm in thickness) spots were measured to give the average value 2.4.7 Differential scanning calorimetry (DSC) The determination of glass-transition temperature (Tg) to assess the interfacial bonding was carried out on a STARe system differential scanning calorimeter (Mettler-Toledo, Switzerland) The curves for samples (3e6 mg) were obtained by heating sample from À80 to 40 C at a rate of 10 C/min under nitrogen atmosphere Appearing as a step in the baseline or heat capacity (Cp), the Tg could be calculated by either the half height of the Cp step, the onset of the transition obtained by extrapolating the tangent of the inflection point to the initial baseline, the inflection point of the step, or the 1/2 DCp between the baselines In our case, Tg was estimated by the inflection point of the step 2.4.8 Dynamic mechanical analysis (DMA)-temperature sweep The storage modulus (G0 ) and internal friction loss (tand) as a function of temperature were measured by a DMTA V dynamic mechanical thermal analyzer (Rheometrics Scientific Inc., Piscataway, New Jersey, United States) with rectangular tension mode of deformation The measurements were carried out at a frequency of 10 Hz, a heating rate of C/min, and a double strain amplitude of 0.1% over a temperature range of À100 to 100 C Each sample was 30 mm in length, mm in width, and mm in thickness The Tg value was taken to be the maximum of the tand versus temperature curve 2.4.9 Rubber process analysis (RPA)-strain sweep Strain sweep experiments (G0 and tand as a function of scanning strain) were performed on vulcanizates by a RPA2000 rubber process analyzer (Alpha Technologies Corporation, Akron, Ohio, United States) at 60 C The strain amplitude (ε%) was varied from 0.28 to 100% and the frequency was Hz 2.4.10 Bound rubber content About g rubber compound was cut into small pieces followed by being placed in a steel wire mesh with an average pore diameter of 75 mm and then was dissolved in toluene solvent Bound rubber content was determined by extracting the unbound materials such as ingredients and free rubbers with toluene for days and acetone for day followed by drying for days at room temperature until a constant mass value The toluene was changed every 24 h The weights of samples before and after the extraction were measured and the bound rubber contents were calculated according to the equation [38]: h ii h i h Wt mr = mf þ mr Rb ð%Þ ¼ 100  Wfg À Wt mf = mf þ mr (1) where Rb was the bound rubber content, Wfg was the weight of filler and gel, Wt was the weight of sample, mf was the fraction of filler in the compound, and mr was the fraction of rubber in the compound 2.4.11 Rheological properties The viscosity (h) and non-Newtonian index (n) of rubber compounds at various shear rates (g) were determined by an Instron3211 capillary rheometer (Instron Corporation, UK) at 100 C under a shear rate ranging from to 104 sÀ1, and the samples were preheated for 10 before the measurement The capillary die had a diameter of 0.1595 cm as well as a length of 2.5557 cm, and the plunger speeds varied at 0.06, 0.2, 0.6, 2.0, 6.0, 20.0 cm/min 2.4.12 Crosslink density (XLD) XLD measurements were carried out on a XLDS-15 crosslink density analyzer and NMR spectrometer (IIC Innovative Imaging Corporation, Blieskastel, Germany) with a magnetic field intensity of 15 MHz at 80 C Rubber sample with a length of mm and a diameter of approximately mm was placed into a glass tube for the measurement Totally 64 measurements at different values were carried out for determining the relaxation time Data analysis was performed according to the IIC Analysis Software package, using a non-linear MarquardteLevenberg algorithm Results and discussion 3.1 Structure and characterizations of A-SSBR 3.1.1 Mechanism and physics of structure formation of A-SSBR/SiO2 composite In this experiment, each polymer was synthesized through anionic polymerization with monomers of styrene and butadiene and initiator of dilithium As far as A-SSBR was concerned, the eSie (OCH3)3 groups were grafted onto the two ends of polymer macromolecular chains after adding CPTMO to polymer solution in the last stage of polymerization After end-group functionalization, A-SSBR can react with silica particles by the condensation reaction between A-SSBR’s eSie(OCH3)3 groups and silica’s eSieOH groups at 85 C for h The structural sketch of synthesis process for ASSBR and condensation reaction between A-SSBR and silica are shown in Fig Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 Fig Sketch of the synthesis process for A-SSBR and condensation reaction between A-SSBR and silica The structure of the final composite with excellent performances was formed through three steps which were illustrated in this schematic representation in detail The first step is conventional anionic polymerization of SSBR The only difference is the initiator (butyl dilithium), which reacted with suitable monomers including butadiene and styrene to form a polymer chain with two anionic sites The second step