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Materials Chemistry and Physics 133 (2012) 87– 94

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

ynthesis and thermal properties of polystyrene-graft-PEG copolymers as newinds of solid–solid phase change materials for thermal energy storage

hmet Sarı ∗, Cemil Alkan, Alper Bic erepartment of Chemistry, Gaziosmanpas a University, 60240 Tokat, Turkey

r t i c l e i n f o

rticle history:eceived 31 May 2011eceived in revised form8 November 2011ccepted 21 December 2011

eywords:hemical synthesisolymershermal properties

a b s t r a c t

A series of polystyrene-graft-PEG6000 copolymers were synthesized as new kinds of polymeric solid–solidphase change materials (SSPCMs). The synthesized SSPCMs storage latent heat as the soft segmentsPEG6000 of the copolymers transform from crystalline phase to amorphous phase and therefore theycan keep its solid state during the phase transition processing. The graft copolymerization reactionbetween polystyrene and PEG was verified by Fourier transform infrared (FT-IR) and 1H NMR spec-troscopy techniques. The morphology of the synthesized SSPCMs was characterized by polarizationoptical microscopy (POM). Thermal energy storage properties, thermal reliability and thermal stabilityof the synthesized SSPCMs were investigated by differential scanning calorimetry (DSC) and thermo-gravimetric (TG) analysis methods. The DSC results showed that the synthesized SSPCMs had typical

◦

ifferential scanning calorimetry (DSC) solid–solid phase transition temperatures in the range of 55–58 C and high latent heat enthalpy inthe range of 116–174 J g−1. The TG analysis findings showed that the synthesized SSPCMs had highthermal durability above their working temperatures. Also, thermal conductivity measurements indi-cated that the synthesized PCMs had higher thermal conductivity compared to that of polystyrene. Thesynthesized polystyrene-graft-PEG6000 copolymers as new kinds of SSPCMs could be used for thermal energy storage.

. Introduction

Thermal energy can be stored as a change in internal energy of aaterial as sensible heat, latent heat and thermochemical or com-

ination of these. Latent heat storage is one of the most efficientethods used for thermal energy storage. Compared with otherethods, by this method much higher amount of energy can be

tored and released at almost constant temperature. Phase changeaterials (PCMs) have been used as latent heat storage materials

ue to their some advantages such as high heat-storage efficiency,emperature stability, and easy of control in the phase change pro-ess [1–4]. They have potential applications for energy storagend temperature control and can be used in such as solar energytoring, smart air-conditioning buildings, agricultural greenhouse,emperature-regulating textiles, heat management of electronics,elecommunications and microprocessor equipment, biomedicalnd biological-carrying systems and so on [5–10]. In recent years,ecause of increasing energy cost and environmental problems, a

reat number of organic, inorganic, polymeric, and eutectic PCMsave been studied [11–16].

∗ Corresponding author. Tel.: +90 3562521616; fax: +90 3562521285.E-mail address: asari@gop.edu.tr (A. Sarı).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.12.056

© 2011 Elsevier B.V. All rights reserved.

PCMs can be classified into three groups according to their phasechange states: solid–solid PCMs, solid–liquid PCMs, and liquid–gasPCMs. Among these phase change types solid–solid phase changematerials (SSPCMs) are major focus of attention. The SSPCM absorbsthermal energy in latent heat form when it transforms from asolid crystalline phase to another crystalline phase or amorphousphase. Solid–solid PCMs include inorganic, organic and polymer-based SSPCMs. Among all these materials, polymer-based SSPCMsare more attractive because of the advantages of no leakage, noliquid or gas generation, small volume change, smaller corrosioneffect on the devices and relatively longer service life among oth-ers, no receptacle needed to seal them, easily being processed intoexpected shape, even being used as a system material directly andthus being required simple and cost effective fabrication proce-dures [17,18]. However, there are several shortcomings in mostof the SSPCMs reported in previous studies, e.g., higher transi-tion temperature, lower transition enthalpy and unstable thermalproperty compared to solid–liquid PCMs. All these disadvantagesseriously limit their applications. In recent years, several investi-gations are focused on developing new materials to overcome theabove-mentioned defects [19–21].

