Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage

Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage

Applied Energy xxx (2014) xxx–xxx Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Synth...

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Applied Energy xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage q Changzhong Chen ⇑, Wenmin Liu ⇑, Hongwei Wang, Kelin Peng School of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China

h i g h l i g h t s  Three new kinds of SSPCMs were synthesized with different skeleton materials.  The phase change properties and thermal stability of SSPCMs were investigated.  The maximum enthalpy in heating (cooling) process is 107.5 kJ/kg (102.9 kJ/kg).  The rigid groups and crosslinking structure of SSPCMs improve the thermal stability.  The SSPCMs could be applied in the temperature range of 30–70 °C.

a r t i c l e

i n f o

Article history: Received 8 February 2014 Received in revised form 24 September 2014 Accepted 2 December 2014 Available online xxxx Keywords: Phase change properties Polyethylene glycol Solid–solid phase change Synthesis Thermal energy storage

a b s t r a c t Three kinds of new polymeric SSPCMs with different crosslinking structures were synthesized and characterized for thermal energy storage. In the SSPCMs, three hexahydroxy compounds (sorbitol, dipentaerythritol and inositol) were individually employed as the molecular skeleton and polyethylene glycol (PEG) was used as the phase change functional chain. The molecular structure, crystalline properties, phase change behaviors, thermal reliability and stability of the synthesized SSPCMs were investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetry (TG), respectively. The results show that the prepared SSPCMs possess high thermal energy storage density and an applicable temperature range of 30–70 °C, and the maximum phase change enthalpy in the heating and cooling process for the SSPCMs is 107.5 kJ/kg and 102.9 kJ/kg, respectively. The prepared SSPCMs have good reusability, excellent thermal reliability and stability from the heating-cooling thermal cycle test and TG curves. The resultant SSPCMs could be potentially applied in the areas of thermal energy storage and temperature-control. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, the shortage of mineral resources and continuous deterioration of environment become a global issue [1,2]. More effective utilization of traditional energy and vigorous development of renewables are common endeavors in all spheres of mankind activity. However, useful energy sources are often available in different place and time than they are needed, which results in a large waste of energy. Thus, improving energy management is an important task in many industries [3]. TES is a promising method for energy savings and improvement of energy efficiency. There

q This paper is included in the Special Issue of Energy Storage edited by Prof. Anthony Roskilly, Prof. Phil Taylor and Prof. Yan. ⇑ Corresponding authors. Tel.: +86 377 63513540. E-mail addresses: [email protected] (C. Chen), [email protected] (W. Liu).

are primarily three TES ways: sensible heat storage, latent heat storage and reversible chemical reaction heat storage [4,5]. Among them, latent heat storage using PCMs is the most efficient way of TES because of the large thermal storage density of PCMs at a nearly constant temperature. Thus, the research and development of PCMs with high thermal properties is the focus and crux of TES for various applications [1–10] such as intelligently air-conditioned buildings, thermostatic regulators, thermoregulated textiles, agricultural greenhouses, and so on. According to the phase change states, PCMs can be generally divided into SLPCMs, SSPCMs and LGPCMs. The application of LGPCMs is greatly restricted because of the large changes in volume during the phase change process. SLPCMs include various inorganic salt hydrates, paraffin waxes, PEG, fatty acids and their eutectics, and they have been researched extensively for TES in last decades [1–9]. Special storage container is an essential part in the TES system of SLPCMs for encapsulating the liquid phase generated

http://dx.doi.org/10.1016/j.apenergy.2014.12.004 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Chen C et al. Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.12.004

