Solar Energy Materials & Solar Cells 95 (2011) 3195–3201

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

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

Synthesis and thermal energy storage characteristics ofpolystyrene-graft-palmitic acid copolymers assolid–solid phase change materials

Ahmet Sarı n, Cemil Alkan, Alper Bic-er, Ali Karaipekli

Department of Chemistry, Gaziosmanpas-a University, 60240 Tokat, Turkey

a r t i c l e i n f o

Article history:

Received 1 November 2010

Received in revised form

25 March 2011

Accepted 6 July 2011Available online 3 August 2011

Keywords:

Solid–solid PCM

Heat storage material

Polystyrene

Palmitic acid

Graft copolymer

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.07.003

esponding author. Tel.: þ90 356 2521616; fa

ail addresses: asari@gop.edu.tr, asari061@hot

a b s t r a c t

A series of polystyrene graft palmitic acid (PA) copolymers as novel polymeric solid–solid phase change

materials (PCMs) were synthesized. In solid–solid PCMs, polystyrene is the skeleton and PA is a functional

side chain that stores and releases heat during its phase transition process. The heat storage of copolymers is

due to phase transition between crystalline and amorphous states of the soft segment PA in copolymer and

the hard segment polystyrene restricted the free movement of molecular chains of the soft segments even

above the phase transition temperature. The copolymers always remain in the solid state during the phase

transition processing and therefore they are described as form-stable PCM. Fourier transform infrared

spectroscopy (FT-IR) and polarization optical microscopy (POM) analyses were performed to investigate the

chemical structures and crystalline morphology. Thermal energy storage properties, thermal reliability and

thermal stability of the PCMs were investigated by differential scanning calorimetry (DSC) and thermogravi-

metric analysis (TGA) methods. Thermal conductivities of the PCMs were also measured using thermal

property analyzer. The analysis results indicated that the PA chains were successfully grafted onto the

polystyrene backbone and the copolymers showed typical solid–solid phase transition properties. Moreover,

thermal cycling test showed that the copolymers have good thermal reliability and chemical stability

although they were subjected to 5000 heating/cooling cycling. The synthesized polystyrene-graft-PA

copolymers as novel solid–solid PCMs have considerable potential for such as underfloor heating, thermo-

regulated fibers and heating and cooling of agricultural greenhouses. Especially, the polystyrene-graft-PA

copolymer including 75% PA is the most attractive PCM due to its highest latent heat storage capacity in the

synthesized copolymer PCMs.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Phase change materials (PCMs) are materials that can store orrelease large amounts of latent heat thermal energy as theychange their phases from one physical state to another. PCMshave many potential applications such as solar energy utilization[1,2], waste heat recovery [3], building air conditioning [4],temperature-control for greenhouses, electric energy-storagekitchen utensil [5], clothing thermal insulation [6] and so on.Therefore, PCMs have attracted a great interest of researchers inrecent years because of increasing energy crisis, energy cost andenvironmental problems.

PCMs can be classified as solid–solid PCMs, solid–liquid PCMs,solid–gas PCMs, liquid–gas PCMs with respect to the type ofphase change. Solid–liquid PCMs such as paraffins, fatty acids,fatty acid esters, salt hydrates, esters, their eutectics and somepolymers are traditional PCMs used in thermal energy storage

ll rights reserved.

x: þ90 356 2521285.

mail.com (A. Sarı).

applications [7–10]. Among the PCMs, fatty acids and theirmixtures have superior properties over the other PCMs such aseasy availability, congruently melting/freezing, good thermal andchemical stability, non-toxicity and suitable phase change tem-perature, high latent heat, no or less volume change during solid–liquid phase transition [11–14]. However, they need containers inenergy storage applications due to leakage problem during theirmelting processes. Form-stable or polymeric solid–solid PCMforms of them are preferred to avoid this phenomenon. Such typePCMs have also some other advantages such as smaller volumechange during the phase change process, no leakage, no corrosionto the container, and long term utility [15–18].

However, many polymeric solid–solid PCMs have severaldefects such as too high phase transition temperature, the lowtransition enthalpy and unstable thermal property in [19]. Allthese defects substantially limit their applications.

