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Synthesis and thermal energy storage properties of the polyurethane solid–solid phase change materials with a novel tetrahydroxy compound
European Polymer Journal 48 (2012) 1295–1303
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis and thermal energy storage properties of the polyurethane solid–solid phase change materials with a novel tetrahydroxy compound Peng Xi a,b,⇑, Yuqing Duan a, Pengfei Fei a, Lei Xia a, Ran Liu a, Bowen Cheng a a b
Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, 100080 Beijing, China
a r t i c l e
i n f o
Article history: Received 28 January 2012 Received in revised form 10 April 2012 Accepted 21 April 2012 Available online 30 April 2012 Keywords: Phase change Thermal performance Crystalline Polyethylene glycol
a b s t r a c t Based on the phase change theory, a novel tetrahydroxy compound (THCD) was designed and prepared. Depending on the spatial structure of the tetrahydroxy compound, a formstable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was synthesized via employing PEG as soft segments, while multi-benzene ring structure made by 4,40 -diphenylmethane diisocyanate and tetrahydroxy compound as hard segments. The composition and structure of THCD and TPUPCM, the TPUPCM’s the weight average molecular weight and number average molecular weight, dissolving and melting abilities, phase change behaviors, thermal performances and crystalline morphology were investigated by Fourier transform infrared spectrometer, 1H nuclear magnetic resonance spectrometer, multiangle laser light scattering apparatus, differential scanning calorimentry, dynamic mechanical thermal analysis, thermogravimetry analysis system, wide-angle X-ray diffraction, polarizing optical microscopy. The results show that the solid–solid phase change material owns excellent phase change properties and a broad processing temperature range. The heating cycle phase change enthalpy is 137.4 J/g, and the cooling cycle phase change enthalpy is 127.6 J/g. The started decomposition temperature and the maximum decomposition temperature are at 323.5 and 396.2 °C, respectively. Furthermore, the solid–solid phase change material is dissolvable, meltable and can be processed directly, and has great potential applications in thermal energy storage. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Phase change materials (PCMs), which are also called latent heat-storage materials, have high capability to store and release large heat energy within a slight or no temperature change as a series of functional materials. Their application fields include solar energy [1], waste heat recovery [2], smart housing [3], temperature-control greenhouses and textiles [4,5], telecommunications and microprocessor equipment [6] and so on. Especially, PCMs
⇑ Corresponding author at: Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China. Tel.: +86 22 27637689; fax: +86 22 83955164. E-mail address: [email protected] (P. Xi). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.04.009
is playing a prominent role while energy crisis is becoming more and more serious. Mondal [7] ever gave a quite comprehensive review of the types of PCMs and the advantages and disadvantages of different types of PCMs. A great number of organic, inorganic, polymeric and eutectic PCMs have latent heat-storage properties. Based on their phase change states, PCMs are divided into three categories: solid–solid PCMs, solid– liquid PCMs and liquid–gas PCMs [8]. Among the various kinds of PCMs, organic solid–solid PCMs are fairly recently developed functional PCMs because they have been found to exhibit many desirable characteristics, for example, no liquid or gas generation, small volume change, no receptacle needed to seal them, and non-corrosiveness and nontoxicity [9]. Unfortunately, the organic solid–solid PCMs have been identified to be relatively few, especially in the polymer-based solid–solid PCMs.
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As a representative linear polymer, polyethylene glycol (PEG) and polyethylene glycol monomethyl ether (MPEG) are mostly employed as the phase change unit in the synthesis of a novel polymer-based solid–solid PCMs owing to their desirable characteristics such as high latent heat storage capacity, a suitable melting point ranging from 3.2 to 68.7 °C, low vapor pressure when melted, and good thermal and chemical stability [10]. At present there are two approaches to prepare formstable polymeric solid–solid PCMs, which are physical approach and chemical approach. In the physical approach, the novel polymer-based PCMs are obtained by dispersing PCMs into higher melting point polymeric materials acting as supporting materials. As long as the temperature is below the melting point of the supporting materials (cellulose [11], PAM [12] or PMMA [13]), the whole PCMs can keep their solid shape even when phase change unit changes from solid to liquid. However, the materials prepared via the physical blending have a tendency to lose their phase change characteristics after several heating–cooling cycles due to the loss of PEG. For the chemical approach, chemical grafting, blocking copolymerization are used to make polymer-based solid– solid PCMs as the energy storage. Zhou [14] prepared a solid–solid PCM via a copolymerization reaction of high molecular weight poly (ethylene glycol) with poly (vinyl alcohol) (PVA) and 4,40 -diphenylmethane diisocyanate (MDI). However, the maximum phase change enthalpy is 72.8 J/g. The value is still very low in comparison with that of pure PEG. Cao et al. [15] prepared a hyperbranched polyurethane solid–solid PCM using hyperbranched polyester (BoltornÒH20) as chain extender via a two-step reaction process of PEG, MDI and H20. The phase transition enthalpy of the PCM is more than 100 J/g. However, the polymer aggregation structure is difficultly to restraint, and the cross-link structure is formed easily. Furthermore, the value of H20 is too expensive to be suitable to extensive application. In the previous studies, we synthesized a copolymer monomer containing polyethylene glycol monomethyl ether (MPEG) phase change unit and a vinyl unit via the modification of hydrogen group of MPEG. By copolymerization of the copolymer monomer and phenyl ethylene, a phase change heat storage material with long-branch chain was prepared [16]. The phase change enthalpy of the polymeric based PCM is 108.5 J/g. However, the PCM has a few defects on the melted processing and mechanical properties, which is not directly processing. It was reported that a polymeric solid–solid PCM (PUPCM) with polyurethane block copolymer structure composed of PEG10000 as soft segment, 4,40 -diphenylmethane diisocyanate and 1,4butanediol as a chain extender possessed an excellent linear segment block copolymer and could be easily fabricated [17]. A cross-linking copolymer PCM (PEG/MDI/PE) prepared via the two-step condensation reaction of PEG10000 with pentaerythritol (PE) and 4,40 -diphenylmethane diisocyanate has phase change enthalpy as 152.97 J/g [18]. Although the research results provide an important way for the synthesis of PCM with high phase change enthalpy, the cross-linking copolymer is not directly processed. In this contribution, we designed and synthesized a novel tetrahydroxy compound. Relying on the spatial struc-
