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Solar Energy 86 (2012) 1149–1154 www.elsevier.com/locate/solener
Synthesis, characterization and thermal properties of novel nanoencapsulated phase change materials for thermal energy storage G.H. Zhang a, S.A.F. Bon b, C.Y. Zhao c,⇑ a School of Engineering, University of Warwick, Coventry CV4 7AL, UK Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK c School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China b
Received 13 October 2011; received in revised form 2 December 2011; accepted 5 January 2012 Available online 8 February 2012 Communicated by: Associate Editor Halime Paksoy
Abstract In this paper, nanocapsules containing n-octadecane with an average 50 nm thick shell of poly(ethyl methacrylate) (PEMA) and poly (methyl methacrylate) (PMMA), and a core/shell weight ratio of 80/20 were synthesized by the direct miniemulsion method, respectively. The average size of the capsules is 140 nm and 119 nm, respectively. The chemical structure of the sample was analyzed using Fourier Transformed Infrared Spectroscopy (FTIR). Crystallography of nanocapsules was investigated by X-ray diﬀractometer. The surface morphology was studied by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The thermal properties and thermal stability of the sample were obtained from Diﬀerential Scanning Calorimeter (DSC) and Thermal Gravimetric Analysis (TGA). The temperatures and latent heats of melting and crystallizing of PEMA nanocapsule were determined as 32.7 and 29.8 °C, 198.5 and 197.1 kJ/kg, respectively. TGA analysis indicated that PEMA/octadecane nanocapsule had good thermal stability. The nanocapsules prepared in this work had a much higher encapsulation ratio (89.5%) and encapsulation eﬃciency (89.5%). Therefore, the ﬁndings of the work lead to the conclusion that the present work provides a novel method for fabricating nanoencapsulated phase change material, and it has a better potential for thermal energy storage. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: n-octadecane; PEMA; Nanocapsules; Miniemulsion; Thermal physical property
1. Introduction Latent thermal energy storage using phase change materials (PCMs) is the most important thermal energy storage technology due to the PCMs can absorb or release a large amount of heat while undergoing phase changes, with small temperature variations. However, most organic PCMs have problems in low thermal conductivity, instability and ﬂammability; and inorganic PCMs are corrosive to most metals and suﬀer from decomposition and sub-cooling. Therefore, encapsulation of PCMs is an attractive solution for above problems in both organic and inorganic PCMs. ⇑ Corresponding author. Tel.: +86 0 21 34204541.
E-mail address: [email protected]
u.edu.cn (C.Y. Zhao). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2012.01.003
The encapsulation cannot only provide a space to control the volume changes during the phase change, but it can also protect PCMs from the outside environment in their applications. In recent research, various kinds of microencapsulated phase change materials (MPCMs) have been fabricated by in situ polymerization (Li et al., 2007; Salau¨n and Vroman, 2008; Yu et al., 2009; Yuan et al., 2008), interface polymerization (Liang et al., 2009; Pascu et al., 2008; Zhang and Wang, 2009), suspension like polymerization (Chang et al., 2009; You et al., 2009) and other polymerization methods (Yuan et al., 2008; Zhang et al., 2007). Nevertheless, in some ﬁelds especially in latent functionally thermal ﬂuids, MPCMs did not performance very well under repeated cycling due to the large particles of the MPCMs
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Nomenclature d, D DHc DHf k Tm Tc
diameter (m) heat of crystallization (kJ/kg) heat of fusion (kJ/kg) Thermal conductivity (W/(m °C)) melting temperature (°C) crystallizing temperature (°C)
