Characterization and Performance of Dot/PS
Nanoencapsulated Phase Change Materials as Latent
Functionally Thermal Fluid
5 FANG Yutang, YANG Guo, LIU Hong, GAO Xuenong, ZHANG Zhengguo
(The Key Laboratory of Enhanced Heat Transfer & Energy Conservation, Ministry of
Education,South China University of Technology, GuangZhou 501640)
Abstract: Latent functionally thermal fluid (LFTF) with encapsulated phase change materials (PCMs) is widely used in thermal control, the cooling of electronic equipments, fluidized beds, and other
10 systems that require high heat transfer efficiency. A novel LFTF with nanoencapsulated phase change material (NEPCM) composed of polystyrene (PS) as shell and n-dotriacontane(Dot) as core was synthesized by ultrasonically initiated miniemulsion polymerization. The composition, morphology and the thermal properties of NEPCM were characterized by particle size analyzer, TEM, FT-IR，DSC and
TG. The results showed that the prepared capsules were regularly spherical with average diameter of-115 163.4 nm and latent heat of 158.4kJ kg. The fluid performance showed that the synthesized latex was
of high specific heat capacity, excellent freeze-thaw resistance, mechanical stability and low viscosity, thus it is very suitable for being used as latent functionally thermal fluid.
Keywords: Nanoencapsulated phase change material; Latent functionally thermal fluid; Ultrasonic initiated miniemulsion polymerization
Latent functionally thermal fluid (LFTF) is a special multiphase fluid with encapsulated phase change materials (PCMs, e.g. microencapsulated PCM)) as disperse phase and heat transfer fluid as continuous phase. Compared with conventional single-phase fluid, LFTF has various
25 advantages such as high-density thermal energy storage, high-speed transportation, low flow drag and less heat loss in the pipe transportation, high specific heat capacity and so on. Therefore, LFTF is a promising material for the applications of thermal control, the cooling of electronic
equipments, fluidized beds, and other systems that require high heat transfer efficiency .
There are various preparation methods for the microencapsulated phase change materials
 30 (MEPCMs), such as in-situ polymerization , interface polymerization and coacervation , etc.
The earlier experimental studies mainly concentrated on enhancing heat conduction performance
[5-8]. [9, 10] of the LFTF that contains MEPCM Recent publications based on numerical simulations
showed that laminar convective heat transfer of MEPCM slurry could be enhanced. Alvarado et al  presented thermo-physical property data of MEPCM slurry. They also presented the impact of
35 using enhanced surface tube in combination with MEPCM slurry under constant heat flux and turbulent conditions. Chen et al  studied the behaviors of the convective heat transfer of MEPCM suspension for laminar flow in a circular tube under a constant heat flux. The results showed that the heat transfer enhancement ratio of 15.8 wt% MPCM suspensions can reach 1.42 times of that of water, and the pump consumption of the MPCM suspension system decreased
40 greatly with a larger heat transfer rate compared with water.
However, the performance of MEPCM turned bad after repeated cycling. The large particles of the microencapsulated PCM not only increased the fluid's viscosity and transfer resistance, but also crushed each other during pump delivery process. Therefore, it is necessary to develop
nanoencap -sulated PCM (NEPCM) with smaller particle size than MEPCM.There is limited
Foundations: National High Technology Research and Development Program (No.2009AA05Z203), Research Fund for the Doctoral Program of Higher Education (No.20090172110015)
Brief author introduction:FANG Yutang, (1964-),Male, Professor，Phase change storage materials. E-mail:
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45 progress for producing NEPCM. Zhang et al  synthesized a kind of nanoencapsulated PCM by
in-situ polymerization, in which melamine-formaldehyde resin was used as the shell, n-octadecane
and cyclopean as the core. Similarly, Momoda et al  prepared nanocapsules with arachidic and
trimethlolethane as core and organicsilicon polymer as shell, then dispersed NEPCM in low
viscosity hydrocarbon as fuel cells coolant. Miniemulsion polymerization was a convenient 50 one-step encapsulation technique for preparing nanocapsules. Luo et al studied the
nanoencapsulation of hydrophobic compounds by miniemulsion polymerization, and it was found
that the thermodynamic factors and the kinetic factors, as well as the nucleation modes all had a
great influence on the latex morphology, but the thermo-physical properties of synthesized
, [17,18] nanocapsules were not mentioned. Parket alFang et al prepared polystyrene (PS)
55 nanoparticles containing paraffin wax as PCM using the ultrasonic-assistant miniemulsion
Generally, the miniemulsion polymerization time is more than 4 h, and the initiator residues
could affect the stability of the NEPCMs emulsion. Utilization of the cavitation and the non-linear
acoustic streaming of ultrasonic radiation, it can effectively synthesize core-shell structure
Since it can remove the post-treatment processes (free initiator) and significantly reduce
reaction time，about 35min！, so ultrasonically initiated polymerization is a synthesis process with
high efficient and environmentally friendly features. In this paper, the nanocapsules with
polystyrene as shell and n-dotriacontane as core were synthesized by ultrasonically initiated free 65 radical-catalyzed miniemulsion in-situ polymerization, and the characterization and performance
of LFTF with NEPCM were also discussed.
