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.
Trang 1Preparation, structure, and properties of solution-polymerized
styrene-butadiene rubber with functionalized end-groups and its
Xiao Liua, Suhe Zhaob,c,*, Xingying Zhangb,c, Xiaolin Lib, Yu Baib
a College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
b Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
c 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
Article history:
Received 30 October 2013
Received in revised form
10 January 2014
Accepted 28 February 2014
Available online xxx
Keywords:
Silica
Solution-polymerized styrene-butadiene
rubber (SSBR)
End-group functionalization
a b s t r a c t
With anionic polymerization, the polymerized styrene-butadiene rubber (SSBR) and solution-polymerized styrene-butadiene rubber with alkoxysilane-functionalization at two ends of macromo-lecular chains (A-SSBR) were synthesized by dilithium as initiator The occurrences of end-group func-tionalization 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 macro-molecular 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 compre-hensive 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
1 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
urgent for researchers to develop and produce the high-performance 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 reinforcingfiller 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 macromo-lecular chains, the properties of SSBR with end groups coupled by SnCl4such as high wet skid resistance, high wear resistance, and low rolling resistance are significantly improved[3,4]
* Corresponding author Key Laboratory of Beijing City on Preparation and
Pro-cessing 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).
Contents lists available atScienceDirect
Polymer
j o u rn a l h o m e p a g e : w w w e l s e v ie r c o m / l o c a t e / p o l y m e r
http://dx.doi.org/10.1016/j.polymer.2014.02.067
0032-3861/Ó 2014 Elsevier Ltd All rights reserved.
Polymer xxx (2014) 1e13
Trang 2With the excellent performances such as low hysteresis loss and
high reinforcement, nanosilica is a widely usedfiller 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 andfiller-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 improvefiller-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
organi-cally 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
exhibi-ted 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
consid-ered According to this assumption, silica particles will be adsorbed
or bonded on the terminals of rubber macromolecular chains This
leads to the strengthenedfiller-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),
poly-butadiene, polyisoprene, polydimethylsiloxane, and polystyrene
terminated by glycidyl methacrylate[20], chlorophosphine[21],
1-[2-(4-chlorobutoxy)ethyl] aziridine [22], (aminopropyl)
dime-thylsiloxy [23], and (tridecafluouo-1,1,2,2-tetrahydrooctyl)
dime-thylchlorosilane[24], respectively As for the SSBR used for tread
material, the published achievements about its end-group
func-tionalization are divided into several categories according to
end-functionalized reagent, including amide-type
(N-phenyl-2-pyrrolidone) [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
(dia-lkylcarbodiimides)[32]to improve impact resilience All these
end-group functionalizations involve the carbon black-filled
vulcani-zates, in which the end-groups physically or chemically react with
carbon black to improve their properties, but for silica-filled
sys-tem, 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
end-functionalized reagent, which can graft onto the end of
macro-molecular 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
solu-tion was treated through the successive steps which include adding
a small amount of silica particles, condensation reaction, and
co-coagulation to obtain an A-SSBR/SiO2co-coagulated rubber Their
molecular structure parameters were characterized and the
end-functionalized 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 re-sults can provide the theoretical basis and novel design for pre-paring a new nanocomposite with excellent performances and potential superiorities as green tire tread material
