Synthesis and characterization of novel solid–solid phase change materials with a polyurethaneurea copolymer structure for thermal energy storage

Abstract

A series of novel polymeric solid–solid phase change materials (SSPCMs) with polyurethaneurea (PUU) copolymer structure were synthesized by the crosslinking copolymerization of polyethylene glycol (PEG) and multiamino compounds (diethylenetriamine, triethylenetetramine, tetraethylenepentamine) with different molar ratios. The results from the polarization optical microscopy (POM) and WAXD patterns indicate that the SSPCMs have typical spherocrystal morphology and characteristic diffraction peaks like neat PEG, but their spherulite size and diffraction peak intensity are smaller than those of PEG. According to the thermal analyses, the SSPCMs possess high phase transition enthalpies, good reusability and high thermal stability for their potential applications in thermal energy storage. With the decrease of crosslinking density of SSPCMs, the crystalline properties and thermal properties of SSPCMs have improved apparently, which is closely related to the free movement of PEG segments in the SSPCMs.

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Introduction

Thermal energy storage (TES) is a technology that stocks surplus thermal energy by heating or cooling a storage medium so that the energy stored can be drawn upon at a later time and usefully re-applied in a given operation.¹ TES systems can reduce energy demand at peak times, energy consumption, carbon dioxide (CO₂) emissions and costs, while increasing overall efficiency of energy systems in buildings and industrial processes.^2,3 Thermal energy can be stored at temperatures from −40 °C to more than 400 °C as sensible heat by heating or cooling a liquid or solid storage medium (e.g., water, sand, molten salts, rocks), latent heat using phase change materials (PCMs) and chemical energy (i.e. thermo-chemical energy storage) using chemical reactions. Among them, latent heat storage has proven to be the most promising and efficient way of TES because PCMs can offer a higher storage capacity and lower heat losses during the storage period.^4,5 A large amount of latent heat can be stored and released as a stable state by the phase transition of PCMs with a small variation of volume and temperature, which could effectively balance the possible mismatch of energy in time and space.

It is well-known that there are two main types of PCMs for the practice applications: solid–liquid PCMs (SLPCMs) and solid–solid PCMs (SSPCMs). A wide range of promising inorganic and organic SLPCMs, including inorganic salt hydrates,^6,7 neat long-chain alkenes and paraffin waxes,^8–12 fatty acids/alcohols and their eutectics,^13–17 and polyols,^18–22 have been extensively studied for TES with various advantages: for instance, large latent heats, numerous species, compactness and easy handling. In spite of these advantages, owing to their fluidity and leakage in molten state, the extra storage containers or supporting materials for sealing or stabilizing molten SLPCMs are necessary in the practical applications.

In the case of SSPCMs, on the contrary, there are no liquid or gas phase generation and small volume changes during the phase transition process of SSPCMs, which is attributed to the reversible transformations of SSPCMs between two different solid crystalline phases or a solid crystalline phase and an amorphous phase as they absorb/release latent heat.^23–25 Consequently, SSPCMs are good candidates for TES and they can apply conveniently with various shapes and dimensions. Many researchers have recently attempted to prepare novel leakage-free polymeric SSPCMs.

PEG belongs to a family of linear polyethers that is extensively studied as the organic SLPCMs for its outstanding characteristics of large latent heat, diversified molecular weight, suitable phase change temperatures for low-temperature applications, good thermal/chemical stability and nontoxicity.^23,24 Due to the unique properties and high reactivity of terminal hydroxyl groups of PEG, it is often utilized as the phase change functional chains (working substance) in many polymeric SSPCMs by the crosslinking, blocking or grafting. We previously reported the PEG/poly(glycidyl methacrylate) (PGMA) crosslinking copolymer as a polymeric SSPCM, in which PEG segments were interlinked with the polymer chains.²³ Sarı et al. prepared the polystyrene (PS)-g-PEG copolymers as new SSPCMs for TES by grafting PEG onto the para-position of benzene ring of PS.²⁴ Li et al. synthesized melamine/formaldehyde/PEG crosslinking copolymers as the polymeric SSPCMs by amine-aldehyde condensation reaction and aldolization.²⁶

However, polyurethane (PU)-based copolymers containing PEG segments are the most popular polymeric SSPCMs, which were easily obtained by the covalently bonding between the terminal hydroxyl groups of PEG and isocyanate groups of diisocyanates with or without chain extender. For instance, Alkan et al. reported a series of PU copolymers by the direct reaction of PEG with three different molecular weights and three different kinds of diisocyanates (hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), and 2,4-toluene diisocyanate (TDI)), and the effects of molecular weight of PEG and type of diisocyanate on the thermal energy storage properties have been discussed.²⁷