The monomer styrene (St, AR, from Guangdong Guanghua Chemical Reagent Co. Ltd., 70 China) was firstly washed three times with sodium hydroxide aqueous solution of 10wt%, then
with deionized water before being used. The comonomer acrylonitrile (AN, AR, from Tianjin
Kermel Chemical Reagent Co. Ltd., China) was used as received. n-dotriacontane (Dot, AR, from
Shanghai Pinchun Chemical Reagent Co. Ltd., China) was used as the core material. Sodium
dodecylsulfate (SDS, AR, from Guangdong Xilong Chemical Reagent Co. Ltd., China) and 75 poly-(ethyleneglycol) monooctyl-phenylether (OP-10, AR, from Shanghai Lingfeng Chemical
Reagent Co. Ltd., China) were used as emulsifiers.
1.2 Preparation of NEPCM
Typically, under 70 water bath and magnetic stirring, 10g ? St, 10g Dot ,1g AN and 0.25g
OP-10 were mixed to obtain oily mixture. Aqueous medium was prepared by mixing together 80 200g deionized water and 0.25g SDS. The oily mixture and the aqueous medium were placed into
a 500ml con-shape flask and pre-emulsified by homogenizer (model FJ200-S, Shanghai Specimen
and Model Factory, China) with 6000 RPM for 10 min. Pre-emulsion was transported into a
500ml three-necked flask with draintube, nitrogen inlet and ultrasonic generator ( Model JYD-900,
Shanghai Zhisun Instruments Co., Ltd , China).After removal of the oxygen in the system with 85 nitrogen for 10min, ultrasonically reacted at 75% amplitude for 35min under the steady
temperature of 55 and reflux ? condensation to obtain nanoencapsulated PCM emulsion, then
naturally cooled to room temperature. Demulsification was accomplished by washing the
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emulsion with 10wt% sodium chloride solution. After the crude white solid was washed three times by petroleum ether and deionized water to remove unencapsulated Dot, the target product
90 was obtained under vacuum drying at 50? for 24h.
1.3 Characterization and performance of NEPCM The particle size of NEPCM was measured with NPA150 (Microtrac Co., Ltd, USA) nanoparticle size analyzer. The latex was diluted to 0.01wt% before measurement. The
morphology of the nanocapsule was observed with H-7500(Hitachi Co., Ltd, Japan) transmission
electron microscopy at an accelerating voltage of 80 kV. The latex was diluted to 1wt %, then 95
mounted on carbon-coated copper grids and left dried at room temperature before analysis. The FT-IR spectra of the samples were recorded on TENSON 27(Bruker Co., Ltd, Germany) in wave
number range from 400 cm-1 to 4000cm-1 and using potassium bromide tablet. DSC measurements of the NEPCM and its latex were carried out on Q20 (TA Instrument, USA)
100 differential scanning calorimeter under N2 atmosphere and 5?min-1 heating or cooling rate. The
thermal stabilities of the dried nanocapsules were evaluated using STA 449C thermo gravimetric analyzer (Netzsch Co., Ltd, Germany) under N2 atmosphere and 10?min-1 scanning rate. The
viscosity of the latex was determined using Brookfield DV-?+ rotation viscometer (Brookfield Co., Ltd, USA) with S61 rotor type at 100RPM and temperature range of 25 to 65?. Identical
samples were tested three times and the average was recorded. The resistance freezing-thaw cycle 105 test was completed by putting the emulsion into the breaker and sealing, then transferring it into a high-low temperature constant temperature chamber (Model QA-FC-40, Young Chenn Instrument
Co., Ltd, China) at temperature range from 0 to 80?. The heating and cooling rate was 3?min-1, the constant temperature time was 30min.The mechanical stability of latex was tested by
centrifugation. Put into the centrifuge tube, the latex was centrifuged for 30min at 1500-3000RPM 110 by Model 800 tabletop centrifuger (Suzhou Weier Laboratory Supplies Co., Ltd. China), then filtrated and vacuum-dried at 50? for 24h to obtain solid product. The mechanical stability of latex was evaluated by the ratio (R) of the solid product after centrifugation to the solid content of latex. The bigger the ratio R was, the worse the mechanical stability of latex was.
