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Polymer Degradation and Stability 94 (2009) 891–897

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Thermal degradation kinetics of polystyrene/cadmium sulfide composites

J. Kuljanin-Jakovljevic, M. Marinovic-Cincovic, Z. Stojanovic, A. Krkljes, N.D. Abazovic, M.I. �Comor*

VIN�CA Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia

a r t i c l e i n f o

Article history:Received 26 January 2009Received in revised form9 March 2009Accepted 10 March 2009Available online 19 March 2009

Keywords:Thermal degradationActivation energyCompositePolystyreneCdS

* Corresponding author. Fax: þ381 11 24 53 986.E-mail address: mirjanac@vinca.rs (M.I. �Comor).

0141-3910/$ – see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.polymdegradstab.2009.03.004

a b s t r a c t

The thermal degradation kinetics of polystyrene/CdS composites were studied by thermogravimetry. Thesamples were heated in nitrogen, with three different heating rates: 5, 20 and 40 �C min�1. We calculatedkinetic parameters using KAS isoconversion method. The results showed that the maximum activationenergy of thermal degradation is achieved for PS/CdS composite with about 10% of the CdS filler. Higherconcentration of CdS in the composite (20%) induced acceleration of the thermal degradation,approaching the rate of degradation of the pure polystyrene matrix.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, polymer composites are widely used in many fieldsof technology. Among them polymers containing semiconductorparticles play a significant role, especially for the manufacturing ofelectronic devices. Inorganic–organic polymer composites haveattracted wide interest, because the addition of inorganic particlesto polymers can enhance conductivity [1], mechanical toughness,optical and catalytic activities. Polymer composites have beenfound successful in many applications, such as organic batteries,microelectronics, non-linear optics and sensors. Therefore it isimportant from a fundamental and practical point of view, tounderstand the effects of the incorporation of particles on thephysical and chemical properties of the composites. There area number of studies devoted to composites where polystyrene (PS)was used as matrix [2–8]. In the present study a simplifiedsynthetic route for preparation of the PS/CdS filled polymer wasused, based on mixing the CdS filler in micrometer size range withthe polymer melt, already developed and described in our previousstudy [4].

We showed, in our previous manuscripts, the dependence ofthermal properties of CdS and Fe2O3/polystyrene composites onfiller concentration as well as morphological and structural char-acterization of filler particles [4,5]. In the scope of this work, ourgoal was to better characterize composites and to examine

ll rights reserved.

influence of the content of inorganic filler (CdS) on the thermaldegradation kinetics of PS/CdS composites using KAS (Kissinger–Akahira–Sunose) isoconversion method [9,10]. Using this method,we calculated activation energies of thermal degradation of pure PSand PS in PS/CdS composites and discuss influence of fillerconcentration on thermal stability and rate of thermal degradation.

2. Experimental

All chemicals used were p.a. grade (Fluka, Akzo Chimie, Merck)and were used without additional purification.

PS/CdS composites were synthesized following procedurespublished elsewhere [4,5]. Briefly, CdS filler particles wereprepared by mixing 500 mL of aqueous solutions at elevatedtemperature (90 �C) containing 7.0�10�2 M Cd(NO3)2 (Merck) and1.0�10�1 M Na2S (Fluka). The precipitate was washed severaltimes with water. In order to make the surface of CdS filler particleshydrophobic 150 mL of castor oil (Akzo Chemie) was added. Thecomposites PS/CdS were prepared by mixing in appropriate ratiothe melt PS and semiconductor filler in a Haake rheometer at200 �C. Mixing was carried out at 32 rpm rotor speed for about10 min. After that, the mixture was shaped into sample sheets0.2 mm thick by compression moulding for 5 min at 180 �C inCarver laboratory press. The pure PS sheets were prepared in thesame manner as the composite in order to obtain samples with thesame thermal history.

The X-ray diffraction (XRD) spectra of the composites wereobtained by using Philips PW 1710 diffractometer.

J. Kuljanin-Jakovljevic et al. / Polymer Degradation and Stability 94 (2009) 891–897892

In order to characterize morphology of the PS/CdS compositeand the mean grain size of the CdS filler particles, samples wereinvestigated by atomic force microscopy (AFM). Measurementswere performed using Quesant Universal SPM instrument oper-ating in non-contact mode in air using Si probes.

