A SAXS and swelling study of cured natural rubber-styrene–butadiene rubber blends

A SAXS and Swelling Study of Cured Natural Rubber/Styrene–Butadiene Rubber Blends

W. SALGUEIRO,1 A. SOMOZA,1 A. J. MARZOCCA,2 I. TORRIANI,3 M. A. MANSILLA2

1IFIMAT- UNCentro and Comision de Investigaciones Cientıficas de la Provincia de Buenos Aires, Pinto 399,B7000GHG Tandil, Argentina

2LPMPyMC, Departamento de Fısica, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Buenos Aires,Ciudad Universitaria, Pabellon I, C1428EGA Buenos Aires, Argentina

3Instituto de Fısica ‘‘Gleb Wataghin’’, Universidade Estadual de Campinas, CP 6165, 13084-971 Campinas,Sao Paulo, Brazil

Received 3 June 2009; revised 15 July 2009; accepted 7 August 2009DOI: 10.1002/polb.21828Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A small-angle X-ray scattering (SAXS) and swelling study of natural rub-ber and styrene–butadiene rubber blends (NR/SBR) is presented. To this aim, speci-mens of NR and SBR and blends with 75/25, 50/50, and 25/75 NR/SBR ratios (in phr)were prepared at a cure temperature of 433 K and the optimum cure time (t100). Thistime was obtained from rheometer torque curves. The system of cure used in the sam-ples was sulfur/n-t-butyl-2-benzothiazole sulfenamide. From swelling tests of the curedsamples, information about the molecular weight of the network chain between chemi-cal crosslinks was obtained. For all cured compounds, in the Lorentz plots built fromSAXS scattering curves, a maximum of the scattering vector q around 0.14 A�1 wasobserved. However, the q position shows a linear-like shift toward lower values whenthe SBR content in the SBR/NR blend increases. In pure NR or SBR the q values showa different tendency. The results obtained are discussed in terms of the existence of dif-ferent levels of vulcanization for each single phase forming the blend and the existenceof a third level of vulcanization located in the interfacial NR/SBR layer. VVC 2009 Wiley

Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2320–2327, 2009

Keywords: blends; NR; rubber; rubber blends; SAXS; SBR; vulcanization

INTRODUCTION

Blends of elastomers are used in rubber productsto improve physical properties, increased servicelife, easier processing, and reduced product cost.It must be considered that the complete miscibil-ity of polymers requires that the free energy ofthe mixing be negative, which implies exothermicmixing or large entropy of mixing.1 In fact, most

blends of elastomers are immiscible because mix-ing is endothermic and the entropic contributionis small because of the high molecular weights.Fortunately, miscibility is not a requirement formost rubber applications. However, adhesionbetween the polymer phases is necessary.

In the cured state of completely miscible elasto-mers, the glass transition process is characterizedby a single narrow peak. On the other hand, forimmiscible or partially compatible elastomers,two (or only one broad) glass transition distribu-tions represent the cured state.2–7

In this scenario, covulcanization of elastomerscontaining different phases and their respective

Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, 2320–2327 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: A. Somoza (E-mail: [email protected])

2320

interfaces are an important subject of study. Asmentioned in the literature, blends possess aninterfacial layer of rubber with a state of cure dif-ferent from that corresponding to the individualphases.6 When dealing with practical applicationsand also from the theoretical point of view, theinterfaces studies become an important goal ofinvestigation. Thus, the NR/SBR system is anexample of not completely miscible elastomers andits use in automobile industry is well known.1,2

Most of the authors of this work have beeninvolved in the study of vulcanization processes innatural rubber (NR) and styrene–butadiene rub-ber (SBR) by means of different experimentaltechniques.8,9 In a recent article, we used small-angle X-ray scattering (SAXS) to analyze thestructural changes in NR vulcanized samples as afunction of the cure temperature.10 On the otherhand, the same technique, and its variants, hassuccessfully been used to investigate elastomers(see, e.g., refs. 3, 4). According to the resultsreported therein, a correlation could be estab-lished between the crosslinking developed duringvulcanization and changes in the broad intensitypeaks of the X-ray scattering patterns.

