Tacticity as a Factor Contributing to the Thermal Stability of Polystyrene

Full Paper

Tacticity as a Factor Contributing to theThermal Stability of Polystyrene

Kai Chen, Kehley Harris, Sergey Vyazovkin*

TGA has been used to study the thermal degradation of PS in atactic (aPS), syndiotactic (sPS),and isotactic (iPS) forms. Thermal stability of sPS is similar to that of aPS, whereas iPS degradesat a notably higher temperature. TGA-FT-IR data do not show any obvious difference in thedegradation products of the three materials. Isoconversional kinetic analysis of non-isothermal TGA data yields somewhat higher activation energy for degradation of iPS thanfor aPS that is found to result in a smaller rate constant foriPS. Another important factor contributing to slowerdegradation rate of iPS is found to be the reaction modelthat has been experimentally determined from isother-mal TGA measurements. The enhanced thermal stabilityof iPS correlates with its hindered molecular mobility thatappears to originate from intramolecular confinement.The hindered mobility has been detected by comparingthe glass transition dynamics in amorphous iPS and aPSsamples. By using multi-frequency temperature modu-lated DSC (TOPEM1) it has been demonstrated that theactivation energy of the glass transition is noticeablylarger for iPS than for aPS that supports the hypothesisof the slowed down molecular mobility.

Introduction

Thermal properties of polymeric materials are largely

determined by the dynamics of the polymer chains. The

chain dynamics can be altered by various treatments such

as crosslinking, plasticizing, filling, drawing, blending and

others. The treatments may either ease or constrain the

motion of the polymer chains, therefore, altering their

conformation compared to that in the virgin non-treated

K. Chen, K. Harris, S. VyazovkinDepartment of Chemistry, University of Alabama at Birmingham,901 S. 14th Street, Birmingham, AL 35294, USAE-mail: [email protected]

Macromol. Chem. Phys. 2007, 208, 2525–2532

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

polymer. In the recent years, polymer nanocomposites

have drawn a great deal of research attention. The

uniqueness of these systems is that they can constrain

the polymer chain motion on the nanometer scale,

creating so-called nanoconfinement. For instance, inter-

calating a polymer chain into 1–5 nm gallery of layered

silicate clay prevents the chain from assuming its natural

conformation of a coil, whose radius of gyration Rg is

typically on the order of 5–20 nm. Molecular dynamics

(MD) simulations[1–4] suggest that nanoconfinement is

also accomplished near the clay surface, where the chains

organize themselves in layers, whose density alternates

being respectively higher and lower than the bulk density.

This nanoconfinement has been linked to the coexistence

DOI: 10.1002/macp.200700426 2525

K. Chen, K. Harris, S. Vyazovkin

2526 �

of fast and slow segmental dynamics revealed by MD

simulations and NMR measurements.[3,4] Nanoconfine-

ment is also accomplished in polymer brushes, whose

bristles are made of uncoiled polymer chains. Our recent

studies[5–9] on a polystyrene (PS)-clay brush system

indicate that average interchain distance in it is about

half that found in regular PS,[8] so that individual chains

experience strong nanoconfinement from their neighbors.

The resulting enhanced interchain interaction correlates

with increases in the activation energy of the glass

transition,[6,7] the temperature and activation energy of

degradation,[5,6] and in the formation of interchain

reaction product a-methylstyrene.[8,9]

The aforementioned types of nanoconfinement are

primarily intermolecular in their nature as they force

interaction of an individual chainwith other chains and/or

with a surface. However, it is possible to confine the

intramolecular motion of individual chains by introduc-

ing, for example, tacticity. Tacticity is known to affect the

polymer chain conformation and dynamics. MD[10] and

Monte Carlo[11] simulations indicate that in the melt

syndiotactic poly(propylene) (PP) adopts conformations,

having larger square end-to-end distance and statistical

segment length than those in isotactic or atactic PP. The

computational predictions agree with small angle neutron

scattering (SANS) measurements[12] and intrinsic viscosity

data.[11] For poly(methyl methacrylate) (PMMA), SANS

measurements[13] demonstrate that the isotactic form has

a larger radius of gyration than the syndiotactic one. For

PS, the solid state 13C NMR and IR spectroscopy indicate[14]

that in the glassy state the syndiotactic form has a larger

content of the trans conformations than the isotactic or

atactic forms.

