Mechanistic Differences in Degradation of Polystyrene and Polystyrene-Clay Nanocomposite: Thermal...

Mechanistic Differences in Degradation of Polystyrene

and Polystyrene-Clay Nanocomposite: Thermal and

Thermo-Oxidative Degradation

Kai Chen, Sergey Vyazovkin*

Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294, USAFax: þ205 975 0070; E-mail: [email protected]

Received: November 21, 2005; Accepted: January 10, 2006; DOI: 10.1002/macp.200500536

Keywords: brush; clay; degradation; nanocomposite; thermal stability

Introduction

Enhanced thermal stability has been widely reported in

various polymer layered-silicate (clay) nanocomposites.

For instance, the degradation temperatures of such systems

are typically larger than those in virgin polymers.[1,2]

Enhancement of thermal stability is found to be critically

dependent on the dispersion of clay platelets into polymer

matrix. Based on the morphology of clay in the polymer

matrix, polymer nanocomposites are usually divided into

two classes. For intercalated nanocomposites, dispersed

silicate clay layers have a swollen interlayer spacing due to

the insertion of polymer chains inside the galleries, but the

registry of clay layers is still retained. For exfoliated

nanocomposites, the originally stacked clay layers become

delaminated, leading to a complete loss of structural

Summary: A conceptual model for degradation of polymer-clay brushes is discussed. The model predicts enhancementof inter-molecular reactions and slowing down of molecularmobility. A polystyrene (PS)-clay brush system is exper-imentally compared with virgin PS under the conditions ofthermal and thermo-oxidative degradation. GC-MS andTGA-FTIR analysis of the gas phase degradation productsof PS-clay composite confirm a dramatic increase in the yieldof inter-molecular reaction product, such as a-methylstyrene.Combining DSC measurements with FTIR analysis of con-densed phase thermo-oxidative degradation of PS-claysuggests that PS-clay nanocomposite is more stable tooxidation, however, oxidation products tend to accumulatein it because of slowing down diffusion of oxidation products.

Gas phase IR spectra from the TGA-FTIR study of virgin PSand PS-clay for different stages of degradation.

Macromol. Chem. Phys. 2006, 207, 587–595 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper DOI: 10.1002/macp.200500536 587

registry. Consequently, individual clay layers are uniformly

dispersed into continuous phase of polymer matrix. The

role of clay in improving thermal stability of polymer

nanocomposites has been rationalized in terms of different

mechanisms. Blumstein[3] has been first to point out that the

increased thermal stability is due to steric factors in

polymer-clay nanocomposites. When poly(methyl metha-

crylate) (PMMA) is intercalated into the gallery of mon-

tmorillonite (MMT) clay, the thermal motion of polymer

chains sandwichedbetween the lamellae ofMMTis restrict-

ed and leads to a greater resistance to thermal degradation

compared to virgin PMMA. Gilman[4,5] invoked the barrier

effect, which assumes the formation of a carbonaceous-

silicate char that builds up on the surface of the polymer

melt and provides the mass and heat transfer barrier.

Wilkie[6] and Brittain[7] argued that clay containing structu-

ral iron can act as free radical scavenger and, therefore,

contribute to the enhanced thermal stability by radical

trapping.

Although the abovemodels explain successfully slowing

down of the degradation of polymer-clay nanocomposites,

they do not predict changes in the mechanism and thermal

effects of degradation. However, it has been noticed[8] in

our previous differential scanning calorimetry (DSC) study

that the heat of degradation for polystyrene (PS)-clay

differs significantly from that for virgin PS. Clay does not

undergo any noticeable thermal degradation at the respec-

tive temperatures. Although the quaternary ammonium

fragments that link PS to clay may degrade, their content

relative to PS is too small to affect the total heat of PS

degradation. Therefore, the change in the total heat of

degradation hints at a change in the mechanism. While it

has been reported that due to its catalytic property clay can

modify the degradation mechanism of polyethylene (PE)

and poly(propylene) (PP) nanocomposites,[9,10] the PS-clay

in our study has not shown the characteristic features of

catalytic cracking such as a decrease in the degradation

temperature and activation energy as well as the change of

the most abundant degradation product from styrene to

benzene.[11,12] Our TGA data[13] show that the degradation

temperature of PS-clay increases by 30–40 8C and its

activation energy of degradation is also markedly larger

than that for virgin PS. Also, no formation of benzene has

been noticed in our analysis of degradation products.[14]

