Fabrication and Performances of Microencapsulated n-Alkanes with

Article pubs.acs.org/IECR

Fabrication and Performances of Microencapsulated n‑Alkanes with Copolymers Having n‑Octadecyl Side Chains As Shells Xiaofen Tang, Wei Li, Xingxiang Zhang,* and Haifeng Shi State Key Lab of Hollow Fiber Membrane Materials and Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China S Supporting Information *

ABSTRACT: Supercooling of microencapsulated phase change materials (MicroPCMs) is the major obstacle for the widespread application of MicroPCMs in the field of latent heat storage. This work develops a feasible approach to suppress supercooling of MicroPCMs. Microcapsules containing even n-alkanes (n-hexadecane, n-octadecane, and n-eicosane) with polymethyl methacrylate (PMMA) and various compositions of n-octadecyl acrylate (ODA)-MMA copolymeric shells (PODAMA) were fabricated through suspension-like polymerization. The fabrication and properties of MicroPCMs were investigated using Fourier transformed infrared spectroscopy (FTIR), a field-emission scanning electron microscope (FE-SEM), particle size distribution analysis, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The results show that a series of microcapsules with spherical shapes were successfully fabricated. The average diameters of the microcapsules were in the range 0.92−1.70 μm. The degree of supercooling of even n-alkanes encapsulated in PMMA microcapsules is approximately 8−10 °C. The degree of supercooling and content of n-alkanes decreased progressively with an increase of molar ratio of ODA in the synthesis; however, the thermal stabilities of the MicroPCMs varied slightly. polymerization.13,14 MicroPCMs are functional and renewable energy sources. Potential applications of MicroPCMs include, for example, energy-saving building, solar energy storage, latent functional thermal fluids, smart fibers, and foam.15−22 Hitherto, various technologies for fabricating microcapsules existed, including in situ polymerization, suspension-like polymerization, and interfacial polymerization, among others. Varieties of materials such as MF, PMMA, PS, polyurethane, and polyurea were used as microcapsule shells.23−26 However, the microencapsulation process causes a significant lowering of n-alkane crystallizing temperature, which is called supercooling, as compared to bulk phase.5,27−32 Moreover, the supercooling phenomenon was also observed on the crystallization of nalkane in emulsified microdroplets.33,34 The reason could be explained by comparing the nucleation mechanisms between in-bulk material and microcapsules or emulsion droplets. When the volume was microscopically divided, the nucleation mechanism of alkane is shifted from heterogeneous nucleation to homogeneous nucleation due to the lack of a nucleating agent. Supercooling causes the microencapsulated n-alkane to be crystallized at a lower temperature, or latent heat to be released at a broader temperature range, which remains a key issue for the potential application of MicroPCMs. According to published literatures, various types and concentrations of nucleating agents have been proposed to suppress the degree of supercooling of MicroPCMs. Yamagishi et al. found that 1tetradecanol was used as an effective nucleating agent to prevent MicroC14 from supercooling, with diameters in the

1. INTRODUCTION Phase change material (PCM) is a clean and renewable energy storage source that possesses the ability to absorb and release a large amount of latent heat via a phase transition over a certain temperature range.1,2 Microencapsulated phase change materials (MicroPCMs) turn PCMs into a solid permanently, and thus, they are beneficial for energy saving and environmental protection.3 Normal alkanes (CnH2n+2, abbreviated Cn), consisting of linear chains of saturated hydrocarbons, have been extensively considered the preferred core materials of microcapsules in latent energy storage. The n-alkanes have several advantages, such as a large amount of latent heat, good stability, and nontoxicity. Microencapsulation of n-alkanes has been widely studied since the late 1970s. Loxley et al. prepared the microcapsules containing C16 (MicroC16) by controlled phase separation.4 Yamagishi et al. developed in situ polymerization and a complex coacervation method to encapsulate C14 with melamine resin and gelatin, respectively.5 Alkan, and Sari et al., microencapsulated even n-alkane (C16, C18, C20, and C22) with poly(methyl methacrylate) (PMMA) shell.6−8 Zhang et al. fabricated microcapsules using C18, C19, and C20 as PCM and melamine-formaldehyde (MF) as shell.9 Qiu et al. investigated the effect of different cross-linked agents on the performance of microcapsules containing C18.10 In addition, several studies focused on microencapsulation of PCM with copolymer shells have attracted widely attention. Shan et al. fabricated microcapsules containing n-octadecane with MMA-methacrylic acid (MAA) copolymeric shell.11 Xu et al. demonstrated microencapsulated wax with MMA-acrylic acid copolymeric shell by radical polymerization.12 Sánchez et al. studied microencapsulated PRS paraffin with MMA-St copolymer, MMA-methyl acrylate (MA) copolymer, and MMA-MAA copolymer shells by means of suspension-like © 2014 American Chemical Society

