Determination of the chemical compostion distribution of

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95Determination of the Chemical Composition
Distributionof Copolymersof Styrene and
Butadieneby GPEC®
P.J.C.H. Cools,F. Maesen,A.M. vanHerk,A.L. German"
Laboratoryof PolymerChemistry
EindhovenUniversityof Technology
P.O. Box513, 5600 MB Eindhoven
The Netherlands
Abstract
In order to determine the chemical composition dislTibution (CCD) of
styrene-butadiene copolymers, Gradient Polymer Elution Chromatography
GPEC_ has been performed. The separation is based on differences in
solubility between the copolymer molecules with different chemical
composition. The solubility of a copolymer is dependent on the following
parameters: temperature, type of solvent/nonsolvent mixture, molar mass of
the polymer, and the chemical compositionof the polymer. The resolution of
the GPEC¢ separation and the molar mass dependency are influenced by
the solvent/nonsolvent combination. In order to obtain a reliable separation
according to chemical composition, the differences in solubility must be
sufficiently high and the molar mass dependency must be negligible. In
order to separate styrene-butadiene copolymers, synthesized by emulsion
polymerization, a tetrahydrofurane/acetonitrile (THF/ACN) gradient was
performed. After calibration of the chromatographic system with styrenebutadiene copolymer standards, the CCD's of styrene-butadiene
copolymers could be calculated.
GPECe is a registeredtxademark
ofWata_
Author to whom correspondence should be sent.
399
Introduction
In the lastfew years polymercharacterizationhas becomemore important.
New techniqueshave been developedin orderto determinethe chemical
compositionof a copolymer. Differenttechniquescan be used in order to
characterizea copolymer.Gel permeationchromatography(GPC) can be
usedto obtainthe molarmassdistribution(MMD). Proton-NMRcan be used
to determine the average mole fraction (F) of monomericunits in a
copolymer.
However, during a batch copolymerization,a chemical composition
distributionis formed due to compositiondrift and statisticalbroadening.
Dependingon the monomercombination,processconditionsand process
strategy, a broad chemical compositiondistributionmay occur.Together
with the MMD, the CCD determinesthe propertiesof the copolymer.Thus
the knowledgeofthe CCD of copolymersis of crucialimportance.
GradientPolymer ElutionChromatographyGPEC® is a high performance
liquid chromatography method that is capable of separating polymer
moleculesaccordingto chemicalcomposition
[1].The separationmechanism
of GPEC®is basedon the differencesin solubilitybetweencopolymerswith
differentchemicalcomposition.
In the literaturegradienthighperformance liquidchromatography(gradient
HPLC) is often called highperformance precipitationliquidchromatography
HPPLC[2"3] or
liquid adsorption chromatography LAC[4's]. HPPLC
emphasizes the precipitation mechanism and LAC focuses on the
adsorption mechanism. In gradient HPLC both mechanismscan occur.
Because HPPLC and LAC only describepart of the mechanism,a better
name for the techniqueis introduced;GPEC®: Gradient Polymer Elution
Chromatography.
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At the beginningof the GPEC® separationprocess the chromatographic
eluent is a nonsolvent.The polymersample is injected in the nonsolvent
and will precipitateon the column. During the GPEC® separation,the
solventcompositionischanginggraduallyin timefrom nonsolventto a good
solvent• Accordingto the chemical compositionof the copolymers,the
polymers will redissolve in a certain solvent composition. Therefore,
polymerswithdifferentchemicalcompositionwill elute at differentretention
times and separationoccurs.
The solubilityof a polymerdepends upon the followingparameters:type
and molar mass of the polymer,solvent/nonsolventcombination,and the
temperature.Duringa GPEC®separationthe temperatureiskept constant.
The solvent/nonsolvent
combinationis an importantparameterin GPEC®.
By choosing the right solvent/nonsolventcombination a successful
separation can be achieved. Since previous research has shown that in
most solvent/nonsolventcombinationsthe molar mass dependency is
negligibleabove molarmass 100,000 (g/mol),in generalpolymersobtained
by emulsion polymerizationcan be separated by GPEC® exclusively
accordingto chemicalcomposition.
Separation on the basisof solubilityis not the only separationmechanism
operativeduringa GPEC® separation,but also adsorptionof the polymer
moleculeson the stationaryphase and exclusionof the polymermolecules
can occur. Adsorption will cause an extra retention of the polymer
molecules. Exclusioncauses the polymer molecules to accelerate with
respect to a given eluent compositionmovingthroughthe column. Both
effects, adsorptionand exclusion,can have a considerableeffect on the
separation.
