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Thermal energy storage characteristics of poly(styrene-co-maleic anhydride)-graft-PEG as polymeric solid–solid phase change materials
Solar Energy Materials & Solar Cells 161 (2017) 219–225
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Thermal energy storage characteristics of poly(styrene-co-maleic anhydride)-graft-PEG as polymeric solid–solid phase change materials
MARK
⁎
Ahmet Sarıa, , Alper Biçerb, Cemil Alkanb a b
Karadeniz Technical University, Department of Metallurgical and Material Engineering, 61080 Trabzon, Turkey Department of Chemistry, Gaziosmanpaşa University, 60240 Tokat, Turkey
A R T I C L E I N F O
A BS T RAC T
Keywords: SMA PEG Solid-solid PCM Latent heat Thermal energy storage
Poly(styrene-co-maleic anhydride)(SMA)-graft-polyethylene glycol)(PEG) copolymers were synthesized as novel polymeric solid–solid phase change materials (S-SPCMs). The synthesized copolymers showed latent heat storage and release ability by means of the phase transition from crystalline phase to amorphous phase of PEG bonded to the skeleton as side chains. The chemical structures of the polymeric S-SPCMs were confirmed by fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR) analysis techniques. The transformation of crystalline phase to amorphous phase were investigated using and polarized optical microscopy (POM) techniques. The latent heat thermal energy storage (LHTES) of the S-SPCMs were measured by differential scanning calorimetry (DSC) analysis method. Thermal degradation temperature limits of the SSPCMs were determined by thermogravimetry analysis (TGA) method. The DSC analysis indicated that the synthesized S-SPCMs demonstrated typical solid-solid phase transitions in the temperature range of about 40– 45 °C and had considerable high latent heat capacity between 107 and 155 J g−1. The TGA results showed that the polymeric S-SPCMs were durable thermally up to at least 300 °C. The thermal cycling test exposed that the S-SPCMs protected their LHTES properties even after 5000 heating/cooling treatment. All findings indicated that the prepared SMA-graft-PEG copolymers posses good thermal energy storage (TES) potential for passive solar heating and cooling applications.
1. Introduction Thermal energy can be stored in sensible, latent, and thermochemical forms. Among all, latent heat storage can be considered as the most attracting method. In this technique, PCM has a function energy storage/release during phase change at a specified temperature. PCMs are advantageous due to their high energy storage capacity, isothermal utility, controlled phase change process, etc [1,2]. Many organic and inorganic PCMs and their mixtures have been used in various applications such as solar water heating, solar heating/cooling of residential and agricultural buildings, temperature-regulating in textiles, heat management of electronics, biomedical and biological carrying systems and so on [3–10]. Depending on the type of phase change, PCMs are distributed to three groups: solid–solid (S-S), solid–liquid (S-L), and liquid–gas (L-G). A S-SPCM can store/release a great deal of latent heat during heating/cooling cycles by transforming from a crystalline phase to another phase or amorphous phase at constant temperature. Such type PCMs have some extra advantages because they show no seepage above phase transition temperature, need not to be encapsulated, and can be processed easily. There are many S-SPCMs [11] ⁎
categorized as inorganic S-SPCMs [12–16], certain hydrocarbon crystal S-SPCMs [17], organic polyols [17–22] and polymer based S-SPCMs. Polymer-based S-SPCMs have developed rapidly in recent years because of direct processing advantage into arbitrary shape, even being used as an element of TES system directly. So, several works were carried out to produce novel polymeric S-SPCMs [23–28]. The physical or chemical pathways were applied to produce S-SPCMs. However, the key difficulty in the physical way is non-homogenous dispersion of PCM in polymer matrix [29–31]. Moreover, the S-SPCMs are produced chemically via grafting, blocking, and cross-linking reactions, which are occurred between polymer material used as hard segment and PCM as soft segment. Poly(ethylene glycol)s (PEGs) are promising polymeric PCMs because of their good LHTES characteristics [11,22,32]. The LHTES characteristics of them mainly depend on the molecular weights [33]. Moreover, the polymeric S-SPCMs which include PEG with lover molecular weights than 4000 do not show satisfying thermal energy storage properties according to literature [34]. In order to obtain novel polymeric S-SCMs with considerable high LHTES capacity, several PEG-based polymeric S-SPCMs such as polyurethane-graft-PEG [35],
Corresponding author. E-mail addresses: [email protected], [email protected] (A. Sarı).
http://dx.doi.org/10.1016/j.solmat.2016.12.001 Received 28 September 2016; Received in revised form 31 October 2016; Accepted 1 December 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.
