Effect of different thermal treatments on the self-assemblednanostructures of a styrene–butadiene–styrene star block copolymer

Elena Serrano, Amaia Zubeldia, Maider Larranaga, Pedro Remiro, Inaki Mondragon*

Materials and Technologies Group, Dpto. Ingenierıa Quımica y Medio Ambiente, Escuela Universitaria Politecnica-Donostia,

Universidad Paıs Vasco/Euskal Herriko Unibertsitatea, Pza. Europa, 120018, Donostia-San Sebastian, Spain

Received 5 June 2003; received in revised form 18 July 2003; accepted 1 August 2003

Abstract

Time evolution of the lamellar self-assembling behaviour of a commercial styrene–butadiene–styrene (SBS) star block copolymer,submitted to two different thermal treatments under thermooxidative degradation conditions, is reported. A microphase separation

process, leading to partially oxidized butadiene and polystyrene-rich copolymer, occurs in the block copolymer exposed at 150 �C inair. The timing of the microphase separation depends upon previous thermal treatment, however, when the microphase separationtakes place, the same evolution can be observed independently of the thermal treatment. The evolution of the morphologies hasbeen observed by Tapping Mode Atomic Force Microscopy (TM-AFM). The evolution of the oxidation reactions and their influ-

ence on the molecular mass has been followed by thermogravimetric analysis (TGA) and gel permeation chromatography (GPC).Fourier transform infrared (FTIR) spectroscopy has been used to analyse the chemical changes involved in the nano/microphaseseparation process.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Self-assembly; Block copolymer; Thermooxidative degradation; Microphase separation

1. Introduction

Block copolymers are a special class of polymershaving the ability to self-assemble into nanoscaleordered structures [1], which can be systematicallydesigned during their synthesis [2–4]. For diblock, AB,and triblock, ABA, copolymers, lamellae and spheresor cylinders arranged on a body-centred cubic orhexagonal packed lattice, whose structure dependsprimarily on molecular composition/architecture,thermodynamic incompatibility of the blocks, andmonomer asymmetry [5], are the first equilibriummorphologies that were confirmed [3,4]. Later, biconti-nous cubic-gyroid and bicontinuous double-diamondwere recognized [6] as equilibrium morphologiesalthough there are some doubts about the stability ofthe last one [7]. Star block, (AB)n, copolymers are parti-cularly interesting because they can form similar nano-structures. However, they have mechanical properties

better than their linear analogues [8]. Multiblock copo-lymers containing three or more segments arranged inlinear, star and branched configurations have produceda variety of other unanticipated and intriguing nano-structures [9]. Most of these morphologies show core-shell type structures such as lamellar, cylindrical orspherical core-shell; and other different ones because ofthe combination of these ones [6]. Upon variation oftemperature, these nanostructures can undergo thermaltransformations from one ordered structure to another(order–order transition) one and from an ordered to adisordered (isotropic) state (order–disorder transition) [2].With the aim of explaining how and under which

conditions the incompatibility of the copolymer blocksleads to the formation of different ordered nanos-tructures, a microscopic statistical theory of phaseequilibrium was developed by Leibler [3] in the eightiesfor block copolymers having a low level of incompat-ibility between the constituents (weak segregationregime). This theory predicts regions of stability of dif-ferent morphologies depending on the degree of mutualmiscibility and the volume fraction of the blocks ofthe copolymer. It can also predict the order–disorder

0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2003.08.009

Polymer Degradation and Stability 83 (2004) 495–507

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +34-943017271; fax: +34-

943017140.

E-mail address: iapmoegi@sc.ehu.es (I. Mondragon).

transition. Afterwards, Leibler and Fredickson [4]extended this theory and determined the phase beha-viour for block copolymers in non-selective good sol-vents using the self-consistent Hartree approximation.Later, Bates and Fredickson [10] and, on the otherhand, Hashimoto [11] found that in the strong segrega-tion regime the morphology basically depends on thevolume fraction of one of the constituent block chains.They concluded that for nearly symmetric block copo-lymers in the strong segregation regime, a lamellarphase is formed.Most studies dealing with the equilibrium morpholo-

gies in block copolymers have employed long annealingtimes under vacuum using relatively large specimenswith characteristic dimensions ranging from tens ofmicrometers to several millimetres. By minimizing sur-face-thermodynamic effects, these conditions allow toremove volatile solvents and to maximize the prob-ability of a given specimen to achieve its thermo-dynamic equilibrium morphology [12,13].One of the most studied block copolymers is poly