To prepare polymeric solid–solid PCMs, there are two generalmethods: one is the physical method in which composite PCMsare obtained by dispersing PCMs into polymeric matrix acting assupporting materials; as long as the temperature is below themelting point of the supporting materials. But, dispersing a PCM

A. Sarı et al. / Solar Energy Materials & Solar Cells 95 (2011) 3195–32013196

in a polymer matrix by physical interaction can cause phasesegregations during repeated melting and freezing processes. Theother kind is to chemically bind the PCMs onto supportingpolymeric materials by methods such as chemical grafting,blocking and cross-linking copolymerization. In this method, thePCMs lose their fluidity at higher temperature than their meltingpoint and thus the liquid leakage problem will be overcome andno encapsulation is needed. Furthermore, these kind polymericsolid–solid PCMs can be easily prepared into desired shapesdirectly.

In recent years, several polymeric solid–solid PCMs such ascellulose-graft-poly(ethylene oxide) [19], cross-linking PEG/MDI/PE copolymer [20], polyurethane-graft-PEG [21,22], cellulosediacetate-graft-poly(ethylene glycol)(PEG) [23–25], chlorinatedpolypropylene-graft-PEG [26], cellulose-graft-PEG [27–29], andpoly(vinyl alcohol)-graft-PEG [30] were prepared and investigatedin terms of their thermal energy storage characteristics.

Polystyrene is one of the aromatic polymers and renewableraw material manufactured from petroleum by the chemicalindustry. It is cheap and therefore many commercial applicationssuch as disposable cutlery, plastic models, smoke detector hous-ings, packing materials, insulation and foam drink cups have beendeveloped for polystyrene [31,32].

The application fields of polystyrene can be extended bygrafting with fatty acids. The obtained polystyrenic materialscan be considered as potential solid–solid PCMs for thermalenergy storage applications. As can be seen from the literaturesurvey, most of the researches have been focused on the prepara-tion and determination of thermal properties of polyurethane/PEG or CDA/PEG copolymers as solid–solid PCMs. However, to thebest of our knowledge, the preparation, crystal morphology,thermal energy storage properties and mechanism of solid–solidphase transition of polystyrene-graft-PA copolymers have neverbeen reported. The synthesized polystyrene-graft-PA copolymersas novel solid–solid PCMs have reasonable phase changeenthalpy, suitable transition temperature, good thermal conduc-tivity and good thermal reliability and stability. Especially,because of suitable phase change temperature the synthesizedcopolymers can be used to store thermal energy in the lowtemperature fields, such as underfloor heating, thermo-regulatedfibers and heating and cooling of agricultural greenhouses.

Fig. 1. Synthesis scheme of polys

In this study, polystyrene copolymers with different palmiticacid (PA) content were synthesized as novel polymeric solid–solidPCMs and they were characterized by FT-IR spectroscopy andPOM methods. Thermal properties and thermal stability of thesolid–solid PCMs were determined by DSC and TG analysis.Thermal reliabilities of the solid–solid PCMs were determinedafter 5000 heating/cooling cycles. In addition, the thermal con-ductivities of the PCMs were measured by thermal propertyanalyzer.

2. Materials and methods

2.1. Materials

Palmitic acid (CH3(CH2)14COOH, 98% pure, PA) was used as softsegment in preparation of polystyrenic solid–solid PCMs. It wasobtained from Merck Company. Polystyrene used as hard seg-ment, was purchased from Aldrich Company. Thionyl chloride(SOCl2), aluminum chloride (AlCl3) and dimethylformamide(DMF) were obtained from Merck Company. Analytical gradetoluene and chloroform were purchased from Merck Companyand they were used as solvent without further purification.

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

Palmitic acid chloride was preferred instead of palmitic acidbecause of the low reactivity of palmitic acid in graft copolymeriza-tion reactions between polystyrene and palmitic acid. The palmiticacid chloride was prepared by refluxing PA (1 mol) and thionylchloride (1 mol) at 85 1C for 6 h in a reflux system. DMF was used ascatalyst in the reaction. Reaction monitoring was done following thedisclosure hydrogen chloride. After the reaction, the unreactedthionyl chloride was removed by heating the mixture to 90 1C for1 h in fume hood and the palmitic acid chloride was obtained.

