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Facile strategy in designing epoxy/paraffin multiple phase change materials for thermal energy storage applications Qingsong Lian, Yan Li, Asim A. S. Sayyed, Jue Cheng, and Junying Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03558 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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ACS Sustainable Chemistry & Engineering

Facile strategy in designing epoxy/paraffin multiple phase change materials for thermal energy storage applications Qingsong Lian, Yan Li, Asim A. S. Sayyed, Jue Cheng*, and Junying Zhang* Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China * Corresponding author. Tel. /fax: +86 10 6442 5439. Mailing address: 15 BeiSanhuan East Road, ChaoYang District, Beijing, 100029. E-mail address: [email protected] (Jue Cheng), [email protected] (Junying Zhang)

ABSTRACT Designing novel phase change materials (PCMs) is of vital importance in achieving the sustainable development of energy. Here, we facilely prepare a series of novel multiple solid-solid PCMs (SSPCMs, EPPa-X systems) by blending paraffin, epoxy resin with crystalline side chains (D18), and poly(propylene oxide)diamine together followed by a one-pot curing process. Strong intermolecular forces between paraffin and D18 (based on their good compatibility) and reliable three-dimensional (3-D) crosslinking network of epoxy resin form a unique encapsulation mechanism, which provides the EPPa-X SSPCMs with excellent shape-stable properties and superior thermal stability (remain stable below 180 oC) without sacrificing too much latent heat of paraffin (only 1.6% latent heat loss). Due to the combination of two melting process at 36 oC and 60 oC derived from D18 and paraffin, respectively, the latent heat of the EPPa-50 system is high to 152.6 J/g. Besides, the supercooling extent of D18 decreased from 13.6 oC to

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10.5 oC with the adding of paraffin due to the heterogeneous nucleation effect. The novel EPPa-X SSPCMs possess tremendous potential for a wide range of applications due to their facile preparation, low cost, high reliability, and excellent phase change properties; while the unique encapsulation mechanism may open a new door for preparing other novel types of PCMs.

KEY WORDS Phase change materials, paraffin, epoxy resin, encapsulation mechanism, supercooling

Introduction The increasing energy crisis, high cost of fossil fuels, and environmental problems motivate researchers to develop sustainable energy resources. Thermal energy, which is a kind of directly usable and ubiquitous form of energy,1 has the potential to substitute for fossil fuels. As a type of advanced thermal energy saving materials, phase change materials (PCMs) have gained a great deal of attention due to their high energy storage density and the isothermal nature during phase change.2-5 According to the type of phase change, there are three types of PCMs: solid-gas PCMs (SGPCMs), solid-liquid PCMs (SLPCMs), and solid-solid PCMs (SSPCMs).6 In general, SGPCMs possess the highest latent heat storage capacity; however, they are not yet practical for use owing to the large volume decrease during their phase change process. As for the SSPCMs, they exhibit many remarkable advantages such as small volume change, no gas or liquid generation, and no seal needed because they can retain their solid state even above the phase change temperature (Tpc). However, due to the limitation of the species of inherent SSPCMs and its supercooling shortcoming, the most commonly used PCMs such as paraffin, fatty acid, and polyethylene glycol (PEG) are SLPCMs, which have superior properties


