Shape Stability of Polyethylene Glycol / Acetylene Black Phase Change Composites for Latent Heat Storage

Sufficient shape stability is essential for a high-performance phase change material (PCM). Although significant advances have been made to develop form-stable composites, technical development in the field of polymer-based PCMs is currently limited by an incomplete understanding of the shape stability. Form-stable polyethylene glycol/acetylene black (PEG/AB) PCMs containing PEGs with different average molecular weights have been obtained by melt mixing to investigate the shape stability of the PEG/AB composites. It was found that the phase change behaviors of the PEG/AB composites were not only attributed to the interactions between the AB and PEG, but also to the intermolecular interactions of the PEG chains, depending on the varying molecular weights of the PEGs. Physically crosslinked structure with temporary junctions was formed through hydrogen bonding, capillary, surface tension forces, intermolecular friction, and macromolecular entanglement, which contributed to the constrained chain motion and thus the solid-solid phase change behavior of the PEG/AB composites. ,e physically crosslinked structure was more stable with longer length of the PEG molecular chains, resulting in higher critical impregnated contents of the PEG into the AB and thus improved latent heat.


Introduction
Energy storage can optimize the energy flows to overcome the mismatch between the demand for energy and the supply of energy, especially for energy generated by variable and renewable resources, such as water, wind, and solar energy [1].Among the principal energy-storage technologies, phase change materials (PCMs) have drawn tremendous attention because they can store and release large amounts of latent heat as they change from one physical state to another for effective use of thermal energy [2][3][4][5].To increase the energystorage ability, PCMs with large latent heat are essential for future thermal energy storage and management [6,7].
Among the PCMs investigated, polyethylene glycol (PEG) is one of the prospective candidates in this regard as it exhibits relatively high latent heat-storage capacity, tunable solid-liquid phase change temperature, and noncorrosiveness, which leads to compact storage devices with high energy-storage density.Moreover, PEG has much better cycling stability during services and higher security than small-molecule organic PCMs due to nondetectable vapor pressure when melts [8][9][10][11].Nevertheless, as a solidliquid PCM, the applications of PEG are largely limited, as the liquid phase leaks above the phase change point.e leakage is usually circumvented by introducing form-stable support.Considerable efforts have been devoted to develop form-stable composites with high energy-storage densities [12][13][14][15][16][17][18][19][20][21][22][23].e form stabilization strategies of PCMs include encapsulation [12,13], grafting [14,15], and physical adsorptions [16][17][18][19][20][21][22][23].Clay, diatomite, halloysite, active carbon, silica, expanded graphite, and graphene have been employed as fillers to prevent melted PCMs from leaking during the phase transition via physical adsorptions, relying on their predominantly polar surface and/or high porous structures [16][17][18][19][20][21][22][23].However, although significant advances have been made, technical development in the field of polymer-based PCMs is currently limited by an incomplete understanding of the shape stability.e affinity of support fillers to PCMs depends on their structures, so the structure of the fillers usually is a primary consideration for the shape stability.us far, few attentions have been paid on the difference in shape stabilities between polymeric PCMs and low molecular weight organic PCMs.
e effect of some critical structure factors for polymeric PCMs, such as molecular weight, on the shape stability was largely neglected.However, the shape stability of polymer-based composite PCMs could not be entirely determined by the affinity of support fillers to PCMs or the interactions between them and might be partly attributed to the structure of the polymeric PCMs.
erefore, a further understanding of the form stabilization mechanisms would ultimately produce a more rational and general approach to the thermal property optimisation of PEG-based composite PCMs.
e present work is an attempt to develop a better understanding of the shape stability of polymeric PCMbased composites by comparing the effects of PEGs with different molecular weights (M w ) on the thermal properties of PCM composites filled by commercially available acetylene black (AB).e phase change behaviors of the formstable PEG/AB PCMs were verified by polarized optical microscopy (POM) and filter paper test.Furthermore, the structures and thermophysical properties of the composite PCMs were analyzed by scanning electron microscopy (SEM), X-ray diffractometer (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC).Finally, the existence of an intermolecular interaction mechanism of solid-solid phase change besides simple physical adsorptions in the PEG/AB composite PCMs was found.