not only increased the ﬂuid’s viscosity, but also were often crushed during pumping, and this would resulting the blocking of the pipe system (Fang et al., 2008). Therefore, there is a trend to develop nanoencapsulated PCMs with smaller particle size as compared with microencapsulated PCMs. In addition, nanocapsules have higher surface-area-to-volume ratio than microcapsules which provides a stronger “driving force” to speed up thermodynamic processes. In recent years, several kinds of nanoencapsulated PCMs synthesized by mini-emulsion polymerization method have been studied. Fang et al. (2008) fabricated a kind of nanocapsule with polystyrene as the shell and n-octadecane as the core. Sari et al. (2009) investigated the nanoencapsulation of n-octacosane with poly(methyl methacrylate) (PMMA) shell. They reported that the nanocapsule had energy storage and release capacity (86.4–88.5 kJ/kg) during its phase change. Less than 100 nm size nanocapsules of n-octadecane were prepared by Kwon et al. (2010). Alay et al. (2010) prepared PMMA/n-hexadecane nanocapsule as a ﬁber additive in textile application. Black et al. (2010) fabricated nanocapsules of n-hexadecane within a 4–40 nm thick shell of poly(alkyl methacrylate). According to the literature review above, there is no work has been done on nanoencapsulated of n-octadecane by poly(ethyl methacrylate) (PEMA). Therefore, the aim of this work is to fabricate nanocapsules of n-octadecane with PEMA shell. Since PS is a rigid plastic and PMMA and PEMA are more softer (PEMA is softer than PMMA), it is suggesting that the soft shell material is favorable for the preparation of microcapsules (Yang et al., 2003). The chemical structure, morphology, diameter and its size distribution, thermal properties of the nanocapsules were obtained from experimental measurements. 2. Method and materials 2.1. Materials N-octadecane, the methyl methacrylate monomers (MMA) and the ethyl methacrylate (EMA) monomers with a purity of 99 wt.% were purchased from Sigma–Aldrich, respectively. Sodium dodecyl sulphate (SDS) and 2,2´-azobisisobutyronitrile (AIBN) were commercially supplied by
Greek letters R the encapsulation ratio (%) E the encapsulation eﬃciency (%) Subscripts c nanocapsule core p particle w wall
VWR international Ltd., UK. All chemicals were of reagent quality and used without further puriﬁcation. 2.2. Synthesis of nanocapsules The PEMA/octadecane nanocapsules and the PMMA/ octadecane nanocapsules were synthesized by the direct mini-emulsion polymerization method. In one beaker, a stock of surfactant solution was prepared by dissolving 0.3 g of SDS in 30 g of deionised water. Then 2 g n-octadecane was added to 0.5 g monomer solution with the desired core/shell ratio: 80:20, and 5 mg of AIBN initiator per gram of monomer was added to the oil/monomer mixture. The mixture solution was added to the surfactant solution and stirred for 0.5 h at 40 °C. The solution was then sonicated with a tip sonicator (Branson Digital Soniﬁer) in a water bath at 70% amplitude for 3 min while the solution was gently stirred. Then the sonicated solution was poured into a 50 mL ﬂask. The ﬂask was sealed with a rubber septum, and the solution was stirred gently under nitrogen ﬂow for 0.5 h at room temperature to remove oxygen. Polymerization was initiated by heating the solution with approximately 600 rpm stirring rate in an oil bath at 70 °C for 24 h to complete the process. The nanocapsules suspension was then dialyzed with dialysis tubing cellulose membrane in the deionised water (keep changing water once a day) for a week to remove any impurities, unreacted monomers and surfactant for characterization. The yield of polymerization reaction of PEMA and PMMA nanocapsules were 50.04% and 61.26%, respectively. 2.3. Characterization Fourier Transform Infrared Spectra (FTIR) of the nanocapsules were obtained using an AVATAR-380 FTIR spectrophotometer with the KBr sampling method. XRD patterns of the samples were obtained using an X-ray diffractometer in the 2h ranges from 10° to 50° (Philips 1820 diﬀractometer with 20 position sample changer). Morphologies were obtained by using a ZEISS SUPRA 55-VP Scanning Electron Microscopy (SEM), and a Jeol 2010F Transmission Electron Microscopy (TEM) at an accelerating voltage of 200 kV. Dynamic Light Scattering
