One-step and solvent-free synthesis of polyethylene glycol-based

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Materials and Interfaces

One-step and solvent-free synthesis of polyethylene glycol-based polyurethane as solid-solid phase change materials for solar thermal energy storage Xiang Lu, Cong Fang, Xinxin Sheng, Li Zhang, and Jinping Qu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05903 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Industrial & Engineering Chemistry Research

One-step and solvent-free synthesis of polyethylene glycol-based polyurethane as solid-solid phase change materials for solar thermal energy storage Xiang Lua, Cong Fanga, Xinxin Shengb,c*, Li Zhangb,c, Jinping Qua** a Key

Laboratory of Polymer Processing Engineering of the Ministry of Education, National Engineering Research

Center of Novel Equipment for Polymer Processing, Guangdong Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou, 510641, China b

Department of Polymeric Materials and Engineering, School of Materials and Energy, Guangdong University of

Technology, Guangzhou, 510006, China c

Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangdong University of

Technology, Guangzhou, 510006, China * Corresponding author: Xinxin Sheng ([email protected]) ** Corresponding author: Jin-ping Qu ([email protected])

Abstract: Polyethylene glycol (PEG)-based solid-solid phase change materials (SSPCMs) were first synthesized using PEG and hexamethylene diisocyanate trimer (HDIT) via one-step and solvent-free approach. When HDIT content is 5 wt%, the obtained SSPCM has no leakage at 80 °C which is higher than the melting temperature of PEG, the phase change temperatures, maximum latent heat and relative enthalpy efficiency of the obtained SSPCM are 65 °C, 136.8 J/g and 87.1%, respectively. After 200 thermal cycles, the latent heat remains nearly constant. It proves that the fabricated SSPCMs has good thermal reliability and reusability. At the same time, the SSPCMs own excellent light-thermal conversion performance and good thermal stability in a broad temperature range of applications. All the above results show that the obtained SSPCMs own great potential as direct solar light absorber for solar thermal energy storage. Keywords: Polyethylene glycol, Solid-solid phase change materials, Light-thermal conversion, Thermal energy storage, Temperature regulation

(For Table of Contents Only) 1. Introduction The need to develop effective storage and recycling for solar thermal energy has become a major concern worldwide, which has led to significant research in the field of solar thermal energy storage (TES) technology. TES technology can be classified into three basic types: sensible heat storage, latent heat storage and chemical reaction heat storage 1-6. Although chemical reaction heat storage has a high heat storage density, it has not been widely used due to technical and economic challenges 7-9. Latent heat Energy storage is based on the heat absorption or release when the phase change materials (PCMs) undergo a phase change from solid to liquid or liquid to gas or vice versa. Latent heat storage materials store 5-15 times more heat per volume than the sensible heat storage materials 10-14. In this regard, latent heat storage technology has become the most widely used and most important energy storage method.

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Polyethylene glycol (PEG) is a kind of typical solid-liquid PCMs with low cost, high enthalpy and good biocompatibility 15-17. The major issue of PEG for TES is the risk of leakage during phase changes 18-21. Thus, various strategies have been proposed to prepare PEG-based SSPCMs that can maintain solid form during phase change in the recent years 22-31 . Chen and coworkers 23 synthesized three kinds of new polymeric SSPCMs employed sorbitol, dipentaerythritol and inosito as the molecular skeleton and PEG as the phase change functional chain. However, the dimethylformamide (DMF) was used as solvent to reduce the viscosity of the reactive system. Fu et al. 24 reported a complicated process via blending the synthesized isocyanate-terminated PEG and tetrahydroxy PEG pre-polymer to prepared a thermosetting SSPCMs. But the maximum relative enthalpy efficiency of prepared SSPCMs is only 59.1 %. Sundararajan et al. 28 prepared a series of hyperbranched (HB) polyurethanes as SSPCMs via A2 +B3 approach with isocyanate terminated PEG as branching unit and phloroglucinol (PG) as aromatic core, the maximum latent heat was found to be 146.6 J/g. Although the obtained SSPCMs have a high enthalpy efficiency, the preparation process is too complicated to be industrially prepared on a large scale. Kong et al. 27 reported a solvent-free and brief synthesis route for preparing thermosetting polyurethane SSPCMs by the bulk polymerization between PEG and polyaryl polymethylene isocyanate (PAPI), but the maximum latent heat of SSPCMs in phase change process is only 111.7 J/g and the relative enthalpy efficiency is 70.8 %. The main reason is that the steric hindrance of rigid benzene ring in PAPI and cross-linking network limit the arrangement and orientation of PEG segment, and then the heat storage density of SSPCMs decrease in a certain degree 23, 26, 32, 33. Huang and coworkers 31 also reported a brief synthesis route for preparing PEG-based SSPCMs through PEG and triallyl isocyanurate (TAIC). However, both PAPI and TAIC, as small monomers, have certain chemical toxicity to the human body. For all the mentioned above, there are many ways to prepare PEG-based SSPCMs. However, there are four defects for the mentioned above: (i) a large amount of organic solvents, such as DMF, were used in the preparation process, resulting in high cost and environmental pollution; (ii) most of the preparation methods reported in the above literature require a plurality of reaction steps, the fabrication process was too complicated; (iii) the arrangement and orientation of PEG segment were limited by rigid group or cross-linking network, resulting in the lower enthalpy efficiency for the obtained SSPCMs; (iv) active monomers have a serious impact on human health. Therefore, the preparation of PEG-based SSPCMs with high enthalpy efficiency via a simple, solvent-free and environmentally friendly process remains a great challenge. Hexamethylene diisocyanate (HDI), a very important and common aliphatic diisocyanate monomer, is widely used in the field of high-performance coatings and thermoplastic polyurethane elastomer. Under the action of catalyst, HDI trimer (HDIT) can be synthesized via addition polymerization between three HDI molecules without any organic solvents. It is a commonly used and very important and non-toxic curing agent for two-component polyurethane coatings and adhesive 34. Three reactive -NCO groups in the HDIT molecular chain skeleton are easier to react with -OH groups at both sides of the PEG molecular chain and provide branching points to generate chemical cross-linked structures. Theoretically, the formed three-dimensional cross-linking network structure (thermoset polyurethane) can provide good support for the extra PEG and avoid the risk of leakage when the PEG is in a molten state. In this paper, we presented a one-step and solvent-free strategy to prepare PEG-based SSPCMs. To the best of our knowledge, it’s the first time to prepare the PEG-based SSPCMs through PEG and HDIT via a facile way. The chemical structure, form-stable performance, crystallization