In most cases, various polyhydroxy polymers and small molecular compounds were also utilized in the synthesis of PU-based polymeric SSPCMs as the molecular skeleton or chain extender, which can effectively increase the chain length of the urethane hard segments and the overall molecular weight of the PU-based polymeric SSPCMs.²⁸ Comb-like or dendrimer-like polymeric SSPCMs with PU structures were prepared by PEG segments with different molecular weights respectively grafting onto the side chains of cellulose,^29,30 cellulose diacetate (CDA),^31,32 and polyvinyl alcohol (PVA)³³ using diisocyanate as crosslinking agent, and the phase change properties of these SSPCMs can regulated by the grafting ratio of polymers. Linear blocking PU-based polymeric SSPCMs were reported by several research groups using 1,4-butanediol^28,34–36 or N-methyldiethanolamine³⁷ as the chain extender and PEG with different molecular weights as the soft segments. Various crosslinking PU-based polymeric SSPCMs also prepared by the crosslinking copolymerization of PEG and compounds with multiple hydroxyl groups such as Span 80 and Tween 80,³⁸ caster oil,³⁹ pentaerythritol,²⁵ Boltorn® H20,⁴⁰ bis(1,3-dihydroxypropan-2-yl)4,4′-methylenebis(1,4-phenylene) dicarbamate.⁴¹ In our previous works,^42–44 a series of crosslinking PU-based polymeric SSPCMs using different chain extender were reported. The hexahydoxy compounds/MDI/PEG crosslinking copolymers were prepared respectively using sorbitol, dipentaerythritol and inositol as chain extender, and the influence of type of chain extender on the thermal properties was discussed;⁴² the glucose/MDI/PEG crosslinking copolymer was synthesized both as polymeric SSPCMs and the supporting material of form stable PCMs, and then the novel binary form stable PCMs based on SLPCM/SSPCM composites were reported;⁴³ the β-cyclodextrin (β-CD)/MDI/PEG crosslinking copolymers with different crosslinking density were obtained by varying the molar ratio of reactants, and the effects of crosslinking density of the copolymers on their properties were investigated.⁴⁴

Theoretically, the isocyanate groups can easily react with any compounds containing active hydrogen atoms. Polymers with PU or polyurea structure are often obtained by the reaction of diisocyanates with polyhydroxy compounds or multiamino compounds. Hence, we think that compounds with multiple amino groups can also act as the chain extender of the polymeric SSPCMs like polyhydroxy compounds with the crosslinking agent of diisocyanates. The polyurethaneurea (PUU) copolymers would be expected to obtain when the isocyanate groups simultaneously react with the hydroxyl groups of PEG and the amino groups of multiamino compounds. Regrettably, as far as we know, the PUU copolymers as polymeric SSPCMs with the chain extender of multiamino compounds have, however, not been reported.

The primary aim of this study was to prepare a series of novel polymeric SSPCMs with PUU copolymer structure using three multiamino compounds (diethylenetriamine, triethylenetetramine and tetraethylenepentamine) individually as the chain extender. The PUU copolymers with different crosslinking densities were obtained by regulating the molar ratios of the reactants, and the chemical structure, crystalline behaviours, phase transition properties, and thermal stability of the synthesized copolymers were systematically investigated by Fourier transform infrared spectroscopy (FTIR), polarizing optical microscopy (POM)/wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetry analyses (TGA), respectively. Besides, the paper emphasizes on the influence of crosslinking density of PUU copolymers on their crystalline behaviours and thermal properties.

Experimental

Materials

Diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), 4,4′-diphenylmethan diisocyanate (MDI) and anhydrous N,N-dimethylformamide (DMF) were purchased from Aladdin Chemistry Co., Ltd., China. PEG with the average molecular weight of 8000 is produced by Amresco Co., USA. The trace amounts of water in DETA, TETA and TEPA were eliminated using the metallic sodium. PEG was dried in vacuum oven at 100 °C for 6 h before use. MDI was kept at 60 °C for 2 h in vacuum oven and then filtered. DMF was used without treatment.