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PEG (2:1) and SMA-graft-PEG (1:1).
cellulose diacetate-graft-PEG [36], chlorinated polypropylene-graftPEG [37], cellulose-graft-PEG [38,39], poly(vinylalcohol)-graft-PEG [40], poly(ethylene terephthalate)-PEG [41] and polystyrene-graftPEG [42] have been prepared using different polymerization methods or modifications. The results reported in above studies showed that the LHTES properties of the S-SPCMs are increased with increase in the molecular weight and mass fraction of PEG grafted to the polymer skeleton. SMA is consisted of styrene and maleic anhydride monomers. SMA at high molecular weights are generally used in engineering plastics as the SMA solutions in alkaline are used in the field of sizing (paper), binders, and coatings [43]. Moreover, the applicability of SMA can be enlarged by PEG modification. The produced copolymers in this have high TES potential due to their S-S phase change facility. According to our best knowledge, the synthesis, LHTES properties, thermal reliability and durability of SMA-graft-PEG copolymers have never been reported in literature. In this regard, SMA-graft-PEG copolymers were synthesized in this work as novel polymeric SSPCMs. In order to investigate the amount effect of PEG side chains on the LHTES properties of whole polymeric structure, it was grafted to SMA skeleton at three different mole ratios of 4:1, 2:1 and 1:1. The synthesized SSPCMs were characterized by FT-IR, 1H NMR and POM techniques. The LHTES of the S-SPCMs were measured by DSC method as their thermal durability properties was determined by TGA technique. Moreover, the thermal reliability of the produced S-SPCMs was studied by exposing them accelerating heating/cooling process including 5000 cycles.
2.3. Characterization SMA-graft-PEG copolymers were characterized by using FT-IR and H NMR spectroscopy analysis techniques. The FT-IR spectra of PEG, SMA and the synthesized copolymers as novel S-SPCMs were taken on a KBr disk in the wave number range of 4000–400 cm−1 using FT-IR spectrophotometer (JASCO 430 model). The 1H NMR spectra of the SSPCMs were taken using a BRUKER 400 NMR spectrometer (Bruker Corp., USA). The copolymer samples were dissolved in CDCl3 with a concentration of 100 mg/0.5 mL. POM investigations were carried out using a Leica DM EP model microscope equipped with a video camera. Thermal energy storage properties of SMA-graft-PEG copolymers were investigated using a Perkin Elmer Jade model DSC instrument at a heating/cooling rate of 10 °C min−1 between −15 °C and 90 °C. The SMA-graft-PEG copolymers were subjected to a thermal cycling test using a thermal cycler (BIOER TC-25/H model). The cycling test was carried out by conducting accelerated heating/cooling performed 5000 times. Thermal reliability of SMA-graft-PEG copolymers was determined by DSC analysis after thermal cycling. The chemical stability of the copolymers after cycling process was checked by FT-IR analysis. Thermal durability of the copolymers was also investigated by using a Perkin- Elmer TGA7 model instrument. The analysis was performed between 25 and 600 °C at a heating rate of 10 °C min−1. 1
3. Results and discussion
2. Material metods
3.1. FT-IR spectroscopy analysis of SMA-graft-PEG copolymers
2.1. Materials
Fig. 2a shows the FT-IR spectra of SMA, PEG and SMA-graft-PEG copolymers. It can be seen from the spectra that SMA had two main functional groups (-C˭O and -C-O) and their stretching vibration peak were observed at 1222 cm−1 and 1778 cm−1, respectively. PEG had three main stretching vibrations of -OH, -C˭O and –C-O groups detected at 3463 cm−1, 1108 cm−1 and 1645 cm−1, respectively. On the other hand, the FT-IR spectra of SMA-graft-PEG(4:1), SMA-graftPEG(2:1) and SMA-graft-PEG(1:1) were shown in Fig. 2b. As seen from the spectral data, the stretching vibration bands of –C˭O, -C-O and -OH groups were detected at 1726, 1109 and 3408 cm−1 for SMAgraft-PEG(4:1) copolymer, at 1722, 1113, and 3415 cm−1 for SMAgraft-PEG(2:1) copolymer and at 1722, 1109, and 3407 cm−1 for SMAgraft-PEG(1:1) copolymer, respectively. The stretching bands of the characteristic groups of precursors shifts slightly in the SSPCMs which confirms grafting reaction. The 1H NMR spectra of PEG, SMA and SMA-graft-PEG(1:1) copolymer were shown as an example in Fig. 3. The following results can derived from these spectra: (1) the g proton of the copolymer observed at 3.3 ppm was not seen in the spectra of SMA and PEG. (2) The c proton of PEG at 3.6 ppm shifted to 3.9 ppm in the copolymer. (3) The c proton of SMA at 2.7 ppm shifted to 2.5 ppm (f proton) in the
SMA (Mw: 224.000 g mol−1, maleic anhydride ∼7 wt%) and PEG Mw: 6000 g mol−1 were purchased from Aldrich Company. Chloroform was obtained from Merck Company and used without further purification. Technical grade phosphoric acid was supplied from Fluka Company. 2.2. Synthesis of SMA-graft-PEG copolymers The SMA-graft-PEG copolymers have been synthesized by grafting PEG to SMA backbone as shown in the reaction scheme (Fig. 1). The reaction was carried out in a reaction set-up consisting of a 500 mL flask equipped with a reflux condenser, a mechanical stirrer and a thermometer. SMA and PEG at calculated amounts and chloroform (200 mL) was solved into a flask. The reaction system was refluxed in the existence of phosphoric acid (1–2 drops) as catalyst at 80 °C for 24 h. After completion of copolymerization reaction, chloroform was removed using a rotary evaporator. Moreover, the same procedure was repeated for the other SMA/PEG mole ratios of 2:1 and 4:1. The produced copolymers were sealed as SMA-graft-PEG (4:1), SMA-graft-
Fig. 1. The reaction schema regarding with the synthesis of the S-SPCMs.