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behavior, phase transition properties and thermal reliability and stability of obtained PEG-based SSPCMs were extensively studied. 2. Experimental 2.1 Materials Polyethylene glycol (analytical grade, 𝑀𝑛=6000 g/mol) was purchased from Amresco Co., USA. Hexamethylene diisocyanate trimer (HDIT) (GB021, industrial grade, the amount of –NCO group in HDIT is 14±0.5 wt%) was obtained from Carpoly Chemical Group Co., Ltd., China. The catalyst was dibutyl tin dilaurate, and it was obtained from Shao Yu Chemical Co., Ltd. (Guangzhou, China). All the chemical materials were used as received. 2.2 Preparation of PEG-based polyurethane PCMs The PEG-based polyurethane PCMs were synthesized by one-step using PEG and HDIT. Firstly, PEG and a small amount of dibutyl tin dilaurate were introduced in to a 250 ml beaker with a magnetic stirring at 80 oC, after melting of PEG, HDIT were poured into the beaker and fully blending was performed for 20 min. Subsequently, thermal curing was conducted at 80 oC for another 1 h in a drying oven. Finally, the PEG-based PCMs were obtained. To investigate the effect of HDIT on the performances of PCMs, the HDIT content was adjusted by varying the mass ratio. the ultimate specimens with 0, 2.5 wt%, 5.0 wt%, and 10.0 wt% HDIT were denoted as pure PEG, PH2.5, PH5, and PH10, respectively. The synthetic route of the PEG-based PCMs are shown in Scheme 1.

Scheme 1. The synthetic route of PEG-based PCMs. 2.3 Characterization 2.3.1 Fourier transform infrared spectroscopy (FT-IR) The chemical composition and structure of the pure PEG, HDIT and obtained PEG-based PCMs were characterized by FT-IR spectrophotometer, and the FT-IR absorption spectra were obtained using a Spectrum 2000 instrument from Perkin-Elmer with a resolution of 2 cm−1 and 32 scans. The scanning range changed from 4000 cm-1 to 400 cm-1. All the specimens were prepared by the KBr pressed disc technique. 2.3.2 X-ray diffraction (XRD) The crystalline structure of pure PEG and obtained PEG-based PCMs were detected by using an



automatic powder diffractormeter (A D8 ADVANCE, Bruker, Germany) at room temperature. Scans were made between Bragg angles of 5° and 50° at a scanning rate of 2 °/min. 2.3.3 Polarizing optical microscopy (POM) An Axioskop-40POL (Germay) polarizing optical microscopy (POM) equipped with a CCD camera was used to observe the crystalline morphology of PEG and obtained PEG-based PCMs at room temperature. A small fragment of the samples was put on a heating stage between two microscope cover glasses. All the specimens were heated from room temperature to 100 oC, and then the temperature was held for 5 min to get rid of the thermal history. Then, all the specimens were cooled to room temperature by the natural cooling. 2.3.4 Differential scanning calorimetry (DSC) The DSC (Netzsch 204c, Germany, equipped with a liquid nitrogen-cooling accessory) was employed to study the phase change properties of pure PEG and obtained PEG-based PCMs. About 10 mg of specimens were first heated from room temperature to 100 oC at a rate of 10 oC /min, kept for 3 min to eliminate any thermal history, and then cooled to 0 oC at a rate of 10 oC /min under a nitrogen atmosphere, kept for another 3 min, and then heated to100 oC again at a rate of 10 oC /min. 2.3.5 Accelerated thermal cycling testing In order to affirm the thermal reliability of the obtained PEG-based SPCMs, the accelerated thermal cycling testing was conducted in high-low temperature chamber (KSON KTHB-415TBS, China) in accordance with the method provide by Fu et al.24. The typical thermal cycling program contained 200 consecutive heating and cooling process. Each thermal cycling was composed of increasing temperature from 10 oC to 80 oC and then decreasing temperature form 80 oC to 10 oC, with a heating/cooling rate 3 oC/min and a respective 5 min isothermal period at 10 oC and 80 oC. Afterwards, the phase change properties and chemical stability of thermal treated PEG-based SSPCMs were characterized by DSC and FTIR analysis, respectively. 2.3.6 The light-thermal conversion performance The photo-thermal conversion and temperature-regulated performance of obtained PEG-based SSPCMs were conducted under simulated solar irradiation in accordance with the method provide by Chen et al. 35. The temperature of inside and around the sample during these periods were recorded by thermocouple 1 and thermocouple 2, respectively. At the same time, the infrared thermography camera (FLIR, SC 3000) was used. 2.3.7 Thermogravimetric analysis (TGA) The thermal stability of pure PEG and obtained PEG-based SSPCMs were investigated on a Netzsch TG209 instrument over 30-700 oC in a N2 atmosphere (250 mL/min) with a 10 oC /min heating ramp, all thermal parameters were determined as an average of three experiments. 3. Results and discussion 3.1 The chemical reaction between PEG and HDIT


