Formation and Thermal Behavior of Polystyrene and Polystyrene/Clay Gels

Full Paper

Formation and Thermal Behavior ofPolystyrene and Polystyrene/Clay Gels

Kai Chen, Ashley N. Baker, Sergey Vyazovkin*

DSC was used to study the thermal behavior of virgin PS as well as of PS/clay nanocompositesof exfoliated brush and of intercalated structure dissolved in carbon disulfide. Compared tovirgin PS, the behavior of the brushmaterial did not show any obvious difference, whereas theintercalated material demonstrated markedlylarger heats of gelation and gel melting. Isocon-versional kinetic analysis of the melting processrevealed that the activation energy was indepen-dent of the conversion for the gels prepared underisothermal conditions, whereas an increase in theactivation energy with the extent of conversionwas observed for the gels prepared under con-tinuous cooling conditions. The melting of thenonisothermally prepared gels apparentlyoccurred alongside with the formation of newgel structures.

Introduction

Gels are formed from polymer solutions (sols) as a result of

crosslinking of macromolecules. A transition from sol to

gel occurs at the point when the extent of crosslinking

becomes such that the solution loses its ability to flow.

When crosslinking occurs at the expense of relativelyweak

intermolecular forces it gives rise to thermoreversible or

physical gels. Such gels can be turned to sols and back to

gels by simply changing the temperature. It has been

reported[1] that solutions of atactic polystyrene (PS) in a

number of solvents can form thermoreversible gels.

Although the gel formation in some solvents has been

questioned by rheological tests,[2] gelation of PS solutions

in carbon disulfide (CS2) is an established experimental

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

Macromol. Chem. Phys. 2008, 209, 2367–2373

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

fact.[3–5] The thermal behavior of PS/CS2 gels has been

studied[6,7] by using differential scanning calorimetry

(DSC) that allowed for determining the heats and

temperatures of the sol/gel (i.e., gelation) and gel/sol

(i.e., melting) conversion. In this work, DSC is applied to

study comparative thermal behavior of PS and PS/clay gels

formed from their respective solutions in CS2. To our

knowledge no systematic studies of the sol/gel and gel/sol

conversion in polymer/clay nanocomposite systems have

yet been conducted. Introduction of nanoparticles is

known to modify a wide variety of processes in polymers

such as glass transition, crystallization, melting and

degradation.[8–12] It is, therefore, quite naturally expected

that the processes of gelation and gel melting can too be

modified in the presence of clay nanoparticles. The

expectation is also supported by the frequently noted

similarities between the processes of gelation and crystal-

lization of polymers. For the latter, introduction of clay

nanoparticles has shown two opposing effects: accelera-

tion that can be linked to nucleating effect of the particles

DOI: 10.1002/macp.200800430 2367

K. Chen, A. N. Baker, S. Vyazovkin

2368

and deceleration that can be associated with slowing

down of polymer chain mobility. Note that crosslinking of

macromolecules by forming crystallites in polymer solu-

tions is considered to be one of the major mechanisms of

gelation even for noncrystallizable polymers such as

atactic PS.[13]

In a previous paper,[14] we have reported on similarities

and differences in the thermal degradation and relaxation

behavior of two PS/montmorillonite clay nanocomposites,

one of which had an intercalated clay structure and the

other an exfoliated clay brush structure. The present work

extends a study of these two nanocomposites to the

thermal behavior of their respective gels and intends to

reveal possible effects of clay on the processes of the sol/

gel and gel/sol conversion. The paper also presents what

appears to be the first application of isoconversional

kinetic analysis[15] to the process of gel melting. The

analysis has been successful in providing physical insights

into a variety of relevant processes,[15] including chemical

crosslinking (curing), crystallization, glass transition, as

well as glass aging,[16] collagen denaturation,[17] gelation

of gelatin[18] and starch[19] solutions.

Experimental Part

Preparation and characterization of the PS/montmorillonite

(MMT) clay nanocomposites used in this study have been

described in detail elsewhere.[20–22] Briefly, the intercalated PS/

clay sample was prepared by a bulk polymerization technique.[20]

The organically modified MMT clay (under commercial name

Cloisite1 10A, containing a surfactant of dimethylbenzyl hydro-

genated tallow ammonium chloride) was used as received from

Southern Clay Product Inc. A radical initiator 2,20-azoisobutyroni-

trile (AIBN) and styrene monomer were mixed with the

organically modified MMT by stirring at room temperature under

flowing nitrogen gas. The bulk polymerization was carried out at

60 8C for 24 h and then at 80 8C for 24 h. The intercalated structure

of the resulting nanocomposites was confirmed by X-ray

diffraction (XRD)[20,23] and transmission electron microscopy

(TEM)[24] for the samples having different clay content. The

resulting material is further referred to as ‘‘intercalated’’ system.