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Nomenclature Abbreviations TES thermal energy storage PCMs phase change materials SLPCMs solid–liquid phase change materials SSPCMs solid–solid phase change materials LGPCMs liquid–gas phase change materials PEG polyethylene glycol MPEG polyethylene glycol monomethyl ether CDA cellulose diacetate PU polyurethane PVA poly (vinyl alcohol) PS polystyrene MDI 4,40 -diphenylmethan diisocyanate PE polyethylene TDI toluene-2,4-diisocyanate PGMA poly (glycidyl methacrylate) DMF N,N-dimethylformamide

during the phase change. It results in the increase of thermal resistance between PCMs and heat transfer fluid, and also augments the running cost of the system [4,5]. Compared with LGPCMs and SLPCMs, SSPCMs have many unique advantages: no gas or liquid generation, small volume changes during phase change process, less stringent container requirements and greater design flexibility [11–13]. Though SSPCMs are potentially efficient and promising latent heat storage materials, the applicable range of available SSPCMs (e.g., polyalcohols, polyethylene, layered perovskites, etc.) is limited to some extent for the drawbacks of lower latent heat and higher phase change temperatures which are undesirable for low temperature applications [11]. Thus, synthesis of novel SSPCMs with high phase change enthalpy and low phase change temperature is a significant and pressing work. Recently, the preparation and application of polymeric SSPCMs has arose growing interests of considerable scientists [14–33], and many novel polymeric SSPCMs were obtained by chemical grafting, blocking and crosslinking copolymerization using the solid frameworks as a hard segment and SLPCMs as a soft segment. PEG and its derivatives are typical SLPCMs with outstanding characteristics of nontoxicity, resistance to corrosion, relatively large enthalpies and wide selectivity of molecular weight. Thus, they are usually used as the phase change functional chains (soft segment) in the molecule of polymeric SSPCMs [17–33]. Jiang et al. [17] reported a series of SSPCMs by grafting polymerization in which CDA acted as the main chain and PEG with different molecular weights acted as the grafting side chain. Later, Cheng et al. [18] also prepared spinnable grafting copolymers as SSPCMs based on MPEG and CDA. Li et al. [19,20] synthesized cellulose-graft-MPEG SSPCMs in common organic solvent and ionic liquid respectively. Other polymeric SSPCMs containing the soft segment of PEG and its derivatives such as PEG-based PU [21–27], PVA-graft-PEG [28], PS-graft-PEG [29], crosslinked PEG/MDI/PE [30], phenyl ethylene/TDI/MPEG [31] were successively reported. In our previous works [32,33], the crosslinked PEG-PGMA and PEG/MDI/glucose copolymers were also synthesized and characterized as novel SSPCMs. It is worth noting that only one skeleton material was used to prepare SSPCMs in every reported work mentioned above, which resulted in the same molecular structure of SSPCMs even varying the molecular weight of PEG. In this way, it is difficult to display and clarify the relationship between the molecular structure of synthesized

FTIR DSC exo XRD TG DTG

Fourier transform infrared spectroscopy differential scanning calorimetry exothermic X-ray diffraction thermogravimetry derivative thermogravimetry

English symbols T phase change temperature (°C), represented by the onset temperature in the DSC curve DH phase change enthalpy (kJ/kg), represented by the area under the DSC curve Subscripts Heating in the heating process Cooling in the cooling process

SSPCMs and their phase change properties. In order to research the influence of molecular structure of SSPCMs on their phase change performances, it is necessary to prepare a series of SSPCMs with the same soft segment and different skeleton materials. As we know, sorbitol, dipentaerythritol and inositol are different polyols with six hydroxyl groups in their molecules. In the present work, PEG was also selected as the phase change functional chain of SSPCMs and sorbitol, dipentaerythritol and inositol were individually selected as the molecular skeleton of SSPCMs, and three kinds of novel SSPCMs with different crosslinked structures were synthesized for TES. The similarities and differences of crystalline properties, phase transition properties and thermal stability for the three kinds of SSPCMs were discussed, which offers reference for the design and application of novel SSPCMs with high phase change performances. 2. Experimental part 2.1. Materials PEG (analytical grade, Mn = 8000, from Amresco Co., USA), D-sorbitol (from Tianjin Kemiou Chemical Reagent Co. Ltd., China), dipentaerythritol (from Aladdin Chemistry Co. Ltd., China) and inositol (from Beijing Aoboxing Biotechnology Co. Ltd., China) were dried under vacuum at 100 °C for 3–4 h before use. MDI (from Aladdin Chemistry Co. Ltd., China) was heated to 60 °C and kept for 2 h under vacuum and then filtered by a heated filter. DMF was dried using 5 Å molecular sieves for 48 h, and then distilled before use. 2.2. Synthesis of SSPCMs The synthetic route of the three kinds of SSPCMs is shown in Fig. 1. The general procedure is as follows: predetermined amounts of PEG and MDI (molar ratio of PEG and MDI = 1:2) were mixed in DMF with stirring in a thermostatic oil-bath at 90 °C under N2 for 3 h. Then, sorbitol (molar ratio of sorbitol and MDI = 1:6) was dissolved in DMF and slowly dropped into the above mixture. After stirring for 24 h, the reaction mixture was poured into a beaker. Thermal curing was conducted at 80 °C for 24 h in the drying oven. Then, the products were kept in vacuum at 40 °C for two weeks to eliminate any volatile matter before testing. The synthetic process