(styrene-b-butadiene-b-styrene) (SBS). Its various equi-librium morphologies as functions of composition arefairly well established [3,12]. When SBS copolymers aresubmitted to processing conditions and/or serviceenvironments, thermodegradative and/or thermo-oxidative reactions take place mainly in the butadieneblock. In the absence of oxygen, the thermal degrada-tion reactions involve cyclisation, isomerisation andcrosslinking and, finally, depolymerisation [14,15]. In anoxygen-containing atmosphere, thermooxidative reac-tions take place in the butadiene block, finally leading tomacrophase separation of polystyrene-rich copolymer[2]. This mechanism is usually described by a free-radi-cal reaction scheme, which involves three steps: initia-tion, propagation and termination. Since SBS rubberscontain an unsaturated rubber midblock, the degrada-tion mechanism is similar to styrene–butadiene rubbersand polybutadiene rubbers. It has been observed that, inthe butadiene block, which contains trans (or cis)-1,4structural units, the radicals mainly attack the allylichydrogen. In comparison, oxidation of vinyl-1,2 poly-butadiene exhibits a more pronounced crosslinking anda lower rate of formation of oxidation products [16].In this paper we present the effect of two different

thermal treatments, consisting on the annealing at twodifferent temperatures, on the self-assembled nanos-tructures of a commercial SBS star block copolymer. Asubsequent oxidizing treatment leads to the evolution ofthese morphologies. The influence of oxidative reactionson the macrophase separation in SBS triblock copoly-mer has been studied by other researchers [2]; however,we present the effect of the oxidizing conditions on themodification of the previously obtained nanostructures,as observed by Atomic Force Microscopy (AFM). Theevolution of the oxidation reactions and their influence

on the molecular mass has been analysed by Thermo-gravimetric Analysis (TGA) and Gel Permeation Chro-matography (GPC). The evolution of crosslinkingreactions has been followed by gel content determina-tion. Fourier Transform Infrared (FTIR) spectroscopyhas also been used since it is an important tool forinvestigating polymer degradation, because of its abilityto detect changes produced in the characteristicabsorptions for SBS.

2. Experimental

2.1. Materials

A commercial SBS star block copolymer (Kraton D-4274), with 0.2 wt.% antioxidant, was supplied by Kra-ton Polymers. It has a number-averaged molecularweight (Mn) of 183,000 referred to polystyrene and apolydispersity index of 1.34 (as measured by GPC).According to quantitative 1H NMR (Bruker 500 MHz)analyses, the SBS block copolymer contains 42.3 wt.%polystyrene, 52.3 wt.% 1,4-polybutadiene and 5.4 wt.%1,2-polybutadiene.

2.2. Sample preparation

Several thermal treatments were performed on SBSsamples in the following way: firstly, pellets of thecopolymer were dissolved in THF to make a 10 wt.%solution. The resultant solution was cast on a slidecover, and the solvent was allowed to evaporate for 12 hat ambient conditions. Then, the obtained film wasannealed in vacuum at 80 �C for 3 h or 150 �C for 30min. Thereafter, the sample was heated in an oven at150 �C in air for several times for the thermooxidativestudy.

2.3. Morphological analysis

The morphology of the specimen tested was studiedby Atomic Force Microscopy (AFM) with a scanningprobe microscope (Nanoscope IIIa, MultimodeTM fromDigital Instruments) operating in tapping mode (TM–AFM). An integrated silicon tip/cantilever, from thesame manufacturer, having a resonance frequency�300 kHz, was used. The TM–AFM images for thesample annealed at 150 �C for 30 min and oxidized at150 �C for several times have been taken aftermaintaining samples for 2 months at room temperature.

2.4. TG analysis

Thermogravimetric analyses (TGA) were performedusing a Setaram thermoanalyser (Model 92) in heliumor air atmosphere at a heating rate of 10 �C/min.

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Thermograms were recorded from room temperature to600 �C. Samples of about 10 mg made of pelletspulverized in liquid N2, were used.

2.5. GPC analysis

Analytical Gel Permeation Chromatography (GPC)was performed with a Perkin-Elmer LC-295 chromato-graph, equipped with a Perkin-Elmer LC-235 UVdetector set at 245 nm and a refractive index detectorLC-30 RI, to determine the changes in molecular weightand the molecular weight distribution with the thermaltreatment. A kit of three Waters styragel columns, HR2, HR 4 and HR 5E, whose molecular weight rangedetection is 500–20 000, 50 000–500 000 and 2000–4 000000, respectively, were used. The mobile phase was tet-rahydrofuran at a flow rate of 1 ml/min. Number andweight average molecular weights were calculated usinga universal calibration method with polystyrene stan-dards. For measuring the gel content, these sampleswere soaked in THF, and the undissolved residueweight was regarded as the gel content.

2.6. Infrared analysis

Fourier Transform Infrared Spectroscopy (FTIR)analyses were performed on a Perkin-Elmer 16PC spec-trometer. The copolymer was analysed as solution-castfilms from a 10 wt.% THF solution on KBr plates. Thespectra were taken with a 2 cm�1 resolution in a wave-number range from 4000 to 400 cm�1. The baseline wascorrected and normalized to the 1493 cm�1 peak height(polystyrene peak), which was seen to be invariant duringthermal and time treatment.