is end-group functionalization of SSBR The selected end-functionalized agent (CPTMO) was added to SSBR solution in the last stage of anionic polymerization Its role is to obtain the siloxane-functionalized SSBR through chemically reacting with the active center at the end of macromolecular chains after the monomers are consumed, providing the chemical basis of subsequent condensation reaction with silica particles The third step is condensation reaction between A-SSBR and silica particles The mechanism of this condensation reaction is essentially the same as that for organic modification of silica particles by CPTMO as silane coupling agent The function of siloxane groups at the end of macromolecular chains is similar to silane coupling agent The condition of reacting at 85 C for h is chosen to ensure the sufficient condensation reaction For A-SSBR/SiO2 compound, the end- Fig 1H NMR spectra of rubbers (a) SSBR (b) A-SSBR-1 (c) A-SSBR-2 (d) SiO2/A-SSBR-1 Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 functionalized polymer macromolecular chains are chemically bounded to the silica particles through eSieOeSie bonds, which in turn decrease the amount of macromolecular chains’ free ends In this way, there are chemical bondings rather than conventional physical adsorption between A-SSBR and silica after adding silica to rubber matrix and condensation reaction at high temperature The eSieOeSie(CH2)3e bonds herein play a role in linking silica and rubber matrix This particular kind of filler-polymer interaction can be beneficial to the improvements of A-SSBR/SiO2 composites’ comprehensive performances 3.1.2 1H NMR characterization and end-functionalized efficiency of A-SSBR After extracted by boiling ethanol solvent for 72 h, SSBR, ASSBR-1, and A-SSBR-2 rubber samples were measured by 1H NMR, which spectra are shown in Fig (a)e(c) respectively According to the reported equations [33], styrene content and vinyl content can be calculated Moreover, the molecular-weight parameters can be determined by GPC measurement All the obtained structural parameters of the synthesized SSBR, A-SSBR-1, and A-SSBR-2 are listed in Table All Fig (a)e(c) spectra exhibit no signal peak in the chemical shift of 6.20e6.85 ppm, indicating that the styrene units are randomly distributed rather than block-copolymerization for these three rubbers [39] Also from Fig (a)e(c) spectra, in the chemical shift of about 3.562 ppm, i.e the position of hydrogen atoms of e Sie(OCH3)3, both Fig (b) and (c) exhibit one peak, but Fig (a) not, indicating that the alkoxysilane groups were successfully grafted onto the ends of macromolecular chains of A-SSBR For each group in 1H NMR spectra, the ratio of its signal peak area to its hydrogen atom number is equivalent According to this principle, the ratio of the number of chain ends containing trimethoxyl-silylpropyl to total chain ends namely the endfunctionalized efficiency therefore can be calculated by the ratio of the peak areas of benzene-H to alkoxysilane-H from the 1H NMR spectra, and the related equation is listed as follows: SBenzeneÀH =SAlkoxysilaneÀH ¼ ð5  St%  Mn=MS Þ=ð9   EÞ (2) SBenzene-H, SAlkoxysilane-H, St%, Mn, MS, and E represent the peak area of hydrogen atoms of benzene, peak area of hydrogen atoms of alkoxysilane, styrene content, number-average molecular weight of polymer, molecular weight of styrene, and end-functionalized efficiency, respectively Moreover, “5” and “9” are respectively the hydrogen atom numbers of benzene and alkoxysilane, i.e., eSie (OCH3)3, and “2” implies the two functionalized ends of macromolecular chains The calculated values of end-functionalized efficiency are shown in Table In addition, 1H NMR spectrum of SiO2/A-SSBR-1 co-coagulated rubber is depicted in Fig (d) to investigate whether the Table End-functionalized efficiency values of SSBR and A-SSBR Sample no SSBR A-SSBR-1 A-SSBR-2 End-functionalized efficiency (%) 75.5 71.0 condensation reaction between A-SSBR-1 and silica is carried out Compared NMR spectrum of A-SSBR-1 (Fig (b)) with that of SiO2/ A-SSBR-1 (Fig (d)), it can be seen that the two curves are similar except for the peak at 3.562 ppm in Fig (b), indicating that the e Sie(OCH3)3 groups of A-SSBR-1 disappear after the addition of silica powder This only is caused by the condensation reaction between eSie(OCH3)3 groups of A-SSBR-1 and eSieOH groups of silica under the condition as described in Section 2.3.2, similarly according to the reported mechanism [40] Thus, it demonstrates that the condensation reaction occurs and the silica-rubber chemical bondings are achieved 3.1.3 FTIR spectrometry characterization FTIR spectra of SSBR and A-SSBR are displayed in Fig It can be seen that both SSBR and A-SSBR spectra exhibit all of the characteristic peaks mentioned in Section 2.3.2, but only A-SSBR spectrum has eSieOeC stretching vibration peaks appeared at 1090e1020 and around 1178 cmÀ1, indicating the occurrence of end-group functionalization reaction 3.1.4 EDS characterization SSBR, A-SSBR-1 