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such as high latent heat storage capacity, little or no supercooling, low cost, little volume change during the phase change process, and so on. However, in contrast with the SSPCMs, the biggest disadvantage of the SLPCMs is their reliability concern and they will have to deal with the leakage problem during the phase change process. To solve this problem, the most popular and feasible thought is the preparation of form-stable PCMs (FSPCMs), which are a kind of composite materials that consist of a type of SLPCMs (provide the latent heat of phase change) and a supporting material (maintain the solid state of the composites). There are two methods to prepare FSPCMs: one is incorporating the SLPCMs into a special material such as porous materials,7-11 aerogels,12-16 hydrogels,17 metal composites,6, 18 and so on; the other is microencapsulating the SLPCMs with different shells such as zirconia,19 silica,20, 21 titanium dioxide,22 calcium carbonate,23 graphene24-27 acrylic polymer,28, 29 polystyrene,30 and melamine–formaldehyde (MF)31 etc. However, the tedious preparation of the supporting materials (especially for the microencapsulated FSPCMs)6 and the relatively high cost of some kinds of the supporting material itself like graphene greatly restrict the application of the above FSPCMs. Besides, the nature of the FSPCMs is still the type of SLPCMs because the intrinsic mechanism to prevent the leakage problem of the FSPCMs is mainly the physical cladding effect, which is not reliable enough because the SLPCMs still have the risk of leakage once the supporting materials were destroyed under external stimulus.20 The other feasible thought is to prepare the polymeric SSPCMs from its pristine SLPCMs.32 Owing to the reliable chemical crosslinking networks, the polymeric SSPCMs can be used directly without encapsulation33 and they are real sense of SSPCMs used in practical. In this field, PEG-based polymeric SSPCMs may be the most successful because they can be easily synthesized according to several steps of chain extension reaction of isocyanates with polyols. Facile preparation, low cost, and reliable



crosslinking network to prevent leakage, all of the above advantages may seem that PEG-based polymeric SSPCMs are much superior to FSPCMs. However, the biggest disadvantage for the PEG-based polymeric SSPCMs is their relatively low latent heat storage capacity ranging from 40 to 110 J/g,33-38 which is lower than the latent heat of most types of FSPCMs. Due to the restriction of the crosslinking structure, the crystalline regions of PEG segments are decreased, which resulted in the fact that the practical latent heat of PEG-based polymeric SSPCMs is much lower than its theoretical latent heat.34-37 Besides, the isocyanates used in this preparation procedure of the PEG-based polymeric SSPCMs are harmful to the environment because aromatic isocyanates will hydrolyze into toxic phenylamine.39 When analyzing the characteristics of the FSPCMs and the polymeric SSPCMs mentioned above, it is obvious that FSPCMs possess relatively high latent heat but might have reliability concern and the cost is relatively high, while the polymeric SSPCMs are reliable and low cost but the latent heat is relatively low due to the restriction of the crosslinking structure. Therefore, it arouses our great interest in finding a polymeric SSPCM as the supporting material of the other SLPCM to prepare a novel type of FSPCMs, and in this case the advantages of FSPCMs and polymeric SSPCMs may be combined together. Besides, the polymeric SSPCM (as the supporting material in this type of FSPCMs) can also provide latent heat, and thus this kind of FSPCM may have higher latent heat than traditional FSPCMs. However, to the best of our knowledge, reports on this kind of FSPCMs are rare, which may be because traditional polymeric SSPCMs can only shape themselves stable during phase change process and are unable to support other SLPCMs well. Fortunately, our group recently synthesized a semicrystalline epoxy resin (SCEP) called D18 by the thiol-ene click reaction of an allyl-based epoxy resin (diglycidyl ether of 4,4’-diallyl bisphenol A, DADGEBA) with 1-Octadecanethiol (ODT). After thermal curing of the synthetic D18 with curing agent


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Jeffamine D230, the grafted ODT (provide the latent heat storage capacity) can be tightly locked in the crosslinking network of epoxy resin and form a novel type of epoxy-based polymeric SSPCM.40 Styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer (SEBS), which has good compatibility with paraffin because the solubility parameter of EB midblock (hydrogenated polybutadiene) of SEBS is similar to paraffin, has been proved a successful supporting material for paraffin by several groups.41, 42 Therefore, we speculate that the curing network of D18 may also be a suitable supporting material for paraffin due to the good compatibility of D18 and paraffin. Among all of the SLPCMs, paraffin own their success to the outstanding properties such as proper melting temperature ranges, good thermal stability, low cost, little or no supercooling, non-toxicity, high latent heat (180 to 250 J/g, which is far superior to the other commonly used SLPCMs), and so on. Therefore, the D18/paraffin PCMs can be facilely prepared by simply blending D18 and paraffin (depending on their good compatibility) followed by the thermal curing process of D18 with curing agent. Apart from the facile preparation, the D18/paraffin PCMs own a great deal of advantages such as high latent heat (both of the D18 and paraffin can provide the latent heat storage capacity), low cost, environmental friendly, and high reliability (because of the reliable crosslinking network of epoxy resin and the strong intermolecular forces between paraffin and D18). Moreover, the melting point of D18 and paraffin is 38 oC and 59 oC, respectively, so the D18/paraffin PCMs are multiple phase change materials that can be applied for many applications in the proper and wide melting temperature ranges. Meanwhile, the supercooling extent might be decreased due to the heterogeneous nucleation effect induced by paraffin. Herein, a new strategy to prepare novel FSPCMs by using a brand new epoxy-based polymeric SSPCM (crosslinking network of D18) as the supporting material of paraffin was designed in this paper.