2 RESULTS AND DISCUSSION115
2.1 Characterization of NEPCM Particle size and morphology
Fig.1 displays the particle size and distribution of the nanoencapsulated PCM. It can be seen that the particle size of NEPCM varied from 50 nm to 300 nm, exhibiting a narrow size
distribution. The Z-average particle size of the nanoencapsulated PCM was 162.4 nm. 120
Fig.2a shows TEM image (Mag.50k) of NEPCM. Fig 2b is the partial enlarged image (Mag.150k).
It can be seen that most of the nanocapsules were regular spherical, and the core of n-dotriacontane (pale part) was located in the shell of polystyrene (dark part). The diameter of the
nanocapsules was about 130~150nm, consistent with the result of the particle size analysis. 125
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Fig. 1 Particle size and distribution of NEPCM
Fig. 2 TEM images of NEPCM 130 FT-IR analysis FT-IR spectra of n-dotriacontane, polystyrene and the nanoencapsulated PCM are displayed in Fig.3. It can be seen from Fig.3a that, the absorption peaks at 2957 cm-1, 1479 cm-1,
correspond to C-H asymmetric stretching vibration and bending vibration of aliphatic methyl, 135
respectively. The absorption peaks at 2919cm-1, 2849 cm-1 and 721 cm-1, were associated with C-H asymmetric, symmetry stretching vibration and C-H in-plane rocking vibration of aliphatic
methylene, respectively. From Fig.3b it can be seen that the absorption peaks at 3067, 3026 cm-1 were associated with the aromatic C–H stretching vibration, the absorption peaks at 1598, 1493
cm-1 were associated with benzene ring C=C stretching vibration, the absorption peak at 698 140
cm-1 was benzene ring deformation vibration, and the absorption peak at 2240 cm-1 was ?N symmetry stretching vibration of cyano-group, which indicated that the associated with C copolymerization took place between styrene and acrylonitrile. All the above
145 Fig. 3 FTIR spectra of Dot (a), PS (b) and NEPCM(c)
characteristic peaks of n-dotriacontane and polystyrene could be observed in the FT-IR spectra of the nanoencapsulated PCM (Fig.3c), and no new peak was observed. The above
analysis results show that the PS is only the carrier to prevent Dot leaking out of capsule shell in 150
the melting process. Thermal performance
Fig. 4 displays the DSC curves of n-dotriacontane and the nanoencapsulated PCM. It can be seen that, the phase change temperature of the nanoencapsulated PCM was very close to that of
n-dotriacontane, suggesting that the thermal property of the nanoencapsulated PCM was similar to 155
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-1, less its core of n-dotriacontane. The latent heat of the nanoencapsulated PCM was 158.4 kJkg than that of pure n-dotriacontane (288.5 kJ kg-1) due to the existence of the polystyrene shell.
The TG and derivative thermogravimetric (DTG) curves of n-dotriacontane, polystyrene and the nanoencapsulated PCM are displayed in Fig. 6. It can be seen that the weight losses of Dot and
160 ntrated on one stage: 98.01% from 177.7? to 383.7?(Fig.6a) due to thePS were mainly conce
gasification of alkane, and 96.01% from 345.9? to 485.7? (Fig.6b) due to the oxidative decomposition of polystyrene, while the weight loss of the nanoencapsulated PCM were mainly
consisted of two stages: 48.44% from 200.5? to 403.2?,corresponding to the gasification of n-dotriacontane, and 33.37% from 410.7 ? to 495.7 ?, corresponding to the decomposition of
polystyrene. Since the\ decomposi-tion temperature of PS is obviously higher than the gasification 165 temperature of Dot, the Dot core is well protected by the PS shell during the melting process, and the thermal stability of encapsulated PCM is enhanced.