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GPEC®can be performedin orderto determinethe chemicalcompositionof
a polymerblend.The methodcan also be usedto determinethe CCD of a
copolymer.
The CCD of a copolymercan be calculatedby using a calibrationcurve,
which can be obtained by measuring the GPEC® retention times of
homogeneouscopolymerswithwell definedchemicalcompositions.
Experimental
In order to determinethe CCD of a copolymer,the properGPEC®conditions
have to be found. The GPEC® conditionscan be found by lookingat the
solubilitybehaviourof the homopolymersof the specificmonomers(styrene
and butadiene)in differentsolvent/nonsolvent
combinations.The solubility
behaviourof a polymercanbe studiedbyturbiditymeasurements.
Bydeterminingthe cloudpointsofthe homopolymer
standardswithdifferent
molarmass,differentsolvent/nonsolvent
combinationscan be verified.The
cloud pointcomposition(CPC) is determinedby titration.In orderto obtain
the turbidity curve (see figure 1), the CPCs of different polystyrenePS
standardsand polybutadienePB standardswere determined:PS 500, PS
2500, PS 18K, PS 102K, PS 2.7M, PB 900, PB 9.3K, PB 120K, and PB
950K (seetable 1).
The molar mass dependency is negligibleabove 100,000 g/mol in the
system THF/ACN for both PS and PB (see figure 1). The difference
betweenCPC of the homopolymerstandardsPS 100Ken PB 100K is about
30% THF (resolutionof GPEC®separation).So the systemTHF/ACN can
be used in order to separatePSB copolymers.
Apparatus
The experimentswere performedwith a WISP (Waters IntelligentSample
Processor)injector,a Waters 600E gradientcontrollerwithpump,a Waters
484 UV-detector,and an EvaporativeLightScatteringDetector(ELSD,ACS
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model750/14). The columnused for the GPEC® separationwas a 7.5 cm
NOVAPAK C18 (35°C). The wavelengthof the UV-detectorwas set at
260nm.The lineargradientusedwas changingfrom 35% THF to 70% THF
in 45 minutes(gradientspeed 1% per minute).The injectionvolumewas 25
!_1.The flowwas 1.0 ml per minute.
Tetrahydrofurane(WESTBURG, HPLC grade) was used as solvent.
Acetonitrile(BIOSOLVE, HPLCgrade) was usedas nonsolvent.The weight
fractionstyrene in the styrene-butadienecopolymerstandards(Scientific
Polymer Products)were: 45%, 23%, and 5%. The homopolymerswere
GPC standardsfrom PolymerLabsandWateis.
GPEC®Technique
The PS standardsand the PB standardswere dissolvedin THF (1 g/i). At
[
the beginningofthe gradientthe eluentwas a non-solvent.The polymers(in
solution)were injectedintothe non-solventand precipitatedon the column
material,because of the poor solvent conditions.When the gradientwas
performed the polymer molecules redissolvedin a specific solvent
compositionand eventuallyeluted from the c_olumn,
leadingto separation
accordingto their chemicalcomposition.
Chemical Composition Distribution CCD
In orderto calculatethe CCD, the GPEC® behaviourof the homopolymers
of each of the two monomerswas studied. The differencebetween the
retentiontimes of the homopolymersmust be sufficientlyhigh, in order to
obtain a high resolution. Also the molar mass dependency of the
homopolymersonthe solubilitywas studied.
With well definedhomogeneousPSB copolymersa calibrationcurve was
determined, in order to obtaina relationbetweenthe retentiontime and the
chemicalcomposition(as in SEC/GPC: retentiontime vs. molarmass).
With the calibrationcurve the CCD of a copolymerwas calculated in the
same way as a MMD can be calculated(see figure2). The chromatogram
was cut in slicesof the same width. Foreach slicethe chemicalcomposition
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was determined using the calibration curve. The height (in the
chromatogram)of each slice (hi) is a measure of the concentration.By
calculatingthe weightfractionof each slice (w_)the chemicalcomposition
distributioncouldbe calculated.For the CCD calculationthe signal of the
evaporativelightscatteringdetectorwas used.