and A-SSBR-2 samples were heated in an alumina crucible at 600 C for h, and then the residues were measured by EDS characterization The data for Si element content are listed in Table From Table 4, the Si element contents of the two A-SSBR samples are much higher than that of SSBR sample; the Si element contents of A-SSBR-1 sample is slightly higher than that of A-SSBR-2 sample This result is in accordance with the endfunctionalized efficiency calculated by 1H NMR The high Si element content of A-SSBR sample implies that it can only be derived from CPTMO-functionalized end groups, but low Si element content of SSBR sample can only be from impurities, confirming the grafting reaction of CPTMO onto the ends of macromolecular chains of A-SSBR Table Structural parameters of SSBR and A-SSBR Parameter a Mn g/mol Mw g/molb Polydispersity index ac Styrene content (%) Vinyl contentd (%) Compound no SSBR A-SSBR-1 A-SSBR-2 140,852 199,189 1.41 19.4 48.5 180,685 255,700 1.42 20.7 49.0 173,457 235,843 1.36 18.5 50.7 a Number-average molecular weight Weight-average molecular weight c Ratio of weight-average molecular weight to number-average molecular weight d Content of 1, 2-butadiene structure b Fig FTIR spectra of SSBR and A-SSBR Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 Table Silicon element contents of SSBR and A-SSBR Sample no SSBR A-SSBR-1 A-SSBR-2 Initial mass of sample (g) Mass of sample after heated (g) Mass fraction of silicon element displayed in EDS (%) Mass of silicon element in sample (g) 0.1500 0.0001 6.41 0.1500 0.0006 48.08 0.1500 0.0006 46.58 6.41  10À6 2.88  10À4 2.79  10À4 From the above three characterization results, the accurate structural information can be obtained The conclusion that A-SSBR is indeed terminated by eSie(OCH3)3 groups is drawn, demonstrating that alkoxysilane functionalizations on the ends of macromolecular chains are achieved 3.2 Processability 3.2.1 Reaction characteristics in mixing process The torqueetime curves of mixing A-SSBR with silica powder in Haake internal mixer at 50 C and 90 C are shown in Fig A significant fluctuation of the two torque curves can be seen in the first 10 min, which is the time of adding silica powder to rubber matrix In Fig 4(a), the torque value directly keeps constant after fluctuation, but in Fig 4(b) the torque value shows a peak value in the 20e27th minute and then remains constant, probably owing to the condensation reaction between alkoxysilane groups on the ends of macromolecular chains and hydroxyl groups of silica particles This indicates that mixing at 90 C is helpful to condensation reaction between silica and alkoxysilane groups of A-SSBR Besides, it is also possible to display good rheological characteristics 3.2.2 Rheological properties The rheological curves of 1#, 2#, and 3# rubber compounds are shown in Fig As is clearly displayed in Fig 5, all these rubbers belong to shear-thinning non-Newtonian fluid In heg curves, all the viscosity values decrease as the shear rate increases, and the three curves are approximately parallel to each other, indicating the similar shear sensitivities The highest viscosity of 1# rubber is exhibited, namely the poorest flowability and highest energy consumption in processing It is probably because the strong interaction among silica particles [41] of 1# rubber results in the formation of filler-aggregates which occlude a part of rubbers [42] and increase the effective volume of filler [43], leading to slip and relaxation difficulties in the flow process of macromolecular chains On the contrary, due to the low occluded rubber content and the weak interaction among silica particles induced by the fewer silanol groups [6] after end-group functionalization, the lower viscosity values of 2# and 3# rubbers are exhibited, which manifest the better processability than 1# rubber In neg curves, the non-Newtonian index values of 2# and 3# rubbers decrease significantly when the shear rate increases, manifested as the non-Newtonian behavior This may be caused by 2# and 3# rubbers’ strong filler-polymer interaction, which also results in good filler dispersion [44], besides may be caused by the diversified conformation of rubber macromolecules in the flow process 3.3 Filler-polymer interaction 3.3.1 Effect of macromolecular chain terminals passivated by silica particles on Tg DSC curves of SiO2/A-SSBR-1 co-coagulated rubber and A-SSBR1 pure rubber are shown in Fig A single transition in the temperature range from À50 C to À30 C with Tg at À38.5 C for ASSBR-1 and that with Tg at À36.7 C for SiO2/A-SSBR-1 are observed, i.e an increment of nearly C after adding silica to rubber This may be partly caused by SiO2/A-SSBR-1’s strengthened interfacial bonding, and partly by an extended crosslinked network among reactive eSi(OCH3)3 groups [45] It enables the increased resistance to the slippage and motion of polymer segments [46], the decrease of chain mobility [47], and the enhancement of Tg 3.3.2 Bound rubber content The bound rubber content is affected by