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Fig. 2. FT-IR spectra of SMA, PEG and the synthesized copolymers.
Fig. 3. 1H NMR spectra of SMA, PEG and the synthesized SMA-graft-PEG(1:1) copolymer
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Fig. 4. POM images of PEG and the synthesized copolymers (a) PEG below the solid–solid phase change temperature (25°C), (b) SMA-graft-PEG(4:1) below the solid–solid phase change temperature (25°C), (c) SMA-PEG(4:1) over the phase change temperature (70 °C), (d) SMA-graft-PEG(2:1) below the phase change temperature (25°C), (e)SMA-graft-PEG(2:1) over the phase change temperature (70°C), (f) SMA-graft-PEG(1:1) below the phase change temperature (25°C), and (g) SMA-graft-PEG(1:1) over the phase change temperature (70°C)
Table 1 The LHTES properties of pure PEG and the synthesized S-SPCMs. Heating period
PEG SMA-graftPEG (4:1) SMA-graftPEG (2:1) SMA-graftPEG (1:1)
s–s,
(°C)
ΔH g−1)
Cooling period s–s,
(J
s–s,
(°C)
ΔH g−1)
s–s,
Phase transition
T
S-L S-S
59.80 39.69
221.31 107.04
36.20 23.68
155.5 109.86
S-S
42.59
127.02
32.29
129.11
S-S
45.03
155.21
33.56
142.62
T
(J
Fig. 5. DSC thermograms of PEG and the synthesized copolymers.
PEG(4:1), SMA-graft-PEG(2:1), and SMA-graft-PEG(1:1) copolymers were characterized by POM analysis and the results were shown in Fig. 4(a–g). As clearly observed from the POM (Fig. 4a), PEG has spherulite crystal structures at room temperature. The spherulite crystalline phases of the copolymers below the S-S phase transition temperatures were also (Fig. 4b, d, and f). The spherulite structures were much smaller than those of pure PEG. There is no change in the morphology of SMA-graft-PEG copolymers up to their phase-transition temperatures. However, as the temperature was approached the phase
copolymer. (4) The b proton of SMA at 3.0 ppm shifted to 3.5 ppm (d proton) in the copolymer. (5) The b proton of PEG was not observed in the spectra of the copolymer. The 1H NMR spectroscopy results indicates that the grafting reaction was achieved successfully. 3.2. Morphology analysis of SMA-graft-PEG copolymers The phase transformations of pristine PEG and SMA-graft222
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Table 2 Comparison of latent heat values of the synthesized S-SPCMs in this work with those of the other S-SPCMs reported in literature.
Table 3 The LHTES properties of the synthesized S-SPCMs after 5000 thermal cycling. Heating period
S-SPCMs
Latent heat capacity (J/g)
References
PU-PEG copolymers PU-PEG block copolymer MDI-PE-PEG cross-linking copolymer CPP-PEG copolymers Cellulose-PEG copolymer PEG/PET copolymers PS-graft-PEG6000 copolymers PS-graft-PA copolymers PS-graft-SA copolymers PScoAA-graft-SA copolymers SMA-graft-PEG copolymers
103–118 139 153 68–143 104 5–22 116–174 26–40 45–55 34–74 107–155
[24] [27] [34] [37] [39] [41] [42] [49] [50] [52] This work
s–s,
(°C)
ΔH
Cooling period s–s,
(J g−1)
T
s–s,
(°C)
ΔH
s–s,
S-SPCMs
T
SMA-graft-PEG (4:1) SMA-graft-PEG (2:1) SMA-graft-PEG (1:1)
40.24
120.82
25.95
115.48
40.45
128.58
35.30
122.30
44.50
150.44
35.80
140.85
(J g−1)
transition points of copolymers, the spherulite phases were destroyed and then disappeared completely (Fig. 4c, e, and g). This process can be also evaluated as transformation of crystal phase of grafted PEG chains to amorphous phase. In this case, the PEG side groups on the skeleton could only be vibrated and rotated, but could not be translated freely due to the destruction of the crystalline structure of copolymers. However, copolymers kept their solid state because of the high mechanical strength of SMA backbone even they were heated above their phase transition temperatures.
3.3. The LHTES properties of the SMA-graft-PEG copolymers The LHTES properties of the produced copolymers such as S-S phase transition temperatures and enthalpies were measured by DSC
Fig. 7. DSC thermograms of the synthesized copolymers after thermal cycling.
Fig. 6. FT-IR spectra of the synthesized copolymers after thermal cycling.