3. Results and discussion

3.1. Effect of thermal treatment

TM–AFM phase images, operating in moderate tap-ping conditions, of SBS films submitted at differentthermal treatments, are shown in Fig. 1. For compar-ison, in Fig. 1a, the AFM topography for as cast unan-nealed SBS film is presented. A disordered morphologyis displayed when the sample remains at ambient con-ditions for 1 week. The disordered structure observed inthe unannealed specimens can be attributed firstly to anuncompleted removal of the solvent. In addition, thekinetic arrest in the polymer might have taken placebefore the chains had enough time to self-assemble intoan equilibrium morphology [12,13,17]. When the speci-mens are annealed in vacuum, the thermal treatmentenhances polymer mobility in absence of solvent andthus provides additional time during which the film

morphology can lead to a more thermodynamicallystable state [12]. Indeed, times longer than a week atroom temperature, if less than 3 h at 80 �C and less thanabout 30 min for the specimens annealed at 150 �C arenecessary for morphology developing.For SBS films annealed in vacuum at 80 �C for 3 h

(Fig. 1b), SBS copolymer is self-assembled into lamellarnanodomains. The lamellae are oriented perpendicu-larly to the free surface with an interlamellar period of35 nm. According to the principles which govern theAFM technique for small drive amplitude and moderateor hard tapping conditions, the soft domain wouldappear brighter in the TM–AFM phase images becauseunder these conditions the tip response is dominated byattractive tip-sample interactions. As a result, it can bededuced that darker segments belong to the styreneblock, while the brighter ones belong to the butadieneblock. A similar assignment for SBS block copolymer isreported in the literature [17–19].Fig. 1c shows morphology for the specimens annealed

at 150 �C for 30 min and later exposed at ambient con-ditions for 2 months. The orientation and the inter-lamellar distance remain the same as those for 80 �Cannealing, however small regions seem to appear notcompletely ordered. After annealing SBS films at 150 �Cfor 1 h, a morphology consisting on agglomerateddomains into island appeared, and the film became yel-lowish. This effect has been attributed [20–29] to ther-mal degradation of SBS block copolymer leading tocrosslinking of a portion of the butadiene block.It has been reported [30,31] that the factors forcing

the nanodomains to lie parallel or perpendicular to thefree surface are, respectively, surface effects, which aregoverned by the enthalpic contribution and, conse-quently, lead to the most stable orientation, and con-finement effects, governed by the entropic contribution.The predominance of one contribution on the otherdepends on the thickness of the film and on its relationwith the natural period of the microdomain, concen-tration of the solution, solvent rate evaporation andannealing treatment. It can be explained in terms ofsurface energy. The block of lower surface energy ispreferentially located at the copolymer/air interface [17].For the SBS block copolymer, the lower surface tensionpolybutadiene would segregate to the free surface lead-ing to a parallel orientation where the butadiene blockwould be located at the copolymer/air interface [13,17].However, for very thin films, as those prepared in thiswork, with a high relation between thickness and nat-ural period, and not very low solvent rate evaporation,the confinement effects seem to be more important thanthe surface ones, so leading to a perpendicular orienta-tion of the nanodomains [17,30]. Thus, for SBS filmsannealed under the previously explained conditions, itcould be deduced that the confinement effects pre-dominate over the surface effects [30] conducting to a

E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507 497

perpendicular orientation between polybutadiene andpolystyrene lamellae.

3.2. Effect of oxidizing conditions

3.2.1. TG analysisFig. 2 shows the derivative thermogravimetric, DTG,

plots for SBS triblock copolymer in air and in helium.The correspondent integral curves are displayed in theinset. As can be seen, in both atmospheres SBS degra-dation occurs in two stages, the first weight loss stagebeing smaller than the second one. However, the weight

loss of the copolymer begins earlier in air than inhelium. This fact shows that initial thermooxidativedegradation takes place in the block copolymer exposedto air atmosphere through the attack of unsaturatedbond of polybutadiene [2,14,16,21,32].In helium atmosphere, around 22 wt.% weight loss

occurs in the first degradation step, which mainly cor-responds to 1,3-polybutadiene and 4-vinylcyclohexanefrom the depolymerization reaction of 1,4-poly-butadiene [15,33]. However, for the oxidizing atmo-sphere, the 30 wt.% weight loss measured correspondsto oxidized polybutadiene segments and some volatile

Fig. 1. TM–AFM phase images of SBS films: (a) unannealed, (b) annealed at 80 �C in vacuum for 3 h, and (c) annealed at 150 �C in vacuum for 30

min and exposed at ambient temperature for 12 months.

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products giving a polystyrene-rich copolymer [2,32],whose degradation is accelerated leading to a yellowishappearance [2,16,21].The temperature for the second degradation step is

less dependent than the correspondent one for the firstdegradation step in both environmental conditions. Thisfact has been attributed [26] to the oxidative degrada-tion that occurs preferentially over the thermal one attemperatures around 300 �C, while it is less importantaround 450 �C, temperature which corresponds to thesecond degradation step [27,28].

