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Preparation of acrylic PCM microcapsules with dual responsivity to temperature and magnetic field changes
European Polymer Journal 101 (2018) 18–28
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Preparation of acrylic PCM microcapsules with dual responsivity to temperature and magnetic field changes
T
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Somayeh Lashgaria,b, Ali Reza Mahdaviana, , Hassan Arabia, Veronica Ambrogic,d, Valentina Marturanoc,d a
Iran Polymer and Petrochemical Institute, P.O. Box: 14965-115, Tehran, Iran National Petrochemical Company, Petrochemical Research & Technology Company, P.O. 1435884711, Tehran, Iran c Department of Chemical, Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy d Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy b
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase change materials Acrylic Magnetic Microcapsule Stimuli-responsive
Energy preservation is one of the serious concerns in recent years to reduce fossil fuels consumption and greenhouse gases. Magnetic microcapsules based on n-hexadecane/Fe3O4 core and poly methyl methacrylate (PMMA) shell were designed as an innovative type of dual-functional phase change materials. Fe3O4 nanoparticles were first synthesized by co-precipitation method and then modified with oleic acid. Then, they were incorporated into suspension polymerization in the presence of MMA and n-hexadecane. FTIR and SEM analyses confirmed the presence of all components in the magnetic microcapsules with a wrinkled morphology and uniform size in averagely 180 µm. They revealed reasonable thermal storage properties and high thermal cycling performance according to DSC thermograms. The encapsulation efficiency and thermal storage ability were calculated by the extracted data from DSC analysis for all the samples and their thermal conductivities were measured too. Magnetometry studies approved superparamagnetic properties of the obtained microcapsules with low remanence and coercivity. With such a dual-functional feature, the introduced magnetic microcapsules could show potential applications in some high-tech fields with providing both thermal regulating and magnetic responsive properties.
1. Introduction Energy preservation has been considered as a major concern in recent years. Phase change materials (PCMs) with spectacular phase transition and functionalities are among successful nominees to overcome this issue [1]. Over the past few decades, PCMs have been known very operative in versatile applications and have exhibited good thermal regulation performance due to their large heat storage capacity and isothermal behavior during phase transitions [2]. PCMs with broad temperature range include organic materials such as paraffins, polyesters, fatty acids, and alcohols or inorganic materials containing pure elements, metals, alloys, salts, and silicates [3–5]. Paraffin waxes (i.e. nalkanes) are of great interest and are preferred than the other PCMs, because of high heat storage capacity, little supercooling and volume change, good chemical and thermal stability, low vapor pressure, no corrosion of the storage container, and non-toxicity [6]. Besides, the melting and crystallization temperatures of paraffin waxes can be easily tuned by their number of carbon [7].
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In spite of many gorgeous features of paraffin-wax-based PCMs, they are not directly used in practice due to the risk of PCM leakage by repeated melting and crystallization, their low thermal conductivity, and poor interfacial combination with accompanying materials [1]. Micro- or nano-encapsulations of PCMs within polymeric shells overcome these complications, as encapsulation can prevent PCMs from leakage during a solid–liquid phase change and can afford a higher heat transfer area per unit volume and consequent higher heat transfer rate relative to the bulk PCMs [8,9]. Moreover, encapsulated PCMs contribute supplementary advantages, such as the ability to tolerate volume changes during the phase change processes [10]. To achieve the disadvantages of low thermal conductivity in organic PCMs, addition of some particles with higher thermal conductivities like metal particles would be a key point [11]. Mazman et al. worked on the improvement of heat transfer using some solid particles like stainless steel and copper pieces and also graphite in fatty acid mixtures [12]. In another technique, Mills et al. exhibited that thermal conductivity of paraffin–EG composites can increase by an order of 20 to 60
Corresponding author. E-mail address: [email protected] (A.R. Mahdavian).
https://doi.org/10.1016/j.eurpolymj.2018.02.011 Received 18 October 2017; Received in revised form 7 January 2018; Accepted 8 February 2018 Available online 09 February 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved.
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2. Experimental
folds and this will increase the expenses of PCMs production in a constant volume [13]. Multi-walled carbon nanotubes with high thermal conductivity have been employed recently and the results showed that addition of just 1 wt% carbon nanotubes increased thermal conductivity and thermal energy storage capacity remarkably [14–16]. In has been demonstrated that addition of nano-SiO2 play a vital role on the heat enthalpies of the composite fibers without any substantial effect on the phase change temperatures [17]. The effect of different contents of Al2O3 nanoparticles on paraffin thermal behavior has been studied by Ho et al. [18] and in a recent attempt, the improvement in thermal conductivity of an organic PCM has been investigated in the presence of copper nanowires [19]. All of these observations demonstrate that addition of nano-materials and minerals to organic PCMs can upsurge both thermal conductivity and thermal storage capacity [20].The effect of calcium carbonate as a component for enhancing thermal conductivity of n-octadecane was investigated before [21]. Sahan and coworkers studied the role of Fe3O4 nanoparticles on heat transfer properties of paraffin and they found it is an effective technique to improve the heat transfer properties of paraffin [22]. The other deficiency of organic PCMs that limits their exploitations is the supercooling phenomenon and is expressed as the difference between melting and crystallization temperatures [23]. Supercooling makes the latent heat to be released at lower temperatures or a broader temperature range, which is a disadvantageous for the energy storage applications. Many attempts have been made to suppress the supercooling process. In 2015, PCM microcapsules were prepared in the presence of Rubitherm RT58 as a nucleating agent and it was observed that by addition of 5 wt% of the nucleating agent, supercooling reduced dramatically [24]. Yang et al. investigated the effect of silicon nitride in n-octadecane/PMMA microcapsules and their results showed that silicon nitride acted as a nucleating agent and suppressed the supercooling to some extent [25]. High thermal conductivity of magnetite (Fe3O4) nanoparticles (9.7 W/m K) [22,25] makes it encouraging to enhance thermal conductivity of organic PCMs. On the other hand, magnetic particles are able to absorb electromagnetic irradiations and thus, their incorporation can add a novel performance to the micro PCMs. Magnetic microcapsules based on n-eicosane core with Fe3O4/SiO2 shell have been synthesized in recent years [26]. The results illustrate proper thermal behavior along with good magnetic properties for the microcapsules. Park et al. prepared PCM/magnetite microcapsules based on polyurea shell with the aim of enhancing thermal conductivity and providing magnetic response to the microcapsules [27]. In 2016, PCM/magnetite microcapsules have been produced through interfacial polymerization [28].The obtained composite films of polyamide and microcapsules represented dual functional feature as thermal regulating and electromagnetic shielding properties. In this study, a new type of such microcapsules containing both nhexadecane and magnetite with PMMA shell have been prepared through suspension polymerization, while Fe3O4 nanoparticles are expected to enhance thermal conductivity of the microcapsules as well as suppressing supercooling by acting as nucleating agents. Moreover, these magnetic PCM microcapsules may offer magnetic responses to increase their application scope. Such a dual-functional feature proposes the potential capability of these microcapsules as smart materials due to both thermal regulating and magnetic properties. With this regard, magnetite nanoparticles were first synthesized using co-precipitation method and then modified by oleic acid to become hydrophobe. Then magnetic PCM microcapsules were prepared through suspension polymerization and the influence of magnetite content on thermal, morphological and magnetic properties of the obtained microcapsules were investigated.