In our recent short communication,[14] we proposed a

conceptual model to explain the alteration of the degrada-

tion mechanism for PS-clay system. In this paper, we

present a more detailed account of the mechanistic data on

the thermal degradation as well as new data on the thermo-

oxidative degradation of the PS-clay system. There have

been relatively few publications on the thermo-oxidative

degradation of polymer-clay nanocomposites.However,we

are not aware of any mechanistic studies of this process. In

our work we combine condensed phase FTIR and DSC data

to better understand the thermo-oxidative degradation of

PS-clay nanocomposites. Obtaining such information is of

crucial importance for practical applications that are

associated with a wide temperature region from ambient

to combustion temperatures. Thermo-oxidative degrada-

tion is usually considered[15] to be a key process that

generates the volatile flammable products during combus-

tion. Therefore, data on the extent and rate of thermo-

oxidative degradation may provide information that

complements other fire related characteristics as the peak

heat release rate in cone calorimeter,[16,17] limiting oxygen

index and UL 94 classification.[18]

Experimental Part

A sample of the PS-clay nanocomposite was kindly providedby Dr. Xiaowu Fan (Northwestern University). The intercala-tion of initiators into the gallery of clay and the exfoliation ofclay in the PS-clay composite were confirmed by X-raydiffraction (XRD), TGA, Fourier Transform Infrared Spectro-scopy (FTIR), X-ray photoelectron spectroscopy (XPS) andatomic force microscopy (AFM) described elsewhere.[19] Abrief summary of the material preparation is as follows: themontmorillonite (MMT) clay (Cloisite1Naþ, cation exchangecapacity (CEC) 92mequiv/100 g, specific surface area 750m2/g) was obtained fromSouthern Clay Product Inc. A free radicalinitiator (2,20-azoisobutyronitrile derivative with a quaternaryammonium group at one end) was immobilized on the claysurface by cation exchange reaction. The surface-attachedinitiators convert the hydrophilic clay surface to an organo-philic one and provide the initiation sites for subsequentin-situ polymerization. Because a polymer chain starts to growfrom the clay surface, this type of polymerization is termed‘‘surface-initiated polymerization’’ (SIP). As the tetheredchains grow, they push apart the stacked clay layers until thestructural registry is lost. As a result, the delaminated clayplatelets become uniformly dispersed into the PS matrix. Theexfoliated PS-clay sample under study contains about 1 wt.-%of MMT[8] and hasMw � 90 000 and PDI� 2.3. For compar-ison purposes, radically polymerized PS (Alfa Aesar, Mw �100 000, PDI �2.4)[13] was used as received. A small amountof neat PS synthesized by using the AIBN initiator wasavailable in our previous study[13] that showed that itsdegradation was practically identical with PS purchased fromAlfa Aesar.

Comparative thermal degradation of PS-clay and virgin PSwas conducted by pyrolysis-GC-MS (Py-GC-MS). All sam-ples were dissolved in tetrahydrofuran (THF, Fisher) andintroduced into Frontier Labs double shot pyrolyzer (FrontierLabs, Japan) where they were purged under a 100 mL �min�1

flow of 99.999% pure (Grade 5) helium for 3 min. Thepyrolyzer was interfaced with a Hewlett Packard 5890 II GCand Hewlett Packard 5970 Mass Selective Detector. Thesamples were heated to 500 8C and the GC-MS measurementsof the evolved gases were taken. The GC-MS heating programstarted at 50 8C and held for 3 min and then followed byramping at 8 K �min�1 for 31.25 min before reaching 300 8Cand holding there for 10.75 min, so that the total GC-MSheating cycle was 45 min.

