Preparation and Properties of Microencapsulated Phase Change

Article pubs.acs.org/IECR

Preparation and Properties of Microencapsulated Phase Change Materials Containing Two-Phase Core Materials He Wang, Jianping P. Wang,* Xuechen Wang, Wei Li, and Xingxiang Zhang Tianjin Municipal Key Lab of Modification and Functional Fibers, Tianjin Polytechnic University, Tianjin 300387, People’s Republic of China ABSTRACT: Microencapsulated phase change materials (MicroPCMs) containing two-phase core materials, in which polypyrrole (PPy) particles were homodispersed in n-octadecane (n-Oct), were synthesized by two-step polymerization technique using poly(methyl methacrylate-co-allyl methacrylate) as shell. The surface morphologies, phase change properties, and thermal stabilities of the microcapsules were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), respectively. The results show the two-phase core materials are well encapsulated in the presence of the emulsifier sodium salt of styrene−maleic anhydride copolymer. The average diameter of the microcapsules is about 35 μm. The crystallization temperature of MicroPCMs is higher than that of MicroPCMs without PPy. At the same time, the enthalpy of the heterogeneous nucleation significantly increased with increasing of the PPy concentration. Microcapsules with 4−14 wt % PPy in the core material were free from obvious supercooling, and PPy had no influence on the morphology, the particle distribution, or the thermal resistant temperature of microcapsules.

1. INTRODUCTION Microencapsulated phase change materials (MicroPCMs) were studied in the late 1970s after phase change materials (PCMs) had long been employed as thermal storage and control materials.1 PCMs, such as water, n-octadecane (n-Oct), poly(ethylene glycol), butyl stearate, and calcium chloride hexahydrate, can absorb or release 150−334 J g−1 of latent heat2 without producing any wastes.3 There are many pieces of literature that have concentrated on MicroPCMs. A literature survey indicates that melamine−formaldehyde resin,4 urea formaldehyde resin,5 polyurethane,6 high-density polyethylene,7−9 styrene-butadiene-styrene copolymer,10 and poly(methyl methacrylate)11,12 are usually selected as microcapsule shell materials for the PCMs protection. Microencapsulation turns PCMs into impermeable solids for various applications. MicroPCMs have been widely investigated for use in the manufacture of building materials,2 thermo-regulated fibers, coatings,13 and foams;14 to date, MicroPCMs have been used as wallboard, ceiling material, and insulation in energy-saving building.6 Although MicroPCMs are widely used in many fields, supercooling is still an important obstacle to the industrial application of MicroPCMs. Supercooling is one of the common features of PCMs whenever they undergo a phase change from liquid to solid. A higher degree of supercooling has an adverse effect on thermal performance when PCMs crystallize from liquid to solid, as energy releasing would be retarded during the process. Therefore, thermal regulation functions cannot be fully utilized. Yamagishi selected a PCM with 2 wt % 1-tetradecanol as a nucleating agent to prevent supercooling of n-tetradecane in MicroPCMs (microcapsule diameters 110−300 μm).15 Lee considered n-alkane derivatives, such as 1-octadecylamine and 1-octadecanol, suitable for preventing PCMs from supercooling and used the appropriate range of about 1−6 wt % with respect to the weight of PCMs.16 Fan and others found microcapsules with approximately 20 wt % paraffin in core materials were free © 2013 American Chemical Society

from supercooling, and paraffin had no influence on the morphology and dispersibility of microcapsules.17 Zhang prepared melamine−formaldehyde MicroPCMs with 9% 1octadecanol and 91% n-octadecane as core materials to restrain supercooling crystallization behavior,18 but it requires lowering the enthalpy of MicroPCMs as the price. Song fabricated melamine−formaldehyde MicroPCMs with n-octadecane containing 3% silver nanoparticles as the core materials, causing its degree of supercooling to be reduced by 2−3 °C.19 Further, Xu and his colleagues20 prepared MicroPCMs containing twophase core materials,21,22 adding 1% multiwall carbon nanotubes into 1-bromooctadecane, which caused the degree of supercooling of MicroPCMs to be reduced by 6.8 °C. As can be seen from these studies, to add higher melting point organic compounds (higher than PCMs) or a heterogeneous nucleation agent in PCMs can effectively suppress the supercooling behavior of MicroPCMs. However, to prepare MicroPCMs containing two-phase core materials, many questionssuch as the homodisperse of heterogeneous nucleation agent in the PCMs, the migrating of the heterogeneous nucleation agent from oil droplets internal to water phase in the emulsification, the influence of the content of heterogeneous nucleation agent on the morphologies, the wall thickness of MicroPCMs, and so onmust be considered. To help solve some of these problems, we suggested a method that could restrain the supercooling of MicroPCMs by generating a heterogeneous nucleation agent in MicroPCMs by in situ polymerization in this paper. We prepared MicroPCMs containing the two-phase core materials, polypyrrole (PPy) homodispersed in n-Oct, with cross-linked poly(methyl Received: Revised: Accepted: Published: 14706