In this work, SAXS was used once more as oneof the main experimental techniques to studystructural details in vulcanized blends preparedwith a NR matrix by addition of SBR using n-t-butyl-2-benzothiazole sulfenamide (TBBS) as anaccelerator and sulfur. As a complementary exper-

imental technique, swelling tests were used todetermine the crosslink density of the cured com-pounds. The results obtained are discussed interms of the existence of different levels of vul-canization for each single phase forming theblend. Special attention was given to the studyof the layer between NR and SBR developedduring the vulcanization process of the blends.Specifically, we have idealized this layer as azone, where the NR has higher crosslink den-sity than the SBR phase due to the migration ofthe curatives from the SBR phase, which is nor-mally overcured.

EXPERIMENTAL

Preparation and Characterizationof the Compounds

Natural rubber (NR-SMR20) and styrene–butadi-ene rubber (SBR–1502) were used to prepare theblends studied in this research. The molecularweights of the polymers were measured by GPCusing a Shimadtzu L-6A liquid chromatographsystem with THF as elute. Thus, the values of Mn

¼ 91,350 g/mol and Mn ¼ 149,910 g/mol wereobtained for SBR and NR, respectively. The den-sities of the polymers used were q (NR) ¼ 0.917g/cm3 and q (SBR) ¼ 0.935 g/cm3.

Five compounds were prepared using the for-mulations given in Table 1. For more details ofthe compounds preparation, see ref. 7. The mixeswere characterized at 433 K by means of torquecurves measured with an Alpha MDR2000 rheo-meter.7 The time to achieve the maximum torque,t100, was calculated for each sample and is givenin Table 2.

Samples sheets of 150� 150� 2 mm3 were curedin a mold at 433 K in a press up to time t100 to makesure that the vulcanization reaction was completed.At the end of the curing cycle, the samples werefinally cooled rapidly in an ice–water mixture.

The density q of each cured compound meas-ured by Archimedes procedure is also given in

Table 1. Compound Formulations (in Parts perHundred Rubber)

Materials A B C D E

SBR1502 0 25 50 75 100NR (SMR20) 100 75 50 25 0Stearic acid 2 2 2 2 2ZnO 5 5 5 5 5Antioxidant 1.2 1.2 1.2 1.2 1.2Sulfur 2.3 2.3 2.3 2.3 2.3TBBS 0.7 0.7 0.7 0.7 0.7

Table 2. Optimum Time Cure t100 (MDR2000, 433 K), Density q, and Tg of theStudied Samples

A B C D E

t100 (min) 10.7 17.0 24.7 34.4 42.3q (g/cm3) 0.9536 0.9566 0.9648 0.9722 0.9786Tg (K) 213.1 214.3/226.7 214.5/226.4 228.2 229.6

In the case of the blend (samples B and C), two values of Tg were detected.7

A SAXS AND SWELLING STUDY OF NR/SBR BLENDS 2321

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

Table 2. As can be seen, q increases when theSBR content in the formulation increases.

On the other hand, the glass transition (Tg) ofthe cured samples shown in Table 2 are thosereported in a previous work.7 As can be seen inthe table, for the samples prepared with only SBRor NR, one Tg was obtained. In the blends B andC, two glass transition temperatures were found.

Swelling Tests

The molecular weight of the network chainbetween chemical crosslinks, Mcs, was determinedfrom swelling tests. To this aim, the methoddescribed by Cunneen and Russell was applied11

and the Flory–Rehner relationship12,13

Mcs ¼ � q 1� 2=/ð ÞV1v1=32m

ln 1� v2mÞ þ vv22m þ v2m� (1)

was used, where q is the density of the rubbernetwork, / the functionality of the crosslinks,v2m the polymer volume fraction at equilibrium(maximum) degree of swelling, and V1 themolar volume of solvent. v is an interactionparameter between the polymer and the swel-ling agent.