588 K. Chen, S. Vyazovkin

Macromol. Chem. Phys. 2006, 207, 587–595 www.mcp-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

TGA-FTIR analysis was performed on a Mettler-ToledoTGA/SDTA851e module interfaced with Nicolet Nexus470 FTIR spectrometer. TGAwas run under a nitrogen purgeat a flow rate of 70 mL �min�1 from 75 to 600 8C at a heatingrate of 10 K �min�1. The evolved gases were passed from thefurnace of TGA to the gas cell of FTIR through a heatedtransfer line (at 230 8C). The IR spectra of degradationproducts were taken at 4 cm�1 resolution. The same FTIRinstrument was used to monitor the thermo-oxidative degra-dation of PS-clay and virgin PS in the condensed phase. Bycasting a sample solution (in THF) onto the KBr substrate, theformed sample filmswere heated in a transmission cell (HT-32,Thermo Spectra-Tech) from a room temperature to 370 8C ata heating rate of 5 K �min�1 and an air flow rate of50 mL �min�1.

The heat released from oxidative degradation of our sampleswas measured by a Mettler-Toledo DSC 822e module whichwas calibrated using an indium standard. Samples of�2.5 mgwere placed in 40 mLAl pans and first heated from 30 to 200 8Cat a heating rate of 20 K �min�1. Then a slow heating rate of2 K �min�1 was used to continue heating up to 400 8C. Allmeasurements were repeated three times and the mean valueswere determined.

Results and Discussion

Conceptual Model

The degradation behavior of polymers generally reflects the

net result of the intra- and inter-molecular motion of

polymer chains. Our conceptual model links the segmental

motion of polymer chains and the mechanism of thermal

degradation of polymer-clay. The conventional classifica-

tion of polymer-clay (intercalated vs exfoliated structure)

focuses on the morphology of silicate layers, but the micro-

structure of polymer matrix is not explicitly considered,

although perturbations of the equilibrium chain conforma-

tions by silicate layers are well-known phenomena. For

establishing structure-property relationship, it is important

to choose a model system with well-defined morphology of

both dispersed clay and matrix polymer chains. The SIP

method grafts polymer chains onto clay surface, giving rise

to exfoliation of clay platelets as well as brush structures

whose morphology is determined by the grafting density,

defined as the number of tethered chains per unit area (nm2)

of the clay surface. According to the theory developed by

Alexander[20] and de Gennes,[21] when grafted chains are

separated by a large distance (d), which is the case of low

grafting density, each chain is able to assume the shape of

coils (Figure 1a) whose size is determined by the radius of

gyration, Rg. However, in the case of high grafting density

where the distance between the grafting sites falls below

2Rg, the space becomes too crowded for each chain to keep

its coil conformation. The strong repulsive interactions

among neighboring chains force them to stretch away from

the clay surface, forming a brush structure (Figure 1b). The

inter-chain distance for polymer brush is directly related to

the grafting density as[22]

d ¼ 1ffiffiffi

sp ð1Þ

where s is the grafting density, which is experimentally

determined by

s ¼ hrNA

Mw

ð2Þ

where r¼ 1.05 g � cm�3 is the density of PS,[23] NA is the

Avogadro’s number,Mw (80 000–90 000) is the molecular

weight of tethered PS, and h is the thickness of the polymer

layer that can bemeasured by ellipsometry. For our PS-clay

sample, h is about 8.8 nm[24] that yields s� 0.07 chains �nm�2 and inter-chain distance d¼ 3.8 nm. This ismore than

4 times smaller than the unperturbed coil diameter,

2Rgffi 18 nm as reported in the literature data[25,26] for the

respectivevalues ofMw. This clearly suggests the formation

of the brush structure.

It is noteworthy that stretching of chains in PS brush

affords very close packing of neighboring chains that can

not be achieved in regular PS. The regular PSmelts have the

structure of entangled coils[27] for which the inter-chain

distance can be estimated as the distance between en-

tanglement points of chains or as the tube diameter in the

repetition model,[28] and is reported to be �8.2 nm.[29,30]

This is the average distance that a monomer unit of one

chain needs to cross to reach a monomer unit of another

chain. At the scale smaller than the inter-chain distance, the

motion of a chain is limited by intra-chain molecular

motion that is practically not affected by neighboring

chains. The amplitude of the intra-molecular motion is

determined by the size of the flexible segment also known as

the Kuhn segment. The Kuhn segment of PS has a length of

�2nm.[31] Therefore, for the PS brushes, a small inter-chain

distance (3.8 nm) actually places two neighboring chains

within the amplitude of their Kuhn segment motions. This

Figure 1. Chain conformation of polymers: (a) random coils and(b) brushes.