Materials.
Chemically pure PEGs were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd.PEG800, PEG4000, PEG6000, and PEG10000 were utilized in this work.AB (purity > 99.99%, particle size � 150-200 nm based on the supplier's information) was purchased from Shanghai Chemical Reagent Co. Ltd. and used after drying.

Preparation of PEG/AB Composites.
To fabricate the PEG/AB composites, the PEGs with various average molecular weights were first melt at 50-70 °C, and then the AB was slowly added under stirring and vacuum.Finally, the mixture was dried in a vacuum oven at 70 °C for 48 h.

Characterization.
e Fourier-transform infrared (FTIR) spectrum was observed at room temperature ( ermo Nicolet Nexus) in the range of 4,000 to 400 cm −1 using KBr pellets.Nitrogen adsorption/desorption isotherms were measured on a Micromeritics Model TriStar II 3020 volumetric analyzer (USA).POM observation was conducted on an Olympus BX51-P polarizing optical microscope equipped with a LTS 350 hot stage.TGA was carried out on a TG/DTA 220U (Seiko Instrument Co. Ltd, Tokyo Japan) with the Exstar 6000 Station.e samples were scanned from 20 to 800 °C with the heating rate of 10 °C/min and nitrogen gas purging.e X-ray measurements were carried out on a Rigaku D/Max-IIIA X-ray diffractometer with Cu-Kα radiation at a wavelength of 1.54 Å. ermal analysis was performed using a DSC (PE Co., USA).e thermal conductivities of the various samples were measured at 28 °C by the guarded heat flow test method (TA Instruments DTC 300, USA).
ermograms were obtained at a heating and cooling rate of 5 °C•min −1 at a temperature range from 20 °C to 80 °C under nitrogen atmosphere.To confirm the reproducibility of the results, at least two samples were measured for each composition.For each sample, at least three DSC traces were recorded.SEM measurements were performed with JSM-IT300A (JEOL Ltd., Japan).

Chemical and Physical Characterizations of the AB
Particles.Chemical characterizations of the AB particles and the PEG/AB composites are carried out using FTIR spectroscopy, as shown in Figure 1.In the spectrum of the AB (Figure 1(a)), the peaks at 2,888 cm −1 , 1,146 cm −1 , and 1,114 cm −1 represent the stretching vibration of C-O groups.
e peak at 1,637 cm −1 is attributed to C�O stretching vibration.O-H stretching vibrations are characterized by the broadband in the region of 3,300-3,650 cm −1 .e spectrum of pristine PEG (Figure 1(b)) is characterized by O-H stretching vibration at 3433 cm −1 and the strong absorption peaks of C-H bonds at 2888 cm −1 , 953 cm −1 , and 839 cm −1 .
e C-O stretching vibration was at 1106 cm −1 .e FTIR results indicate that no significant new peaks were observed other than characteristic peaks of PEGs and AB at FTIR spectrums of the PEG/AB composites (Figure 1(b)), which proves that the interactions between the functional groups of PEG and AB are physical in nature.Moreover, the existence of weak hydrogen bonding between the AB and PEG, which could be able to increase PEG-absorbing capacity of the AB to some extent.
To further examine the pore adsorption capacity of the AB particles, nitrogen adsorption/desorption isotherms are measured.e pore-size distribution of the AB is shown in Figure 2.
e AB particles possess a specific Brunauer-Emmett-Teller (BET) surface area of 53.79 m 2 /g, with a most probable pore size of about 2.6 nm. e specific surface area is much lower than those of porous carbon materials, such as expanded graphite or active carbon.Moreover, the average pore size of the AB particles is smaller than the equivalent sphere diameter of PEG4000 or PEG with higher M w .
e equivalent radius of PEG random coils can be calculated according to previous reports [24].
e above analysis shows a relatively low pore adsorption capacity of the AB particles.