The tacticity-induced conformational changes are

accompanied by a change in the chain dynamics. MD

simulations and NMR experiments demonstrate an

increase in the rate of the segmental relaxation dynamics

of PP in the row: syndiotactic, atactic, isotactic.[10,15]

Rheological measurements show that syndiotactic PP

exhibits a 10 times higher zero shear rate viscosity[16]

and 10–15 kJ �mol�1 larger activation energy of viscous

flow[16,17] than isotactic PP. Similarly, a larger zero shear

rate viscosity is found in syndiotactic PMMA.[18–20]

Undoubtedly, the segmental dynamics plays an impor-

tant role in the polymer reactivity because it is closely

related to the basic kinetic factors such as the frequency of

collisions and the rate of mass transfer. For this reason,

tacticity can be a factor affecting the thermal degradation

and, therefore, thermal stability of polymers. Surprisingly,

the literature reports very little on the effect of tacticity on

the thermal degradation kinetic of polymers. In an inert

atmosphere, the syndiotactic form of PMMA displays a

higher degradation temperature than the isotactic one[21]

that is attributed to the slower chain mobility of the


2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

former. A study of the thermo-oxidative degradation of PP

in air demonstrates that the syndiotactic form is more

stable than isotactic one.[22]

There have been a few thermal degradation studies of

isotactic[23,24] (iPS) and syndiotactic[25] (sPS) PS. However, no

degradation studies of all three tactic forms under

comparable conditions (i.e., similar molecular weight,

atmosphere, experimental setup) have been performed

and no reliable kinetic parameters have been reported.

Nevertheless, the rotational isomeric state model pre-

dicts[26] iPS and sPS to have a larger intrinsic viscosity

than atactic PS (aPS) that is consistent with the melt

viscosity data[27] obtained for iPS and aPS of the similar

molecular weight. Also, simulations[28,29] suggest that

within the same chain the interactions between nearby

phenyl rings are stronger in iPS than in sPS so that the

former experiences a larger rotational energy barrier and,

thus, larger steric hindrance. Based on these data one can

certainly expect an effect of tacticity on the thermal

degradation of PS. Revealing such effect is the purpose of the

present study that explores the kinetics of the thermal

degradation and relaxation of PS of different tacticity. To

minimize other effects, the present study has been

performed on PS of similar molecular weights and under

a similar set of conditions.

It should be noted that tacticity can only introduce some

limited confinement within short chain segments so that

the resulting conformation of thewhole chain aswell as its

interaction with other chains are not as dramatically

affected as in typical nanocomposites whose properties

originate from the enhancement of intermolecular inter-

actions. Therefore, alteration of the thermal stability due

to tacticity is expected to be smaller than that due to

intermolecular confinement in nanocomposites. Never-

theless, exploring this phenomenon is of fundamental

significance for elucidating the effect of confined chain

dynamics on the thermal stability of polymers.

Experimental Part

Materials

iPS (90% mass fraction of isotactic triads, Mw ¼ 400000) and sPS

(90% mass fraction of syndiotactic triads, Mw ¼300000) were

purchased from Scientific Polymer Products, Inc. A reference aPS

sample (Mw ¼ 410000, Mw=Mn � 1.06) was obtained from Poly-

mer Laboratories Ltd. The polydispersity of the tactic PS samples

was not reported by the supplier, although it can be expected to

have a rather large value for iPS that is typically polymerized by

Ziegler-Natta catalysts. The resulting increase in lower molecular

weight fraction may, in principle, diminish to some extent the

thermal stability of PS. Nevertheless, our preliminary data showed

that the degradation temperatures of iPS are noticeably larger

than those for three different aPS samples (Mw ¼100000,

Mw=Mn � 2.4 from Alfa Aesar, Mw ¼350000, Mw=Mn �2.1 from

DOI: 10.1002/macp.200700426

Tacticity as a Factor Contributing to the Thermal Stability of Polystyrene

Aldrich and Mw ¼410000, Mw=Mn �1.06 from Polymer Labora-

tories Ltd). On the other hand, the three aPS samples barely

showed any difference in their decomposition temperatures that

suggests that the effect of polydispersity is inferior to that of

tacticity. Therefore, even if iPS has a large polydispersity, the

tacticity effect discussed in this paper can only be larger in iPS of a

smaller polydispersity.