The physical and chemical methods have been usually usedto prepare polymeric SSPCMs. In the physical method compositePCMs are obtained by dispersing PCMs into higher melting point-polymeric matrix acting as supporting materials; as long as the

88 A. Sarı et al. / Materials Chemistry a

tasitlhflas

wpmrtcoc[[gpaaldg

mtapdpnipccatmsm

fotwtaata

Fig. 1. The reaction schema regarding with the bromination of polystyrene.

emperature is below the melting point of the supporting materi-ls [22–24]. The main problem encountered in this method is phaseegregation during repeated thermal cycles due to dispersing PCMn polymer matrix by physical interaction. In the chemical methodhe SSPCMs are fabricated by chemical grafting, blocking and cross-inking copolymerization reaction between the solid frameworks asard segment and PCMs as soft segment in monomer or polymer

orm. The solid–solid functional polymer storages a great deal ofatent heat as it passing from crystalline phase to amorphous phaset constant temperature. During phase transition process the wholeystem always remains in the solid state.

Poly(ethylene glycols) (PEG) is well-defined macromoleculeith good characteristics such as nontoxicity, good biocom-atibility, biodegradability, hydrophilicity, and ease of chemicalodification. Moreover, PEGs are promising PCMs because of its

elatively large fusion heat, congruent melting behavior, resistanceo corrosion, and wide melting-temperature range [25]. The phase-hange characteristics of PEG depend on their molecular weightf PEG [26]. Recently, different PEG based-copolymers such asellulose-graft-poly(ethylene oxide)[27], cross linking PEG/MDI/PE18], polyurethane-graft-PEG [17,20], cellulose diacetate-graft-PEG21,22,28], chlorinated polypropylene-graft-PEG [29], cellulose-raft-PEG [30,19,31,32], poly(vinyl alcohol)-graft-PEG [33] andolyethylene terephalate-PEG [34] were prepared as SSPCMsnd characterized their thermal energy storage characteristics. Inbove-mentioned studies the experimental results showed that theatent heats and phase transition temperatures of the SSPCMs wereepended on the molecular weight and weight percentages of PEGrafted to the polymer chain.

Polystyrene is one of the aromatic polymers and renewable rawaterials manufactured from petroleum by the chemical indus-

ry. It is cheap and therefore many commercial applications suchs disposable cutlery, plastic models, smoke detector housings,acking materials, insulation, and foam drink cups have beeneveloped for polystyrene [35,36]. The chemical modification ofolystyrene has continued to attract much attention due to theumerous applications of functionalized resins in areas as varied as

on exchange [37], peptide synthesis [38], and other polymer sup-orted reactions [39]. Thus, the application fields of polystyrenean be extended via its modification by using PEG at differentoncentration levels. The obtained copolymers can be considereds potential solid–solid PCMs for thermal energy storage applica-ions. However, the best of our knowledge, the preparation, crystal

orphology, thermal energy storage properties and mechanism ofolid–solid phase transition of polystyrene-graft-PEG6000 copoly-ers have never been reported.In this study, polystyrene-graft-PEG6000 copolymers with dif-

erent amount of PEG6000 content were synthesized as new kindsf polymeric SSPCMs and characterized by FT-IR and NMR spec-roscopy methods. The morphology of the synthesized copolymersas investigated by using POM analysis. Thermal properties and

hermal stability of the SSPCMs were determined by DSC and TG

nalysis. Thermal reliabilities of the SSPCMs were also determinedfter 5000 heating/cooling cycles. In addition, the thermal conduc-ivities of the SSPCMs were measured by using a thermal propertynalyzer.

nd Physics 133 (2012) 87– 94

2. Materials and methods

2.1. Materials

Polystyrene used as hard segment, was purchased from AldrichCompany. PEG6000 was used as soft segment in preparation ofSSPCMs and obtained from Alfa Aesar Company. Bromine and N,N-dimethyl formamide (DMF) were obtained from Merck Company.Analytical grade toluene and chloroform were purchased fromMerck Company and they were used as solvent without furtherpurification.