Received: Revised: Accepted: Published: 1678

October 21, 2013 December 23, 2013 January 10, 2014 January 10, 2014 dx.doi.org/10.1021/ie403542h | Ind. Eng. Chem. Res. 2014, 53, 1678−1687

Industrial & Engineering Chemistry Research

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range 110−300 μm.5 Zhang et al. demonstrated that the degree of supercooling of microencapsulated n-alkanes (C18, C19, and C20) with MF shell was further increased as the diameter of the microcapsules depresses to a critical size, specifically when the average diameter is lower than 4.3 μm. The degree of supercooling could be suppressed when 1-octadecanol and paraffin were employed as nucleating agents.27 The silica fume nanoparticles were ineffective in suppressing supercooling of microcapsules containing C14 (MicroC14) with diameters in the range 90−125 μm.28 Okubo et al. indicated that the crystallization temperatures of microcapsules containing nhexadecane (MicroC16) with divinylbenzene (DVB)-based polymer shells were approximately 10 °C lower than that of pure n-hexadecane with a cooling rate of 5 °C min−1.29,30 On the other hand, Black et al. conducted research on the supercooling of MicroC16 with a series of poly(alkyl methacrylate) (C1−C6) shell. They proposed that the degree of supercooling of MicroC16 increased when increasing the length of ester side chains on the shell polymer.31 As can be seen from the literatures, almost all of the researches has been devoted suppressing the supercooling of microcapsules by adding various nucleating agents into the core material. However, this approach is not satisfactory and also has several disadvantages. 1-octadecanol as a nucleating agent made the MicroPCMs easily coagulate. The encapsulation efficiency of MicroPCMs decreases when increasing the concentration of the nucleating agents. The utilization of nanoparticle as nucleating agent for supercooling suppression is subject to a major restriction because of the poor dispersibility of nanoparticles in the liquid.35 It is, therefore, essential to develop a new approach to deal with this problem. Aliphatic alkanol with high crystallizing temperature and nanoparticles have been used as nucleating agents to suppress the supercooling of MicroPCMs, as mentioned above. Based on the nucleation mechanism, supercooling of MicroPCMs might also be suppressed using unique shell with a high crystallizing temperature, which can support solidification nuclei that can promote heterogeneous nucleation of molten PCM upon cooling. Su et al. described a method to reduce the supercooling of microencapsulated indium nanoparticles with high-melting point semicrystalline silica shell. The silica derived from sodium silicate facilitates heterogeneous nucleation and decreases the degree of supercooling up to 30 °C.36 On the other hand, the crystallization behaviors of n-alkanes confined in emulsified n-alkane microdroplets, a three-dimension soft confined system, have been investigated. Several studies showed that surfactants with similar alkyl chain lengths as the emulsified lipid have been shown to increase the crystallization point of emulsion droplets.37−40 The dense adsorption layers of long chain surfactant molecules could act as templates for n-alkane crystallization, and such emulsion droplets exhibited a low degree of supercooling.41 These results enlighten a new approach to fabricate MicroPCMs with low supercooling by using copolymeric shell having long n-alkyl side chains as shell. Comb-like polymers with crystallizable poly(n-alkyl) side chains attached to the amorphous main chain known to pack into small n-alkyl nanodomains with an alternating crystalline side chain region and amorphous main chain region in the solid state.42 In general, the crystalline structures and thermal properties of copolymers that include side chain monomers are similar to those of the comb-like polymers from those side chain monomers. 43 Qiu et al. fabricated n-octadecane containing microcapsules using a series of MMA-based