A series of D18/paraffin PCMs were facilely prepared by simply blending paraffin, D18, and Jeffamine D230 together followed by a one-pot curing process. Systematic characterization including FTIR, scanning electron microscope (SEM), differential scanning calorimeter (DSC), X-ray diffraction (XRD), polarized optical microscope (POM), and Thermogravimetric (TG) experiments were used to evaluate the phase change properties of the D18/paraffin PCMs. Besides, the unique encapsulation mechanism including the intermolecular forces between paraffin and D18 (based on their good compatibility) and the reliable three-dimensional (3-D) crosslinking network of epoxy resin were fully discussed, which may open a new door for preparing other novel types of PCMs.

Experimental Section Materials.

Figure 1. Chemicals used in this study. (n for paraffin is from 18 to 45; n for D230 is 2.7)

D18 (epoxy equivalent 594 g/mol) was synthesized via thiol-ene click reaction of DADGEBA and ODT in our laboratory according to our previous study, the characterization of D18 (FTIR and 1H NMR spectra) can also be seen in this literature.40 Poly (propylene oxide) diamine (Jeffamine D230, amine equivalent 61 g/mol) was purchased from Huntsman. Paraffin wax (melting point is 60 to 62 oC) was obtained from


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Aladdin reagent Ltd. The structure of D18, D230, and paraffin can be seen in Figure 1.

Preparation of the D18/Paraffin SSPCMs

Scheme 1. Preparation of the EPPa-X PCMs and its phase change behavior.

Paraffin and D18 was well mixed with the mass ratio of 55/45, 50/50, 40/60, 30/70, 20/80, 10/90 and 0/100 at 80 oC, and then all the mixtures were poured into D230 according to the stoichiometric ratio, respectively. After well mixed of the D18/Paraffin/D230 curing systems, all the final mixtures were pre-cured at 100 oC for 4h and cured at 125 oC for 8h. The cured samples (also the SSPCMs) were named as EPPa-55, EPPa-50, EPPa-40, EPPa-30, EPPa-20, EPPa-10, and EPPa-0 according to the paraffin content. The preparation procedure of the EPPa-X PCMs and its structure can be seen in Scheme 1. It should be noted



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that the mixture is transparent during the whole curing process. Besides, before curing, solid paraffin began to separate out from the mixture below 60 oC, which is also the melting point of paraffin. Therefore, the D18-D230-Paraffin system is likely an upper critical solution system and the upper critical solution temperature is around 60 oC. This means that the D18-D230-Paraffin mixture is homogeneous when the temperature is above 60 oC, below which phase separation might occur.

Measurements. FTIR experiments were tested on a Bruker Alpha FTIR instrument at a resolution of 4 cm-1 using KBr pellet pressing method, the wavenumber range is 375 to 4000 cm-1 and all the samples were scanned for 32 times. Leakage test was carried out as the method described in the Supporting Information. Differential scanning calorimeter (DSC) tests were carried out on a TA Instruments Q20 equipped with an RCS 90 cooling system. The phase change properties of all the samples were tested at a heating rate of 10 oC/ min under N2 atmosphere between -70 oC and 150 oC. XRD analysis was examined to characterize the crystalline property of the samples on a Rigaku D/Max 2500 VB2+/PC diffractometer with Cu Kα radiation. POM spectra were recorded on an Olympus BX51 polarizing microscope to further characterize the crystalline properties of the samples. SEM images of the fracture morphology (broken in liquid nitrogen) were obtained by a Hitachi Limited

S4700

scanning electron microscope equipped with an

energy dispersive

X-ray

spectrophotometer (EDS). TG measurements were carried out on a TGA Q50 analyzer at a heating rate of 10 °C/min under N2 atmosphere to test the thermal stability of the samples.

