Polymer International Polym Int 54:667–672 (2005)DOI: 10.1002/pi.1739

Curing behaviour of syndiotacticpolystyrene–epoxy blends: 1. Kinetics ofcuring and phase separation processNicolas Salmon,1,4 Veronique Carlier,2 Jaap Schut,3 Pedro M Remiro4

and Inaki Mondragon4∗1Dip Chimica Industriale e Ingegneria Chimica ‘Giulio Natta’, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy2Unite de Physique et de Chimie des Hauts Polymeres, Universite Catholique de Louvain, Place Croix du Sud 1, 1348 Louvain-la-Neuve,Belgium3Institut fur Polymerforschung Dresden eV, Hohe Straße 6, Dresden 01069, Germany4Escuela Universitaria Politecnica, Departamento Ingenierıa Quımica y Medio Ambiente, Universidad del Paıs Vasco/Euskal HerrikoUnibertsitatea, Plaza de Europa, 1 20018 Donostia/San Sebastian, Spain

Abstract: The modification of the curing behaviour and the phase separation process for an epoxyresin blended with a crystalline thermoplastic was investigated in the case of the diglycidyletherof bisphenol-A (DGEBA)/4,4′-methylene bis(3-chloro-2,6-diethylaniline) (MCDEA) blended withsyndiotactic polystyrene (sPS) and cured at 220 ◦C. Phase separation taking place during curing ofthe blend was investigated by differential scanning calorimetry (DSC) and optical microscopy in orderto get a better understanding of the complex interactions between cure kinetics of epoxy matrix andcrystallisation of sPS, both influenced by blend composition. Results suggested that phase separationand crystallisation of sPS occurred at almost similar times, with phase separation just being aheadof crystallisation. DSC and near-infrared measurements were used for the determination of the curekinetics. Slow delays on the cure reactions were observed during the first minutes for the sPS-containingblends compared with the neat DGEBA/MCDEA system but, after some time, the reaction rate becamefaster for the blends than for the neat matrix. Phase separation occurring in the mixtures may explainthis particular phenomenon. 2004 Society of Chemical Industry

Keywords: diglycidylether of bisphenol-A; 4,4′-methylene bis(3-chloro-2,6-diethylaniline); syndiotacticpolystyrene; cure kinetics; reaction-induced phase separation; crystallisation-induced phase separation

INTRODUCTIONPolymer blends composed of thermoplastic andthermoset constituents have been widely studiedbecause they offer the possibility of obtainingnew materials with improved properties. Toughnessimprovement of thermoset networks modified withhigh-performance ductile thermoplastics1–3 and newprocessing possibilities for thermoplastics using thethermoset as a reactive diluent are the main goals.3–6

Depending on the cure conditions and on theblend composition, different morphologies rangingfrom particulate to semi-interpenetrating networks(semi-IPN) are obtained. The morphology obtaineddetermines the final properties of the blend.

Many studies have been published dealingwith epoxy–amine networks blended with dif-ferent thermoplastics,4,7–21 but very few stud-ies has been done on blending such thermosets

with semi-crystalline thermoplastics. Among these,epoxy has been blended with poly(ε-caprolactone),14

poly(ethylene terephthalate)22,23 and poly(butyleneterephthalate).24–28 Syndiotactic polystyrene (sPS) isan engineering semi-crystalline thermoplastic whichhas a high melting temperature (270 ◦C), high resis-tance to solvents, low dielectric constant and goodprocessability, as well as high elastic modulus, highheat resistance, good dimensional stability and attrac-tive price.29 Most of these properties are due to itscrystallinity. Moreover, the crystalline nature of sPSoffers the possibility of generating various new mor-phologies during curing of its mixtures with thermosetresins just by changing the curing conditions and thesample preparation procedure.