G.H. Zhang et al. / Solar Energy 86 (2012) 1149–1154
2.4. Thermal properties of nanocapsules Diﬀerential Scanning Calorimeter (DSC) was performed to determine the melting temperature and heat of fusion during the heating process, crystallizing temperature and crystallization heat during the cooling process by using a SETARAM Instrumentation-D/SENSYS-2A diﬀerential scanning calorimeter, and all measurements were carried out under an air atmosphere at a heating or cooling rate of 0.2 °C/min. Thermal Gravimetric Analysis (TGA) was applied to measure the thermal stability of these nanocapsules by a Polymer laboratories-STA 1500. The thermal conductivity of the nanocapsule was calculated using the composite sphere approach (Karaipekli et al., 2007): 1 1 dpdc ¼ þ kp d p kcd c kwd p d c
3. Results and discussions Fig. 1 shows the FTIR spectra of the pure n-octadecane, PEMA/octadecane nanocapsules and PMMA/octadecane nanocapsules. In the spectrum of pure n-octadecane, the strong alkyl C–H stretching vibrations of methyl and methylene groups are observed at 2846, 2912 and 2952 cm1, the peak at 1469 cm1 is due to C–H bending vibrations of methylene bridges, the characteristic transmission peak at 715 cm1 corresponds to the in-plane rocking vibration of the methylene group. It is observed that the spectra of two kinds of nanocapsules are similar to the spectra of the pure n-octadecane. Nevertheless, the C–H stretching vibration of the polymer shells are found at 1725 cm1
n-octadecane Nanocapsule PEMA
Diffraction Intensity (CPS)
measurements (DLS) were carried out by using a Malvern Instruments-Mastersizer 2000 laser particle size analyzer. The size distribution and the mean diameters of the nanocapsules were determined.
50000 40000 30000 20000 10000 0 10
2-Theta ( C) Fig. 2. X-ray patterns of n-octadecane, PEMA/octadecane nanocapsule and PEMA shell. (For interpretation to colors in this ﬁgure, the reader is referred to the web version of this paper.)
and the peaks at 1147 cm1 is associated with the C–O stretching of the ester group. Therefore, it is demonstrated that all the characteristic peaks of nanocapsules can be distinguished in the spectra of the pure n-octadecane. X-ray patterns of n-octadecane, nanoencapsulated noctadecane and polymer shell are shown in Fig. 2. Four obvious diﬀraction peaks are found at 12°, 15°, 19°, and 23° (2h), respectively in the X-ray pattern of the n-octadecane, which can be indexed as (0 1 1), (0 1 2), (1 0 1) and (1 0 2). These peaks also appear in the X-ray patterns of the nanocapsule, indicating that the n-octadecane and the n-octadecane within the nanocapsules are both triclinic (Zhang and Wang, 2009). Therefore, the crystal line of the n-octadecane in the capsules is the same as that of the bulk. However, the diﬀraction intensities decreased dramatically after the nanoencapsulation due to the motion of the molecular chains of the n-octadecane is inhibited by a small space of the capsule, preventing the n-octadecane
n-octadecane PEMA/octadecane nanocapsules PMMA/octadecane nanocapsules
20 0 500
1000 1500 2000 2500 3000 3500 4000 4500
Wavenumber (cm-1) Fig. 1. FTIR spectra of the n-octadecane, PEMA/octadecane nanocapsules and PMMA/octadecane nanocapsules.
Fig. 3. Particle size distribution of (a) the PEMA/octadecane nanocapsules and (b) the PMMA/octadecane nanocapsules.
G.H. Zhang et al. / Solar Energy 86 (2012) 1149–1154
Fig. 4. SEM images of nanocapsule with PEMA shell (left) and PMMA shell (right).