copolymer as shells, in which butyl acrylate, butyl methacrylate, lauryl methacrylate, and octadecyl methacrylate (ODMA) were employed as comonomer. Phase change enthalpies of MicroPCMs increased with the length decreasing of the side chain of the monomers.44 However, the effects of copolymeric shells on the supercooling of MicroPCMs have not been investigated. Additionally, there is still little information focused on the crystallization behavior of microencapsulated n-alkane variations of alkyl carbon numbers with copolymer shell having long side chains. The physical properties of poly(n-alkyl acrylates) have been studied since the early 1940s.45 For poly(n-alkyl acrylates), nine carbons is the minimal crystallizable side chain length.46 ODA is more reactive than ODMA and is a suitable monomer for introducing n-octadecyl side chains into polymers.47 In this paper, microencapsulation of even n-alkanes (C16, C18, and C20) with PMMA and PMMA-based copolymer composed of various compositions of n-octadecyl acrylate (ODA) as a functional copolymer shell was carried out through suspensionlike polymerization. Supercooling of MicroPCMs caused by the microencapsulation process was demonstrated as their diameters depressed to their limit upon cooling. The effects of various shell materials on the formation, crystallization behaviors, and thermal stabilities MicroPCMs were also investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. ODA (purity 95 wt %, Tianjin Tianjiao Chemical Co., Ltd.) and MMA (purity 99.0 wt %, Tianjin Fuchen Chemical Reagents Factory) were used as monomers in the composition of the shell. ODA and MMA was washed with 10 wt % sodium hydroxide aqueous solution three times to remove the inhibitor, and then dried over anhydrous MgSO4. 1, 4-butylene glycol diacrylate (BDDA, purity 80 wt %, Tianjin Tianjiao Chemical Reagent Co., Ltd.) was used as the crosslinking agent. Benzoyl peroxide (BPO, purity 99.0 wt %, Tianjin Fuchen Chemical Reagents Factory) was employed as a hydrophobic initiator. Sodium salt emulsion of styrene−maleic anhydride copolymer (SMA, 19 wt % aqueous solutions, Shanghai Leather Chemical Works) was applied as surfactant. C16, C20 (purity 95 wt %, Union Lab. Supplies Limited, Hong Kong), and C18 (purity 90 wt %, Chevron Phillips Chemical Company, U.S.A.) were used as core material. 2.2. Fabrication of MicroPCMs. MicroPCMs with poly(ODA-co-MMA) (PODAMA) and PMMA as shells were each fabricated by suspension-like polymerization. MicroC16 was used to illustrate the fabrication of MicroPCMs, with the following as an example of the formation process: The polymerization system consisted of an oil phase and a continuous phase. Predetermined amounts of ODA, MMA, BDDA, and C16 were mixed with the initiator as an oil phase. An aqueous phase was prepared by dissolving 10 g of SMA in 100 g deionized water. The oil phase was added dropwise into the aqueous phase with constant stirring, and the resultant mixture was emulsified by a homogenizer to form a stable emulsion system in a beaker. The obtained emulsion was then poured into a four-necked flask equipped with a mechanical stirrer, condenser, and inlet for nitrogen gas. Before starting the reaction, the reactor was purged with nitrogen for 15 min to remove oxygen. The reaction was carried out for 5 h with a stirring rate of 400 rpm at 85 °C in a nitrogen atmosphere. The resultants were repeatedly washed with 50 wt % ethanol solution at 50 °C, and filtered to remove the unreacted 1679

dx.doi.org/10.1021/ie403542h | Ind. Eng. Chem. Res. 2014, 53, 1678−1687


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monomer and C16 on the surface of the microcapsules. Finally, they were dried to a constant weight in an oven at about 50 °C. The synthetic procedure of PODAMA is the same as the formation process of MicroC16 in the absence of C16. 2.3. Characterization. FTIR spectra of C16, C18, C20, PMMA, MicroC16, MicroC18, MicroC20, and PODAMA were obtained using Fourier transformed infrared spectroscopy (FTIR, Tensor37 Bruker) at room temperature. The surface morphologies of MicroPCMs and PODAMA were characterized using a field-emission scanning electron microscope (FE-SEM; Hitachi S4800). The specimens were coated with gold. The diameter distributions of MicroPCMs were examined using a diameter distribution analyzer (Horiba LA-300). The phase change properties of C16, C18, C20, MicroC16, MicroC18, MicroC20, and PODAMA were measured using a differential scanning calorimeter (DSC 200 F3). Specimens were heated at a rate of 10 °C min−1 from −20 to 100 °C, and then cooled to −20 °C at the same rate, followed by heating again to 100 °C. The first cooling and second heat thermograms were recorded. The content of n-alkanes in the MicroPCMs can be calculated according to the measured enthalpies, x = (ΔHm1 − ΔHm2)/ΔHm

Figure 1. FTIR spectra of PMMA, even n-alkanes and MicroPCMs with PMMA shell: (a) PMMA; (b) C16; (c) C18; (d) C20; (e) MicroC160; (f) MicroC180; (g) MicroC200.

(1)

where ΔHm1 is the melting enthalpy of MicroPCMs; ΔHm2 is the melting enthalpy of PODAMA among the endothermic interval of MicroPCMs; and ΔHm is the melting enthalpy of even n-alkanes. The degree of supercooling (ΔT) is defined as the temperature difference between the onset crystallizing temperature of even n-alkanes and that of MicroPCMs. Thermal stabilities of PODAMA, MicroC18, and C18 were investigated using thermogravimetric analysis (TGA, Netzsch STA409PC) at a scanning rate of 10 °C min−1 in the range 25− 800 °C in a nitrogen atmosphere. The temperature of 5% weight loss (Td5%) was recorded.