Mechanistic Differences in Degradation of Polystyrene and Polystyrene-Clay Nanocomposite: . . . 589


dramatically enhances the interactions between neighbor-

ing chains. Therefore, the movement of each individual

chain in the ‘‘brushmelt’’ becomes strongly correlated with

its neighbors giving rise to significantly increased molec-

ular cooperativity. The latter can be estimated from the heat

capacity measurement.[32] According to our data,[33] the

volume of the cooperatively rearranging region in the PS-

clay brush is 36.7 nm3 which is 1.8 times larger for virgin

PS (20.9 nm3). The increasing degree of cooperativity

between neighboring chains is also manifested in the

activation energy of the a-relaxation that is markedly larger

for PS-clay system (�340 kJ �mol�1) than virgin PS

(�270 kJ �mol�1).

To summarize, the conceptual model predicts that due to

the increased cooperativity, the motion of individual chains

in the brush system should slow down the transport of

radicals, oxygen and degradation products. More impor-

tantly, by reducing the inter-chain distance it should

increase the probability of inter-molecular reactions and

yield inter-molecular reaction products. In the following

sections, the mechanistic predictions of the conceptual

model are examined by using actual experimental data on

the thermal and thermo-oxidative degradation of PS-clay

system.

Thermal Degradation under Nitrogen

The thermal degradation of PS-clay has been studied by

analyzing evolved gas products in TGA-FTIR runs.

Comparative vapor phase FTIR spectra (Figure 2) clearly

show a difference in the composition of degradation

products of PS-clay composite and virgin PS, suggesting

a change in the degradation pathway. The first distinct

difference in the IR spectrum is found in the region near

1 600 cm�1. The spectra of virgin PS degradation products

always display a medium peak at 1 630 cm�1 (C C

stretching in the vinyl group) with a shoulder at 1 600 cm�1

(‘‘benzene ring breathing’’).[34] Conversely, the PS-clay

degradation products at the early stage (10% mass loss)

show a reverse pattern with a stronger peak at 1 600 cm�1

and a weaker one at 1 630 cm�1. At the high extent of PS-

clay degradation (70% mass loss), the spectra in the region

of�1 600 cm�1 become quite similar to those for degrading

virgin PS. The change of absorption pattern is even

more obvious in the evolution of bands at 1 600 cm�1 and

1 630 cm�1 (Figure 3). In Figure 3, out-of-plane C–H

bending bands at 910 and 989 cm�1 are used as reference for

the vinyl group in styrene. Note that the band at 1 630 cm�1

follows the same profile as 910 and 989 cm�1 bending

bands in both virgin PS and PS-clay systems, which

suggests that the band at 1 630 cm�1 is due to the formation

of styrene. As for the benzene ring mode at 1 600 cm�1, it is

due to the stretching and contracting of the carbon-carbon

bonds in the aromatic ring. In PS-clay, 1 600 cm�1 peak

reaches its maximum faster than the trace of styrene,

implying that a larger ratio of aromatic rings per vinyl group

is formed at the earlier stage of degradation than at the later.

This means that some products containing aromatic ring

structure but no vinyl group are formed predominantly at

the beginning of degradation. On the other hand, the peaks

at 1 600 and 1 630 cm�1 in virgin PS have very similar

shapes with maximum occurring practically at the same

time as the vinyl peaks at 910 and 989 cm�1. Another major

difference in the IR spectra (Figure 2) of PS and PS-clay is

found in the absorption region of 2 800–3 000 cm�1, which

is characteristic of C–H stretching in methyl or methylene

groups. In styrene the methylene absorption lies above

3 000 cm�1. However, the methyl stretch of a-methylstyr-

ene is found within this region. Therefore, the band at

2 974 cm�1 is likely associated with the formation of

a-methylstyrene. Similar to the 1 600 cm�1 band, this band

evolves most quickly at the earlier stage of degradation

(Figure 3). On the other hand, band at 2 974 cm�1 is barely

seen in the spectrum of virgin PS.