In order to have a better understanding of thedeveloping of these morphologies, we focused ourwork on the curing of such blends. The influence

∗ Correspondence to: Inaki Mondragon, Escuela Universitaria Politecnica, Departamento Ingenierıa Quımica y Medio Ambiente, Universidaddel Paıs Vasco/Euskal Herriko Unibertsitatea, Plaza de Europa, 1 20018 Donostia/San Sebastian, SpainE-mail: iapmoegi@sc.ehu.es(Received 23 October 2003; revised version received 17 June 2004; accepted 5 August 2004)Published online 24 December 2004

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of sPS on the cure kinetics of the epoxy–aminewas investigated. Bonnet et al10 reported a suddenincrease in the reaction rate at the onset ofphase separation for epoxy–polystyrene and forepoxy–polyetherimide blends with high thermoplasticcontent (at least 30 %). In this paper we also report onthe possibility of using this criterion for determiningthe onset of phase separation for epoxy–sPS blendsin the low sPS concentration range. For suchstudies, differential scanning calorimetry (DSC) andnear-infrared spectroscopy (NIR) were used ascomplementary techniques to avoid experimentaluncertainty.

EXPERIMENTALSyndiotactic polystyrene was supplied by Dow Chem-ical under the trade name Questra QA 101. Number-average and weight-average molar mass are 94 100 and192 000 g mol−1, respectively. The used epoxy resinwas diglycidylether of bisphenol-A (DGEBA) withn=0.15, supplied by Dow Chemical under the nameDER 330. The hardener is an aromatic diamine 4,4′-methylene bis(3-chloro-2,6-diethylaniline) (MCDEA)supplied by Lonza (Germany).

The low content (less than 25 %) sPS blends studiedin this work were prepared by the following procedure.First, weighed amounts of sPS and DGEBA wereplaced together in a test tube and preheated in an oilbath at 220 ◦C for 10 min. The test tube was thentransferred to a metal bath at 290 ◦C and maintainedthere for 10 min during which the blend was vigorouslystirred in order to melt the sPS and to dissolve itin DGEBA. Meanwhile, a stoichiometric amount ofMCDEA was weighed in another test tube and meltedin an oil bath at 220 ◦C. After 10 min at 290 ◦C,the test tube containing sPS and DGEBA solutionwas brought back to the oil bath at 220 ◦C and themolten MCDEA was quickly poured into this solutionand vigorously mixed for about 30 s in order to startthe curing reaction. From these blends, several kindsof samples were prepared according to experimentalrequirements.

For the cure kinetics study, neat DGEBA/MCDEAmixtures and blends containing 5, 9.9 and 24.7 wt%sPS were prepared. The sample was divided in severalportions which were cured in an air oven at 220 ◦Cfor times ranging from 1 to 45 min. To stop the curereactions, samples were taken out of the oven at regularintervals and quenched in cold water. The residualheat of reaction of these partially reacted blends wasmeasured by an isothermal DSC scan at 220 ◦C. Theglobal conversion, X , was calculated as:

X = 1 − (�H/Xt)

where �H is the measured residual enthalpy and Xt isthe total reaction enthalpy for the DGEBA/MCDEAsystem, which was taken from the literature9,30–33 asXt = 100 kJ mol epoxy−1. This method for kinetics

calculation has already been used by Zvetkov in itskinetics study of DGEBA with m-phenylenediamine.34

Moreover, for the phase separation study, neatDGEBA/MCDEA mixtures and blends containing2.1, 5.1, 10.1, 17.1, 20.1 and 24.7 wt% sPS wereprepared and quenched in liquid nitrogen just afterblending. The quenching in liquid nitrogen wasnecessary in order to avoid the sPS crystallisationwhich takes place immediately at temperatures lowerthan 220 ◦C.

For the kinetics study, DSC isothermal measure-ments were carried out in a Perkin-Elmer DSC-7instrument in a dry nitrogen atmosphere and using6–8 mg samples sealed in aluminium pans. Theinstrument was previously calibrated with an indiumstandard.

Near-infrared measurements were performed ina Perkin-Elmer NIR-FT Raman 2000R Systemspectrometer. This instrument is configured to operatein the NIR spectral region (4000 cm−1 —10 000 cm−1)

with a KBr beamsplitter. Spectra were recorded at a4 cm−1 resolution by co-adding 32 scans.