Please cite this article in press as: Chen C et al. Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.12.004

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Fig. 1. The synthetic route of SSPCMs.

Fig. 2. The photographs of experimental apparatus of synthesis reaction and the final products of SSPCMs.

of SSPCMs based on the skeleton of dipentaerythritol and inositol is the same as the above procedure. The synthesized SSPCMs using sorbitol, dipentaerythritol and inositol as skeleton were labeled

as PCM-S, PCM-D and PCM-I, respectively. The images of experimental apparatus of the synthesis reaction and the final product of SSPCMs are shown in Fig. 2.

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2.3. Characterization 2.3.1. FTIR The chemical structure of the samples were measured by FTIR, and the FTIR spectra of all the reactants and products were obtained using Infrared Spectrophotometer (NICOLET-760, Nicolet Co., USA) in the wavenumber range of 400–4000 cm1. Before the measurement, every sample was mixed with KBr powders and then pressed into a small pellet. 2.3.2. DSC The phase change properties (phase change temperatures and enthalpies) of pristine PEG, sorbitol, dipentaerythritol, inositol and the synthesized SSPCMs were measured by DSC curves using a Simultaneous Thermal Analysis Apparatus (STA 449 F3 JupiterÒ, Netzsch, Germany). Indium was used for calibration. The DSC analyses were carried out at 2 K/min heating/cooling rate under a constant stream of nitrogen at a flow rate of 30 mL/min. To eliminate the heat history of material, the sample (about 10 mg) loaded in an aluminum crucible was firstly undergone a thermal cycle (containing a heating process from room temperature to 100 °C and a cooling process from 100 °C to 25 °C). Next, another thermal cycle consisted of a consecutive heating and cooling process (in the temperature interval of 25–80 °C) was carried out for the reactants, and another five thermal cycles with the same process were conducted for SSPCMs. All DSC curves of the samples were recorded from the second thermal cycle. 2.3.3. XRD The crystalline properties of the samples were characterized by XRD, and the XRD patterns of the samples were recorded at room temperature on an automatic powder diffractometer (D8 Advance, Bruker-AXS, Germany) with Ni-filtered Cu Ka radiation at 40 kV and 30 mA. The scans were obtained by using a 0.02° step programmed with a collection time of 0.1 s per step. Measurements were performed in the range of 5° < 2h < 50°. 2.3.4. TG The thermal stability of the samples was characterized by TG curves also using a Simultaneous Thermal Analysis Apparatus (STA 449 F3 JupiterÒ, Netzsch, Germany). About 10 mg of sample was placed into an aluminum oxide crucible and the profiles were recorded from room temperature to 600 °C under a nitrogen atmosphere at a heating rate of 10 K/min. During the process, the simultaneous DSC curves were also recorded.