April 3, 2013 July 3, 2013 September 10, 2013 October 7, 2013 dx.doi.org/10.1021/ie401055r | Ind. Eng. Chem. Res. 2013, 52, 14706−14712

Industrial & Engineering Chemistry Research

Article

suspension was dropped on a stainless steel SEM stub and airdried overnight. Then the samples were gold-coated. 2.3.2. Fourier Transform Infrared Spectroscopy (FTIR). FTIR of MicroPCMs and PPy were obtained using Fourier transform infrared spectroscopy (FTIR, Bruker Uecior-22, the wavenumber of 4000−400 cm−1) at room temperature. 2.3.3. Measurement of Microcapsules Diameter. The diameter distribution MicroPCMs was examined by using a diameter distribution analyzer (HORIBA LA-300). 2.3.4. Differential Scanning Calorimetry (DSC). Thermal properties of MicroPCMs were measured using a differential scanning calorimeter (DSC, Perkin-Elmer, DSC-7) in the range of −20 to 80 °C with a heating or cooling rate of 10 °C min−1 in a nitrogen atmosphere. The encapsulation ratio and efficiency, as two important parameters, are used to describe the phase change properties of microencapsulated n-Oct, which were determined by analysis of the DSC thermograms. The encapsulation ratios can be calculated by eq 1 on the basis of the fusion heat obtained from the DSC analysis

methacrylate) as the shell by a two-step polymerization technique. Some key parameters, involving morphologies, size distribution, phase change properties, and the thermostability of MicroPCMs with the presence of different concentrations of PPy, were investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. Pyrrole (Py) was obtained from SigmaAldrich. Benzoyl peroxide (BPO, purity 99 wt %, Tianjin Chemical Reagent) was used as an initiator and oxidizer. Azodiisobutyronitrile (AIBN, purity 99 wt %, Tianjin Guangfu Chemical Reagent) was used as an initiator. n-Octadecane (nOct, purity 95 wt %, Union Lab. Supplies Limited, Hong Kong) was used as core material. Methyl methacrylate (MMA, purity 99.5 wt %, Tianjin Kermel Chemical Reagent) were used as shell-forming monomers. MMA was purified by vacuum distillation, washed with a 5 wt % sodium hydroxide aqueous solution to remove the inhibitor bisphenol A, and dried in CaCl2 overnight. Allyl methacrylate (AMA, Tianjin Chemical Reagent Research Institute) was used as a cross-linking agent. The sodium salt of styrene−maleic anhydride copolymer (SMA, 19% of the aqueous solution, Shanghai Leather Chemical Plant) was used as an emulsifier. 2.2. Fabrication of MicroPCMs Containing Two-Phase Core Materials. Cross-linked PMMA MicroPCMs containing PPy homodispersed in n-Oct were fabricated by a two-step method, free radical polymerization and in situ polymerization. In the first polymerization, microencapsulation was carried out in a 250 mL three-necked round-bottom flask equipped with a mechanical stirrer, a reflux condenser, and a nitrogen gas inlet tube. The flask was put in a water thermostat bath. The polymerization consisted of an oil phase and a water phase. The oil phase formula is presented in Table 1. Specific reaction steps

R=

shell

E=

AMA (g)

n-Oct (g)

AIBN (g)

BPO (g)

Py (g)

S1 S2 S3 S4 S5

10 10 10 10 10

1.0 1.0 1.0 1.0 1.0

12 12 12 12 12

0.15 0.15 0.15 0.30 0.30

0.15 0.15 0.15 0.30 0.30

0 0.5 1.0 1.5 2.0

× 100% (1)

