A STUDY OF STYRENE BUTADIENE RUBBER USING GPC-FTIRby

James N. Willis and Xiaojun LiuLab Connections, Inc.5 Mount Royal Avenue

Marlborough, MA 01752

Rubber is one of the oldestandmostusefulpolymerproductsavailableto man.The chemicalcharacterizationof thismaterialhas provedto be a challengingone since its commercialintroductionin1823. The firstapplicationof gelpermeationchromatographywas inthe studyof rubberand continuesto be animportantmethodof determinationof molecularweightbuthas littlecapability indeterminingcomposition.Infraredspectroscopywas used duringWorld War II.asa QC method in the manufactureof syntheticrubber buthas generallybeenusedonly inthe studyof bulkmaterial.Only recently,however, has FTIR beenusedin conjunctionwith GPC as a methodof completelycharacterizingelastomericmaterial.

The only commercialsystemwhichhas been used successfullyto couple GPCwith FTIR is the LC-Transform.Currentlytwoversionsof the LC-Transformareavailable, one which usesa pneumaticnozzleto removethe mobilephase andonewhich usesan ultrasonicnozzle. Eithernozzleworksfor SBR type polymersandboth wer? used in this study.Detailsof each systemare found elsewhere 1.Any commercialFTIR systemcan be used to collect IR data butthe systemused in this studywas a Nicolet510P. The chromatographicsystemusedwas aWaters 510 pumpwith UltrastyragelGPC columns.UnstabilizedTHF was themobilephase. Once the sampleswere collectedfromthe GPC system,thecollectiondisc holdingthe samplewas placedin an opticsmodulewhichwaslocated in the sample compartmentof the FTIR. The IR spectra of the completesamplewere then collectedand subsequentlyanalyzed.

Five sampleswere examined;two commercialsamplesfrom ScientificPolymerProducts,part numbersC199 and C200, which were reportedto have 5% and23% styrenecontent, respectively,two commercialproductsfroma U.S.manufacturer,AMP1721 and AMP1502, whichwere reported to have 40% and23.5% styrene, and oneTurkishsample,PETKIM, which was supposeto have24.6 % styrene. SamplesC199, C200 and PETKIMwere examinedby bothNMRand photodiodearray detection=

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1

EXPERIMENTAL

Chromatoaraphv LC-Transform FTIR

Waters 510 Model 102 Nicolet 510Pcolumn-mixedbed(7.8x300mm) Temp 55°C DTGS detectormobile phase-1 ml/minTHF nozzle flow-180 ul/min 4 cm-1 resolutionsample conc. 0.5% 10 deg/mindisc rotation 16 coadds/data set100ullmin injection transfergas-dryair purgedbench

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DISCUSSION

Of primary interestinthe studyis the determinationof the distributionof styreneand butadiene in the copolymerand the identityof the additiveswhich arepresent.The IR spectrumof styrene is characterized by the presenceof bandsat 3027 and 3063 cm-1 which arise fromC-H stretchesassociatedwith thephenyl ring, bandscenteredaround1650 cm-1 from the C-C modesin thephenyl ring,and the strongbandat 700 cm-1 which is attributedto the out ofplane ring bendingmode.The 700 cm-1 band is well separatedfromotherbandswhich appear in the spectrumof SBR and can be used to monitortheconcentrationof the styrenecontentin the copolymer.It is possiblethat both cisand trans formsof butadieneare presentin the copolymer.The cis form shouldhave bandsbetween 1662 and 1652 cm-1 which arisesfrom the C=C stretchingmode and an our-of-planebendingmodefromthe C-H at approximately690 cm-1. The trans formhas a similarband between 1678 and 1668 cm-1 from theC=C mode and anotherbetween980 to 965 cm-1 for the C-H out-of-planebendingmodea.No indicationin the spectraof the compoundsstudiedwasfound indicatingthe presence of the cisform of butadiene.Bymonitoringthe 700and 968 cm-1 bandsof the two materialsone shouldbe able to determine therelative concentrationof one co-monomerwithinthe copolymer.There are, ofcourse, otherbandswhich may be candidatesfor the identificationof thespecies presentbut the modespickedare normallyquite intenseand are wellseparated fromother bandswhich may interfere.The other materialswhich areexpected to be presentare stabilizers(reportedto be 1.25% of total) andsoap(fattyacids)which are addedas slippageagents. The spectra of theadditiveshave been well characterized"and fattyacidscan be easilydistinguishedby a rathersimplealiphaticlike IR spectrumplusthe presence of astrongC=O band in the 1700-1750 cm-1 region.

Once the IR data was collected, 3D/IR softwarewas used to reduce the data tosimple plotswhich representedconcentrationvs. molecularweight or plotsof thedistributionof the monomerscomparedto one another.