Polystyrenic solid–solid PCMs were prepared using graft poly-merization method. The synthesis scheme of polystyrene-graft-PAcopolymer PCMs is shown in Fig. 1. The copolymerization reactionwas carried out by taking the calculated amount of polystyrene,and palmitoyl chloride (molar ratio: 3:1 for styrene:palmitoylchloride) in chloroform and in a reaction system equipped with a

tyrene-graft-PA copolymers.

A. Sarı et al. / Solar Energy Materials & Solar Cells 95 (2011) 3195–3201 3197

reflux condenser and thermometer. AlCl3 was used as catalyst inthis reaction. The reaction was continued at 65 1C for 6 h andreaction monitoring was performed by observing color change oflitmus paper during the reflux process. After the reaction wascompleted, the product was filtered and washed four times usingdeionized water including mild alkaline solution. The samesynthesis process was also applied to molar ratio: 1:1 and 1:3of polystyrene:palmitoyl chloride. In the rest of the text, obtainedpolystyrene-graft-PA copolymers were named as poly(S-PA-S)(25% PA) for polystyrene-graft-PA copolymer with 25% PA,poly(S-PA-S)(50% PA) for polystyrene-graft-PA copolymer with50% PA and poly(S-PA-S)(75% PA) for polystyrene-graft-PA copo-lymer with 75% PA.

2.3. Characterization

The chemical characterization of the synthesized copolymerswas performed using FT-IR technique. The FT-IR spectra ofpalmitoyl chloride, polystyrene and polystyrene-graft-PA copoly-mer PCMs were taken on a KBr disk at the wavenumber range of4000–400 cm�1 using JASCO 430 model FT-IR spectrophot-ometer. POM analyses were performed on a Leica DM EP modelmicroscope equipped with a video camera.

Thermal properties of polystyrene-graft-PA copolymer PCMssuch as phase transition temperatures and enthalpies weremeasured by DSC technique (Perkin Elmer Jade, DSC). The DSCanalyses were carried out at 5 1C/min heating rate under aconstant stream of nitrogen at a flow rate of 60 mL/min. Repro-ducibility was tested by conducting three measurements. In orderto determine thermal reliability of the copolymers, acceleratedthermal cycling test was conducted. The tests were performed upto 5000 heating/cooling processes using a thermal cycler (BIOERTC-25/H model). The changes in thermal properties of polystyr-ene-graft-PA copolymer PCMs after thermal cycling were evalu-ated using DSC analysis. In addition, the chemical stability of thePCMs after thermal cycling test was investigated by FT-IRanalysis.

Thermal stability characterization of the PCMs was carried outon a thermal analyzer (Perkin-Elmer TGA7). The measurements

Fig. 2. FT-IR spectra obtained for polystyrene, palmitoyl

were performed at temperature range of 25–600 1C at heatingrate of 10 1C min�1 under a static air atmosphere. Thermalconductivities of the copolymers were measured at room tem-perature using KD2 thermal property analyzer.

3. Results and discussion

3.1. FT-IR analysis

Fig. 2 shows the FT-IR spectra of polystyrene, palmitoylchloride and polystyrene-graft-PA copolymer PCMs. As can beseen from the spectrum of polystyrene in Fig. 2(a), the peaksobserved at 2920 cm�1 show the asymmetric stretching of C–Hband. The transmission band of –CH2 and –CH also appeared at2850 cm�1. Moreover, the peaks at 3060, 1590, 1538 and1450 cm�1 belong to the characteristic peaks of benzene ring ofthe polystyrene. As can be also seen from Fig. 2(a), the peaks at2923 and 2850 cm�1 represent the asymmetric stretching of –CH2 and –CH bands of palmitoyl chloride. The strong peak at1801 cm�1 corresponds to the stretching vibration peak of thecarbonyl (–C¼O) group of palmitoyl chloride. The peakat 1463 cm�1 is the –CH2 bending peak, 1403 cm�1 representsC–H and C–C bending and 717 and 678 cm�1 correspond torocking vibration and bending, respectively, which are all char-acteristics for aliphatic chain of palmitoyl chloride.