3.2.2. GPC analysisIn order to gain a deeper insight on the thermo-

oxidative process for SBS, GPC experiments were con-ducted on films annealed at 80 or 150 �C and lateroxidized in air in an oven at 150 �C for different times.Initially, the samples were fully soluble in THF, but atoxidizing times larger than 12 min for samples annealedat 80 �C or 5 min for samples annealed at 150 �C, a gelfraction was detected. Fig. 3 shows GPC chromato-grams for the soluble portion for films annealed at80 �C. The original SBS presents three peaks; the firstone at a retention time of 20.0 min corresponds to theblock copolymer. Another two smaller peaks areobserved at longer retention times, 21.5 and 23.5 min,which could be attributed either to low molecular frac-tions of SB diblock copolymer and polystyrene homo-polymer, respectively, resulting from the synthesis of thetriblock SBS. When the oxidizing time increases to 12min the first peak begins to decrease in intensity and thesecond and third peaks start to increase in intensitysuggesting that the chain scission reaction in the blockcopolymer is beginning. At longer oxidizing times, theintensity of peak corresponding to SBS block copoly-mer decreases in intensity continuously, but it does notdisappear completely even at 100 min. It is worth notingthat the second peak does not show any evident change

with the oxidation time, however the polystyrene peakincreases continuously, thus suggesting that the chainscission reaction leads to a polystyrene-rich copolymer,and volatile oxidation products from the butadieneportion that flush.Specifics changes for number-average and weight-

average molecular weights, and also gel content, for thefilms annealed at 80 �C are shown in Fig. 4. Both, thenumber-average and weight-average molecular weightsbegin to decrease at 6 min, thus suggesting that thechain scission reaction is beginning. On the other hand,the number-average molecular weight drops to almost1/3 of its original value at 31 min, however, the weight-average molecular weight drops to almost 1/2 of its ori-ginal value at 50 min. Thus, it seems that a polystyrene-rich copolymer is formed between 31 and 50 min ofexposure at 150 �C. For the gel content, it can be seenthan initially it is virtually zero and jumps to 1.2 wt.%at 12 min. At higher times, the gel content remainsalmost constant. This evolution suggests that, after 12min of exposure at 150 �C in air, crosslinking reactionshave already take place for the films annealed at 80 �C.The results show than the chain scission reaction beginsearlier than the crosslinking reaction (6 vs 12 min,respectively), and leads to a polystyrene-rich copolymerat times between 31 and 50 min. Moreover, taking intoaccount than the maximum gel percentage is only 1.4wt.%, for the films annealed at 80 �C and oxidized at150 �C for 1 h, it can be deduced that, for the analysingtimes, the thermooxidative reaction is governed bychain scission reactions in SBS [32].A similar evolution of the peaks is observed for the

soluble portion of films annealed at 150 �C (Fig. 5).When the oxidizing time increases to 7 min, the firstpeak decreases in intensity and the third peak, whichappears at retention times of 23.5 min, starts to increasein intensity. At longer oxidizing times, both, the secondand the third peak grow up in intensity until 15 min atthe expense of the first peak but this one does not dis-appear completely even at 50 min of exposure. Fig. 6shows the correspondent changes in number-averageand weight-average molecular weights, and also gelcontent. Chain scission reactions drive to a continuousdecrease of Mn and Mw, nevertheless the changes areirregular, which could indicate the formation of highermolecular weights by recombination [25]. In addition,the gel formation is already evident by 5 min and a greatincrease in gel content up to 30 wt.% is observed after10 min which increases to 40 wt.% after 15 min, thenremaining almost constant for the higher oxidizingtimes analysed. Therefore, for films annealed at 150 �C,in opposite to the ones annealed at 80 �C, the recombi-nation and crosslinking reactions become dominantfrom 7 min of exposure at 150 �C. However, by com-paring the GPC chromatograms after 50 min for bothsamples, annealed at 80 or 150 �C, it can be seen that

Fig. 2. DTG thermograms for SBS pellets in: (—) helium and (– . –)

air atmospheres. The inset shows the TGA thermograms. Heating rate

10 �C/min.

E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507 499

Fig. 3. GPC chromatograms for SBS films annealed at 80 �C in vacuum for 3 h and oxidized at 150 �C in air for different times.

Fig. 4. Evolution of weight-average molecular weight (-&-), number-average molecular weight (-~-) and gel content (-�-) as a function of oxidizing

time at 150 �C for SBS films annealed at 80 �C in vacuum for 3 h.

500 E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507

Fig. 5. GPC chromatograms for SBS films annealed at 150 �C in vacuum for 30 min and oxidized at 150 �C in air for different times.