2 Experimental 2.1 Materials Styrene (analytical reagent) was from Beijing Chemical Reagents Company (Beijing, China) Butadiene (industrial grade) and cyclo-hexane (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 ad-ditives, 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, 4 parts zinc oxide, 1 part stearic acid, 1.5 parts polymerized 2,2,4-trimethyl-1,2-dihydroquinoline, 1.2 parts benzothiazyl disulfide, 1.2 parts diphenyl guanidine, 1 part triethanolamine, and 1.8 parts sulfur
2.3 Synthesis and preparation 2.3.1 Purification
The reaction conditions of anionic polymerizations were so se-vere 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
at15C The THF, a structure regulator, was purified by using the same methods as the styrene except for the atmospheric distilla-tion The cyclohexane as a solvent was bathed in calcium hydride for 24 h followed by atmospheric distillation to collect the fraction
at 65e70C, 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 min 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 78e80C, 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
X Liu et al / Polymer xxx (2014) 1e13 2
Trang 3initiated by the ready-made naphthalene-lithium at 25C for 2 h
in cyclohexane solution Next, the cyclohexane, styrene,
buta-diene, 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 3 h at 50C with a stirring
speed of 250r/min Finally, SSBR was prepared by adding alcohol
to SSBR solution to terminate the reaction The two
solution-polymerized styrene-butadiene rubbers with alkoxysilane
func-tionalized end-groups, i.e A-SSBR-1 and A-SSBR-2, were
pre-pared by adding CPTMO to SSBR solution in the last stage of
polymerization and then reacting for half an hour at 65C The
amounts or concentration of all the used chemicals are listed in
Table 1
To examine the kinetics of anionic polymerization of SSBR,
A-SSBR-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% (t¼ 30 min), 81.2%, 79.4%, 81.8% (t ¼ 60 min),
91.7%, 90.5%, 92.1% (t¼ 90 min), 95.4%, 95.1%, 95.5% (t ¼ 120 min),
and 98.9%, 98.6%, 99% (t¼ 150 min) For A-SSBR-1, the total, styrene,
butadiene conversion respectively were 63.7%, 60.4%, 64.8%
(t¼ 30 min), 79.7%, 77%, 80.6% (t ¼ 60 min), 90.9%, 90.3%, 91.1%
(t¼ 90 min), 95.2%, 94.9%, 95.3% (t ¼ 120 min), and 98.8%, 98.5%,
98.9% (t¼ 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% (t¼ 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%
(t¼ 150 min)
1H 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
pro-ton 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 ineSie(OCH3)3)[33e36]
FTIR (KBr): 3080e3020 (CeH stretching vibration peak of
ben-zene), 1495e1453 (skeleton vibration peak of benzene ring), 968
(CeH bending vibration peaks of polybutadiene’s trans-1,4
struc-tures), 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 cm1(eSieOeC stretching
vibration peaks)[37]
Thereafter, co-coagulated SiO2/A-SSBR-1 was prepared through
a successive process of adding 5 hr (parts per hundred of rubber)
silica powder to rubber solution, stirring and reflux at 85C for 3 h,
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 co-coagulated 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 6-inch 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 pre-pare vulcanizates, and the curing condition was 150C t90(cure time) The hydraulic pressure was 15 MPa on the mould and each vulcanizate had a thickness of about 2 mm The cure times of rubber compounds were determined at 150C with a P3555B2 oscillating disk rheometer (Huanfeng Chemical Technology and Experiment Machine Plant, Beijing, China) About 8 g of rubber compound was used for each test and a 1arc 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
Waters150-C gel permeation chromatograph (Waters Waters150-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 105g/mol THF was used as the eluent at aflow rate of 1.0 mL/ min at 40C
2.4.2 1H nuclear magnetic resonance (1H NMR) The characteristic groups including functionalized end-groups
of polymers were tested by1H NMR measurement carried out on
a Bruker AV600 high-resolution NMR spectrometer (Bruker Cor-poration, Bremen, Germany) with a frequency of 600 MHz at room temperature (25C) The polymer samples were dissolved in CDCl3
in a 5 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 deuter-ated 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 con-centration of about 10% The sample films were prepared by spreading a small amount of rubber solution on a KBr pellet uni-formly after the evaporation of solvent In all cases, 64 scans in a
Table 1
Amounts of all the used chemicals.