from crystallizing. There are only two small amorphous diﬀraction peaks in the pattern of the polymer shell, which locate at 12° and 18° (2h). The particle size distribution (PSD) of the nanocapsules is shown in Fig. 3. In the Fig. 3a, the PSD in intensity shows the PEMA/octadecane particle size varies from 60 nm to 360 nm, and presenting a narrow size distribution. The average diameter of the nanocapsule is about 140 nm. Fig. 3b shows the PMMA/octadecane particle size ranges from 50 nm to 300 nm and it is also presenting a narrow size distribution with an average diameter of the 119 nm. The size of the nanocapsule was primarily controlled through the sonication time before the polymerization, and Asua (Asua, 2002) indicated that the particle size decreased with sonication time. High particle distribution index (PDI) is typical for mini-emulsions polymerization method in comparison with emulsions, and the PDI
obtained in this experiment are 0.081 and 0.076 for PEMA nanocapsule and PMMA nanocapsule, respectively. The phenomenon of high PDI can be improved by longer sonication times. Fig. 4 shows the SEM images of the samples with various diameters. It is observed that the regular spheres are seen on these nanocapsules and the capsules may have rough and smooth surface structures, although the sizes of these special capsules are not uniform. The diameters of the sample are around 100 nm, which matches the results from the Dynamic Light Scattering (DLS) measurements. The TEM images of the samples are shown in Figs. 5 and 6. The main ﬁgure shows a negative stained image with a few large particles and the diameters of these capsules are in the range of 150–200 nm, consistent with the measured diameter from the DLS. It is observed that most of the nanocapsules were regular spherical. There are also several
Fig. 5. TEM image of the PEMA/octadecane nanocapsules. The scale bar is 200 nm. The inset ﬁgure shows a Cryo-TEM image. The scale bar is 100 nm.
Fig. 6. TEM image of the PMMA/octadecane nanocapsules. The scale bar is 200 nm. The inset ﬁgure shows a Cryo-TEM image. The scale bar is 100 nm.
G.H. Zhang et al. / Solar Energy 86 (2012) 1149–1154
Heating (PEMA nanocapsule) Cooling (PEMA nanocapsule) Heating (n-octadecane) Cooling (n-octadecane)
60 50 40
Heat flow (mW)
30 20 10 0 -10 -20 -30 -40 -50 10
Heat flow (mW)
Temperature ( C)
70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 10
Heating (n-octadecane) Cooling (n-octadecane) Heating (PMMA nanocapsule) Cooling (PMMA nanocapsule)
Temperature ( C) Fig. 7. DSC curves of (a) n-octadecane and the PEMA/octadecane nanocapsule and (b) n-octadecane and the PMMA/octadecane nanocapsule. (For interpretation to colors in this ﬁgure, the reader is referred to the web version of this paper.)
small particles in the 50 nm range and they are likely residual surfactant micelles. The inset ﬁgure shows a Cryo-TEM image of a few capsules. The image presents that those nanocapsules may have very smooth surface structure. The DSC curves of the n-octadecane, the PEMA/octadecane nanocapsule and the PMMA/octadecane nanocapsule are presented in Fig. 7, and the thermal properties of them are summarized in Table 1. Both PEMA and PMMA nanocapsules showed relatively good heat storage property, and PMMA nanocapsule was slightly better than PEMA nanocapsule. The n-octadecane revealed a signiﬁcant super-cooling phenomenon during the cooling process and this supercooling phenomenon reduced dramatically after nanoencapsulation. It is interesting to note that the melting temperature and the crystallizing temperature of the nanocapsule
Fig. 8. TGA curves of PEMA (a), PEMA/octadecane nanocapsules (b) and n-octadecane (c).