As clearly seen from the spectra of polystyrene-graft-PAcopolymers in Fig. 2(b), the asymmetric stretching bands of–CH2 and –CH were observed at 2920 cm�1 and 2920 cm�1.The stretching vibration peak of the carbonyl (–C¼O) group wasseen at 1737 cm�1. Also, the peak at 1454 cm�1 is the –CH2

bending peak and 1371 cm�1 represents the C–H and C–Cbending.

When compared the FT-IR results of the polystyrene-graft-PAcopolymers with that of polystyrene and palmitoyl chloride, it canbe noted that there are no available absorption band of carbonylgroup (C¼O) in polystyrene spectrum whereas it can be seen thatthe polystyrene-graft-PA copolymers have stretching vibrationpeak of the carbonyl group at about 1801 cm�1 due to thepalmitoyl chloride grafted to polystyrene. In other words, in the

chloride, and polystyrene-graft-PA copolymer PCMs.

Fig. 4. DSC thermograms of polystyrene-graft-PA copolymer PCMs.

Table 1Thermal properties of synthesized polystyrene-graft-PA copolymer PCMs.

Thermal properties

Ts–s, heating (1C) DHs–s, heating

(J/g)

Ts–s, cooling

(1C)DHs–s, cooling

(J/g)

Poly(S-PA-S)(25% mole) 21.47 26.20 17.65 20.02

Poly(S-PA-S)(50% mole) 18.72 31.16 18.51 28.04

Poly(S-PA-S)(75% mole) 19.18 39.78 18.72 39.19

A. Sarı et al. / Solar Energy Materials & Solar Cells 95 (2011) 3195–32013198

spectra of the polystyrene-graft-PA copolymer PCMs, the char-acteristic absorption band of carbonyl group of palmitoyl chlorideappearing at 1801 cm�1 was observed at 1737 cm�1 for poly(S-PA-S)(25% PA), 1739 cm�1 for poly(S-PA-S)(50% PA) and1702 cm�1 for poly(S-PA-S)(75% PA). The shift to lower wave-number of these absorption bands was most probably due tobinding of PA chains to the polystyrene backbone. This conclusionis supported by shifting of aromatic –C¼C stretch bands at about1500 cm�1 on aromatic rings of the copolymer chain [33].

3.2. Crystalline morphology

Fig. 3 shows POM micrographs of pure PA and poly(S-PA-S)(25% PA) at lower and upper temperatures than its solid–solidphase transition temperature. As can be seen from Fig. 3(a) and (b),the pure PA and poly(S-PA-S)(25% PA) are crystalline below theirphase transition temperatures. In the other words, crystal structureof PA has not changed with grafting of PA on polystyrene. However,the crystal size of poly(S-PA-S)(25% PA) is much smaller than thatof PA, indicating that the crystallization of PA in copolymer isconfined by the hard segment polystyrene, and the crystallineperfection of PA is destroyed. Thus, in the copolymer, big crystalstructure could not be formed. Similar POM micrographs were alsoobserved for the poly(S-PA-S)(50% PA) and poly(S-PA-S)(75% PA)copolymer samples.

On the other hand, while the poly(S-PA-S)(25% PA) was heated,no change in its crystal morphology was observed under thephase-transition temperature. The crystal structure starts to bedestroyed when the temperature reaches the solid–solid phasetransition temperature (e.g., 25 1C), and the crystal structure isdisappeared completely at last (Fig. 3(c)).This result confirms thetransition from crystalline phase to amorphous phase as thetemperature is over the phase transition temperatures ofthe PCMs.