Synthesis of the PUU copolymers

PUU-based polymeric SSPCMs containing PEG segments were prepared similarly to our previously reported protocol for the PU-based SSPCMs.^42–44 Stoichiometric PEG and MDI (PEG : MDI = 1 : 2, molar ratio) were individually dissolved in DMF, and then the PEG solution was heated to 90 °C in a thermostatic oil-bath under dried N₂. The MDI solution was dropped slowly to PEG solution with stirring. The reaction was carried out for 5 h. Whereafter, the multiamino compound (DETA, TETA or TEPA) with different proportions was added by dropwise into the above reaction system. The reaction mixture was poured into a beaker after the reaction continued for 24 h. The yellowish solid products were obtained after the thermal curing at 80 °C in drying oven for another 24 h. Last, the obtained solid products were kept at 60 °C in vacuum oven for two weeks in order to remove any volatile components. The synthesized PUU copolymers using DETA, TETA and TEPA as the chain extender were labelled as DETA/MDI/PEG copolymers, TETA/MDI/PEG copolymers and TEPA/MDI/PEG copolymers, respectively.

Analytical characterization

The FTIR experiments were conducted using a Nicolet 760 spectrometer (Nicolet Co., USA) between the wavelengths 4000 to 400 cm⁻¹. The homogeneous mixture of every sample and KBr powders was pressed into a small disk before testing. The POM images of samples were obtained by a polarization optical microscopy (XPN-300E, Shanghai Changfang Optical Instrument Co., Ltd, China). A very thin casting membrane of samples on the glass side was prepared before testing. WAXD analyses were performed for neat PEG and the synthesized copolymers using a wide-angle X-ray diffractometer (D8 Advance, Bruker-AXS, Germany) with a Cu Kα radiation source. All diffraction data were acquired in the range of 5° < 2θ < 50°. Both the DSC and TGA experiments were conducted using a Simultaneous Thermal Analysis Apparatus (STA 449 F3 Jupiter®, Netzsch, Germany) under N₂ atmosphere. Small amounts of samples (5–10 mg) were placed in an aluminum pans, sealed, and four heat–cool cycles were employed between the temperatures 25–100 °C at a scanning rate 2 °C min⁻¹. All the DSC curves were recorded from the second thermal cycle. In order to minimize the uncertainty, the data of thermal properties (phase change enthalpy and phase change temperature) were reported as average values of the recorded DSC curves with standard deviations. The testing temperature range of TGA curves is 25–600 °C, and the heating rate is set as 10 °C min⁻¹.

The thermal reliability and reusability of synthesized copolymers were evaluated by the thermal cycling test. 1000 times continuous heating–cooling thermal cycles (the temperature is kept in the range of 25–100 °C) were performed by the hot stage for the sample loaded in an aluminum pan. Each heating–cooling thermal cycle took 20 min, and the total time of the test lasted for about two weeks. The FTIR spectra and DSC curves of copolymers after thermal cycling test were also characterized with the above corresponding measurement conditions.

Results and discussion

The molecular structure of the PUU copolymers

It is clear that theoretically there are 5 mol (for DETA), 6 mol (for TETA) and 7 mol (for TEPA) active N–H bonds respectively in 1 mol multiamino compound from their molecular structures, which can react with –NCO groups of MDI to form uramido. Undoubtedly, the N–H bonds in different locations of the molecules of multiamino compounds have different reactivity, so the numbers of N–H bonds took part in the crosslinking reaction could be regulated by adjusting the molar ratio of reactants. We assume that all –NCO groups of MDI would react with –OH or –NH–, namely the moles of –NCO groups in MDI are equal to the sum of moles of –OH in PEG and –NH– in the multiamino compound. Thus, when the input of MDI and PEG in the reaction system is fixed, we only need vary the amount of multiamino compounds. Table 1 listed the product codes and the corresponding molar ratio of reactants.

Table 1 The product codes and corresponding molar ratio of reactants for the synthesized SSPCMs

Samples	Molar ratio of reactants
SSPCM-1a	DETA : MDI : PEG = 1 : 2 : 1
SSPCM-1b	DETA : MDI : PEG = 2/3 : 2 : 1
SSPCM-1c	DETA : MDI : PEG = 1/2 : 2 : 1
SSPCM-1d	DETA : MDI : PEG = 2/5 : 2 : 1
SSPCM-2a	TETA : MDI : PEG = 1 : 2 : 1
SSPCM-2b	TETA : MDI : PEG = 1/2 : 2 : 1
SSPCM-2c	TETA : MDI : PEG = 1/3 : 2 : 1
SSPCM-3a	TEPA : MDI : PEG = 1 : 2 : 1
SSPCM-3b	TEPA : MDI : PEG = 1/2 : 2 : 1
SSPCM-3c	TEPA : MDI : PEG = 2/5 : 2 : 1
SSPCM-3d	TEPA : MDI : PEG = 2/7 : 2 : 1