Fig. 6. Evolution of weight-average molecular weight (-&-), number-average molecular weight (-~-) and gel content (-�-) as a function of oxidizing

time at 150 �C for SBS films annealed at 150 �C in vacuum for 30 min.

E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507 501

the shape is similar. Moreover, the Mn and Mw valuesare quite similar. Thus, it seems that the thermo-oxidative reaction follows the same way but it is fasterfor the films annealed at 150 �C. Besides, it seems thatduring annealing at this temperature an initial degrada-tion occur in SBS that could explain the higher gel per-centage for these films in comparison with the filmsannealed at 80 �C, which could be responsible for smalldisordered regions observed in Fig. 1c. Probably, longeroxidizing times for the films annealed at 80 �C for 3 hlead to similar gel percentages.

3.2.3. Infrared analysisThe evolution of the oxidation reactions for SBS films

has also been followed by FTIR. Spectra for SBS sam-ples annealed in vacuum at 80 and 150 �C, and afterbeing oxidized at 150 �C for several times are shown inFigs. 7 and 8, respectively. The characteristics absorp-tions for SBS that can undergo any transformationduring the oxidation treatment are specifically shown inTable 1 [34].Thermooxidative degradation produces oxygen-con-

taining groups. This chemical process can be followedby observing the appearance of new peaks in thehydroxyl (3600–3200 cm�1) and carbonyl (1800–1600cm�1) regions, and changes in the reference signals for

the SBS [7]. By comparing the spectra for films annealedat 80 �C (Fig. 7), at t=0 and after 6 min of exposure at150 �C in air two small peaks are visible in the carbonylregion, at 1700 and 1696 cm�1. Nevertheless, thesepeaks were already visible at t=0 and, as have beendetected by NMR, they could be attributed to residuesof emulsifier and other acidic components from thesynthesis of the block copolymer, which absorb at thesewavenunbers [34]. At longer oxidizing times, 12 min,three different carbonyl groups arise at 1793, 1735 and1730 cm�1. In addition, hydroxyl groups are alreadyclearly observed as a broad absorption band between3600 and 3300 cm�1 with a maximum at 3535 cm�1.With the oxidizing time, the carbonyl group thatappeared at 1730 cm�1 decrease in intensity while thecorresponding ones at 1735 and 1793 cm�1 increase inintensity. This last peak displaces towards lower wave-numbers, which might suggest that the acidic groupsreact with some alkene carbons of 1,2- and E-1,4-poly-butadiene giving carbonyl and carboxylate groups.Important differences can be also noted in the buta-

diene and polystyrene regions. After 12 min of exposureat 150 �C in air, the bands assigned to vibration of thealkene carbons of 1,2-polybutadiene, which appear at1634 cm�1, and to deformation of hydrogen atoms of1,2- and E-1,4-polybutadiene, which appear at 996 and

Fig. 7. FTIR spectrum in several ranges of SBS films annealed in vacuum at 80 �C for 3 h and oxidized at 150 �C for 0, 6, 12, 18 and 50 min.

502 E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507

911 cm�1, and 965 cm�1 respectively, decrease in inten-sity. It is worth noting that the band at 1655 cm�1,assigned to alkene carbons of Z-1,4-polybutadiene, andthe corresponding one for hydrogen deformation, whichappears at 743 cm�1 as a little shoulder of the poly-styrene peak at 755 cm�1, do not seem to change.However, a decrease in intensity of 1240 cm�1 band can

be detected. It is at this time that an important reduc-tion in molecular weight is driven by chain scission reac-tions, and a small percentage of crosslinking reactionstakes place. Thus, it could suggest that the thermo-oxidative degradation at 150 �C for the specimensannealed at 80 �C begins by direct oxidation, after 12 minof exposure, of mainly 1,2- and E-1,4-polybutadiene

Fig. 8. FTIR spectrum in several ranges of SBS films annealed in vacuum at 150 �C for 30 min and oxidized at 150 �C for 0, 3, 6, 12 and 50 min.

Table 1

FTIR characteristics absorptions for SBS [34]

Absorptions (cm�1)

Assignment

3005

Polybutadiene C–H stretching

2000–1700

Overtones and combination vibration for polystyrene ring

1655

Z-1,4-polybutadiene C¼C stretching vibrations

1650

E-1,4-polybutadiene C¼C stretching vibrations

1634

1,2-polybutadiene C¼C stretching vibrations

1600

Polystyrene ring modes

1493 and 1450

Polystyrene CH2 scissoring modes

1405

1,2-polybutadiene CH2 scissoring modes

1240

1,2-polybutadiene vibration mode

996

Z-1,4 and 1,2-polybutadiene C–H deformation

965

E-1,4-polybutadiene C–H deformation

911

1,2-polybutadiene C–H deformation

760, 700

Polystyrene monosubstitued harmonics

Z-1,4-polybutadiene C–H deformation (740 cm�1)