ture of the tetrahydroxy compound, a form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared via employing PEG as soft segments, while 4,40 -diphenylmethane diisocyanate (MDI) and THCD comprising of multi-benzene ring structure as hard segments. The material’s composition and structure and properties were investigated by various techniques. The results show that the solid–solid PCM owns excellent phase change properties. It can be processed directly and is likely to be produced in large scale. 2. Experimental 2.1. Materials PEG (analytical grade, M n = 6000, from Shanghai Medical Chemical Reagent Co. Inc., China) is dried at 80 °C under high vacuum (20 Pa) for overnight prior to use. MPEG (analytical grade, M n = 4000, from China Medical Group, China) is degassed and dried at 100–110 °C under high vacuum (20 Pa) for 3–4 h. Benzaldehyde glycerin acetal (BGA, analytical grade, from Shanghai Ruiteng Chemical Reagent Co. Inc., China), Tetrahydrofuran (THF, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), acetic acid (HOAc, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), petroleum ether (analytical grade, from Shanghai Medical Chemical Reagent Co. Inc., China), ethyl acetate (analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China) and N,N-dimethylformamide (DMF, analytical grade, from Tianjin third Chemical Reagent Co. Inc., China) were dried by 5 Å molecular sieve for 24 h followed by distillation before use. MDI (analytical grade, from Tianjin Kemiou Chemical Reagent Co. Inc., China) was used as received. 2.2. Synthesis of THCD The synthetic route of THCD is shown in Scheme 1. THCD was prepared in a three-necks round bottomed flask fitted with an overhead stirrer, nitrogen inlet and an addition funnel. First, MDI (2.50 g, 10 mmol) was introduced into the flask and dissolved in THF (20 ml). And then, BGA (3.60 g, 20 mmol) was dropped into the reaction flask. Under nitrogen, the mixture was stirred at 60 °C. Completion of the reaction was followed by monitoring the disappearance of strong absorbed peaks of AOH and ANCO using Fourier transform infrared (FT-IR) spectroscopy. Then THF was evaporated and recovered. The white precipitate was subjected to column chromatography on silica gel (eluent; petroleum ether:ethyl acetate = 2:1(v/v)) to give 1 (5.55 g, 9.1 mmol) in 91.0% yield. Under nitrogen, to a solution of 1 (3.00 g, 4.91 mmol) in mixture of THF and HOAc (THF: HOAc = 1:2 (v/v)) (40 ml) was stirred at 50 °C for 24 h, according to literature procedures [19]. The concentrate was precipitated using petroleum ether (20 ml), and the obtained precipitate was isolated by filtration and dried under reduced pressure to give a crude product. The crude product was washed for twice with petroleum ether. And then, the mixture was filtered and dried to give 2 (2.03 g, 4.67 mmol). The sample
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Scheme 1. Synthesis route to THCD.
was kept in a vacuum oven at 80 °C until further characterization (white powder). 2.3. Synthesis of TPUPCM The synthesis was conducted in a two step polymerization process under an inert atmosphere of nitrogen in three-necks round bottomed flask fitted with an overhead stirrer. The synthesis route is shown in Scheme 2. First, under nitrogen, MDI (0.50 g, 2 mmol) was dissolved with freshly distilled DMF (20 ml) at 0 °C. And then, the dried PEG6000 (6 g, 1 mmol) was dropped into the solution and stirred for 5 min, and the resulting mixture was stirred at 80 °C. Completion of the reaction was indicated by the disappearance of strong absorbed peaks of AOH in PEG molecule using FT-IR spectroscopy. The ANCO terminated modified MPEG (MMPEG) was synthesized using similar procedure as mentioned earlier. In the second step reaction, under nitrogen, to a solution of 2 (0.43 g, 1 mmol) in freshly distilled DMF (10 ml) was added into the three-necks round bottomed flask by dropping. The mixture was stirred at 80 °C for 6 h, and MMPEG (4.25 g, 2 mmol) was added. The reaction continued at 80 °C for 2 h. At last, the reaction mixture was cast in a special glass pan and set in vacuum oven further reaction at 80 °C for 48 h, so that the reactions would be proceed completely, and the DMF was volatilized. The samples were kept in vacuum at room temperature for two weeks prior to testing. In whole reaction, the total amount of ANCO groups is kept to equal to that of AOH groups. 2.4. Characterization FT-IR spectra of the MDI, BGA, THCD, TPUPCM and PEG6000 were obtained on a Nicollet NEXUS-670 FT-IR
spectrometer. Transition of attenuated total reflectance (ATR) spectra of sample between 500 and 4000 cm1 were collected by 32 scans for each spectrum and the resolution for the ATR spectra was 2 cm1. 1H Nuclear magnetic resonance (1H NMR) spectra of THCD and TPUPCM were collected using a JEOL EX-400 spectrometer at room temperature with DMSO-d6 as the solvent.The weight average molecular weight (M w ) and number average molecular weight (M n ) of the prepared TPUPCM were determined with a multiangle laser light scattering apparatus (DAWN-DSP, Wyatt Technology Co., Santa Barbara, CA) combined with a P100 pump (Thermo Separation Products) equipped with a TSKGEL G5000 HHR column (7.8–300 mm). The solvent used was high-performance liquid chromatograph grade dimethylformamide (DMF). Differential scanning calorimentry (DSC) analysis of the samples was carried out with a Perkin Elmer DSC-7 to monitor the variations in melting temperature (Tm), crystallizing temperature (Tc), latent heat of fusion (DHf) and latent heat of crystallization (DHc) of the samples. A typical thermal cycling consisted of a consecutive heating and cooling process (in the temperature interval 0–100 °C). The heating and cooling process times were maintained at approximately 15 and 25 min, respectively. Thermal cycling test continued until 100 complete heating–cooling processes of TPUPCM. Dynamic mechanical thermal analysis (DMTA) was performed on molded films with a NETZSCH DMA242C. 50 mg sample was processed in the special model. The relaxation spectrum was scanned from 0 to 220 °C with a frequency of 1 Hz and a heating rate of 3 °C/min. Thermogravimetry and derivative of thermogravimetry (TGA) curves of the TPUPCM were obtained using a Netzsch STA409PC thermal analysis system. About 10 mg dried sample was set into an alumina crucible and
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Scheme 2. Synthesis route to TPUPCM.