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As shown in Table 1, a series of PUU copolymers were synthesized using different molar ratios of reactants. According to the previous discussion, less the moles of multiamino compounds in the reactive system, more the numbers of N–H bonds would take part in the crosslinking reaction, which results in a more compact crosslinking structure for the synthesized copolymers. Fig. 1 demonstrates the molecular structure of SSPCM-1a and SSPCM-1d, respectively. Only the most active N–H bonds of the terminal amino groups had participated in the reaction for SSPCM-1a, which caused the formation of linear PUU copolymer structure for them. In contrast, the PUU copolymers with densest crosslinking structure had obtained for SSPCM-1d because all the N–H bonds of the amino groups had took part in the reaction. The crosslinking density of SSPCM-1b and SSPCM-1c falls in between these two extremes. It is easy to conclude that the molecular structures of TETA/MDI/PEG copolymers and TETA/MDI/PEG copolymers are similar with those of DETA/MDI/PEG copolymers. Therefore, the order of crosslinking density for DETA/MDI/PEG copolymers, TETA/MDI/PEG copolymers and TETA/MDI/PEG copolymers is SSPCM-1a < SSPCM-1b < SSPCM-1c < SSPCM-1d, SSPCM-2a < SSPCM-2b < SSPCM-2c and SSPCM-3a < SSPCM-3b < SSPCM-3c < SSPCM-3d, respectively.

Fig. 1 Schematic illustrations showing the linear copolymer for SSPCM-1a and crosslinking copolymer for SSPCM-1d.

FTIR study

The FTIR spectra of PEG, DETA and the DETA/MDI/PEG copolymers were shown in Fig. 2. The characteristic peaks of DETA at 3348 cm⁻¹, 1576 cm⁻¹ and 1316 cm⁻¹ were observed, and correspond to N–H antisymmetry stretching, N–H bending and C–N stretching vibrations, respectively. In addition, several peaks were observed at 2937 cm⁻¹, 1478 cm⁻¹, 942 cm⁻¹, and 820 cm⁻¹ corresponding to the stretching and bending vibrations of C–H bonds in –CH₂– groups. Salient neat PEG peaks are observed at 3432 cm⁻¹, 2889 cm⁻¹ and 1100 cm⁻¹, corresponding to O–H stretching, CH₂ stretching and C–O stretching vibrations, respectively.²¹ In the case of DETA/MDI/PEG copolymers, the C–H stretching/bending vibration peaks (at 2886 cm⁻¹, 959 cm⁻¹ and 843 cm⁻¹) and the C–O stretching vibration peak (at 1105 cm⁻¹) also exist, and the characteristic absorption peak of –NCO groups from MDI at 2265 cm⁻¹ disappears completely. Meanwhile, the absorption peak around 3300–3500 cm⁻¹ in the spectra of the copolymers became wider and weaker compared with DETA (at 3348 cm⁻¹ for the N–H stretching vibration) and PEG (at 3432 cm⁻¹ for the O–H stretching vibration), and the intensity of the peak at that region gradually decreases from SSPCM-1a to SSPCM-1d. On the other hand, the amide and carbonyl vibration peaks of urethane and uramido of the copolymers at 1541 cm⁻¹ and 1727 cm⁻¹,^29,30 the C–N stretching vibration peak at 1316 cm⁻¹, and the stretching vibration peak of benzene rings at 1600 cm⁻¹ also appear in the FTIR spectra of the DETA/MDI/PEG copolymers. These changes of characteristic peaks indicate that the –NCO groups from MDI, the O–H bonds from PEG and the N–H bonds from DETA with different ratios participated in the reaction and the PUU copolymers with different crosslinking density were successfully formed. Similarly, the TETA/MDI/PEG copolymers and TEPA/MDI/PEG copolymers were also obtained.

Fig. 2 FTIR spectra of PEG, DETA and DETA/MDI/PEG copolymers.