the filler-polymer interaction [48] The bound rubber contents of 1#, 2#, and 3# rubber compounds are displayed in Table In Table 5, the higher bound rubber contents of 2# and 3# rubber compounds are exhibited, indicating more chemical bondings between macromolecular chains and silica particles This performance is the result of alkoxysilane functionalization on the ends of macromolecular chains, but also can contribute good mechanical properties to the corresponding vulcanizates discussed in Section 3.5 Also by contrast in Table 5, the bound rubber content of 2# rubber compound is the highest, which is close to that of 3# and that of 1# is the lowest This result is directly proportional to the result of endfunctionalized efficiency; further illustrating the end-group functionalization is an effective way to strengthen filler-polymer interaction A similar literature reported by Mélé [15] showed that the amount of bound rubber increased with the addition of the silane coupling agent in silica/SBR compounds and this could result from the increase of the specific surface induced by the better dispersion of fillers Fig Torqueetime curves of mixing A-SSBR with silica powder in Haake internal mixer (a) 50 C (b) 90 C Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 Fig Rheological curves of rubber compounds 3.3.3 Crosslink density To further study the filler-polymer interaction, the data of 1H NMR relaxation parameters measured by a NMR crosslink density analyzer are listed in Table 1H NMR relaxation is produced by intermolecular and intramolecular magnetized-dipole interaction [49] When the testing temperature is above Tg, the intermolecular dipole interaction can be neglected and hence the main part is intramolecular dipole interaction, which is affected by surroundings and can reflect molecular activity ability Signal decay data and molecular weight between crosslinking points (Mc) are analyzed according to a Gaussian-exponential function [50] and a formula reported by Kuhn [51], respectively From Table 6, the lowest physical crosslink density and the highest chemical crosslink density of 2# rubber are exhibited, indicating that the crosslinking bond formed by condensation reaction between functionalized macromolecules and hydroxyl groups of silica particles can increase the amount of chemical bonding and can reduce the physical filler-polymer adsorption and the physical entanglement among macromolecular chains Also in Table 6, the Mc, T2, A(T2) values of all the vulcanizates are lower and the A(Mc) values of all the vulcanizates are higher than those of the corresponding rubber compounds This result shows that the vulcanization can bring on more crosslinking points, fewer activity units, and lower activity ability Also from Table 6, the highest A(Mc) values and lowest A(T2) values of 2# rubber are exhibited; the second is 3# and the last is 1# It demonstrates that the larger amount of chemical bonding derived from condensation reaction between silica and functionalized polymer corresponds to the larger crosslinking point amount and the lower mobile fraction It is proved that the free movement of the molecular chain ends is restrained after the macromolecular chains’ end-group functionalization and condensation reaction with silica Furthermore, the result of the lowest Mc values for 2# rubber also implies its densest chemical crosslinking and strongest interfacial bonding between silica and A-SSBR This is in accordance with the data of bound rubber content and S J Park [52] ’s investigation reporting that the organic functional groups on the silica surface make an increase of the adhesion at interfaces between silica and rubber matrix, resulting in improved crosslink density 3.4 Dynamic mechanical properties 3.4.1 Temperature sweep The curves of both G0 and tand as a function of temperature with a constant frequency for all the vulcanizates are shown in Fig The approximate G0 values in glassy state for three vulcanizates are observed, but in high-elastic state there are two distinct differences First, the highest value of 1# vulcanizate is attributed to the low mobility of matrix inside its silica aggregates [53] induced by the strong fillerefiller interaction [54] Next, G0 value of 2# is close to that of 3#, and both are lower than that of 1#, indicating that the better filler dispersion of 2# and 3# vulcanizates can decrease the effects of filler aggregates on G0 value The reason, according to some researchers’ viewpoints [53,55], is likely that the presence of CPTMO favors the filler dispersion or reduces the strengthening effect of rigid inorganic particles thus, decreasing the storage shear modulus The similar results were also reported in the literature [56] The glass-transition characteristic data obtained from Fig are shown in Table Generally, the temperature associated with the peak magnitude of the tand plot is defined as Tg [57] It can be seen Table Bound rubber contents of rubber compounds Fig DSC curves of A-SSBR-1 pure rubber and SiO2/A-SSBR-1 