weighed. The curve was recorded at a heating rate of 5 °C/min in nitrogen atmosphere. The analysis of the PEG and TPUPCM by wide-angle Xray diffraction (WAXD) was carried out with a Philip PW 1710 at 30 kV and 20 mA. Bragg’s angle 2h is set from 10° to 50° with a rate of 3 °C/min. An observation of the samples by polarizing optical microscopy (POM) was performed on Olympus BX51 Polarizing Microscope. The sample was placed between a microscope glass and a cover slip, and then heated with hot stage. 3. Results and discussion 3.1. Analysis of the composition and structure for THCD The results of FT-IR and 1H NMR spectra for THCD (Figs. 1 and 2) verify that the composition and structure
of THCD agree with Scheme 1. In the FT-IR spectrum of THCD (shown in Fig. 1), the characteristic peak of ANCO for MDI at 2279 cm1 and the stretching vibration peak of AOH of BGA at 3437 cm1 disappear. The stretching vibration peaks of ANH, AC@O and ACAOA emerge at 3313, 1723, and 1259–1001 cm1, respectively. These results prove that the chemical bond of ANHCOOA has formed. The strong absorption peak at 3415 cm1 is assigned to the stretching vibration peaks of terminal hydroxyl group of THCD. The peaks locating at 3005, 1598, 1506 and 1445 cm1 are the characteristic peaks of benzene ring [20]. In order to further verify the FT-IR analysis results, the 1 H NMR analysis was performed. Through the analysis results of 1H NMR spectrum of the sample, it can be found that the structure of THCD in the Scheme 1 corresponds to the 1H NMR spectrum. Both the IR spectrum and 1H
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PEG6000 were also shown in Fig. 3. From the Fig. 3, it can be found that the hydroxyl groups of THCD at 3415 cm1 tend to polymerize. Moreover, the strong peaks emerge at 2884 and 1101 cm1, which belong to the stretching vibration peaks of ACH2A and ACAOA for soft segments. The stretching vibration peaks of ANAH locate at 3313 and 1610 cm1. The carbonyl group stretching vibration peak of the ester group emerges at 1725 cm1. The peaks at 1596, 1546, 1512 and 1465 cm1 are assigned to the characteristic peaks of benzene ring. These results prove that TPUPCM has been synthesized. In 1H NMR spectrum of TPUPCM, the proton peaks correspond to the structure of TPUPCM. The analysis results of 1H NMR spectrum of TPUPCM verify further those of FT-IR spectra. 3.3. Physical properties of TPUPCM
Fig. 1. FT-IR spectrum of THCD: (a) THCD; (b) BGA; (c) MDI.
NMR spectrum of THCD indicate that the novel tetrahydroxy compound has been successfully prepared. 3.2. Analysis of the composition and structure for TPUPCM To reveal composition and structure of TPUPCM, FT-IR and 1H NMR spectra test were performed as shown in Figs. 3 and 4. For a comparison, FT-IR spectra of THCD and
The physical properties of the novel polyurethane solid– solid phase change material synthesized are tested. The results show that the sample as-obtained owns excellent dissolving and melting-processing properties. It is dissolvable in tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively. The TPUPCM obtained has an M w of 2.56 105 and an M n of 1.68 105. The melting point and the first degradation temperature of the polyurethane phase change material is 186 and 323 °C (TGA curve), respectively. It can be manufactured to form any given shape (film, strip, particle and filament) when the temperature is at 186–323 °C. These results verify that the novel polyurethane phase change material
Fig. 2. 1H NMR spectrum of THCD.
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Fig. 3. FT-IR spectrum of TPUPCM: (a) TPUPCM; (b) PEG6000; (c) THCD.
as-synthesized is thermoplastic polymer, which benefits from the steric structure of THCD. In the synthesis of TPUPCM, the multi-benzene rings structure in the main chain is inflexible due to the hyperconjugation effect among multi-benzene rings and acyl groups. The space steric effect inhibits the second hydroxyl to participate reaction and leads to the linear polymer main chain to be formed. 3.4. Phase change properties of TPUPCM In order to study the phase change properties of the TPUPCM, the Differential scanning calorimetry (DSC) was employed. The DSC curves of pure PEG6000 and TPUPCM are shown in Fig. 5, and the data of their transition temperature and enthalpy are summarized in Table 1. The results show that there is a strong melting peak at 56.1 °C in the
heating scanning DSC curve of TPUPCM and the phase change enthalpy is 137.4 J/g. Through the analysis of DSC, it can be found that both of PEG and TPUPCM undergo all phase transition, but their phase transition states are quite different. When the temperature is raised to about 65.9 °C, the pure PEG undergoes a change from a white crystal solid to a transparent liquid [8]. While TPUPCM remains solid in the whole heating process, even if the temperature is raised to 100 °C or higher. This indicates that TPUPCM is a solid–solid phase change material. The cooling scanning DSC curves of PEG and TPUPCM are presented in Fig. 5 (II). They show that similar quantities of heat are released. Compared with PEG/MDI/PE cross-linking copolymer [18], the phase change enthalpy of TPUPCM is 137.4 J/g, lower than that of PEG/MDI/PE cross-linking copolymer, the phase change enthalpy of PEG/MDI/PE cross-linking copolymer is 152.97 J/g. This is because the phase change enthalpy of the PCM is intimately related to the weight percentage of phase change unit. In the molecular structure of TPUPCM, the phase change units include PEG6000 (in the main chain) and MPEG4000 (in the branch chain). When the main chain structure remains unchanged, the weight percentage of phase change unit of TPUPCM gradually increases with the increase of molecular weight of MPEG. In the synthesis of PEG/MDI/PE cross-linking copolymer PCM, PEG10000 was chosen as phase change unit. In view of the mechanical properties of TPUPCM, we chose MPEG4000 as branch chain of TPUPCM. This affects the phase change enthalpy of TPUPCM to an extent. Liao et al. [21] synthesized a solid–solid PCM (D-HBPU) via two steps reaction of PEG6000 with MDI and pentaerythritol (PE). The phase change enthalpy of D-HBPU is 125.0 J/g. The result verifies the structural advantage of THCD on the synthesis of PCM with high phase change enthalpy. To verify the stability of phase change for TPUPCM, the 100 time heating and cooling cycles were carried out, as shown in Fig. 6. It can be found that the phase change temperatures and latent heat values of the TPUPCM slightly change after 100 time cycles. This is quite a good and
Fig. 4. 1H NMR spectrum of TPUPCM.
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Fig. 5. DSC curves of PEG6000 (a) and TPUPCM (b): (I) Heating cycle; (II) Cooling cycle.
Table 1 Phase change temperature and enthalpy of PEG6000 and TPUPCM. Sample
Phase transition
PEG TPUPCM
Solid–liquid Solid–solid
DH (J/g)
Tp (°C)
Heating cycle
Cooling cycle
Heating cycle
Cooling cycle
189.6 137.4
167.2 127.6
65.9 56.1
37.6 37.5
Tp: peak transition temperature of samples DH: phase transition enthalpy of samples.