Characterization

The thermal degradation kinetics have been measured as the

temperature- and time-dependent mass loss by using thermo-

gravimetric analyzer (Mettler-Toledo TGA/SDTA851e). For non-

isothermal measurements, samples of �5 mg have been placed in

40 mL aluminum pans and heated in the flowing atmosphere of

N2 at a flow rate of 70 ml �min�1, from 25 to 600 8C at the heating

rates of 2.5, 5.0, 7.5, 10.0 and 12.5 K �min�1. The buoyancy effect in

TGA has been accounted for by performing empty pan runs

and subtracting the resulting data from the subsequent sample

mass measurements. For isothermal measurements, the samples

have been heated at the temperature 375 8C until complete

degradation.

The degradation products evolved during the TGA runs have

been analyzed in-situ by IR Spectroscopy (Nicolet Nexus 470 FTIR)

coupledwithMettler-Toledo TGA/SDTA851e. The heating rate was

10 K �min�1, and FT-IR spectrawere collected at 4 cm�1 resolution.

The glass transition (relaxation) kinetics have been determined by

using the recently invented[30] technique of multi-frequency

temperature-modulated differential scanning calorimetry (TOPEM1

DSC 823e by Mettler Toledo). By overlaying a series of stochastic

temperature pulses of different durationwith a temperature rampat

a constant underlying rate, one single TOPEM1 run allows for

determining the frequency dependence of the complex heat capacity

that can be conveniently used to characterize the glass transition

dynamics. To prepare glassy samples for DSCmeasurements,�5mg

of amaterial has been placed in 40mL Al pan, heated to 250 8C (iPS) or

200 8C (aPS), and quenched into liquid nitrogen. The glass transition

has beenmeasured in a heatingmode. The temperature program for

all measurements was defined by superimposing the underlying

heating rate of 0.5 K �min�1 and a series of stochastic temperature

pulses of the 1 K amplitude and the time between the pulses ranging

from 25 s to 60 s. DSC has been calibrated by using Indium and Zinc

standards.

Analysis of the Degradation Kinetics

Because of its complexity, polymer degradation kinetics can rarely

be described by a single-step rate equation[31]

Macrom

� 2007

da

dt¼ kðTÞfðaÞ ¼ A exp

�E

RT

� �f ðaÞ (1)

that holds throughout the whole range of the degradation

temperatures, T, and conversions, a. In Equation (1), t is the time,

T is the temperature, R is the gas constant,A is the pre-exponential

factor, E is the activation energy, k(T) is the rate constant, and f(a)

is the reaction model. As discussed in a review paper,[32]

ol. Chem. Phys. 2007, 208, 2525–2532

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

isoconversional methods provide an effective way of dealing

with complex kinetics of thermally stimulated processes in

polymericmaterials. The underlying assumption of themethods is

that the reaction model is invariant to temperature or heating

rate. By virtue of this assumption, at a constant extent of

conversion the reaction rate is only a function of the temperature:

d ln da=dtð ÞdT�1

� �a

¼ � EaR

(2)

Although Equation (2) takes its origin in the single step rate

Equation (1), Ea is assumed constant only for a certain extent of

conversion and the narrow temperature region related to this

conversion. That is, the isoconversional methods analyze the

degradation kinetics by using a set of Equation (1), each ofwhich is

associated with a certain a and has its respective value of Ea. The

latter has a meaning of the effective activation energy, and its

variation with a and/or T can provide valuable insights into the

mechanism and kinetics of complex processes.[32]

The present study makes use of an advanced isoconversional

method developed by Vyazovkin.[33,34] The method has two key

advantages over the popular methods of Flynn and Wall[35] and

Ozawa.[36] Firstly, it can treat the kinetics, occurring under

arbitrary variation in temperature, T(t) that allows for accounting

of experimentally detected self-heating/cooling. For a series of n

experiments conducted under different programs, Ti(t), the

activation energy is estimated at any given a by finding Ea,

which minimizes the function[33,34]

FðEaÞ ¼Xni¼1

Xnj 6¼i

J½Ea; TiðtaÞ�J½Ea; TjðtaÞ�

(3)

where

J½Ea; TiðtaÞ� �Z ta

ta�Da

exp�EaRTiðtÞ

� �dt (4)

Secondly, performing integration over small time segments

[Equation (4)] eliminates a systematic error[34] found in the Flynn

andWall and Ozawamethodswhen Ea varies considerably with a.