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passive TES applications for heating and cooling purposes. Especially SMA-graft-PEG(1:1) copolymer is the most promising owing to its highest latent heat capacity. Additionally, Table 2 presents the comparison of the latent heat values of the synthesized S-SPCMs with those of other S-SPCMs reported in the literature. As can be clearly seen from the tabulated data, the latent heat capacity of the copolymers synthesized in this work is higher than the most of S-SPCMs in the previous studies [24,27,34,37,39,41,42,49,50,52]. 3.4. Thermal reliability of SMA-graft-PEG copolymers Thermal reliability property is one of the most important factors for PCMs used in passive TES systems. Significant changes in LHTES property of a PCM depending on thermal cycling means its unreliability for TES targets. The cycling test is consisted of repeated heating/ cooling cycling 5000 times above/below phase transition temperatures of the SMA-graft-PEG copolymers. The DSC analysis results of the produced copolymers and the measured LHTES properties after the cycling test were shown in Fig. 6 and in Table 3, respectively. The results for the related materials show that there are no significant changes LHTES properties of the copolymers. Therefore, it is remarkably noted that the prepared copolymers have good thermal reliability after repeated 5000 thermal cycling. The chemical structures of the copolymers are also checked after the cycling test. As seen from the FTIR spectrums shown in Fig. 7, the shape of the characteristic bands and their wavenumbers were not changed after the cycling test. These result proved good long-period structural stability of the synthesized SSPCMs.
Fig. 8. TGA curves of the synthesized copolymers.
Table 4 TGA results obtained for pure SMA, PEG and the synthesized S-SPCMs.
SMA PEG SMA-graft-PEG (4:1) SMA-graft-PEG (2:1) SMA-graft-PEG (1:1)
Degradation interval (°C)
Mass loss (%wt)
194–476 321–456 306–455 309–460 302–461
95.01 97.83 90.30 95.80 88.92
3.5. Thermal durability of SMA-graft-PEG copolymers analysis. The DSC curves of SMA-graft-PEG polymers were shown in Fig. 5 and thermal properties were also summarized in Table 1. The phase transition process of pure PEG is carried out from solid to liquid and white crystal of pure PEG changes to a transparent liquid temperature is raised to about 70 °C. On the other hand, the synthesized S-SPCMs remains their solid states during the heating process although the temperature is above 70 °C. The on-set peak values of endothermic and exothermic of synthesized S-SPCMs were measured at 39.69 and 23.68 °C for SMA-graft-PEG(4:1), 42.59 and 32.29 °C for SMA-graft-PEG(2:1), 45.03 and 33.56 °C for SMA-graft-PEG(1:1). The phase transition temperatures of the synthesized S-SPCMs is very suitable for the passive cooling of electronic devices, thermo-regulating of fibers and solar passive heating applications in building agricultural greenhouse envelopes. Moreover, as can be seen from these results, there is considerable difference between the S-S phase transition temperatures obtained for heating and cooling periods of the SSPCMs. This case is was due to two reasons: (i) because of the subcooling behavior of PEG side chains in the copolymer structures. The similar behavior were observed in case of the polyurethane-PEG block copolymer [24,27]. (ii) due to the effect of heating/cooling rates on the phase change temperature range and the hysteresis between heating and cooling curves. Higher heating/cooling rates increase the hysteresis and shift the phase change temperature range towards colder temperatures [44]. The effects of hysteresis and subcooling on thermal performance of PCM [45-51]. As also seen from Table 1, the latent heat capacities of the synthesized SMA-graft-PEG(4:1), SMA-graft-PEG(2:1) and SMAgraft-PEG(1:1) copolymers were measured as 107.04, 127.02, 155.21 Jg−1 for heating processes and 109.86, 129.11 and 142.62 Jg−1 for cooling processes, respectively. As can be seen from these data, the crystalline domains was enhanced with increasing of PEG segments, which resulted in considerable increase in the latent heat capacities of the copolymers. Based on the DSC data in Table 1, it can be concluded that the produced copolymers had practical latent heat capacities and appropriate phase transition temperature for solar
Thermal decomposition or degradation just above the ambient temperatures limits the usage of PCMs. PCMs works isothermally however temperature fluctuates frequently during usage. For this reason, PCMs should be satisfactorily durable, which depends on the thermal degradation temperature. The thermal durability of SMA, PEG and synthesized copolymers were investigated by TGA method and its results were presented in Fig. 8. Also the thermal degradation temperatures derived from the curves are given in Table 4. As can be seen from Fig. 8 that SMA and pure PEG show one-step thermal degradation in the temperature range of 321–456 °C and 194–476 °C, respectively. Moreover, in contradiction of the expectations the copolymers degraded at one-step like pure SMA and PEG. This may be due to the fact that the degradation temperatures of SMA and PEG used in the synthesis are close to each other. Furthermore, Table 4 tabulates the degradation temperatures in the temperature range of 306–461 °C, which indicates that the produced copolymers can be used as S-SPCMs up to 306 °C in solar passive TES systems. 4. Conclusions In this work, SMA-graft-PEG copolymers were successfully synthesized as new S-SPCMs for solar passive TES applications. The chemical structures of the copolymers were confirmed by using FT-IR and 1H NMR spectroscopy analyses as morphological investigation was performed by using POM. The produced copolymers kept their solid states although crystalline phase of PEG grafted to the SMA backbone was transformed to amorphous phase. The DSC results revealed that the copolymers had appropriate phase transition temperatures in the range of 39–45 °C and high latent heat capacity in the range of 107–155 Jg−1. Moreover, the phase transition enthalpies and temperatures of the copolymers were increased by depending on the amount of PEG side chains bonded to the backbone. Although the copolymers were subjected to 5000 heating-cooling cycling noteworthy change in their LHTES properties was no observed and they protected their chemical structures. The TGA results exhibited that the synthesized SMA-graft224