Crystalline properties of the PUU copolymers

To observe the crystallization morphology of the synthesized PUU copolymers, the polarized-light microscope was utilized. Fig. 3 is the POM images of neat PEG and the DETA/MDI/PEG copolymers. It is well-known that PEG is a highly symmetric and non-branched linear polymer, which leads to an increased segment mobility and enable better geometrical conditions for formation of well-ordered crystalline structures.²⁸ Thus, PEG exhibits a typical spherulite with cross petal shape and large size in the polarized light photographs at room temperature shown in Fig. 3(a). Similarly, the four DETA/MDI/PEG copolymers also have the typical spherocrystal structure under the same measurement conditions (Fig. 3(b)–(e)). DETA as the chain extender in this work is a liquid and there is no crystal in the room temperature. Thus, it is reasonably concluded that the spherocrystal morphology for DETA/MDI/PEG copolymers is resulted from the (CH₂CH₂O)_n units of PEG in the copolymer molecules, which agrees with the conclusions in our previous studies about the reported PU-based SSPCMs.^42–44 Specially, the spherocrystal size of the copolymers is obviously smaller than that of neat PEG, which shows that the crystallization structure and crystallinity of PEG segments in the copolymers has declined to some extent compared with neat PEG. As shown in Fig. 1, the molecular chains of PUU copolymers are composed of hard segments and soft segments, and the solid–solid phase transition behaviours of copolymers is caused by the transitions of soft segments (PEG chains) between crystalline state and amorphous state. Due to the restriction of hard segments containing benzene ring and/or the formation of crosslinking network structure, PEG segments in the copolymer molecules are fixed strictly in a finite interspace. Thus, the arrangement and orientation of PEG segments in the molecules of copolymers are partially confined, which make some PEG segments in copolymers cannot form crystals. As a consequence, the crystalline regions of PEG segments in PUU copolymers (namely the crystallinity of copolymers) decrease compared with pure PEG.

Fig. 3 POM images: (a) PEG; (b) SSPCM-1a; (c) SSPCM-1b; (d) SSPCM-1c; (e) SSPCM-1d; (f) SSPCM-1a above 60 °C.

The in situ observations of samples were performed by the polarized-light microscope equipped with a hot stage. When the copolymers on the hot stage were heated to the temperature below their phase transition temperatures, the spherocrystal morphology of all the DETA/MDI/PEG copolymers had no change observed. As the sample was heated above their corresponding phase transition temperature (e.g. 60 °C for SSPCM-1a), the spherulites of the copolymers disappeared completely and the black region appeared under polarized light (Fig. 3(f)). During the heating process, all the copolymers had no liquid flow phase generated under a normal light microscope (even the heating temperature higher than 100 °C), which proves that the synthesized PUU copolymers had underwent actual solid–solid phase change process instead of solid–liquid phase change.

It is worth noting that the spherocrystal size of DETA/MDI/PEG copolymers obviously decreases from SSPCM-1a to SSPCM-1d as shown in Fig. 3(b)–(e). In other word, the spherocrystal of SSPCM-1a has the largest size among the four DETA/MDI/PEG copolymers, which indicates that SSPCM-1a has more perfect crystalline structure and highest crystallinity. Due to the linear structure composed of hard segments and soft segments for SSPCM-1a, the movement of PEG segments is relative flexible and the crystallization of PEG segments is more perfect because they are only partly fettered by the hard segments. With the increase of crosslinking density of copolymers (SSPCM-1a < SSPCM-1b < SSPCM-1c < SSPCM-1d), more dense crosslinking network structures had formed, which results in the stricter restriction of the free movement of PEG segments. Thus, SSPCM-1a has the biggest spherocrystal size from Fig. 3 and most perfect crystallization compared with SSPCM-1b, SSPCM-1c and SSPCM-1d. It is easy to deduce that the crosslinking density of the synthesized copolymers has negative impact on their crystalline properties, which is accordance with the conclusion in our previous study.⁴⁴

Fig. 4 and 5 give the POM images of TETA/MDI/PEG copolymers and TEPA/MDI/PEG copolymers, respectively. Similarly, all the synthesized copolymers have the typical spherocrystal structure with different sizes, and the order of spherocrystal size is SSPCM-2a > SSPCM-2b > SSPCM-2c and SSPCM-3a > SSPCM-3b > SSPCM-3c > SSPCM-3d, respectively. The change tendency of spherocrystal size for TETA/MDI/PEG copolymers and TEPA/MDI/PEG copolymers is the same as that of DETA/MDI/PEG copolymers, namely higher the crosslinking density of the PUU copolymers, smaller the spherocrystal size for the corresponding copolymers.