In Equation (4), a is varied fromDa to 1–Dawith a stepDa typically

chosen to be 0.02. The integral, J in Equation (3), is computed by

the trapezoid rule. The minimization procedure is repeated for

each value of a to establish the dependence Ea on a.

The pre-exponential factor has been evaluated by using

so-called artificial isokinetic relationship,[37]

lnAj ¼ c� dEj (5)

where c and d are constants. Twelve pairs of lnAj and Ej have

been determined by the Coats-Redfern method[38] by using

twelve different reaction models.[39] The parameters c and d

have been evaluated by fitting lnAj and Ej data to Equation (5).

Then the isoconversional Ea values have been substituted for Ejin Equation (5) to calculate the corresponding lnAa values.

The reaction model f(a) has been determined independently by

running isothermal TGA measurements at the temperature

www.mcp-journal.de 2527


2528 �

375 8C. At a constant degradation temperature, the shape of the

mass loss curve is only controlled by f(a) because k(T) in

Equation (1) remains constant throughout an isothermal meas-

urement. The time-dependent mass loss is used to calculate the

extent of conversion of degradation as a function of time by

Macrom

2007

a ¼ mi �mt

mi �mf(6)

where mi and mf are, respectively, the sample mass at the initial

and final time of measurement, and mt is the mass measured at

different degradation times. The first derivative of a versus t (da/

dt) plot has been used for curve-fitting to calculate and compare

the reaction model for PS in different tactic forms.
Figure 1. TGA curves for degradation of aPS, sPS and iPS at aheating rate of 2.5 K �min�1 in nitrogen. Inset shows the firstderivative of TGA curves (DTG).
Analysis of the Glass Transition Dynamics

The glass transition is the process of relaxation from the non-

equilibrium glassy state to the equilibrium liquid state. The rate of

the process can be expressed by a first-order kinetic equation[40]

dj

dt¼ �1

tðj� jeÞ (7)

where j and je are respectively the non-equilibrium and equilib-

rium values of an order parameter such as enthalpy, and t is the

relaxation time. For a narrow temperature region, the tempera-

ture dependence of t can be approximated by the Arrhenius

equation

t ¼ A expE

RT

� �(8)

The activation energy in Equation (8) can be determined from

the frequency-dependence of the glass transition temperature (Tg).

The dependence is readily obtained by TOPEM1 DSC thatmeasures

the glass transition at different frequencies, f, as the heat capacity

step, themidpoint ofwhich can be used as an estimate of Tg. Since t

and f are reciprocally related, Equation (8) can be used to determine

the activation energy of the glass transition as follows

E ¼ �Rd ln f

dT�1g

(9)

The TOPEM1 DSC technique has an advantage of evaluating

the activation energy in a single experiment. This eliminates

run-to-run experimental errors unavoidable in a series of single

frequency runs in temperature modulated DSC as well as in a

series of single heating rate runs in regular DSC, which can also be

used for estimating the value of E.[41]

Figure 2. Gas phase IR spectra of degradation products for aPS,sPS and iPS.

Results and Discussion

Degradation

Figure 1 displays TGA curves of three PS samples degraded

in the atmosphere of N2. It shows that themass loss curves

ol. Chem. Phys. 2007, 208, 2525–2532


of sPS and iPS are shifted to a higher temperature

compared to aPS. The shift is the largest for iPS. From

the first derivative of TGA curves, the temperature of a

maximum degradation rate for the sPS and iPS is

respectively found to be 3 8C and 7 8C higher than that

for aPS, indicating that iPS is the most thermally stable

form of PS.

The gas phase degradation products of PS samples of

different tacticities have been compared by performing

in-situ IR analysis. As shown in Figure 2, iPS and sPS

samples display practically the same IR absorption pattern

as their atactic counterpart. The major degradation

product in all three cases is styrene detected by the

absorption bands at 910, 989 cm�1 (––C–H bending) and

1 630 cm�1 (C––C stretch). Note that intermolecular

confinement of aPS in clay nanocomposites gives rise to

an unusual degradation product a-methylstyrene that can

be detected by 2974 cm�1 absorption band.[8,9] The band

does not appear in the spectra of degradation products of

iPS and sPS. Apparently, the chain tacticity does not alter

DOI: 10.1002/macp.200700426


Figure 4. Dependence of the pre-exponential factor (A) on theextent of conversion (a) for the non-isothermal degradation ofaPS, sPS and iPS.

the PS degradation pathway and the respective yield of

degradation products. Since the degradationmechanism of

PS is independent on its tacticity, our following study is

focused on the effect of tacticity on the kinetics of PS

degradation.

