Encapsulation of Phase Change Material with Water-Absorbable Shell

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Encapsulation of Phase Change Material with Waterabsorbable Shell for Thermal Energy Storage Taegu Do, Young Gun Ko, Youngsang Chun, and Ung Su Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00807 • Publication Date (Web): 28 Sep 2015 Downloaded from http://pubs.acs.org on October 2, 2015

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ACS Sustainable Chemistry & Engineering

Encapsulation of Phase Change Material with Water-absorbable Shell for Thermal Energy Storage Taegu Do,†,‡ Young Gun Ko,*,§ Youngsang Chun,† and Ung Su Choi*,†,‡ †

Center for Urban Energy Systems, Korea Institute of Science and Technology, Hwarang-ro 14-

gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea ‡

Department of Energy and Environmental Engineering, University of Science and Technology,

217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea §

Environmental Radioactivity Assessment Team, Korea Atomic Energy Research Institute, 989-

111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea

Corresponding Author *(Y.G.K.) Tel: +82-42-866-6104. Fax: +82-42-863-1289. E-mail: [email protected]. *(U.S.C.) Tel: +82-2-958-5657. Fax: +82-2-958-5659. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: To enhance the efficiency of heat transfer fluids, encapsulated phase change materials (PCMs) have been researched as the media for storing and release of a latent heat. The efficiency of thermal energy transportation and the durability of the PCM capsules are quite dependent on the physicochemical and mechanical properties of their shell. Herein, we have prepared the poly(2-hydroxyethyl methacrylate) (pHEMA) encapsulated paraffin-wax sphere as the PCM capsule with high encapsulation ratio, encapsulation efficiency, and thermal storage capability of 97.67, 97.33, and 99.65%, respectively. A hydrous PCM capsule exhibits improved thermal conductivity from ca. 0.26 to 0.47 W/(m·K) at 25 oC in comparison with a dry PCM capsule, and good durability in the melting-freezing cycle test. Our study demonstrates that the encapsulation of phase change material with the hydrous-flexible polymer shell is an effective strategy to enhance the thermal conductivity and durability of PCM capsules.

KEYWORDS: phase change material, encapsulation, thermal energy storage, water-absorbable shell, Pickering emulsion


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INTRODUCTION

The problems of depletion of fossil fuels and the emission of carbon dioxide have come to assume a grave aspect because the amount of energy consumption has increased according to population growth and industrial development.1 To solve these problems, the need of researches such as developments of alternative fuels2 and new energy sources,3 and improvement of energy efficiency4 has been highlighted. The thermal energy storage can be one of the solution which leads to saving of energy fuels and makes systems more cost-effective by the short- and longterm storage of wasted thermal energy.5 The encapsulated phase change materials (PCMs) absorbing, storing and releasing the latent heat by a phase transition has recently attracted considerable attention to make systems more cost-effective by using residual thermal energy.6 The technique based on PCM capsules can be used to eliminate/shave the peak of electricity demand or transfer the thermal energy as a form of PCM capsules dispersed fluid. The absorption, storage and release efficiencies of thermal energy for PCM capsules quite depend on the properties of the capsule shell.7 The capsule shell acts as a barrier for a jumble of a melted core (PCM) and a fluid, and the shell material requires some properties of high thermal conductivity, good sealing tightness, high endurance and good flexibility.8 Metal shell materials offer high thermal conductivity and high endurance. However, they are corrodible in an aqueous fluid.9 Polymeric shell materials are enough flexible to offer PCM capsules a great endurance to large volume change during their repetitive phase change processes. However, they still have a limitation of low thermal conductivity.10 The PCM capsules have been prepared by various techniques such coacervation,11 electrospinning,12 sol-gel method,13 interfacial polymerization with surfactants,14 polymerization