The exfoliated PS/clay nanocomposite containing 1% byweight

of MMT was prepared by a solution surface-initiated polymeriza-

tion (SIP) method.[21,22] A monocationic initiator, (AIBN-analogue

compound with quaternized amine group at one end) was

intercalated inside the gallery of pristine MMT via a cation

exchange reaction in which the cationic end of the initiator was

ionically attached to the negatively charged silicate surface.[21]

The initiator-modified clay particles were mixed with styrene

monomer in THF solvent. At 60 8C, the in situ polymerization

under nitrogen atmosphere was directly initiated from the clay

surface[22] to which the initiators have been attached. In the final

products, the exfoliation of MMT clay and the attachment of the

initiator and PS chains onto the clay surface were demon-

strated[21,22] by XRD, infrared spectroscopy (IR), X-ray photoelec-



tron spectroscopy (XPS) and atomic force microscopy (AFM). The

respective material is further referred to as ‘‘brush’’ system. The

brush system containing 1 wt.-% of clay was compared to the

intercalated system having 5 wt.-% of clay because these two

systems demonstrated significant similarity in a previous

study[14] of the processes of degradation and relaxation. For

comparison purposes, radically polymerized PS (Alfa Aesar) was

used as received and is referred to as ‘‘virgin’’ PS. The Mw of PS in

the intercalated system was determined[14] to be 1.61�105

g �mol�1. The Mw values are comparable to the previously

determined values for the brush system (0.90�105 g �mol�1)

and for virgin PS (1.00� 105 g �mol�1).[22]

High purity carbon disulfide (CS2, Spectranalyzed1, Fisher

Scientific) was used as solvent to make PS solutions. Pure PS and

PS/clay nanocomposites were dissolved in CS2 in hermetically

closed vials at the temperature of 45 8C over period of no less than

24 h to ensure complete dissolution. The concentration of resulting

solutions for all samples is 300 mg �mL�1 or�19 wt.-% (the density

of CS2 is 1.26 g � cm�3 [25]).

The gel formation and its melting behavior for all PS systems

were characterized by heat flux DSC (Mettler-Toledo, 822e).

Temperature, heat flow and tau-lag calibrations were performed

by using indium and zinc standards. The experiments were

conducted in the atmosphere of nitrogen flow (80 mL �min�1). A

typical sample size of PS solutionwas about 60–80mg. All samples

were hermetically sealed in 100 mL Al pans. The absence of leaks

was confirmed by measuring the sample mass before and after

each DSC run. The PS gels were formed under either isothermal or

continuous cooling conditions. Under isothermal condition, the PS

solutions were cooled down to �50 8C at a cooling rate of

10 8C �min�1, then the samples were held at this temperature for

30 min. Since no thermal events were detected during cooling at

10 8C �min�1, it is believed that either no PS gel was formed or it

was produced in a negligibly small amount undetectable by DSC.

That is, PS gels were predominately formed during the isothermal

hold at�50 8C. The gel formation processwas directlymeasured as

an exothermic DSC peak under continuous cooling conditions at a

very slow cooling rate of 1 8C �min�1. At this rate, the PS solutions

were cooled from 50 to �70 8C. Once PS gels were formed, either

under isothermal or continuous cooling condition, they were

immediately heated to 50 8C by using various heating rates: 5, 7.5,

10, 12.5, and 15 8C �min�1. The melting behavior of PS gel was

characterized by the melting peak temperature, the heat of

melting, and the effective activation energy of melting. All DSC

measurements were repeated at least 2 times.

Isoconversional Kinetic Computations

The DSC data on gel melting were subjected to isoconversional

kinetic analysis, for which purpose the effective activation energy,

Ea, was determined as a function of the extent of conversion, a, by

employing the advanced isoconversional method proposed by

Vyazovkin.[26,27] The method affords evaluating a Ea dependence

from a set of n experimental runs performed under different

temperature programs, Ti(t). The effective activation energy is

estimated at any given value of a by finding Ea, which minimizes

the function:

F Eað Þ ¼Xni¼1

Xnj 6¼i

J Ea; Ti tað Þ½ �J Ea; Tj tað Þ� � (1)