Fig. 4 POM images: (a) SSPCM-2a; (b) SSPCM-2b; (c) SSPCM-2c.

Fig. 5 POM images: (a) SSPCM-3a; (b) SSPCM-3b; (c) SSPCM-3c; (d) SSPCM-3d.

To further verify the crystalline properties of the synthesized PUU copolymers, the WAXD patterns of PEG and SSPCMs are shown in Fig. 6. Neat PEG exhibits two main characteristic peaks at 19.3° and 24.6°, while all the copolymers also exhibit two similar characteristic diffraction peaks in the same positions like PEG, which reveals that the copolymers and PEG have similar crystal structure and crystal cell type.^25,33,34 However, the crystalline properties of the PUU copolymers apparently declined compared with PEG for the lower diffraction peak intensity. Meanwhile, the order of diffraction peak intensity for the PUU copolymers is respectively SSPCM-1a > SSPCM-1b > SSPCM-1c > SSPCM-1d, SSPCM-2a > SSPCM-2b > SSPCM-2c, and SSPCM-3a > SSPCM-3b > SSPCM-3c > SSPCM-3d, which is completely opposite with the change tendency of the crosslinking density of copolymers. To sum up: the higher crosslinking density of the PUU copolymers goes against the free movement of PEG segments and then has a negative impact on the crystallization properties of copolymers. All the results from WAXD patterns are consistent with the POM images discussed above.

Fig. 6 WAXD patterns of PEG and the PUU copolymers.

Phase transition heat storage properties of the PUU copolymers

Differential scanning calorimetry is a conventional testing technique which can provide valuable and important information about phase transition enthalpies and phase transition temperatures. From our previous works,^42–44 the DSC curve of PEG8000 shows the strong and sharp endothermic and exothermic peaks due to its solid–liquid phase transition, and its endothermic/exothermic enthalpy (ΔH_endo/ΔH_exo) and endothermic/exothermic phase change temperature (ΔT_endo/ΔT_exo) from DSC curve are 148.12 J g⁻¹/139.31 J g⁻¹ and 60.40 °C/47.63 °C, respectively. The second heating and cooling thermograms of all the synthesized PUU copolymers are shown in Fig. 7. Strong endothermic and exothermic peaks below 100 °C also can be found in all the DSC curves of the PUU copolymers, which means that the synthesized copolymers have reversible and balanced latent heat storage/release properties in that temperature range.

Fig. 7 DSC curves of all the PUU copolymers.

The phase transition enthalpies and phase transition temperatures of all the PUU copolymers from the DSC curves are depicted in Fig. 8. Clearly, T_endo and T_exo of copolymers have a small variation in the range of 53.9–59.7 °C and 41.6–43.9 °C respectively, which are lower than those of pure PEG. The reason for this phenomenon is that the PEG segments in the PUU copolymers cannot crystallize perfectly due to the steric hindrance effects of the crosslinking network structure.²⁵ Meanwhile, ΔH_endo (in the range of 70.08–119.90 J g⁻¹) and ΔH_exo (in the range of 53.90–115.98 J g⁻¹) of all the PUU copolymers are within 120 J g⁻¹, which is also smaller than those of PEG. The decline of crystalline properties of PUU copolymers compared with neat PEG is the main reason for the decrease of endothermic/exothermic capacity of PUU copolymers.

Fig. 8 Phase transition enthalpies and phase transition temperatures of all the PUU copolymers.

Table 2 listed the phase change enthalpy and enthalpy ratio (namely the ratio between the enthalpies of the SSPCM and neat PEG)⁴⁴ of the PU-based SSPCMs in the literatures and the PUU-based SSPCMs in the present work. It is worth noting that the scanning rate of DSC experiments is 2 °C min⁻¹ in this study instead of 10 °C min⁻¹ in most reported literatures, which results in the relatively low measured values of phase transition enthalpy.^42,45 Even so, the experimental values of ΔH_endo of PUU copolymers in this work are higher than those of most of the reported PU-based polymeric SSPCMs from Table 2.