ΔHm,MicroPCMs + ΔHc,MicroPCMs ΔHm,PCM + ΔHc,PCM

× 100% (2)

where: ΔHm,MicroPCMs and ΔHc,MicroPCMs are the enthalpies of the microencapsulated n-Oct fusion and crystallization, respectively; ΔHm,PCM and ΔHc,PCM are the enthalpies of pure n-Oct fusion and crystallization, respectively; R and E are the encapsulation ratio and the encapsulation efficiency, respectively. 2.3.5. Thermogravimetric Analysis (TGA). The thermal resistance of MicroPCMs was investigated by using a thermogravimetric analyzer (TGA, NETZSCH STA 409 PC/ PG TG-DT) at a scanning rate of 10 °C min−1 in the range of 25−600 °C in a nitrogen atmosphere.

core

MMA (g)

ΔHm,PCM

Furthermore, the encapsulation efficiency can be calculated by eq 2

Table 1. Oil Phase Formula sample

ΔHm,MicroPCMs

3. RESULTS AND DISCUSSION 3.1. Morphology of MicroPCMs. Figure 1 show scanning electron micrograph of the microcapsules in which a, b, c, d, and e represent samples 1−5, respectively. The resulting core− shell microcapsules have relatively uniform sizes and a regular spherical shape. There are many dimples on the microcapsules all of samples. The reason can be explained by the production of reserved expansion space in the preparation. First, the density of monomer was smaller than that of the polymer shell P(MMA-co-AMA) and greater expansion space would be produced in the reaction because of the density change. We know that the density of the monomer MMA and AMA is about 0.94 g/cm3; that of the generated polymer shell P(MMA-co-AMA) is about 1.18 g/cm3. In the early stage of the first polymerization, the microcapsules’ embryonic form was forming (as shown in Scheme 1). If we ignore the consumption of monomers, the volume of W (g) monomers in the oil drop will be calculated by a formula: V = W/ρ, in which V, W and ρ represent the volume, mass, and density of the monomer, respectively. Then the volume of monomers Vm can be written as Vm = 0.94/W (cm3). In like manner, the volumes (VP) of polymer P(MMA-co-AMA) in the first polymerization can be written as VP = 1.18/W (cm3). The change rate of the volume

are as follows: SMA (10 g) was dissolved in 200 g of distilled water to form a water phase. The water and oil phase was placed in a water bath at 50 °C and preheated at least 10 min. Then the oil phase was added to the water phase, and the mixture was emulsified mechanically with a stirring speed of 4000 rpm to form an oil/water (O/W) emulsion in a 250 mL reactor equipped with circulating cooling water. The reaction continued with a stirring speed of 200 rpm for 120 min at 68 °C and MicroPCMs were obtained. In the second polymerization, the temperature was increased to 75 °C and kept for 2 h at a stirring rate of 200 rpm. The resultant microcapsules were filtered and washed with distilled water at 60 °C twice to remove the remaining SMA and then dried in an oven at 100 °C. The procedure of the two-step polymerization is shown in Scheme 1. 2.3. Characterization of MicroPCMs. 2.3.1. Scanning Electronic Microscope (SEM). The surface morphologies of MicroPCMs were examined by using a scanning electronic microscope (SEM, Quanta-200). A drop of the microcapsule 14707

dx.doi.org/10.1021/ie401055r | Ind. Eng. Chem. Res. 2013, 52, 14706−14712


Article

Scheme 1. Schematic Diagram for the Fabrication of MicroPCMs Containing Two-Phase Core Materials