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Leakage Behavior of the PEG/AB Composites.
e leakage behavior of the PEG/AB composites is investigated by POM and lter paper test.e POM images are taken below the crystallization temperature of the PEG10,000/AB composites, as shown in Figure 3. e darker regions correspond to the saturated AB, whereas the lighter regions marked by arrows indicate the excess PEG, as shown in Figure 3(a).When the mass fraction of the AB is increased to 11.6 wt.%, the excess PEG1,000 cannot be observed in the POM image in Figure 3(b), indicating the complete absorption of the PEG in the AB particles.
e leakage behavior of the PEG/AB composites at elevated temperature above the phase change point is further studied.Typically, the PEG/AB samples are kept in lter paper envelopes above the phase change temperatures for 1.5 h and the change in the weight of the lter paper envelopes before and after the test are measured, and the results were presented in Table 1.No signi cant change in weight is observed under 80 °C, indicating that no obvious PEG leakage occurred during the test.Consequently, the optimum absorption ratio of PEG800, PEG4,000, PEG6,000, and PEG10,000 in AB should be around 0.76, 0.84, 0.87, and 0.88 (responding to the results obtained by TGA below), respectively, and these mass fractions are adequate to obtain form-stable PEG/AB composite PCMs.

ermal Reliability of the PEG/AB Composites.
e TG curves of the pure PEG and PEG/AB composite PCMs are shown in Figure 4.As can be seen from the curves, all samples have a single thermal degradation stage.e degradation of the PEGs with relatively large molecular weights and their composite PCMs starts around 350 °C and ends at 430 °C, while the PEG800 starts below 300 °C and ends at 430 °C.It indicates that the AB does not signi cantly a ect the thermal stability of the PEGs with relatively large M w but improves the thermal stability of PEG with relatively small M w (PEG800).is is attributed to the stronger interaction between AB and PEG800 induced by pore adsorption due to the smaller equivalent sphere diameter of PEG800 than the average pore size of AB. at is, the PEG chains are trapped inside the AB pores.In addition, these results also demonstrate that no chemical reactions take place during simple melt blending and impregnation process.Moreover, the TG curves of the form-stable composite PCMs also con rm the optimum impregnation ratios of PEG800, PEG4,000, PEG6,000, and PEG10,000 as 76.4 wt%, 83.9 wt%, 86.8 wt%, and 88.4 wt%, respectively.

Crystalline Properties of the PEG/AB Composites.
e XRD patterns of the PEG and PEG/AB composites at room temperature are displayed in Figure 5.It is observed that the AB only has a weak and broad peak at 25 °, indicating an amorphous structure.e XRD patterns of the PEG4,000, PEG6,000, and PEG10,000 are characterized by two strong 2θ peaks at 19.1 °and 23.6 °, which are assigned to the typical plane of (120) and (112) of the PEGs.
e XRD curves of the PEG/AB composites are similar to those pure PEGs, and the position of the di raction peaks remains unchanged after the Advances in Materials Science and Engineering melt blending process, which demonstrates that the pristine PEG and the PEG/AB have the similar crystal structure and crystal cell type.

3.5.
ermal Conductivities of the PEG/AB Composites.e thermal conductivities of di erent mass ratios of AB are presented in Figure 6.It can be seen that almost linear correlation exists between the thermal conductivity and content of the AB for each PEG system.A slow increase in thermal conductivity from ∼0.15 W/mK to ∼0.26 W/mK with the increase of the mass ratio of the AB is observed due to the intrinsically low thermal conductivity of the AB induced by its amorphous structure.AB is chosen as the supporting material to verify the e ect of molecular weight of PEG on the shape stability so as to exclude the strong interactions between PEGs and supporting material, but actually it is not a promising thermally conductive ller for PCMs.