are close to that of n-octadecane. This indicates that the phase change behaviors of the n-octadecane and nanocapsule are similar to each other. In comparison to the nanocapsule, the n-octadecane has a high DHm of 220.4 kJ/kg and a high DHc of 222.6 kJ/kg. Encapsulation ratio and encapsulation eﬃciency are two important parameters in determining the thermal properties of the nanocapsule and they can be obtained from the DSC results. The encapsulation ratio and encapsulation eﬃciency can be calculated from the following equations (Zhang et al., 2010): R¼
DH m;Micro-PCMs 100% DH m;PCM
DH m;Micro-PCMs þ DH c;Micro-PCMs 100% DH m;PCM þ DH c;PCM
where DHm,PCM and DHc,PCM are the heat of fusion and heat of crystallization of the bulk n-octadecane, respectively; DHm,Micro-PCMs and DHc,Micro-PCMs are the fusion heat and crystallization heat of the nanoencapsulated PCMs, respectively. According to the above two equations, the encapsulation ratio and encapsulation eﬃciency of the PEMA/octadecane nanocapsule are 89.5% and 88.9%, respectively; and they are 94.7% and 93.6% for the PMMA/octadecane nanocapsule, respectively. It can be found that the thermal conductivity of the nanocapsule is as low as 0.16 W m1 K1 and 0.14 W m1 K1 in Table 1 and this could be caused by their low thermal-conductive core material and shell material. Therefore, heat transfer
Table 1 Thermal properties of the n-octadecane, the PEMA/octadecane nanocapsule and the PMMA/octadecane nanocapsule. Sample name
D Hf (kJ/kg)
Onset Tm (°C)
Oﬀset Tm (°C)
D Hc (kJ/kg)
k (W m1 K1)
N-octadecane PEMA Capsule PMMA Capsule
30.1 32.2 31.9
220.4 198.5 208.7
29.4 29.2 31.1
30.5 33.6 32.3
25.8 29.8 30.2
222.6 197.1 205.9
0.15 Zhang et al. (2010) 0.16 0.14
G.H. Zhang et al. / Solar Energy 86 (2012) 1149–1154
enhancement of the nanocapsule can be improved by using the higher thermal-conductive shell material in the further study due to the thermal conductivity plays a key role in the heat transfer process. The low thermal conductivity of the nanocapsule can be a drawback reducing the release rates of thermal storage during the heating and cooling process. The TGA curves of PEMA, PEMA/octadecane nanocapsules and n-octadecane are shown in Fig. 8. Thermal Gravimetric Analysis (TGA) was calibrated by the zinc. As it can be seen from the Fig. 8, the pure n-octadecane lost its weight in one stage as PEMA/octadecane nanocapsules lost their weight in two stages. There are about 80% of weight loss from 150 °C to 300 °C due to the gasiﬁcation of n-octadecane (Fig. 8c), and about 20% of weight loss from 310 °C to 400 °C due to the decomposition of PEMA shell (Fig. 8a). 4. Conclusions In this work, PEMA/octadecane and PMMA/octadecane nanocapsules with average particle sizes of 140 nm and 119 nm in diameter were synthesized by the direct mini-emulsion polymerization method. According to the FITR spectra, X-ray scattering measurement, SEM image and TEM image, it was conﬁrmed that the polymer shell was successfully fabricated on the surface of the core materials. The temperatures and latent heats of melting and crystallizing of PEMA nanocapsule were determined as 32.2 and 29.8 °C, 198.5 and 197.1 kJ/kg, respectively, by DSC analysis, and the temperatures and latent heats of melting and crystallizing of PMMA nanocapsule were determined as 31.9 and 30.2 °C, 208.7 and 205.9 kJ/kg, respectively. The super-cooling problem reduced dramatically after nanoencapsulation for both PEMA and PMMA nanocapsules. TGA analysis indicated that PEMA/octadecane nanocapsule had good thermal stability. The results also showed that both samples had a much higher encapsulation eﬃciency and encapsulation ratio. Based on the above results, it can be concluded that the prepared nanocapsules have good energy storage potential. Acknowledgments This work is supported by the UK Engineering and Physical Science Research Council (EPSRC Grant No: EP/F061439/1), the National Natural Science Foundation of China (Grant No: 51176110). The authors are thankful to the Birmingham Science City: Energy Eﬃciency and Demand Project (Project Ref: SY/SP8008). References Alay, S., Go¨de, F., Alkan, C., 2010. Preparation and characterization of poly(methylmethacrylate-coglycidyl methacrylate)/n-hexadecane nanocapsules as a ﬁber additive for thermal energy storage. Fibers and Polymers 11, 1089–1093.