2.1. Materials n-Hexadecane (HD) as the hydrophobic core, methyl methacrylate (MMA) as the shell monomer, ethylene glycol dimethacrylate (EGDMA) as the crosslinking agent and benzoyl peroxide(BPO) as the initiator were all purchased from Merck Chemical Company. Poly (vinyl alcohol) (PVA, Sigma-Aldrich, USA, molecular weight: 40,000–88,000, degree of hydrolysis: 88%) was used as the stabilizer. Iron salt hydrates (FeCl3·6H2O, FeCl2·4H2O) oleic acid (OA), ammonium hydroxide (NH4OH, 25% ammonia) and HCl (38%) were used for preparation of Fe3O4 magnetic nanoparticles. All chemicals for Fe3O4 synthesis were purchased from Merck Chemical Company. Deionized water (DI water, conductivity < 2 µs/cm) was employed in all experiments. All materials were used as received without further purification. 2.2. Synthesis of Fe3O4 nanoparticles Fe3O4 magnetic nanoparticles were first prepared by co-precipitation of Fe(II) and Fe(III) chlorides in an ammonia solution according to our previous report [22,29]. Typically, FeCl2.4H2O (2.0 g) and FeCl3·6H2O (5.2 g) were added into a 500-mL three-necked roundbottom flask equipped with a nitrogen gas inlet, mechanical stirrer and dropping funnel. Meanwhile, 25 mL DI water was poured into the flask and the mixture was stirred at 500 rpm to form a clear yellow solution under nitrogen atmosphere. Then, 250 mL of 1.5 M ammonia solution was added dropwise into the mixture during 2 h. With stirring over 40 min, black fine Fe3O4nanoparticles were formed and finally, the nanoparticles were carefully separated by an external magnetic field. The fine particles were washed with DI water and rinsed by ethanol several times, followed by 48 h drying at ambient temperature and also vacuum drying for 3 h to give 2.12 g magnetite nanoparticles (Fe3O4 NPs). 2.3. Modification of Fe3O4 NPs In order to obtain stabilized hydrophobic Fe3O4 NPs, the particles were modified with oleic acid to improve their hydrophobicity and avoid their agglomeration. At first, 4 g Fe3O4 NPs and 70 mL DI water were probe-sonicated for 5 min and 1.8 mL oleic acid was added and homogenized by sonication for 5 min. Afterward, 6 mL ammonium hydroxide aqueous solution (25%) was added, following 5 min sonication and vigorous mixing for 2 h. Then, pH of the solution was adjusted from alkaline to acidic by adding concentrated HCl which resulted in separation of unreacted oleic acid. The mixture was centrifuged for 10 min at 3000 rpm. Unreacted oleic acid was decanted as a yellow layer on the water phase. The precipitate was rinsed three times by water/ethanol solution (3:1 by volume), and the solid was vacuumdried at 20 °C for 20 h. The product was coded as m-Fe3O4 NPs. Fig. 1 shows the dispersibility of Fe3O4 NPs and m-Fe3O4 NPs in water and heptane (oil phase). 2.4. Preparation of magnetic PCM microcapsules A 250 mL double-jacketed glass reactor equipped with stirrer, nitrogen gas inlet and outlet and circulating cooling system were used for preparation of the PCM microcapsules with respect to our previous report [6]. First, the aqueous phase containing 132 g DI water and 0.05 g PVA were added to the reactor and mixed to give a homogenous solution. The oil phase including MMA (32.68 g), EGDMA (1.72 g), HD, m-Fe3O4 NPs and 0.21 g BPO was then transferred into the reactor. The mixture was stirred mechanically for 40 min at 800 rpm and the reaction temperature reached to 70 °C. The polymerization conducted for 5 h at 300 rpm under N2 atmosphere. Finally, the obtained microcapsules were separated by filter paper and washed twice with hexane 19
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instruments, New castle, USA) was carried out for observation of thermal decomposition behavior of the microcapsules. The heating rate was 20 °C/min in the temperature range of 30–600 °C under N2 atmosphere. Magnetic properties of nanoparticles and Mag-PCM microcapsules were characterized by vibration sample magnetometer (VSM, 155, Princeton Applied Research (PAR), USA) in applied field up to 10,000 Oe at room temperature. The saturation magnetization, remanence magnetization, and coercive force were calculated from the resulting magnetization curves. Thermal conductivity of Mag-PCM samples were measured using thermal conductivity meter (EKO HC-110, Simultech, Australia) according to ASTM C-518 standard. 3. Results and discussion Suspension polymerization is one of the favorite methods for microencapsulation process [6]. Paraffin, as an important family of PCMs, could be employed in suspension polymerization easier than emulsion techniques for preparation of PCMs microcapsules. Thermal storage property of such microcapsules makes them a good candidate in several industries. Moreover, providing magnetic properties to PCM microcapsules will develop their capabilities in the hi-tech fields, where responses to electromagnetic waves are essential. So, magnetic PCM microcapsules include thermal storage and thermo-regulating properties together with response to external magnetic fields and electromagnetic waves. Here, PMMA as a known shell in PCM microcapsules was selected and magnetite/HD/PMMA microcapsules were designed to be prepared through suspension polymerization and their thermal, morphological and magnetic properties were investigated extensively.
Fig. 1. Dispersion of the prepared Fe3O4 NPs and m-Fe3O4 NPs (hydrophobic) in heptane/ water mixture.
to remove any adsorbed organic impurities. This process was carried out with different magnetite contents (from 0 to 4 wt% relative to the total amount of monomers and HD) to give Mag-PCM 0–4 samples, which the numbers address to the m-Fe3O4 NPs weight percent in the recipe. Moreover, a sample without any HD and m-Fe3O4 NPs (PMMA-N) was prepared as the control one. The quantities of materials in preparation of different samples have been summarized in Table 1.