co-coagulated rubber Sample no 1# rubber compound 2# rubber compound 3# rubber compound Bound rubber content (%) 14.69 59.52 48.95 Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 Table Relaxation parameters of rubbers Sample no 1# 1# 2# 2# 3# 3# a b c d e f g Parameter rubber compound vulcanizate rubber compound vulcanizate rubber compound vulcanizate Physical XLD  10À5a (mol/cm3) Chemical XLD  10À5b (mol/cm3) Mcc (kg/mol) A(Mc)d (%) T2e (ms) A(T2)f (%) qM2  104g (s2) 7.64 4.57 6.54 3.18 7.43 4.03 e 4.11 e 5.68 e 4.84 12.43 10.95 14.53 10.72 12.78 10.97 35.49 57.78 46.51 60.08 44.39 58.24 2.12 1.60 2.00 1.44 2.02 1.52 64.50 41.70 53.30 39.55 55.30 41.78 62.43 80.49 45.70 84.06 59.12 80.25 Physical crosslink density Chemical crosslink density Molecular weight between crosslinking points Percentage of crosslinking fractions Relaxation time Percentage of high-mobile fractions Residual dipolar interaction that Tg values of 1#, 2#, and 3# vulcanizates are 4.1 C, 5.9 C, and 5.5 C higher than those of corresponding pure rubbers respectively, and there is a similar feature between this result and DSC data It may be in relation with the constrained dynamics of segmental motions of the rubber molecules interacting with filler surface, as supported by NMR experiments [58] This result presents the larger increments for 2# and 3# vulcanizates, which shows that the end-group functionalization helps to achieve stronger constraints to macromolecular motions and stronger fillerpolymer interactions In tire industry, tand values at C and 60 C usually are used as the indexes to evaluate wet skid resistance and rolling resistance respectively [59,60] Also from Table 7, 2# vulcanizate exhibits the highest C tand value and lowest 60 C tand value, implying a balance between high wet skid resistance and low rolling resistance can be achieved through end-group functionalization This result indicates that the firm eSieOeSie bonds formed by condensation reaction between silica and functionalized end-group can enhance the filler-polymer interaction Besides, the constrained terminals also can greatly reduce the irregular thermal motion among macromolecular chains, thus resulting in the low tand value at 60 C 3# vulcanizate is not better than 2# vulcanizate in above aspects, for its relatively lower degree of end-group functionalization 3.4.2 Strain sweep G0 -ε% and tand-ε% curves of all the vulcanizates are shown in Fig Payne reported [61] that G0 decreased with the increase of strain This result was explained by the breakdown of aggregated secondary network of filler particles or agglomerates formed by van der Waals-London attraction forces This consideration was supported by Kraus [62] Therefore, Payne effect [61,63,64] usually is used as an evaluation of the three-dimensional filler network for fillerefiller and fillerepolymer interaction The lower Payne effect implies the stronger filler-polymer interaction and better filler dispersion [65] From G0 -ε% curve, the DG’ values (G0 difference between ε ¼ 0.28% and ε ¼ 100%) and Payne effect of both 2# and 3# vulcanizates are lower than those of 1# vulcanizate, i.e., fewer agglomerates and stronger filler-polymer interaction This reveals that functionalized groups at the end of macromolecular chains can Table Glass-transition parameters of rubbers Sample no Glasstransition SSBR A-SSBR-1 A-SSBR-2 parameter 1# pure 1# 2# pure 2# 3# pure 3# rubber vulcanizate rubber vulcanizate rubber vulcanizate Fig G0 -T and tand-T curves of vulcanizates Tg ( C) À26.5 Tand value e at C Tand value e at 60 C À22.4 0.269 0.076 À25.8 e e À19.9 0.417 0.069 À25.1 e e À19.6 0.400 0.071 Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 10 X Liu et al / Polymer xxx (2014) 1e13 Fig G0 -ε% and tand-ε% curves of vulcanizates (60 C) improve the silica-rubber linkage and silica dispersion in rubber matrix From tand-ε% curve, tand value of 1# vulcanizate is the highest and increases rapidly with the increase of strain This result shows that its filler aggregates are gradually split with the increase of strain, and thus the released occluded rubber can enhance the internal friction loss [56] The tand values of 2# and 3# vulcanizates are significantly lower than that of 1# vulcanizate, and rise slightly with the increase of strain This indicates that there are few large aggregates in 2# and 3# vulcanizates, and the generated friction heats are derived from the split of small aggregates This further indicates that the functionalized groups at the end of macromolecular chains immobilize the mobility of the free chain terminals and reduce the friction loss of the chain terminals These G0 -ε% and tand-ε% results are basically consistent with the other published research finding [56] The above statements show that the filler-polymer interaction and interfacial bonding are strengthened as well as the internal friction loss and heat are reduced after the condensation between hydroxyl groups of silica and functionalized end-groups of A-SSBR occurs at the filler-rubber