3. RESULTS AND DISCUSSION 3.1. Formation of MicroPCMs. FTIR spectra of PMMA, even n-alkanes (C16, C18, and C20) and microencapsulated even n-alkanes with PMMA shell are shown in Figure 1. All the peaks appearing at 2958, 2922, and 2850 cm−1 are attributed to the symmetrical stretching vibration of C−H group. The peak at approximately 1475 cm−1 is attributed to the C−H bending. The absorption band at 720 cm−1 is attributed to the in-plane rocking vibration of the CH2 group. These peaks are the characteristic bands for the even n-alkanes and can also be observed in the spectra of MicroPCMs. However, none of these peaks could be observed in the FTIR spectrum of PMMA. The FTIR spectra of PODAMA and microencapsulated even n-alkanes with ODA-MMA copolymeric shells are demonstrated in Figure 2. The peaks at about 1734 and 1149 cm−1 were assigned to the carbonyl group and C−O stretching of the ester group. For PODAMA, the absorption band at 720 cm−1 associated to the n-octadecyl group of ODA can also be observed, which confirmed the incorporation of ODA. However, there is not the characteristic peak at 720 cm−1 in the spectrum of PMMA. The spectra of PODAMA are nearly identical to that of MicroPCMs with various compositions of ODA-MMA copolymeric shells. The characteristic peaks of PODAMA are overlapped with those of MicroPCMs. Such

Figure 2. FTIR spectra of PODAMA and MicroPCMs with PODAMA shell: (a) PODAMA1; (b) PODAMA3; (c) PODAMA4; (d) MicroC161; (e) MicroC163; (f) MicroC182; (g) MicroC183; (h) MicroC202; (i) MicroC204.

results infer that the even n-alkanes have been encapsulated as core within PMMA and PODAMA microcapsules. 3.2. Size Distributions of MicroPCMs. The average diameters of MicroPCMs with various compositions of ODAMMA copolymeric shells are shown in Figure 3. The diameters of MicroPCMs are in the range 0.4−4 μm. The average diameters of MicroC16, MicroC18, and MicroC20 are in the range 0.92−1.06, 1.20−1.31, and 1.65−1.73 μm for various compositions of copolymer shell, respectively. The supercooling of MicroPCMs is associated with the diameter of microcapsules. The three kinds of MicroPCMs show a slight difference in their average diameter and polydispersity, which is conductive to analyze their crystallization behavior, respectively.27,31 It is found that the composition of shell has hardly any effect on the diameter and average diameter of MicroPCMs when regulating the speed of the mixing emulsification machine and emulsification time during the emulsifying process. Little intermicelle exchange between the emulsion droplets is 1680



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even n-alkanes is equivalent to that of n-alkanes. This means that the melting behavior of even n-alkanes is slightly affected by the microencapsulation process. Upon cooling, there are two obvious exothermic peaks in the DSC curves of MicroC160, MicroC180, and MicroC200. To facilitate the discussion, the peaks are marked as α and β from places of higher temperature toward lower temperature, respectively. According to the previous reports,27,32 the multiple peaks are associated with the appearance of a new metastable rotator phase in the cooling process of the microencapsulated n-alkanes. The α-peak with small supercooling (ΔT ≈ 1 °C), is assigned to the heterogeneously nucleated liquid-rotator transition. The βpeak is attributed to the transition from the metastable rotator phase to the stable triclinic phase.27,32 The degrees of supercooling of MicroC160, MicroC180 and MicroC200 are 7.9, 9.8, and 6.2 °C, respectively. The mechanism of this nucleation transition is homogeneous, so the degree of supercooling of β-peak is higher than that of the α-peak. Supercooling causes the latent heat to be released at a lower temperature or a broader temperature range, which limits the application of microencapsulated n-alkanes. 3.5. Phase Change Behavior of MicroPCMs with ODAMMA Copolymeric Shell. The DSC curves of MicroC16 with various compositions of ODA-MMA copolymeric shells are shown in Figure 7, and their thermal properties are presented in Table 1. There are two endothermic peaks in the heating process of MicroC16. The melting peak appearing at higher temperature is ascribed to the fusion of PODAMA. It is clear that the onset melting temperatures (about 11.0 °C) of MicroC16 with ODA-MMA copolymeric shells are lower than that of MicroC16 with PMMA shell (15.3 °C). Moreover, the onset melting temperatures display a gradual increase with an increasing concentration of long side chains. Upon cooling, two obvious exothermic peaks upon cooling can be observed, and the onset and peak temperature of the two peaks increased with increasing the molar ratio of ODA. The onset temperature of the α-peak is higher in comparison than that of C16. It is in good agreement well with the onset crystallizing temperature of PODAMA. The β-peak is attributed to the crystallization of C16 within microcapsules. It is observable that the onset and peak crystallizing temperatures display a gradual increase as ODA molar fraction increases in the synthesis. The onset crystallizing temperatures of ODA-MMA copolymeric shell increase from 19.0 to 25.9 °C with the molar ratio of ODA changes from 0.1 to 0.4 (by ODA/monomer ratio in synthetic formulation), while the corresponding onset crystallizing temperatures of MicroC16 increase from 9.3 to 15.3 °C. As shown in Figure 8, the composition of copolymer shell plays a significant role in the degree of supercooling and the C16 content in MicroC16. With increasing the molar fraction of ODA monomer in the synthesis, the degree of supercooling of MicroC16 decreases from 7.9 to 1.2 °C, and the content of C16 falls to 29 wt % from 37 wt %. The DSC curves of MicroC18 with various compositions of ODA-MMA copolymeric shells are shown in Figure 9, and their thermal properties are listed in Table 2. In the heating process of MicroC18, only one endothermic peak was observed. The onset melting temperatures of MicroC18 with copolymeric shell range from 20.6 to 21.4 °C, which increase with an increasing molar ratio of ODA. Nevertheless, their onset temperatures are lower than that of MicroC180 (24.8 °C). These behaviors are similar to MicroC16 with ODA-MMA copolymeric shells. Upon cooling, there are two exothermic