Fig. 4 DSC curves of Dot (a) and NEPCM (b) 170 Fig. 5 TG and DTG curves of Dot (a), PS (b) and NEPCM(c)
2.2 Performance of NEPCM as Latent Functionally Thermal Fluid Specific heat capacity 175 Fig.6 shows the specific heat capacity with different latex concentration determined from DSC during the latex heating process. When the temperature was lower than 60 ?, the n-dotriacontane inside the capsule was in the solid state, the specific heat capacities with different latex concentration had an approximately constant value. When the temperature rose to 60-70?,
due to the melting of the n-dotriacontane and the generating of melting heat, the specific heat 180 capacity of the latex rapidly increased with temperature ascension. The maximum specific heat -1-1 capacities were 6.08, 4.89 and 4.54 kJ kgKwith the latex concentration of 10%, 5.0% and 2.5wt%, respectively. Their values were about 1.46, 1.17 and 1.09 times of that of the latex during
the n-dotriacontane in the solid state, respectively. When the temperature was higher than 70?, the n-dotriacontane inside the capsule was in completed liquid state, the specific heat capacity
185 declined to a constant value. It indicates that the latex with NEPCM has high heat capacity.
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Fig. 7 Specific heat capacity with different latex concentration 190
Viscosity The latex viscosity with different concentration is shown in Fig.7. The slurry viscosity increased slowly with the increase of the latex percentage. Considering the viscosity of deionized
water at 30? (0.99cP), it was found that even at a relatively high latex concentration (10wt %), the slurry viscosity was still relatively low (2.23 cP). The viscosity of slurry decreased with the 195 increase of temperature.
Moreover, the higher the latex concentration was, the bigger the fluctuation of the viscosity along with the temperature change was. For example, considering the phase change process, as
10wt% latex concentration at 70?, the slurry viscosity was only 1.45cP, about 1.69 times of that of pure water. 200
Therefore, the latex is suitable for being used as latent functionally thermal fluid. Fig. 7 Viscosity with different latex concentration Stability of latex 205 The resistance freezing-thaw cycle test of latex was measured. It was found that, after 50
cycles, neither the concentrated layer at the superior part, nor the deposit in the bottom appeared, namely, no sedimentation nor creaming took place. The solid product by treating the latex with
freezing-thaw cycle, demulsification, filtration and desiccation, was characterized by DSC. The results showed that the phase change temperature was almost the same as the untreated NEPCM, 210 -1 and the latent heat was about 145.5kJkg, with only little decline. Therefore, the emulsion displays considerable stability of the resistance freezing-thaw.
The mechanical stability was tested by centrifugation. The experimental results were summarized in Table 1. As shown in Table 1, after centrifugation at 1500RPM, no solid sediment produced, when the speed of rotation ascended to 3000RPM, the precipitate amount was 2.17mg, 215
and the calculated R value (the ratio of the solid product obtained by centrifugation and the one by
demulsification for original emulsion) was only 0.0385%. Therefore, the synthesized emulsion
displays considerable mechanical stability, no nanocapsule ruptures.
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Table.1 The mechanical stability of original emulsion Speed of rotation Mass of precipitation Latex before Latex after R* /RPM /mg centrifugation centrifugation / % no creaming no creaming, 1500 0 0 no sedimentation no sedimentation no creaming little 3000 2.17 0.0386 no sedimentation sedimentation
*R denotes the ratio of the solid product obtained by centrifugation and the one by demulsification for original emulsion
225 3 Conclusions New environment-friendly energy-saving materials, latent functionally thermal fluid with nanoencapsulated phase change material (NEPCM) composed of polystyrene (PS) as the shell and
n-dotriacontane (Dot) as the core was synthesized by the ultrasonically initiated free radical-catalyzed miniemulsion polymerization. It has stable structure and broad application
prospects. The detection methods above effectively characterized the particle size, morphology, 230
composition and performance, providing a better condition for the further development of NEPCMs.