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using microfluidic devices15 and spray-drying.16 The diameter of the previously developed PCM capsules is less than 50 µm. Small PCM capsules provide large specific surface area for heat transfer. However, the small PCM capsules cause difficult handling in the separation process of used PCM capsules from the heat fluid. Although the size of PCM capsule increases, the shell thickness does not changes. Therefore, in considering weight ratio of shell to core of the PCM capsule, larger PCM capsule than 50 µm is better than smaller capsules if there is enough time for the fully phase change of the core material. Herein, we manufactured ca. 450 µm of water-absorbable PCM capsule with paraffin wax and poly(2-hydroxyethyl methacrylate) (pHEMA) as a core and a shell of the capsule respectively. Paraffin has been widely used as phase change material due to their properties such as affordability, non-toxic, high heat storage capacity, and low supercooling degree.17 The pHEMA has been used to contact lens, drug delivery systems and biomaterials owing to its nontoxic, elasticity, and noncorrosive properties.18 Especially, pHEMA shows high thermal conductivity after the water absorption in its matrix.19 The capsules were manufactured by redox polymerization of pHEMA on the surface of amino-functionalized silica particles coated paraffin in aqueous. Morphology of the capsules was characterized by optical microscope and field emission gun scanning electron microscope (FEG-SEM). Universal testing machine (UTM) and laser flash analysis (LFA) were used to confirm the durability and thermal conductivity of capsules according to the absorbed water content in the shell material. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out to measure a latent heat capacity of PCM capsules and thermal stability of their shell.


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EXPERIMENTAL SECTION

Materials.

Tetraethyl

orthosilicate

(TEOS,

Sigma-Aldrich,

>97%),

(3-

aminopropyl)trimethoxy silane (APTMS, Sigma-Aldrich, >97%), ethylene glycol dimethacrylate (EGDMA, Sigma-Aldrich, >98%), ammonium hydroxide solution (Sigma-Aldrich, 28% NH3 in H2O, >99.99% trace metals basis), 2,2’-Bipyridyl (Sigma-Aldrich, 99%) and toluene (SigmaAldrich, anhydrous) were used as received. 2-Hydroxyethyl methacrylate (HEMA, SigmaAldrich, 97%) was passed through a basic alumina column before use to remove the inhibitors. Copper (I) chloride (Sigma-Aldrich, 97%) was purified by stirring in acetic acid (Sigma-Aldrich, >99%), washing with ethanol (Acros, 99.5%), collecting particles, and drying under vacuum at room temperature, in that order. Preparation of Silica Particles. 100 ml of deionized water, 75 ml of ethanol and 60 ml of 28% ammonium hydroxide solution were added to 500 ml 3-neck round bottom flask, and stirred for 1 h at room temperature. 50 g of TEOS (0.24 mol) was added in drops into the solution, and then the reaction mixture was stirred for 24 h at room temperature. After the reaction, the silica particles were collected by filtration, and then washed with deionized water and ethanol in that order. Preparation of Amino-Functionalized Silica (Si-NH2) Particles. 2 g of the prepared silica particles were added into the 150 ml of anhydrous toluene containing 6.3 ml of APTMS (0.036 mol), and then the mixture solution was kept at room temperature for 24 h under gentle stirring with N2. After the reaction, the reacted silica particles were collected by filtration, and then washed with deionized water and ethanol in that order. The amine groups were immobilized on


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the silica particles as ligands to catch Cu catalysts to induce the polymerization of HEMA only on the surface of paraffin spheres. Preparation of Si-NH2 Particle Coated Paraffin (Si-NH2@paraffin) Spheres. 0.1 g of Si-NH2 particles were dispersed in 10 g of melted paraffin by sonication at 80 oC, and then 25 g of preheated deionized water were poured into the Si-NH2 particles dispersed melted paraffin. The mixture was stirred vigorously for 5 min at 80 oC to form the Pickering emulsion. The SiNH2@paraffin spheres were prepared by cooling the Pickering emulsion at room temperature, and washed with deionized water. Manufacturing of PCM Capsules. 1 g of Si-NH2@paraffin spheres were poured into 20 ml of deionized water, and then 5 g of HEMA was added into mixture solution. The aqueous solution was stirred gently under N2 for 1 h. Then, 53.46 mg of copper (I) chloride (0.54 mmol) and 168.69 mg of 2,2’-bipyridyl (1.08 mmol) were added into the solution. And the solution was stirred for 6 h at 35 oC under N2. After the reaction, the poly(2-hydroxyethyl methacrylate) coated paraffin capsules (pHEMA@paraffin) were collected by filtration, and washed 3 times with deionized water. Measurements. Morphologies of the pHEMA@paraffin capsules were observed using a field emission gun scanning electron microscope (FEG-SEM) (Inspect F50, FEI, Hillsboro, US), equipped with energy dispersive X-ray (EDX) spectroscopy, at 10 kV. The samples for FEGSEM were prepared by dropping onto a double-sided adhesive carbon disk and sputter-coated with a thin layer of Pt/Pd (E-1010, Hitachi, Japan). Fourier transform infrared spectroscopy (FTIR) (Frontier, PerkinElmer, Waltham, US) was used to confirm the synthesis of silica, Si-NH2 and pHEMA@paraffin capsule. Samples were blended with potassium bromide, and then pressed at 10 tons for 2.5 min for analysis. The image series of melted and solidified cores of a