FTIR spectra of PEG, sorbitol, dipentaerythritol and inositol were also shown in Fig. 3 for comparison. In the FTIR spectrum of pristine PEG shown in Fig. 3(a), the characteristic absorption peaks at 3432 cm1 and 1099 cm1 are attributed to the O–H stretching vibration and the C–O–C symmetrical stretching vibration respectively, and the strong absorption peaks of the C–H bonds appears at 2889 cm1, 962 cm1 and 842 cm1. The strong peak of O–H stretching vibration at about 3392 cm1, 3435 cm1 and 3392 cm1 appears in the FTIR spectra of sorbitol, dipentaerythritol and inositol shown in Fig. 3(b), (d) and (f) respectively. It is clear that the C–O–C symmetrical stretching vibration at 1099 cm1 and C–H stretching/bending vibration at 2889 cm1, 962 cm1and 842 cm1 also appears in the FTIR spectra of PCM-S, PCM-D and PCM-I shown in Fig. 3(c), (e) and (g), but the strong hydroxyl absorption peaks above 3000 cm1 and characteristic absorption peak of isocyanate from MDI at 2265 cm1 disappears completely. Moreover, the newly formed peaks at 1542 cm1 and 1726 cm1 are assigned to the amide and carbonyl vibration of –NHCOO– groups of the SSPCMs, and the peak at 1599 cm1 associated with the stretching vibration of benzene rings is identified in Fig. 3(c), (e) and (g). It is obvious that the chemical reactions occurred between the isocyanate groups of MDI and hydroxyl groups of PEG and polyols. Therefore, we could conclude that PCM-S, PCMD and PCM-I copolymer with the anticipant crosslinked structure had been gained. 3.2. Crystalline properties of SSPCMs To reveal the crystalline properties of the synthesized SSPCMs, the XRD patterns of reactants and SSPCMs at room temperature are shown in Fig. 4. It is easily observed that all the samples have sharp diffraction peaks at different diffraction angles, which indicates that they show good crystallization. Clearly, there are two main diffraction peaks at about 19.3° and 24.6° shown in the XRD curve of pristine PEG, and the latter is evidently stronger than the former. The XRD curve of PCM-S has diffraction patterns similar to those of pure PEG, and the position of the diffraction peaks remains unchanged after the crosslinking reaction, which demonstrates that pristine PEG and PCM-S have the similar crystal structure and crystal cell type [21,24,28]. The main difference between the XRD curve of PEG and PCM-S is the intensity of diffraction

2.4. Accelerated thermal cycling test In order to confirm the reusability, thermal reliability and stability of the synthesized SSPCMs, the accelerated thermal cycling test of the materials was conducted. The test was performed by a hot stage (STC200, Instec, USA) as the following step. The aluminum pan loaded appropriate amount of sample was put on a hot stage with a typical thermal cycling program consisted of 500 times consecutive heating and cooling process (in the temperature interval of 25–80 °C). The time of each thermal cycling was set 20 min for a heating–cooling process, and the total time of the thermal cycling test lasted for about one week. Afterwards, thermal properties and chemical stability of SSPCMs after thermal cycling were performed by DSC and FTIR, respectively. 3. Results and discussion 3.1. FTIR study To reveal the composition and chemical structure of the synthesized SSPCMs, FTIR spectra of the samples were shown in Fig. 3.

Fig. 3. FT-IR spectra of (a) PEG; (b) sorbitol; (c) PCM-S; (d) dipentaerythritol; (e) PCM-D; (f) inositol; and (g) PCM-I.

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PCM-I and PCM-D has been destroyed to some extent by the crosslinking structure of SSPCMs. From Fig. 4, it is clear that the diffraction peaks of PCM-D have biggest difference with those of PEG, which shows that the crystallization of soft segments in PCM-D was most affected by the skeleton. 3.3. Phase change properties of SSPCMs

Fig. 4. XRD patterns of the reactants and the synthesized SSPCMs.

peaks. Apparently, the intensity of diffraction peaks of PCM-S is lower than that of PEG, which indicates that the crystalline region of the PEG segments in PCM-S is smaller than that of pure PEG. Moreover, there are also two main diffraction peaks in the XRD curve of PCM-D and PCM-I. Unlike the XRD curve of PEG and PCM-S, the diffraction peak in the XRD curve of PCM-D at lower diffraction angle is merged with the strong and sharp diffraction peak at 19.2° from dipentaerythritol, and the two diffraction peaks in the XRD curve of PCM-I have almost the same intensity, which indicates the crystalline region of PEG segments in PCM-D and PCM-I is far less perfect compared with pure PEG and PCM-S. It is concluded that the crystalline structure of PEG segment in