The above IR analysis is corroborated byPy-GC-MSdata

(Figure 4). For virgin PS, it shows three major peaks at

retention times 6.1, 21.3, and 29.8 min, which have been

identified to be monomer (styrene), dimer (2,4-diphenyl-1-

butene) and trimer (2,4,6-triphenyl-1-hexene), by compar-

ing their mass-spectra with the NIST database. This

degradation product pattern is consistent with the

Figure 2. Gas phase IR spectra from the TGA-FTIR study ofvirgin PS (a) and PS-clay (b) for the earlier (10% mass loss) andlater (70% mass loss) stages of degradation.



well-established free radical mechanism.[35] It suggests that

themost abundant product, styrene, is formed viab-scissionof chain-end radical (unzipping). The dimer and trimer are

produced via intra-molecular transfer (backbiting) reaction

in which a chain end radical folds back onto the same chain

and abstracts hydrogen from the 3rd and 5th carbon of the

chain, so-called 1,3- and 1,5-transfer reactions. The higher

yield of trimer agrees with calculations[36,37] that 1,5-

transfer reaction via the formation of a six-membered

transition state is energetically favored. For PS-clay, the

pyrogram is different, although styrene remains the most

abundant product. In addition to regular dimer (2,4-

diphenyl-1-butene) and trimer (2,4,6-triphenyl-1-hexene)

observed in virgin PS, a series of aromatic compounds,

denoted as dimer and trimer derivatives are also observed in

significant amounts at the retention time 20.3–22.3min and

28.9–30.8 min respectively. It appears that these oligomer

derivatives are responsible for the increased absorption of

benzene ring mode at 1 600 cm�1 in the IR spectrum of PS-

clay degradation products (Figure 2). The peak at 8.1 min

has been matched with a-methylstyrene. It should be

stressed that the yield of a-methylstyrene for PS-clay

degradation is abnormally high compared to that of virgin

PS (tiny peak at 8.1min in Figure 4a). It is remarkable that a

similarly large yield of a-methyl styrene has recently been

reported[38] for degradation of intercalated PS-clay com-

posites, when the clay load reaches 5 wt.-%.

Figure 3. Evolution of bands at 2 974, 1 630, 1 600, 989, and 910 cm�1 as a function ofdegradation time. (a) and (b) virgin PS; (c) and (d) PS-clay composite.

Figure 4. Gas chromatography programs of (a) virgin PS and (b)PS/clay at 500 8C in a helium carrier gas. Peaks: 1¼ styrene;2¼ 2,4-diphenyl-1-butene; 20 ¼ dimer derivatives; 3¼ 2,4,6-triphenyl-1-hexene; 30 ¼ trimer derivatives; 4¼ a-methylstyr-ene.



As noticed by a number of workers,[39–42] the trace

amounts of a-methylstyrene are produced by degradation of

virgin PS as a result of intermolecular transfer reactions.

The significant yield of a-methylstyrene in the PS-clay

degradation provides an important evidence of enhance-

ment of inter-molecular reaction in PS brushes as predicted

by our conceptual model. A possible pathway for the for-

mation of a-methylstyrene including inter-molecular rad-

ical transfer is shown as follows.

The key step of this mechanism is the hydrogen transfer

between two different chains (step 1), where amacroradical

(R) abstracts hydrogen atom from anothermolecule (A) and

yields a mid-chain radical (B). The b-scission of (B)

produces an unsaturated chain end (D), which creates

a-methylstyrene radical (E) by a further b-scission. Finally,a-methylstyrene radical (E) converts to a-methylstyrenevia

hydrogen abstraction (step 1). Note that the increased

probability of inter-molecular reaction (step 1) in PS-clay

system follows directly from the decreased inter-chain

distance that is within the amplitude of Kuhn segment

motion. However, for virgin PS, the chains are separated by

a distance larger than their Kuhn segment motion region, so

that a monomer unit has a larger chance to react with

monomer unit of the same chain (backbiting) than with

monomer unit of another chains. That is why virgin PS

chain ismore likely to fold back and abstract hydrogen from

its own chain than from the other chains. But this type of

folding of chain segments in the PS-clay system is

considerably hindered by the closely positioned neighbor-

ing chains, leading to a relative decrease in the yield of

ordinary intra-molecular reaction products, such as dimer

(2,4-diphenyl-1-butene) and trimer (2,4,6-triphenyl-1-hex-

ene). On the other hand, we observe increased yield of

dimer and trimer derivatives that is explainable by intensi-

fication of inter-molecular reactions.