The dynamic mechanical thermal analysis (DMTA) is an effective method to research the thermal performances of 0 TPUPCM. The elastic modulus (E ) and loss tangent (Tand) of TPUPCM over the temperature ranging from 0 to 0 210 °C are presented in Fig. 7. At around 56.1 °C, E curve appears an obvious step and Tand curve shows a sharp
peak in the same temperature zone. These can be ascribed to the melted transition of PEG soft segments. The test results are agreed with that of differential scanning calorim0 etry. When the temperature was risen continually, the E curve becomes a platform until the temperature reached 186 °C. This indicates that the whole polymer molecule will start to move at 186 °C. However, the elastic modulus of material is 40 MPa in the 60–186 °C, which is completely contributed by hard segments of TPUPCM. The thermogravimetric and derivative thermogravimetric analysis curves of TPUPCM are shown in Fig. 8. The mechanism of thermal decomposition involves two steps,
Fig. 6. The 100th time heating and cooling DSC curves of TPUPCM.
Fig. 7. DMTA curves of TPUPCM.
essential property for PCMs. These results indicate that TPUPCM has a quite good energy storage and release effect and great application prospects. 3.5. The thermal performances of TPUPCM
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3.6. Crystalline morphology investigations of TPUPCM
Fig. 8. TGA curve of TPUPCM.
as can be seen in TGA curves. When the temperature reaches 323 °C, slight thermal decomposition appears. The decomposition of TPUPCM reaches to a maximum at 396.2 °C. These results indicate that TPUPCM has a wider processing temperature range. This is very important for fabrication and application of TPUPCM.
To reveal the crystalline morphology of TPUPCM, WAXD patterns of PEG6000 and TPUPCM are shown in Fig. 9. It shows that pure PEG and TPUPCM have similar diffraction patterns, and so are their diffraction angles and crystal plane distances. In the diffraction pattern of pure PEG and TPUPCM, two strong diffraction peaks appear at 18.8° and 23.2°. The result indicates that PEG and TPUPCM possess similar crystal structure and crystal cell type [22]. The differences between them are that the diffraction peak height of TPUPCM is lower and the half width is broader than that of pure PEG, which means the crystallites become smaller and the degree of crystallinity decreases. To further verify the crystallization properties of TPUPCM, the POM was used for recording the micro-morphologies of pure PEG and TPUPCM. Fig. 10 (a) and (b) show the POM photos of PEG6000 and TPUPCM at room temperature. It is observed that both micrographs show obvious cross-extinction patterns, which suggests that both of them are crystalline and their crystalline morphologies are spherulites [23]. However, the soft segments were limited by the hard segments in TPUPCM, the crystallization process of PEG becomes heterogeneous nucleation
Fig. 9. WAXD profiles of the PEG6000 and TPUPCM: (a) PEG 6000 and (b) TPUPCM.
Fig. 10. POM photos of PEG and TPUPCM: (a) PEG and (b) TPUPCM.
P. Xi et al. / European Polymer Journal 48 (2012) 1295–1303
and confined crystallization, and the crystallites are smaller [24]. When the temperatures approaches melting point, the spherulites fade away and eventually disappear, and the soft segments experience the transformation from crystalline state to amorphous one. These results are in good agreement with WXRD results. 4. Conclusion Based on the above analysis, a novel tetrahydroxy compound was synthesized. A form-stable thermoplastic polyurethane solid–solid phase change material was prepared with the tetrahydroxy compound. In the molecular structure of TPUPCM, as the main chain and branch chain contain all phase change unites, the weight percentage of the phase change unite is increased, which enhances the phase change properties of the solid–solid PCM. Furthermore, when the structure of main chain is invariant with the increase of the molecular weight of MPEG, the aggregated structure of TPUPCM will change; this provides a very wide range of application space for TPUPCM. When MPEG4000 was chosen as branch chain, TPUPCM shows outstanding processing and mechanical properties, and can be processed directly. These results indicate that TPUPCM has a great potential for thermal energy storage applications. Acknowledgments The authors thank for the financial supports from Application Fundamental and Advanced Technology Research Proposal Project of Tianjin, China (No. 10 JCYBJC03100) and Key project of Chinese Ministry of Education (No. 208005). References [1] El-Dessouky H, Al-Juwayhel F. Effectiveness of a thermal energy storage system using phase-change materials. Energy Conv Manage 1997;38:601–17. [2] Farid MM, Khudhair AM, Razack SAK, Al S. A review on phase change energy storage: materials and applications. Energy Conv Manage 2004;45:1597–615. [3] Li H, Fang GY. Experimental investigation on the characteristics of polyethylene glycol/cement composites thermal energy storage materials. Chem Eng Technol 2010;33:1650–4. [4] Chen CZ, Wang LG, Huang Y. Crosslinking of the electrospun polyethylene glycol/cellulose acetate composite fibers as shapestabilized phase change materials. Mater Lett 2009;63:569–71.