The thermal stability of pure PS and the PS/CdS was investigatedby non-isothermal thermogravimetric analysis (TG) using a PerkinElmer TGS-2 instrument. The measurements were conducted atheating rates of 5, 20 and 40 �C min�1 in a dynamic nitrogenatmosphere (flow rate 25 cm3 min�1), in the temperature rangefrom 50 to 600 �C. The average sample mass was about 8 mg. Theoverall activation energies of thermal degradation were calculatedfrom the TG data according to the KAS isoconversion method.

3. Results and discussion

3.1. Structure and morphology

In Fig. 1 the XRD spectrum of PS/CdS is presented. All XRD peaksare assigned to characteristic crystallographic planes indicatingcubic crystalline structure of CdS particles [11]. The XRD peaks arerelatively broad due to the small crystalline domains in theparticles.

The Scherrer diffraction formula was used to estimate thecrystalline domain size (D):

D ¼ kl=b cos q (1)

where k¼ 1 for the CdS cubic structure, l¼ 1.541 Å is the X-raywavelength, b is the most intense peak angular width and q is thediffraction angle. The crystalline domain size was found to beD w 5 nm.

Typical AFM images are presented in Fig. 2. In contrast toamorphous structure usual for PS, the composite is characterizedby a granular one (Fig. 2A and B, respectively). The fine structure ofagglomerates made of spherical particles with 150–300 nm diam-eter is clearly observable (Fig. 2B). Agglomerates of CdS particleswith around 2 mm diameter are presented in Fig. 2C. This value is ingood agreement with SEM results [4].

3.2. Thermal degradation of PS/CdS composites

In order to investigate changes in thermal stability of PS/CdScomposites as well as kinetics of degradation, dynamic thermog-ravimetric measurements were performed. TG curves of pure PS and

Fig. 1. XRD spectrum of the PS/CdS composite.

PS/CdS composites are presented in Fig. 3. It is obvious (Fig. 3) thatthermal degradation of pure PS and PS/CdS composites proceeds inone step, between 250 and 600 �C. Also, it can be seen that residualmasses correspond well with concentration of the CdS filler parti-cles, which means that CdS filler remains unchanged after thermaldegradation. That is in agreement with data obtained by Paul et al.[12] who studied thermal stability of different shapes of CdS parti-cles and found out that all kinds of CdS particles are stable whenexposed to temperatures up to 700 �C, in dynamic nitrogenatmosphere. Due to that fact we normalized TG curves only to the PScontent ðmPSð%Þ ¼ ð100ððmT

com �mCdSÞ=ðm0com �mCdSÞÞÞÞ in

a working temperature range. The obtained conversion curves arepresented in Fig. 4.

The temperatures of the maximum rate of degradation of purePS and PS in composites, obtained from conversion curves, arepresented in Table 1.

From TG curves (Fig. 3) and from conversion curves (Fig. 4) aswell as from Table 1, we can conclude that thermal stability of PS inPS/CdS composites is higher than in pure PS, when TG measure-ments were conducted with lowest heating rate (5 �C min�1),although the change of thermal stability (and Tmax) is not a linearfunction of the filler concentration. Higher heating rates (20 and40 �C min�1) revealed more obviously that the thermal stability ofPS in the composites increases with increasing CdS filler concen-tration up to 10%. PS in PS/CdS (80/20) composite showed lower, oralmost the same, thermal stability as in pure PS. Improved thermalstability of composites with respect to the pure PS can be assignedto partially altered molecular mobility of the polymer chains due totheir adsorption on the surface of the filler particles [13–15]. Also,exfoliated CdS filler particles have significant barrier effect to slowdown product volatilisation and thermal transport during decom-position of the polymer, which assists composites with highthermal stability. Simultaneously, adsorption of polymer chainsonto the surface of CdS filler particles results in restriction ofsegmental mobility and serves to suppress redistribution and chaintransfer reactions [4,5,16–18].

These statements can also be used as explanation for loweringthermal stability of the PS in PS/CdS (80/20) composite. We believethat agglomeration of CdS particles occurred due to their relativelyhigh concentration (20%) in polystyrene matrix, during compositesynthesis. Agglomeration was followed by reduction of free CdSsurface, on which adsorption of the polymer chains can occur.Polymer adsorption on the surface of filler particles is a crucial stepfor explanation of changed thermal stability of polystyrene/semi-conductor composites, as was stated earlier [2,4,5].