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segments, to anhydrides, ketones and acidic compoundsleading to chain scission reactions in the block copoly-mer. At even longer times, the bands assigned to 1,2 andZ-1,4-polybutadiene decrease in intensity, some of themuntil disappearing. However, although the deformationfor hydrogen atoms of E-1,4-polybutadiene diminishes,the band assigned to alkene carbons begins to increaseat 31 min probably due [28,29,33] to the formation ofbutadiene homopolymers and other alkenes as propeneor pentene, some of them oxidized.In agreement with GPC results, which show that the

chain scission reaction leads to microphase separation,FTIR results confirm the chain scission reactions andshow that they are conducted by the direct oxidation ofpolybutadiene segments that, at higher oxidation times,can lead to phase-separation between polystyrene-richcopolymer and partially oxidized butadiene.Fig. 8 shows the spectra for films annealed at 150 �C.

At t=0, it is noticeable the emergence of carbonylgroups, strong band around 1700 cm�1 and another twoweaker at higher wavenumbers—bands around 1720and 1780 cm�1—, which can be assigned [10,34] to car-

boxylic acid, ketone and anhydride, respectively. More-over, it can be seen the absorption of hydroxyl groups—broad band between 3600 and 3200 cm�1 with a max-imum at 3506 cm�1. Thus, it seems to be clear that, asabove commented in GPC analysis, an initial degrada-tion in SBS films occurs during annealing at 150 �C for30 min leading to the small disordered regions in themorphology (Fig. 1c). For higher oxidizing times, theevolution of the peaks is similar to the correspondingones for annealing at 80 �C, although the changes occurat earlier oxidizing times. Nevertheless, the alkene car-bon region, bands around 1730 cm�1, seems not tochange probably due to the formation of species ofhigher molecular weight [25] and crosslinking reactions,as above demonstrated by GPC. However, when thespectra after 50 min of exposure at 150 �C for bothannealing are compared, and taking into account thatthe spectra for the films annealed at 80 �C are satu-rated, no evident changes can be detected. Thus, it canbe deduced that the previous annealing treatmentsdetermine the earlier stages of the thermooxidationreaction but when the separation process has taken

Fig. 9. TM–AFM phase images of the temporal evolution of nano/microstructure of SBS film annealed in vacuum at 80 �C for 3 h and exposed in

air at 150 �C: (a) 6 min, (b) 12 min, (c) 21 min, (d) 31 min and (e) 50 min.

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place, as above demonstrated by GPC, the evolution ofthe oxidation is similar. Furthermore, the fasterappearance of stable degradation products indicates thelower stability of the annealing at higher temperatures[22].

3.2.4. Morphological analysisThe effect of environmental conditions on the mor-

phology was also followed by TM–AFM. Fig. 9 showsthe temporal evolution of self-assembled nanos-tructures. Microphase separation takes place amongpolystyrene-rich copolymer and partially oxidized buta-diene of SBS films exposed at 150 �C in air afterannealing upon vacuum at 80 �C for 3 h. The originalmorphology has been shown in Fig. 1b. No significantchanges in the original lamellar morphology can bedetected after 6 min of exposure in an oxidative atmo-sphere (Fig. 9a). Even after 12 min, Fig. 9b, the originallamellar morphology is maintained, although slightlyaltered. When the oxidizing time increases to 21 min(Fig. 9c), small regions of a disordered morphologyarise between the lamellae, which grow at even longertimes, 31 min (Fig. 9d), but the ordered regions do not

show any evident changes, and the interlamellar dis-tance remains almost constant. A dramatic change inthe morphology is noticed after 50 min of oxidizingtime, where the small disordered regions grow leadingto a microphase-separated structure, in which thelamellar self-assembly for the copolymer is completelyremoved (Fig. 9e).The AFM images confirm GPC analysis, which seems

to indicate that a microphase separation between 31 and50 min of exposure is produced in the block copolymerannealed at 80 �C. Moreover, as above demonstrated byGPC and FTIR analysis, the thermooxidative degrada-tion is governed by chain scission reactions in the timeinterval studied, since the crumbling of the orderednanostructures does not occur until an oxidizing time of50 min.The morphological study was also performed on

samples annealed at 150 �C. It is worth remarking thatthe images have been taken after maintaining samples atroom temperature for 2 months. After 1 min of expo-sure at 150 �C in air (Fig. 10a), the lamellar structureobserved in the original sample (Fig. 1b), althoughslightly altered, does remain, though some small regions

Fig. 10. TM–AFM phase images of temporal evolution of nano/microstructure of SBS films annealed in vacuum at 150 �C for 30 min, and exposed

in air at 150 �C: (a) 1 min, (b) 3 min, (c) 5 min, (d) 6 min and (e) 12 min. The images were recorded after two months at ambient temperature.