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PCM capsule were obtained with an optical microscope (Reichert Metaplan 2, Leica), equipped with software (Nex Measure Pro 5, Bestecvision, Korea). The PCM capsule was observed in the heating coil-equipped water bath with a heating and cooling rate of 10 oC/min. Elongation at break, elongation at constant load of 1N/mm2, and repeated tension test of the pHEMA film at various amounts of water content in the film were measured according to ASTM D790 and D638 using universal testing machine (UTM) (Instron 4442, Instron Corp, Canton, MA) at a crosshead speed of 5 mm/min. Thermal stability and weight loss percentage of pHEMA was studied by thermogravimetric analyzer (TGA) (TGA Q500, TA Instrument) at the heating rate of 10 oC/min under nitrogen atmosphere. Laser flash analysis (LFA) (Netzsch, LFA 447 NanoFlash) with InSb sensor was carried out to measure the thermal conductivity of pHEMA film according to water content in the film at 25 oC. The phase change temperatures and latent heat of pHEMA@paraffin capsules were characterized by a differential scanning calorimeter (DSC) (DSC 4000, Perkin-Elmer) at the heating and cooling rate of 10 oC/min under nitrogen atmosphere.


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RESULTS AND DISCUSSION

The thermal stability of the shell of PCM capsules is very important owing to the repetitive phase change processes at high and low temperature.20 The TGA curve of pHEMA was obtained to confirm its thermal stability (Figure 1a). The weight of the pHEMA decreased slightly at ca. 100 o

C owing to the absorbed moisture in it, and fell down dramatically at 300 oC due to its

decomposition. Therefore, the pHEMA can be stably used as a shell of PCM capsules in the hot aqueous fluid. The pHEMA was used as a shell material of the PCM capsule in this study. The hydrous pHEMA should be characterized because the PCM capsules are in the aqueous fluid when they are used as heat transfer carriers. The pHEMA absorbs large amount of water owing to its hydrophilic property, and shows various changes of its chemicophysical properties such as flexibility and thermal conductivity according to the absorbed water amount.21 The volume expansion ratio (%) and the absorbed water content (wt%) of pHEMA versus the immersion time of pHEMA in water was shown in Figure 1b. The volume expansion ratio (%) and the absorbed water content (wt%) were calculated by dividing the volume change (∆V) and the weight change (∆m) of PHEMA by initial volume (Vi) and final weight (mf) of PHEMA, respectively. For the accurate measurement, the pHEMA was dried in a vacuum oven for 1 week at room temperature before its immersion into 20 oC water. At the equilibrium state, the values of volume expansion ratio and absorbed water content were ca. 54 % and ca. 35 %, respectively. To evaluate the mechanical properties of water-absorbed pHEMA, the elongations at break and constant load of pHEMA on the absorbed water content were measured according to ASTM D 790 (Figure 2a). The elongation at break of the fully water-absorbed pHEMA is 13


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times higher than that of dry pHEMA. The elongation at 1 N/mm2 of constant load also increased with the increase of absorbed water amount in pHEMA. The absorbed water increase of the mobility of polymer chain in pHEMA which influences the impact strength and fracture toughness of the polymer.22,23 The repetitive tension test was carried out to measure the recovery ability of the hydrous pHEMA according to ASTM D638 using UTM (Figure 2b) because the shell of PCM capsule should endure rapidly repetitive volume change of its core by melting and solidifying the capsule.24 The volume of used paraffin expands to ca. 10 vol% when it melts fully. Therefore, the liner expansion of paraffin wax is ca. 3.23% according to eq 1. Linear expansion rate % = 1 +