3.3. Kinetic analysis

Temperature changes can stimulate a variety of chemical andphysical processes in polymer systems [19]. Important examples ofthese processes include thermal degradation, crosslinking, crys-tallization, glass transition, etc. The overall or macroscopic kineticsof these processes are conveniently measured by using thermalanalysis methods such as thermogravimetry (TGA), differentialscanning calorimetry (DSC), and other techniques [19]. By theirnature the macroscopic kinetics are complex as they includeinformation about simultaneously occurring multiple steps. Dis-entangling macroscopic kinetics presents a certain challenge thatcan only be met by the computational methods that allow fordetecting and treating multi-step processes. Results of the ICTACKinetics Project [20] showed that isoconversion methods areamong the few methods that are up to this challenge. The iso-conversion methods may be best known through their mostpopular representatives, the methods of Friedman [21], Kissinger–Akahira–Sunose (usually denoted as KAS method) [9,10], Ozawa

Fig. 2. Typical AFM micrograph of pure PS (A) and PS/CdS composite (B, C).

J. Kuljanin-Jakovljevic et al. / Polymer Degradation and Stability 94 (2009) 891–897 893

[22] and Flynn and Wall [23]. All four methods were started by theresearchers working in the field of thermal degradation of poly-mers and since then they have been mostly used in polymerkinetics studies. The isoconversion methods require performinga series of experiments at different temperature program rates andyield the values of effective activation energy as a function ofconversion. More often than not, the activation energy (Ea) is foundto vary with the extent of conversion. The full potential of theisoconversion methods has been appreciated as Vyazovkin [24]brought analysis of the Ea dependences to the forefront anddemonstrated that they can be used for exploring the mechanismsof processes and for predicting kinetics. These two features makea foundation of the isoconversion kinetic analysis or so-called‘‘model-free kinetics’’ [25–30].

The overall rate of polymer degradation is commonly describedby the Eq. (2):

da

dt¼ A exp

��Ea

RT

�f ðaÞ (2)

where a is the extent of polymer conversion, t is the time, T is thetemperature, R is the gas constant, A is the pre-exponential factor,Ea is the activation energy – assumed to be constant for a certainvalue of conversion a, and f(a) is the reaction model. Degradation ofpolymers tends to demonstrate complex kinetics that cannot bedescribed only by Eq. (2) throughout the whole temperature region.A simpler alternative is to use a Vyazovkin model-free iso-conversion method. The model-free kinetics method is based on anisoconversion computational technique that calculates the effective

activation energy (Ea) as a function of the conversion (a) ofa chemical reaction, Ea¼ f(a). A degradation process is followed atleast at three different heating rates (b) and the respectiveconversion curves are calculated. Taking the reaction rate pre-sented as f(a), Eq. (2), and dividing by the heating rate b ¼ dT=dtcan be obtained:

da

dt¼ k f ðaÞ0da

dt¼ k

bf ðaÞ (3)

where da/dt¼ reaction rate (s�1); k¼ velocity constant (s�1);a¼ conversion; b¼ heating rate (�C s�1). Substituting k in Eq. (3) byArrhenius equation k ¼ k0 expð�ðEa=RTÞÞ and rearranging gives:

1f ðaÞda ¼ k0

bexp

��Ea

RT

�dT (4)

The integration of Eq. (3) up to conversion a (at temperature T)gives:

Za

0

1f ðaÞda ¼ k0

b

ZT

T0

exp��Ea

RT

�dT (5)

whereR a

0 ð1=f ðaÞÞda ¼ ga. Since Ea/RT in Eq. (5) is much biggerthan 1, the temperature integral can be approximated by:

ZT

T0

exp��Ea

RT

�dTz

RT2

Eaexp

��Ea

RT

�(6)

Fig. 4. Conversion of PS in PS/CdS composite: pure PS (A); PS/CdS (95/5) (B); PS/CdS (90/10) (C); PS/CdS (80/20) (D) as a function of temperature.

Fig. 3. TG curves at different heating rates of pure PS (A) and composite PS/CdS (95/5) (B); PS/CdS (90/10) (C); PS/CdS (80/20) (D).

J. Kuljanin-Jakovljevic et al. / Polymer Degradation and Stability 94 (2009) 891–897894

Table 1Temperatures (�C) of maximum rate of degradation (Tmax) for pure PS and PS inPS/CdS composites.