3.3. Thermal properties

Fig. 4 shows the DSC curves of polystyrene-graft-copolymersincluding PA with different mole ratio. The thermal propertiesobtained from the DSC curves were also summarized in Table 1.As can be seen from this table, the polystyrene-graft-PA copoly-mers show reversible transition. The enthalpies of the positiveprocess and the reverse process are close. However, there is aslight decrease in thermal properties of the polystyrene-graft-PAcopolymers for their cooling processes. This phenomenonobserved during the cooling processes simultaneously agreeswith the previous conclusion found by many researchers[20,22,29]. It can be due to the fact that the hard segment incopolymer can destroy the perfection of the crystallization of PAin the structure. So the defects of crystal lattices may be lead tothe low transition temperature and latent heat during cooling

Fig. 3. POM micrographs of pure PA (a) at 15 1C, poly(S-PA-S)(25% PA) cop

process. On the other hand, in the copolymer samples, only PAsegments are crystalline and so it is possible that the phasetransition temperature changes with the increasing of PA percen-tage. However, the defects in crystal structure due to the hardsegment in polystyrene-graft-PA copolymer can cause irregularchanges in solid–solid phase transition temperatures of softsegment (PA) of copolymer PCM.

The molecular movement of soft segment is increased withtemperature rise and thus the crystalline structure of soft segmentis destroyed during the phase transition process of a polymericsolid–solid PCM. So PA segment in the copolymers turns intoamorphous state. But the amorphous PA is connected with thehard segment and it can make only movements of vibration androtation. Therefore it shows solid–solid phase change behavior. Inthis case, the solid–solid PCM stores latent heat during its transitionfrom low entropy state to high entropy state [21,22]. The latentheats of phase transition were found to be in the range of 20.02–26.20 kJ/kg for poly(S-PA-S)(25% PA), 28.04–31.16 kJ/kg for poly(S-PA-S)(50% PA) and 39.19–39.78 kJ/kg for poly(S-PA-S)(75% PA).These results indicated that the phase transition enthalpy valuesare increased with increasing PA content.

Based on the DSC data in Table 1 it can be also concluded thatpolystyrene-graft-PA copolymers have reasonable transition

olymer, (b) at 15 1C and poly(S-PA-S)(25% PA) copolymer (c) at 25 1C.

A. Sarı et al. / Solar Energy Materials & Solar Cells 95 (2011) 3195–3201 3199

enthalpy value, and the suitable transition temperature point fora several thermal energy storage based-heating and coolingapplications such as in phase change floor, or smart housing,thermo-regulated fibers and agricultural greenhouse. Especially,the polystyrene-graft-PA copolymer including 75% PA is the mostattractive PCM due to its highest latent heat storage capacity inthe synthesized copolymer PCMs.

Comparing the synthesized polystyrene-graft-PA copolymers inthis study with the polymeric solid–solid PCMs reported in previousworks in terms of latent heat storage capacity, it has much higherthan that of some solid–solid PCMs. For example, Vigo and hiscolleagues use PEG to graft onto the surface of natural cellulose fiberto obtain solid–solid PCM and reported its enthalpy as less than 15 J/g[34]. Hu et al. found the latent heat values of the PEG4000/PET andPEG6000/PET copolymers including 40–60% PEG as about 5–22 J/g[35]. However, the latent heat capacities of synthesized polystyrene-graft-PA copolymers are lower than that of some solid–solid PCMsreported in the literature. Li and Ding showed that PEG10000/MDI/PEtertiary cross-linking copolymer had typical solid–solid phase transi-tion property and a high enthalpy of 152.97 J/g [20]. Su and Liumeasured the latent heat values of the novel solid–solid PCM withPUPCM composed of PEG as soft segment for heating and coolingprocess as 138.7 and 126.2 J/g, respectively [21]. Cao and Liumeasured the latent heat capacity of the hyperbranched polyurethanecopolymers including 70% and 80% soft segment during heatingprocess as 102.8 and 118.1 J/g, respectively, and as 115.8 and100.9 J/g during cooling process, respectively [22]. Zhang and Dingdetermined the latent heat values of chlorinated polypropylene (CPP)grafted by PEG6000 and PEG10000/CPP for heating process as 67.5

Fig. 5. DSC thermograms for polystyrene-graft-

and 142.5 J/g, respectively [26]. The latent heat storage capacity ofnano-crystalline cellulose/polyethylene glycol as a new kind of solid–solid PCM was found to be 103.8 J/g by Yuan and Ding [29]. Liao andLiao determined the solid–solid transition enthalpy of doped hyper-branched polyurethane with 90 wt% soft segment as 125.0 J/g [36].