Thermo-Oxidative Degradation under Air

The degradation mechanism of PS in the presence of

oxygen is entirely different from that without oxygen. It is

generally agreed that polystyryl radicals react easily with

oxygen to form peroxy radicals, whose thermal decom-

position can produce numerous oxygenated species. The

evolved gas product analysis (EGA) has confirmed[43] that

the common thermo-oxidative degradation products of PS

include carbon monoxide, carbon dioxide, and oxidative

Scheme 1. Formation of a-methylstyrene via inter-molecular transfer reactions.

Figure 5. Condensed phase IR spectra of virgin PS and PS-claysamples taken at 330 8C (heating rate of 5 K �min�1).



hydrocarbons, such as benzaldehyde, acetophenone, phe-

nol, styrene oxide, and etc. While the identity of degrada-

tion products provides valuable information about

the degradation mechanism, it is yet a great challenge to

derive the degradation pathways which usually involve

multiple steps for most polymeric materials. In addition to

the EGA method, condensed phase FTIR is suitable for

obtaining in-situ information about the change in polymer

structure and the formation of intermediate products that

may not be detectable by EGA. To compare the course of

the degradation in virgin PS and PS-clay, the condensed

phase IR spectra of the degrading materials have been

monitored in-situ at the moderate degradation temperature

(<400 8C).As shown in Figure 5, partially degraded PS-clay at

330 8C displays absorptions of aromatic C-H stretching

above 3 000 cm�1, methylene asymmetric/symmetric

stretching at 2 923/2 850 cm�1, benzene ring modes at

1 600 cm�1 and 1 492 cm�1, out-of-plane C–H bending of

aromatic ring hydrogen at 756 cm�1 and ring bending at

698 cm�1, which are the characteristic absorptions due to

the main chain structure of virgin PS. Meanwhile, some

important difference between PS-clay and virgin PS is

found in the following three regions: (A)� 1 700 cm�1,

(B)� 1 200 cm�1, (C)� 1 000 cm�1. For the purpose of

comparison, the evolution of IR bands in these regions is

recorded as a function of degradation temperature in

Figure 6 and 7. As the temperature increases, an increase

in absorption of carbonyl group at 1 685 cm�1 (indicated as

peak A) is observed. This absorption is usually assigned to

acetophenone structure confirmed by UV and NMR

measurements.[44] To account for the variation of sample

thickness and mass loss during degradation, the intensity of

1 685 cm�1 band and of other peaks in Figure 6 and 7 are

normalized to the strongest peak at 698 cm�1. It is seen in

Figure 7 that from 300 8C and above PS-clay displays

relatively stronger absorption at 1 685 cm�1 than virgin PS

(Figure 6). A similar trend is found in the region (B) around

1 200 cm�1 which is related to the C–O stretch. For PS-clay

samples, a clear absorption peak at 1 260 cm�1 starts to

develop at 320 8C (Figure 7) and continues to grow for the

duration of degradation. In the same temperature range,

virgin PS has a much weaker absorption at 1 260 cm�1 than

PS-clay.

Finally, in the region (C) near 1 000 cm�1, virgin PS

exhibits practically the same absorption intensity whereas

PS-clay shows broadening and intensifying of absorption

with increasing the temperature. We believe the develop-

ment of this absorption is due to clay present in the PS-clay

system. Pure clay (MMT) shows a strong and broad ab-

sorption at 1 040 cm�1 due to the Si–O–Si bond.[19] This

absorption overlaps with the aromatic in-plane C–H

bending band (at 1 028 cm�1) of PS.[45] Since the content

of clay in our nanocomposite is very small (1 wt.-%), the

contribution of clay to the total absorption near 1 000 cm�1

is not significant before degradation, and therefore the IR

spectrum of PS-clay at room temperature appears to be

quite similar to that of virgin PS. Upon heating, PS chains

degraded to low molecular weight and leave the condensed

phase, but clay remains there in the same amount. As a

result, the relative concentration of clay in the nano-

composite increases in the progress of degradation and the

contribution from the Si–O–Si bond becomes more evident

as seen in Figure 7.