interface during the stretching [70,71] The modulus at 300% elongation, tensile strength, tear strength, and lowest permanent set of 2# vulcanizate are the highest in Table Polmanteer [72] and Pal [73] discovered that some properties of sulfur-cured rubbers, such as tensile strength and tear strength, were improved as the quality of silica dispersion increased This result therefore indicates that the filler dispersion [74], filler-polymer interaction, and external force resistance are improved and the propagation paths of tear crack are lengthened after the reactive blending of A-SSBR-1/silica by two steps In Table 8, the order from low to high for the three vulcanizates’ compression heat build-up values is 2#, 3#, and 1#, exhibiting the same feature as their tand values at 60 C This result indicates the interfacial bonding between rubber and silica for 2# vulcanizate can be enhanced by its strong filler-polymer interaction [75] Due to the formation of effective interface which avoids severe friction [19], it leads to the reduction of friction heat loss in the process of dynamic compression 3.5 Mechanical properties TEM photographs of all the vulcanizates are shown in Fig In these photographs, the dark color part is silica and the light color part is SSBR or A-SSBR matrix In Fig 9(a), there are the evident filler-aggregation and poor dispersion, which can weaken the rubber by creating structural flaws and damage to properties [76], whereas in Fig 9(b) and (c), the only tiny aggregates which hinder the formation of local stress concentrations where fracture is easy to start [53], and the isolated single silica particles which are uniformly distributed in rubber matrix in a scale of less than 50 nm with good filler dispersion are observed This result can serve as the direct proof of good filler dispersion used for analyzing the improvements in such as bound rubber content, dynamic, rheological, and mechanical properties, and further indicates that silica dispersion in nano-scale can be effectively achieved by condensation reaction between alkoxysilane groups on the ends of macromolecular chains and hydroxyl groups of silica particles Mechanical properties of all the vulcanizates are shown in Table From Table 8, the Shore A hardness value of 1# vulcanizate is the highest, which is related to the poor silica dispersion in rubber matrix and is in accordance with its highest modulus value in G0 -T curve It is well known that the tensile properties are affected by the size of agglomerates formed by the silica [66,67] and rubber/silica interaction [68,69] For the samples with weak or without filler-rubber chemical bonding, the dewetting firstly Table Mechanical properties of vulcanizates Mechanical parameter Sample no 1# vulcanizate Shore A hardness Modulus at 100% elongation (MPa) Modulus at 300% elongation (MPa) Tensile strength (MPa) Elongation at break (%) Permanent set (%) Tear strength (KN$mÀ1) Compression heat build-up ( C) 2# vulcanizate 3# vulcanizate 68 1.7 60 1.6 60 1.7 4.2 7.3 6.7 14.3 593 18 31.9 14.9 17.6 440 10 38.0 8.5 16.0 459 10 37.4 10.2 3.6 Microstructure 3.7 Relationship between novel chemical structure and properties According to the theory of network elasticity, the hysteresis loss of rubber material is mainly from its free terminals which have no contribution to elasticity and increase the internal resistance Nagata [77] considered the modification of the molecular chain ends was the principal means endowing SSBR with energy-saving properties This implies both hysteresis loss and heat build-up are decreased via reducing the amount of free chain terminals In this Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 11 Fig TEM photographs of vulcanizates (a) 1# (b) 2# (c) 3# study, controlled/“living” anionic polymerization, which makes the methods of molecular structure modification diversified, has been adopted to perform end modification of SSBR Besides, one special silane coupling agent with one end which can terminate the active center of anionic polymerization and the other end of siloxane groups which can react with silica particles, was selected as endfunctionalized agent As a result, it can not only reduce significantly the free ends of macromolecular chains, but also chemically anchor these rubber chains on the surfaces of silica particles to further strengthen the silica-rubber interaction The covering layer of end-functionalized macromolecular chains around the silica particles is beneficial to reducing the silica agglomeration and improving the silica dispersion Some characterizations and measurements were carried out to investigate the relationship between this novel structure and properties There are three aspects about this relationship by summarizing all the experimental results The first is the degree of structural modification This is represented by end-functionalized efficiency, which is an important parameter and calculated from H NMR spectra The higher the end-functionalized efficiency is, the greater the number of chain