respectively. The peak at 1470 cm−1, which is associated with C−H bending, is characteristic for n-Oct. None of these specific peaks are watched in the spectrum of the polymer shell, however. In addition, some of the specific peaks are only found in the spectra of the polymer shell and MicroPCMs. For instance, the peak at 1730 cm−1 is assigned to the carbonyl group. The peak at 1220 cm−1 is assigned to the C−O stretching of an ester group in MMA. The FTIR transmission spectrum of PPy nanoparticles exhibited characteristic vibration bands at 1531, 1480, 1469 cm−1 for Py ring stretching, 1458 cm−1 for conjugated C−N stretching, and 781 cm−1 for C−H wagging vibrations.24,25 None of these specific peaks are observed in the spectrum of the MicroPCMs containing twophase core materials, however. These cases indicate that n-Oct and PPy as core materials have been successfully encapsulated by the shell of P(MMA-co-AMA). 3.3. Diameter Distribution of MicroPCMs Containing Two-Phase Core Materials. The overall distribution of the size of the microcapsules plays an essential role in establishing the surface area in which contents are unconfined. As the microcapsules originate from the solution, various sizes of spherical capsules will be developed. The size distribution curves for MicroPCM samples S2−4 are shown in Figure 3. All the preparation conditions were based on the stirring rates and were set at 4000 rpm for about 10 min during the emulsion. From Figure 3, three samples have similar curve profiles and particle sizes. Their size distribution is considerably wide, which covers approximately the range from 1 to 200 μm. The mean diameter is about 35 μm for all the samples. There are smaller

(ΔV%) can be calculated by formula ΔV% = ((VP − Vm)/Vm) × 100, that is, ΔV% = ((1.18/W − 0.94/W)/(0.94/W)) × 100 = 25.5%. From these calculations, it can be seen that the volume of the first polymerization reduced 25.5% compared to that of the O/W drop. Second, n-Oct would shrink from melting state to crystal state during cooling. Therefore, these large dimples are mainly attributed to the shrinkage of the expansion space.23 From Figure 1, MicroPCMs (S1) and those containing the two-phase core materials (S2, S3, S4, and S5) not only are consistent in diameter but also had similar surface morphologies. It is clear that the surface morphology of MicroPCMs does not depend on whether Py was added or not. We also discovered that the system will keep a cream color, which is unchanged in the overall first step of the polyreaction. When raising the temperature to 75 °C, which means that the reaction has entered the second polymerization stage, the system changes color from cream to a cinereous color. Optical microscope results show that color changes originate in the internal space of the microcapsules. This indicates that PPy has been generated within the microcapsules. 3.2. Infrared Spectrum of MicroPCMs. The FTIR spectra of dried samples for MicroPCMs, those containing two-phase core materials (S2), n-Oct, P(MMA-co-AMA), and PPy, are shown in Figure 2. Multiple strong absorption peaks located at approximately 2848−2916 and 717 cm−1 in the spectra of n-Oct and the microcapsules are associated with the aliphatic C−H stretching vibration and the in-plane rocking vibration of the CH2 group, 14708



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Figure 1. SEM micrographs of MicroPCMs. (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5.

Figure 2. FTIR spectra of (a) S2, (b) S1, (c) n-Oct, (d) P(MMA-coAMA), and (e) PPy.

Figure 3. Diameter distribution curves for MicroPCMs (a) S2, (b) S3, and (c) S4.

differences in the particle size distribution and the average particle diameter for three samples. This may be because of the instability of the emulsifying conditions such as a lower shearing rate, a shorter time, types of emulsifiers, and using

different container.26 It shows that the addition of the Py will not cause too much impact on the particle size of the microcapsules. 14709



Article

3.4. Thermal Storage Capacity, Encapsulation Ratio and Efficiency, and Phase Change Behavior of MicroPCMs. The efficiency of PCMs is dependent on the encapsulated quantity and energy storage capacity per unit mass during its melting and solidifying. The encapsulation ratio and efficiencies, shown in Table 2, were calculated with the Table 2. Heat Absorbing and Evolving Properties of MicroPCMs S1 α

Tc1 (°C) ΔHc1 (J/g) β Tc2 (°C) ΔHc2 (J/g) γ Tc3 (°C) ΔHc3 (J/g) Tm (°C) ΔHm (J/g) encapsulation ratio (%) encapsulation efficiency (%) percentage of PPy in core (%) (Aγ/ (Aα+Aβ+Aγ)) × 100% (%) Avrami exponent

n-Oct

S2

S3

S4

S5

10.6 37.3

11.7 3.3

11.9 3.4

11.7 7.6

10.5 26.3

20

17.6 4.8

19.1 2.9

20 1.4

17.3 2.4

27.1 12.7

22.7 28.3

22.8 32.1

22.8 20.2

22.8 26.6

26 222

27.8 68.5 30.9

28.2 118.3 53.3

27.9 119.1 53.6

28.2 141.4 63.7

28.4 120.4 54.2

31 222

26.7

34.8

35.5

38.4

39.6

0

3.9

7.5

10.6

13.7

25.4

77.7

83.6

69.2

48.1

1.7

3.1

1.2

1.1

1.2, 5.8

Figure 4. DSC curves for MicroPCMs (a0) PPy, (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, and (f) n-Oct. 100