Heating rate (�C min�1) Composite PS/CdS

(100/0) (95/5) (90/10) (80/20)

5 345 400 440 37320 420 470 508 41740 440 500 528 420

Fig. 5. Dependence of activation energy on conversion (a) for pure PS and PS/CdScomposites.

J. Kuljanin-Jakovljevic et al. / Polymer Degradation and Stability 94 (2009) 891–897 895

Substituting Eq. (6) in Eq. (5), rearranging and taking logarithms,give:

lnb

T2a

¼ ln�

Rk0

Ea;agðaÞ

�� Ea;a

R1Ta

(7)

Eq. (7) is defined as a dynamic equation which is used for thedetermination of the activation energy (Ea) for all conversion (a).This equation was first derived by Kissinger [9] and Akahira andSunose [10], and methods based on this equation can be denoted asKAS methods. For each conversion (a) ln b/T2 is plotted versus 1/Ta,

Fig. 6. Conversion of pure PS (A) and PS in PS/Cd

giving rise to a straight line with slope �Ea,a/R, therefore providingthe activation energy as a function of conversion. The standarddeviation was in the range from 0.02 for pure PS to 0.13 for PS/CdS(90/10) nanocomposite. The activation energies (Ea) for the thermaldegradation process of pure PS and PS in PS/CdS composites arecalculated using data from conversion curves (Fig. 4). Obtainedvalues presented as a function of conversion, are shown in Fig. 5.The activation energies for pure PS are lower than that for degra-dation of PS in all composites for all conversions. The maximumincrease of activation energy of 42 kJ mol�1 was obtained forcomposite PS/CdS (90/10) and 20% conversion.

The largest activation energies of thermal degradation in wholerange of conversion, was obtained for PS/CdS (90/10) composite.Also, the isoconversion method allows complex (i.e., multi-step)processes to be detected via a variation of Ea with a [31]. Conversely,independence of Ea on a is a sign of a single-step process. From theshapes of our curves that describe dependence of Ea on a (Fig. 5) itcan be assumed that thermal degradation of PS is most probablya one-step process.

Conversion curves presented in Fig. 4 can also be presented inthe form of the plots of degree of conversion versus time, as isshown in Fig. 6. a¼ f(t) dependence was obtained using Vyazovkinmodel-free equation [24]:

ta ¼�

b exp��Ea

RT

���1ZTa

0

exp��Ea

RT

�dT (8)

where Ta is an experimental value of the temperature corre-sponding to a given conversion at the heating rate b. Eq. (8) enablescalculating the time at which a given conversion will be reached inan arbitrary isothermal degradation. Solving Eq. (8) for differentconversions, one can predict a dependence of a on t at an arbitrarytemperature [24]. These graphs show comparative curves of purePS and PS in composites, at four sets of temperature: 350, 400, 450and 500 �C. The time needed for degradation of pure PS (Fig. 6A)and PS in PS/CdS (95/5) composite (Fig. 6B), vary considerably asa function of temperature, higher temperatures result in fasterdegradation process of PS. Pure PS thermally degrades fastercompared to PS in PS/CdS (95/5) composite at all temperatures. At350 �C, 10% of pure PS degrades in 5 min, but for degradation of 10%

S (95/5) composite (B) as a function of time.

Fig. 7. The predicted conversion curves as a function of time for pure PS and PS/CdScomposites, considering the reference degradation temperatures of 350 �C.

Table 3Predicted temperatures (�C) of the degradation process for PS in composite PS/CdS(95/5).

Time (min) Conversion (%)

10 20 30 40 50 60 70 80 90

10 641.8 652.9 659.2 663.9 668.4 671.7 675.3 681.3 689.620 610.4 620.9 626.8 631.4 635.5 638.3 641.6 646.1 652.330 593.4 603.6 609.3 613.9 617.7 620.3 623.4 627.2 632.340 581.9 591.9 597.5 602.0 605.7 608.2 611.1 614.4 618.850 573.2 583.1 588.6 593.1 596.7 599.1 601.9 604.8 608.860 566.4 576.2 581.5 586.0 589.5 591.8 594.6 597.2 600.870 560.7 570.4 575.7 580.2 583.6 585.8 588.5 591.0 594.280 555.9 565.5 570.7 575.2 578.5 580.7 583.4 585.6 588.690 551.7 561.2 566.4 570.9 574.1 576.3 578.9 581.0 583.8100 548.0 557.5 562.6 567.0 570.3 572.4 575.0 576.9 579.5110 544.7 554.1 559.2 563.6 566.8 568.9 571.5 573.3 575.7120 541.8 551.1 556.2 560.6 563.7 565.8 568.3 570.0 572.3