SSBR A-SSBR-1 A-SSBR-2
Butyl dilithium (purifying agent) (ml) 1.12 1.87 1.49
Butyl dilithium (initiator) (ml) 3.45 4.85 6
CPTMO (end-functionalized reagent) (ml) 0 0.33 0.41
Molar ratio of THF to active center 68.18:1 48.5:1 47.6:1
Molar ratio of and CPTMO to active center 0 1:1 1:1
Trang 4wavenumber range of 400e4000 cm1at a resolution of 0.6 cm1
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-4300field emission scanning electron
microscope (FE-SEM) equipped with a Genesis-60 energy
disper-sive spectrometer (EDAX Inc., United States)
2.4.5 Transmission electron microscopy (TEM) observation
The micrographs of vulcanizates were observed by a Hitachi
H-800-1 transmission electron microscope (Hitachi Corporation,
Tokyo, Japan) with an acceleration voltage of 200 kV and a
magnification of 5 104 The samples were ultramicrotomed
at 100 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 carbonfilm
2.4.6 Mechanical properties
The tensile strength, tear strength, Shore A hardness, and
dy-namic compression properties of vulcanizates were measured
ac-cording 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,
Guang-dong, China) at 25 2 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
compres-sion fatigue testing machine (Shanghai Chemical Machinery No.4
Factory, Shanghai, China), respectively The dynamic compression
measurement lasted 25 min at 55C with a load of 1.01 MPa, a
compression stroke of 4.45 mm, and a compression frequency of
1800 min1 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 6 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
differ-ential scanning calorimeter (Mettler-Toledo, Switzerland) The
curves for samples (3e6 mg) were obtained by heating sample
from 80 to 40C at a rate of 10 C/min under nitrogen
atmo-sphere 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.,
Piscat-away, 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 3C/min, and a double strain amplitude of
0.1% over a temperature range of100 to 100C Each sample was
30 mm in length, 6 mm in width, and 2 mm in thickness The Tg
value was taken to be the maximum of the tandversus temperature
curve
2.4.9 Rubber process analysis (RPA)-strain sweep Strain sweep experiments (G0and tandas a function of scanning strain) were performed on vulcanizates by a RPA2000 rubber pro-cess analyzer (Alpha Technologies Corporation, Akron, Ohio, United States) at 60C The strain amplitude (ε%) was varied from 0.28 to 100% and the frequency was 1 Hz
2.4.10 Bound rubber content About 2 g rubber compound was cut into small pieces followed
by being placed in a steel wire mesh with an average pore diameter
of 75mm 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 3 days and acetone for 1 day followed by drying for 2 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]:
Rbð%Þ ¼ 100 hWfg Wt
h
mf=mfþ mr
ii
Wth
mr=mfþ mr
i (1)
where Rbwas the bound rubber content, Wfgwas the weight offiller and gel, Wtwas the weight of sample, mfwas the fraction offiller in the compound, and mrwas the fraction of rubber in the compound 2.4.11 Rheological properties
The viscosity (h) and non-Newtonian index (n) of rubber com-pounds at various shear rates (g) were determined by an Ins-tron3211 capillary rheometer (Instron Corporation, UK) at 100C under a shear rate ranging from 1 to 104s1, and the samples were preheated for 10 min 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 magneticfield intensity
of 15 MHz at 80C Rubber sample with a length of 8 mm and a diameter of approximately 5 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
3 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, theeSie (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)3groups and silica’s eSieOH groups
at 85C for 3 h The structural sketch of synthesis process for A-SSBR and condensation reaction between A-A-SSBR and silica are shown inFig 1
X Liu et al / Polymer xxx (2014) 1e13 4
Trang 5The structure of the final composite with excellent
perfor-mances was formed through three steps which were illustrated in
this schematic representation in detail Thefirst step is
conven-tional 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 85C for 3 h is chosen to ensure the suf-ficient condensation reaction For A-SSBR/SiO2compound, the
end-Fig 1 Sketch of the synthesis process for A-SSBR and condensation reaction between A-SSBR and silica.
Fig 2 1 H NMR spectra of rubbers (a) SSBR (b) A-SSBR-1 (c) A-SSBR-2 (d) SiO 2 /A-SSBR-1.
Trang 6functionalized polymer macromolecular chains are chemically
bounded to the silica particles througheSieOeSie 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 offiller-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,
A-SSBR-1, and A-SSBR-2 rubber samples were measured by1H NMR,
which spectra are shown inFig 2 (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
ob-tained structural parameters of the synthesized SSBR, A-SSBR-1,
and A-SSBR-2 are listed inTable 2
AllFig 2 (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 fromFig 2 (a)e(c)spectra, in the chemical
shift of about 3.562 ppm, i.e the position of hydrogen atoms ofe
Sie(OCH3)3, bothFig 2 (b) and (c)exhibit one peak, butFig 2 (a)
not, indicating that the alkoxysilane groups were successfully
grafted onto the ends of macromolecular chains of A-SSBR
For each group in1H 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
end-functionalized efficiency therefore can be calculated by the ratio
of the peak areas of benzene-H to alkoxysilane-H from the1H NMR
spectra, and the related equation is listed as follows:
SBenzeneH=SAlkoxysilaneH¼ ð5 St% Mn=MSÞ=ð9 2 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
ef-ficiency, 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
macro-molecular chains The calculated values of end-functionalized
ef-ficiency are shown inTable 3
In addition,1H NMR spectrum of SiO2/A-SSBR-1 co-coagulated
rubber is depicted in Fig 2 (d) to investigate whether the
condensation reaction between A-SSBR-1 and silica is carried out Compared NMR spectrum of A-SSBR-1 (Fig 2 (b)) with that of SiO2/ A-SSBR-1 (Fig 2 (d)), it can be seen that the two curves are similar except for the peak at 3.562 ppm inFig 2 (b), indicating that thee
Sie(OCH3)3groups of A-SSBR-1 disappear after the addition of silica powder This only is caused by the condensation reaction between eSie(OCH3)3groups of A-SSBR-1 andeSieOH groups of silica un-der the condition as described in Section2.3.2, 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 inFig 3 It can be seen that both SSBR and A-SSBR spectra exhibit all of the charac-teristic peaks mentioned in Section2.3.2, but only A-SSBR spectrum haseSieOeC stretching vibration peaks appeared at 1090e1020 and around 1178 cm1, 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 600C for 6 h, and then the residues were measured by EDS characterization The data for Si element content are listed inTable 4 FromTable 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 end-functionalized 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 2
Structural parameters of SSBR and A-SSBR.