3.1. Characterization of magnetic PCM microcapsules
2.5. Characterization
FTIR spectroscopy was used to characterize chemical structure and composition of the magnetic PCM microcapsules (Fig. 2). Modified magnetite with oleic acid (m-Fe3O4) was washed with ethanol to remove non-adsorbed oleic acid thoroughly. FTIR spectrum of m-Fe3O4 in Fig. 2 represents strong absorption bands at 577 and 449 cm−1 relating to Fe–O vibrations as the characteristic peaks of Fe3O4. The absorption band at 1718 cm−1 is relevant to the stretching vibration of carbonyl groups in the adsorbed oleic acid through hydrogen bonding [30]. The characteristic peaks of magnetite (577 and 449 cm−1) have been appeared in Mag-PCM 4 spectrum too. The comparison between FTIR spectra of HD and PMMA-N and peaks in Mag-PCM 4 shows the presence of HD and PMMA in the prepared magnetic PCM microcapsules. The crystalline structure of m-Fe3O4 NPs and magnetic PCM microcapsules were investigated by XRD and the corresponding patterns have been given in Fig. 3. Since HD is in molten state in the analysis condition, there would be no interference between crystalline structure of HD and magnetite particles in the XRD analysis. XRD pattern of mFe3O4 NPs displays a set of diffraction peaks at 2θ of 30.2°, 35.6°, 43.4°, 53.8°, 57.2°, 63.1°, and 74.2°, which are well assigned to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) planes, respectively [26,29]. This depicts that the magnetite nanoparticles have properly been crystallized and the position and relative intensity of the diffraction peaks match well with the standard XRD data for the bulk magnetite (JCPDS File No. 19-0629) crystalline structure with inverse spinal. Also, Mag-PCM 2 and Mag-PCM4 XRD patterns including mFe3O4 NPs revealed similar crystalline behavior, owing to all set of diffraction peaks for Fe3O4 NPs. On the other hand, no characteristic diffraction peak regarding to PMMA could be found in their XRD patterns, implying the amorphous PMMA matrix.
Fourier transform infrared (FTIR) spectra were recorded using Perkin-Elmer Spectrum RX1 spectrophotometer (USA) over the range of 400 to 4000 cm−1 using KBr pellet. Powder X-ray diffraction (XRD) measurement was performed to confirm the crystalline structure of magnetic particles on a Siemens D5000 (Germany) X-ray diffractometer with Cu-Kα radiation (k = 0.154 nm) and was operated at 40 kV and 40 mA with a scan rate of 1.2°/min in the range of 5–70°. Particle size and the morphology of the prepared magnetite and microcapsules were assessed by means of a scanning electron microscope (SEM, FEI Quanta 200 FEG, Eindhoven, Netherlands) equipped with a secondary electron detector. SEM micrographs were collected at operating in low vacuum mode and using an acceleration voltage of 30 kV. Thermal behavior of the microcapsules was determined by differential scanning calorimetry (DSC, Q20, TA instruments, New castle, USA). Samples of about 5 mg in aluminum pans were cooled and heated in the temperature range of −30 to 60 °C at the ramp rate of 10 °C/min under N2 atmosphere. Thermal gravimetric analysis (TGA, Q5000, TA Table 1 The recipe for preparation of magnetic PCM microcapsules. Sample
PMMA-N Mag-PCM 0 Mag-PCM0.5 Mag-PCM1 Mag-PCM2 Mag-PCM3 Mag-PCM4 a
HD (g)
0 8.60 8.6 8.6 8.6 8.6 8.6
m-Fe3O4 NPs g
wt%a
0 0 0.215 0.43 0.86 1.29 1.72
0 0 0.5 1 2 3 4
3.2. Morphological studies Microcapsules morphology has an important role for obtaining the
Relative to the total amount of monomers and HD.
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the recipe in Mag-PCM 0, the particles became wrinkled with size of about 180 μm. This could be attributed to the shell shrinkage during polymerization to minimize internal stresses between PMMA shell and HD as the core material. High interfacial tension between PMMA and HD could impose internal stress on polymer chains and by considering the polymerization temperature (70 °C, which is lower than the Tg of PMMA homopolymer), rigid PMMA chains are not flexible enough to tolerate such a stress and tend to relax to give a wrinkled surface (Fig. 5b). However, no more changes in the morphology of PCM microcapsules were observed with the addition of m-Fe3O4 NPs (Fig. 5c and d). This demonstrates the proper modification of Fe3O4 NPs in a way that the addition of m-Fe3O4 NPs did not affect on the interfacial tension between core (HD) and shell (PMMA) remarkably. So, the final morphologies of magnetic PCM microcapsules were similar to MagPCM 0. In addition, no coalescence in microcapsules was found in these samples. In order to investigate core–shell structure of the prepared magnetic PCM microcapsules, the samples were crushed to observe the fractured surfaces and SEM images were taken (Fig. 6). For PMMA-N, filled particles were observed, on the contrary to multi-nucleus or pomegranate-like morphology for Mag-PCM 0 and Mag-PCM 4. These are the consequence of some thermodynamic (i.e. interfacial tension between HD and PMMA) and kinetic (i.e. polymerization rate) features which control the morphology of magnetic PCM microcapsules and were discussed in our previous work [6]. It is believed that the microencapsulation process and formation of core/shell morphology were mainly influenced by the thermodynamic features, but the high polymerization rate of MMA did not permit the phase separation to proceed completely. So, new micron-sized spherical PMMA particles were produced beside the formation of main PCM microcapsules and inside them (Fig. 6b and c). This dual particle growth was not affected by the added m-Fe3O4 NPs and the magnetite nanoparticles did not involve in the nucleation and particle growth stages (Fig. 6d and e). These multinucleus or pomegranate-like morphologies are quit fascinating due to the better encapsulation and protection of the core content against undesirable leakage. Moreover, EDX mapping was utilized to investigate the dispersion of m-Fe3O4 NPs in the Mag-PCM series samples (Fig. 7). It is evident that m-Fe3O4 NPs had good dispersion inside the magnetic PCM microcapsules and no serious agglomeration was found by increasing mFe3O4 NPs content. This is also another indication of proper modification of m-Fe3O4 NPs for reaching to efficient dispersion in the droplets during microencapsulation.
Fig. 2. FTIR spectra of Mag-PCM 4, m-Fe3O4, m-Fe3O4, PMMA-N and HD.
3.3. Thermal properties Thermal stability of Mag-PCM microcapsules and the actual content of Fe3O4 NPs in the microcapsules were investigated by TGA and differential thermogravimetric analysis (DTG) (Fig. 8). Pure n-hexadecane and neat PMMA (PMMA-N) exhibited a typical one-step thermal degradation with almost no residue and maximum decomposition temperature (Tdmax) at 190 and 380 °C, respectively. As expected, Fe3O4 NPs represented no specific weight loss upon heating up to 700 °C because of the inorganic nature of these nanoparticles. m-Fe3O4 NPs showed weight loss through a two-step thermal degradation with two Tdmax at 250 and 385 °C. Total weight loss at 700 °C for Fe3O4 NPs and m-Fe3O4 NPs were found to be 4% and 26%, respectively. Therefore, the weight percent of adsorbed OA onto Fe3O4 NPs during modification process was about 22 wt%. It is reasonable to say that thermal degradation profiles for MagPCM series samples look a cumulative of those for HD, PMMA-N and mFe3O4 NPs (Fig. 8b and d). However, the maximum degradation temperatures shifted slightly to higher ones with the increase in magnetite content. Moreover, Td5 (temperature for 5% weight loss) of the samples were moved from 128 to 168 °C by the incorporation of magnetite into the microcapsules (Table 2). Furthermore, Td95 (temperature for 95%
Fig. 3. XRD patterns of (a) m-Fe3O4 NPs, (b) Mag-PCM 2 and (c) Mag-PCM 4.
desired thermal storage properties. Here, the formation of Fe3O4 NPs was investigated by SEM analysis before and after stabilization with oleic acid (Fig. 4). It seems that Fe3O4 NPs are spherical-like with average size of 35–40 nm and addition of OA didn’t have any specific influence on particle size distribution. In other words, OA has just achieved to hydrophobize Fe3O4 NPs without affecting on their size distribution. Fig. 5 shows SEM images of PMMA-N and the prepared magnetic PCM microcapsules. Mag-PCM samples with size of 180–200 μm showed a wrinkled surface and opposite to PMMA-N with smooth surface. Based on our previous study, the interfacial tension between PMMA and HD plays a crucial role for obtaining core/shell structure with such a morphology [6]. PMMA-N sample contained spherical particles with average size of 140 μm. When HD was added to 21
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Fig. 4. SEM images of (a) Fe3O4 NPs and (b) m-Fe3O4 NPs.