2.2. Synthesis of the polystyrene-graft-PEG6000 copolymer PCMs

2.2.1. Bromination of polystyreneIn order to increase the reactivity of polystyrene in the copoly-

merization reaction it was firstly reacted with bromine whichhave high reaction ability in especially substitution reactions. Forthis aim, the bromination reaction of polystyrene was carried outin a 500 mL three-neck round-bottomed flask equipped with amechanical stirrer and a thermometer. The flask was filled with thecalculated amount of polystyrene and bromine (the mass ratio ofpolystyrene/bromine is 1/1) was solved in 80 mL methylene chlo-ride and the reaction flask was kept in oil bath at 50 ◦C for 5 h. Thereaction is catalyzed by iodine (the mass ratio of bromine/iodineis 10/1). After the reaction was completed, the methylene chlorideused as solution was removed by rotary evaporation. The reactionschema regarding with the bromination of polystyrene is shownin Fig. 1. The reaction method was repeated for the synthesis ofgrafted polystyrene by bromine at the mass ratio of 2/1 and 4/1.

The synthesized product, polystyrene-graft-bromine was char-acterized by 1H NMR spectroscopy method. Fig. 2 shows thecharacteristic 1H NMR peaks of the polystyrene and polystyrene-graft-bromine. When the peaks related to the protons of the phenylgroup of polystyrene chain was observed at 7.11 ppm (labeledas a + c) and 6.53 ppm (labeled as b) they shifted to 7.28 ppm(labeled as c) and 6.38 ppm (labeled as b) in case of polystyrene-graft-bromine, respectively. This result clearly indicated that thebromination reactions of polystyrene were successfully completed.

2.2.2. Synthesis of polystyrene-graft-PEG6000 copolymersIn this study, polystyrene-based SSPCMs were prepared by using

graft polymerization method. The synthesis scheme of polystyrene-graft-PEG copolymer is shown in Fig. 3. The reaction was carriedout in a 500 mL flask equipped with a mechanical stirrer and athermometer. The flask was filled with the calculated amount ofpolystyrene-graft-bromine, PEG6000 and N,N-dimethyl formamide(DMF) (the mass ratio of polystyrene-graft-bromine/PEG6000 is1/1). The reaction mixture was maintained for 6 h at 140–150 ◦Cby using oil bath. The copolymerization reaction was catalyzedby cupper dust (Cu) and potassium hydroxide (the molar ratioof polystyrene-graft-bromine/Cu/KOH is 5/1/1). After the reac-tion was completed, the product was filtered and dried in fumehood. The same experimental procedure was also repeated byusing different mass ratios of polystyrene-graft-bromine/PEG6000as 2:1 and 4:1. In the rest of the text, the synthesized copolymers,polystyrene-graft-PEG6000 (mass ratio: 4:1) and polystyrene-graft-PEG6000 (mass ratio: 2:1) polystyrene-graft-PEG6000 (mass ratio:1:1) were named as SSPCM1, SSPCM2 and SSPCM3, respectively.

2.3. Characterization

The chemical characterization of the synthesized copolymerswas performed by using FT-IR technique. The FT-IR spectra ofPEG6000 and synthesized polystyrene-graft-PEG6000 copolymersas SSPCMs were taken on a KBr disk at the wavenumber range

A. Sarı et al. / Materials Chemistry and Physics 133 (2012) 87– 94 89

rene and the brominated polystyrene.

otm

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Si(tac

uEasou

3

3

u

Fig. 2. 1H NMR spectra of polysty

f 4000–400 cm−1 by using JASCO 430 model FT-IR spectropho-ometer. POM analyses were performed on a Leica DM EP model

icroscope equipped with a video camera.Thermal properties of the fabricated SSPCMs such as solid–solid

hase transition temperatures and enthalpies were measured bySC analysis technique (Perkin Elmer Jade, DSC). The DSC analysesere carried out at 5 ◦C min−1 heating rate under a constant stream

f nitrogen at a flow rate of 60 mL min−1. Reproducibility was testedy repeating measurements for three times.

In order to determine thermal reliability of the fabricatedSPCMs, an accelerated thermal cycling test consisted of 5000 heat-ng/cooling processes was performed by using a thermal cyclerBIOER TC-25/H model). The changes in thermal properties of syn-hesized SSPCMs after thermal cycling were measured using DSCnalysis again. The chemical stability of the SSPCMs after thermalycling test was also investigated by FT-IR analysis.