liquid−rotator transition, the peak β is attributed to rotator− crystal transition, and peak α is attributed to a homogeneously nucleated liquid−crystal transition.29 For MicroPCMs without PPy S1, the number of nuclei inside MicroPCMs is rarely because of the decrease of the MicroPCMs size. As a result, the exothermic peak γ, the heterogeneously nucleated liquid− rotator transition, has a lower temperature and a smaller peak area. Contrarily, there are a number of nuclei inside the MicroPCMs containing two-phase core materials because of the PPy particles generated by the second polymerization. When increasing the PPy concentrations, the peak γ shifts to the higher temperature and the integrated area of the peak γ increases compared with that of S1. Simultaneously, it is obvious that the integrated area of the peak α decreases. A ratio of the exothermic peak γ to the total exothermic peak increases from 25.4% for S1 to 83.6% for S3, which is a maximum when the percentage of PPy in the core is 7.5%. It then reduces to 48.1% for S5. To investigate the nucleation mechanism of microencapsulated n-Oct, we also studied the crystallization kinetics of MicroPCMs. The Avrami exponents of all the samples calculated by using the crystallization exotherms in Figure 4 according to the literature30 are listed in Table 2. After microencapsulating, Avrami exponents are 1.2 or 5.8, 1.7, 3.1, 1.2, and 1.1 for S1, S2, S3, S4, and S5, respectively. From the values, it could be speculated that the crystallization of the nOct and the correlated microcapsules is associated with heterogeneous nucleation. It is clear that there is a typical heterogeneous nucleation process for S3. Some lower Avrami exponents shown in Table 2 are attributed to the inhomogeneous distribution of nuclei.31 These facts indicate that PPy, as the nucleating agent, promotes the heterogeneous nucleation of n-Oct. 3.5. Thermal Stability of MicroPCMs. TGA curves of nOct and microencapsulated n-Oct are presented in Figure 5. The weight of n-Oct and microcapsules decreased with

0.2

*

Tc1: α crystallization point; Tc2: β crystallization point; Tc3: γ crystallization point; ΔHc1: α crystallization enthalpy; ΔHc2: β crystallization point; ΔHc3: γ crystallization point; Tm: melting point; ΔHm: melting enthalpy; Aα, Aβ, and Aγ are areas the endothermic peaks α, β, and γ, respectively.

above-mentioned eqs 1 and 2 on the basis of the enthalpy values measured by the DSC. From the data of the encapsulation ratio and efficiency, the encapsulation ability is good for the two-step polymerization due to its mechanism and strong ability for the MMA-AMA to integrate into the polymer shells. The melting and crystallizing properties of n-Oct and microcapsules with and without PPy at a heating and cooling rate of 10.0 °C min−1 are presented in Table 2, too. The DSC curves of PPy, n-Oct, and microcapsules are shown in Figure 4. It is observed that the melting peak temperatures of all the microcapsules are lower than that of n-Oct, which may be attributed to the fact that MicroPCMs have bigger specific surface areas than that of n-Oct. There are two peaks (α, γ) and three peaks (α, β, γ) on the DSC cooling curve of the microcapsules for S1 and S2, S3, S4, and S5, respectively, but there is only one peak on that of n-Oct. In comparison, PPy has none of them. A similar behavior has been described in a similar MicroPCMs systems27,28 in which the supercooling phenomena occur in MicroPCMs that are smaller than 100 μm in diameter. PPy has no effect on the endothermic peak but drastically effects the exothermic peaks as compared to the control. Multiple exothermic peaks due to different nucleation mechanisms are shown in the DSC cooling curves. On the basis of Yamagishi’s results,15 the exothermic peak γ on the DSC cooling curve is likely the heterogeneously nucleated 14710