restrainedly bonded on silica particle is The end-functionalized efficiency is directly proportional to the excellence of most properties, including EDS, viscosity, bound rubber content, chemical crosslink density, G0 value in highelastic state or at lower strain, internal friction heat, tensile and tear strengths, wet skid resistance, and rolling resistance This further implies the final properties can be improved by increasing the degree of this structural modification The second is filler-polymer interaction The advantages of silica’s high reinforcement and low heat build-up can be largely taken only when the interfacial adhesion between added silica and rubber chain (i.e., filler-polymer interaction) is strengthened This aim can be achieved by condensation reaction between end-functionalized macromolecular chains and silica particles The silica particles and rubber chains are chemically linked by the resultant eSieOeSie(CH2)3e bonds instead of conventional physical adsorption This action can significantly strengthen the filler-polymer interaction so that the firm eSieOeSie bond can well resist the external force or dynamic strain As a macroscopic result of filler-rubber interactions, the bound rubber content is consequently considered, and its importance is stressed as the obvious link towards excellent properties The bound rubber content of A-SSBR is significantly higher than that of SSBR, testifying the improvement of filler-rubber interaction by this end-group functionalization method The third is filler dispersion This has relevance to the filler-rubber interaction The increased filler-rubber interaction results in the decreased fillere filler interaction, and thus silica particles’ well-known agglomeration is weaken to improve the silica dispersion The situation is totally different with silica whose surface is occupied by sizable quantities of siloxane and silanol groups, giving rise to strong interparticles interactions through hydrogen bonding Fillerefiller interaction leads to the formation of silica networks and the associated difficulties in dispersing such a material in non-polar elastomers [78] Generally, silica particles are organically modified by using silane coupling agent to loosen their networks Nevertheless this cannot immobilize the free movement of the molecular chain ends On the contrary, the more positive experimental results, which are consistent with our expected results from original molecular design, can be obtained by our end-group functionalization method The most intuitionistic results are shown by TEM photographs, illustrating the improved silica dispersion in nanoscale Furthermore, good silica dispersion effectively reduces the stress concentration and molecular friction heat, manifesting other excellent properties such as low Payne effect, low tand value under temperature sweep or strain sweep, high wet skid resistance, low rolling resistance, and good mechanical properties In a word, the filler dispersion in rubber matrix and interfacial bonding between silica particles and macromolecules are improved Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 12 X Liu et al / Polymer xxx (2014) 1e13 significantly after the end-group functionalization An effective technique (i.e., end-group functionalization and then condensation reaction with silica) is provided in this study It widens the way of thinking to achieve the reduction of free macromolecular chain ends and the chemical bonds of silica particles on rubber chains, since silica only naturally interact with certain specialty elastomers such as polydimethylsiloxane, thank to their similar chemistry Therefore, A-SSBR/SiO2 composite exhibiting excellent rheological, mechanical and dynamic properties, high wet skid resistance, and low rolling resistance will be as an ideal material for preparing the green tire tread Conclusion SSBRs with alkoxysilane-functionalization at two ends of macromolecular chains (A-SSBRs) were successfully synthesized, and were characterized by 1H NMR, FTIR, and EDS, respectively The end-functionalized efficiency reaches 71e76% The filler-polymer interaction and silica dispersion can be significantly improved by using A-SSBR as matrix As a result, SiO2/A-SSBR composites exhibit low viscosities, good flowabilities, low energy consumptions in process, high bound rubber contents, good mechanical properties, short relaxation times, low internal friction loss and heat build-up, as well as high wet skid resistance and low rolling resistance Particularly, the comprehensive properties of 2# vulcanizate prepared by reactive blending in two steps are the most excellent Its novel process of end-group functionalization and then condensation reaction with silica can not only reduce significantly the free ends of macromolecular chains to restrain their irregular thermal motions, but also promote these rubber chains chemically anchor on the surfaces of silica particles In brief, A-SSBR is considered to be appropriate as green tire tread material Acknowledgments