Results and discussion


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Structure characterization of the EPPa-X SSPCMs

Figure 2. FTIR analysis of the EPPa-X SSPCMs and paraffin.

All the characteristic peaks of paraffin can be seen in Figure 2, the peaks at 2921, 2849, 1465, and 724 cm-1 were attributed to the asymmetrical stretching vibration of -CH2-, symmetrical stretching vibration of -CH2-, bending vibration of -CH2-, and in-plane rocking vibration of -CH2-, respectively. As for the EPPa-0 (D18-D230) system, the peaks at 3431, 1606, 1249, and 810 cm-1 can be assigned to the stretching vibration of -OH (generated by the ring-opening reaction of D18 and D230), C=C stretching vibration

of

benzene

skeleton,

C-O

stretching

vibration

of

aromatic

ether,

and

C-H

out-of-plane bending vibration of benzene framework. With the increase content of paraffin (from 0% to 50%), the characteristic peaks of D18 at 3431, 1606, 1249, and 810 cm-1 gradually decreased. Therefore, the chemical structure of the EPPa-X SSPCMs was proved by the FTIR analysis.

Shape-stable properties



Figure 3. Visual images of the EPPa-X SSPCMs at 25 oC (room temperature) and 120 oC for 60 min.

Shape-stable properties were characterized by the leakage test described in the 2.3 part of this article. Unfortunately, the EPPa-55 sample can cause slightly trace amounts of paraffin leaking during the leakage test (leakage loss is around 7% of the mass of EPPa-55 sample). Except for the EPPa-55 system, all the other EPPa-X (X≤50) samples can remain their solid state at 120 oC without the leakage problem shown in Figure 3. The leakage test reveals that 50% paraffin content is probably the maximum value for the EPPa-X systems in preventing the leakage problem. Although EPPa-0 sample is a simple epoxy system, there still existed very little opacity due to the crystalline behavior of ODT side chains at 25 oC. With the increase content of paraffin, the transparency of the EPPa-X samples became lower and lower. There are two reasons for this phenomenon: one is the decrease of the refractive index due to the crystalline properties of paraffin and ODT side chains in D18; the other is attributed to the thermodynamic phase diagram, we have mentioned that the D18-D230-Paraffin system is likely an upper critical solution system with the UCST of around 60 oC, so phase separation might occur at 25 oC, demonstrating the opacity of the EPPa-X systems at this time. However, when all the samples were heated to 120 oC and remained for 60 min, on the one hand paraffin as well as ODT side chains had entirely transformed from crystalline state to amorphous state, on the other hand phase separation also disappeared because the temperature is above the UCST, and thus demonstrating the transparent state of all the samples at 120 oC. In fact, in our previous study, we have proved that D18 is more compatible with ODT rather than diglycidyl ether of bisphenol A (DGEBA) although D18 is a type of ODT grafted epoxy resin, leading to the conclusion that the nature of D18 is


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much similar to ODT instead of epoxy resin.40 Considering that the chemical structure of paraffin is quite similar to ODT, it is reasonable that D18 and paraffin have so excellent compatibility that phase separation did not occur when the temperature is above 60 oC, which guaranteed the wonderful shape-stable properties of the EPPa-X systems.

Surface morphology analysis

Figure 4. SEM images of the cold-fractured interface of the EPPa-X SSPCMs. (600X magnification)