Table 2 Comparison of phase change properties of the PU-based SSPCMs in literatures and the PUU-based SSPCMs in the present work

Ref.	SSPCMs	Heating rate (°C min^−1)	ΔHendo of PEG (J g^−1)	ΔHendo of SSPCM (J g^−1)	Enthalpy ratio (%)
32	CDA-IPDI-PEG5000 (30.1 wt%)	5	176.09	25.45	33.47
33	PEG4000/MDI/PVA	10	168.4	72.8	43.23
23	PEG10000/PGMA	10	166.6	73.2	43.94
38	PEG6000/MDI/Span 80	10	234.4	122.4	52.22
	PEG6000/MDI/Tween 80	10	234.4	127.7	54.48
30	Cellulose-graft-PEG1100 (77.4 wt% PEG)	10	140.3	73.3	52.25
	Cellulose-graft-PEG5000 (90.1 wt% PEG)	10	204.7	153.6	75.04
39	PEG4000/HDI/castor oil	10	174.20	99.47	57.10
	PEG6000/HDI/castor oil	10	196.30	117.70	59.96
35	PEG3400-IPDI-BDO	10	174.1	121	69.50
42	PEG8000/MDI/sorbitol	2	148.1	107.5	72.59
	PEG8000/MDI/dipentaerythritol	2	148.1	91.0	61.44
	PEG8000/MDI/inositol	2	148.1	92.8	62.66
34	PEG10000/MDI/BDO	10	189.6	138.7	73.15
43	PEG8000/MDI/glucose	2	148.1	108.7	73.39
29	Cellulose-graft-PEG2000 (85 wt% PEG)	10	199.7	149.1	74.66
44	PEG8000/MDI/β-CD	2	148.12	115.20	77.77
25	PEG10000/MDI/pentaerythritol	10	189.45	152.97	80.74
Present study	SSPCM-1a	2	148.12	119.90	80.95
	SSPCM-2a	2	148.12	108.15	73.02
	SSPCM-3a	2	148.12	106.18	71.69

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In order to exclude the interference of DSC measurement conditions to the experimental results, enthalpy ratio is introduced here for the comparison of phase change heat storage properties, which can better and more sensitively reflect the real performances of the synthesized SSPCMs. From Table 2, the values of enthalpy ratio of SSPCM-1a, SSPCM-2a and SSPCM-3a are obviously higher than those of most of PU-based SSPCMs in literatures. Especially, SSPCM-1a has the highest enthalpy ratio among all the PU-based and PUU-based SSPCMs. Consequently, the PUU-based polymeric SSPCMs in this study have extensive potential applications in area of the thermal energy storage.

On the other hand, the phase transition enthalpies of the PUU copolymers are very different from each other, which is resulted from the different crosslinking density of the PUU copolymers. The order of ΔH_endo or ΔH_exo for the PUU copolymers is SSPCM-1a > SSPCM-1b > SSPCM-1c > SSPCM-1d, SSPCM-2a > SSPCM-2b > SSPCM-2c, SSPCM-3a > SSPCM-3b > SSPCM-3c > SSPCM-3d, which exactly contrasts with the order of their crosslinking density. For SSPCM-1a, SSPCM-2a and SSPCM-3a, the lower crosslinking density results in smaller spatial hindrance, which can help the free movement and crystallization of PEG segments of the PUU copolymers. For SSPCM-1d, SSPCM-2c and SSPCM-3d, denser crosslinking network structure formed because theoretically all N–H bonds in the molecules of multiamino compounds (DETA, TETA and TEPA) took part in the crosslinking reaction. Therefore, there is an obvious rule that the PUU copolymers with lower crosslinking density have larger phase change enthalpies.

Reusability and thermal stability of the PUU copolymers

Thermal cycling test containing 1000 heating–cooling thermal cycles was performed for evaluating the thermal reliability and reusability of copolymers in this work. The mass loss of the PUU copolymers would be negligible by compared the weight of sample before and after the thermal cycling test. The FTIR spectra of SSPCM-1a, SSPCM-2a and SSPCM-3a before and after thermal cycles were shown in Fig. 9. Compared with the original samples, no characteristic absorption peaks disappear or location shift and no new absorption peaks generate for the thermal cycled samples, which could effectively confirm that the chemical structure of the PUU copolymers has no variation after cycles.

Fig. 9 FTIR spectra of SSPCM-1a, SSPCM-2a and SSPCM-3a before and after thermal cycling test.

Fig. 10 shows the DSC curves of SSPCM-1a, SSPCM-2a and SSPCM-3a before and after thermal cycling test, and Table 3 lists the corresponding data of phase transition properties. Likewise, the DSC curves of the PUU copolymers after thermal cycling have good repeatability with the original ones. From Table 3, the phase transition properties of all the samples after thermal cycling have few changes compared with those of the corresponding original samples. The present results conclusively indicate that the synthesized PUU copolymers have high thermal reliability and good reusability below 100 °C.