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RESULTS

Identification of materialsFigure I show the IR spectrumfrom 3500 to 600 cm-1 of a portionof sampleC200. This general patternis shownin the IR spectrumof all the samplesexamined exceptPETKIM whichwillbe discussedlater.Variationswere seenbetween the samplesbutthese changeswere relatedto the amountof styrenewhich was present inthe individualsamples.A 3D plotof C200 is shown inFigure 2 in which the butadieneand phenylmodeswhichwere used to monitorthe presence of each speciesare indicated.This plotshowsthe variation intheamountof sampleas a functionof timewhich inthiscase is also decreasingmolecularweight as onegoesfrom the front to backof the plot.This figurerepresents60 IR spectrasets collectedover the entiremolecularweight rangeofthe copolymer.Equivalentdata was collectedfor each sample.The majorportionof the sample is SBR, howeverone can see towardsthe end of the sampleevidenceof a new materialarisingwith a peak at 1705 cm-1 which is notfoundin SBR. This is the additive,a fattyacid ester. Figure3 showsthechemigram(CGM)of the sampleon collectiondiscvs. time. Fromthis data threepeak areas were integratedand shownas individualspectrain Figure 4. Peaks1 and 2 are clearly SBR butPeak 3 is notand can be easily recognized as afatty acid ester.

Composition distribution

By measuringthe intensityof either the butadienebandor the styreneband,corrected for the changing massdistribution,and plottingthisagainst molecularweight, one can measurehow eitherof the co-monomersvarieswith molecularweight.The procedureusedwas to measurethe total intensityof the region from980-957 cm-1(diene), the region705-694 cm-l(phenyl), and 3100-2800 cm-1(totalC-H), and then to ratiothe diene to totalC-H or the phenylto total C-Hand plot the resultvs. time. Figure5 presentsthe resultsfor five samples.Thepercentages listedafter each sample is the average percentstyrene as givenbythe manufacturer.The Y axis is the ratio(x100)determinedby the IR methodgivenabove. While the trendwhich we obtainiswithin the range expected, thereis additionalvalue inthe fact that usingthe IR methodone can see trends incompositionaldistributionas well as beingable to measurethe styrene(or diene)content at any point inthe distribution.

Table 1 compares the styrenecontent providedby the supplierswith theaverage value obtainedusingthe IR method.The data agrees reasonablywell ifone keeps in mindthat the values reportedfrom the manufacturersare totalaverage valuesand the data from the IR methodis the average value acrossthecenter of the distribution.

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TABLE 1STYRENE CONTENT IN A SERIES OF SBR SAMPLES

SAMPLE REPORTED VALUE(%) RATIO(X1000) byIR(avg.)

C199 5 6.7C200 23 32.41502 23.5 28.41721 40 45.1PETKIM 24.6 32.8

Generally the CGM of all the samplesshowa smoothdistribution,varying onlywhen shoulderswere present.These shouldersarisewhen the mass distributionof the samplevaries. PETKIM presenteda very differentshape curve,seeFigure6. In the case of PETKIM, the additivewas notisolatedby the GPCtechniquesused and isfound to lie underthe SBR sampleas can be seen bythe sharp peak at 15 minutes.The totalCGM( dottedline) representsthe totalmassof material on the disc.Usingsoftwareto extractthe diene and phenyldistribution(diene/total CGM and phenylltotalCGM), one can see that there isno contributionto the copolymermaterialby the additive-itis anothermaterialentirely-and itsdistributionwithinthe massof materialcan be easilydetrained.To verify this the additivewas extracted fromthe sampleusing acetone and itsIR spectrumwas obtained(Figure 7). It is identicalto the spectrumobtained bysubtractingspectral files fromthe CGM beforethe additiveeludes and at the15.5 minutepoint.

CONCLUSION

The use of GPC combinedwith FTIR can be veryvaluable in the determinationof compositionaldistributionin elastomermaterialsand can in principalbe usedto determinequantitativelythe amountof bothdieneand styreneas a functionofmolecularweight.Additivematerialscan be identifiedusing FTIR and can beseparatedfromthe base stockmaterialwithouthavingcompletechromatographicisolation.

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REFERENCES

1. Lab Connections,Inc., 5 MountRoyalAve., Marlborough, MA. 01752

2. W.A. Dark, ElastomerCharacterizationwith PhotodiodeArrayDetector, 1989InternationalGPC Symposium.

3. R.N. Jones, InfraredSpectra of OrganicCompounds,SummaryCharts ofPrincipalGroup Frequencies,BulletinNo.6, NationalResearch Council,Ottawa,Canada, 1959

4. Analysisof PolymerAdditives,AN-i0, LabConnections,1993

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