The low latent heat capacities of polystyrene-graft-PA copoly-mers synthesized as solid–solid PCMs in this study may be due tothe presence of crosslinking structure as well as rigid benzenering groups on the polystyrene chain. These hard segments wouldreduce the crystallinity of the polystyrene-graft-PA copolymers,which consequentially led to the decline of enthalpy. Conse-quently the arrangement and orientation of PA molecules bondedto benzene rings on the polystyrene chain would be partiallysuppressed by steric effect and the crystalline regions turnedsmaller, which caused the falling down of transition enthalpy.

3.4. Thermal reliability of the polystyrene-graft-PA copolymers

PCMs

The PCMs must be thermally and chemically stable in practiceafter long term utility period. Therefore, there should be nosignificant change in their thermal properties and chemicalstructures after the repeated phase transition processes. Thermalcycling test was conducted to determine thermal reliability ofpolystyrene-graft-PA copolymer PCMs. Fig. 5 shows the DSCcurves for the copolymers before and after thermal cycling.Thermal properties obtained from the DSC curves were also givenin Table 2. The results showed that the phase transition tempera-tures had a little change, which were not significant for thermal

PA copolymer PCMs after thermal cycling.

Table 2Thermal properties of the polystyrene-graft-PA copolymer PCMs after 5000

thermal cycling.

Thermal properties

Ts–s, heating

(1C)DHs–s, heating

(J/g)

Ts–s, cooling

(1C)DHs–s, cooling

(J/g)

Poly(S-PA-S)(25% PA) 21.62 23.08 17.44 21.05

Poly(S-PA-S)(50% PA) 18.89 29.96 17.46 28.68

Poly(S-PA-S)(75% PA) 19.23 39.40 18.43 40.81

Fig. 6. FT-IR spectra for polystyrene-graft-PA copolymer before thermal cycling

(a) poly(S-PA-S)(25% PA), (c) poly(S-PA-S)(50% PA), (e) poly(S-PA-S)(75% PA) and

after thermal cycling (b) poly(S-PA-S)(25% PA), (d) poly(S-PA-S)(50% PA),

(f) poly(S-PA-S)(75% PA).

Fig. 7. TG curves of polystyrene and the polystyrene-graft-PA copolymer PCMs.

Table 3TG analysis data of polystyrene and polystyrene-graft-PA copolymer PCMs.

Degradation interval (1C) Mass loss (%wt)

Polystyrene 341–441 98.53

Poly(S-PA-S)(25% PA) 176–331 (1. step) 40.18

349–448 (2. step) 58.46

Poly(S-PA-S)(50% PA) 160–285 (1. step) 53.93

352–441 (2. step) 44.28

Poly(S-PA-S)(75% PA) 180–300 (1. step) 58.96

359–457 (2. step) 38.39

A. Sarı et al. / Solar Energy Materials & Solar Cells 95 (2011) 3195–32013200

energy storage applications. Therefore, it can be said that thepolystyrene-graft-PA copolymer PCMs have good thermal relia-bility in terms of the changes in their phase transition tempera-tures. After repeating 5000 thermal cycling, the latent heat valuesof the copolymer PCMs were changed as irregular, but thechanges were negligible for thermal energy storage-basedapplications.

On the other hand, the chemical stability of the polystyrene-graft-PA copolymer PCMs after repeating thermal cycling was alsoinvestigated by FT-IR analysis. When the FT-IR spectra werecompared in Fig. 6, it can be seen that the peak positions andshapes are consistent before and after thermal cycling. Theseresults indicated that chemical structure of the polystyrene-graft-PA copolymer PCMs was not affected by thermal cycling and anychemical degradation in the PCMs did not occur during thermalcycling.