Figure 3. Average diameters of MicroPCMs as a function of the molar ratio of ODA in the synthesis. The inset figure shows the size distribution curves of typical MicroPCMs (MicroC182 and MicroC202).

observed due to the suppression of Ostwald ripening through the introduction of ODA. 3.3. Morphology of MicroPCMs. Figure 4 shows the SEM micrographs of PMMA, PODAMA, and MicroPCMs with various compositions of ODA-MMA copolymeric shells. PMMA and PODAMA have regular spherical profiles, and their surfaces are smooth and compact. However, the surface morphologies of MicroPCMs are clearly distinct from that of the copolymers. All microcapsules show a spherical shape, and their sizes match the measured diameter from the particle size distribution analysis. The crushed microcapsules confirm the formation of MicroPCMs with core/shell structure. It is noticeable that obvious concaves and collapses exist on the surface of MicroPCMs with PMMA shell (MicroC160, MicroC180, and MicroC200). This can be explained by the generation of reserved expansion space in the encapsulation process, which is caused by the difference in density between the reactive monomers and the copolymer, as well as the shrinkage of C18 from the melting state to the crystal state in the cooling process.11,48 There are fewer concaves and a more compact surface with increased the molar ratio of ODA. According to literature, the apparent morphology of MicroPCMs is really relevant to several factors, such as shell materials, types of cross-linker and the amount of cross-linker used.7,10,11 Compared to MicroPCMs with BDDA as crosslinker, fewer collapses were observed on the surface of MicroPCMs with divinylbenzene (DVB). The number of concaves reduced with an increase of the number of crosslinking functional moieties and amount of cross-linker used. In brief, the higher mechanical strength of shell to resist external pressure resulting from the shrinkage, the fewer concaves and collapses emerge. Here, by increasing the molar ratio of ODA and keeping the cross-linker at a constant level in the recipes, more ordered crystalline structure of copolymer is increased and formed microcapsules with a higher mechanical strength to resist external pressure.10,49 Consequently, the amount of concaves is decreased with an increase of molar fraction of ODA. 3.4. Phase Change Behavior of Microencapsulated Even n-Alkanes with PMMA Shell. The DSC curves of even n-alkanes and microencapsulated even n-alkanes are presented in Figure 5 and Figure 6, and their thermal properties are summarized in Tables 1, 2, and 3, respectively. In the heating process, the endothermic temperature of microencapsulated 1681



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Figure 4. SEM micrographs of dried MicroPCMs: (a) PMMA; (b) PODAMA2; (c) MicroC160; (d) MicroC161; (e) MicroC164; (f) MicroC180; (g) MicroC182; (h) MicroC183; (i) MicroC200; (j) MicroC203.

crystallizing temperature of the first phase transition (peak α) displays a slight increase as the ODA molar ratio increases in the synthesis. The degree of supercooling on the transition isotropic liquid to the rotator phase is not immediately obvious.

peaks in the cooling process of MicroC181, MicroC182, and MicroC183, which is similar to that of MicroC180. However, only one exothermic peak was observed on the DSC cooling curve of MicroC184. The DSC data shows that the onset 1682