Fig. 5. SEM images of (a) PMMA-N, (b) Mag PCM 0, (c) Mag PCM 2 and (d) Mag PCM 4.
samples were obtained according to the residue at 600 °C and upon complete degradation of the organic phases and they were calculated with respect to the residue for Mag-PCM 0 (Table 2). The results revealed a good agreement with the added m-Fe3O4 NPs in the recipes, depicting almost complete encapsulation of the primary used m-Fe3O4 NPs.
weight loss) in these samples were increased from 391 to 414 °C from Mag-PCM 0 to Mag-PCM 4. These elevations in Td5 and Td95 could be attributed to the capability of m-Fe3O4 NPs to absorb heat and improve thermal resistance of the obtained magnetic PCM microcapsules and retard HD leakage and shell degradation. However, the encapsulated magnetite content in Mag-PCM series
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Fig. 6. SEM images of the crushed (a) PMMA-N, (b, c) Mag-PCM 0, (c, d) Mag-PCM 4 samples.
Fig. 7. EDX mapping of (a) Mag-PCM 0.5, (b) Mag-PCM 2 and (c) Mag-PCM4 samples.
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Fig. 8. TGA (a) and DTG (b) of PMMA-N, Fe3O4 NPs, m-Fe3O4 NPs and HD; and TGA (c) and DTG (d) of Mag-PCM series samples.
agent in the crystallization process. This results in the appearance of two different Tcs in Mag-PCM 1 and resumes coherently until reaching to another single Tc in Mag-PCM 4. Observation of one Tc in the sample with highest level of m-Fe3O4 NPs returns to the dominant crystallization through nucleation process. Despite the crystallization peaks, all samples revealed a single melting temperature (Tm) between 18 and 20 °C, corresponding to Tm of pure HD (18 °C) (Fig. 9c). This illustrates that although two crystallization peaks have been emerged, the obtained HD crystalline structures are identical and m-Fe3O4 NPs can only influence on the crystallization process. In addition and due to the nucleating effect of m-Fe3O4 NPs, the number of HD crystal domains increases. These appear in more narrow and sharper melting peaks which is evident in Fig. 9c. Furthermore, the increase in thermal conductivity of the resulting microcapsules after incorporation of m-Fe3O4 NPs is another reason for this observation. Supercooling phenomenon is considered as a major obstacle to the practical application of PCM microcapsules [9]. Supercooling temperatures ((ΔTs) is determined from crystallization-melting hysteresis of a PCM (ΔTsi = Tm − Tci), where “i” refers to each crystallization temperature. As shown in Table 3, ΔTs2 for Mag-PCM samples is lower than ΔTs1 for Mag-PCM 0due to the incorporation of m-Fe3O4 NPs. This would be attributed to the increase in thermal conductivity by the presence of m-Fe3O4 NPs and the role of m-Fe3O4 NPs as a nucleating source for crystallization of HD [26]. The crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) are two important indexes for representing thermal storage-release capability of a PCM. It is understood (Table 3) that pure HD has considerably high phase-change enthalpy of 218 J/g, indicating an excellent phase change material. However, the encapsulation of HD into PMMA shell (Mag-PCM series) resulted in reduction of phase change enthalpies obviously. This reduction is quite expectable, as the heat changes in DSC are measured relative to the sample weight. and for Mag-PCM series samples, the quantity of HD as the responsible component for heat storage and release is lower with respect to pure HD.
Table 2 Thermal degradation information of the samples from TGA. Sample
Td5 (°C)
Td95 (°C)
Residue at 600 °C (wt%)
HD Fe3O4 m-Fe3O4 Mag-PCM Mag-PCM Mag-PCM Mag-PCM Mag-PCM Mag-PCM
126 – 202 128 129 166 169 168 168
244 – – 391 410 398 407 410 414
0 96 74 ∼0 0.5 1 2 3 4
0 0.5 1 2 3 4
The phase-change behavior and thermal storage-release performance were investigated by DSC scans for the magnetic PCM microcapsules as well as pure HD as a control sample. The resulting DSC thermograms have been shown in Fig. 9 and meanwhile, the extracted phase change characteristic data from DSC thermograms have been summarized in Table 3. It is evident that the phase-change behavior of HD has been notably affected by the incorporation of m-Fe3O4 NPs. The presence of Fe3O4 in microcapsules causes a bimodal crystallization temperature from DSC thermograms [26]. Pure HD showed distinct and sharp melting and crystallization temperatures at 18 and 11 °C, respectively (Fig. 9a), but an interesting phenomenon was observed during crystallization process (Fig. 9b). For the samples with no (Mag-PCM 0) or low (Mag-PCM 0.5) or even higher (Mag-PCM 3 and 4) magnetite contents, the crystallization of HD occurred in a broad range with two different temperatures (Tc1 and Tc2) and the details have been reported in Table 3. With the increase in m-Fe3O4 NPs from 0.5 to 3 wt%, bimodality in crystallization temperature is quite obvious. This is a remarkable indicator of incorporation of m-Fe3O4 NPs into the prepared PCM microcapsules. In other words, magnetite nanoparticles were involved as a nucleating 24
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Fig. 9. DSC thermograms of (a) crystallization and melting of pure HD, (b) crystallization and (c) melting of the prepared Mag-PCM series samples.
Table 3 Thermal characteristics of pure HD and magnetic PCM microcapsules extracted from DSC thermograms. Tc1a (°C)
Tc2b (°C)
Tm (°C)
ΔTs1 (°C)
ΔTs2 (°C)
ΔHc (J/ g)
ΔH(Tc1) (J/g)
ΔH(Tc2) (J/g)
ΔHm (J/ g)
ΔH′ (J/ g)
C
0
11 7 8
– – –
18 19 18
7 12 10
– – –
218 53.1 52
53.1 52
– –
218 53.2 52.2
– 52.9 51.8
1 2 3 4
8 8 10 –
12 13 12 12
21 19 19 19
13 11 9 –
9 6 7 7
53 51.1 52.3 51.4
38.6 22.9 – –
14.1 28.4 52.3 51.4
53.1 51.3 52.5 51.5
52.7 51.0 52.3 51.2
Sample
HD Mag-PCM Mag-PCM 0.5 Mag-PCM Mag-PCM Mag-PCM Mag-PCM a b
E (%)
TCP (%)
Magnetite content from TGA (%)
– 24.4 23.9
– 97.6 95.6
– 99.4 99.2
– – 0.5
24.4 23.6 24.1 23.6
97.6 94.4 96.4 94.4
99.2 99.4 99.6 99.4
1 2 3 4
E
(%)
First crystallization peak at lower temperature. Second crystallization peak at higher temperature.