∆

− 1 × 100%

(1)

The repetitive tension test was carried out at 10% strain that is three times higher than the linear expansion of paraffin. During 10 cycles, there was no breakage of the pHEMA film. The test indicates that pHEMA shell can endure the thermal cycling without the breakage of PCM capsule. After 7 cycle, the increase of tensile strength was observed owing to the evaporation of absorbed water in the pHEMA film. If the test was conducted in the water, the increase of tensile strength will not be observed. The thermal conductivity of pHEMA film on absorbed-water content in the film was measured according to ASTM E1461 (Figure 3). The obtained thermal conductivity of completely dried pHEMA is ca. 0.26 W/(m·K) at 25 oC, and higher than that of polystyrene, polyurethane,25 or polymethylmethacrylate.26 The thermal conductivity of pHEMA increased with the increase of absorbed-water content in the polymer, and approximately 0.47 W/(m·K) at 25 oC was measured when the pHEMA contained ca. 35 wt% of water. The thermal conductivity of pHEMA dramatically increased at 10 ~ 20 wt% of the absorbed-water content owing to the formation of water bridge in the structure of pHEMA at the range of absorbed-water content.27


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PCM capsules are in the aqueous fluid when they are used,28 therefore shows always good thermal conductivity unlike other polymeric shells of PCM capsules. PCM capsules consisted of pHEMA and paraffin, as a shell and a core, have been prepared as presented schematically in Figure 4. Amino-functionalized silica particles (Si-NH2) were synthesized by the sol-gel process of TEOS and the immobilization of APTMS on the fabricated silica particles. Then, the Si-NH2 coated paraffin (Si-NH2@paraffin) spheres were fabricated by the Pickering emulsion technique.29 Finally, the PCM capsules (pHEMA@paraffin) were prepared by the polymerization of HEMA on the surface of Si-NH2@paraffin spheres. The FT-IR spectra of paraffin, silica particles, Si-NH2 particles, and pHEMA@paraffin capsules are shown in Figure 5. In the spectrum of paraffin, the strong peaks at 2915 and 2847 cm-1 were assigned to the aliphatic C-H stretching vibration, the peak at around 1465 cm-1 was the characteristic peak of the C-H deformation vibration, and the peak for the in-plane rocking vibration of -CH2 appeared at 720 cm-1. In the spectrum of silica, the broad peak at around 32003600 cm-1 corresponded to the stretching vibration of hydroxyl group (-OH). A sharp peak of SiO stretching vibration was observed at 1100 cm-1, and the peaks of bending vibration of Si-OH and Si-O-Si arose at 950 and 800 cm-1 respectively. After the immobilization of APTMS on to the surface of silica particles, two peaks at 1460 and 1380 cm-1 newly arose for the characteristic of deformation vibration of -CH2-. The bending vibration for primary amine (-NH2) was also observed at around 1550 cm-1. The emergence of these peaks, which can be observed in the inset, demonstrated that amine functional groups were successfully immobilized onto the surface of silica particles. In the spectrum of pHEMA@paraffin, the peaks of paraffin and Si-NH2 were still existed, and C=O peak for the ester group was newly observed at 1725 cm-1.


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The diameter for each sphere is presented in Figure 6. The average diameters of silica particles, Si-NH2@paraffin spheres, and pHEMA@paraffin capsules were ca. 150 nm, ca. 440 µm, and ca. 455 µm, respectively. The thickness of the shell of pHEMA@paraffin was ca. 7 µm. The range of diameter distribution for pHEMA@paraffin capsules was not wide, and their diameter was larger than reported diameters of other PCM capsules.30-34 As shown in Figure 6, the pHEMA@paraffin capsules were successfully prepared with a large diameter just as we had planned. After encapsulation, the morphologies of Si-NH2@paraffin spheres and pHEMA@paraffin capsules were characterized by the FEG-SEM (Figure 7) equipped with an energy dispersive Xray (EDX) spectroscope. The Si-NH2@paraffin spheres prepared by Pickering emulsion method showed that the paraffin spheres were well coated with the Si-NH2 particles onto their surface (Figure 7a-c). Figure 7d shows the morphologies that after encapsulation of the Si-NH2@paraffin spheres with pHEMA. After encapsulation, it can be observed that the surface of the paraffin became smooth. The cross section of hollow pHEMA@paraffin capsule was prepared to observe the shell (Figure 7e and f). Elementary mapping of the cross section of pHEMA@paraffin capsule was carried out to confirm the interior structure visually (Figure 7g). The images of the elementary mapping were obtained by cutting the pHEMA@paraffin capsules using a razor, and dissolving the paraffin core using n-hexane. Paraffin, Si-NH2 particles, and pHEMA can be clearly distinguished through the mapping of C, Si and O, C and O, respectively. The carbon, C, as the element of paraffin and pHEMA, was found in the area of the core and the shell of pHEMA@paraffin. The oxygen, O, as the element of pHEMA and Si-NH2, was shown in the shell area. The silicon, Si, as the element of Si-NH2, was represented in the area of the shell of pHEMA@paraffin around the core (paraffin sphere). With the results of the elementary mapping