Furthermore, to investigate the phase separationprocess, DSC measurements were performed on aDSC 822 from Mettler-Toledo equipped with aSample Robot 801 RO. All scans were performedunder nitrogen atmosphere on samples weighingbetween 5 and 7 mg. Dynamic scans were performedat a heating rate of 10 ◦C min−1.

To observe the morphology of the blends duringcuring, a Nikon Eclipse E600 optical microscopeequipped with ×5, ×10, ×20 and ×50 objectives anda ×10 ocular was used. Images were recorded andanalysed with the AnalySIS Auto 3.2 software fromSoft Imaging System GmbH. Blends were heatedusing a Mettler FP82 HT Hot Stage connected toa Central Processor FP90 from Mettler for dataacquisition.

RESULTS AND DISCUSSIONPhase separation processPhase separation can occur in DGEBA/MCDEA/sPSblends by two different mechanisms. It can be theresult of a liquid–solid phase separation, in whichphases become separated as a consequence of thecrystallisation of sPS in the homogeneous reactingblend. This is called crystallisation-induced phaseseparation (CIPS). Or it can be a liquid–liquid phaseseparation resulting from a decrease of the mixingentropy due to the increasing molar mass of thereacting epoxy–amine network. Phase separation isthus induced in this case by the reaction (RIPS).Thus, the investigation of phase separation inDGEBA/MCDEA/sPS blends is complicated due tothe possible crystallisation of sPS. DSC analysis andmicroscopic observations were carried out on blendsduring curing in order to understand their behaviour.

Figure 1 shows DSC scans performed at 220 ◦Cfor neat DGEBA/MCDEA and for its blends with

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Curing behaviour of syndiotactic polystyrene–epoxy blends

Figure 1. DSC isothermal scans of sPS–epoxy mixtures curing at220 ◦C.

different amounts of sPS. For all sPS-containingblends, besides the exothermic peak correspondingto the curing of epoxy already seen for the neatDGEBA/MCDEA, the thermograms show anothersmall exothermic peak at about 5–6 min after thebeginning of the scan. There are slight differencesbetween each blend in both area of this peak andtime at which it appears, possibly related with smallexperimental errors. This peak corresponds to theappearance of small spherical domains revealed byoptical microscopy observations (Fig 2), which areresponsible for the cloudiness after 6 min of curing.

The exothermic peak observed on DSC curves mightbe due to crystallisation of sPS, ie to CIPS, the crystalsbeing the observed domains. However, RIPS couldalso be responsible of such a peak. In that case, anotherpeak, corresponding to sPS crystallisation, shouldappear slightly later. As only one peak is detected,it seems possible that CIPS and RIPS occurred at thesame time. It remains difficult at the present time toelucidate whether the phase separation was inducedby reaction (RIPS) or by crystallisation of sPS (CIPS).

However, the behaviour of the cold crystallisationpeak displayed by amorphous blends, which hadbeen quenched in liquid nitrogen, can help toclarify this point. On DSC dynamic scans from25 ◦C to 300 ◦C at 10 ◦C min−1, these amorphoussamples gave a crystallization peak whose positionwas found to depend on the sPS content of theblend. Figure 3 shows the cold crystallisation peakvalues obtained from these scans. Two groups inwhich the crystallisation temperature is compositiondependent can be observed. The first group is madeup of blends with low content of sPS, from 2.1to 10.1 wt% sPS, which crystallise at temperaturesbetween 76 and 88 ◦C. The crystallisation temperatureincreased with increasing sPS content. The secondgroup, composed of blends from 17.1 to 24.7 wt%sPS, showed a similar trend, crystallising from 78to 85 ◦C. This crystallisation peak shifting according

(a) (b)

(c) (d)

Figure 2. Optical microscopy pictures of the 20.1 wt% sPS mixture for various curing times: (a) 30 s, (b) 305 s, (c) 325 s, (d) 335 s.