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supercooling and the content of C18 in microcapsules with ODA-MMA copolymeric shells as a function of molar ratio of ODA are shown in Figure 10. With increasing the molar fraction of ODA monomer in the synthesis, the degree of supercooling of MicroC18 declines to 0 °C from 7.9 °C, and the content of C18 in MicroC18 reduces from 37 wt % to 29 wt %. The phase change behaviors of MicroC20 with various compositions of ODA-MMA copolymeric shells were also investigated. The DSC melting and cooling curves of MicroC201 and MicroC202 are shown in Figure 11. The melting behavior of MicroC20 is similar to that of MicroC16 and MicroC18 with ODA-MMA copolymeric shells. Only one endothermic peak was observed in the heating process of MicroC201 and MicroC202, and their onset melting temperatures are about 2−3 °C below that of MicroC200. In the cooling process of MicroC20, a metastable rotator phase was observed while the molten C20 crystallized into a stable triclinic phase. The onset and peak crystallization temperatures of the rotator phase (β) appear at lower temperature in comparison with that of MicroC200. The DSC investigation suggests that ODA-MMA copolymeric shell, in spite of the high content of ODA, has a negative effect on the supercooling suppression of MicroC20 upon cooling. The degree of supercooling of MicroC201 and MicroC202 are 10.3 and 9.1 °C, respectively. Compared to MicroC20 0 , MicroC20 with ODA-MMA copolymeric shells has a greater degree of supercooling. However, the degree of supercooling decreases with an increase of molar ratio of ODA. The DSC results suggest that supercooling of MicroC16 and MicroC18 is effectively suppressed through copolymerization of ODA with MMA as shell, and the degree of supercooling decreased with an increase of the molar ratio of ODA. These results indicate dense layers of long n-alkyl chains could act as templates for n-alkane crystallization. The mechanism is explained as follows (Figure 12): there is a number of small alkyl nanodomains formed accordingly in the interface between the inner wall of microencapsulated n-alkane and n-alkane when PODAMA is crystallized through an n-octadecyl side chain packing in the cooling process of microencapsulated n-alkane. The ordered long n-alkyl chains act as templates for n-alkane crystallization and the small alkyl nanodomains formed before the C16 will act as a crystal nucleus.40,41,50 In addition, the occurrence of the metastable rotator phase was related to the nucleating barrier; the ordered n-octadecyl side chains adjacent to the interface between the nanocapsule inner wall and nalkane could decrease the nucleating barrier of n-alkane, and thus induces the metastable rotator phase to appear at higher temperature.38,50 Subsequently, with the help of the alkyl

Figure 5. DSC curves of C16 (a) and MicroC160 (b).

Figure 6. DSC curves of C20 (a) and MicroC200 (b).

Nevertheless, the onset and peak temperature of the transition from the rotator phase to the triclinic phase displays a remarkable increase. With increasing the molar ratio of ODA from 0.1 to 0.3, the corresponding onset crystallizing temperatures of β-peak increase from 18.8 to 21.6 °C. As the molar ratio of ODA reaches 0.4, crystallization behavior of MicroC184 is almost the same as that of C18. In addition, the endset crystallizing temperatures of MicroC184 (9.5 °C) are higher than that of MicroC180 (6.2 °C). The degree of Table 1. Thermal Properties of C16 and MicroC16a sample

Tmo1 (°C)

Tmp1 (°C)

C16 MicroC160 MicroC161 MicroC162 MicroC163 MicroC164

17.3 15.3 10.5 10.6 10.8 11.0

24.0 21.6 17.3 16.8 18.3 19.5

Tmo2 (°C)

21.8 20.9 24.6 30.5

Tmp2 (°C)

Tme (°C)

ΔHm (J/g)

Tco1 (°C)

Tcp1 (°C)

26.0 29.1 31.9 36.5

27.5 25.3 29.8 33.6 38.2 41.7

269 96 103 106 107 108

16.5 15.2 19.0 21.1 23.4 25.9

11.2 12.6 13.9 15.9 18.5 21.4

Tco2 (°C) 8.6 9.3 10.1 12.6 15.3

Tcp2 (°C)

Tce (°C)

ΔHc (J/g)

3.2 3.4 6.3 7.0 8.3

5.7 −4.3 −3.7 −1.9 −0.4 0.7

−268 −100 −104 −105 −107 −109

a

Tmo = the onset temperature on DSC heating run; Tmp = the peak temperature on DSC heating run; Tme = the endset temperature on DSC heating run; ΔHm = enthalpy on DSC heating run; Tco = the onset temperature on DSC cooling run; Tcp = the peak temperature on DSC cooling run; Tce = the endset temperature on DSC cooling run; ΔHc = enthalpy on DSC cooling run. 1683