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[5] Chen CZ, Wang LG, Huang Y. Role of Mn of PEG in the morphology and properties of electrospun PEG/CA composite fibers for thermal energy storage. AICHE J 2009;55:820–7. [6] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sust Energy Rev 2009;13:318–45. [7] Mondal S. Phase change materials for smart textiles-An overview. Appl Therm Eng 2008;28:1536–50. [8] Meng QH, Hu JL. A poly (ethylene glycol)-based smart phase change material. Sol Energy Mater Sol Cells 2008;92:1260–8. [9] Gu XH, Xi P, Cheng BW. Synthesis and characterization of a novel solid–solid phase change luminescence material. Polym Int 2010;59:772–7. [10] Ke GZ, Xie HF, Ruan RP. Preparation and performance of porous phase change polyethylene glycol/polyurethane membrane. Energy Conv Manage 2010;51:2294–8. [11] Senturk SB, Kahraman D, Alkan C. Biodegradable PEG/cellulose, PEG/ agarose and PEG/chitosan blends as shape stabilized phase change materials for latent heat energy storage. Carbohydr Polym 2011;84: 141–4. [12] Zhou XM. Study on phase change characteristics of PEG/PAM coupling blend. J Appl Polym Sci 2010;116:1591–5. [13] Sari A, Alkan C, Karaipekli A. Poly (ethylene glycol)/poly (methyl methacrylate) blends as novel form-stable phase-change materials for thermal energy storage. J Appl Polym Sci 2010;116:929–33. [14] Zhou XM. Preparation and characterization of PEG/MDI/PVA copolymer as solid–solid phase change heat storage material. J Appl Polym Sci 2009;113:2041–5. [15] Cao Q, Liao L, Xu HL. Study on the influence of thermal characteristics of hyperbranched polyurethane phase change materials for energy storage. J Appl Polym Sci 2010;115:2228–35. [16] Xi P, Gu XH, Cheng BW, Wang YF. Preparation and characterization of a novel polymeric based solid–solid phase change heat storage material. Energy Conv Manage 2009;50:1522–8. [17] Su JC, Liu PS. A novel solid–solid phase change heat storage material with polyurethane block copolymer structure. Energy Conv Manage 2006;47:3185–91. [18] Li WD, Ding EY. Preparation and characterization of cross-linking PEG/MDI/PE copolymer as solid–solid phase change heat storage material. Sol Energy Mater Sol Cells 2007;91:764–8. [19] Silva Paulo HR, Goncalves Valter LC, Mota Claudio JA. Glycerol acetals as anti-freezing additives for biodiesel. Bioresource Tech 2010;101:6225–9. [20] Xi P, Gu XH, Huang XA. Synthesis, characterization and luminescent properties of new highly luminescent organic ligand and complexes of trivalent rare earth. Spectrochim Acta Part A 2007;66:667–71. [21] Liao L, Cao Q, Liao HQ. Investigation of a hyperbranched polyurethane as a solid-state phase change material. J Mater Sci 2010;45:2436–41. [22] Cao Q, Liu PS. Crystalline-amorphous phase transition of hyperbranched polyurethane phase change materials for energy storage. J Appl Polym Sci 2007;42:5661–5. [23] Alkan C, Sari A, Uzun O. Poly(ethylene glycol)/acrylic polymer blends for latent heat thermal energy storage. AICHE J 2006;52:3310–4. [24] Mao SR, Shuai XT, Unger F. Synthesis, characterization and cytotoxicity of poly(ethylene glycol)-graft-trimethyl chitosan block copolymers. Biomaterials 2005;26:6343–56.
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis and thermal energy storage properties of the polyurethane solid–solid phase change materials with a novel tetrahydroxy compound Peng Xi a,b,⇑, Yuqing Duan a, Pengfei Fei a, Lei Xia a, Ran Liu a, Bowen Cheng a a b
Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, 100080 Beijing, China
a r t i c l e
i n f o
Article history: Received 28 January 2012 Received in revised form 10 April 2012 Accepted 21 April 2012 Available online 30 April 2012 Keywords: Phase change Thermal performance Crystalline Polyethylene glycol
a b s t r a c t Based on the phase change theory, a novel tetrahydroxy compound (THCD) was designed and prepared. Depending on the spatial structure of the tetrahydroxy compound, a formstable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was synthesized via employing PEG as soft segments, while multi-benzene ring structure made by 4,40 -diphenylmethane diisocyanate and tetrahydroxy compound as hard segments. The composition and structure of THCD and TPUPCM, the TPUPCM’s the weight average molecular weight and number average molecular weight, dissolving and melting abilities, phase change behaviors, thermal performances and crystalline morphology were investigated by Fourier transform infrared spectrometer, 1H nuclear magnetic resonance spectrometer, multiangle laser light scattering apparatus, differential scanning calorimentry, dynamic mechanical thermal analysis, thermogravimetry analysis system, wide-angle X-ray diffraction, polarizing optical microscopy. The results show that the solid–solid phase change material owns excellent phase change properties and a broad processing temperature range. The heating cycle phase change enthalpy is 137.4 J/g, and the cooling cycle phase change enthalpy is 127.6 J/g. The started decomposition temperature and the maximum decomposition temperature are at 323.5 and 396.2 °C, respectively. Furthermore, the solid–solid phase change material is dissolvable, meltable and can be processed directly, and has great potential applications in thermal energy storage. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Phase change materials (PCMs), which are also called latent heat-storage materials, have high capability to store and release large heat energy within a slight or no temperature change as a series of functional materials. Their application fields include solar energy [1], waste heat recovery [2], smart housing [3], temperature-control greenhouses and textiles [4,5], telecommunications and microprocessor equipment [6] and so on. Especially, PCMs
⇑ Corresponding author at: Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China. Tel.: +86 22 27637689; fax: +86 22 83955164. E-mail address: [email protected] (P. Xi). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.04.009
is playing a prominent role while energy crisis is becoming more and more serious. Mondal [7] ever gave a quite comprehensive review of the types of PCMs and the advantages and disadvantages of different types of PCMs. A great number of organic, inorganic, polymeric and eutectic PCMs have latent heat-storage properties. Based on their phase change states, PCMs are divided into three categories: solid–solid PCMs, solid– liquid PCMs and liquid–gas PCMs [8]. Among the various kinds of PCMs, organic solid–solid PCMs are fairly recently developed functional PCMs because they have been found to exhibit many desirable characteristics, for example, no liquid or gas generation, small volume change, no receptacle needed to seal them, and non-corrosiveness and nontoxicity [9]. Unfortunately, the organic solid–solid PCMs have been identified to be relatively few, especially in the polymer-based solid–solid PCMs.