To investigate the phase change properties of samples, the DSC measurement was employed. In most previous literatures about the research of SSPCMs, the heating/cooling rate during DSC measurement was usually set as 10 K/min [10,11,13,14,17– 20,22,26,28–30,32] or 5 K/min [12,15,16,21,23,27]. As we know, the measuring conditions especially the used heating/cooling rate have significant influences on the results of DSC measurement. Generally, the phase change properties of samples degrade obviously with the decrease of heating/cooling rate [34]. In the present work, the heating/cooling rate with 2 K/min was used for all the samples during the DSC measurement, which is a balance we found between improving test resolution and obtaining reasonable phase change enthalpies. Fig. 5 gives the DSC curves of PEG, sorbitol, dipentaerythritol and inositol. Obviously, the strong endothermic and exothermic peaks in the DSC curve of PEG show that PEG has solid–liquid phase change in the temperature range of 35–70 °C. However, there is no endothermic and exothermic peak in the DSC curves of three hexahydroxy compounds in the same temperature interval, which shows that phase change had not underwent for sorbitol, dipentaerythritol and inositol in that temperature range. Fig. 6 shows the DSC curves of the synthesized SSPCMs, and the corresponding data of thermal properties (phase change enthalpies and phase change temperatures) are tabulated in Table 1. Similar to pristine PEG, PCM-S, PCM-D and PCM-I also have strong endothermic and exothermic peaks in the temperature range of 30–70 °C from Fig. 6, which indicates that the three types of

Fig. 5. DSC curves of PEG, sorbitol, dipentaerythritol and inositol.

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Fig. 6. DSC curves of the synthesized SSPCMs.

Table 1 Phase change properties of PEG and the synthesized SSPCMs from the DSC curves. Samples

Theating (°C)

DHheating (kJ/kg)

Tcooling (°C)

DHcooling (kJ/kg)

Phase transition

PEG PCM-S PCM-D PCM-I

60.4 59.7 56.1 59.9

148.1 107.5 91.0 92.8

47.6 44.0 45.6 45.8

139.3 102.9 86.1 87.8

Solid–liquid Solid–solid Solid–solid Solid–solid

synthesized materials have reversible thermal storage and release properties below 100 °C. It is worth noting that the phase transition process of PEG and the synthesized SSPCMs are quite different. As a typical SLPCM, PEG changes from a white crystal solid to a transparent liquid when the temperature is raised to above 61 °C. Contrary, three types of SSPCMs remain solid state even if the temperature is higher than 100 °C, which confirms that the latent heat storage/releasing properties of the synthesized SSPCMs at 30–70 °C are caused by their solid–solid phase transitions between the crystalline state and amorphous state. From Fig. 6, the DSC curves of the five thermal cycles for each sample are almost overlapped, which indicates that these SSPCMs have repeatability for thermal energy storage. Apparently, DHheating and DHcooling for PCM-S, PCM-D and PCM-I shown in Table 1 are lower than those of pristine PEG, which indicates that the endothermic and exothermic capacity of all synthesized SSPCMs has been weakened to some extent compared with pristine PEG. As shown in Fig. 1, the molecular chain of SSCPMs is composed of hard segments and soft segments, and the solid– solid phase change of SSPCMs are caused by the variation of PEG segments from crystalline state to amorphous state. It is clear that the arrangement and orientation of PEG segments in the molecules of SSPCMs were partially confined due to the crosslinking structure and the introduction of rigid benzene ring groups. Consequently, the crystalline regions of PEG segments in SSPCMs decrease compared with pure PEG, which is confirmed by XRD patterns of SSPCMs discussed above. Therefore, the phase transition enthalpies of the SSPCMs decline compared with pristine PEG, which is also accordance with the conclusion of the previous reports by many researchers [17,18,24,30]. From the results of DSC measurement, it is clear that the phase change properties (phase change enthalpies and phase change temperatures) of the synthesized SSPCMs heavily depend on the performances of the used phase change functional chain (e.g., PEG in the present work). Therefore, the phase change properties of synthesized SSPCMs can be regulated and controlled by varying the content and molecular weight of PEG in the reaction. Compared with the available SSPCMs (e.g. polyalcohols, PE, layered perovskites, etc.) on the market, the synthesized polymer-based SSPCMs by this method would have more extensive applications in the