The kinetics of the thermal degradation of PS in the

three tactic forms have been further analyzed by using the

advanced isoconversional method. The isoconversional

activation energies of degradation are plotted in Figure 3. It

shows that the effective activation energies (Ea) of all three

PS samples are practically independent of the degradation

conversion that suggests that the overall degradation

process is limited by a single step.[32]

Figure 3 also shows that aPS has an effective activation

energy about 10 kJ �mol�1 lower than sPS and iPS, while

the difference in Ea between iPS and sPS is insignificant.

The values of Ea have been used to calculate the

pre-exponential factor, another important parameter in

Arrhenius law that controls the rate of the degradation

process. As seen in Figure 4, lnAa is not practically

dependent on the conversion, and aPS has the smallest

value of lnAa among the three stereoisomers. The obtained

values of Aa and Ea have been employed to determine the

rate constant of degradation. Since both values are

practically constant during thewhole degradation process,

the respective averages have been used to determine the

k(T) values. The latter are presented in Figure 5 in the form

of the Arrhenius plots. It is seen that of all three forms, iPS

has the smallest rate constant throughout the whole

temperature region of degradation. The result is obviously

consistent with the TGA data (Figure 1) that suggest iPS

being more thermally stable. It is noteworthy that Aa and

Ea make opposing contributions to k(T) for the thermal

degradation of iPS. Compared to aPS, the respective Aa

value is larger for iPS that suggests that degradation of the

latter should be faster. On the contrary, iPS demonstrates

Figure 3. Dependence of the effective activation energy (Ea) onthe extent of conversion (a) for the nonisothermal degradation ofaPS, sPS and iPS.



larger Ea so that its degradation should be slower than that

of aPS. Since the combined effect of these two parameters

results in k(T) being smaller for iPS than for aPS, we can

conclude that an increase in the activation energy makes

the dominant contribution to the enhanced thermal

stability of iPS.

As indicated by Equation (1), the rate of a degradation is

also determined by the reaction model f(a) that has been

independently evaluated from the time-dependence of

mass loss in isothermal TGA measurements. Figure 6

displays that the degradation of iPS reaches a smaller

extent of conversion than aPS at the same degradation

time and temperature, indicating a slower degradation

rate for iPS. This is in agreement with our dynamic TGA

measurements (Figure 1) that demonstrate delayed

degradation of iPS relative to aPS. The inset of Figure 6

further shows that both aPS and iPS display the maximum

Figure 5. Comparison of rate constants [k(T)] for the nonisother-mal degradation of aPS, sPS and iPS.



Figure 6. The time dependence of the extent of conversion (a) forisothermal degradation of aPS and iPS at 375 8C. Inset shows theplots of the rate versus conversion.

2530 �

rate at middle conversion that is characteristic of

autocatalytic processes, whose rate can be described as

FigiPS

Macrom

2007

da

dt¼ kðTÞamð1� aÞn (10)

By fitting the experimental rate curves (inset of Figure 6)

to Equation (10), the reaction models for aPS and iPS have

been determined and compared in Figure 7. It is clearly

seen that for iPS the values of f(a) are smaller than those for

aPS throughout the whole region of degradation. That is,

the decreased value of f(a) also contributes to slowing the

degradation rate of iPS. Putting the above results together,

we can conclude that the tacticity of iPS shows the

degradation rate by decreasing both f(a) and k(T) or, in

other words, by making respectively weaker the depend-

ence of the rate on both conversion and temperature.

The altered degradation kinetics of tactic PS dem-

onstrated in our TGA measurements compliments the

ure 7. The fitted reactionmodels [f(a)¼am (1�a)n] for aPS and.

ol. Chem. Phys. 2007, 208, 2525–2532


previous literature reports[21,22,42,43] addressing tacticity as

factor that can affect the degradation behavior of

polymers. Since the reported stability of tactic PP has

been investigated[22] in the reactive atmosphere of air,

which is fundamentally different from the inert environ-

ment used in the present study, it seems more appropriate

to compare our results with the results of degradation

study of tactic PMMA performed in the inert atmosphere.