The interaction parameter polymer–solvent vwas evaluated using a mixture law, starting fromthe v for the systems NR/toluene and SBR/tolu-ene. These values were v(NR) ¼ 0.43 þ 0.05v2m,

14

v(SBR) ¼ 0.524 � 0.285v2m,15 and V1 (toluene) ¼

106.29 mL/mol.16

Finally, the values of Mcs obtained for each vul-canized blend are given in Table 3.

It is important to point out that a crosslink isconsidered as a small region in a macromoleculefrom which at least four chains emanate. Specifi-cally, a crosslink behaves as a central core inwhich macromolecules are attached and theirnumber defines the crosslink functionality /.From the structural point of view, there is ahigher electron density at the site of the cross-links. Besides, sulfur cured elastomers are usu-ally considered a four-functional network,17 and

precisely for this work this functionality, / ¼ 4,was used in eq 1.

To obtain the experimental Mcs, 17-mm dia-meter discs of the cured sheets were cut with adie. These discs remained in pyridine for 16 h atroom temperature. Then, they were continuouslyextracted in acetone for 24 h (ASTM D 297-93(2006)) and dried. One probe of each sample wascompletely immersed in pure toluene, in asealed glass bottle, at room temperature untilequilibrium swelling occurred. Normally, thisprocess takes more than 48 h. When this stepwas completed, the samples were removed fromthe bottles, the excess toluene from the surfaceof the samples was wiped off and the swollenweight immediately measured using an elec-tronic balance with an accuracy of 10�4 g. Thesamples were dried at 333 K and weightedagain when all the solvent was evaporated.

The volume fraction v2m was calculated basedon the following equation

v2m ¼ b Wd �Wfð Þ=qcWd �Wfð Þ=qð Þ þ Ws �Wdð Þ=qsð Þ½ � ; (2)

where Wd is the weight of the sample after swel-ling and drying, Ws the weight of the swollen sam-ple, and qs the density of the solvent (0.8669g/cm3 for toluene16). Wf is the weight of the nonex-tractable filler in the sample. The compoundsused in this research do not contain filler as car-bon black or silica, but as it can be observed inTable 1, there is ZnO in the composition and wehave used the ASTM D 297-93 (2006) method toevaluate Wf. The volume fractions v2m for eachblend composition are given in Table 3 and Figure1. The values of v2m for samples 90NR/10SBR and10NR/90SBR were also measured and included inFigure 1. These cured samples were prepared fora previous research work to study the thermal dif-fusivity on these blends.7

SAXS Measurements

From 2-mm thick sheets, 8-mm diameter sampleswere cut with a die. The samples were set up inthe sample holder device of the D11A-SAXS1beamline of the Brazilian National SynchrotronLaboratory (LNLS, Campinas, Brazil) at roomtemperature. A wavelength k ¼ 1.608 A wasselected for the monochromatic beam used in theexperiments. The scattering intensity distribu-tions I(q) as a function of the scattering vector q (q¼ (4p/k) sin h; 2h being the scattering angle) wereobtained in the q-range between �0.02 and 0.40

Table 3. Equilibrium Volume Fraction in theSwollen State (v2m) and Molecular Weight BetweenCrosslinks Mcs of the Studied Samples

A B C D E

v2m 0.198 0.178 0.180 0.173 0.166Mcs (g/mol) 5,451 7562 8,080 9,653 11,958

2322 SALGUEIRO ET AL.


A�1. A typical spectrum was obtained after meas-uring for 15 min. This time was enough to get reli-able results for this type of measurements. A one-dimensional position sensitive detector was usedwith a 729.8-mm sample-detector distance. Toreduce parasitic scattering, the samples were keptunder vacuum during the exposures. Dataobtained were normalized by the integrated inten-sity and detector response curve. Background sub-traction was subsequently performed for everysample measurement. Because of the strong scat-tering signal, the data were acquired using a slitin front of the detector window for the full q-range,and in a second step, blocking the low q regionwith a shield to allow a longer time exposure andimprove the statistics in the region around q ¼0.14 A�1. The two sets of data were later spliced to-gether (as shown in Fig. 2).