Experimental core content (CE, Eq. (1)) and encapsulation efficiency (E, Eq. (2)) of the prepared magnetic PCM microcapsules were used to evaluate their thermal storage properties (Table 3).
CE (%) =
E (%) =
ΔHm × 100 ΔHHD CE × 100 Ct
(1)
(2)
The encapsulation efficiencies were all above 94%, revealing successful encapsulation of HD by PMMA after polymerization. Moreover, no obvious change in E and CE of the microcapsules was observed by adding magnetite nanoparticles, depicting that the incorporation of mFe3O4 NPs did not sacrifice their thermal properties. Also, due to the presence of two crystallization temperatures, two crystallization enthalpies could be considered (ΔH (Tc1), ΔH (Tc2)). So, the crystallization curves were deconvoluted by “Origin V.9” software and the quantity of each deconvoluted peak has been given in Table 3. The enthalpies ratio
Fig. 10. ΔH (Tc1)/ΔHm and ΔH (Tc2)/ΔHm variations vs. m-Fe3O4 NPs content.
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Table 4 Magnetic parameters of the prepared samples. Sample
Ms (emu/ g)
Mr (emu/ g)
Hc (Oe)
Mr/Ms
Magnetization loss (%)a
Fe3O4 m-Fe3O4 Mag-PCM 0.5 Mag-PCM Mag-PCM Mag-PCM Mag-PCM
65 58 0.095
12 12 0.02
82 108 124.5
0.18 0.21 0.21
– 10.76 67.24
0.33 0.8 1.2 1.45
0.08 0.2 0.2 0.3
140 142 135 125
0.24 0.25 0.17 0.21
42.10 31.03 31.03 37.5
1 2 3 4
a Magnetization loss for m-Fe3O4 was calculated with respect to Fe3O4 NPs and for Mag-PCM series was calculated relative to m-Fe3O4 NPs.
65 emu/g with Mr/Ms. of 0.18 which demonstrates the achievement in preparation of almost superparamagnetic Fe3O4 NPs. Ms. for m-Fe3O4 NPs was 58 emu/g with 10.7% reduction relative to Fe3O4 NPs. This could be explained by two reasons: (i) 22 wt% surface modification of Fe3O4 NPs (from TGA results) in which the adsorbed OA on the surface of magnetite acts as an impurity and decreases the saturation magnetization[31], (ii) employment of ultrasonic irradiation in the procedure that causes partial oxidation of Fe3O4 and results in reducing magnetization per weight unit of magnetite [32]. Magnetization curve of Mag-PCMs have been demonstrated in Fig. 12 and the numerical data have been listed in Table 4. Low magnetization remanence and coercivity were observed, representing superparamagnetic behavior of the Mag-PCM samples. On the other hand, the observed Ms amounts from VSM analysis were lower than the expected ones. For instance, Mag-PCM 4 with 4 wt% magnetite content is expected to have Ms of 2.32 emu/g (relative to Ms of m-Fe3O4 NPs), but the observed Ms was 1.45 emu/g with 37.5% loss in magnetization. The amounts of magnetization loss for all the samples have been listed in Table 4 too. This would be attributed to the oxidation of magnetite during suspension polymerization. It is worth mentioning that the presence of peroxide initiators along with high shear rate of mixing during polymerization are responsible for promoting the above oxidation [32]. However, the increasing trend in magnetization from 0.095 to 1.45 emu/g for Mag-PCM 0.5 to Mag-PCM 4 is consistent with the aforementioned increase in magnetite content. Mr/Ms values imply that superparamagnetic properties of the obtained microcapsules have been preserved during this suspension polymerization. This means that these magnetic microcapsules can be attracted to by an external magnetic field and retain no residual magnetization when the magnetic field is removed at room temperature. Such a feature makes the magnetic microcapsules do not tend to aggregate together after removal of the magnetic field [33]. The prepared magnetic PCM microcapsules were readily dispersed in water to form an aqueous suspension by simple ultrasonication, and
Fig. 11. Thermal performance of Mag-PCM 1 and Mag-PCM 4 before (solid line) and after 300 heating-cooling cycles (dashed line).
of deconvoluted peaks to total melting peak showed a specific trend (Fig. 10). Both trends demonstrate a turning point in which, there will be no difference in crystallization behavior. Here, this point was appeared at 3 wt% of m-Fe3O4 NPs. Performance of the prepared magnetic PCM microcapsules was controlled by heating-cooling cycles for 300 times. The test gives useful information about efficiency of protecting HD by PMMA shell in the obtained PCM microcapsules. For this reason, DSC thermograms of the samples were recorded before and after thermal cycles. The observed thermal cycling performances (TCP) have been given in Table 3 and calculated from Eq. (3).
TCP (%) =
ΔH ′ × 100 ΔHm
(3)
ΔH′ is the melt enthalpy of washed samples after thermal cycling for 300 times. Fig. 11 shows the performance of the two samples before and after heating-cooling cycles. No significant difference was observed for the enthalpies of Mag-PCMs after thermal cycling process, revealing the merit of these Mag-PCMs for versatile applications as dual-responsive phase change materials. 3.4. Studies on magnetic properties Magnetic properties of the prepared Fe3O4 NPs, m-Fe3O4 NPs and Mag-PCMs were evaluated by a vibrating sample magnetometer (VSM) at room temperature (25 °C) and the resulting magnetic hysteresis loops have been presented in Fig. 12. The superparamagnetic behavior is determined by high saturation magnetization (Ms), low remanence magnetization (Mr) and coercive force (Hc) values and also approaching of Mr/Ms to zero. The saturation magnetization of Fe3O4 NPs was
Fig. 12. Magnetic hysteresis loops of (a) Fe3O4 NPs and m-Fe3O4 NPs; and (b) Mag-PCM series microcapsules.
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Fig. 13. Thermal conductivity of the prepared Mag-PCM series samples (a) and its growth with increase in magnetite content (b).
they were shortly attracted by an external magnet. They were quickly redispersed with a slight shaking, once the magnetic field was removed. Such a good magnetic response may render the prepared magnetic PCM microcapsules in fast magnetic manipulation in actual applications.
and superparamagnetic characteristics were measured. Thermal conductivity of the Mag-PCM microcapsules increased linearly with the increase in Fe3O4 NPs content. It is believed that such Mag-PCM microcapsules could be considered in the future generation of PCMs that are called smart PCM materials. The introduced microcapsules with such dual functional features open a route toward application of these materials in manufacturing of microelectronic elements, electronic devices, textile and even special buildings such as hospitals with antijamming and thermal regulating effectiveness.