Thermal stability of the fabricated SSPCMs was investigated bysing TGA method. The TG analysis was performed by using Perkin-lmer TGA7 model instrument. The measurements were carried outt temperature range of at heating rate of 10 ◦C min−1 and undertatic air atmosphere. Furthermore, thermal conductivity valuesf the fabricated SSPCMs were measured at room temperature bysing KD2 thermal property analyzer.

. Results and discussion

.1. FT-IR spectroscopy analysis of synthesized SSPCMs

The synthesized SSPCMs were characterized structurally bysing 1H NMR and FT-IR spectroscopy methods. Fig. 4 and Table 1

Fig. 3. The reaction schema regarding w

Fig. 4. The FT-IR spectrum of PEG6000 and the brominated polystyrene.

show the FT-IR analysis results of PEG6000 and polystyrene-graft-bromine products. As can be seen from the spectra, there are notpresent the vibration bands of C O and OH groups in the spectrumof polystyrene-graft-bromine whereas the PEG6000 has stretchingvibration peak of the OH group at 3463 cm−1 and C O group

at 1108 cm−1. On the other hand, the FT-IR spectrum of SSPCM1,SSPCM2 and SSPCM3 were shown in Fig. 5. It can be seen thatthe vibration bands of C O and OH groups were observed at1106 cm−1 and 3473 cm−1 for SSPCM1, 1108 cm−1 and 3401 cm−1

ith the synthesis of the SSPCMs.

90 A. Sarı et al. / Materials Chemistry a

Table 1FT-IR spectra of PEG6000, the brominated polystyrene and polystyrene-graft-PEG6000

copolymers.

v(C O) v(C H) v(OH)

PEG6000 1108 2883 3463P(S-BrS)(4:1 wt%) – 2929 –P(S-BrS)(2:1 wt%) – 2929 –P(S-BrS)(1:1 wt%) – 2929 –SSPCM1 1106 2879 3473SSPCM2 1108 2883 3401SSPCM3 1108 2883 3428

ftbc

ip

SSPCMs to amorphous phase. Thus, the groups of PEG of SSPCMs in

Fig. 5. FT-IR spectrum of synthesized SSPCMs.

or SSPCM2 and 1108 cm−1 and 3428 cm−1 for SSPCM3, respec-ively. This result verified the grafting copolymerization reactionetween polystyrene-graft-bromine and PEG6000 were successfullyompleted.

The 1H NMR spectra of the PEG6000 and SSPCM3 were shownn Fig. 6 as an example. The appearance of peaks related to therotons of phenyl group of the copolymer labeled as b and c at

Fig. 6. 1H NMR spectra of P

nd Physics 133 (2012) 87– 94

7.02 ppm and 7.27 ppm means that the PEG molecules bondedaromatic rings of the polystyrene. Moreover, the alkyl protons ofPEG6000 resonanced at 4.73 ppm (labeled as a) and the alkyl protonsof PEG6000 molecules (labeled as a) grafted to polystyrene chainwere resonanced at 3.62 ppm. All these results showed that thegraft polymerization reaction between the polystyrene and PEG6000were carried out successfully. The similar 1H NMR spectral findingswere found for the other synthesized SSPCM1 and SSPCM2.

3.2. Morphology analysis of synthesized SSPCMs

The synthesized SSPCMs are copolymers consisted ofpolystyrene backbone as hard segment and PEG units as softsegment. In thermo-mechanical processes, the PEG units with lowmelting temperature are incompatible with the polystyrene thathave high melting temperature. This leads to the microphase sepa-ration with increasing temperature which results in the forming ofdomain structure. The soft segments (PEG units) are dispersed inthe hard segments (polystyrene) that acts cross-linking. However,the existence of this hard segments has negatively affected theforming of PEG’ crystal structure.