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of the pHEMA@paraffin capsule, it can be confirmed visually that the paraffin sphere was well encapsulated with pHEMA. To confirm the sealing tightness and the endurance of the shell of pHEMA@paraffin capsules, the repetitive melting-solidification process of the capsules was conducted. At first, we slightly dissolved the dried capsules by using n-hexane to verify visually whether there is a hole in the shell or not (Figure 8a). Because the crack or the hole is not well observed owing to the transparent property of the shell after complete removal of core, the core was removed partially to see the crack or the hole in the shell of capsule. In the optical microscope image of the slightly removed core of the pHEMA@paraffin capsules (right panel of Figure 8a), any hole or crack was not observed in the shell of the capsule. The pHEMA@paraffin capsules were added into a small water bath equipped with the temperature sensor and the heating coil, and then the repetitive heating-cooling process was carried out in the temperature of 30 ~ 80 oC. The obtained optical images from the process at 30, 50 and 80 oC were shown in Figure 8b, and it was confirmed that the shell of capsule was well maintained without the breakage during the process. As we had expected, the prepared shell of pHEMA@paraffin capsule showed good sealing tightness and high endurance owing to its reversibly stretchy property for the repetitive heatingcooling process that causes the volume change of the core of capsule. The phase change temperature and latent heat of paraffin and pHEMA@paraffin capsule were measured using DSC (Figure 9a). The paraffin was compared with the pHEMA@paraffin capsules that after 10 cycles of the repetitive melting-solidification process. The DSC curves of the paraffin and the capsule were slightly broad owing to contained hydrocarbons of low molecular weight in them.35 After the encapsulation of the paraffin spheres with the pHEMA shell, the melting point increased by 2.3 oC, and the freezing point decreased by 1.6 oC. These


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increase and decrease of the melting and freezing temperatures are due to the heterogeneous nucleating effect of the pHEMA shell on the solidification of the paraffin sphere.36,37 With this result, it can be confirmed that the whole time of melting and cooling of the paraffin core in the pHEMA@paraffin capsule takes about 13.8 and 9.6 seconds, respectively, based on the calculation using the temperature difference between the onset and the end of the peaks divided by the 10 oC/min of the heating and cooling rate. The calculated melting and freezing latent heats by integration of the peaks of DSC thermograms, were measured to be 172 J/g and 172.5 J/g for the paraffin, and 168 J/g and 167.3 J/g for the pHEMA@paraffin capsule, respectively. The decrease of latent heat per weight after the encapsulation of paraffin is caused by the weight of pHEMA shell which is not able to absorb the latent heat. The thermal stability of pHEMA@paraffin capsules were evaluated by melting and solidifying the capsules repeatedly 10 times using DSC with heating-cooling rate of 10 oC /min (Figure 9b). With the results of DSC curves, we were able to confirm that the pHEMA@paraffin capsules were not broken while the repetitive phase change processes were performed for 10 cycles, because there was not any peak shift or change of latent heat in the DSC curves. The enthalpy for phase change, as one of the most important factor for the heat transfer and storage performance, is highly up to the encapsulation efficiency and the encapsulation ratio of the pHEMA@paraffin capsules.38 Using the data of DSC, the encapsulation efficiency (Ee) and encapsulation ratio (Er) of pHEMA@paraffin capsules can be calculated by following eqs 2 and 3, respectively.39 ∆ ,"#$%&'( )∆*,"#$%&'( ,+#,#**-. )∆*,+#,#**-.