3.5. Thermal conductivity Although pure n-hexadecane has many desirable properties as a useful PCM, its organic nature with no free electron results in a low thermal conductivity [1]. This may delay its thermal response to the storage and release of the latent heat. Therefore, enhancement in thermal conductivity is absolutely essential when designing microencapsulated PCMs. It is expected that the encapsulation of n-hexadecane together with an inorganic material with higher thermal conductivity compensates this defect. Pure HD (in solid state) and PMMA display thermal conductivities as low as 0.15 [34] and 0.19 [35] W m−1 K−1, respectively. Thermal conductivities of the prepared magnetic PCM microcapsules were measured and their significant growth has been shown in Fig. 13. Interestingly, all of the prepared microcapsules showed thermal conductivities above than 0.19 W m−1 K−1. This observation is anticipated to the presence of Fe3O4 NPs in the microcapsules, while thermal conductivity of Fe3O4 NPs is 9.7 W m−1 K−1 [22,27]. These results confirm the encapsulation of m-Fe3O4 NPs in the presence of n-hexadecane and they actively impart more thermal conductivity to the resulting PCM microcapsules linearly. It is also noteworthy that the thermal conductivities for Mag-PCMs are associated with the quantity of included m-Fe3O4 NPs, and the higher thermal conductivity is attainable at higher magnetite contents. This depicts that m-Fe3O4 NPs play a critical role in enhancing heat transfer in Mag-PCMs with providing a continuous phase to the encapsulated HD. Such a continuous phase can be considered as a virtual heat transfer network and thus enhances the heat transfer rate over the whole microcapsule.
Acknowledgment We wish to express our gratitude to Iran Polymer and Petro-chemical Institute (IPPI) for partial financial support of this work (Grant#2207). References [1] V. Kapsalis, D. Karamanis, Solar thermal energy storage and heat pumps with phase change materials, Appl. Therm. Eng. 99 (2016) 1212–1224. [2] S. Kahwaji, M.B. Johnson, A.C. Kheirabadi, D. Groulx, M.A. White, Stable, low-cost phase change material for building applications: the eutectic mixture of decanoic acid and tetradecanoic acid, Appl. Energy 168 (2016) 457–464. [3] R. Baetens, B.P. Jelle, A. Gustavsen, Phase change materials for building applications: a state-of-the-art review, Energy Build. 42 (9) (2010) 1361–1368. [4] M.K. Rathod, J. Banerjee, Thermal stability of phase change materials used in latent heat energy storage systems: a review, Renew. Sustain. Energy Rev. 18 (2013) 246–258. [5] L.F. Cabeza, A. Castell, C. Barreneche, A.De Gracia, A.I. Fernández, Materials used as PCM in thermal energy storage in buildings: a review, Renew. Sustain. Energy Rev. 15 (3) (2011) 1675–1695. [6] S. Lashgari, H. Arabi, A.R. Mahdavian, V. Ambrogi, Thermal and morphological studies on novel PCM microcapsules containing N-hexadecane as the core in a flexible shell, Appl. Energy 190 (2017) 612–622. [7] N. Ukrainczyk, S. Kurajica, J. Šipušiæ, Thermophysical comparison of five commercial paraffin waxes as latent heat storage materials, Chem. Biochem. Eng. Q. 24 (2) (2010) 129–137. [8] C.Y. Zhao, G.H. Zhang, Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications, Renew. Sustain. Energy Rev. 15 (8) (2011) 3813–3832. [9] W. Su, J. Darkwa, G. Kokogiannakis, Review of solid–liquid phase change materials and their encapsulation technologies, Renew. Sustain. Energy Rev. 48 (2015) 373–391. [10] R. Jacob, F. Bruno, Review on shell materials used in the encapsulation of phase change materials for high temperature thermal energy storage, Renew. Sustain. Energy Rev. 48 (2015) 79–87. [11] S. Lingamneni, M. Asheghi, K.E. Goodson, A parametric study of microporous metal matrix-phase change material composite heat spreaders for transient thermal applications, Fourteenth Intersoc. Conf. Therm. Thermomech. Phenom. Electron. Syst. (2014) 870–875. [12] M. Mazman, L.F. Cabeza, H. Mehling, H.O. Paksoy, H. Evliya, Heat transfer enhancement of fatty acids when used as PCMs in thermal energy storage, Int. J. Energy Res. 32 (2008) 135–143. [13] A. Mills, M. Farid, J.R. Selman, S. Al-Hallaj, Thermal conductivity enhancement of phase change materials using a graphite matrix, Appl. Therm. Eng. 26 (14–15) (2006) 1652–1661. [14] J. Wang, H. Xie, Z. Xin, Y. Li, L. Chen, Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers, Sol. Energy 84
4. Conclusion In this work, novel dual functional hexadecane-PMMA PCM microcapsules containing Fe3O4 nanoparticles were designed and prepared by suspension polymerization. Hydrophobic Fe3O4 NPs were prepared by co-precipitation method and their subsequent modification with oleic acid for better dispersion in the paraffinic core. A complete morphological study was carried out and the results showed a wrinkled surface for the magnetic PCM microcapsules. Extensive thermogravimetric and calorimetric observations were performed to elucidate thermal behavior of the microcapsules. The obtained Mag-PCM microcapsules were responsive to an external magnetic field due to the presence of Fe3O4 NPs and their magnetic parameters such as Ms, Mr, Hc 27
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[26] F. Jiang, X. Wang, D. Wu, Design and synthesis of magnetic microcapsules based on N-eicosane core and Fe3O4/SiO2 hybrid shell for dual-functional phase change materials, Appl. Energy 134 (2014) 456–468. [27] S. Park, Y. Lee, Y.S. Kim, H.M. Lee, J.H. Kim, I.W. Cheong, W.-G. Koh, Magnetic nanoparticle-embedded PCM nanocapsules based on paraffin core and polyurea shell, Colloids Surf. A Physicochem. Eng. Asp. 450 (2014) 46–51. [28] F. Jiang, X. Wang, D. Wu, Magnetic microencapsulated phase change materials with an organo-silica shell: design, synthesis and application for electromagnetic shielding and thermal regulating polyimide films, Energy 98 (April) (2016) 225–239. [29] A.R. Mahdavian, M.A.S. Mirrahimi, Efficient separation of heavy metal cations by anchoring polyacrylic acid on superparamagnetic magnetite nanoparticles through surface modification, Chem. Eng. J. 159 (1–3) (2010) 264–271. [30] M. Ashjari, A.R. Mahdavian, N.G. Ebrahimi, Y. Mosleh, Efficient dispersion of magnetite nanoparticles in the polyurethane matrix through solution mixing and investigation of the nanocomposite properties, J. Inorg. Organomet. Polym. Mater. 20 (2) (2010) 213–219. [31] A.R. Mahdavian, M. Ashjari, H.S. Mobarakeh, Nanocomposite particles with coreshell morphology. I. Preparation and characterization of Fe3O4-Poly(butyl acrylatestyrene) particles via miniemulsion polymerization, J. Appl. Polym. Sci. 110 (2) (2008) 1242–1249. [32] A. Mahdieh, A.R. Mahdavian, H. Salehi-Mobarakeh, Chemical modification of magnetite nanoparticles and preparation of acrylic-base magnetic nanocomposite particles via miniemulsion polymerization, J. Magn. Magn. Mater. 2017 (426) (November 2016) 230–238. [33] F. Jiang, X. Wang, D. Wu, Design and synthesis of magnetic microcapsules based on N-eicosane core and Fe3O4/SiO2 hybrid shell for dual-functional phase change materials, Appl. Energy 134 (2014) 456–468. [34] L. Fan, J.M. Khodadadi, Thermal conductivity enhancement of phase change materials for thermal energy storage: a review, Renew. Sustain. Energy Rev. 15 (1) (2011) 24–46. [35] M.J. Assael, S. Botsios, K. Gialou, I.N. Metaxa, Thermal conductivity of polymethyl methacrylate (PMMA) and borosilicate crown glass BK7, Int. J. Thermophys. 26 (5) (2005) 1595–1605.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Preparation of acrylic PCM microcapsules with dual responsivity to temperature and magnetic field changes
T
⁎
Somayeh Lashgaria,b, Ali Reza Mahdaviana, , Hassan Arabia, Veronica Ambrogic,d, Valentina Marturanoc,d a
Iran Polymer and Petrochemical Institute, P.O. Box: 14965-115, Tehran, Iran National Petrochemical Company, Petrochemical Research & Technology Company, P.O. 1435884711, Tehran, Iran c Department of Chemical, Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy d Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy b