Asua, J.M., 2002. Miniemulsion polymerization. Progress in Polymer Science 27, 1283–1346. Black, J.K., Tracy, L.E., Roche, C.P., Henry, P.J., Pesavento, J.B., Adalsteinsson, T., 2010. Phase Transitions of Hexadecane in Poly(alkyl methacrylate) CoreShell Microcapsules. The Journal of Physical Chemistry B 114, 4130–4137. Chang, C.C., Tsai, Y.L., Chiu, J.J., Chen, H., 2009. Preparation of phase change materials microcapsules by using PMMA network-silica hybrid shell via sol–gel process. Journal of Applied Polymer Science 112, 1850–1857. Fang, Y., Kuang, S., Gao, X., Zhang, Z., 2008. Preparation and characterization of novel nanoencapsulated phase change materials. Energy Conversion and Management 49, 3704–3707. Karaipekli, A., SarI, A., Kaygusuz, K., 2007. Thermal conductivity improvement of stearic acid using expanded graphite and carbon ﬁber for energy storage applications. Renewable Energy 32, 2201–2210. Kwon, H., Cheong, I., Kim, J., 2010. Preparation of n-octadecane nanocapsules by using interfacial redox initiation in miniemulsion polymerization. Macromolecular Research 18, 923–926. Li, W., Zhang, X.-X., Wang, X.-C., Niu, J.-J., 2007. Preparation and characterization of microencapsulated phase change material with low remnant formaldehyde content. Materials Chemistry and Physics 106, 437–442. Liang, C., Lingling, X., Hongbo, S., Zhibin, Z., 2009. Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system. Energy Conversion and Management 50, 723–729. Pascu, O., Garcia-Valls, R., Giamberini, M., 2008. Interfacial polymerization of an epoxy resin and carboxylic acids for the synthesis of microcapsules. Polymer International 57, 995–1006. Salau¨n, F., Vroman, I., 2008. Inﬂuence of core materials on thermal properties of melamine-formaldehyde microcapsules. European Polymer Journal 44, 849–860. SarI, A., Alkan, C., Karaipekli, A., Uzun, O., 2009. Microencapsulated noctacosane as phase change material for thermal energy storage. Solar Energy 83, 1757–1763. Yang, R., Xu, H., Zhang, Y., 2003. Preparation, physical property and thermal physical property of phase change microcapsule slurry and phase change emulsion. Solar Energy Materials and Solar Cells 80, 405–416. You, M., Zhang, X., Wang, J., Wang, X., 2009. Polyurethane foam containing microencapsulated phase-change materials with styrene– divinybenzene co-polymer shells. Journal of Materials Science 44, 3141–3147. Yu, F., Chen, Z.-H., Zeng, X.-R., 2009. Preparation, characterization, and thermal properties of microPCMs containing n-dodecanol by using diﬀerent types of styrene-maleic anhydride as emulsiﬁer. Colloid & Polymer Science 287, 549–560. Yuan, L., Gu, A., Liang, G., 2008. Preparation and properties of poly(urea-formaldehyde) microcapsules ﬁlled with epoxy resins. Materials Chemistry and Physics 110, 417–425. Yuan, Y.C., Rong, M.Z., Zhang, M.Q., 2008. Preparation and characterization of microencapsulated polythiol. Polymer 49, 2531–2541. Zhang, H., Wang, X., 2009. Fabrication and performances of microencapsulated phase change materials based on n-octadecane core and resorcinol-modiﬁed melamine-formaldehyde shell. Colloids and Surfaces A: Physicochemical and Engineering Aspects 332, 129–138. Zhang, H., Wang, X., 2009. Synthesis and properties of microencapsulated n-octadecane with polyurea shells containing diﬀerent soft segments for heat energy storage and thermal regulation. Solar Energy Materials and Solar Cells 93, 1366–1376. Zhang, Y., Lin, W., Yang, R., Zhang, Y., Zhang, Q., 2007. Preparation and thermal property of phase change material microcapsules by phase separation. Materials Science Forum, PRICM 6, 2293–2296. Zhang, H., Wang, X., Wu, D., 2010. Silica encapsulation of n-octadecane via sol–gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance. Journal of Colloid and Interface Science 343, 246–255.