The authors gratefully acknowledge the financial supports of the National Tenth-five Year Plan (Grant number: 2004BA310A41), National Natural Science Foundation of China (Grant number: 50573005), National Natural Science Foundation of China (Grant number: 51208012) and Research Fund of New Teachers for the Doctoral Program of Higher Education of China (Grant number: 3c009011201301) References [1] Castellano M, Conzatti L, Costa G, Falqui L, Turturro A, Valenti B, et al Polymer 2005;46(3):695e703 [2] Tsutsumi F, Sakakibara M, Oshima N Rubber Chem Technol 1990;63(1):8e22 [3] Manley K, Barry M Benefits of solution polymerized SBR In: Alan B, editor Tire technology international Dorking: UK & International Press; 1998 pp 78e82 [4] Sierra CA, Fatou JMG, Quiteria VRS, Galan C Rubber Chem Technol 1995;68(2): 259e66 [5] Wolff S Rubber Chem Technol 1996;69(3):325e46 [6] Wolff S, Görl U, Wang MJ, Wolff W Eur Rubber J 1994;16(1):16e9 [7] 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World 1964;151(3):41e3 13 [77] Nagata N, Kobatake T, Watanabe H, Ueda A, Yoshioka A Rubber Chem Technol 1987;60(5):837e55 [78] Leblanc JL Prog Polym Sci 2002;27(4):627e87 Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 [...]... al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 12 X Liu et al / Polymer xxx (2014) 1e13 significantly after the end-group functionalization An effective technique (i.e., end-group functionalization and then condensation reaction with. .. NA, Railsback HE Rubber World 1964;151(3):41e3 13 [77] Nagata N, Kobatake T, Watanabe H, Ueda A, Yoshioka A Rubber Chem Technol 1987;60(5):837e55 [78] Leblanc JL Prog Polym Sci 2002;27(4):627e87 Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer... 1975;48(5):795e809 [73] Pal PK, De SK Rubber Chem Technol 1982;55(5):1370e88 Please cite this article in press as: Liu X, et al., Preparation, structure, and properties of solution-polymerized styrene-butadiene rubber with functionalized end-groups and its silica-filled composites, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.02.067 X Liu et al / Polymer xxx (2014) 1e13 [74] Peng Z, Kong... high bound rubber contents, good mechanical properties, short relaxation times, low internal friction loss and heat build-up, as well as high wet skid resistance and low rolling resistance Particularly, the comprehensive properties of 2# vulcanizate prepared by reactive blending in two steps are the most excellent Its novel process of end-group functionalization and then condensation reaction with silica... way of thinking to achieve the reduction of free macromolecular chain ends and the chemical bonds of silica particles on rubber chains, since silica only naturally interact with certain specialty elastomers such as polydimethylsiloxane, thank to their similar chemistry Therefore, A-SSBR/SiO2 composite exhibiting excellent rheological, mechanical and dynamic properties, high wet skid resistance, and. .. link towards excellent properties The bound rubber content of A-SSBR is significantly higher than that of SSBR, testifying the improvement of filler -rubber interaction by this end-group functionalization method The third is filler dispersion This has relevance to the filler -rubber interaction The increased filler -rubber interaction results in the decreased fillere filler interaction, and thus silica particles’... selected as endfunctionalized agent As a result, it can not only reduce significantly the free ends of macromolecular chains, but also chemically anchor these rubber chains on the surfaces of silica particles to further strengthen the silica -rubber interaction The covering layer of end -functionalized macromolecular chains around the silica particles is beneficial to reducing the silica agglomeration and improving... number of chain restrainedly bonded on silica particle is The end -functionalized efficiency is directly proportional to the excellence of most properties, including EDS, viscosity, bound rubber content, chemical crosslink density, G0 value in highelastic state or at lower strain, internal friction heat, tensile and tear strengths, wet skid resistance, and rolling resistance This further implies the final properties. .. particles and rubber chains are chemically linked by the resultant eSieOeSie(CH2)3e bonds instead of conventional physical adsorption This action can significantly strengthen the filler-polymer interaction so that the firm eSieOeSie bond can well resist the external force or dynamic strain As a macroscopic result of filler -rubber interactions, the bound rubber content is consequently considered, and its importance... photographs of vulcanizates (a) 1# (b) 2# (c) 3# study, controlled/“living” anionic polymerization, which makes the methods of molecular structure modification diversified, has been adopted to perform end modification of SSBR Besides, one special silane coupling agent with one end which can terminate the active center of anionic polymerization and the other end of siloxane groups which can react with silica