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Figure 1. FTIR spectra of pure PEG, HDIT and PEG-based PCM. Figure 1 shows the FT-IR spectra of pure PEG, HDIT and synthesized PCMs. From Figure 1(a), in the spectrum of pure PEG, the characteristic absorption peaks at 3433 cm−1 and 1110 cm−1 are assigned to stretching vibrations of −O−H and −C−O, the absorptions at 2886 cm−1, 1468 cm−1, 1340 cm−1, 1280 cm−1, 960 cm−1, and 840 cm−1 are attributed to the C−H vibrations. HDIT shows the absorption peaks of −CH2 at 2941 cm−1 and 2862 cm−1, strong peak of −NCO at 2272 cm−1, and the strong band occurs at 1686 cm−1, corresponding to the −C=O of isocyanurate ring 36-39. In the spectra of obtained PCMs, the characteristic absorption peak of isocyanate from HDIT at around 2272 cm−1 disappears completely, indicating that the cross-linking reaction between PEG and HDIT is successfully occurred. In addition, the N−H stretching vibration (3433 cm−1) of PCMs is covered by the −O−H vibration (3440 cm−1), the newly formed absorption peaks at 1720 cm−1 and 1560 cm−1 resulted from hydrogen bonded “disordered” carbonyl band and deformation vibrations of N−H are indicated in Figure 1(b) 40, verifying the formation of –NHCOO– groups in the synthesized PCMs. Based on the above FTIR results, PEG-based PU PCMs are successfully prepared through chemical reaction of PEG and HDIT. 3.2 Leakage test

Figure 2. Photographs of pure PEG and PEG-based PCMs heated at 80 oC for 60 min. The form stability has the most important impact on the application of PCMs. Thus, the pure PEG and PEG-based PCMs with different HDIT content were heated up to 80 oC for 60 min to evaluate the form stable behavior. The photographs of pure PEG, PH2.5, PH5, PH10 at 30 oC and 80 oC are



shown in Figure 2. It can be seen that all the samples are solid state at 30 oC. After being heated to 80 oC, which is higher than the phase transition temperature of PEG, the pure PEG quickly melted and liquid leakage on the bottom could be observed, while PH2.5 mostly melted into liquid but presented a small amount of solid state on the bottom. These solids are the crosslinked thermoset polyurethane formed by the reaction between PEG and HDIT. It indicates, for PH2.5, the amount of crosslinked polyurethane formed by the reaction between PEG and HDIT is not sufficient to provide support for extra PEG. By contrast, during the phase change process, there is no liquid was observed for PH5 and PH10. This indicates the crosslinked polyurethane structure in PH5 can effectively limit the leakage of the extra PEG and maintain the initially defined shapes well. However, the shape of PH10 has changed a lot. The reason is that the adhesion of PH10 powder is poor due to the high degree of cross-linking, after being heated, the PH10 cylinder expands and changes its shape. All the results suggest that the PH5 and PH10 are SSPCMs and the minimum HDIT content in SSPCMs without any leakage of melted PEG is 5 wt%. 3.3 Phase change properties

Figure 3. DSC curves of pure PEG, PH5 and PH10. The phase change properties of pure PEG, PH5 and PH10 were analyzed by DSC. The DSC curves are shown in Figure 3 and the corresponding date of phase change enthalpy in the melting process (∆𝐻𝑚), melting temperature (𝑇𝑚) and freezing temperature (𝑇𝑓) are summarized in Table 1. What’s more, the relative enthalpy efficiency (λ) 41is used to weigh the crystallization ability of PEG in SSPCMs. The equation is given by ∆𝐻𝑚 ― 𝑃𝐶𝑀