Based on the above discussion, it appears that oxidation

reaction is intensified in the PS-clay system since PS-clay

samples show stronger absorptions of oxygen-containing

species at 1 685 cm�1 and 1 260 cm�1 than virgin PS. If this

is the case, the oxidative degradation of PS-clay should

produce more heat than virgin PS. The respective heats of

oxidation have been measured by DSC. The preparation of

sample films has been carefully controlled to make sample

Figure 6. Condensed phase IR spectra of partially degradedvirgin PS as function of degradation temperature.

Figure 7. Condense phase IR spectra of partially degraded PS-clay as function of degradation temperature.



films of comparable thickness and mass. All samples are

first heated from 30 to 200 8C at a rate of 20 K �min�1, and

then followed by a rather slow heating at 2 K �min�1 to

allow more time for diffusion of oxygen.

Our DSC data (Figure 8) demonstrates that exothermic

effect in PS-clay is noticeably smaller than that in virgin PS.

The heat released from the oxidative degradation of PS-clay

is�220 J � g�1 compared with the value of�430 J � g�1 for

virgin PS. The exothermic peak in PS-clay is markedly

shifted to a higher temperature, indicating the oxidation

process in PS-clay system is greatly inhibited. This result

appears to contradict to the FTIR data that PS-clay seems to

be more readily oxidized than virgin PS. The DSC data

clearly show that PS-clay is more resistant to the oxidation

than virgin PS, and call for an alternative explanation of the

FTIRdata.We suggest that the observed greater absorptions

related to the oxygen-containing species are due to the

accumulationeffect.Theoxygenated specieswhichare con-

tinuously generated during degradation may not leave the

condensed phase immediately because of the slow diffusion

motion that controls their escape to the gas phase. There-

fore, the IR spectra reflect the cumulative concentration of

oxygenated products. Note that polymer-clay nanocompo-

sites create a remarkable barrier for diffusion of various

small molecules, such as O2, CO2 and vapor water in many

polymer matrices. A significant decrease in the gas

permeability has been reported for polyimide,[46] poly-

ethylene,[47] poly(methyl methacrylate),[48] poly(vinyl

alcohol)[49] and styrene-butadiene rubber[50] composites

with clay. Since the oxygen-containing species can be chain

fragments withmuch largermolecular weight than O2, their

diffusion in the PS-clay system should be even more hin-

dered, especially in the brush region. This picture is

obviously consistent with our conceptual model that

predicts that strongly correlated chain motion will decrease

the mobility of degradation products. In other words, the

‘‘trapped’’ oxygen-containing species will stay longer in

the PS-clay sample than in virgin PS, leading to the higher

IR absorption. That does not necessarily mean a high extent

of oxidation.

Conclusion

A conceptual model has been discussed that puts the

degradation mechanisms in direct dependence on the

degree of nanoconfinement, which can be estimated in

polymer brush for the nanocomposite as the inter-chain

distance. A significant enhancement of inter-molecular

reactions is predicted for systems in which inter-chain

distance reaches double the size of the Kuhn segment in the

respective polymer. In virgin PS, the average inter-chain

distance is significantly larger than double the Kuhn

segment size that results in negligible yield of inter-

molecular degradation products such as a-methylstyrene. In

PS-clay brush system, the inter-chain distance is compar-

able to double the Kuhn segment size that intensifies the

inter-molecular transfer reactions and dramatically in-

crease the yield of a-methylstyrene. The increased inter-

molecular interactions in PS-clay system also result in slow

diffusion of radicals, oxygen and degradation products

of PS-clay nanocomposite. The condensed phase FTIR

measurements provide evidence of trapping of oxidation

products in the condensed phase of PS-clay, whereas DSC

data indicate a smaller heat of oxidation compared to virgin

PS. The hindered thermo-oxidative degradation observed in

the PS-clay nanocomposite suggests that the flammability

of this system is likely to be reduced due to the smaller

amount of thegenerated heat aswell as due to the slower rate

of generating both heat and volatile flammable products.

Acknowledgements: We thank Dr. Xiaowu Fan for providingthe PS-clay sample and Dr. Tony Gies for the Py-GC-MS analysis.Wewould also like to acknowledgeMettler-Toledo for donation ofthe TGA instrument used in this work.

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