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Thermal energy storage characteristics of poly(styrene-co-maleic anhydride)-graft-PEG as polymeric solid–solid phase change materials
MARK
⁎
Ahmet Sarıa, , Alper Biçerb, Cemil Alkanb a b
Karadeniz Technical University, Department of Metallurgical and Material Engineering, 61080 Trabzon, Turkey Department of Chemistry, Gaziosmanpaşa University, 60240 Tokat, Turkey
A R T I C L E I N F O
A BS T RAC T
Keywords: SMA PEG Solid-solid PCM Latent heat Thermal energy storage
Poly(styrene-co-maleic anhydride)(SMA)-graft-polyethylene glycol)(PEG) copolymers were synthesized as novel polymeric solid–solid phase change materials (S-SPCMs). The synthesized copolymers showed latent heat storage and release ability by means of the phase transition from crystalline phase to amorphous phase of PEG bonded to the skeleton as side chains. The chemical structures of the polymeric S-SPCMs were confirmed by fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR) analysis techniques. The transformation of crystalline phase to amorphous phase were investigated using and polarized optical microscopy (POM) techniques. The latent heat thermal energy storage (LHTES) of the S-SPCMs were measured by differential scanning calorimetry (DSC) analysis method. Thermal degradation temperature limits of the SSPCMs were determined by thermogravimetry analysis (TGA) method. The DSC analysis indicated that the synthesized S-SPCMs demonstrated typical solid-solid phase transitions in the temperature range of about 40– 45 °C and had considerable high latent heat capacity between 107 and 155 J g−1. The TGA results showed that the polymeric S-SPCMs were durable thermally up to at least 300 °C. The thermal cycling test exposed that the S-SPCMs protected their LHTES properties even after 5000 heating/cooling treatment. All findings indicated that the prepared SMA-graft-PEG copolymers posses good thermal energy storage (TES) potential for passive solar heating and cooling applications.
1. Introduction Thermal energy can be stored in sensible, latent, and thermochemical forms. Among all, latent heat storage can be considered as the most attracting method. In this technique, PCM has a function energy storage/release during phase change at a specified temperature. PCMs are advantageous due to their high energy storage capacity, isothermal utility, controlled phase change process, etc [1,2]. Many organic and inorganic PCMs and their mixtures have been used in various applications such as solar water heating, solar heating/cooling of residential and agricultural buildings, temperature-regulating in textiles, heat management of electronics, biomedical and biological carrying systems and so on [3–10]. Depending on the type of phase change, PCMs are distributed to three groups: solid–solid (S-S), solid–liquid (S-L), and liquid–gas (L-G). A S-SPCM can store/release a great deal of latent heat during heating/cooling cycles by transforming from a crystalline phase to another phase or amorphous phase at constant temperature. Such type PCMs have some extra advantages because they show no seepage above phase transition temperature, need not to be encapsulated, and can be processed easily. There are many S-SPCMs [11] ⁎
categorized as inorganic S-SPCMs [12–16], certain hydrocarbon crystal S-SPCMs [17], organic polyols [17–22] and polymer based S-SPCMs. Polymer-based S-SPCMs have developed rapidly in recent years because of direct processing advantage into arbitrary shape, even being used as an element of TES system directly. So, several works were carried out to produce novel polymeric S-SPCMs [23–28]. The physical or chemical pathways were applied to produce S-SPCMs. However, the key difficulty in the physical way is non-homogenous dispersion of PCM in polymer matrix [29–31]. Moreover, the S-SPCMs are produced chemically via grafting, blocking, and cross-linking reactions, which are occurred between polymer material used as hard segment and PCM as soft segment. Poly(ethylene glycol)s (PEGs) are promising polymeric PCMs because of their good LHTES characteristics [11,22,32]. The LHTES characteristics of them mainly depend on the molecular weights [33]. Moreover, the polymeric S-SPCMs which include PEG with lover molecular weights than 4000 do not show satisfying thermal energy storage properties according to literature [34]. In order to obtain novel polymeric S-SCMs with considerable high LHTES capacity, several PEG-based polymeric S-SPCMs such as polyurethane-graft-PEG [35],
Corresponding author. E-mail addresses: [email protected], [email protected] (A. Sarı).
http://dx.doi.org/10.1016/j.solmat.2016.12.001 Received 28 September 2016; Received in revised form 31 October 2016; Accepted 1 December 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.