3.5. Thermal stability of the synthesized solid–solid PCMs

Thermal stability of PCMs is one of the most importantparameters for thermal energy storage applications because oflimiting the usability of them by thermal decomposition, degra-dation and sublimation. The thermal stabilities of polystyrene and

the polystyrene-graft-PA copolymers were investigated by TGanalysis. The degradation data obtained from the TG thermo-grams in Fig. 7 are given in Table 3. As shown from Fig. 7, thatpolystyrene starts to lose weight at approximately 341 1C, and itcompletely loses its weight at 441 1C. The polystyrene-graft-PAcopolymers are not degraded and almost no weight loss can beobserved at the temperature lower than 160 1C. It means thatpolystyrene-graft-PA copolymers are very stable in the workingtemperature region, or in the temperature range of phase transi-tion for energy storage application. However, the copolymersbegin to degrade as the temperature is above 160 1C.

On the other hand, as can be seen from Fig.7, the TG curve ofpolystyrene is sharp and one step whereas the TG curves forpolystyrene-graft-PA copolymers have two steps. The first stepsare decomposition steps corresponding to the thermal degrada-tion of PA molecular chains. The second steps are assigned to thethermal degradation of the polystyrene main chains. The degra-dation with two steps for the polystyrene-graft-PA copolymersindicates the independent decomposition of two components ofthe copolymer. The TG results are in agreement with thatreported by Vigo et al. and Araki et al. [34,37].

3.6. Thermal conductivity of the polystyrene-graft-PA copolymer

PCMs

Thermal conductivity of PCM can be considered as importantparameter in thermal energy storage applications as well as its phasetransition temperature and latent heat of PCM. The energy storageperformance of PCM depends on this parameter because it has asignificant effect on the rates of energy storage and release ofPCM. Thermal conductivity of the synthesized solid–solid PCMswas measured as 0.13 Wm�1 K�1 for poly(S-PA-S)(25% PA),0.18 Wm�1 K�1 for poly(S-PA-S)(50% PA) and 0.20 Wm�1 K�1 for

A. Sarı et al. / Solar Energy Materials & Solar Cells 95 (2011) 3195–3201 3201

poly(S-PA-S)(75% PA). These results indicated that the thermalconductivity of the copolymers is increased by increasing the molepercentage of PA. Nevertheless thermal conductivity data of thePCMs were in acceptable level for a solid–solid PCM, which will beused for latent heat energy storage applications.

4. Conclusions

Polystyrene-graft-PA copolymer PCMs were synthesized asnovel solid–solid PCMs by graft polymerization technique. TheFT-IR results confirmed that the PA was successfully grafted onthe polystyrene used as backbone chain. DSC thermal analysesshowed that the synthesized graft copolymers have typical solid–solid phase transition behavior with good energy storage densityfor thermal energy storage applications. The POM investigationsshowed that the crystalline phase of soft segment PA of poly-styrene copolymers was transformed to amorphous phase duringthe solid–solid phase transition of the copolymer PCMs. The phasetransition temperature of copolymer PCMs could be adjusted bychanging the content of soft segment PA. TG analysis resultsshowed that the polystyrenic copolymers degrade in two stepsand are resistant above their working temperature. In addition,the FT-IR results showed that the repeated 5000 thermal cyclingdid not cause any degradation in the chemical structure of thecopolymers. DSC analysis results obtained after thermal cyclingrevealed that a significant change was not observed in thermalproperties of the copolymer PCMs. It can be also concluded thatthe synthesized polystyrene-graft-PA copolymers as novel solid–solid PCMs have considerable potential for low thermal energystorage applications such as underfloor heating, thermo-regulatedfibers and heating and cooling of agricultural greenhouses.Especially, the polystyrene-graft-PA copolymer including 75% PAis the most attractive PCM due to its highest latent heat storagecapacity in the synthesized copolymer PCMs.

Acknowledgments

We would like to thank the Scientific and Technical ResearchCouncil of Turkey (TUBITAK) for their financial support for thisstudy (The Project Code: 109T190-TBAG). Authors also thankAltınay Boyraz (Erciyes University Technology Research andDeveloping Center) for TG analysis.

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