DOI: 10.1002/macp.200800430

Formation and Thermal Behavior of Polystyrene and Polystyrene/Clay Gels

where

Fig1 8CHersam

Macrom

� 2008

J Ea; Ti tað Þ½ � �Zta

ta�Da

exp�EaRTi tð Þ

� �dt (2)

In Equation 2, a varies fromDa to 1–Dawith a stepDa, typically

chosen to be 0.02. The integral, J, is calculated numerically by

using the trapezoid rule. Theminimization routine is performed at

each value of a to establish the dependence of the activation

energy on the extent of conversion. The mean relative error of the

Ea values determined in the present study was around 20% in

agreement with the results of the previous analysis[28] of the

method. The advanced isoconversional method has two crucial

advantages over the simpler isoconversional methods of Flynn

and Wall[29] and Ozawa.[30] First, it is capable of treating data

obtained under arbitrary temperature programs that secures its

validity in the case when a preset program is distorted by self-

heating or cooling of a sample. Second, because of performing

integration over small time segments (Equation 2) it eliminates a

systematic error[27] present in the Flynn and Wall and Ozawa

methods when Ea varies significantly with a.

Results and Discussion

Sol-to-Gel Conversion

Figure 1 presents data obtained on cooling of PS sols. It is

seen that in all three systems the exothermic process of

ure 1. Heat release detected by DSC on cooling PS sols at�min�1. Each curve is an average of five measurements.e and in all other figures the heat is given per gram ofple.

ol. Chem. Phys. 2008, 209, 2367–2373

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

crosslinking (sol/gel conversion) starts roughly below

�10 8C and continues down to �60 8C. Unexpectedly, theintercalated and brush samples have shown significantly

different behavior. The DSC peak for the brush sample

appears to be quite similar to that for the virgin PS sample.

There is no obvious difference in the respective heights and

width of the peaks. The integral heat for the brush sample

is only a little smaller (0.4 J � g�1) than that for the virgin

sample (0.5 J � g�1). Compared to the brush sample, the

peak for the intercalated sample is significantly taller and

its area is significantly larger (2.0 J � g�1). This means that

the presence of clay in the intercalated system promotes

the process of sol-gel conversion, whereas in the brush

system it shows no obvious effect. Such a difference is very

surprising because these two materials have previously

shown[14] very similar effects in terms of the thermal

degradation and glass transition behavior. Apparently, the

thermal behavior of these two systems changes in the

presence of the solvent. It should be recalled that the

interaction between PS and clay in these two materials is

of quite different nature. In the brush system the PS chains

are tethered to the clay surface via strong ionic forces. On

the other hand, in the intercalated system the PS chains are

attached to the organically modified clay surface primarily

via much weaker van der Waals forces. Therefore, in the

presence of the solvent some of the PS chainsmay separate

from the clay surface while in solution. As a result, the

nanoconfinement[14] of PS chains diminishes and their

mobility increases. Since the intercalated system has �5

times more clay than the brush system, it should have a

substantial vacant clay surface that may act as a site for

initiation of crosslinking when the temperature is lowered

below the gel temperature, Tgel. It seems quite reasonable

to expect that the dominant effect of intercalated clay

nanoparticles should be promoting the gel formation

because several PS chains anchored to a certain spot on the

clay surface would more readily crosslink than regular

chains in the solution.

However no enhanced crosslinking seems to be

observed in the brush system, in which the PS chains

are strongly bound to the surface. The reason for this is not

only the smaller clay content, but also that the clay surface

is effectively shielded by the dense brushwhose interchain

distance and grafting density are estimated[31] to be about

3.8 nm and 0.07 chains per nm2, respectively. In this

situation, the clay surface could only promote crosslinking

of the chains directly attached to the surface. However, this

effect is likely to be offset by the slow chainmobility due to

nanoconfinement of the PS chains that spreads much

farther away from the clay surface than in the case of the

intercalated system.[14] These reasons can explain why the

presence of clay in the brush system does not show any

obvious accelerating effect as in the case of the inter-

calated system.

www.mcp-journal.de 2369


Figure 3. Heat absorption measured by DSC on 10 8C �min�1 heat-ing of PS gels prepared under isothermal and continuous coolingconditions. The curves for isothermally prepared gels are insidethe dashed rectangle.