Results and Discussion
In orderto determinethe CCD of styrene-butadienecopolymersa THF/ACN
gradientwas used. For the solvent/nonsolventmixtureTHFIACN the molar
mass dependencyon the solubilityabove molarmass 100,000 (g/mol) is
negligible.The differencebetween the cloud pointsof high molar mass
polystyreneand highmolarmass polybutadienein the systemTHF/ACN is
sufficientlyhighto obtaina separationaccordingto chemicalcomposition
(see figure1).
A GPEC® chromatogramof a styrene-butadienecopolymeris presentedin
figure 3. The chromatogramindicates that beside a copolymer also
homopolymeris present in the copolymersample. So due to composition
driftpolystyrenehasbeen formed,probablyat the end of the reaction.
The calibrationcurveis shown in figure 4. The curve is only valid for low
styrene contents (is -< 0.50), because no styrene-butadienecopolymer
standardswithhighstyrenecontentwere available.
The CCD of the chromatogram(plottedin figure 3) has been calculated
accordingto the method describedschematicallyin figure 2. The CCD is
plottedin figure5.
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The response(h-_of the detector is assumed to be independentof the
copolymer composition.Any possible dependence of the response on
copolymercomposition,may resultin a slightcorrectionof the CCD trace.
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Conclusions
Foreach type of copolymera newsolvent/nonsolvent
combinationhasto be
found in order to obtain a successfulseparation according to chemical
composition.In the systemTHF/ACN the molar mass dependencyon the
GPEC® separationof styrene-butadienecopolymersis negligible,but in
other systems,the molar mass of the polymermay disturbthe separation
according to chemica! composition. Hence, similar
to
SEC/GPC
separations,the separationsperformedby GPEC® shouldbe interpreted
with great care. Therefore,the dependencyof copolymermolar mass on
GPEC®separationsrequiresfurtherstudy.
In behalf of the separation of styrene-butadienecopolymers,a suitable
solvent/nonsolventcombinationwas found. With this solvent/nonsolvent
combination(THF/ACN), reproducibleGPEC® chromatogramsof styrenebutadienecopolymerswereobtained.
GPEC®can be used to determinethe copolymerCCD's, whichare of great
value in predictingand understandingcopolymerproduct properties.Also
informationaboutthe kineticsof the polymerizationcan be obtained.
Thus GPECe is a powerful tool in order to characterize copolymers
accordingto chemicalcomposition.
Literature
1.
Staai,w, CoolsP.J.C.H.,van HerkA.M., German A.L., J. Liq.
Chrom., 17_,
2.
3191-3199, 1994.
GI0cknerG., 'Gradient HPLC of Copolymersand Chromatographic
Cross Fractionation',Springer-VedagBedinHeidelberg,1991.
3.
MoriS, Tazid H., J. Liq. Chrom., 17 (14.15), 3055-3068, 1994.
4.
MoriS., Anal. Sci., 4, 365-268, 1988.
5.
MoriS., Anal. Chem.,60, 1125-1128, 1988.
405
Table 1 Cloud Point Compositions (CPC) of PS and PB standards.
M,,,(g/mol)
..
CPC (voI%THFin ACN)--
PS 500
500
PS 2500
2,500
PS 18K
18,000
29.2
PS 102K
102,000
42.0
--
PS 2.7M
2,700,000
48.2
-
PB 900
900
PB 9.3K
9,300
61.4
PB 120K
120,000
73.7
PB 950K
950,000
76.0
--
-
-a-iS
.-.o_I=B
l__n0F____
P
10000.
1000
do
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%TI-F
do
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do
Figure 1 Turbidity Plot of PS(II) and PB(O) in THF/ACN.
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time
time
Chromatogram
Calibration
Ft
(_urve
CCD
Figure 2 Determination of the CCD of a Copo/ymer.
copolymer
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50o.00.
polystyrene
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20.00
Figure 3 GPEC ® Chromatogram
/inear gradient
of Styrene-Butadiene
ACN-THF, No_pak
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Copolyrnet; ELSD,
C18, #ow=l.0
mgmin.
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Fs 0.60
0.50 =
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55
50
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.................
65
70
t
80
75
%TBF
Figure4 CalibrationCurveof Styrene-Butadiene
Copolymersin THFIACN,molefraction
styrenein copo/yrner
F=vs. %THF.
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Figure5 ChemicalComposition
Distributionof a Styrene-Butadiene
Copolymer.
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