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PEG (2:1) and SMA-graft-PEG (1:1).
cellulose diacetate-graft-PEG [36], chlorinated polypropylene-graftPEG [37], cellulose-graft-PEG [38,39], poly(vinylalcohol)-graft-PEG [40], poly(ethylene terephthalate)-PEG [41] and polystyrene-graftPEG [42] have been prepared using different polymerization methods or modifications. The results reported in above studies showed that the LHTES properties of the S-SPCMs are increased with increase in the molecular weight and mass fraction of PEG grafted to the polymer skeleton. SMA is consisted of styrene and maleic anhydride monomers. SMA at high molecular weights are generally used in engineering plastics as the SMA solutions in alkaline are used in the field of sizing (paper), binders, and coatings [43]. Moreover, the applicability of SMA can be enlarged by PEG modification. The produced copolymers in this have high TES potential due to their S-S phase change facility. According to our best knowledge, the synthesis, LHTES properties, thermal reliability and durability of SMA-graft-PEG copolymers have never been reported in literature. In this regard, SMA-graft-PEG copolymers were synthesized in this work as novel polymeric SSPCMs. In order to investigate the amount effect of PEG side chains on the LHTES properties of whole polymeric structure, it was grafted to SMA skeleton at three different mole ratios of 4:1, 2:1 and 1:1. The synthesized SSPCMs were characterized by FT-IR, 1H NMR and POM techniques. The LHTES of the S-SPCMs were measured by DSC method as their thermal durability properties was determined by TGA technique. Moreover, the thermal reliability of the produced S-SPCMs was studied by exposing them accelerating heating/cooling process including 5000 cycles.
2.3. Characterization SMA-graft-PEG copolymers were characterized by using FT-IR and H NMR spectroscopy analysis techniques. The FT-IR spectra of PEG, SMA and the synthesized copolymers as novel S-SPCMs were taken on a KBr disk in the wave number range of 4000–400 cm−1 using FT-IR spectrophotometer (JASCO 430 model). The 1H NMR spectra of the SSPCMs were taken using a BRUKER 400 NMR spectrometer (Bruker Corp., USA). The copolymer samples were dissolved in CDCl3 with a concentration of 100 mg/0.5 mL. POM investigations were carried out using a Leica DM EP model microscope equipped with a video camera. Thermal energy storage properties of SMA-graft-PEG copolymers were investigated using a Perkin Elmer Jade model DSC instrument at a heating/cooling rate of 10 °C min−1 between −15 °C and 90 °C. The SMA-graft-PEG copolymers were subjected to a thermal cycling test using a thermal cycler (BIOER TC-25/H model). The cycling test was carried out by conducting accelerated heating/cooling performed 5000 times. Thermal reliability of SMA-graft-PEG copolymers was determined by DSC analysis after thermal cycling. The chemical stability of the copolymers after cycling process was checked by FT-IR analysis. Thermal durability of the copolymers was also investigated by using a Perkin- Elmer TGA7 model instrument. The analysis was performed between 25 and 600 °C at a heating rate of 10 °C min−1. 1
3. Results and discussion
2. Material metods
3.1. FT-IR spectroscopy analysis of SMA-graft-PEG copolymers
2.1. Materials
Fig. 2a shows the FT-IR spectra of SMA, PEG and SMA-graft-PEG copolymers. It can be seen from the spectra that SMA had two main functional groups (-C˭O and -C-O) and their stretching vibration peak were observed at 1222 cm−1 and 1778 cm−1, respectively. PEG had three main stretching vibrations of -OH, -C˭O and –C-O groups detected at 3463 cm−1, 1108 cm−1 and 1645 cm−1, respectively. On the other hand, the FT-IR spectra of SMA-graft-PEG(4:1), SMA-graftPEG(2:1) and SMA-graft-PEG(1:1) were shown in Fig. 2b. As seen from the spectral data, the stretching vibration bands of –C˭O, -C-O and -OH groups were detected at 1726, 1109 and 3408 cm−1 for SMAgraft-PEG(4:1) copolymer, at 1722, 1113, and 3415 cm−1 for SMAgraft-PEG(2:1) copolymer and at 1722, 1109, and 3407 cm−1 for SMAgraft-PEG(1:1) copolymer, respectively. The stretching bands of the characteristic groups of precursors shifts slightly in the SSPCMs which confirms grafting reaction. The 1H NMR spectra of PEG, SMA and SMA-graft-PEG(1:1) copolymer were shown as an example in Fig. 3. The following results can derived from these spectra: (1) the g proton of the copolymer observed at 3.3 ppm was not seen in the spectra of SMA and PEG. (2) The c proton of PEG at 3.6 ppm shifted to 3.9 ppm in the copolymer. (3) The c proton of SMA at 2.7 ppm shifted to 2.5 ppm (f proton) in the
SMA (Mw: 224.000 g mol−1, maleic anhydride ∼7 wt%) and PEG Mw: 6000 g mol−1 were purchased from Aldrich Company. Chloroform was obtained from Merck Company and used without further purification. Technical grade phosphoric acid was supplied from Fluka Company. 2.2. Synthesis of SMA-graft-PEG copolymers The SMA-graft-PEG copolymers have been synthesized by grafting PEG to SMA backbone as shown in the reaction scheme (Fig. 1). The reaction was carried out in a reaction set-up consisting of a 500 mL flask equipped with a reflux condenser, a mechanical stirrer and a thermometer. SMA and PEG at calculated amounts and chloroform (200 mL) was solved into a flask. The reaction system was refluxed in the existence of phosphoric acid (1–2 drops) as catalyst at 80 °C for 24 h. After completion of copolymerization reaction, chloroform was removed using a rotary evaporator. Moreover, the same procedure was repeated for the other SMA/PEG mole ratios of 2:1 and 4:1. The produced copolymers were sealed as SMA-graft-PEG (4:1), SMA-graft-
Fig. 1. The reaction schema regarding with the synthesis of the S-SPCMs.