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REFERENCES

(1) Colvin, D. P. Advances in Heat and Mass Transfer in Biotechnology, HTD-Vol. 363/BED-Vol. 44; ASME: New York, 1999; pp 199. (2) Khudhair, M.; Farid, M. M. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Convers. Manag. 2004, 45, 263−275. (3) Chaurasia, P. B. L. Solar Energy Thermal Storage System Based on Encapsulated Phase Change Material. Res. Ind. 1981, 26, 159−161. (4) Su, J. F.; Wang, L. X.; Ren, L. Preparation and characterization of double-MF shell microPCMs used in building materials. J. Appl. Polym. Sci. 2005, 97, 1755−62. (5) Tseng, Y. H.; Fang, M. H.; Tsai, P. S.; Yang, Y. M. Preparation of microencapsulated phase-change materials (MCPCMs) by means of interfacial polycondensation. J. Microencapsulation 2005, 22, 37−46. (6) Kim, E. Y.; Kim, H. D. Preparation and properties of microencapsulated octadecane with waterborne polyurethane. J. Appl. Polym. Sci. 2005, 96, 1596−604. (7) Cai, Y. B.; Song, L.; He, Q. L. Preparation, thermal and flammability properties of a novel form-stable phase change materials based on high density polyethylene/poly(ethylene-co-vinyl acetate)/ organophilic montmorillonite nanocomposites/paraffin compounds. Energy Convers. Manag. 2008, 49, 2055−62. (8) Cai, Y. B.; Wei, Q. F.; Huang, F. L. Thermal stability, latent heat and flame retardant properties of the thermal energy storage phase change materials based on paraffin/high density polyethylene composites. Renewable Energy 2009, 34, 2117−23. (9) Xiao, M.; Feng, B.; Gong, K. C. Preparation and performance of shape stabilized phase change thermal storage materials with high thermal conductivity. Energy Convers. Manag. 2002, 43, 103−8. (10) Zhang, Y. P.; Lin, K. P.; Yang, R. Preparation, thermal performance and application of shape-stabilized PCM in energy efficient buildings. Energy Build. 2006, 38, 1262−9. (11) Alkan, C.; Sari, A. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Sol. Energy 2008, 82, 118−24. (12) Wang, L. J.; Meng, D. Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage. Appl. Energy 2010, 87, 2660−5. (13) Bryant Y. G.; Colvin D. P. Fiber with reversible enhanced thermal storage properties and fabrics made therefrom. U. S. Patent 4,756,958, July 12, 1988. (14) You, M.; Zhang, X. X.; Li, W.; Wang, X. C. Effects of MicroPCMs on the fabrication of MicroPCMs/polyurethane composite foams. Thermochim. Acta 2008, 472, 20−24. (15) Yamagishi, Y.; Tomohisa, S.; Takashi, I. An evaluation of microencapsulated PCM for use in cold energy transportation medium. Proc. Intersoc. Energy Convers. Eng. Conf. 1996, 547−564. (16) Lee W. M.; Song, H. C. Microcapsule containing phase change material and article having same. WO Patent 2002/053370 A1, July 11, 2002. (17) Fan, Y. F.; Zhang, X. X.; Wang, X. C.; Li, J.; Zhu, Q. B. Supercooling prevention of microencapsulated phase change material. Thermochim. Acta 2004, 413, 1−6. (18) Zhang, X. X.; Fan, Y. F.; Tao, X. M.; Yick, K. L. Crystallization and prevention of super cooling of microencapsulated n-alkanes. J. Colloid Interface Sci. 2005, 281, 299−306. (19) Song, Q. W.; Li, Y.; Yuen, M. Supercooling of Silver Nano Composite PCM Microcapsules. J. Fiber Bioeng. Inf. 2008, 1, 239−248. (20) Xu, D. B.; Song, Q. W.; Wang, J. M. Super-cooling of phase change microcapsules in corporate with carbon nanotube. J. Tianjin Polytech. Univ. 2011, 30, 15−18. (21) Wang, J. P.; Zhao, X. P.; Guo, H. L.; Zheng, Q. Preparation of Microcapsules Containing Two-Phase Core Materials. Langmuir 2004, 20, 10845−10850. (22) Wang, J. P.; Zhao, X. P.; Wang, D. W. Preparation of nanocapsules containing the two-phase core materials. J. Microencapsulation 2007, 24, 757−766. (23) You, M.; Zhang, X. X.; Wang, J. P.; Wang, X. C. Polyurethane foam containing microencapsulated phase-change materials with

Figure 5. TGA curves for n-Oct and samples of MicroPCMs.