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase change materials Acrylic Magnetic Microcapsule Stimuli-responsive
Energy preservation is one of the serious concerns in recent years to reduce fossil fuels consumption and greenhouse gases. Magnetic microcapsules based on n-hexadecane/Fe3O4 core and poly methyl methacrylate (PMMA) shell were designed as an innovative type of dual-functional phase change materials. Fe3O4 nanoparticles were first synthesized by co-precipitation method and then modified with oleic acid. Then, they were incorporated into suspension polymerization in the presence of MMA and n-hexadecane. FTIR and SEM analyses confirmed the presence of all components in the magnetic microcapsules with a wrinkled morphology and uniform size in averagely 180 µm. They revealed reasonable thermal storage properties and high thermal cycling performance according to DSC thermograms. The encapsulation efficiency and thermal storage ability were calculated by the extracted data from DSC analysis for all the samples and their thermal conductivities were measured too. Magnetometry studies approved superparamagnetic properties of the obtained microcapsules with low remanence and coercivity. With such a dual-functional feature, the introduced magnetic microcapsules could show potential applications in some high-tech fields with providing both thermal regulating and magnetic responsive properties.
1. Introduction Energy preservation has been considered as a major concern in recent years. Phase change materials (PCMs) with spectacular phase transition and functionalities are among successful nominees to overcome this issue [1]. Over the past few decades, PCMs have been known very operative in versatile applications and have exhibited good thermal regulation performance due to their large heat storage capacity and isothermal behavior during phase transitions [2]. PCMs with broad temperature range include organic materials such as paraffins, polyesters, fatty acids, and alcohols or inorganic materials containing pure elements, metals, alloys, salts, and silicates [3–5]. Paraffin waxes (i.e. nalkanes) are of great interest and are preferred than the other PCMs, because of high heat storage capacity, little supercooling and volume change, good chemical and thermal stability, low vapor pressure, no corrosion of the storage container, and non-toxicity [6]. Besides, the melting and crystallization temperatures of paraffin waxes can be easily tuned by their number of carbon [7].
⁎
In spite of many gorgeous features of paraffin-wax-based PCMs, they are not directly used in practice due to the risk of PCM leakage by repeated melting and crystallization, their low thermal conductivity, and poor interfacial combination with accompanying materials [1]. Micro- or nano-encapsulations of PCMs within polymeric shells overcome these complications, as encapsulation can prevent PCMs from leakage during a solid–liquid phase change and can afford a higher heat transfer area per unit volume and consequent higher heat transfer rate relative to the bulk PCMs [8,9]. Moreover, encapsulated PCMs contribute supplementary advantages, such as the ability to tolerate volume changes during the phase change processes [10]. To achieve the disadvantages of low thermal conductivity in organic PCMs, addition of some particles with higher thermal conductivities like metal particles would be a key point [11]. Mazman et al. worked on the improvement of heat transfer using some solid particles like stainless steel and copper pieces and also graphite in fatty acid mixtures [12]. In another technique, Mills et al. exhibited that thermal conductivity of paraffin–EG composites can increase by an order of 20 to 60
Corresponding author. E-mail address: [email protected] (A.R. Mahdavian).
https://doi.org/10.1016/j.eurpolymj.2018.02.011 Received 18 October 2017; Received in revised form 7 January 2018; Accepted 8 February 2018 Available online 09 February 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved.
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2. Experimental
folds and this will increase the expenses of PCMs production in a constant volume [13]. Multi-walled carbon nanotubes with high thermal conductivity have been employed recently and the results showed that addition of just 1 wt% carbon nanotubes increased thermal conductivity and thermal energy storage capacity remarkably [14–16]. In has been demonstrated that addition of nano-SiO2 play a vital role on the heat enthalpies of the composite fibers without any substantial effect on the phase change temperatures [17]. The effect of different contents of Al2O3 nanoparticles on paraffin thermal behavior has been studied by Ho et al. [18] and in a recent attempt, the improvement in thermal conductivity of an organic PCM has been investigated in the presence of copper nanowires [19]. All of these observations demonstrate that addition of nano-materials and minerals to organic PCMs can upsurge both thermal conductivity and thermal storage capacity [20].The effect of calcium carbonate as a component for enhancing thermal conductivity of n-octadecane was investigated before [21]. Sahan and coworkers studied the role of Fe3O4 nanoparticles on heat transfer properties of paraffin and they found it is an effective technique to improve the heat transfer properties of paraffin [22]. The other deficiency of organic PCMs that limits their exploitations is the supercooling phenomenon and is expressed as the difference between melting and crystallization temperatures [23]. Supercooling makes the latent heat to be released at lower temperatures or a broader temperature range, which is a disadvantageous for the energy storage applications. Many attempts have been made to suppress the supercooling process. In 2015, PCM microcapsules were prepared in the presence of Rubitherm RT58 as a nucleating agent and it was observed that by addition of 5 wt% of the nucleating agent, supercooling reduced dramatically [24]. Yang et al. investigated the effect of silicon nitride in n-octadecane/PMMA microcapsules and their results showed that silicon nitride acted as a nucleating agent and suppressed the supercooling to some extent [25]. High thermal conductivity of magnetite (Fe3O4) nanoparticles (9.7 W/m K) [22,25] makes it encouraging to enhance thermal conductivity of organic PCMs. On the other hand, magnetic particles are able to absorb electromagnetic irradiations and thus, their incorporation can add a novel performance to the micro PCMs. Magnetic microcapsules based on n-eicosane core with Fe3O4/SiO2 shell have been synthesized in recent years [26]. The results illustrate proper thermal behavior along with good magnetic properties for the microcapsules. Park et al. prepared PCM/magnetite microcapsules based on polyurea shell with the aim of enhancing thermal conductivity and providing magnetic response to the microcapsules [27]. In 2016, PCM/magnetite microcapsules have been produced through interfacial polymerization [28].The obtained composite films of polyamide and microcapsules represented dual functional feature as thermal regulating and electromagnetic shielding properties. In this study, a new type of such microcapsules containing both nhexadecane and magnetite with PMMA shell have been prepared through suspension polymerization, while Fe3O4 nanoparticles are expected to enhance thermal conductivity of the microcapsules as well as suppressing supercooling by acting as nucleating agents. Moreover, these magnetic PCM microcapsules may offer magnetic responses to increase their application scope. Such a dual-functional feature proposes the potential capability of these microcapsules as smart materials due to both thermal regulating and magnetic properties. With this regard, magnetite nanoparticles were first synthesized using co-precipitation method and then modified by oleic acid to become hydrophobe. Then magnetic PCM microcapsules were prepared through suspension polymerization and the influence of magnetite content on thermal, morphological and magnetic properties of the obtained microcapsules were investigated.