λ(%) = ∆𝐻𝑚 ― 𝑃𝐸𝐺 × 𝑤𝑃𝐸𝐺 × 100%

(1)

where ∆𝐻𝑚 ― 𝑃𝐶𝑀 and ∆𝐻𝑚 ― 𝑃𝐸𝐺 are the melting enthalpy of synthesized PEG-based SSPCMs and pure PEG, respectively; 𝑤𝑃𝐸𝐺 is the weight percent of used PEG, and λ reflects the free movement degree of soft segments in PEG-based SSPCMs. A large value of λ indicates the small latent heat loss of PEG-based SSPCMs. It can be easily seen that pure PEG is an excellent candidate for solar energy storage owing to its high latent heat (165.3 J/g) and suitable phase transition temperature (34.3 -65.1 oC). Similar to pure PEG, the PH5 has reversible heat storage and release performances in the temperature range of 30 - 65 oC. When PEG reacts with 5.0 wt% HDIT, the latent heat and λ of PH5 decrease to 136.8 J/g and 87.1 %, respectively. With the content of HDIT


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increase to 10 wt%, the latent heat decreases from 165.3 J/g (pure PEG) to 89.7 J/g and the relative enthalpy efficiency reduces to 60.3 % due to excessive crosslinking. And there is no significant change in the 𝑇𝑚 and 𝑇𝑓. The decreased latent heat and λ indicate that the endothermic and exothermic capacities of PH5 and PH10 are weakened to some extent after the cross-linking reaction between PEG and HDIT, which is attributed to chemical restriction and physical entanglements 27, 31. Especially for PH10, as discussed in section 3.2, the molar ratio of –OH: –NCO is about 0.9, all the PEG reacted with HDIT to form the crosslinked polyurethane and without any extra PEG, while the crosslinked polyurethane structure restricts the motion of the PEG molecular chain segments and leads to a sharp drop in latent heat. However, the latent heat and λ of PH5 as SSPCM in present work are higher than that of most traditional PEG-based SSPCMs. Therefore, these results indicate that the introducing of HDIT is an effective method to preparing novel SSPCMs with high latent heat. Table 1. Thermal energy storage characteristics of different PEG-based SSPCMs in literatures and present study. SSPCMs

𝑻𝒇

𝑻𝒎

Theoretical enthalpy (J/g)

Latent heat (J/g)

λ (%)

Reference

PEG8000/MDI/Sortibol(93.4%)

44.0

59.7

138.3

109.4

79.1

23

PEG4000/MDI/DA (91.3%)

38.4

49.9

159.5

98.2

61.6

24

PEG6000/HDI/CO (85.4%)

42.3

51.4

167.6

117.7

70.2

26

PEG8000/MDI/Xylitol (91.4%)

29.7

41.6

162.8

76.4

46.9

29

PEG6000/PAPI (96.2%)

41.4

50.5

157.7

111.7

70.8

27

PEG4000/TAIC (90%)

31.2

57.1

145.4

136.9

94.2

31

PEG2000/Cellulose (85%)

28.9

50.4

175.4

149.1

85.0

42

PEG6000

34.3

65.1

165.3

165.3

100

Present

PH5 (95%)

34.1

64.8

157.0

136.8

87.1

Present

PH10 (90%)

33.9

63.9

148.8

89.7

60.3

Present

3.4 Crystalline performances

Figure 4. XRD curves of pure PEG and PH5. As discussed above, with the introduction of HDIT, the endothermic and exothermic capacities



of PH5 are weakened (compared to pure PEG). The reason may be that the crystalline structure of PEG component was destroyed by the chemical restriction and physical entanglements of chemical cross-linking points 26, 33. Therefore, the XRD and POM are employed to reveal the crystalline structure and crystalline morphology of pure PEG and PH5. As shown in Figure 4, there are two typical strong and sharp diffraction peaks at around 19.3 o and 23.4 o for pure PEG, corresponding to the diffractions of (120) and (132) crystal planes 15, 43, respectively. The PH5 shows similar typical diffraction peaks with the pure PEG, revealing that the PH5 possesses the same crystal structure as PEG. However, with the introduction of HDIT, the diffraction peak intensity at around 23.4 o decreased, and the diffraction peak intensity at around 28.3 o, 29.2 o, 32.3 o 34.0 o and 38.8 o increased. It indicates that, compared with pure PEG, the crystalline regions of PEG chains in the PH5 have been decreased to some extent owing to the formation of crosslinking network structure 26, 29, 31, 32. In that case, the capacity of movement and arrangement of PEG segments in PH5 are weakened by the limit of chemical bonds and the disruption of HDIT (acting as the impurity). Thus, the crystallization ability and crystalline regions of PEG reduce simultaneously. Figure 5 shows the POM images of pure PEG and PH5 with time. For pure PEG, only a large cross petal-shaped crystal spherulite can be observed due to the highly symmetric, non-branched structure and good flexibility. However, compared to pure PEG, there are several spherulites grow simultaneously for PH5 in the same scale range, it indicates that the chemical cross-linking point of PH5 act as a nucleus. In addition, the spherulite sizes of the PH5 is smaller than that of the pure PEG, implying the movement flexibility of PEG segments is limited and destroyed partly by crosslinking network. The spherulite crystalline structure of PEG segments in PH5 does not alter by the crosslinking network, but the degree of crystallization reduces. These results are in good agreement with the XRD and DSC finding discussed above. Fig. 5 (a) and (a’) show the POM image of pure PEG and PH5 80 °C (higher than the phase change temperature), respectively. All the spherulites fade away, and the visual field of POM becomes dark, suggesting the melting of PEG segments 44. Moreover, no liquid phase of PH5 is observed under normal light during the process, implying the PH5 is actual and excellent SSPCM.