The thermal stability of PMMA has been found[21] to

depend on its tacticity, and chain mobility has been

proposed as a key factor controlling the polymer

degradation. In comparison with isotactic PMMA, syndio-

tactic PMMA has more entangled chains with a closer

entanglement spacing (a smaller entanglement molecular

weight).[18,44] The stronger intermolecular interactions,

presumably associated with stronger carbonyl-carbonyl

dipole interactions of ester group in syndiotactic

PMMA,[45] impose a larger confinement on the mobility

of syndiotactic chains.[19] The resulting constrained chain

mobility contributes to the higher thermal stability of

syndiotactic PMMA.[21] It should be noted that in PMMA

the syndiotactic form is more stable than the isotactic one,

whereas in PS the situation is reverse (Figure 1). This,

however, is not very surprising because in PS the isotactic

form experiences larger intramolecular confinement than

the syndiotactic one. The origins of the confined molecular

mobility in tactic PS can be understood from the results of

conformational analysis.[28] It shows that in iPS there are

strong interactions between side phenyl rings along the

same chain. This steric hindrance (intramolecular effect)

due to bulky side groups gives rise to a higher rotational

energy barrier because the skeletal rotations of iPS require

more consecutive monomer units to move cooperatively

than in sPS.[28,29] Since the rotations around chain bonds

are a part of the segmental mobility, an increased

rotational cooperativity in iPS appears to be an important

factor that causes intramolecular confinement of the PS

chain in a wide temperature region, covering both glassy

and melt state.

Relaxation

The glass transition dynamics has been characterized by

applying TOPEM1 multi-frequency DSC. sPS and iPS are

semi-crystalline materials and aPS is an entirely amor-

phous material. A confinement effect of crystalline phase

on themobility of the amorphous phase has been reported

for iPS[46] and other semi-crystalline polymers.[47,48] In

order to identify the effect of tacticity on the PS chain

mobility, the effect of crystalline phase must be removed.

Our glass transition study has focused on iPS, which,

according to the thermal degradation data, demonstrates a

larger effect of tacticity than sPS. Amorphous iPS has been

analyzed in comparison to aPS.

DOI: 10.1002/macp.200700426


Figure 9. Activation energy (E) of the glass transition for iPSdetermined from the frequency-dependence of Tg.

The activation energy (E) of the glass transition has

been evaluated from the frequency dependence of Tg[Equation (9)] determined in TOPEM1 measurements. An

example of a typical TOPEM1 measurement (on iPS

sample) is given in Figure 8. The real part of the complex

heat capacity is evaluated at the frequencies, f, from 10 to

25 mHz. It shows that the glass transition temperature

increases with increasing the frequency. The frequency

dependence of the glass transition temperature is plotted

in Figure 9. The slope of lnf versus 1/Tg plots yields an

activation energy for the glass transition. For iPS the E

value is 504� 7 kJ �mol�1, which is ca. 27 kJ �mol�1 larger

than that for aPS (E¼ 477� 5 kJ �mol�1). The larger E for iPS

indicates that a process of molecular motion in the

isotactic amorphous state encounters a larger energetic

barrier than in the atactic amorphous state, therefore,

making the chain mobility more hindered.

Although the obtained result demonstrates hindered iPS

chain mobility in the transitional glass to liquid state

(lower temperature region), we believe that this effect is

relevant to themelt state (higher temperature region). This

conjecture is supported by the reported viscosity data of PS

which are clearly tacticity-dependent. The rotational

isomeric state model predicts[26] that iPS and sPS have a

larger intrinsic viscosity than aPS. The measured melt

viscosity of iPS is higher than that of aPS when they have

the same molecular weight.[27] By extrapolating the

viscosity data[49] of sPS and aPS to the same temperature,

we find that sPS also displays a larger viscosity than aPS in

the melt state. An increase in the viscosity can signifi-

cantly affect the kinetics of degradation that occurs in

viscous reaction media,[50] because the effective time of a

process, tef, is generally determined by the sum of the

characteristic time of a chemical reaction, tr, and the

characteristic time for reactants to diffuse in the viscous

Figure 8. Frequency dependence of the heat capacity for iPSevaluated by TOPEM1.



medium, t:[51]

tef ¼ tr þ t (11)

In its turn, the characteristic time of diffusion is directly

related to viscosity as follows,[52]

t ¼ 4pa3h

kBT(12)

where h is the viscosity of the medium, a is the molecular

radius, and kB is Boltzmann’s constant. Equation (11) and

(12) suggest that an increase in viscosity can slow down

the overall process by increasing its effective time.