RESULTS AND DISCUSSION

In Figure 2 the SAXS curves I(q) as a function ofthe scattering vector q for the different com-pounds studied are shown. The curve obtained forthe sample named A (100 phr NR) is in very goodagreement with that previously reported by theauthors in ref. 10 for the same elastomer. As canbe seen, the curves presented in the figure show awide band for scattering vector values between�0.12 and �0.18 A�1. When SBR is added to theNR matrix, increasing SBR contents (samples Bto D) correspond to a higher I(q) value reaching amaximum for 100% SBR (sample E). Summariz-ing, the systematic increase of the intensity maxi-

mum is directly proportional to the SBR contentin the blend. Besides, the difference in the I(q)versus q curves corresponding to 100 phr NR and100 phr SBR indicates that the scattering proper-ties of the two main components in the blend havesome similarities, but present differences in theintensity and maximum position.

As usual, to get a more precise localization ofthe angular position of the intensity maximum,Lorentz plots (I(q)q2 vs. q)18 for the different com-pounds are presented in Figure 3. In agreementwith the aim of this work, we have focused ourattention in the region in which the different max-ima are present, that is, a region for the scatteringvector q between 0.10 and 0.18 A�1.

It is well accepted that for dense (bulk) polymernetworks, like the compounds studied in this work,that in the experimental SAXS curves the angularposition of the intensity maximum (qmax) in theLorentz plots is frequently used to better definea structural characteristic correlation distancebetween the center of the scatterers, n, by means of

n ¼ 2pqmax

� �(3)

As can be seen in Figure 3, only the qmax valuescorresponding to the blends B, C, and D presentwell-defined systematic changes. The experimen-tal data corresponding to each compound werewell fitted by using a single Gaussian distribution(see solid lines). From the first-order statisticalmomentum of each Gaussian function, the qmax

was obtained for all curves.

Figure 2. Scattering curve I(q) as a function of thescattering vector q for all compounds studied. SampleA: 0SBR/100NR; B: 25SBR/75NR; C: 50SBR/50NR; D:75SBR/25NR; and E: 100SBR/0NR.

Figure 1. Equilibrium volume fraction in the swol-len state (v2m) as function of the SBR content in thecured samples. Dash line represents the mixture lawobtained from eq 5. Dash dot line is only an eye guide.



In Figure 4 the angular positions of the inten-sity maxima as a function of the SBR content inthe compounds are presented. As can be seen, theanalysis of the data can be divided in two parts:one corresponds to the elastomers 100 phr NR and100 phr SBR, in which a increase difference inqmax from 0.1415 A�1 for NR to 0.1435 A�1 forSBR is observed. The other one corresponds to theblend compounds. In this case, the qmax values sys-tematically decrease from 0.1425 A�1 for a SBRcontent of 25 phr to 0.140 A�1 when the SBR con-tent is 75 phr. This three angular positions corre-sponding to the intensity maxima for the blends B,C, and D could be satisfactorily fitted by using alinear function (with a correlation coefficient of0.995), which is represented by a solid line in thefigure. We want to point out that the assumptionregarding the use of a linear fitting on only threedata points must be considered as a first approxi-mation to the analysis of the experimental data.However, the most important result obtained ana-lyzing this figure is that qmax decreases when theSBR content in the blends increases.

For a general discussion of the experimentalresults reported earlier, it must be taken intoaccount that all the compounds studied in thiswork contain zinc oxide and sulfur in their formu-lation (see Table 1). As was previously dis-cussed,10 the ZnO and carbon particles do not con-

tribute to the SAXS patterns in the q-range ana-lyzed in this experiments. In fact, as the sulfurgroups localize heavier elements in the rubbermatrix, they produce a local increase in the elec-tron density, creating specific inhomogeneities forNR and SBR, as discussed further later.