= ∆

∆ ,"#$%&'(

/ = ∆

,+#,#**-.

× 100%

× 100%

(2) (3)


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Here, ∆Hm,Paraffin and ∆Hm,Capsule are the melting latent heat of the paraffin and pHEMA@paraffin capsule, respectively, and ∆Hf,Paraffin and ∆Hf,Capsule represent the freezing latent heat of the paraffin and pHEMA@paraffin capsule, respectively. The encapsulation ratio (Er) shows the effective encapsulation of paraffin in the PCM capsule, while the volume fraction is the considered as the total volume of paraffin in the capsule. If the encapsulation ratio is lower than the volume fraction, it means that all paraffin in the PCM capsule does not carry out the thermal

storage and

release performance.

Encapsulation

ratio

of paraffin

in

the

pHEMA@paraffin capsule was determined to be 97.67%, and the calculated encapsulation ratio was well matched with the volume fraction (ca. 97%) of the paraffin core and shell which was measured by FEG-SEM. The encapsulation efficiency, which is an indicator that can accurately evaluate the working efficiency of pHEMA@paraffin capsule by comparing the latent heat storage capacity of before and after encapsulation, is calculated to be 97.33%. To evaluate the heat storage and release performance of the pHEMA@paraffin capsules, the thermal storage capability (η) was calculated by eq 4.40 0 =

1 ,+#,#**-. 2∆ ,"#$%&'( )∆*,"#$%&'( 3 × 1 ,"#$%&'( 21 ,+#,#**-. )1*,+#,#**-. 3

100%

(4)

The thermal storage capability of pHEMA@paraffin capsule was determined to be 99.65%, which is higher thermal storage capability compared with previously reported values for the encapsulated paraffin.41,42 The enhancement of thermal storage capability of pHEMA@paraffin capsule is attributable to the increased ratio of paraffin in the PCM capsule.


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CONCLUSIONS

In this work, we manufactured phase change capsules larger than 400 µm by the encapsulation of paraffin spheres with poly(2-hydroxyethyl methacrylate) (pHEMA) (pHEMA@paraffin) to enhance the high thermal conductivity, good sealing tightness, high endurance and good flexibility. Although the pHEMA showed the thermal conductivity of 0.26 W/m·K which is higher than those of commercial polymers such as polystyrene, polyurethane and poly(methyl methacrylate), the hydrous pHEMA exhibited the high thermal conductivity of 0.47 W/m·K. The prepared shell of pHEMA@paraffin capsule showed good sealing tightness and high endurance owing to its reversibly stretchy property for the repetitive heating-cooling process that causes the volume change of the core of capsule. The prepared pHEMA@paraffin capsules showed the latent heat of ca. 167 J/g for the storage and release of thermal energy. The calculated encapsulation ratio and efficiency of pHEMA@paraffin based on the value of latent heat were 97.67% and 97.33%, respectively. The determined thermal storage capability of the pHEMA@paraffin capsules was also as high as ca. 99.65%. In addition to the use of the prepared pHEMA@paraffin capsules as thermal energy carriers in the fluid, the capsules can be a promising material for various applications such as a wall material of buildings or houses, a cloth material of outdoor wears, and a material of lunch boxes for temperature sensitive foods.


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AUTHOR INFORMATION Corresponding Author *(Y.G.K.) Tel: +82-42-866-6104. Fax: +82-42-863-1289. E-mail: [email protected]. *(U.S.C.) Tel: +82-2-958-5657. Fax: +82-2-958-5659. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Green City Technology Institutional Program funded by Korea Institute of Science and Technology (KIST-2015-2E25302).