Figure 5. POM images of (a-e) pure PEG and (a’-e’) PH5 cooled from 80 oC to room temperature by the natural cooling. 3.5 Thermal reliability and reusability


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Figure 6. (a) FTIR specttra, (b) XRD curves and (c) DSC curves of PH5 before and after 200 thermal cycles. The thermal reliability and reusability after a long-time practical utility is important for PCMs. In this work, 200 times of heating-cooling cycling were carried out to confirm whether the PH5 had good thermal reliability and reusability. The thermal cycling started from 10 °C and ended at 80 °C. The FT-IR spectra, XRD curves and thermal properties before and after 200 times of heatingcooling cycling are compared. And the corresponding thermal data from the DSC analysis are summarized in Table 2. As shown in Figure 6 (a) and (b), after 200 thermal cycles, the FT-IR spectra and XRD curves of PH5 give the same characteristic band positions as those obtained before the cycling test. This indicates that the PH5 has good structural stability. From the Figure 6 (c), no apparent differences are observed in the DSC curves of PH5 before and after 200 thermal cycles, which demonstrates that PH5 has a good thermal reliability. In addition, Table 2 shows that the 𝑇𝑚, 𝑇𝑓, ∆𝐻𝑚 and phase change enthalpy in the crystallization process (∆𝐻𝑓) of PH5 remain nearly constant before and after 200 thermal cycles. At the same time, the relative coefficient (η) is introduced to calculate the loss of phase change enthalpy before and after the cycle test. After 200 thermal cycles, the η for the melting process and crystallization process are 0.2 % and 0.3 %, respectively. The slight change for the phase transition enthalpy (∆𝐻𝑚 and ∆𝐻𝑓) is negligible for thermal energy storage applications. All the results prove that the fabricated PH5 has good thermal reliability and shows promise for use in thermal energy storage materials. Table 2. The thermal data of PH5 before and after 200 thermal cycles. Sample

∆𝑯𝒇(J/g)

𝑻𝒇𝒐d(oC)

𝑻𝒇𝒑

𝛈𝒇g(%)

∆𝑯𝒎

𝑻𝒎𝒐

𝑻𝒎𝒑

𝛈𝒎

(J/g)

a(oC)

b(oC)

c(%)

Before

136.8

54.5

64.8

-

135.7

39.4

33.9

-

After

136.5

54.5

65.3

0.2

135.3

39.8

33.8

0.3

e(oC)

a: Onset temperature of the melting point. b: Peak temperature of the melting point. (∆𝐻𝑚(𝑏𝑒𝑓𝑜𝑟𝑒) ― ∆𝐻𝑚(𝑎𝑓𝑡𝑒𝑟)) × 100 ∆𝐻 𝑚(𝑏𝑒𝑓𝑜𝑟𝑒) c: η𝑚 = d: Onset temperature of the crystallization point. e: Peak temperature of the crystallization point. (∆𝐻𝑓(𝑏𝑒𝑓𝑜𝑟𝑒) ― ∆𝐻𝑓(𝑎𝑓𝑡𝑒𝑟)) × 100 ∆𝐻 𝑓(𝑏𝑒𝑓𝑜𝑟𝑒). g: η𝑓 =

3.6 Light-thermal conversion and temperature regulation performance



Figure 7. (a) Schematic illustration for light-thermal conversion and (b) temperature-time curves inside and around of the PH5. The light-thermal conversion and temperature-regulated performance testing was performed under simulated solar illumination at a constant power of 500 W, and the temperature evolution inside and around the sample were recorded by thermocouple 1 and thermocouple 2, respectively, as illustrated in Figure 7 (a). Figure 7 (b) shows the temperature-time curves inside and around of the PH5. With the solar irradiation time increases, both the temperature inside and around the sample rise rapidly. When the temperature reaches about 55 oC (onset temperature of phase transition), the temperature rise of the PH5 becomes slow, but the temperature around the sample continues to rise rapidly. In this process, PH5 continuously converts solar light energy to heat energy and store it. After the sample temperature reaches 65 oC (end temperature of phase transition), the temperature around the sample has reached as high as 77 oC. At this time, the solar irradiation is turn off. Both the temperature inside and around the sample decreases sharply. When the temperature drops to 45 oC, the temperature-time curve of the PH5 appears a phase transition platform, implying the release of stored energy as the solidification of sample. In this process, the temperature of the sample is maintained between 35 - 45 oC (from 720 s to 1700 s). However, no obvious platform appears during cooling process for the temperature-time curve around the sample. The temperature around the sample falling from 45 oC to 35 oC takes only about 500 s. Simultaneously, the light-thermal conversion performance of the PH5 was observed clearly through an infrared thermography camera. As shown in Figure 8, the cylindrical PH5 is placed in the middle of the dish. The red portion represents the high temperature and the blue portion represents the low temperature. At the beginning (0 s), the temperature of the sample is close to the temperature of the surrounding environment, so the image is blue overall. Under the solar irradiation, the temperature both inside and around the sample rise rapidly, and the color of the image gradually change to red. Between 240 s to 430 s, the sample is undergoing the phase transition and the temperature is maintained between 55 - 65 oC, which is lower than the ambient temperature. Therefore, the color of the sample in the image is bluer than the surrounding color. After the solar irradiation is turn off at 435 s, the temperature both inside and around the sample begin to decrease. From 720 s to 1700 s, the phase change working substance in the PH5 changes from liquid phase to solid phase, and releases the stored energy. During this process, the temperature of the sample is higher than that around the sample. Thus, the color of the sample in the image is redder than the surrounding color. It is well consistent with the above result of temperature-time curves. Moreover, the shape of the sample did not change throughout the process, which is good agreement with the


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leakage test results. Above results demonstrate that the PH5 with excellent photo-thermal performance and show great potential as direct solar light absorber for industrial thermal utilization, such as hot water heater, preheat burner.