Particle size analysis indicated that the particle size and distribution of the prepared nanocapsules was narrow with 162.4 nm in the Z-average diameter. SEM revealed that the
nanocapsules had perfect regular spherical. All the characteristic peaks of n-dotriacontane and 235
polystyrene were observed in the FT-IR spectrum of the nanoencapsulated PCM. DSC analysis showed that the phase change latent heat of NEPCMs was above 158.4kJ kg-1, and the latex
specific heat capacity was obviously increased during the phase transition process, with a peak at -1-1 6.08 kJ kgKunder the latex concentration of 10wt%. Moreover, the stability and viscosity
testing of synthesized latex showed that this fluid had excellent resistance freeze-thaw and 240 mechanical stability and had low viscosity (2.23 cP at 30? and 10wt% latex concentration), thus
it is very suitable for being used as latent functionally thermal fluid. References  MONICA D, ANA L, JAVIER M, BELÉN Z. Review on phase change material emulsions and 245 microencapsulated phase change material slurries: Materials, heat transfer studies and applications [J]. Renew. Sust. Energ Rev., 2012, 16 (1) :253-273  SARIER N, ONDER E. The manufacture of microencapsulated phase change materials suitable for the design of thermally enhanced fabrics[J]. Thermochimica Acta, 2007, 452(2):149-160  CHO J S, KWON A, CHO C G. Microencapsulation of octadecane as a phase-change material by interfacial 250 polymerization in an emulsion system[J].Colloid Polym. Sci., 2006, 280(3): 260-266.  OZONUR Y, MAZMAN M, PAKSOY H O and EVLIYA H. Microencapsulation of coco fatty acid mixture for thermal energy storage with phase change material [J]. Int. J. Energy Res., 2006, 30(10): 741-749  KASZA K E, CHEN M M, Improvement of the performance of solar energy or waste heat utilization systems by using phase-change slurry as an enhanced heat-transfer storage fluid [J]. J. Sol. Energy Eng., 1985, 255 107(3):229-236  COLVIN D P, MULLIGAN J C, BRYANT Y G. Enhanced heat transport in environmental systems using microencapsulated phase change materials [J]. S.A.E. Technical Paper Series,1992,1-9  YANG R, XU H, ZHANG Y. Preparation, physical property and thermal physical property of phase change microcapsule slurry and phase change emulsion[J]. Sol. Energy Mater. Sol. Cells, 2003, 80(4):405-416. 260  ALVARADO J L, MARSH C, SOHN C and PHETTEPLACE G. Phetteplace, T.Newell. Thermal performance of microencapsulated phase change material slurry in turbulent flow under constant heat flux[J]. Int J Heat Mass Transfer, 2007, 50(9-10): 1938-1952  ZHAO Z, HAO R, SHI Y. Parametric analysis of enhanced heat transfer for laminar flow of microencapsulated phase change suspension in a circular tube with constant wall temperature [J]. Heat Transfer Eng., 2008, 29(1): 265 97-106.
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280 Dextran-methylprednisolonesuccinate as a prodrug of methylprednisolone: in vitro immunosuppressive effects on rat blood and spleen lymphocytes [J]. Int. J. Pharm., 2000, 208(1-2):71-76  PARK S J, KIM K S, HONG S K. Preparation and thermal properties of polystyrene nanoparticles containing phase change materials as thermal storage medium[J]. Polymer-Korea. 2000, 29(1):8-13  FANG Y T , Kuang S Y, Gao X N, Zhang Z G. Preparation and characterization of novel nanoencapsulated 285 phase change materials[J]. Energy Conver Manag., 2008,49 (8): 3704-3707
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latent functionally thermal fluid[J]. J Phy D-Appl Phys, 2009, 42 (3): 035407, 7  CHOU J H C, STOFFER J, Ultrasonically initiated Free radical-catalyzed emulsion polymerization of methyl methacrylate (I)[J]. J. Appl. Polym. Sci.,1999,72 (6) :797–825 290 潜热型功能热流体用三十二烷/聚苯乙烯纳米胶囊相
变材料的表征及性能 方玉堂，杨果，刘洪，高学农，张正国 ，华南理工大学传热强化与过程节能教育部重点实验室，广州 501640！ 摘要(含 胶囊化相变材料的潜热型功能热流体，LFTF！广泛应用于热控制、电子设备冷却、 流化床以 及其它需要高热传输效率的系统。本文介绍一种新型的纳米相变胶囊功能热流体， 该胶囊以 正三十二烷，Dot！为芯材，聚苯乙烯，PS！为壁材，采用超声辐射细乳液聚合法 制备。纳295 米相变胶囊的组成、大小、形态及热性能采用纳米粒度分析仪、透射电子显微镜、 傅里叶变 换红外、差示扫描量热、热失重等进行表征。流体的比热容、抗高低温性能、机械 性能及粘 度等性能进行测试，结果显示(制备的纳米胶囊为规整球形，其平均粒径为 163.4 nm，相热 -1焓达到 158.4kJ kg，合成乳液在相变范围内具有高的比热容、优异的抗高低温性 能和机械 稳定性，较低的粘度，适合做潜热型功能热流体。 关键词(纳米相变胶囊；潜热型功能热流
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