a Number-average molecular weight.
b Weight-average molecular weight.
c Ratio of weight-average molecular weight to number-average molecular
weight.
d Content of 1, 2-butadiene structure.
Table 3 End-functionalized efficiency values of SSBR and A-SSBR.
End-functionalized efficiency (%)
Fig 3 FTIR spectra of SSBR and A-SSBR.
X Liu et al / Polymer xxx (2014) 1e13 6
Trang 7From the above three characterization results, the accurate
structural information can be obtained The conclusion that A-SSBR
is indeed terminated byeSie(OCH3)3 groups is drawn,
demon-strating 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 50C and 90 C are shown inFig 4 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 InFig 4(a), the torque value directly keeps constant after
fluctuation, but inFig 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
par-ticles This indicates that mixing at 90C 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 inFig 5 As is clearly displayed inFig 5, all these rubbers
belong to shear-thinning non-Newtonianfluid Inhegcurves, 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 poorestflowability and highest energy
con-sumption in processing It is probably because the strong
interac-tion among silica particles[41]of 1# rubber results in the formation
offiller-aggregates which occlude a part of rubbers[42]and
in-crease the effective volume offiller[43], leading to slip and
relax-ation 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 negcurves, 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 goodfiller 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 SiO2/1 co-coagulated rubber and
A-SSBR-1 pure rubber are shown inFig 6 A single transition in the tem-perature range from50C to30C with Tg at38.5C for A-SSBR-1 and that with Tg at 36.7 C for SiO2/A-SSBR-1 are observed, i.e an increment of nearly 2 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 reactiveeSi(OCH3)3groups[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 inTable 5 InTable 5, the higher bound rubber contents of 2# and 3# rubber compounds are exhibited, indicating more chemical bondings between macromo-lecular 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 inTable 5, the bound rubber content of 2# rubber com-pound 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 end-functionalized efficiency; further illustrating the end-group func-tionalization 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 offillers
Table 4
Silicon element contents of SSBR and A-SSBR.
Initial mass of sample (g) 0.1500 0.1500 0.1500
Mass of sample after heated (g) 0.0001 0.0006 0.0006
Mass fraction of silicon element
displayed in EDS (%)
Mass of silicon element in
sample (g)
6.41 10 6 2.88 10 4 2.79 10 4
Fig 4 Torqueetime curves of mixing A-SSBR with silica powder in Haake internal mixer (a) 50 C (b) 90C.
Trang 83.3.3 Crosslink density
To further study thefiller-polymer interaction, the data of1H
NMR relaxation parameters measured by a NMR crosslink density
analyzer are listed inTable 6.1H 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
surround-ings 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
re-action between functionalized macromolecules and hydroxyl
groups of silica particles can increase the amount of chemical
bonding and can reduce the physicalfiller-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 fromTable 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 function-alized 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 Mcvalues 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 inves-tigation 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 G0and tandas a function of temperature with
a constant frequency for all the vulcanizates are shown inFig 7 The approximate G0 values in glassy state for three vulcanizates are observed, but in high-elastic state there are two distinct differ-ences First, the highest value of 1# vulcanizate is attributed to the low mobility of matrix inside its silica aggregates[53]induced by the strongfillerefiller interaction[54] Next, G0value of 2# is close
to that of 3#, and both are lower than that of 1#, indicating that the betterfiller dispersion of 2# and 3# vulcanizates can decrease the effects offiller aggregates on G0value The reason, according to some researchers’ viewpoints[53,55], is likely that the presence of CPTMO favors thefiller 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 fromFig 7are shown inTable 7 Generally, the temperature associated with the peak magnitude of the tandplot is defined as Tg[57] It can be seen
Fig 5 Rheological curves of rubber compounds.