low temperature range (below 100 °C) due to the adjustability of phase change properties. Nevertheless, how to cut down the cost for the synthesized SSPCMs is a problem that needs solving before the practical application. Though sorbitol, dipentaerythritol and inositol have the same number of hydroxy group in their molecules, the phase change enthalpies are quite different for the three kinds of SSPCMs. From Table 1, the order of DHheating or DHcooling for the SSPCMs is PCM-S > PCM-I > PCM-D. There are several factors influencing the phase transition properties of SSPCMs, such as the weight percent of the soft segments, spatial hindrance of the skeleton and the crystalline state of soft segments. According to the input amount of the reactants, the theoretical weight percent of PEG segments in PCM-S, PCM-D and PCM-I is 93.44%, 93.18% and 93.45%, respectively. There is no obvious difference in the weight percent of PEG segments in the three kinds of SSPCMs, so we conclude that the spatial hindrance of the skeleton and the crystalline state of soft segments are key factors affecting the phase transition properties of SSPCMs in this study. From the molecular structure of sorbitol, dipentaerythritol and inositol shown in Fig. 1, it is apparent that the order of spatial hindrance for the synthesized SSPCMs is PCM-I > PCM-S > PCM-D. Undoubtedly, the smaller spatial hindrance of the skeleton helps the movement of soft segments of the SSPCMs, and then results in the higher phase change enthalpies. However, PCM-D has the lowest phase change enthalpies from DSC measurements, which fully shows that the crystalline structure of soft segment in PCM-D is seriously suppressed and influenced by the skeleton. The conclusion agrees with the results by the XRD patterns. Therefore, to design and prepare novel SSPCMs with high phase change enthalpy, all of the above factors influencing the phase transition properties must be considered together. Table 2 Phase change properties of the synthesized SSPCMs after 500 thermal cycles. Samples

Theating (°C)

DHheating (kJ/kg)

Tcooling (°C)

DHcooling (kJ/kg)

PCM-S PCM-D PCM-I

59.2 56.0 59.5

106.9 90.6 93.1

43.3 45.5 45.8

103.3 85.8 88.2

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Fig. 7. DSC curves of the synthesized SSPCMs after 500 thermal cycles.

synthesized SSPCMs have good thermal stability during a long utility period. TG technique was also employed to verify the thermal stability of the synthesized SSPCMs, and the heating rate of TG measurement was often set as 20 K/min [16,21,28,30] or 10 K/min [14,15,19,20,27,29,32]. In this work, 10 K/min was used as the heating rate. Fig. 9 shows the TG, simultaneous DSC and DTG curves of PEG and three types of SSPCMs, and the corresponding

Fig. 8. FT-IR spectra of (a) PCM-S; (b) PCM-D; (c) PCM-I. (Dot line: original sample; solid line: sample after 500 thermal cycles).

3.4. Thermal reliability and stability of SSPCMs A lot of PCMs are limited to use due to their thermal decomposition, degradation, evaporation, and sublimation [19], so it is necessary to assess the thermal reliability and stability of novel PCMs. The accelerated thermal cycling test containing 500 heating– cooling thermal cycles was carried out for the synthesized SSPCMs, and the SSPCMs were weighed before and after thermal cycling. It is found that the weight of the samples had no variation with an accuracy to three decimal places of the dollar after the test. Therefore, the mass loss of the SSPCMs would be negligible in the potential applications. The DSC curves of the synthesized SSPCMs before and after thermal cycling test are shown in Fig. 7, and the data of phase transition properties are listed in Table 2. From Fig. 7, the DSC curves of the synthesized SSPCMs after thermal cycling have no obvious variation compared with the corresponding original samples. Moreover, the phase change temperatures and phase change enthalpies of the SSPCMs after thermal cycling are in agreement with those of the corresponding original sample within the measurement uncertainties from Table 2. Based on these results, it is concluded that the synthesized SSPCMs have good thermal reliability and excellent reusability in terms of thermal energy storing and releasing properties. The FTIR spectra of synthesized SSPCMs before and after thermal cycling are shown in Fig. 8. It can be clearly found that the FTIR spectrum of PCM-S, PCM-D and PCM-I after thermal cycling (solid line in Fig. 8) is almost the same as that of the corresponding original sample (dot line in Fig. 8), which indicates that thermal degradation did not occur in the chemical structure of the SSPCMs after thermal cycling. Therefore, it is concluded that the