In order to further understand the compatibility of paraffin and D18 as well as the crystalline morphology of the EPPa-X SSPCMs, SEM analysis was used to characterize the cold-fractured interface (broken in liquid nitrogen) morphology. In Figure 4, it was observed that crystalline ODT side chains that grafted on DADGEBA exhibited white needle-like structures and was well dispersed in the epoxy networks, thus demonstrating very low opacity of the EPPa-0 sample at 25 oC shown in Figure 3 due to the low size of these needle-like structures. Besides, the EPPa-0 sample, which is a type of liquid crystalline (LC) grafted copolymer according to our previous research40, can exhibit bicontinuous phase morphology like other crystalline-amorphous LC block copolymers (BCPs)43-46 induced by the huge polarity



difference between DADGEBA and ODT; microphase separation occurred at the same time. EDS result in Figure S1 and Table S1 further proves the bicontinuous phase morphology of the EPPa-0 system. When paraffin was incorporated into the D18-D230 curing system, many spherical structures can be observed and the average diameter of these spherical structures increased from 5 µm (for the EPPa-10 system shown in Figure S2) to 40 µm (for the EPPa-50 system shown in Figure 4) with the increase content of paraffin. Besides, SEM images were the morphology of the EPPa-X (X>0) systems displayed at 25 oC, at which phase separation was very likely occurred because the temperature is lower than the LCST of the EPPa-X (X>0) systems. It should be noted that all the EPPa-X samples were naturally cooled to room temperature (from their final curing temperature at 125 oC) in the oven. In this case the microphase separation has enough time to occur. The typical sea-island structural morphology of the SEM images of the EPPa-X (X>0) systems proves our speculation. Besides, EDS study proves that D18/D230 is the continuous phase, while paraffin and ODT side chains constitute the dispersed phase according to Figure S3 and Table S2. Actually, the excellent compatibility of D18 and paraffin can only be attributed to the ODT side chains because the compatibility of common epoxy resin (like DGEBA) and paraffin is very poor. Therefore, the spherical structures displayed in the SEM images can be most likely formed by the self-assemble effect of paraffin on the ODT side chains, and only in this way paraffin can keep away from the crosslinking network of epoxy resin and D230 in the maximum extent by the intrinsic motive force (derived from the huge incompatibility between paraffin and the DADGEBA part of D18). EDS result listed in Table S3 proves our speculation that paraffin was self-assembled on the ODT side chains and formed the spherical structures observed in the SEM images. Moreover, the fact that the average diameter of these spherical structures increased with the increase content of paraffin for the EPPa-X system reveals that the self-assemble effect of paraffin became more and more obvious with the increase content of paraffin. The sea-island structural morphology suggests that paraffin can be tightly encapsulated in the D18-D230 curing network, which provides excellent shape-stable properties for the EPPa-X (10≤X≤50)


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SSPCMs. It should be noted that the SEM image of the EPPa-55 sample (in Figure S4) exhibits quite different morphology because phase reversion was occurred at this time. The above result can explain the reason why the EPPa-55 sample has leakage problem. In conclusion, the sea-island structural morphology of the EPPa-X (10≤X≤50) systems proves the excellent shape-stable properties of the PCMs, while the leakage problem of the EPPa-55 sample can be revealed by its SEM image of the combination of the sea-island structural morphology and the bicontinuous phase morphology for this sample.

Thermal Analysis

Figure 5. DSC curves of paraffin and the EPPa-X SSPCMs.



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Table 1. Thermal characteristics of paraffin and EPPa-X samples. Melting Process

Freezing Process T

System

Tm-EP (oC)

Tm-Pa (oC)

∆Hm (Jg-1)

∆Hm (Jg-1)

∆Hm loss (%)

Tf-EP (oC)

Tf-Pa (oC)

∆Hf (Jg-1)

∆HfT (Jg-1)

∆Hm loss (%)

Paraffin

/

59.0

240.1

240.1

0.0

/

55.0

236.3

236.3

0.0

EPPa-0

35.9

/

70.1

70.1

0.0

22.3

/

64.6

64.6

0.0

EPPa-10

36.3

49.9

79.2

87.1

9.1

24.5

41.5

74.4

81.8

9.0

EPPa-20

35.9

60.0

96.1

104.1

7.7

24.2

47.0

91.7

98.9

7.3

EPPa-30

36.5

59.9

113.3

121.1

6.4

25.1

53.1

108.9

116.1

6.2

EPPa-40

36.4

59.3

134.7

138.1

2.5

25.5

54.4

130.2

133.3

2.3

EPPa-50

37.0

60.2

152.6

155.1

1.6

26.5

54.1

148.3

150.5

1.4

T

Notes: Tm, Tf, ∆Hm, and ∆Hf can be obtained directly from DSC curves; ∆Hm and

∆HfT

of the EPPa-X samples were calculated

by sum the melting or freezing enthalpies of the EPPa-0 parts (multiplying the weight percentage of EPPa-0 with the melting or freezing enthalpies of EPPa-0 sample) and the paraffin parts (multiplying the weight percentage of paraffin with the melting or freezing enthalpies of paraffin).