In the compounds studied in this work, thecrosslinks are formed with sulfur as a crosslinkagent; therefore, the local density fluctuations atthe crosslink sites should be different from thesurrounding.

Such differences would produce differences inthe size of the sulfur clusters, which are detectedin our SAXS experiments. In fact, as it can beseen in Table 3, Mcs measured in all the com-pounds analyzed by swelling test are different.

In Figure 5, the correlation distance n obtainedusing eq 3 is presented as a function of the SBRcontent in the elastomers.

A preliminary analysis of the data presented inthis figure can be divided into two groups:

• Pure elastomers, 100 phr NR and 100 phrSBR (samples A and E)

In this case, it can be observed that n islower (43.8 A) in the cured 100 phr SBRsample than in the 100 phr NR one (44.4 A).This last value of n is different than thatestimated from Figure 5 of ref. 10 for thesame vulcanization temperature T ¼ 433 K(n � 43.9 A). But, this disagreement can beeasily explainable on the basis that in bothcases the sulfur and accelerant contents ofthe 100 phr NR samples were different (S ¼

Figure 4. Position of the maximum qmax (A�1) as afunction of the SBR content in the compounds. A lin-ear fit to experimental points obtained in blends isdepicted in solid line.

Figure 3. A selected region from the Lorentz repre-sentation for scattering vector between 0.10 and 0.185A�1 is presented to evidence the curves fitted (solidlines). The results corresponding to different compoundsare labeled (Sample A: 0SBR/100NR; B: 25SBR/75NR;C: 50SBR/50NR; D: 75SBR/25NR; and E: 100SBR/0NR).The arrow evidences the shift of the maxima qmax

obtained from the fitted curves for each blend.



1.8 phr and TBBS ¼ 1.2 phr for the samplesstudied in ref. 10; and S ¼ 2.3 phr andTBBS ¼ 0.7 phr for the samples preparedfor this work). The difference in sulfur/accel-erator ratio produces different networkstructure in the cured NR.19,20

For four-functional networks, the cross-link density is defined by (see ref. 10 andreferences therein):

lc ¼q2

1

Mcs� 1

Mn

!;

(4)

where q is density of the polymer. From eq 4,it is clear that the crosslink density is relatedto the inverse of the molecular weight betweencrosslinks Mcs; therefore, from Table 3 resultsthat the cured SBR sample has a lower cross-link density than that of the NR elastomer.Both NR as well as SBR have many allylichydrogens in its structure and, during the vul-canization process, the free radicals can reactwith these hydrogens much easier than withother hydrogens present in the molecules. It isusual to expect a higher reactivity in NR thanin SBR, because statistically the first one hasmore allylic hydrogens in its structure.21

Then, it would be expected a higher crosslinkdensity in NR than in SBR when the same for-mulation of curatives is used in both elasto-mers. The relationship between networkstructure and vulcanization chemistry is verycomplex for elastomers cured with a sulfur/ac-

celerator system. The amount of monosulfide,disulfides, and polysulfides crosslinks andcyclic structures formed during vulcanizationat a given temperature are not necessary thesame in cured NR and cured SBR.22,23

Qualitatively, it could be thought that thevalues of n are directly related to the cross-link density. However, in a recent SAXSstudy reported by the authors10 on curedNR at different temperatures, a reductionin the correlation distance when the cross-link density decreases was found. Thisbehavior was attributed to a different ratiobetween the number of polysulfidic anddisulfidic crosslinks as well as the numberof monosulfidic crosslinks, which produce amodification in the correlation distance.

Following the ideas described earlier, itmust be considered that the slight differencein n between the cured samples of 100 phrNR and 100 phr SBR seems to be specifi-cally related to the crosslinks type presentin the vulcanized network created duringthe vulcanization process.