Fig 6 DSC curves of A-SSBR-1 pure rubber and SiO 2 /A-SSBR-1 co-coagulated rubber.
Table 5 Bound rubber contents of rubber compounds.
compound
2# rubber compound
3# rubber compound Bound rubber
content (%)
X Liu et al / Polymer xxx (2014) 1e13 8
Trang 9that Tg values of 1#, 2#, and 3# vulcanizates are 4.1C, 5.9C, and
5.5C higher than those of corresponding pure rubbers
respec-tively, 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 withfiller
surface, as supported by NMR experiments[58] This result
pre-sents 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 filler-polymer interactions
In tire industry, tandvalues at 0C and 60C usually are used as the indexes to evaluate wet skid resistance and rolling resistance respectively[59,60] Also fromTable 7, 2# vulcanizate exhibits the highest 0C tand value and lowest 60C tandvalue, implying a balance between high wet skid resistance and low rolling resis-tance 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 thefiller-polymer interaction Besides, the constrained terminals also can greatly reduce the irregular thermal motion among macromolecular chains, thus resulting in the low tandvalue
at 60C 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 8 Payne reported[61]that G0decreased with the increase of strain This result was explained by the breakdown of aggregated secondary network offiller particles or agglomerates formed by van der Waals-London attraction forces This consideration was sup-ported by Kraus[62] Therefore, Payne effect[61,63,64]usually is used as an evaluation of the three-dimensionalfiller 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 D ’ values (G0 difference betweenε 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 strongerfiller-polymer interaction This reveals that functionalized groups at the end of macromolecular chains can
Table 6
Relaxation parameters of rubbers.
Physical XLD 10 5 a (mol/cm 3 )
Chemical XLD 10 5 b (mol/cm 3 )
M c
(kg/mol)
A(M c ) d (%)
T 2
(ms)
A(T 2 ) f (%)
qM 2 10 4
(s 2 )
a Physical crosslink density.
b Chemical crosslink density.
c Molecular weight between crosslinking points.
d Percentage of crosslinking fractions.
e Relaxation time.
f Percentage of high-mobile fractions.
g Residual dipolar interaction.
Table 7 Glass-transition parameters of rubbers.
Glass-transition parameter
Sample no.
1# pure rubber 1#
vulcanizate
2# pure rubber 2#
vulcanizate
3# pure rubber 3# vulcanizate
Tg ( C) 26.5 22.4 25.8 19.9 25.1 19.6 Tandvalue
at 0 C
Tandvalue
at 60 C
Fig 7 G0-T and tand-T curves of vulcanizates.
Trang 10improve the silica-rubber linkage and silica dispersion in rubber
matrix
From tand-ε% curve, tandvalue 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
in-ternal friction loss[56] The tandvalues 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
macromo-lecular 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
researchfinding[56]
The above statements show that thefiller-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
3.5 Mechanical properties
Mechanical properties of all the vulcanizates are shown in
Table 8 FromTable 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
occurs at thefiller-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 8 Polmanteer[72]and Pal[73]discovered that some prop-erties 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 thefiller 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
InTable 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 tandvalues at 60C This result indicates the interfacial bonding between rubber and silica for 2# vulcanizate can be enhanced by its strongfiller-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
3.6 Microstructure TEM photographs of all the vulcanizates are shown inFig 9 In these photographs, the dark color part is silica and the light color part is SSBR or A-SSBR matrix InFig 9(a), there are the evident filler-aggregation and poor dispersion, which can weaken the rubber by creating structuralflaws and damage to properties[76], whereas inFig 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 uni-formly distributed in rubber matrix in a scale of less than 50 nm with goodfiller dispersion are observed This result can serve as the direct proof of goodfiller dispersion used for analyzing the im-provements 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 condensa-tion reaccondensa-tion between alkoxysilane groups on the ends of macro-molecular chains and hydroxyl groups of silica particles
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
Fig 8 G 0 -ε% and tand-ε% curves of vulcanizates (60 C).
Table 8
Mechanical properties of vulcanizates.
Mechanical parameter Sample no.
1# vulcanizate 2# vulcanizate 3# vulcanizate
Modulus at 100%
elongation (MPa)
Modulus at 300%
elongation (MPa)
Compression heat
build-up ( C)
X Liu et al / Polymer xxx (2014) 1e13 10