Table 2. Theoretical and practical latent heat provided by paraffin part and epoxy resin part in the EPPa-X systems. Melting Process

Freezing Process

System

∆HmT-EP (Jg-1)

∆Hm-EP (Jg-1)

∆HmT-Pa (Jg-1)

∆Hm-Pa (Jg-1)

∆HfT-EP (Jg-1)

∆Hf-EP (Jg-1)

∆HfT-Pa (Jg-1)

∆Hf-Pa (Jg-1)

Paraffin

/

/

240.1

240.1

/

/

236.3

236.3

EPPa-0

70.1

70.1

/

/

64.6

64.6

/

/

EPPa-10

63.1

62.8

24.0

17.4

58.1

57.6

23.6

16.8

EPPa-20

56.1

55.4

48.0

40.7

51.7

51.3

47.3

40.4

EPPa-30

49.1

48.5

72.0

64.8

45.2

46.9

70.9

62.0

EPPa-40

42.1

41.8

96.0

92.9

38.8

38.2

94.5

92.0

EPPa-50

35.1

34.6

120.1

118.0

32.3

31.8

118.2

116.5

Phase change properties are of vital importance for PCMs. Tpc and latent heat, which can be directly obtained from DSC curves, are the two dominant parameters for determining the application range and application value of the PCMs,36 respectively. DSC curves of paraffin and the EPPa-X SSPCMs can be seen in Figure 5. Besides, the corresponding melting temperature (Tm), freezing temperature (Tf), melting latent heat (∆Hm), and freezing latent heat (∆Hf) obtained by DSC analysis were listed in Table 1. EPPa-0


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system only exhibits one melting peak at 35.9 oC, which is the melting point of the ODT side chains in D18. However, pristine paraffin has two melting peaks, the first melting peak at 44.2 oC is attributed to the solid-solid phase change of paraffin, while the second melting peak at 59.0 oC can be attributed to the solid-liquid phase change of paraffin.47 Here, we defined the major melting peak (59.0 oC for pristine paraffin) as the melting temperature of paraffin described as Tm-Pa shown in Table 1; this definition of Tm-Pa was also used for the EPPa-X (X>0) SSPCMs. When paraffin was incorporated into the D18-D230 curing network, it can be observed two obvious melting and freezing peaks for the EPPa-X (X>0) SSPCMs: one is the melting or freezing temperature of ODT side chains of D18 (Tm-EP or Tf-EP); the other is the melting or freezing temperature of paraffin (Tm-Pa or Tf-Pa). It is interesting to observe that the Tm-EP essentially remained constant, while the Tf-EP increased from 22.3 oC (for the EPPa-0 system) to 26.5 oC (for the EPPa-50 system) with the increase content of paraffin. Here comes the concept of supercooling, which is a phenomenon that liquid PCMs cannot solidify until the temperature is far below the Tm due to the relatively slow nucleation process compared to the cooling rate.48 It is obvious that excessive supercooling extent can be a huge obstacle to the applications of PCMs. Usually supercooling extent is expressed as the difference between Tm and Tf. Calculated by the Tm-EP and Tf-EP data shown in Table 1, the supercooling extent of the ODT side chains in the EPPa-0, EPPa-10, EPPa-20, EPPa-30, EPPa-40, and EPPa-50 system is 13.6, 11.8, 11.7, 11.4, 10.8, and 10.5 oC, respectively. The decrease of the supercooling extent of ODT side chains in the EPPa-X systems (with the increase of paraffin content) can be attributed to the heterogeneous nucleation effect49 induced by paraffin, which is a crystalline impurity for the ODT side chains. Besides, it should be noted that the latent heat of the EPPa-X SSPCMs is the combination of the EPPa-0 sample and paraffin; further, the latent heat of paraffin (240.1 J/g) is much higher than that of the EPPa-0 sample (70.1 J/g). Therefore, it is not surprising that the latent heat of the EPPa-X SSPCMs increased with the increase content of paraffin. However, the ∆Hm loss (or ∆Hf loss), which equals to the (∆HT-∆H)/∆HT, decreased from 9.1% (for the EPPa-10 system) to 1.6% (for the EPPa-50 system). This phenomenon aroused