Fig. 7 presents POM images of PEG6000 and SSPCM1, SSPCM2and SSPCM3. A typical spherulite crystalline morphology with anobvious cross-extinction pattern was observed at room tempera-ture for PEG6000 (Fig. 7a). The spherulite crystalline morphologywas also observed from the POM images of SSPCM1, SSPCM2and SSPCM3 below their solid–solid phase change temperatures(Fig. 7 b, d, and f). However, the spherulites were much smallerthan those of pure PEG6000. While the samples were heated untiltheir phase-transition temperatures, no change in the morphol-ogy of the SSPCMs was seen. When they were heated abovetheir phase-transition temperatures, the spherulite structure (crys-talline phase) of the soft segments of the copolymers (PEGsegments) disappeared rapidly (Fig. 7c, d, and g). This process canbe also considered as transformation of crystal phase of PEG into

amorphous phase can only vibrate and rotate but they cannot trans-late freely because the crystalline perfection of SSPCMs has beendestroyed. However, the synthesized SSPCMs keep their solid states

EG6000 and SSPCM3.

A. Sarı et al. / Materials Chemistry and Physics 133 (2012) 87– 94 91

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dda

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ig. 7. POM images of (a) PEG6000 (25 ◦C), (b) SSPCM1 below the solid–solid phase cSPCM2 above the phase change temperature (25 ◦C), (e) SSPCM2 over the phase chg) SSPCM3 above the phase change temperature (70 ◦C).

uring this phase transition process due to the high mechanicalurability of polystyrene backbone although the working temper-ture is above their phase transition temperatures.

.3. Thermal properties of the synthesized SSPCMs

Thermal properties of the synthesized SSPCMs such asolid–solid phase change temperature for heating processTs–s, heating (◦C)), solid–solid phase change temperature for coolingrocess (Ts–s, cooling (◦C)), phase change enthalpy for heating process�Hs–s (J g−1)) and solid–solid phase change enthalpy for coolingrocess (�H (J g−1)) were measured by using DSC analysis

s–s, coolingethod. The DSC curves of the SSPCMs were shown in Fig. 8 and the

hermal properties obtained from these curves were also summa-ized in Table 2. As can be seen from Table 2, there are considerable

Fig. 8. DSC curves of PEG6000 and the synthesized SSPCMs.

temperature (25 ◦C), (c) SSPCM1 above the phase change temperature (70 ◦C), (d) temperature (70 ◦C), (f) SSPCM3 below the phase change temperature (25 ◦C), and

differences between the solid–solid phase transition temperaturesof the SSPCMs obtained for heating and cooling processes. It wasdue to the subcooling behavior of PEG6000 groups into SSPCMs.The similar results were found the polyurethane block copolymercomposed of PEG10000 [17,20].

On the other hand, the latent heat values of the copolymer PCMsmeasured by DSC for their heating and cooling process were alsosummarized in Table 2. As can be seen from this table, phase tran-sition enthalpies of the copolymers increased with the PEG6000content. That is, the concentration of the crystalline domainsincreased with the mass percentages of PEG segments. The latentheat values were found to be 111.48, 130.98, 179.4 kJ kg−1 forheating processes and 99.43, 115.07, 160.44 kJ kg−1 for cooling pro-cesses of SSPCM1, SSPCM2 and SSPCM3, respectively. Moreover, theenthalpies of the heating process and the reverse process are close.However, there is a slightly decrease in latent heat capacity of thecopolymers for their cooling processes. This phenomenon observedduring the cooling processes simultaneously agrees with the previ-

ous results found by many researchers [17,18,20]. It can be due tothe fact that the hard segment in copolymer can destroy the perfec-tion of the crystallization of PEG in the structure. So the defects of

Table 2Thermal properties of PEG6000 and the synthesized SSPCMs.

Thermal properties

Ts–s, heating

(◦C)�Hs–s, heating

(J g−1)Ts–s, cooling

(◦C)�Hs–s, cooling

(J g−1)

PEG6000 59.80 221.31 36.20 155.5SSPCM1 58.04 111.48 38.48 99.43SSPCM2 56.50 130.98 37.39 115.07SSPCM3 44.95 179.47 34.36 143.6

92 A. Sarı et al. / Materials Chemistry and Physics 133 (2012) 87– 94

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3

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Table 3Thermal properties of the synthesized SSPCMs after 5000 thermal cycling.