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Table 2. Thermal Properties of C18 and MicroC18 sample

Tmo (°C)

Tmp (°C)

Tme (°C)

ΔHm (J/g)

Tco1 (°C)

Tcp1 (°C)

C18 MicroC180 MicroC181 MicroC182 MicroC183 MicroC184

24.3 24.8 20.6 20.8 21.0 21.4

32.9 29.0 29.1 28.4 28.2 27.9

35.8 31.3 34.1 34.5 36.3 37.1

225 91 96 95 98 98

25.2 24.4 24.2 24.3 24.6 25.4

18.5 22.6 22.0 21.1 22.8 18.2

Tcp2 (°C)

Tce (°C)

ΔHc (J/g)

15.4 18.8 19.4 21.6

9.8 11.9 13.3 15.9

14.0 6.2 2.6 7.1 8.9 9.5

−225 −90 −94 −96 −100 −99

Tco2 (°C)

Tcp2 (°C)

Tce (°C)

ΔHc (J/g)

22.0 14.6 14.0

24.2 12.8 4.5 6.3

−263 −111 −111 −110

Tco2 (°C)

Table 3. Thermal Properties of C20 and MicroC20 sample

Tmo (°C)

Tmp (°C)

Tme (°C)

ΔHm (J/g)

Tco1 (°C)

Tcp1 (°C)

C20 MicroC200 MicroC201 MicroC202

34.9 34.8 31.9 32.7

42.6 40.2 39.5 39.2

47.9 46.3 50.1 48.7

261 110 112 113

34.4 34.3 32.5 33.1

28.2 32.7 30.2 31.4

28.2 24.1 25.3

Figure 9. DSC curves of MicroC18 with various compositions of ODA-MMA copolymeric shells: (a) MicroC181; (b) MicroC182; (c) MicroC183; MicroC184.

Figure 7. DSC curves of MicroC16 with various compositions of ODA-MMA copolymeric shells: (a) MicroC161; (b) MicroC162; (c) MicroC164.

Figure 10. Degree of supercooling of MicroC18 and the content of C18 in microcapsules with ODA-MMA copolymeric shells as a function of molar ratio of ODA.

Figure 8. Degree of supercooling of MicroC16 and the content of C16 in microcapsules with ODA-MMA copolymeric shell as a function of molar ratio of ODA.

crystallization temperature of microencapsulated n-alkane to appear at higher temperature. Consequently, the degree of supercooling can be further decreased with an increased content of ODA in the recipes.

nanodomains, which act as nucleating sites, the microencapsulated n-alkane crystallizes to a stable triclinic phase via the heterogeneous nucleation mechanism upon cooling. The higher crystallizing temperature of nanodomains will induce the 1684



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than ODMA, resulting from the absence of 2-methyl. The different hydrophilicity of these comonomers may underlie the content of C18 changes during the fabrication of MicroC18. 3.6. Effects of ODA-MMA Copolymeric Shell on the Thermal Stability of MicroC18. Thermal stability is an important factor in the assessment of encapsulated PCMs for their practical application. It is indispensable to investigate the thermal stability of MicroPCMs with ODA-MMA copolymeric shells. In the case of MicroC18, the incorporation of ODA on the thermal properties of MicroPCMs has been observed. TGA thermograms of MicroC18 are shown in Figure 13; and

Figure 11. DSC curves of MicroC20 with various compositions of ODA-MMA copolymeric shells: (a) MicroC201; (b) MicroC202.

Figure 12. Schematic illustration of the mechanism of supercooling suppression of MicroPCMs with ODA-MMA copolymeric shells.

Figure 13. TGA curves of specimens: (a) C18; (b) MicroC181; (c) MicroC182; (d) MicroC183; (e) MicroC184; (f) PODAMA3.