• Blends (samples B, C, and D)As can be seen in Figure 5, the correlation

distance systematically increases when theSBR content increases. Taking into account theMcs values reported in Table 3, the total cross-link density decreases for higher SBR contentin the blend. Then, there exists an inverse cor-relation between n and the total crosslink den-sity for all blends studied. Based on the ele-ments discussed in the previous item, thisbehavior should be considered as consistent.

In addition to the general discussions givenearlier, we consider that it is necessary to godeeper into the interpretation of the data pre-sented in Figure 5. To this aim, aspects linked tothe role of the interphase stability in the blendsand the level of vulcanization of each elastomerwill be discussed later.

When dealing with blends, as it can be seenin Table 2, we note that the t100 value for thesample with 100 phr NR is about four timeslower than that measured for the sample with100 phr SBR. On the other hand, a systematicincrease is observed for t100 when the SBR con-tent increases in the different compounds. Inother words, we know that t100 (NR)\ t100 (NR/SBR)\ t100 (SBR). This fact implies that the NRphase is always overcured in the blends, indicat-ing the existence of two vulcanization levels,

Figure 5. Correlation distance n (A) as a function ofSBR content in the compounds. Dash dot line is onlyan eye guide. Mcs (in g/mol) values are given in brack-ets beside each point (see text).



one in each phase of the different blends. Such abehavior is clearly deduced from Figure 1 of ref.7: for all the times t100 obtained measuring eachblend, the rheometer curve corresponding to thecured 100 phr NR already shows reversion. Thisassertion could be reinforced considering thechange of the values of both glass transitiontemperatures detected in the blend compounds(see Table 2). Specifically, the Tg associated witheach phase of the blends depends on the blendcomposition and the curing level of each phaseinto the blend. Furthermore, it is well acceptedthat the NR phase is less stable than the SBRphase19 and mainly polysulfidic linkages changeto disulfidic and monosulfidic linkages in over-cured NR.24–27

Assuming that when preparing the compoundsthe distribution of chemicals is the same in bothNR and SBR phases, and considering the differ-ent reactivities of NR and SBR in the vulcaniza-tion process the solubility of sulfur and TBBS isdifferent in both elastomeric phases.

Curative diffusion between the phases of anelastomer blend takes place during the vulcaniza-tion process but not during mixing.28 It is knownthat NR crosslinks more rapidly than SBR, and arapid depletion of the curatives in the NR phase ofthe blend and a replenishment of the curatives dueto the diffusion from the SBR phase werereported.6 The consequence of these mechanisms isthe development of a zone of high crosslink density(i.e., layer) in the NR phase close to the interphase,higher than in the case of the NR sample and withdifferent state of cure than the bulk.1,6 Theincrease in the Tg values associated with the NRphase in the NR-rich blends would confirm this hy-pothesis. This behavior is observed in Table 2.

At the same time, the migration of curativesfrom SBR to NR produces a diminution in thecrosslink density into the SBR phase when com-pared with that corresponding to the 100 phr SBRsample. Therefore, a lower Tg value associatedwith the SBR phase would be expected. In fact,this behavior is precisely reported in Table 2.

Under the aforementioned assumptions, it canbe concluded that there exists an interphase layerbetween the two elastomeric phases. This inter-phase should have a different vulcanization levelin comparison with those corresponding to thesamples of pure NR or pure SBR. The samebehavior was reported by Mallon and McGill6

from the study of polyisoprene/SBR blends.On the other hand, it must be considered that a

blend of two elastomers 1 and 2 can theoretically

produce different types of vulcanizates such as:Type I, a dispersion of a cured 1-phase in anuncured 2-matrix; Type II, a dispersion of anuncured 1-phase in a cured 2-matrix; Type III, withboth 1 and 2 phases cured.29 In the last case, manydifferent situations can be established dependingon the crosslink level of each phase. Consideringthe samples B, C, and D of this study, we couldassume that we are dealing with blends of Type III.