E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507 505

where the lamellae are broken arise between the lamel-lae. For the films oxidized for 3 min in air (Fig. 10b), nochanges with respect to the films exposed for 1 min canbe observed. Even at 5 min, Fig. 10c, although moredisordered regions can be detected, the ordered regionsdo not show any evident changes, and the interlamellardistance remains almost constant. Full microphaseseparation between butadiene-rich and polystyrene-richcopolymer, both of them partially oxidized, has alreadyhappened after 6 min of oxidizing time (Fig. 10d). Fur-thermore, no sign of nanoordered morphology in thecopolymer phase can be detected. At longer times, 12min, the microphase-separated structure is alsoobserved (Fig. 10e).By comparing Figs. 9e and 10d, the AFM images

show a similar morphology thus confirming that oncethe microphase-separated structure is formed, the evo-lution of the reaction is similar independently ofannealing being performed at 80 or 150 �C. Never-theless, the changes, as above demonstrated by GPCand FTIR, occur earlier for the films annealed at 150 �C.

4. Conclusions

In this work the influence of the thermooxidativedegradation of SBS star block copolymers self-assem-bled at different annealing conditions has been investi-gated. A competition between chain scission reactionand recombination and crosslinking reactions, bothinducing a microphase separation, in which the copoly-mer is not able to maintain the nanostructure after theoxidation process, takes place during the thermo-oxidative degradation at 150 �C. The timing of micro-phase separation depends upon annealing treatment.For the films previously annealed at 80 �C, the oxida-tion of mainly E-1,4- and 1,2-polybutadiene segmentsleads to chain scission reactions that determine thecourse of the microphase separation to a polystyrene-rich copolymer. For the films annealed at 150 �C,however, an initial degradation during the annealingtreatment determines the earlier stages of thethermooxidation leading to recombination and cross-linking reactions, in addition to the chain scission.However, once the microphase separation process takesplace, the evolution of the thermooxidative degradationat 150 �C for SBS star block copolymer is similarlyindependent of whether annealing has been performedat 80 or 150 �C.

Acknowledgements

Funding of this work was provided by Ministerio deCiencia y Tecnologıa (Spain), Grants MAT2000-0293and MAT2001-0714.

References

[1] Bates FS, Fredickson GH. Block copolymers–designer soft

materials. Phys Today 1999;52(2):32–8.

[2] Fan S, Kyu T. Macrophase separation in styrene–butadiene

block copolymers driven by thermooxidative reactions. Macro-

molecules 2000;33(26):9568–74.

[3] Leibler L. Theory of microphase separation in block copolymers.

Macromolecules 1980;13(6):1602–17.

[4] Fredickson GH, Leibler L. Theory of block copolymer solutions:

nonselective good solvents. Macromolecules 1989;22(3):1238–50.

[5] Roberge RL, Patel NP, White SA, Thongruang W, Smith SD,

Spontak RJ. Block copolymer/homopolymer mesoblends: prepara-

tion and characterization. Macromolecules 2002;35(6):2268–76.

[6] Thomas EL, Alward DB, Kinning DJ, Martin DC, Handling Jr

DL, Fetters LJ. Ordered bicontinuous double-diamond structure

of star block copolymers: a new equilibrium microdomain mor-

phology. Macromolecules 1986;19(8):2197–202.

[7] Likhtman AE, Semenov AN. Stability of the OBDD structure for

diblock copolymer melts in the strong segregation limit. Macro-

molecules 1994;27(11):3103–6.

[8] Michler GH, Adhikari R, Lebek W, Goerlitz R, Weidisch R,

Knoll K. Morphology and micromechanical deformation beha-

viour of styrene/butadiene–block copolymers. I. Toughening

mechanisms in asymmetric star block copolymers. J Appl Polym

Sci 2002;85(4):683–700.

[9] Zhao J, Majumdar B, Schulz MF, Bates FS, Almdal K, Morten-

sen K, Hajduk DA, Gruner S. Phase behaviour of pure diblocks

and binary diblock blends of poly(ethylene)-poly(ethylethylene).

Macromolecules 1996;29(4):1204–15.

[10] Bates FS, Fredickson GH. Block copolymer thermodynamics:

theory and experiment. Annu Rev Phys Chem 1990;41:525–57.

[11] Hashimoto T. In: Legge NR, Holden G, Schroeder HE, editors.

Thermoplastic elastomers. A comprehensive review. Munich:

Hanser; 1987.

[12] Kim G, Libera. Morphological development in solvent-cast

polystyrene–polybutadiene–polystyrene (SBS) triblock copolymer

thin films. Macromolecules 1998;31(8):2569–77.

[13] Ton-That C, Shard AG, Daley R, Bradley RH. Effects of

annealing on the surface composition and morphology of PS/

PMMA blend. Macromolecules 2000;33(22):8453–9.

[14] Schnabel W, Levchik GF, Wilkie CA, Jiang DD, Levchick SV.