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As a representative linear polymer, polyethylene glycol (PEG) and polyethylene glycol monomethyl ether (MPEG) are mostly employed as the phase change unit in the synthesis of a novel polymer-based solid–solid PCMs owing to their desirable characteristics such as high latent heat storage capacity, a suitable melting point ranging from 3.2 to 68.7 °C, low vapor pressure when melted, and good thermal and chemical stability [10]. At present there are two approaches to prepare formstable polymeric solid–solid PCMs, which are physical approach and chemical approach. In the physical approach, the novel polymer-based PCMs are obtained by dispersing PCMs into higher melting point polymeric materials acting as supporting materials. As long as the temperature is below the melting point of the supporting materials (cellulose [11], PAM [12] or PMMA [13]), the whole PCMs can keep their solid shape even when phase change unit changes from solid to liquid. However, the materials prepared via the physical blending have a tendency to lose their phase change characteristics after several heating–cooling cycles due to the loss of PEG. For the chemical approach, chemical grafting, blocking copolymerization are used to make polymer-based solid– solid PCMs as the energy storage. Zhou [14] prepared a solid–solid PCM via a copolymerization reaction of high molecular weight poly (ethylene glycol) with poly (vinyl alcohol) (PVA) and 4,40 -diphenylmethane diisocyanate (MDI). However, the maximum phase change enthalpy is 72.8 J/g. The value is still very low in comparison with that of pure PEG. Cao et al. [15] prepared a hyperbranched polyurethane solid–solid PCM using hyperbranched polyester (BoltornÒH20) as chain extender via a two-step reaction process of PEG, MDI and H20. The phase transition enthalpy of the PCM is more than 100 J/g. However, the polymer aggregation structure is difficultly to restraint, and the cross-link structure is formed easily. Furthermore, the value of H20 is too expensive to be suitable to extensive application. In the previous studies, we synthesized a copolymer monomer containing polyethylene glycol monomethyl ether (MPEG) phase change unit and a vinyl unit via the modification of hydrogen group of MPEG. By copolymerization of the copolymer monomer and phenyl ethylene, a phase change heat storage material with long-branch chain was prepared [16]. The phase change enthalpy of the polymeric based PCM is 108.5 J/g. However, the PCM has a few defects on the melted processing and mechanical properties, which is not directly processing. It was reported that a polymeric solid–solid PCM (PUPCM) with polyurethane block copolymer structure composed of PEG10000 as soft segment, 4,40 -diphenylmethane diisocyanate and 1,4butanediol as a chain extender possessed an excellent linear segment block copolymer and could be easily fabricated [17]. A cross-linking copolymer PCM (PEG/MDI/PE) prepared via the two-step condensation reaction of PEG10000 with pentaerythritol (PE) and 4,40 -diphenylmethane diisocyanate has phase change enthalpy as 152.97 J/g [18]. Although the research results provide an important way for the synthesis of PCM with high phase change enthalpy, the cross-linking copolymer is not directly processed. In this contribution, we designed and synthesized a novel tetrahydroxy compound. Relying on the spatial struc-
ture of the tetrahydroxy compound, a form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared via employing PEG as soft segments, while 4,40 -diphenylmethane diisocyanate (MDI) and THCD comprising of multi-benzene ring structure as hard segments. The material’s composition and structure and properties were investigated by various techniques. The results show that the solid–solid PCM owns excellent phase change properties. It can be processed directly and is likely to be produced in large scale. 2. Experimental 2.1. Materials PEG (analytical grade, M n = 6000, from Shanghai Medical Chemical Reagent Co. Inc., China) is dried at 80 °C under high vacuum (20 Pa) for overnight prior to use. MPEG (analytical grade, M n = 4000, from China Medical Group, China) is degassed and dried at 100–110 °C under high vacuum (20 Pa) for 3–4 h. Benzaldehyde glycerin acetal (BGA, analytical grade, from Shanghai Ruiteng Chemical Reagent Co. Inc., China), Tetrahydrofuran (THF, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), acetic acid (HOAc, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), petroleum ether (analytical grade, from Shanghai Medical Chemical Reagent Co. Inc., China), ethyl acetate (analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China) and N,N-dimethylformamide (DMF, analytical grade, from Tianjin third Chemical Reagent Co. Inc., China) were dried by 5 Å molecular sieve for 24 h followed by distillation before use. MDI (analytical grade, from Tianjin Kemiou Chemical Reagent Co. Inc., China) was used as received. 2.2. Synthesis of THCD The synthetic route of THCD is shown in Scheme 1. THCD was prepared in a three-necks round bottomed flask fitted with an overhead stirrer, nitrogen inlet and an addition funnel. First, MDI (2.50 g, 10 mmol) was introduced into the flask and dissolved in THF (20 ml). And then, BGA (3.60 g, 20 mmol) was dropped into the reaction flask. Under nitrogen, the mixture was stirred at 60 °C. Completion of the reaction was followed by monitoring the disappearance of strong absorbed peaks of AOH and ANCO using Fourier transform infrared (FT-IR) spectroscopy. Then THF was evaporated and recovered. The white precipitate was subjected to column chromatography on silica gel (eluent; petroleum ether:ethyl acetate = 2:1(v/v)) to give 1 (5.55 g, 9.1 mmol) in 91.0% yield. Under nitrogen, to a solution of 1 (3.00 g, 4.91 mmol) in mixture of THF and HOAc (THF: HOAc = 1:2 (v/v)) (40 ml) was stirred at 50 °C for 24 h, according to literature procedures [19]. The concentrate was precipitated using petroleum ether (20 ml), and the obtained precipitate was isolated by filtration and dried under reduced pressure to give a crude product. The crude product was washed for twice with petroleum ether. And then, the mixture was filtered and dried to give 2 (2.03 g, 4.67 mmol). The sample
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Scheme 1. Synthesis route to THCD.
was kept in a vacuum oven at 80 °C until further characterization (white powder). 2.3. Synthesis of TPUPCM The synthesis was conducted in a two step polymerization process under an inert atmosphere of nitrogen in three-necks round bottomed flask fitted with an overhead stirrer. The synthesis route is shown in Scheme 2. First, under nitrogen, MDI (0.50 g, 2 mmol) was dissolved with freshly distilled DMF (20 ml) at 0 °C. And then, the dried PEG6000 (6 g, 1 mmol) was dropped into the solution and stirred for 5 min, and the resulting mixture was stirred at 80 °C. Completion of the reaction was indicated by the disappearance of strong absorbed peaks of AOH in PEG molecule using FT-IR spectroscopy. The ANCO terminated modified MPEG (MMPEG) was synthesized using similar procedure as mentioned earlier. In the second step reaction, under nitrogen, to a solution of 2 (0.43 g, 1 mmol) in freshly distilled DMF (10 ml) was added into the three-necks round bottomed flask by dropping. The mixture was stirred at 80 °C for 6 h, and MMPEG (4.25 g, 2 mmol) was added. The reaction continued at 80 °C for 2 h. At last, the reaction mixture was cast in a special glass pan and set in vacuum oven further reaction at 80 °C for 48 h, so that the reactions would be proceed completely, and the DMF was volatilized. The samples were kept in vacuum at room temperature for two weeks prior to testing. In whole reaction, the total amount of ANCO groups is kept to equal to that of AOH groups. 2.4. Characterization FT-IR spectra of the MDI, BGA, THCD, TPUPCM and PEG6000 were obtained on a Nicollet NEXUS-670 FT-IR
spectrometer. Transition of attenuated total reflectance (ATR) spectra of sample between 500 and 4000 cm1 were collected by 32 scans for each spectrum and the resolution for the ATR spectra was 2 cm1. 1H Nuclear magnetic resonance (1H NMR) spectra of THCD and TPUPCM were collected using a JEOL EX-400 spectrometer at room temperature with DMSO-d6 as the solvent.The weight average molecular weight (M w ) and number average molecular weight (M n ) of the prepared TPUPCM were determined with a multiangle laser light scattering apparatus (DAWN-DSP, Wyatt Technology Co., Santa Barbara, CA) combined with a P100 pump (Thermo Separation Products) equipped with a TSKGEL G5000 HHR column (7.8–300 mm). The solvent used was high-performance liquid chromatograph grade dimethylformamide (DMF). Differential scanning calorimentry (DSC) analysis of the samples was carried out with a Perkin Elmer DSC-7 to monitor the variations in melting temperature (Tm), crystallizing temperature (Tc), latent heat of fusion (DHf) and latent heat of crystallization (DHc) of the samples. A typical thermal cycling consisted of a consecutive heating and cooling process (in the temperature interval 0–100 °C). The heating and cooling process times were maintained at approximately 15 and 25 min, respectively. Thermal cycling test continued until 100 complete heating–cooling processes of TPUPCM. Dynamic mechanical thermal analysis (DMTA) was performed on molded films with a NETZSCH DMA242C. 50 mg sample was processed in the special model. The relaxation spectrum was scanned from 0 to 220 °C with a frequency of 1 Hz and a heating rate of 3 °C/min. Thermogravimetry and derivative of thermogravimetry (TGA) curves of the TPUPCM were obtained using a Netzsch STA409PC thermal analysis system. About 10 mg dried sample was set into an alumina crucible and
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Scheme 2. Synthesis route to TPUPCM.