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to blend composition had already been observed bySchut et al in their study of sPS crystallisation whenblended with DGEBA monomer.35 They observed adepression of the crystallisation peak depending on thecontent of sPS in the blend, but they did not observethe separation into two groups. This behaviour wasattributed to a dilution effect.

In contrast, the time at which the small exothermicpeak appearing during the isothermal curing of thesPS containing blends was not dependent on thesPS content, and thus it may be concluded that theorigin of this peak is not the crystallization of sPSbecause, as seen with the cold crystallization peak,the crystallisation of sPS in the DGEBA/MCDEA/sPSblends depends on the sPS content. Thus, the peakappearing in dynamic DSC scans would correspondto a RIPS immediately followed by the crystallisationof sPS. Indeed, once separated from epoxy, sPS wouldcrystallise very quickly as DGEBA would not play itsrole of solvent any more.

Kinetics of curingFigure 4 shows the global conversion versus cure timecurves for DGEBA/MCDEA/sPS blends with differentamounts of sPS, as obtained by DSC. As can beseen, the reaction of DGEBA with MCDEA occurredquickly at this high temperature of 220 ◦C. Thus, thegel point conversion (taken as about 0.636) is reachedin about 8 min. For conversions lower than 75–80 %,the reaction rate for sPS-containing blends appears tobe slightly slower than for the neat DGEBA/MCDEA.In addition, only very small differences can be observedamong blends containing different sPS concentrations.These differences are close to the experimental errorbut, by comparing the 24.7 wt% sPS sample with neatDGEBA/MCDEA, the delay in reaction rate with theaddition of sPS becomes evident. The slower reactionrate for sPS-containing mixtures could be attributedto the cooperative effect, on the one hand, of a dilutioneffect of the reacting groups in presence of sPS, and,on the other hand, to the increase in viscosity of the

Figure 3. Composition dependence of the cold crystallisation peak ofsPS–epoxy mixtures.

Figure 4. Cure kinetics of sPS-epoxy mixtures as obtained by DSC.

reacting medium which hinders the mobility of thereacting species at increasing thermoplastic contents.

With the aim of clarifying these results, the kineticsstudy was completed with NIR measurements. Variousbands can be taken as references because of theirstability during the reaction. The most frequentlyused band is that at 4680 cm−1, since aromatic ringsdo not participate in the curing reaction and sotheir absorption bands do not change with time. Inaddition, the band at 4623 cm−1 could be used but,for polystyrene containing blends, it may overlap withthat appearing at 4570 cm−1 coming from sPS.

To follow the evolution of the epoxy groups withreaction time, the bands located at 4525 cm−1 and6060 cm−1 could be used. The band at 4525 cm−1

is usually preferred37 because it appears more clearlyand has less risk of overlapping with other bandsthan that at 6060 cm−1, although an absorption dueto the primary amine groups have also been reportedin this region.37 Before performing kinetic calculationsthe following treatment was applied to the spectra. Ini-tially, the spectra were normalised using the 4680 cm−1

aromatic band as a reference. Then, the normalisedspectrum of a fully cured sample was subtracted fromeach of the normalised spectra.38,39 This treatment,which does not change the quantitative aspect of themeasurement, presents the evidence the evolution ofthe different absorptions bands more clearly.

The superposition of the treated NIR spectrafor DGEBA/MCDEA/sPS blends containing 5 wt%sPS and cured for times from 1 to 35 min isreported in Fig 5. The disappearance of epoxy(4525 cm−1) and primary amine (5060 cm−1) bandsduring the reaction is clearly seen. From thesecurves, the corresponding conversion versus time plotswere obtained. Overall conversion was calculatedby measuring the absorbance decrease for the4525 cm−1 epoxy band. To make this calculation,two assumptions were necessary. Since for the less-cured sample the curing reaction has already takenplace for 1 min during the sample preparation, itsinitial conversion was defined to be equal to the oneobtained by DSC measurements. It was also assumedthat for the fully cured sample (2 h at 220 ◦C) theconversion had reached 100 %. As the spectrum of

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Figure 5. Superposition of NIR spectra of 5 wt% sPS–epoxy mixtures cured for different times at 220 ◦C.