3.6.
ermal Performances of the PEG/AB Composites.e latent heat and phase change temperature of the samples are measured using DSC. Figure 7 shows the melting and freezing DSC curves of the pristine PEGs and form-stable PEG/AB   1. e theoretical melting/crystallization enthalpies of the PEG/AB composites are calculated based on the impregnated contents of the PEG into the AB and the measured melting/crystallization enthalpies of pristine PEGs, assuming that the AB cannot contribute to the phase change latent heat.
e latent heat capacities of the composite PCMs containing the PEGs with di erent M w are obviously lower than those of the pristine PEGs due to the incorporation of the AB that does not undergo phase change.Consequently, in most cases in this study, the melting enthalpies of the PEG/AB composites almost show agreement with the relevant mixture rules.e measured latent heats of the PEG/AB composites with varying PEG M w from ∼4,000 to ∼10,000 are comparable with the theoretical latent heats.By adding a proper loading of the AB, relatively high latent heats are retained, especially for the PEG/AB composites containing the PEGs with relatively large M w .
e incorporation of the AB does not induce a signi cant change of the crystal structure of the PEG with relatively large M w .Notably, in the case of the PEG800/AB composite, the melting enthalpy is signi cantly lower by 55.7% than the theoretical value at the optimum loading ratio. is is partially expected given that some of the PCM weight (23.6%) is replaced by the AB particles that are not subject to phase change.In addition, it is quite possible that other mechanisms also a ect the phase change process of the PEG800 in the PEG800/AB composites.e presumed reason may be that some of the PEG chains are trapped inside the AB pores when M w of the PEG is low (PEG800) due to the small equivalent sphere diameter of PEG800 compared to the most probable pore size of the AB. e phase transition of the PEG from amorphous phase into stable crystal phase can be inhibited to a large extent due to the con ned chain motion of the PEG induced by the strong adsorbability of the AB pores, and thus the obviously reduced latent heat of phase change is detected by DSC.As a result, the phase change of the composite is prone to take place at lower temperature (Table 2).After AB incorporation, the changes in crystallization temperature identify their tendency to supercool when cooling.However, the incorporation of the AB does not induce a signi cant supercooling of the PEGs with relatively large M w because of decreased restriction from the AB pores.By contrast, when the M w of the PEG increased to ∼4,000, most of the PEG chains are completely outside of the AB pores.
ese chains, di erent from both the free PEG chains and the chains trapped into the AB pores, are con ned by the surface of the AB particles.e longer PEG chains may o er more interacting site for the surface absorption of the PEG on the AB particles.Furthermore, it should be noted that the optimum impregnation ratios of the PEG into the AB for the PEG/AB composites increase with higher M w of the PEG.ese imply the existence of some other mechanisms leading to solid-solid phase change in the PEG/AB composite PCMs besides physical adsorptions.
e phase change behavior of the PEG as a polymeric PCM is signi cantly di erent from small-molecule PCM, namely, much stronger intermolecular interactions than them.It thus explains in terms of the stronger intermolecular interactions, such as intermolecular friction and macromolecular entanglement, between the PEG4000 chains than those of the PEG800.erefore, both the critical impregnated content of the PEG into the AB and enthalpy of the PEG4000/AB composite increase remarkably.When M w increases to ∼6,000, the con nement e ect from intermolecular interactions is strengthened further with the increase of chain length, and thereby the declining amplitude in T m of the PEG/AB composite is reduced; the critical impregnated content of the PEG into the AB also shows an increasing trend.With the increase of M w to 10,000, the critical impregnated content of the PEG into the AB increases further and thus the enthalpy of the PEG10,000/AB composite is improved as well.In other words, the phase change behaviors for the PEG/AB composites are not only attributed to the interactions between the AB and PEG, but also to the intermolecular interactions of the PEG chains depending on the varying M w of the PEG.

Shape-Stability Model of the PEG/AB Composites.
A model is proposed to explain the shape stability of the PEG/AB composites, based on a physically crosslinked structure with temporary junctions (Figure 8).A part of PEG Advances in Materials Science and Engineering random coils with small equivalent radius and segments are trapped into the pores of the AB particles by physical interactions, such as capillary and surface tension forces (abbreviated as trapped PEG in Figure 8).In addition, some of the PEG chains that are closely adjacent to the AB are adsorbed on the surface of the AB particles by hydrogen   6 Advances in Materials Science and Engineering bonding (abbreviated as adsorbed PEG in Figure 8); and some of the PEG chains with relatively large M w are stabilized by the macromolecular entanglement and intermolecular friction (abbreviated as entangled PEG in Figure 8).e melting process of the PEG in the composite is con ned by a physically crosslinked structure with temporary junctions above T m of the PEG, which inhibits the phase change of form-stabilized solid phase into liquid phase.e structure becomes more stable with longer length of the PEG molecular chains, resulting in higher critical impregnated content of the PEG into the AB.When the PEG content is increased further above the critical impregnated content, some of the PEG chains would be unbound above the phase change point of the composite, eliminating the shape-stabilized composites.