our great interest. The data of theoretical and practical latent heat provided by paraffin part and epoxy resin part in the EPPa-X systems was listed in Table 2. It can be observed that the theoretical and practical latent heat derived from ODT side chains in D18 did not change a lot, while the practical latent heat derived from paraffin parts is lower than its theoretical latent heat in all the EPPa-X systems. The result shows that the ∆H loss of EPPa-X systems is mainly attributed to the ∆H loss of paraffin encapsulated in the D18-D230 curing network. Besides, in Figure 5, it can be observed that the second melting or freezing peak of paraffin is too small to be identified in the EPPa-10 system (this phenomenon can be observed more clearly in Figure S7), which shows that the solid-liquid phase change process of paraffin is greatly restricted and the majority of paraffin can only experience the solid-solid phase change, leading to the highest ∆Hm loss of 9.1% and the lowest freezing point of paraffin at 41.5 oC. With the increase of paraffin content, the second melting or freezing peak became more and more obvious and the freezing point of paraffin gradually increased to approach to the freezing point of pure paraffin (55.0 oC), suggesting that the solid-liquid phase change of paraffin gradually played the dominant role like pure paraffin and the ∆Hm loss can be decreased. Therefore, the EPPa-50 sample owns the highest latent heat of 152.6 J/g, while the ∆Hm loss is the lowest in the EPPa-X systems. Here comes another question, what is the internal cause that restricts the solid-liquid phase change of paraffin and further leads to the ∆Hm loss of the EPPa-X systems? Actually, we have mentioned that the practical latent heat of polymeric SSPCMs is far lower than their theoretical latent heat due to the restriction of crosslinking network, while the practical latent heat of the traditional FSPCMs is very close to their theoretical latent heat because there is no crosslinking network in this type of PCMs. As for the EPPa-X systems, there is no chemical crosslinking between paraffin and the D18-D230 curing network. There might be two reasons for the fact that the ∆Hm loss of the EPPa-X systems decreases with the increase content of paraffin. Firstly, although the glass transition platform cannot be observed in the DSC curves, the glass transition temperature (Tg) should be existed in all the EPPa-X systems. In our previous study, we have pointed out that Tg of the EPPa-0 sample is around 36 oC.40 When paraffin was


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incorporated into the D18-D230 curing network, Tg of the EPPa-X systems will decrease with the increasing content of paraffin due to the plasticizing effect. Therefore, the flexibility of the epoxy matrix of the EPPa-X systems increases with the increase content of paraffin. Actually, a flexible network is more adequate for the swelling and deswelling processes of the PCMs than a glassy network, leading to the decrease of the ∆H loss value of the EPPa-X systems with the increase content of paraffin. Secondly, a little amount of paraffin can be dissolved in the epoxy matrix and the solid-liquid phase change of this part of paraffin can be greatly limited, while the solid-liquid phase change of the self-assembled paraffin (that cannot be dissolved in the epoxy matrix) is hard to be limited except for the surface part of paraffin of the self-assembled spherical structures. With the increase content of paraffin, the proportion of the self-assembled paraffin (of all the paraffin) increases greatly because paraffin dissolved in the epoxy matrix remains the same. This could also be a possibility for the fact that the ∆H loss value of the EPPa-X systems decreases with the increasing content of paraffin. In conclusion, with the increase content of paraffin, the ∆Hm value increases from 70.1 to 152.6 J/g. Meanwhile, intermolecular forces between paraffin and D18-D230 curing network may restrict the solid-liquid phase change of paraffin. Tg, specific surface area of self-assembled paraffin sphericals, and the proportion of paraffin dissolved in the epoxy matrix are three factors that may cause the slightly ∆Hm loss of the EPPa-X systems from 9.1% (for the EPPa-10 system) to 1.6% (for the EPPa-50 system).