Thermal degradation of polystyrene, poly(1,4-butadiene) and

copolymers of styrene and 1,4-butadiene irradiated under air or

argon with 60Co-g-rays. Polym Degr and Stab 1999;63(3):365–75.

[15] Luda MP, Guaita M, Chiantore O. Thermal degradation of

polybutadiene, 2. Makromol Chem 1992;193:113–21.

[16] Ahlblad G, Reitberger T, Terselius B, Stenberg B. Thermal

oxidation of hydroxyl-terminated polybutadiene rubber I. Chemi-

luminiscence studies. Polym Degr and Stab 1999;65(2):179–84.

[17] Zhang Q, Tsui OKC, Du B, Zhang F, Tang T, He T. Observation

of inverted phases in poly(styrene-b-butadiene-b-styrene) triblock

copolymer by solvent-induced order-disorder phase transition.

Macromolecules 2000;33(26):9561–7.

[18] Bar G, Thomann Y, Brandsch R, Cantow HJ, Whangbo MH.

Factors affecting the height and phase images in tapping mode

atomic force microscopy. Study of phase-separated polymer

blends of poly(ethene-co-styrene) and poly(2,6-dimethyl-1,4-

phenylene oxide). Langmuir 1997;13(14):3807–12.

[19] Raghavan D, Gu X, Nguyen T, VanLandingham M, Karim A.

Mapping polymer heterogeneity using atomic force microscopy

phase imaging and nanoscale indentation. Macromolecules 2000;

33(7):2573–83.

[20] Wang SM, Chang JR, Tsiang RCC. Infrared studies of thermal

oxidative degradation of polystyrene–block–polybutadiene–

block–polystyrene thermoplastic elastomers. Polym Degr and

Stab 1996;52(1):51–7.

506 E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507

[21] Singh RP, Desai SM, Solanky SS, Thanki PN. Photodegradation

and stabilization of styrene–butadiene–styrene rubber. J Appl

Polym Sci 2000;75(9):1103–14.

[22] Sakurai S, Kawada H, Hashimoto T, Fetters LJ. Thermo-

reversible morphology transition between spherical and cylind-

rical microdomains of block copolymers. Macromolecules 1993;

26(21):5796–802.

[23] Motyakin MV, Schlick S. Thermal degradation at 393 K of

poly(acrylonitrile–butadiene–styrene) (ABS) containing a hin-

dered amine stabilizer: a study by 1D and 2D electron spin reso-

nance imaging (ESRI) and ATR-FTIR. Polym Degr and Stab

2002;76(1):25–36.

[24] Belfiore LA, Sun X, Das P, Lee JY. High-temperature infrared

kinetics of transition–metal–catalyzed chemical reactions in solid

state complexes of polybutadienes with palladium chloride.

Polymer 1999;40(20):5583–99.

[25] Kaczmarak H. Accelerated photodegradation of cis 1,4-poly-

butadiene in the presence of hydrogen peroxide. Polym Bull 1995;

34(2):211–8.

[26] Park Y, Hool JN, Curtis W, Roberts CB. Depolymerization of

styrene–butadiene copolymer in near-critical and supercritical

water. Ind Eng Chem Res 2001;40(3):756–67.

[27] Brazier DW, Schwartz NV. The effect of heating rate on the

thermal degradation of polybutadiene. J Appl Polym Sci 1978;

22(1):113–24.

[28] McNeill IC, Ackerman L, Gupta SN. Degradation of polymer

mixtures. IX. Blends of polystyrene with polybutadiene. J Polym

Sci, Polym Chem Ed 1978;16(9):2169–81.

[29] McCreedy K, Keskkula H. Effect of thermal crosslinking on

decomposition of polybutadiene. Polymer 1979;20(9):1155–9.

[30] Kellogg GJ, Walton DG, Mayes AM, Lambooy P, Russell TP,

Gallagher PD, Satja SK. Observed surface energy effects in con-

fined diblock copolymers. Phys Rev Lett 1996;76(14):2503–6.

[31] Henkee CS, Thomas EL, Fetters LJ. The effect of surface con-

straints on the ordering of block copolymer domains. J Mater Sci

1988;23(5):1685–94.

[32] Fan S, Kyu T. Reaction kinetics of thermooxidative degradation

in a styrene-b-butadiene diblock copolymer. Macromolecules

2001;34(3):645–9.

[33] Golub MA, Gargiulo RJ. Thermal degradation of 1,4-poly-

isoprene and 1,4-polybutadiene. J Polym Sci, Polym Lett Ed

1972;10(1):41–9.

[34] Hummel DO, Scholl F. Atlas of Polymer and Plastics Analysis.

Vol 1: Polymers: Structures and Spectra. Munich: Hanser, 1978.

Vol 2: Plastics, Fibres, Rubbers, Resins; Starting and Auxiliary

Materials, Degradation Products. 2nd. Ed. Munich: Hanser, 1984.

E. Serrano et al. / Polymer Degradation and Stability 83 (2004) 495–507 507