Under conditions of free swelling, the equilib-rium swelling behavior of the blend should beexpressed by

v2m ¼ f1v12m þ f2v

22m

v2m ¼ v12m þ f2 v22m � v12m� � ; (5)

where v12m and v22m are the corresponding volumefractions of two components phases equilibrium(maximum) degree of swelling. f1 and f2 are thevolume fractions of the two elastomers present inthe blend29 and f1 þ f2 ¼ 1.

The comparison of the experimental data of v2mwith eq 5 for the blend compositions analyzed isgiven in Figure 1. It can be observed that eq 5 doesnot fit well the experimental data, especially forthose blends in which NR is the main phase. Thisbehavior points out that the existence of an inter-phase that behaves as a third phase should be takeninto account in the analysis given in this work.

Unfortunately, SAXS results give only an ‘‘av-erage’’ information regarding the bulk structureof the elastomers studied. The nonmonotonicbehavior of the correlation distance when theSBR content increases (see Fig. 5) is hard toexplain in terms of the presence of two or moredifferent phases in the blend compounds.

To analyze the scattering curves, the contribu-tion of each phase should be identified. In ourexperiments, the correlation distance n increasesat higher SBR content in the blend, and as can beseen in Figure 3, there is a noticeable change inthe peak broadening, which can be attributed toadditional electron density contrast between thephases in the blend. According to a previouswork,10 n changes with the type of crosslinks gen-erated in the cured samples. In fact, this valuewould decrease when a reduction of polysulfidelinkages followed by a moderate increase of themonosulfide crosslinks is produced. It is interest-ing to notice that the region of SBR near to theinterface has also an increase in the crosslinklevel compared with the bulk of the SBR phase.In this layer, because of the migration of cura-tives, it is expected a higher concentration of



polysulfide linkages at higher concentration ofSBR in the blend. Recent studies on vulcanizedSBR, with the same system of cure as the oneused in this research, confirm this assertion.15

Thus, the increased presence of polysulfide link-ages and a less diffuse interphase boundary mayresult in a narrower distribution of the interdo-main correlation distance in the blend.

CONCLUSIONS

A SAXS and swelling study of vulcanized NR andSBR blends (NR/SBR) with 75/25, 50/50, and 25/75 phr ratios and 100 phr NR and 100 phr SBRwas carried out. Samples were cured at 433 K atthe optimum time.

Swelling of cured samples shows that the cross-link density increases when the SBR phaseincreases in the blend. However, the volume frac-tion of the blend at equilibrium (maximum) degreeof swelling does not follow a mixture law consider-ing only the two elastomeric phases, NR and SBR,in the blend. Then, it is clear that the interphasebetween the two elastomeric domains has to beconsidered as another phase into the analysis.

Evidences reported in the literature have con-firmed that the diffusion of curatives from SBR toNR takes place during vulcanization. In thiswork, we have associated the shift of the scatter-ing vector qmax in the SAXS experiments towardlower values when the SBR content increases inthe formulation of the blend with the curativesdiffusion. Furthermore, this change would beassociated with changes in the type of crosslinksin the interphase layer due to the different levelof cure of the interphase, depending on the curetime of the samples.

The authors acknowledge the support of the LNLS, Bra-zil (Project: D11A-SAXS1 # 4181/05). Agencia Nacionalde Promocion Cientıfica y Tecnologica (PID 2003-0435and PICT 2006-1650), Comision de InvestigacionesCientıficas de la Provincia de Buenos Aires and SECAT(UNCentro), Argentina. The authors are very gratefulto F Queiruga Rey and Tomas Plivelic for the technicalsupport during measurements at LNLS. This work waspartially supported by the Universidad de BuenosAires, Argentina (Research Project 2006-2009 X808).

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A SAXS and swelling study of cured natural rubber-styrene–butadiene rubber blends

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Transcript of A SAXS and swelling study of cured natural rubber-styrene–butadiene rubber blends