Figure 8. Representative thermal images about the photo-thermal performance. 3.7 Thermal stability

Figure 9. TG and DTG curves of pure PEG and PH5. Thermal stability is another crucial concerning to practical applications. Figure 9 shows the TG and corresponding DTG curves of pure PEG and PH5. And Table 3 summarizes the onset degradation temperature (𝑇5) (the temperature at which 5 wt% degradation occurred), the maximum degradation temperature (𝑇𝑚𝑎𝑥) (the peak temperature of the DTG curve) and the mass loss at 100 oC,

300 oC and 500 oC, respectively. As shown in Fig. 9, both pure PEG and PH5 show only a single thermal degradation stage in the temperature range of 300 oC to 500 oC, corresponding to the thermal decomposition of PEG segments 45. Compared with pure PEG (377.3 oC), the 𝑇5 of PH5 reduces to 372.5 oC, but within acceptable limits. However, the 𝑇𝑚𝑎𝑥 of PH5 increased to 422.9 oC, which is higher than that of pure PEG (418.8 oC). Moreover, no apparent weight loss can be observed below 100 oC and 300 oC for PH5. On the whole, the PH5 shows no degradation in the temperature during phase change process and exhibits good thermal stability in a broad temperature range of applications.



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Table 3. Thermal stability parameters for PEG and PH5. Sample

Mass loss (wt%)

𝑻𝟓

𝑻𝒎𝒂𝒙

(oC)

(oC)

At 100

PEG

377.3

418.8

PH5

372.5

422.9

oC

At 300 oC

At 500 oC

0.2

0.5

97.8

0

0.1

65.3

4. Conclusion PEG-based PU as SSPCMs were obtained through one-step and solvent-free synthesis method using PEG as functional moiety and HDIT as crosslinking agent as well as supporting skeleton. Only use 5 wt% HDIT, the cross-linking product of PEG and HDIT can provide strong support for the extra PEG to obtain SSPCM within the phase change temperature range, and the relative enthalpy efficiency is as high as 87.1 %, which is significantly higher than that of most traditional SSPCMs reported in the literature. However, with the content of HDIT increase to 10 wt%, the latent heat decreases from 165.3 J/g to 89.7 J/g and the relative enthalpy efficiency reduces to 60.3 % due to excessive crosslinking. Due to the movement flexibility of PEG segments is restricted and destroyed partly by crosslinking network, compared with pure PEG, the spherulite sizes and the degree of crystallization of the PEG segments in PEG-based PCMs are smaller and lower, respectively. The results suggest that the minimum HDIT content in SSPCMs without any leakage of melted PEG is 5 wt%. Moreover, the 200 times accelerated thermal cycling and TGA results reveal that the prepared PH5 exhibits good thermal reliability and stability for long-time applications. In conclusion, the obtained PH5 SSPCM has a great potential as solar water heaters for solar thermal energy storage. Acknowledgements We acknowledge the National Key Research and Development Program of China (Grant No. 2016YFB0302300), the Key Program of National Natural Science Foundation of China (Grant No. 51435005), the National Natural Science Foundation of China (Grant No.51505153), the PhD Startup Fund of Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030310429), the Science and Technology Program of Guangzhou, China (Grant No. 201607010240), the Natural Science Foundation of Guangdong Province (2016A030313486 and 2018A030313275), the Program of Nanhai Talented Team (201609180006) and the Program of Foshan Innovative Entrepreneurial Team (2016IT100152). References (1) Kong, W.; Yang, Y.; Zhou, C.; Lei, J. Novel thermosetting phase change materials with polycarbonatediol based curing agent as supporting skeleton for thermal energy storage. Energ. Buildings 2017, 146, 12-18. (2) Zhu, Y.; Qin, Y.; Wei, C.; Liang, S.; Luo, X.; Wang, J.; Zhang, L. Nanoencapsulated phase change materials with polymer-SiO 2 hybrid shell materials: Compositions, morphologies, and properties. Energ. Convers. Manage. 2018, 164, 83-92. (3) Khan, Z.; Khan, Z.; Ghafoor, A. A review of performance enhancement of PCM based latent heat storage system within the context of materials, thermal stability and compatibility. Energ. Convers. Manage. 2016, 115, 132-158.