Fig. 10 DSC curves of SSPCM-1a, SSPCM-2a and SSPCM-3a before and after thermal cycling test.

Table 3 Phase transition properties of the PUU copolymers before and after thermal cycling test

Sample		ΔHendo^a (J g^−1)	Tendo (°C)	ΔHexo (J g^−1)	Texo (°C)
a ΔHendo and ΔHexo is the phase transition enthalpy in the endothermic and exothermic process, respectively; Tendo and Texo is the phase transition temperature in the endothermic and exothermic process, respectively.
SSPCM-1a	Before cycles	119.90 ± 1.20	59.62 ± 0.40	115.98 ± 1.18	43.90 ± 0.45
	After cycles	119.08 ± 1.43	59.60 ± 0.30	115.16 ± 1.02	43.92 ± 0.25
SSPCM-2a	Before cycles	108.15 ± 1.05	57.22 ± 0.20	104.45 ± 0.92	43.02 ± 0.10
	After cycles	107.60 ± 0.98	57.20 ± 0.10	103.72 ± 1.03	43.10 ± 0.15
SSPCM-3a	Before cycles	106.18 ± 1.08	56.64 ± 0.11	102.55 ± 0.96	41.60 ± 0.15
	After cycles	105.55 ± 1.25	56.60 ± 0.20	101.94 ± 1.36	41.60 ± 0.20

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The thermal stability of all the PUU copolymers was tested by TGA curves shown in Fig. 11, and the corresponding results are collected in Table 4. From the previous study,⁴³ there is a sole degradation step in the TGA curve of PEG, which appears in the temperature interval of 376.2–412.9 °C. As shown in Table 4, the degradation temperatures of all the PUU copolymers are higher than those of PEG, which indicates that the introduction of hard segments and/or crosslinking network structure of copolymers have effectively enhanced their thermal stability. Moreover, the mass loss of the PUU copolymers decreases with the increase of the crosslinking density, and it is lower than 1%, 2%, and 4% at 100 °C, 250 °C and 300 °C, respectively. Combined the results of thermal cycling test and TGA curves, it is easily concluded that all the PUU copolymers have good thermal stability in their phase change temperature range, which facilitates the reusability of the copolymers for TES.

Fig. 11 TGA curves of all the PUU copolymers.

Table 4 Degradation data from TGA curves

Samples	Degradation interval^a (°C)	Tpeak^b (°C)	Mass loss (%)
			At 100 °C	At 250 °C	At 300 °C
a Degradation interval is from Tonset (the onset degradation temperature of TGA curve) to Tend (the end degradation temperature of TGA curve).b Tpeak is the peak temperature of DTG curve.
SSPCM-1a	386.2–418.3	398.0	0.99	1.95	3.46
SSPCM-1b	386.4–418.7	399.2	0.76	1.78	2.68
SSPCM-1c	387.4–418.0	401.4	0.70	1.47	2.18
SSPCM-1d	387.6–418.6	403.0	0.66	1.32	2.08
SSPCM-2a	386.6–418.2	398.9	0.74	1.38	2.06
SSPCM-2b	388.0–419.1	402.3	0.69	1.19	2.02
SSPCM-2c	388.5–418.5	403.8	0.39	1.13	1.90
SSPCM-3a	385.9–420.7	397.7	0.59	1.96	3.02
SSPCM-3b	387.0–419.7	397.8	0.41	1.53	2.79
SSPCM-3c	386.6–420.7	401.3	0.34	1.31	2.14
SSPCM-3d	396.1–418.9	404.9	0.21	0.92	1.69

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Conclusions

A series of PUU copolymers with different crosslinking density as novel polymeric SSPCMs were firstly synthesized individually using DETA, TETA and TEPA as the chain extender. Through extensive use of characterization techniques, the synthesized PUU copolymers show typical solid–solid phase change characteristic, regular spherocrystal morphology and high phase transition enthalpies. The crystallization properties and phase transition properties of the PUU copolymers have a decreasing tendency with the increase of crosslinking density of the copolymers. Furthermore, the PUU copolymers have good reusability and high thermal stability in their phase transition temperature range. The synthesis of PUU-based polymeric SSPCMs can enrich the varieties and selectivity of polymeric SSPCMs for thermal energy storage.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21404061) and the STP project of Nanyang Normal University (Grant No. STP2016009).

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