A correlation between the confined chain mobility and

enhanced thermal stability of iPS stresses the importance

of molecular mobility in controlling the reactivity of a

polymer. Our results combined with the results of an

earlier study of stereoregular PMMA systems,[21] suggest

that stereoisomers with lower chain mobility (e.g., iso-

tactic PS and syndiotactic PMMA) should generally posses

higher thermal stability and degrade slower.

Conclusion

The thermal degradation study of PS of different tacticity

has demonstrated that iPS has measurably larger thermal

stability than aPS. No evidence has been found to suggest

that the tacticity of PSmay affect its degradation pathway.

The enhanced thermal stability of iPS reveals itself in the

rate constant and reaction model, the respective values of

which are smaller than those found for aPS. While repro-

ducibly detectable, both effects are fairly small, which is

not unexpected because the experimentally measured

increase in the degradation temperature for iPS is also

relatively small. The relaxation kinetics study has yielded

an activation energywhich is noticeably larger for iPS than



2532 �

for aPS that is indicative of more confined molecular

mobility in the former. The observed correlation between

the confined intramolecular chain mobility and enhanced

thermal stability is consistent with the literature report on

degradation of tactic PMMA. It can be concluded that the

tactic form that forces stronger intramolecular confine-

ment and slower segmental mobility should generally

have larger thermal stability. It is noteworthy that the

effect of intramolecular confinement on degradation and

relaxation of tactic polymers appears qualitatively similar,

albeit quantitatively smaller, to the effect of intermolec-

ular confinement frequently found in degradation and

relaxation studies of polymer nanocomposites.

Acknowledgements: We thank Mettler-Toledo Inc. for donatingthe TGA instrument and loaning multi-frequency TMDSC(TOPEM1).

Received: August 7, 2007; Revised: August 27, 2007; Accepted:August 30, 2007; DOI: 10.1002/macp.200700426

Keywords: confinement; degradation; isotactic; mobility; syn-diotactic

[1] D. Schmidt, D. Shah, E. P. Giannelis, Curr. Opin. Solid StateMater. Sci. 2002, 6, 205.

[2] E. Manias, V. Kuppa, Eur. Phys. J. E 2002, 8, 193.[3] E. Manias, V. Kuppa, D. K. Yang, D. B. Zax, Colloid Surf. A 2001,

187–188, 509.[4] D. B. Zax, D. K. Yang, R. A. Santos, H. Hegemann, E. P. Giannelis,

E. Manias, J. Chem. Phys. 2000, 112, 2945.[5] S. Vyazovkin, I. Dranca, X. W. Fan, R. Advincula, Macromol.

Rapid Commun. 2004, 25, 498.[6] S. Vyazovkin, I. Dranca, X. W. Fan, R. Advincula, J. Phys.

Chem. B 2004, 108, 11672.[7] S. Vyazovkin, I. Dranca, J. Phys. Chem. B 2004, 108, 11981.[8] K. Chen, M. A. Susner, S. Vyazovkin, Macromol. Rapid Com-

mun. 2005, 26, 690.[9] K. Chen, S. Vyazovkin, Macromol. Chem. Phys. 2006, 207, 587.[10] S. J. Antoniadis, C. T. Samara, D. N. Theodorou, Macromol-

ecules 1999, 32, 8635.[11] T. M. Madkour, A. Soldera, Eur. Polym. J. 2001, 37, 1105.[12] T. D. Jones, K. A. Chaffin, F. S. Bates, B. K. Annis,

E. W. Hagaman, M. H. Kim, G. D. Wignall, W. Fan,R. Waymouth, Macromolecules 2002, 35, 5061.

[13] J. M. O’Reilly, D. M. Teegarden, G. D. Wignall,Macromolecules1985, 18, 2747.

[14] T. Nakaoki, M. Kobayashi, J. Mol. Struct. 2003, 655, 343.[15] V. Arrighi, D. Batt-Coutrot, C. H. Zhang, M. T. F. Telling,

A. Triolo, J. Chem. Phys. 2003, 119, 1271.[16] A. Eckstein, C. Friedrich, A. Lobbrecht, R. Spitz, R. Mulhaupt,

Acta Polym. 1997, 48, 41.