Fig. 9. TG, simultaneous DSC and DTG curves of PEG and the synthesized SSPCMs.

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Table 3 Degradation data of PEG and the synthesized SSPCMs from the TG curves with heating rate of 10 K/min. Samples

PEG PCM-S PCM-D PCM-I

Degradation interval (°C)

376.2–412.9 387.2–420.1 383.9–420.8 384.8–420.4

Funds for Young Teachers of Nanyang Normal University (QN2013050) and the STP project of Nanyang Normal University (STP2012010).

Mass loss (%) At 100 °C

At 300 °C

At 500 °C

0.10 0.12 0.15 0.05

0.22 2.36 1.60 2.45

98.85 96.54 94.79 97.50

data are summarized in Table 3. As shown in Fig. 9, TG/DTG curves of PEG and all SSPCMs are almost straight line from the room temperature to 100 °C, and the mass loss of samples at 100 °C is near zero within the measurement uncertainties (shown in Table 3), which is accordance with the results in the thermal cycling test. In fact, the mass loss of the synthesized SSPCMs at 300 °C is also below 2.5%, which indicates that the weight of the materials has little loss below 300 °C. At the same time, there is a sharp endothermic peak below 100 °C in the DSC curves of all samples, which is attributed to the phase change process of PEG (solid–liquid phase change) and SSPCMs (solid–solid phase change). A broad endothermic peak appears in the DSC curves of all the samples around 400 °C, and there is a strong peak in DTG curves of all the samples in the same temperature range, which is caused by the thermal decomposition of PEG and SSPCMs. From Table 3, the onset temperature of decomposition for PCM-S, PCM-D and PCMI are higher than those of pristine PEG. Thus, thermal stability of the synthesized SSPCMs improves apparently because the rigid phenyl groups from MDI and crosslinking network play a very important role in elevating the thermal resistance of SSPCMs. The results show that the synthesized materials are very stable below 100 °C, which is helpful for their applications in low temperature range. 4. Conclusions Three types of novel polymeric SSPCMs (PCM-S, PCM-I and PCMD) with different crosslinked structures, in which sorbitol, dipentaerythritol and inositol were individually employed as the molecular skeleton and PEG was selected as the phase change functional chain, were successfully prepared for TES. It is found that the synthesized materials have typical solid–solid phase transition property, balanced thermal energy storing and releasing behaviors, high phase change enthalpies and low phase change temperature range of 30–70 °C from the DSC curves. According to the heating– cooling thermal cycling test, the three kinds of SSPCMs have excellent reusability and thermal reliability. Due to the influence of spatial hindrance of the skeleton and crystalline structure of soft segments, there are different phase transition properties for the three SSPCMs, and the order of their phase change enthalpies is PCM-S > PCM-I > PCM-D. Moreover, the SSPCMs exhibited better thermal stability than that of pristine PEG because of the rigid groups and appropriate crosslinking in the molecular structure of SSPCMs. The synthesized SSPCMs have potentials as heat storage material for energy efficient buildings, thermostatic regulators and electronic equipments. In addition, the study would provide reference and consulting for the molecule design of novel polymeric SSPCMs with high performance of TES. Acknowledgement The authors thank for the financial supports by the National Natural Science Foundation of China (No. 21404061), the Foundation of Henan Educational Committee (No. 2011A150022), the

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Please cite this article in press as: Chen C et al. Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.12.004