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Melting of Gels Formed under Isothermal Conditions

According to rheological data,[4] PS-CS2 sols of a similar

concentration (200 g � L�1) and similar Mw of PS (1.8�105 g �mol�1) gel at Tgel around �17 8C. Similar to the rate

of polymer crystallization,[32,33] the rate of the gel formation

has a negative temperature dependence. That is, cooling a

PS/CS2 sol to a lower temperature leads to faster formation

of the gel.[4] In our study, we have tested three

temperatures (�20, �30, and �50 8C) to which PS/CS2 sols

were cooled and held for a certain time. Heating of the

resulting gels andmeasuring the heat of melting has given

us an idea about the amount of the gel formed. It has been

observed that the rate of the sol/gel conversion at �50 8Cwas markedly faster than at �30 and �20 8C. At the latter

temperature the melting effects have been barely detect-

able. Figure 2 demonstrates the evolution of the heat of

melting and the melting peak temperature of a virgin PS

gel as a function of the time of aging at �50 8C. It is seenthat both parameters continue evolving over a period of

8 h without reaching a plateau (i.e., equilibrium). The

amount of heat detected on melting of a gel aged for 8 h is

�0.25 J per gram of gel that corresponds approximately to

1.3 J per gram of PS. The amount of heat released on

melting of the gels aged for 30 min at �50 8C has been

found sufficient to perform a comparative study ofmelting

of the PS and PS/clay gels.

Figure 3 presents the melting DSC peaks for PS gels

formed after aging for 30 min at �50 8C. It is observed that

the peaks for the brush and virgin PS systems are quite

similar. Compared to these peaks, the peak for the

intercalated system is markedly larger. This result is

Figure 2. Evolution of the heat of melting and the melting peaktemperature in virgin PS gel aged at �50 8C for different periodsof time up to 8 h.



consistent with the fact that the presence of clay in this

system promotes the process of crosslinking (Figure 1).

Being crosslinked to a greater extent the respective gel

should obviously demonstrate a larger heat ofmelting. The

heat of melting in either of the three systems has been

practically independent of the heating rate. Figure 4 shows

a representative set of the melting peaks for the brush

system at five different heating rates. The heating rate

average value of the heat of melting for this system is

Figure 4. Heat absorption measured by DSC on heating of thebrush sample at different heating rates. The numbers by the linesdenote the respective values of the heating rate.

DOI: 10.1002/macp.200800430


Figure 5. Effective activation energies evaluated as a function ofconversion for melting of the gels prepared under isothermalconditions.

�0.10 J � g�1. The same average value has been determined

for the virgin PS system. For melting of the intercalated

system, the heating rate average value is 50% larger, i.e.,

�0.15 J � g�1.

The gel melting data obtained at different heating rates

have been subjected to isoconversional analysis. Figure 5

presents the effective activation energies of the process as

a function of the gel to sol conversion in all three systems.

For each of these systems, the values of Ea are practically

independent of a that suggests that the gel melting

kinetics is limited by a single step pathway. The average

value of Ea for virgin PS system is �88 kJ �mol�1.

It is somewhat larger for the intercalated system

(�100 kJ �mol�1) and slightly smaller for the brush system

(�78 kJ �mol�1). However, these small differences are not

necessarily significant considering that, as mentioned in

above, the average error in Ea reaches 20%.

Figure 6. Heat absorption measured by DSC on heating of thevirgin sample at different heating rates. The numbers by the linesdenote the respective values of the heating rate.

Melting of Gels Formed on Continuous Cooling

The gels formed under nonisothermal conditions (constant

cooling rate 1 8C �min�1) demonstrate a complex melting

behavior which is quite different from that of the gels

formed isothermally. First of all, one can immediately see

(Figure 3) that for any of the three systems the DSCmelting

peaks are significantly broader than those for the

isothermally formed gels. The latter melt in the tempera-

ture range from �40 to �20 8C, whereas the former start

melting at around �50 8C and continue to melt up to 0 8C.As in the case of isothermally formed gels, the intercalated



system shows a larger melting DSC peak than the virgin

and brush systems. On average the heats of melting of the

intercalated systems are about 2 times larger than those of

the virgin systems when compared at respectively similar

heating rates (Figure 6 and 7). This result is expected

considering the enhanced crosslinking process in the

intercalated system (Figure 1). Unexpected is the fact that

the melting peaks for the brush system have been

considerably smaller than those observed for the virgin

system. They have also consistently demonstrated a rather

poor signal to noise ratio because the process rate (i.e.,

intensity of DSC signal) has been quite low.

Another unexpected result is that the height and the

area of the DSC melting peaks have demonstrated a

systematic decrease with the heating rate (Figure 6 and 7).