2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[17] A. Eckstein, J. Suhm, C. Friedrich, R. D. Maier, J. Sassmann-shausen, M. Bochmann, R. Mulhaupt, Macromolecules 1998,31, 1335.

[18] K. Fuchs, C. Friedrich, J. Weese, Macromolecules 1996, 29,5893.

[19] S. Doulut, P. Demont, C. Lacabanne,Macromolecules 2000, 33,3425.

[20] A. Soldera, Y. Grohens, Polymer 2004, 45, 1307.[21] K. Hatada, T. Kitayama, N. Fujimoto, T. Nishiura, J. Macromol.

Sci., Part A: Pure Appl. Chem. 1993, 30, 645.[22] T. Hatanaka, H. Mori, M. Terano, Polym. Degrad. Stab. 1999,

64, 313.[23] J. Boon, G. Challa, Makromol. Chem. 1965, 84, 25.[24] A. Nakajima, F. Hamada, T. Shimizu, Makromol. Chem. 1966,

90, 229.[25] G. P. Ravanetti, M. Zini, Thermochim. Acta 1992, 207, 53.[26] H. Z. Ma, L. X. Zhang, J. M. Xu, Acta Polym. Sin. 2001, 6, 769.[27] J. Boon, G. Challa, P. H. Hermans, Makromol. Chem. 1964, 74,

129.[28] S. Bruckner, G. Allegra, P. Corradini,Macromolecules 2002, 35,

3928.[29] G. Allegra, Macromol. Symp. 2004, 218, 89.[30] J. E. K. Schawe, T. Hutter, C. Heitz, I. Alig, D. Lellinger, Thermo-

chim. Acta 2006, 446, 147.[31] J. H. Flynn, ‘‘Encyclopedia of polymer science and engineer-

ing’’, Vol. Suppl., Wiley, New York 1989.[32] S. Vyazovkin, N. Sbirrazzuoli, Macromol. Rapid Commun.

2006, 27, 1515.[33] S. Vyazovkin, J. Comput. Chem. 1997, 18, 393.[34] S. Vyazovkin, J. Comput. Chem. 2001, 22, 178.[35] J. H. Flynn, L. A.Wall, J. Polym. Sci., Polym. Lett. Ed. 1966, 4, 323.[36] T. Ozawa, Bull. Chem. Soc. Jpn. 1965, 38, 1881.[37] S. Vyazovkin, Int. J. Chem. Kinet. 1996, 28, 95.[38] A. W. Coats, J. P. Redfern, Nature 1964, 201, 68.[39] S. Vyazovkin, Int. Rev. Phys. Chem. 2000, 19, 45.[40] G. R. Strobl, ‘‘The physics of polymers: concepts for under-

standing their structures and behavior’’, Springer, Berlin, NewYork 1997.

[41] C. T. Moynihan, A. J. Easteal, J. Wilder, J. Tucker, J. Phys. Chem.1974, 78, 2673.

[42] M. Lazzari, T. Kitayama, K. Hatada, O. Chiantore, Macromol-ecules 1998, 31, 8075.

[43] H. Nakatani, S. Suzuki, T. Tanaka,M. Terano, Polymer 2005, 46,12366.

[44] W. Souheng, J. Polym. Sci., Part B: Polym. Phys. 1989, 27, 723.[45] O. N. Tretinnikov, K. Ohta, Macromolecules 2002, 35, 7343.[46] B. Natesan, H. Xu, B. S. Ince, P. Cebe, J. Polym. Sci., Part B:

Polym. Phys. 2004, 42, 777.[47] S. Z. D. Cheng, M. Y. Cao, B. Wunderlich,Macromolecules 1986,

19, 1868.[48] C. Schick, A. Wurm, A. Mohammed, Thermochim. Acta 2003,

396, 119.[49] R. Pantani, A. Sorrentino, Polym. Bull. 2005, 54, 365.[50] S. Vyazovkin, N. Sbirrazzuoli, Macromol. Chem. Phys. 2000,

201, 199.[51] D. A. Frank-Kamenetskii, ‘‘Diffusion and heat transfer in

chemical kinetics’’, Plenum, New York 1969.[52] P. J. W. Debye, ‘‘Polar molecules’’, Dover Publications, New

York 1960.

DOI: 10.1002/macp.200700426

Tacticity as a Factor Contributing to the Thermal Stability of Polystyrene

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