weighed. The curve was recorded at a heating rate of 5 °C/min in nitrogen atmosphere. The analysis of the PEG and TPUPCM by wide-angle Xray diffraction (WAXD) was carried out with a Philip PW 1710 at 30 kV and 20 mA. Bragg’s angle 2h is set from 10° to 50° with a rate of 3 °C/min. An observation of the samples by polarizing optical microscopy (POM) was performed on Olympus BX51 Polarizing Microscope. The sample was placed between a microscope glass and a cover slip, and then heated with hot stage. 3. Results and discussion 3.1. Analysis of the composition and structure for THCD The results of FT-IR and 1H NMR spectra for THCD (Figs. 1 and 2) verify that the composition and structure
of THCD agree with Scheme 1. In the FT-IR spectrum of THCD (shown in Fig. 1), the characteristic peak of ANCO for MDI at 2279 cm1 and the stretching vibration peak of AOH of BGA at 3437 cm1 disappear. The stretching vibration peaks of ANH, AC@O and ACAOA emerge at 3313, 1723, and 1259–1001 cm1, respectively. These results prove that the chemical bond of ANHCOOA has formed. The strong absorption peak at 3415 cm1 is assigned to the stretching vibration peaks of terminal hydroxyl group of THCD. The peaks locating at 3005, 1598, 1506 and 1445 cm1 are the characteristic peaks of benzene ring [20]. In order to further verify the FT-IR analysis results, the 1 H NMR analysis was performed. Through the analysis results of 1H NMR spectrum of the sample, it can be found that the structure of THCD in the Scheme 1 corresponds to the 1H NMR spectrum. Both the IR spectrum and 1H
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PEG6000 were also shown in Fig. 3. From the Fig. 3, it can be found that the hydroxyl groups of THCD at 3415 cm1 tend to polymerize. Moreover, the strong peaks emerge at 2884 and 1101 cm1, which belong to the stretching vibration peaks of ACH2A and ACAOA for soft segments. The stretching vibration peaks of ANAH locate at 3313 and 1610 cm1. The carbonyl group stretching vibration peak of the ester group emerges at 1725 cm1. The peaks at 1596, 1546, 1512 and 1465 cm1 are assigned to the characteristic peaks of benzene ring. These results prove that TPUPCM has been synthesized. In 1H NMR spectrum of TPUPCM, the proton peaks correspond to the structure of TPUPCM. The analysis results of 1H NMR spectrum of TPUPCM verify further those of FT-IR spectra. 3.3. Physical properties of TPUPCM
Fig. 1. FT-IR spectrum of THCD: (a) THCD; (b) BGA; (c) MDI.
NMR spectrum of THCD indicate that the novel tetrahydroxy compound has been successfully prepared. 3.2. Analysis of the composition and structure for TPUPCM To reveal composition and structure of TPUPCM, FT-IR and 1H NMR spectra test were performed as shown in Figs. 3 and 4. For a comparison, FT-IR spectra of THCD and
The physical properties of the novel polyurethane solid– solid phase change material synthesized are tested. The results show that the sample as-obtained owns excellent dissolving and melting-processing properties. It is dissolvable in tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively. The TPUPCM obtained has an M w of 2.56 105 and an M n of 1.68 105. The melting point and the first degradation temperature of the polyurethane phase change material is 186 and 323 °C (TGA curve), respectively. It can be manufactured to form any given shape (film, strip, particle and filament) when the temperature is at 186–323 °C. These results verify that the novel polyurethane phase change material
Fig. 2. 1H NMR spectrum of THCD.
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Fig. 3. FT-IR spectrum of TPUPCM: (a) TPUPCM; (b) PEG6000; (c) THCD.
as-synthesized is thermoplastic polymer, which benefits from the steric structure of THCD. In the synthesis of TPUPCM, the multi-benzene rings structure in the main chain is inflexible due to the hyperconjugation effect among multi-benzene rings and acyl groups. The space steric effect inhibits the second hydroxyl to participate reaction and leads to the linear polymer main chain to be formed. 3.4. Phase change properties of TPUPCM In order to study the phase change properties of the TPUPCM, the Differential scanning calorimetry (DSC) was employed. The DSC curves of pure PEG6000 and TPUPCM are shown in Fig. 5, and the data of their transition temperature and enthalpy are summarized in Table 1. The results show that there is a strong melting peak at 56.1 °C in the
heating scanning DSC curve of TPUPCM and the phase change enthalpy is 137.4 J/g. Through the analysis of DSC, it can be found that both of PEG and TPUPCM undergo all phase transition, but their phase transition states are quite different. When the temperature is raised to about 65.9 °C, the pure PEG undergoes a change from a white crystal solid to a transparent liquid [8]. While TPUPCM remains solid in the whole heating process, even if the temperature is raised to 100 °C or higher. This indicates that TPUPCM is a solid–solid phase change material. The cooling scanning DSC curves of PEG and TPUPCM are presented in Fig. 5 (II). They show that similar quantities of heat are released. Compared with PEG/MDI/PE cross-linking copolymer [18], the phase change enthalpy of TPUPCM is 137.4 J/g, lower than that of PEG/MDI/PE cross-linking copolymer, the phase change enthalpy of PEG/MDI/PE cross-linking copolymer is 152.97 J/g. This is because the phase change enthalpy of the PCM is intimately related to the weight percentage of phase change unit. In the molecular structure of TPUPCM, the phase change units include PEG6000 (in the main chain) and MPEG4000 (in the branch chain). When the main chain structure remains unchanged, the weight percentage of phase change unit of TPUPCM gradually increases with the increase of molecular weight of MPEG. In the synthesis of PEG/MDI/PE cross-linking copolymer PCM, PEG10000 was chosen as phase change unit. In view of the mechanical properties of TPUPCM, we chose MPEG4000 as branch chain of TPUPCM. This affects the phase change enthalpy of TPUPCM to an extent. Liao et al. [21] synthesized a solid–solid PCM (D-HBPU) via two steps reaction of PEG6000 with MDI and pentaerythritol (PE). The phase change enthalpy of D-HBPU is 125.0 J/g. The result verifies the structural advantage of THCD on the synthesis of PCM with high phase change enthalpy. To verify the stability of phase change for TPUPCM, the 100 time heating and cooling cycles were carried out, as shown in Fig. 6. It can be found that the phase change temperatures and latent heat values of the TPUPCM slightly change after 100 time cycles. This is quite a good and
Fig. 4. 1H NMR spectrum of TPUPCM.