Page 13 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Liu, Y.; Duan, J.; He, X.; Wang, Y. Experimental investigation on the heat transfer enhancement in a novel latent heat thermal storage equipment. Appl. Therm. Eng. 2018, 142, 361370. (5) Alkan, C.; Sari, A. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Sol. Energy 2008, 82, 118-124. (6) Pielichowski, K.; Flejtuch, K. Recent developments in polymeric phase change materials for energy storage: poly(ethylene oxide)/stearic acid blends. Polym. Advan. Technol. 2005, 16, (2-3), 127-132. (7) Zhang, P.; Xiao, X.; Ma, Z. W. A review of the composite phase change materials: Fabrication, characterization, mathematical modeling and application to performance enhancement. Appl. Energ. 2016, 165, 472-510. (8) Wu, W.; Yang, X.; Zhang, G.; Chen, K.; Wang, S. Experimental investigation on the thermal performance of heat pipe-assisted phase change material based battery thermal management system. Energ. Convers. Manage. 2017, 138, 486-492. (9) Xu, T.; Li, Y.; Chen, J.; Wu, H.; Zhou, X.; Zhang, Z. Improving thermal management of electronic apparatus with paraffin (PA)/expanded graphite (EG)/graphene (GN) composite material. Appl. Therm. Eng. 2018, 140, 13-22. (10) Iten, M.; Liu, S.; Shukla, A. A review on the air-PCM-TES application for free cooling and heating in the buildings. Renew. Sust. Energ. Rev. 2016, 61, 175-186. (11) Kumarasamy, K.; An, J.; Yang, J.; Yang, E.-H. Novel CFD-based numerical schemes for conduction dominant encapsulated phase change materials (EPCM) with temperature hysteresis for thermal energy storage applications. Energy 2017, 132, 31-40. (12) Ren, Y.; Xu, C.; Yuan, M.; Ye, F.; Ju, X.; Du, X. Ca(NO 3 ) 2 -NaNO 3 /expanded graphite composite as a novel shape-stable phase change material for mid- to high-temperature thermal energy storage. Energ. Convers. Manage. 2018, 163, 50-58. (13) Kee, S. Y.; Munusamy, Y.; Ong, K. S. Review of solar water heaters incorporating solid-liquid organic phase change materials as thermal storage. Appl. Therm. Eng. 2018, 131, 455-471. (14) Ye, H.; Ge, X.S. Preparation of polyethylene paraffin compound as a form-stable solid-liquid phase change material . Sol. Energ. Mat. Sol. C. 2000, 64, 37-44. (15) Zhou, Y.; Sheng, D.; Liu, X.; Lin, C.; Ji, F.; Dong, L.; Xu, S.; Yang, Y. Synthesis and properties of crosslinking halloysite nanotubes/polyurethane-based solid-solid phase change materials. Sol. Energ. Mat. Sol. C. 2018, 174, 84-93. (16) Zhou, Y.; Liu, X.; Sheng, D.; Lin, C.; Ji, F.; Dong, L.; Xu, S.; Wu, H.; Yang, Y. Polyurethanebased solid-solid phase change materials with in situ reduced graphene oxide for light-thermal energy conversion and storage. Chem. Eng. J. 2018, 338, 117-125. (17) Feng, L.; Wang, C.; Song, P.; Wang, H.; Zhang, X. The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage. Appl. Therm. Eng. 2015, 90, 952-956. (18) Liang, K.; Shi, L.; Zhang, J.; Cheng, J.; Wang, X. Fabrication of shape-stable composite phase change materials based on lauric acid and graphene/graphene oxide complex aerogels for enhancement of thermal energy storage and electrical conduction. Thermochim. Acta 2018, 664, 115. (19) Sun, K.; Kou, Y.; Zheng, H.; Liu, X.; Tan, Z.; Shi, Q. Using silicagel industrial wastes to synthesize polyethylene glycol/silica-hydroxyl form-stable phase change materials for thermal