Thermal properties

Ts–s, heating

(◦C)�Hs–s, heating

(J g−1)Ts–s, cooling

(◦C)�Hs–s, cooling

(J g−1)

SSPCM1 55.83 116.22 37.32 103.71SSPCM2 58.35 113.82 36.01 117.09SSPCM3 56.44 174.18 39.66 157.91

Table 4TG analysis data of polystyrene, PEG6000 and the synthesized SSPCMs.

Degradation interval (◦C) Mass loss (%wt)

Polystyrene 341–441 98.53PEG6000 321–456 97.83

SSPCM1, SSPCM2 and SSPCM3 measured as 0.06, 0.14, 0.10, 0.11

Fig. 9. DSC curves of the synthesized SSPCMs after thermal cycling.

rystal lattices may be lead to the low transition temperature andatent heat during cooling process.

Based on the DSC data in Table 2 it can be also concluded thathe SSPCMs have reasonable latent heat values, and the suitablehase transition temperature points for several thermal energytorage applications such as in phase change floor, or smart hous-ng, thermo-regulated fibers and agricultural greenhouse. Amonghe synthesized polymeric SSPCMs, Especially, the SSPCM3 is the

ost attractive one due to its highest latent heat storage capacity.n addition, the latent heat capacity of SSPCM3 is much higher thanhat of the most of the SPCMs reported in the following studies:ao and Liu measured the latent heat value of the hyperbranchedolyurethane copolymers with 70% and 80% PEG60000 content as02 .8 and 118.1 J g−1 for heating process respectively, and as 115.8nd 100.9 J g−1 during cooling process, respectively [17]. Li and Dingeported the latent heat capacity of PEG10.000/MDI/PE tertiary cross-inking copolymer as 152.97 J g−1 [18]. Su and Liu measured theatent heat value of PU-PEG10.000 block copolymer for heating andooling process as 138.7 and 126.2 J g−1, respectively [20]. Zang anding measured the phase transition enthalpy as 67.5 J g−1 for chlo-

inated polypropylene (CPP) grafted by PEG6000 and 142.5 J g−1 forPP grafted by EG10.000 [29]. Yuan and Ding found the latent heattorage capacity of nano-crystalline cellulose/PEG4000 as 103.8 J g−1

31]. Vigo and his colleagues measured the latent heat enthalpy ofhe SSPCMs prepared by grafting PEG to the surface of natural cel-ulose fiber as less than 15 J g−1 [32]. Hu et al. found the latent heatf the PEG4000/PET and PEG6000/PET copolymers including 40–60%EG4000 as about 5–22 J g−1 [34]. Liao and Liao reported the phaseransition enthalpy of hyperbranched polyurethane doped with0 wt% PEG6000 as 125.0 J g−1 [40].

.4. Thermal reliability of the synthesized SSPCMs

The PCMs must be stable for long term utility. Therefore, therehould be no significant change in their thermal properties andhemical structures after repeated phase transition processes. Inrder to determine thermal reliability of the synthesized SSPCMs,he samples were subjected to thermal cycling test. The cyclingest is consisted of 5000 heating process above their phase transi-ion temperatures and following 5000 cooling process below theirhase transition temperatures. The DSC curves of the synthesizedSPCMs after thermal cycling were shown in Fig. 9. Thermal energytorage properties obtained from these curves are also presentedn Table 3. As seen from these results, solid–solid phase transition

emperatures and enthalpies of the copolymers show irregular lit-le changes for thermal energy storage applications. Therefore, itan be said that the copolymer PCMs have good thermal reliability

SSPCM1 289–463 98.64SSPCM2 273–459 97.61SSPCM3 276–457 97.20

in terms of the changes in thermal energy storage properties afterrepeated 5000 thermal cycling.

The chemical stability of the synthesized copolymers was alsoinvestigated after thermal cycling test by using FT-IR analysismethod. When compared the FT-IR spectra of the copolymers inFig. 10, it can be seen that the peak positions and shapes did notchange after thermal cycling. This means that that the copolymerspreserved their chemical structures even after repeated 500 ther-mal cycling.