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Fig. 2. FT-IR spectra of SMA, PEG and the synthesized copolymers.
Fig. 3. 1H NMR spectra of SMA, PEG and the synthesized SMA-graft-PEG(1:1) copolymer
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Fig. 4. POM images of PEG and the synthesized copolymers (a) PEG below the solid–solid phase change temperature (25°C), (b) SMA-graft-PEG(4:1) below the solid–solid phase change temperature (25°C), (c) SMA-PEG(4:1) over the phase change temperature (70 °C), (d) SMA-graft-PEG(2:1) below the phase change temperature (25°C), (e)SMA-graft-PEG(2:1) over the phase change temperature (70°C), (f) SMA-graft-PEG(1:1) below the phase change temperature (25°C), and (g) SMA-graft-PEG(1:1) over the phase change temperature (70°C)
Table 1 The LHTES properties of pure PEG and the synthesized S-SPCMs. Heating period
PEG SMA-graftPEG (4:1) SMA-graftPEG (2:1) SMA-graftPEG (1:1)
s–s,
(°C)
ΔH g−1)
Cooling period s–s,
(J
s–s,
(°C)
ΔH g−1)
s–s,
Phase transition
T
S-L S-S
59.80 39.69
221.31 107.04
36.20 23.68
155.5 109.86
S-S
42.59
127.02
32.29
129.11
S-S
45.03
155.21
33.56
142.62
T
(J
Fig. 5. DSC thermograms of PEG and the synthesized copolymers.
PEG(4:1), SMA-graft-PEG(2:1), and SMA-graft-PEG(1:1) copolymers were characterized by POM analysis and the results were shown in Fig. 4(a–g). As clearly observed from the POM (Fig. 4a), PEG has spherulite crystal structures at room temperature. The spherulite crystalline phases of the copolymers below the S-S phase transition temperatures were also (Fig. 4b, d, and f). The spherulite structures were much smaller than those of pure PEG. There is no change in the morphology of SMA-graft-PEG copolymers up to their phase-transition temperatures. However, as the temperature was approached the phase
copolymer. (4) The b proton of SMA at 3.0 ppm shifted to 3.5 ppm (d proton) in the copolymer. (5) The b proton of PEG was not observed in the spectra of the copolymer. The 1H NMR spectroscopy results indicates that the grafting reaction was achieved successfully. 3.2. Morphology analysis of SMA-graft-PEG copolymers The phase transformations of pristine PEG and SMA-graft222
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Table 2 Comparison of latent heat values of the synthesized S-SPCMs in this work with those of the other S-SPCMs reported in literature.
Table 3 The LHTES properties of the synthesized S-SPCMs after 5000 thermal cycling. Heating period
S-SPCMs
Latent heat capacity (J/g)
References
PU-PEG copolymers PU-PEG block copolymer MDI-PE-PEG cross-linking copolymer CPP-PEG copolymers Cellulose-PEG copolymer PEG/PET copolymers PS-graft-PEG6000 copolymers PS-graft-PA copolymers PS-graft-SA copolymers PScoAA-graft-SA copolymers SMA-graft-PEG copolymers
103–118 139 153 68–143 104 5–22 116–174 26–40 45–55 34–74 107–155
[24] [27] [34] [37] [39] [41] [42] [49] [50] [52] This work
s–s,
(°C)
ΔH
Cooling period s–s,
(J g−1)
T
s–s,
(°C)
ΔH
s–s,
S-SPCMs
T
SMA-graft-PEG (4:1) SMA-graft-PEG (2:1) SMA-graft-PEG (1:1)
40.24
120.82
25.95
115.48
40.45
128.58
35.30
122.30
44.50
150.44
35.80
140.85
(J g−1)
transition points of copolymers, the spherulite phases were destroyed and then disappeared completely (Fig. 4c, e, and g). This process can be also evaluated as transformation of crystal phase of grafted PEG chains to amorphous phase. In this case, the PEG side groups on the skeleton could only be vibrated and rotated, but could not be translated freely due to the destruction of the crystalline structure of copolymers. However, copolymers kept their solid state because of the high mechanical strength of SMA backbone even they were heated above their phase transition temperatures.
3.3. The LHTES properties of the SMA-graft-PEG copolymers The LHTES properties of the produced copolymers such as S-S phase transition temperatures and enthalpies were measured by DSC
Fig. 7. DSC thermograms of the synthesized copolymers after thermal cycling.
Fig. 6. FT-IR spectra of the synthesized copolymers after thermal cycling.