increasing temperature. The n-Oct began to lose weight at approximately 134.3 °C and lost weight completely at approximately 210 °C. The boiling point of n-Oct is 308 °C;32 however, n-Oct evaporated before boiling. By contrast, the weight loss rate of samples S1, S2, S3, S4, and S5 are obviously lower than that of the bulk n-Oct. The P(MMA-coAMA) shell protected n-Oct from losing weight quickly. Further, the thermal resistant temperature (starting point T0.5) of samples S1, S2, S3, S4, and S5 are 191.7, 185.9, 187.6, 194.3 and 187.8 °C, respectively. With increasing PPy, the thermal resistant temperature of the microcapsules changes very little. This shows that adding PPy had no effect on the thermal resistant temperature of MicroPCMs.

4. CONCLUSIONS In this paper, P(MMA-co-AMA) microcapsules containing twophase core materials, PPy homodispersed in n-Oct, have been prepared by a two-step polymerization technique. Under the experimental conditions, MicroPCMs have an uneven surface and are 1−100 μm in size. The thermal resistant temperature of microencapsulated n-Oct is above 180 °C. Generated PPy makes the peak area represented the heterogeneous nucleation of the microcapsules significantly increase; the peak area representing homogeneous nucleation is significantly reduced in DSC cooling curves. When the content of PPy was 7.5 wt %, the enthalpy of the heterogeneous nucleation increased to a maximum of 83.6%. The generation of PPy inside MicroPCMs affects neither the particle size distribution and morphologies of the microcapsules nor the thermal stability.

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AUTHOR INFORMATION

Corresponding Author

*J. P. Wang. Phone: +86 22 8395 5368. Fax: +86 22 8395 5282. E-mail:[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the National Natural Science Fund of China (no. 50573058), the National Natural Science Foundation of China (grant no. 51203113), Aeronautical Science Foundation of China (grant no.201229Q2002), and the Science and Technology Development Plan of Tianjin Municipal (09ZCKFGX02200) for their financial support. 14711



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styrene-divinybenzene co-polymer shells. J. Mater. Sci. 2009, 44, 3141−3147. (24) Tian, B.; Zerbi, G. Lattice-Dynamics and Vibrational-Spectra of Polypyrrole. J. Chem. Phys. 2009, 92, 3886−3891. (25) Arora, K.; Chaubey, A.; Singhal, R.; Singh, R. P.; Pandey, M. K.; Samanta, S. B.; Malhotra, B. D.; Chand, S. Application of Electrochemically Prepared Polypyrrole-Polyvinyl Sulphonate Films to DNA Biosensor. Biosens. Bioelectron. 2006, 21, 1777−1783. (26) Ouzineb, K.; Lord, C.; Lesauze, N.; Graillat, C.; Tanguy, P. A.; McKenna, T. Homogenisation devices for the production of miniemulsions. Chem. Eng. Sci. 2006, 61, 2994−3000. (27) Luz, S.; Paula, S.; Antonio, D. L.; Manuel, C.; Juan, F. R. Microencapsulation of PCMs with a polystyrene shell. Colloid Polym. Sci. 2007, 285, 1377−1385. (28) Luz, S. S.; John, T.; Donald, S.; Sanchez, P.; Juan, F. R. Synthesis and Characterization of Paraffin Wax Microcapsules with AcrylicBased Polymer Shells. Ind. Eng. Chem. Res. 2010, 49, 12204−12211. (29) Kraack, H.; Sirota, E. B.; Deutsch, M. Measurements of homogeneous nucleation in normal-alkanes. J. Chem. Phys. 2000, 112, 6873. (30) Neil, W.; Richard, W. H.; Suresh, N. Triacylglyceride Crystallization in Vegetable Oils: Application of Models, Measurements, and Limitations. In Crystallization and Solidification Properties of Lipids; Widlak, N., Hartel, R. Narine, S., Eds.; Elsevier: Amsterdam, 1982; pp 200−214. (31) Sun, N. X.; Liu, X. D.; Lu, K. An explanation to the anomalous Avrami exponent. Scr. Mater. 1996, 34, 1201−1207. (32) Freund, M.; Csikós, R.; Keszthelyi, S.; Mózes, G. Y. Paraffin products, properties, technologies, applications; Elsevier: Amsterdam, 1982; pp14−15.

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Preparation and Properties of Microencapsulated Phase Change

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