For MicroC20, it is reasonable to postulate that the nucleating barrier of C20 is still too high, or the crystallizing temperature of nanodomains is much lower than that of C20, resulting from the shortage of molar ratio of ODA in the synthesis, or the short length of the n-octadecyl side chain. The nanodomains cannot act as the active solidification nuclei to promote the nucleation of C20. Instead, the n-octadecyl side chains at the interface between the microcapsule inner wall and C20 act as an impurity,37 leading to the greater supercooling of MicroC20 with MMA-co-ODA copolymeric shells. Moreover, it is interesting to note that the content of nalkanes decreases gradually with increased molar ratio of ODA, while the monomer/n-alkanes mass ratio and the mass of crosslinker keeps as a constant. However, there is little variation in the enthalpy sum of MicroPCMs resulting from the enthalpy of ODA-MMA copolymeric shell. The enthalpy sum of MicroC16 and Micro18 is in the range 104−109 J/g and 94−100 J/g. The enthalpy of MicroC20 ranges from 110 J/g to 113 J/g. The content of n-alkanes encapsulated within microcapsules is associated with the strength of the polymer shell and the phase separation ability between the polymer shell and n-alkanes.10,51 Both reduced with increased ODA molar ratio in the recipes because the long side chain of the comb-like copolymer shell is slightly miscible with C18, resulting in a lower content of nalkanes in microcapsules. Okubo et al. indicated that an increase in the hydrophilicity of DVB-based polymer shells with increased acrylic monomers (butyl acrylate and ethyl acrylate) could achieve high content of n-hexadecane. When the content of acrylic monomers is greater, an increased content of nhexadecane will be gained.30 The ODMA is a high hydrophobic monomer. The ODA, however, exhibits better hydrophilicity

Figure 14. Define weight losses at various temperatures for C18 bulk and MicroC18 with various compositions of copolymer shells. Td30%, Td50%, and Td80% exhibits the required temperature for weight losses of 30, 50, and 80 wt %, respectively.

quantitative results have been plotted in Figure 14. As observed, the weight loss profiles of all MicroC18 are quite similar, and their thermal decomposition performs through a two-step degradation stage, whereas PODAMA3 and pure C18 lose its weight with only one step. The first stage of weight loss from 165 to 380 °C occurs due to the gasification of C18,10,11 and 1685



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the second stage of weight loss from 380 to 500 °C corresponds to the decomposition of the polymer shell.49 The Td5% of MicroC18 is about 230 °C, which is equivalent to that of C18-cantaining microcapsules with methacrylic acid (MAA)-co-MMA and styrene (St)-co-DVB copolymeric shell.11,48 The extracted value at the temperature for occurrence of weight losses of 5 wt %, 10 wt % (Td10%), and 20 wt % (Td20%) for MicroC18 was approximately 50 °C above that of pure C18. It indicates that the core material has been well protected by the shell. The results are consistent with that of nanoencapsulated C18 in our previous report. On the other hand, it also clearly indicates that the thermal stability of MicroPCMs displays a gradual increased increasing the molar ratio of ODA. This may be attributed to the high content of long side chains, which can retard the decomposition of the molecular main chain during the pyrolysis.49

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (No. 51203113, 20904040, 21174105), the Key Project of Tianjin Municipal Natural Science Foundation (No. 12JCZDJC26800) and the Youth Science Fund Project (No. 2051203113).

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4. CONCLUSIONS Microencapsulation of n-alkanes (i.e., n-hexadecane, n-octadecane, and n-eicosane) with polymethyl methacrylate (PMMA), and various compositions of poly(MMA-co-octadecyl acrylate (ODA)) copolymeric shells was carried out through suspension-like polymerization. The average diameter of microencapsulated n-hexadecane, n-octadecane, and n-eicosane (MicroC16, MicroC18, and MicroC20) was in the range 0.92− 1.06, 1.20−1.31, and 1.65−1.73 μm, respectively. Two exothermic peaks were observed in the process of MicroC16, MicroC18, and MicroC20 with PMMA shell, and their degree of supercooling are approximately 7.9, 9.8, and 6.2 °C. The onset melting temperatures of microencapsulated n-alkanes with ODA-MMA copolymeric shells were about 2−5 °C below that of microencapsulated n-alkanes with PMMA shell. The degree of supercooling and the content of PCMs decreased with increasing content of ODA in the recipes. As the molar ratio of ODA reaches 0.4, the degree of supercooling of MicroC16 and MicroC18 declined to 1.2 and 0 °C, respectively. However, the ODA-MMA copolymeric shell causes the crystallization temperature of MicroC20 to appear at a lower temperature. Thermal stability of MicroC18 increased with increasing the content of ODA, and the Td5% of MicroC18 is approximately 230 °C. This result presents an efficient strategy to fabricate MicroC16 and MicroC18 with low degree of supercooling and better thermal stability by using copolymers having n-octadecyl side chains as shells. The nucleating potential barrier of microencapsulated n-alkane was further decreased by the n-octadecyl side chain layer at the interface between the microcapsule inner wall and n-alkane (core). The alkyl nanodomains, which act as active solidification nuclei that promote the heterogeneous nucleation of PCM upon cooling, could be the underlying reason for the supercooling suppression of MicroPCMs.

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Fabrication and Performances of Microencapsulated n-Alkanes with

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