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REFFERENCES

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(16) Borreguero, A. M.; Valverde, J. L.; Rodriguez, J. F.; Barber, A. H.; Cubillo, J. J.; Carmona, M. Synthesis and characterization of microcapsules containing Rubitherm (R) RT27 obtained by spray drying. Chem. Eng. J. 2011, 166, 384-390. (17) Jin, Z. G.; Wang, Y. D.; Liu, J. G.; Yang, Z. Z. Synthesis and properties of paraffin capsules as phase change materials. Polymer 2008, 49, 2903-2910. (18) Guiseppi-Elie, A.; Dong, C. B.; Dinu, C. Z. Crosslink density of a biomimetic poly(HEMA)-based hydrogel influences growth and proliferation of attachment dependent RMS 13 cells. J. Mater. Chem. 2012, 22, 19529-19539. (19) Jerman, M.; Cerny, R. Effect of moisture content on heat and moisture transport and storage properties of thermal insulation materials. Energy Build. 2012, 53, 39-46. (20) Jiang, Y. B.; Wang, D. J.; Zhao, T. Preparation, characterization, and prominent thermal stability of phase-change microcapsules with phenolic resin shell and n-hexadecane core. J. Appl. Polym. Sci. 2007, 104, 2799-2806. (21) Kulasinski, K.; Guyer, R.; Keten, S.; Derome, D.; Carmeliet, J. Impact of Moisture Adsorption on Structure and Physical Properties of Amorphous Biopolymers. Macromolecules 2015. (22) Alamri, H.; Low, I. M. Effect of water absorption on the mechanical properties of nanofiller reinforced epoxy nanocomposites. Mater. Des. 2012, 42, 214-222. (23) Yang, B.; Huang, W. M.; Li, C.; Li, L. Effects of moisture on the thermomechanical properties of a polyurethane shape memory polymer. Polymer 2006, 47, 1348-1356. (24) Cheng, W. L.; Liu, N.; Wu, W. F. Studies on thermal properties and thermal control effectiveness of a new shape-stabilized phase change material with high thermal conductivity. Appl. Therm. Eng. 2012, 36, 345-352.


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Figure Captions

Figure 1. (a) TGA and DTA curves of pHEMA, and (b) volume expansion ratio of pHEMA and absorbed water content in pHEMA according to the immersion time of pHEMA in water.

Figure 2. (a) Elongation at break and fixed load (1 N/mm2) on the absorbed water content in pHEMA, and (b) repetitive tension test of pHEMA containing 35 wt% of water. Data are presented as mean ± standard deviation, n = 5.

Figure 3. Thermal conductivity of pHEMA on the absorbed water content in pHEMA. Data are presented as mean ± standard deviation, n = 5.

Figure 4. Schematic representation of pHEMA encapsulated paraffin (pHEMA@paraffin) capsules.

Figure 5. FT-IR spectra of paraffin, silica particles, Si-NH2 and pHEMA@paraffin.

Figure 6. Diameter distribution for (a) Si-NH2, (b) Si-NH2@paraffin, and (c) pHEMA@paraffin. (d) Thickness distribution for pHEMA layer of pHEMA@paraffin.


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Figure 7. FEG-SEM images of Si-NH2@paraffin spheres and pHEMA@paraffin capsules. (a) zoom-out and (b) zoom-in of Si-NH2@paraffin spheres. (c) Magnified image of red-dotted area in (b). (d) pHEMA@paraffin capsules (inset: magnified image of pHEMA@paraffin capsule). (e) and (f) Cross-sectional images of pHEMA@paraffin capsule after removal of paraffin core using n-hexane. (g) Cross-sectional image of pHEMA@paraffin capsule (right four panels: elementary mappings of C, O, Si, and merge for pHEMA@paraffin capsule).

Figure 8. (a) Optical images of pHEMA@paraffin capsules before and after slight removal of the paraffin spheres using n-hexane. (b) Optical images of pHEMA@paraffin capsule during a melting/freezing cycle.

Figure 9. (a) Heating-cooling DSC curves for paraffin and 10th cycle of pHEMA@paraffin capsule, and paraffin. (b) DSC curves for 1, 2, 5, and 10 cycles of pHEMA@paraffin capsules.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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Figure 9


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for Table of Contents Graphic use only

Encapsulation of Phase Change Material with Water-absorbable Shell for Thermal Energy Storage Taegu Do, Young Gun Ko,* Youngsang Chun, and Ung Su Choi*

Synopsis Phase change material (PCM) capsules were prepared for the efficient harvest and delivery of waste heat from power plants to houses or office buildings.


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Encapsulation of Phase Change Material with Water-Absorbable Shell

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