Structural Analysis of the PEG/AB Composites.
Comparison between the AB and the PEG/AB composites is made by SEM. Figure 9 shows the SEM images of the AB before PCM impregnation and the form-stable PEG/AB composites containing the critical impregnated contents of the PEGs.
e morphology of the PEG800/AB composite is similar to that of the AB particles.e AB particles as the supporting material are covered with PEG layers, and the structure of AB framework can be seen because the PEG layer is thin.A part of the PEG800 di uses into the pores of the AB particles, which is consistent with the DSC results.As discussed before, the PEG molecules with relatively small M w can be easily trapped by AB, despite the fact that the AB can only provide limited adsorption capability due to the relatively low speci c surface area and small pore size.In the case of the composite containing the PEG with relatively large M w , they appear to follow the di erent trend.rough SEM analysis, we con rm that most of the PEGs are not dispersed into the pores of the AB.With an increase of M w to ∼4,000 (Figure 9(c)), the AB framework is not obvious since the PEG layer outside of the AB is thicker.When M w increases to ∼6,000 (Figure 9(d)) or ∼10,000 (Figure 9(e)), the AB is covered with PEG completely and an irregular morphology can be found since the PEG layer is much thicker due to the longer molecular chain and higher mass fraction of the PEG.Based on the physically crosslinked structure model as discussed above, a part of the PEG segments and random coils with small equivalent radius are trapped into pores of the AB particles by capillary or surface tension forces.e hydrophilic character of the AB particles and cavities in them help us to improve the interfacial compatibility and wetting between the PEG and AB particles; thus some of the PEG chains adjacent to the AB are adsorbed on AB particle surfaces by hydrogen bonding, capillary, or surface tension forces.Other PEG chains are anchored by intermolecular friction and entanglement because there are more chances of intermolecular interactions between the longer PEG chains compared to the PEG with relatively low M w .Physically crosslinked structure comprising the PEG chains and AB particles is formed through hydrogen bonding, capillary, surface tension forces, intermolecular friction, and macromolecular entanglement, which contributes to constrained chain motion and thus the solid-solid phase change behavior of the PEG/AB composites.
erefore, the optimum impregnation ratio of the PEG into the AB for the PEG/AB composites without seepage of the melted PEG increases with the increasing M w of the PEG.

Conclusions
e PEG/AB composite PCMs exhibited desirable properties in term of shape stability and large latent heats.e AB particles kept the composite form stable without inducing a large reduction in their phase change enthalpies.
e fusion enthalpies of the form-stable PEG4,000/AB, PEG6,000/AB, and PEG10,000/AB composite PCMs reached 144.68, 157.24, and 161.22 J/g, respectively.Our results suggest that the phase change behaviors for the PEG/AB composites are attributed not only to the interactions between the AB and PEGs but also to the intermolecular interactions of the PEG chains depending on the varying M w of the PEGs.
e shape stability of the PEG/AB composites were explained based on a physically crosslinked structure model.Physically crosslinked structure with temporary junctions was formed through hydrogen bonding, capillary, surface tension forces, intermolecular friction, and macromolecular entanglement.e structure became stabler with increasing length of the PEG molecular chains, resulting in the increase of the critical impregnated content of the PEG into the AB.Advances in Materials Science and Engineering

Figure 4 :Figure 3 :
Figure 4: TG plots of the PEGs and PEG/AB composites.

Figure 6 :
Figure 6: ermal conductivities with various mass ratios of AB.

Table 1 :
Relative weight changes of the PEG/AB composites before and after the leakage tests at 80˚C for 1.5 h.
* ) obtained are presented in Table

Table 2 :
ermal analysis of the PEG/AB composites and pure PEGs with varying molecular weights.