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Fig. 5. DSC curves of PEG6000 (a) and TPUPCM (b): (I) Heating cycle; (II) Cooling cycle.
Table 1 Phase change temperature and enthalpy of PEG6000 and TPUPCM. Sample
Phase transition
PEG TPUPCM
Solid–liquid Solid–solid
DH (J/g)
Tp (°C)
Heating cycle
Cooling cycle
Heating cycle
Cooling cycle
189.6 137.4
167.2 127.6
65.9 56.1
37.6 37.5
Tp: peak transition temperature of samples DH: phase transition enthalpy of samples.
The dynamic mechanical thermal analysis (DMTA) is an effective method to research the thermal performances of 0 TPUPCM. The elastic modulus (E ) and loss tangent (Tand) of TPUPCM over the temperature ranging from 0 to 0 210 °C are presented in Fig. 7. At around 56.1 °C, E curve appears an obvious step and Tand curve shows a sharp
peak in the same temperature zone. These can be ascribed to the melted transition of PEG soft segments. The test results are agreed with that of differential scanning calorim0 etry. When the temperature was risen continually, the E curve becomes a platform until the temperature reached 186 °C. This indicates that the whole polymer molecule will start to move at 186 °C. However, the elastic modulus of material is 40 MPa in the 60–186 °C, which is completely contributed by hard segments of TPUPCM. The thermogravimetric and derivative thermogravimetric analysis curves of TPUPCM are shown in Fig. 8. The mechanism of thermal decomposition involves two steps,
Fig. 6. The 100th time heating and cooling DSC curves of TPUPCM.
Fig. 7. DMTA curves of TPUPCM.
essential property for PCMs. These results indicate that TPUPCM has a quite good energy storage and release effect and great application prospects. 3.5. The thermal performances of TPUPCM
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3.6. Crystalline morphology investigations of TPUPCM
Fig. 8. TGA curve of TPUPCM.
as can be seen in TGA curves. When the temperature reaches 323 °C, slight thermal decomposition appears. The decomposition of TPUPCM reaches to a maximum at 396.2 °C. These results indicate that TPUPCM has a wider processing temperature range. This is very important for fabrication and application of TPUPCM.
To reveal the crystalline morphology of TPUPCM, WAXD patterns of PEG6000 and TPUPCM are shown in Fig. 9. It shows that pure PEG and TPUPCM have similar diffraction patterns, and so are their diffraction angles and crystal plane distances. In the diffraction pattern of pure PEG and TPUPCM, two strong diffraction peaks appear at 18.8° and 23.2°. The result indicates that PEG and TPUPCM possess similar crystal structure and crystal cell type [22]. The differences between them are that the diffraction peak height of TPUPCM is lower and the half width is broader than that of pure PEG, which means the crystallites become smaller and the degree of crystallinity decreases. To further verify the crystallization properties of TPUPCM, the POM was used for recording the micro-morphologies of pure PEG and TPUPCM. Fig. 10 (a) and (b) show the POM photos of PEG6000 and TPUPCM at room temperature. It is observed that both micrographs show obvious cross-extinction patterns, which suggests that both of them are crystalline and their crystalline morphologies are spherulites [23]. However, the soft segments were limited by the hard segments in TPUPCM, the crystallization process of PEG becomes heterogeneous nucleation
Fig. 9. WAXD profiles of the PEG6000 and TPUPCM: (a) PEG 6000 and (b) TPUPCM.
Fig. 10. POM photos of PEG and TPUPCM: (a) PEG and (b) TPUPCM.
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and confined crystallization, and the crystallites are smaller [24]. When the temperatures approaches melting point, the spherulites fade away and eventually disappear, and the soft segments experience the transformation from crystalline state to amorphous one. These results are in good agreement with WXRD results. 4. Conclusion Based on the above analysis, a novel tetrahydroxy compound was synthesized. A form-stable thermoplastic polyurethane solid–solid phase change material was prepared with the tetrahydroxy compound. In the molecular structure of TPUPCM, as the main chain and branch chain contain all phase change unites, the weight percentage of the phase change unite is increased, which enhances the phase change properties of the solid–solid PCM. Furthermore, when the structure of main chain is invariant with the increase of the molecular weight of MPEG, the aggregated structure of TPUPCM will change; this provides a very wide range of application space for TPUPCM. When MPEG4000 was chosen as branch chain, TPUPCM shows outstanding processing and mechanical properties, and can be processed directly. These results indicate that TPUPCM has a great potential for thermal energy storage applications. Acknowledgments The authors thank for the financial supports from Application Fundamental and Advanced Technology Research Proposal Project of Tianjin, China (No. 10 JCYBJC03100) and Key project of Chinese Ministry of Education (No. 208005). References [1] El-Dessouky H, Al-Juwayhel F. Effectiveness of a thermal energy storage system using phase-change materials. Energy Conv Manage 1997;38:601–17. [2] Farid MM, Khudhair AM, Razack SAK, Al S. A review on phase change energy storage: materials and applications. Energy Conv Manage 2004;45:1597–615. [3] Li H, Fang GY. Experimental investigation on the characteristics of polyethylene glycol/cement composites thermal energy storage materials. Chem Eng Technol 2010;33:1650–4. [4] Chen CZ, Wang LG, Huang Y. Crosslinking of the electrospun polyethylene glycol/cellulose acetate composite fibers as shapestabilized phase change materials. Mater Lett 2009;63:569–71.
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