The expectation has been that the integral heats ofmelting

should be comparable to the integral heats of the sol-gel

conversion in the respective systems (Figure 1). However,

the comparable heats could only be obtained at slow

heating rates around 2 8C �min�1, whereas at progressively

faster heating rates the heats of melting have been

consecutively smaller in all types of gels. We hypothesize

that this phenomenon is associated with the complex

structure of the gels formed on continuous cooling. It is

known that a sol cooled to and aged at a certain

temperature, Ta below Tgel forms a gel whose melting

point is larger than Ta (e.g., see Figure 2). Continuous

cooling of a sol is equivalent to going through a series of

very short aging periods at progressively smaller aging

temperatures. This would result in the formation of

various gel structures, having progressively smaller



Figure 8. Effective activation energies evaluated as a function ofconversion for melting of the gels prepared under continuouscooling conditions.

Figure 7. Heat absorption measured by DSC on heating of theintercalated sample at different heating rates. The numbers bythe lines denote the respective values of the heating rate.

2372

melting temperatures. This type of behavior has been

observed for gelatin gels.[34] When the lower melting point

gel structures start melting on heating, they cannot form a

stable sol because the temperature is below Tgel. That

means that the just formed supercooled sol would tend to

form new gel structures, having a larger melting

temperature. Consequently, the endothermic heat flow

associated with the melting would partially be offset by

the exothermic heat flow related to the formation of the

new gel. Thus, the overall process of the gel melting can

be envisioned roughly as a competition of two pathways:

the gel to sol conversion and the new gel formation.

Because the two pathways are likely to have different

activation energies, a relative contribution of a larger

activation energy pathway to the overall process rate

would increase with increasing temperature. On the other

hand, the temperature of the melting process increases

with increasing the heating rate as can be seen from the

shift in the DSC peaks displayed in Figure 6 and 7.

Therefore, the decrease in the endothermic heat flow

observed upon increasing the heating rate is indicative of

an increasing contribution of the exothermic pathway

associated with the new gel formation. As the overall

endothermic heat flow becomes progressively smaller

with increasing the heating rate, the progressively larger

portion of the overall heat of melting falls below the

detection limit of the DSC instrument.

The fact that the melting occurs via more than one

pathway can be readily detected[15] by using isoconver-

sional kinetic analysis. The application of the latter to the

melting of the intercalated and virgin systems has resulted



in the Ea dependencies shown in Figure 8. Because of the

poor signal to noise ratio the brush system melting data

have been unsuitable for the analysis. As seen in Figure 8,

both systems demonstrate increasing Ea dependencies

that are characteristic[35] of the competing pathways

kinetics. The initial stages of the melting yield effective

activation energy of around 45 kJ �mol�1 that apparently

represents disintegration of the least stable (low melting

temperature gel structures). As the melting proceeds the

effective activation energy of the overall processes quickly

rises which reflects an increasing relative contribution of

the pathway, having larger activation energy. According to

our hypothesis this is the exothermic pathway that leads

to the new gel formation. At any rate, isoconversional

analysis clearly demonstrates that the melting kinetics of

the gels formed on continuous cooling is definitely more

complex than of the gels formed isothermally. The latter

shows the Ea values that are practically independent of a

(Figure 5) which is characteristic of a single pathway

melting process.

Conclusion

Although the gel formation and gel melting in the system

PS/CS2 is accompanied by release and absorption of very

small amounts of heat, DSC has been found suitable for

detecting the effects of clay on the thermal behavior of PS/

clay/CS2 systems. The effect is dependent on the structure

of PS/clay composite. While negligible in the brush system,

the effect has been obvious in the intercalated system that,

DOI: 10.1002/macp.200800430


compared to the virgin PS system, has demonstrated a

significant increase in the heats of both gel formation and

gel melting. Significant differences have been discovered

in the melting behavior of the gels prepared under

isothermal and continuous cooling conditions. The melt-

ing of isothermally prepared gels appears to be dominated

by a single pathway of the gel/sol conversion. On the other

hand, the results obtained for the melting of the gels

prepared under continuous cooling conditions suggest

that the process occurs via competition of the gel/sol

conversion and the formation of new gel structures.

Acknowledgements: Support from the American Chemical SocietyPetroleum Research Fund under grant 46760-AC7 is gratefullyacknowledged, as is support for the SUMR Scholar (A. N. Baker)under grant 46760.01-AC7.

Received: August 25, 2008; Revised: September 24, 2008;Accepted: September 25, 2008; DOI: 10.1002/macp.200800430

Keywords: activation energy; differential scanning calorimetry(DSC); gelation; kinetics; nanoparticles

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Formation and Thermal Behavior of Polystyrene and Polystyrene/Clay Gels

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