Figure 6. (a) Cure kinetics of mixtures as obtained by NIR.(b) Superposition of DSC and NIR results of cure kinetics.

this sample was previously subtracted from the otherspectra, the absorbance 0 for the epoxy peak in Fig 5corresponds to the 100 % conversion. At time 0, theconversion was arbitrarily fixed at 0 %.

The kinetic curves obtained from NIR measure-ments are shown in Fig 6a. As already remarked forthe DSC results, neat DGEBA/MCDEA showed aslightly faster rate at the earlier stages of reactioncompared with the sPS-containing blends. Again, verylittle difference, within the range of the experimentaluncertainty, is observed between the 5 and 9.9 wt%sPS blends. However, they show faster kinetics than

the 24.7 wt% sPS blend, as expected, and a slowerkinetics than neat DGEBA/MCDEA system.

For comparison, Fig 6b shows the superpositionof kinetic curves for 0 and 24.7 wt% sPS mixturesobtained from DSC and NIR measurements. Similartrends can be seen by both techniques. A delay inreaction rate is observed initially for sPS-containingblends. This is seen by both techniques whencomparing neat DGEBA/MCDEA and 24.7 wt% sPS-containing blends. This slight delay may be due to adilution effect and to the increase in viscosity of thereacting medium at increasing sPS content, as at thistime the blend is not phase separated.

The second interesting point comes from the fasterkinetics of blends with sPS after phase separation,as already observed by Bonnet et al.10 From theseventh minute of curing, the reaction rate for theneat DGEBA/MCDEA showed a progressive slowingwhile sPS containing blends did not show such aphenomenon but was almost linear. The change inthe kinetics is not as marked as that observed byBonnet et al, probably because the sPS concentrationin the blend is lower, but the difference between neatDGEBA/MCDEA and sPS-containing blends is clearwhen the conversion curve of the former crosses thecurve of the others. Phase separation, occurring atthis temperature at about 5–7 min of curing, mightbe the explanation for this observation. When phaseseparation takes place, a DGEBA/MCDEA-rich phasecontaining some sPS, and a sPS-rich phase with somedissolved DGEBA and MCDEA are formed. In theepoxy/amine-rich phase, the reaction rate would befaster than in the homogeneous miscible mixture sincesPS is present in this phase in lower concentrations andthus the dilution and viscosity effects responsible forthe delay in the reaction rate would be less important.

CONCLUSIONSThe phase separation process and the cure kineticsfor DGEBA/MCDEA/sPS blends have been studied.It appears that reaction-induced phase separation

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and sPS crystallisation occurred almost at the sametime. From direct observations of the curing of theblend and from comparisons with trends observedin the same blends for cold crystallisation, it seemsthat reaction-induced phase separation occurs first,about 5 min after the beginning of curing. It wouldbe followed immediately by the crystallisation of sPS,which takes place very quickly at 220 ◦C, since thistemperature is appreciably lower than the meltingpoint of pure sPS. These DGEBA/MCDEA/sPSblends would thus present a reaction-induced phaseseparation (RIPS) when cured at 220 ◦C. More workon the phase separation behaviour in such blends has tobe done for better understanding of its consequences.Observations performed on blends containing highersPS content should be interesting as the effect onthe kinetics described above should then appear moreclearly.

The kinetic study of the curing of theDGEBA/MCDEA system blended with sPS wasperformed by DSC and NIR. A small influence ofsPS on the curing kinetics was observed. In the firstminutes of curing, sPS slowed down the reaction rate,probably because of a dilution effect and an increasein the viscosity of the reacting medium at increasingthermoplastic content. However, at longer times, thereaction rate for sPS-containing blends increased,probably because of phase separation, thus cancellingthe dilution and viscosity effects in the blend and thusmodifying its kinetics.

ACKNOWLEDGEMENTSThe research undertaken in the present study hasbeen funded by the Research Training Networks(RTN) program under the name POLYNETSET,contract number HPRN-CT-2000-00 146, under theauspices of the Directorate-General for Research ofthe European Commission.

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