Crystalline Characterization



Figure 6. XRD diagrams of paraffin and the EPPa-X SSPCMs.

XRD analysis was used to prove the crystalline property of the EPPa-X SSPCMs. In Figure 6, the EPPa-0 sample only have one sharp diffraction peak at around 22o, which is attributed to the regular crystallization of the grafted ODT side chains according to our previous study.40 Besides, a relatively broad and smooth peak at around 20o is attributed to the amorphous epoxy curing network. When paraffin was incorporated into the D18-D230 system, we can observe several characteristic crystalline peaks of paraffin at 21o, 23o, 29o, 36o, and 40o, respectively.50 With the increase of the paraffin content, the intensities of the above five peaks of paraffin increased. Besides, it should be noted that the sharp crystalline peaks of paraffin greatly covered up the diffraction peaks of epoxy resin (amorphous part) and grafted ODT (crystalline part), which cannot be identified in the EPPa-10 system (only contain 10% paraffin), not even for the other EPPa-X systems that contain more paraffin content. The result shows that the crystalline property of paraffin was not significantly destroyed by the epoxy curing network. We have proved that the crystalline property of grafted ODT in the EPPa-0 sample was greatly suppressed by the epoxy curing network due to the restriction of the chemical crosslinking points40 like other polymeric SSPCMs. However, there are no chemical crosslinking points between epoxy curing network (D18-D230) and paraffin, which was only physically interpenetrated by the epoxy curing network due to the good


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compatibility of paraffin and D18. Therefore, it is not surprising that the crystalline property of paraffin cannot be significantly destroyed by the epoxy curing network. The result is consistent with our previous speculation. Besides, POM test was also used to characterize the crystalline properties of the EPPa-X systems and further proves the increasing crystallinity of the EPPa-X systems with the increase content of paraffin (Figure S5-S6). In conclusion, both XRD spectra and POM images prove the increasing crystallinity of the EPPa-X systems with the increase content of paraffin. Moreover, XRD results also prove that the physical interpenetration between paraffin and D18-D230 curing network can effectively prevent the leakage problem of paraffin without significantly destroying the crystalline properties of paraffin. While the POM results not only prove the excellent dispersion of paraffin in the epoxy curing network due to the good compatibility of D18 and paraffin, but also further prove our speculation that intermolecular forces between paraffin and D18-D230 curing network may restrict the solid-liquid phase change of paraffin that are on the surface of the spherical structures induced by the self-assemble effect of paraffin.

Thermal Recycling Properties Table 3. Thermal recycling properties of the EPPa-X PCMs. Melting latent heat (∆Hm) System

Freezing latent heat (∆Hf)

1 cycle (Jg-1)

50 cycles (Jg-1)

∆Hm loss (%)

1 cycle (Jg-1)

50 cycles (Jg-1)

∆Hf loss (%)

EPPa-0

70.1

69.4

0.99

64.6

64.0

0.93

EPPa-10

79.2

78.6

0.76

74.4

73.8

0.81

EPPa-20

96.1

95.3

0.83

91.7

90.9

0.87

EPPa-30

113.3

112.3

0.88

108.9

107.9

0.92

EPPa-40

134.7

133.9

0.59

130.2

129.4

0.61

EPPa-50

152.6

151.3

0.85

148.3

147.1

0.81

Thermal recycling property is a vital index which determines the long-term working reliability of the PCMs. Fifty loops of DSC scans were carried out to investigate the phase change performance of the EPPa-X systems. DSC curves of the EPPa-X systems before and after 50 thermal cycles were shown in



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Figure S3, while ∆H value and ∆H loss value obtained from the above DSC curves were listed in Table 3. It should be noted that no significant change was observed in the melting or freezing temperature of the EPPa-X PCMs in Figure S3. Meanwhile, in Table 3, only very slight decrease (

paraffin multiple ... - ACS Publications

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