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passive TES applications for heating and cooling purposes. Especially SMA-graft-PEG(1:1) copolymer is the most promising owing to its highest latent heat capacity. Additionally, Table 2 presents the comparison of the latent heat values of the synthesized S-SPCMs with those of other S-SPCMs reported in the literature. As can be clearly seen from the tabulated data, the latent heat capacity of the copolymers synthesized in this work is higher than the most of S-SPCMs in the previous studies [24,27,34,37,39,41,42,49,50,52]. 3.4. Thermal reliability of SMA-graft-PEG copolymers Thermal reliability property is one of the most important factors for PCMs used in passive TES systems. Significant changes in LHTES property of a PCM depending on thermal cycling means its unreliability for TES targets. The cycling test is consisted of repeated heating/ cooling cycling 5000 times above/below phase transition temperatures of the SMA-graft-PEG copolymers. The DSC analysis results of the produced copolymers and the measured LHTES properties after the cycling test were shown in Fig. 6 and in Table 3, respectively. The results for the related materials show that there are no significant changes LHTES properties of the copolymers. Therefore, it is remarkably noted that the prepared copolymers have good thermal reliability after repeated 5000 thermal cycling. The chemical structures of the copolymers are also checked after the cycling test. As seen from the FTIR spectrums shown in Fig. 7, the shape of the characteristic bands and their wavenumbers were not changed after the cycling test. These result proved good long-period structural stability of the synthesized SSPCMs.
Fig. 8. TGA curves of the synthesized copolymers.
Table 4 TGA results obtained for pure SMA, PEG and the synthesized S-SPCMs.
SMA PEG SMA-graft-PEG (4:1) SMA-graft-PEG (2:1) SMA-graft-PEG (1:1)
Degradation interval (°C)
Mass loss (%wt)
194–476 321–456 306–455 309–460 302–461
95.01 97.83 90.30 95.80 88.92
3.5. Thermal durability of SMA-graft-PEG copolymers analysis. The DSC curves of SMA-graft-PEG polymers were shown in Fig. 5 and thermal properties were also summarized in Table 1. The phase transition process of pure PEG is carried out from solid to liquid and white crystal of pure PEG changes to a transparent liquid temperature is raised to about 70 °C. On the other hand, the synthesized S-SPCMs remains their solid states during the heating process although the temperature is above 70 °C. The on-set peak values of endothermic and exothermic of synthesized S-SPCMs were measured at 39.69 and 23.68 °C for SMA-graft-PEG(4:1), 42.59 and 32.29 °C for SMA-graft-PEG(2:1), 45.03 and 33.56 °C for SMA-graft-PEG(1:1). The phase transition temperatures of the synthesized S-SPCMs is very suitable for the passive cooling of electronic devices, thermo-regulating of fibers and solar passive heating applications in building agricultural greenhouse envelopes. Moreover, as can be seen from these results, there is considerable difference between the S-S phase transition temperatures obtained for heating and cooling periods of the SSPCMs. This case is was due to two reasons: (i) because of the subcooling behavior of PEG side chains in the copolymer structures. The similar behavior were observed in case of the polyurethane-PEG block copolymer [24,27]. (ii) due to the effect of heating/cooling rates on the phase change temperature range and the hysteresis between heating and cooling curves. Higher heating/cooling rates increase the hysteresis and shift the phase change temperature range towards colder temperatures [44]. The effects of hysteresis and subcooling on thermal performance of PCM [45-51]. As also seen from Table 1, the latent heat capacities of the synthesized SMA-graft-PEG(4:1), SMA-graft-PEG(2:1) and SMAgraft-PEG(1:1) copolymers were measured as 107.04, 127.02, 155.21 Jg−1 for heating processes and 109.86, 129.11 and 142.62 Jg−1 for cooling processes, respectively. As can be seen from these data, the crystalline domains was enhanced with increasing of PEG segments, which resulted in considerable increase in the latent heat capacities of the copolymers. Based on the DSC data in Table 1, it can be concluded that the produced copolymers had practical latent heat capacities and appropriate phase transition temperature for solar
Thermal decomposition or degradation just above the ambient temperatures limits the usage of PCMs. PCMs works isothermally however temperature fluctuates frequently during usage. For this reason, PCMs should be satisfactorily durable, which depends on the thermal degradation temperature. The thermal durability of SMA, PEG and synthesized copolymers were investigated by TGA method and its results were presented in Fig. 8. Also the thermal degradation temperatures derived from the curves are given in Table 4. As can be seen from Fig. 8 that SMA and pure PEG show one-step thermal degradation in the temperature range of 321–456 °C and 194–476 °C, respectively. Moreover, in contradiction of the expectations the copolymers degraded at one-step like pure SMA and PEG. This may be due to the fact that the degradation temperatures of SMA and PEG used in the synthesis are close to each other. Furthermore, Table 4 tabulates the degradation temperatures in the temperature range of 306–461 °C, which indicates that the produced copolymers can be used as S-SPCMs up to 306 °C in solar passive TES systems. 4. Conclusions In this work, SMA-graft-PEG copolymers were successfully synthesized as new S-SPCMs for solar passive TES applications. The chemical structures of the copolymers were confirmed by using FT-IR and 1H NMR spectroscopy analyses as morphological investigation was performed by using POM. The produced copolymers kept their solid states although crystalline phase of PEG grafted to the SMA backbone was transformed to amorphous phase. The DSC results revealed that the copolymers had appropriate phase transition temperatures in the range of 39–45 °C and high latent heat capacity in the range of 107–155 Jg−1. Moreover, the phase transition enthalpies and temperatures of the copolymers were increased by depending on the amount of PEG side chains bonded to the backbone. Although the copolymers were subjected to 5000 heating-cooling cycling noteworthy change in their LHTES properties was no observed and they protected their chemical structures. The TGA results exhibited that the synthesized SMA-graft224
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