3.5. Thermal stability of the synthesized solid–solid PCMs

Thermal stability of PCMs is one of the most important param-eters for thermal energy storage applications because of negativeeffects of thermal decomposition, degradation, and sublimation onthe useful life of PCMs. The thermal durability of polystyrene andthe synthesized copolymers were investigated by TG analysis. TheTG curves were presented in Fig. 11 and the thermal degradationdata derived from the curves are tabulated in Table 4. As shownfrom Fig. 11, that polystyrene showed one-step thermal decompo-sition that started at approximately 341 ◦C and ended completelyat 441 ◦C. Similarly the pure PEG6000 decomposed at one step withtemperature range of 321–456 ◦C.

On the other hand, contrary to the expectations the degra-dation of the synthesized copolymers occurred by one-step. Thismay be due to the fact that the decomposition temperatures ofpolystyrene and PEG6000 are close to each other. As can be seenfrom Table 4, the decomposition temperatures are in the temper-ature range of 270–465 ◦C. This means that the fabricated SSPCMsdo not degrade or lose weight below 270 ◦C. It can be concludedthat the SSPCMs have high thermal durability because the lowerlimit of decomposition temperature (270 ◦C) are much higher thanworking temperature region, or the solid–solid phase transitiontemperature range.

3.6. Thermal conductivity of the synthesized SSPCMs

Thermal conductivity of PCM can be considered as one of impor-tant parameters in thermal energy storage applications as wellas its phase transition temperature and latent heat capacity. Thisparameter significantly affects the heat charging and dischargingperformance of a thermal energy storage system integrated withPCM. Thermal conductivity of the PEG6000, polystyrene, and the

and 0.13 Wm−1 K−1, respectively. These results indicated that thethermal conductivity of the copolymers is about two times thatof polystyrene due to relatively high thermal conductivity of PEG

A. Sarı et al. / Materials Chemistry and Physics 133 (2012) 87– 94 93

thesiz

cvctr

Fig. 10. FT-IR spectrum of the syn

ontent. These results also showed that the thermal conductivityalues of the copolymers were increased with rising the mass per-

entage of PEG content. Moreover, it can be remarkably noted thathe thermal conductivity data of the synthesized SSPCMs were ineasonable level for thermal energy storage applications.

Fig. 11. TG curves of PEG6000 and the SSPCMs.

ed SSPCMs after thermal cycling.

4. Conclusions

In this study, polystyrene-graft-PEG6000 copolymers were syn-thesized as new kind SSPCMs by graft polymerization technique.The copolymerization reactions between PEG and polystyreneusing different mass percentage of PEG content was confirmed bythe FT-IR and 1H NMR spectroscopy methods. The POM investiga-tions shoved that the crystalline phase of soft segment (PEG6000)was transformed to amorphous phase during the solid–solid phasetransition process of the SSPCMs. The DSC results showed that thefabricated SSPCMs had suitable solid–solid phase transition tem-peratures in the range of 55–58 ◦C and high latent heat enthalpy inthe range of 116–174 J g−1 for thermal energy storage. The phasetransition enthalpy of the SSPCMs could be increased by raising themass ratio of PEG6000. The DSC analysis results also revealed that aconsiderable change was not observed in energy storage propertiesof the SSPCMs after thermal cycling. The FT-IR results confirmedthat the copolymers protected their chemical structures even theywere subjected to 5000 thermal cycling. TG analysis results showedthat the copolymers degraded at one step and had high thermaldurability above their working temperatures. Furthermore, the fab-ricated SSPCMs had higher thermal conductivity compared to thatof polystyrene. Based on all results, it can be also concluded that

the SSPCMs can be used as new kinds of energy storage materi-als because of their good thermal properties such as high energystorage capacity, suitable phase transition temperature, reversiblephase transition behavior and relatively high thermal conductivity,

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4 A. Sarı et al. / Materials Chem

ood thermal reliability and thermal durability. Moreover, theyan be considered as energy storage materials in different ther-al energy storage applications such as solar space hating, smart

ousing, thermo-regulating fibers and agricultural greenhouses.

cknowledgments

We would like to thank the Scientific & Technical Researchouncil of Turkey (TUBITAK) for their financial support for thistudy (The Project Code: 109T190-TBAG). Authors also thankltınay Boyraz (Erciyes University Technology Research & Devel-ping Center) for TG analysis.

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