J. Kuljanin-Jakovljevic et al. / Polymer Degradation and Stability 94 (2009) 891–897896

of PS in PS/CdS (95/5) composite, time longer than 15 min isnecessary, as can be seen in Fig. 6. The predicted conversion curvesas a function of time for pure PS and PS in PS/CdS composites with5%, 10% and 20% CdS considering the reference degradationtemperatures of 350 �C, are presented in Fig. 7. These results shouldbe used only for a qualitative discussion of the thermal stability ofinvestigated composites.

Data presented in Fig. 7 indicate that the rate of degradationdepends on concentration of CdS filler particles. PS/CdS (95/5) andPS/CdS (90/10) composites lose 20% of PS after 20 and 60 min at350 �C, respectively, but PS/CdS (80/20) composite loses 20% of PSafter 6 min, almost the same like pure PS. This effect was expectedconsidering temperatures of the maximum degradation rate pre-sented in Table 1 and variation of the activating energies withrespect to filler concentration in the composite. Already proposedmechanism of aggregation of the CdS particles during synthesis ofPS/CdS (80/20), induced lower activation energies and fasterthermal degradation compared to other composites.

Also, it was possible to predict the temperature of the degra-dation for PS and PS in composites, obtained by model-free data,providing an estimation of the time required for the degradationreaction as summarized in Tables 2 and 3, respectively (althoughthese data should be used only for qualitative discussion). It can beobserved, for instance, that for pure PS to achieve 40% of degra-dation, it is necessary for it to be exposed at 610 �C for a period oftime of 10 min; whereas for the PS/CdS (95/5) composite, to achievethe same degradation in the same time, it must be exposed toa temperature of 664 �C. These results show the potential of the

Table 2Predicted temperatures (�C) of the degradation process for pure PS.

Time (min) Conversion (%)

10 20 30 40 50 60 70 80 90

10 586.4 598.7 604.7 609.8 613.7 616.9 618.3 625.4 631.620 552.6 564.9 571.4 576.6 580.4 583.6 586.9 591.7 596.730 534.6 546.9 553.5 558.7 562.5 565.8 570.0 573.6 578.040 522.6 534.7 541.5 546.7 550.5 553.8 558.6 561.4 565.450 513.6 525.7 532.6 537.8 541.5 544.8 550.0 552.3 556.060 506.4 518.6 525.5 530.7 534.4 537.7 543.2 545.1 548.670 500.6 512.6 519.6 524.8 528.5 531.8 537.6 539.1 542.580 495.6 507.6 514.7 519.8 523.5 526.8 532.8 534.1 537.390 491.3 503.3 510.3 515.5 519.2 522.5 528.7 529.7 532.8100 487.5 499.5 506.6 511.7 515.4 518.7 525.0 525.8 528.8110 484.1 496.1 503.2 508.3 512.0 515.3 521.8 522.4 525.3120 481.1 493.0 500.1 505.3 508.9 512.2 518.8 519.3 522.1

isoconversion method in the prediction of time and temperaturesnecessary for the degradation of polymers and other organicsubstances [32].

4. Conclusions

The thermal stability of composites PS/CdS with variouscompositions has been investigated by non-isothermal thermog-ravimetric analysis with different temperature programs.Improvement of the thermal stability of the PS in composites withrespect to the pure PS matrix is demonstrated for all compositions.The kinetics of the degradation of composites and pure PS matrixhave been characterized by calculating activation energies usingthe KAS isoconversion method from experimental thermogravi-metric data. The thermal degradation activation energies ofcomposites were higher than activation energies of pure poly-styrene. The model-free kinetics applied in this investigation hasproven to be a useful evaluation tool in the study of the thermaldegradation process of PS with and without CdS filler. The ther-mogravimetric study provides an important link between thecharacteristic thermal degradation temperatures and fillerconcentration in composites.

Acknowledgement

The authors are grateful to Dr Zorica Kacarevic-Popovic foruseful comments. Financial support for this study was granted bythe Ministry of Science and Technological Development of theRepublic of Serbia (Project No. 142066).

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