energy storage applications. Sol. Energ. Mat. Sol. C. 2018, 178, 139-145. (20) S, I. H.; A, A. R.; S, K. Bifunctional nanoencapsulated eutectic phase change material core with SiO 2 /SnO 2 nanosphere shell for thermal and electrical energy storage. Mater. Design. 2018, 154, 291-301. (21) Sundararajan, S.; Samui, A. B.; Kulkarni, P. S. Shape-stabilized poly(ethylene glycol) (PEG)cellulose acetate blend preparation with superior PEG loading via microwave-assisted blending. Sol. Energy 2017, 144, 32-39. (22) Yanshan, L.; Shujun, W.; Hongyan, L.; Fanbin, M.; Huanqing, M.; Wangang, Z. Preparation and characterization of melamine/formaldehyde/polyethylene glycol crosslinking copolymers as solid–solid phase change materials. Sol. Energ. Mat. Sol. C. 2014, 127, 92-97. (23) Chen, C.; Liu, W.; Wang, H.; Peng, K. Synthesis and performances of novel solid–solid phase change materials with hexahydroxy compounds for thermal energy storage. Appl. Energ. 2015, 152, 198-206. (24) Fu, X.; Xiao, Y.; Hu, K.; Wang, J.; Lei, J.; Zhou, C. Thermosetting solid–solid phase change materials composed of poly(ethylene glycol)-based two components: Flexible application for thermal energy storage. Chem. Eng. J. 2016, 291, 138-148. (25) Li, W.D.; Ding, E.Y. Preparation and characterization of cross-linking PEG/MDI/PE copolymer as solid–solid phase change heat storage material. Sol. Energ. Mat. Sol. C. 2007, 91, 764-768. (26) Liu, Z.; Fu, X.; Jiang, L.; Wu, B.; Wang, J.; Lei, J. Solvent-free synthesis and properties of novel solid–solid phase change materials with biodegradable castor oil for thermal energy storage. Sol. Energ. Mat. Sol. C. 2016, 147, 177-184. (27) Kong, W.; Fu, X.; Liu, Z.; Zhou, C.; Lei, J. A facile synthesis of solid-solid phase change material for thermal energy storage. Appl. Therm. Eng. 2017, 117, 622-628. (28) Sundararajan, S.; Samui, A. B.; Kulkarni, P. S. Thermal Energy Storage Using Poly(ethylene glycol) Incorporated Hyperbranched Polyurethane as Solid–Solid Phase Change Material. Ind. Eng. Chem. Res. 2017, 56, 14401-14409. (29) Yang, Y.; Kong, W.; Cai, X. Solvent-free preparation and performance of novel xylitol based solid-solid phase change materials for thermal energy storage. Energ. Buildings 2018, 158, 37-42. (30) Sarı, A.; Biçer, A.; Alkan, C. Thermal energy storage characteristics of poly(styrene-co-maleic anhydride)-graft-PEG as polymeric solid–solid phase change materials. Sol. Energ. Mat. Sol. C. 2017, 161, 219-225. (31) Huang, X.; Guo, J.; Gong, Y.; Li, S.; Mu, S.; Zhang, S. In-situ preparation of a shape stable phase change material. Renew. Energ. 2017, 108, 244-249. (32) Cao, Q.; Liu, P. Hyperbranched polyurethane as novel solid–solid phase change material for thermal energy storage. Eur. Polym. J. 2006, 42, 2931-2939. (33) Alkan, C.; Günther, E.; Hiebler, S.; Ensari, Ö. F.; Kahraman, D. Polyurethanes as solid–solid phase change materials for thermal energy storage. Sol. Energ. 2012, 86, 1761-1769. (34) Hu, J.; Chen, Z.; He, Y.; Huang, H.; Zhang, X. Synthesis and structure investigation of hexamethylene diisocyanate (HDI)-based polyisocyanates. Res. Chem. Intermediat. 2016, 43, 27992816. (35) Chen, Y.; Zhang, Q.; Wen, X.; Yin, H.; Liu, J. A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance. Sol. Energ. Mat. Sol. C. 2018, 184, 82-90.


Page 14 of 15

Page 15 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) He, Y.; Xie, D.; Zhang, X. The structure, microphase-separated morphology, and property of polyurethanes and polyureas. J. Mater. Sci. 2014, 49, 7339-7352. (37) He, Y.; Zhang, X.; Runt, J., The role of diisocyanate structure on microphase separation of solution polymerized polyureas. Polymer 2014, 55, 906-913. (38) Lu, X.; Huang, J.; He, G.; Yang, L.; Zhang, N.; Zhao, Y.; Qu, J. Preparation and Characterization of Cross-Linked Poly(butylene succinate) by Multifunctional Toluene Diisocyanate–Trimethylolpropane Polyurethane Prepolymer. Ind. Eng. Chem. Res. 2013, 52, 13677-13684. (39) He, Y.; Zhang, X.; Zhang, X.; Huang, H.; Chang, J.; Chen, H. Structural investigations of toluene diisocyanate (TDI) and trimethylolpropane (TMP)-based polyurethane prepolymer. J. Ind. Eng. Chem. 2012, 18, 1620-1627. (40) Mattia, J.; Painter, P. A Comparison of Hydrogen Bonding and Order in a Polyurethane and Poly(urethane-urea) and Their Blends with Poly(ethylene glycol). Macromolecules 2007, 40, 15461554 (41) Cao, R.R.; Li, X.; Chen, S.; Yuan, H.R.; Zhang, X.X. Fabrication and characterization of novel shape-stabilized synergistic phase change materials based on PHDA/GO composites. Energy 2017, 138, 157-166. (42) Li, Y.; Liu, R.; Huang, Y. Synthesis and phase transition of cellulose-graft-poly(ethylene glycol) copolymers. J. Appl. Polym. Sci. 2008, 110, 1797-1803. (43) Zhang, H.; Sun, Q.; Yuan, Y.; Zhang, Z.; Cao, X. A novel form-stable phase change composite with excellent thermal and electrical conductivities. Chem. Eng. J. 2018, 336, 342-351. (44) Chen, C.; Liu, W.; Wang, Z.; Peng, K.; Pan, W.; Xie, Q. Novel form stable phase change materials based on the composites of polyethylene glycol/polymeric solid-solid phase change material. Sol. Energ. Mat. Sol. C. 2015, 134, 80-88. (45) Deng, Y.; Li, J.; Qian, T.; Guan, W.; Li, Y.; Yin, X. Thermal conductivity enhancement of polyethylene glycol/expanded vermiculite shape-stabilized composite phase change materials with silver nanowire for thermal energy storage. Chem. Eng. J. 2016, 295, 427-435.


One-step and solvent-free synthesis of polyethylene glycol-based

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