2.1. Materials n-Hexadecane (HD) as the hydrophobic core, methyl methacrylate (MMA) as the shell monomer, ethylene glycol dimethacrylate (EGDMA) as the crosslinking agent and benzoyl peroxide(BPO) as the initiator were all purchased from Merck Chemical Company. Poly (vinyl alcohol) (PVA, Sigma-Aldrich, USA, molecular weight: 40,000–88,000, degree of hydrolysis: 88%) was used as the stabilizer. Iron salt hydrates (FeCl3·6H2O, FeCl2·4H2O) oleic acid (OA), ammonium hydroxide (NH4OH, 25% ammonia) and HCl (38%) were used for preparation of Fe3O4 magnetic nanoparticles. All chemicals for Fe3O4 synthesis were purchased from Merck Chemical Company. Deionized water (DI water, conductivity < 2 µs/cm) was employed in all experiments. All materials were used as received without further purification. 2.2. Synthesis of Fe3O4 nanoparticles Fe3O4 magnetic nanoparticles were first prepared by co-precipitation of Fe(II) and Fe(III) chlorides in an ammonia solution according to our previous report [22,29]. Typically, FeCl2.4H2O (2.0 g) and FeCl3·6H2O (5.2 g) were added into a 500-mL three-necked roundbottom flask equipped with a nitrogen gas inlet, mechanical stirrer and dropping funnel. Meanwhile, 25 mL DI water was poured into the flask and the mixture was stirred at 500 rpm to form a clear yellow solution under nitrogen atmosphere. Then, 250 mL of 1.5 M ammonia solution was added dropwise into the mixture during 2 h. With stirring over 40 min, black fine Fe3O4nanoparticles were formed and finally, the nanoparticles were carefully separated by an external magnetic field. The fine particles were washed with DI water and rinsed by ethanol several times, followed by 48 h drying at ambient temperature and also vacuum drying for 3 h to give 2.12 g magnetite nanoparticles (Fe3O4 NPs). 2.3. Modification of Fe3O4 NPs In order to obtain stabilized hydrophobic Fe3O4 NPs, the particles were modified with oleic acid to improve their hydrophobicity and avoid their agglomeration. At first, 4 g Fe3O4 NPs and 70 mL DI water were probe-sonicated for 5 min and 1.8 mL oleic acid was added and homogenized by sonication for 5 min. Afterward, 6 mL ammonium hydroxide aqueous solution (25%) was added, following 5 min sonication and vigorous mixing for 2 h. Then, pH of the solution was adjusted from alkaline to acidic by adding concentrated HCl which resulted in separation of unreacted oleic acid. The mixture was centrifuged for 10 min at 3000 rpm. Unreacted oleic acid was decanted as a yellow layer on the water phase. The precipitate was rinsed three times by water/ethanol solution (3:1 by volume), and the solid was vacuumdried at 20 °C for 20 h. The product was coded as m-Fe3O4 NPs. Fig. 1 shows the dispersibility of Fe3O4 NPs and m-Fe3O4 NPs in water and heptane (oil phase). 2.4. Preparation of magnetic PCM microcapsules A 250 mL double-jacketed glass reactor equipped with stirrer, nitrogen gas inlet and outlet and circulating cooling system were used for preparation of the PCM microcapsules with respect to our previous report [6]. First, the aqueous phase containing 132 g DI water and 0.05 g PVA were added to the reactor and mixed to give a homogenous solution. The oil phase including MMA (32.68 g), EGDMA (1.72 g), HD, m-Fe3O4 NPs and 0.21 g BPO was then transferred into the reactor. The mixture was stirred mechanically for 40 min at 800 rpm and the reaction temperature reached to 70 °C. The polymerization conducted for 5 h at 300 rpm under N2 atmosphere. Finally, the obtained microcapsules were separated by filter paper and washed twice with hexane 19
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instruments, New castle, USA) was carried out for observation of thermal decomposition behavior of the microcapsules. The heating rate was 20 °C/min in the temperature range of 30–600 °C under N2 atmosphere. Magnetic properties of nanoparticles and Mag-PCM microcapsules were characterized by vibration sample magnetometer (VSM, 155, Princeton Applied Research (PAR), USA) in applied field up to 10,000 Oe at room temperature. The saturation magnetization, remanence magnetization, and coercive force were calculated from the resulting magnetization curves. Thermal conductivity of Mag-PCM samples were measured using thermal conductivity meter (EKO HC-110, Simultech, Australia) according to ASTM C-518 standard. 3. Results and discussion Suspension polymerization is one of the favorite methods for microencapsulation process [6]. Paraffin, as an important family of PCMs, could be employed in suspension polymerization easier than emulsion techniques for preparation of PCMs microcapsules. Thermal storage property of such microcapsules makes them a good candidate in several industries. Moreover, providing magnetic properties to PCM microcapsules will develop their capabilities in the hi-tech fields, where responses to electromagnetic waves are essential. So, magnetic PCM microcapsules include thermal storage and thermo-regulating properties together with response to external magnetic fields and electromagnetic waves. Here, PMMA as a known shell in PCM microcapsules was selected and magnetite/HD/PMMA microcapsules were designed to be prepared through suspension polymerization and their thermal, morphological and magnetic properties were investigated extensively.
Fig. 1. Dispersion of the prepared Fe3O4 NPs and m-Fe3O4 NPs (hydrophobic) in heptane/ water mixture.
to remove any adsorbed organic impurities. This process was carried out with different magnetite contents (from 0 to 4 wt% relative to the total amount of monomers and HD) to give Mag-PCM 0–4 samples, which the numbers address to the m-Fe3O4 NPs weight percent in the recipe. Moreover, a sample without any HD and m-Fe3O4 NPs (PMMA-N) was prepared as the control one. The quantities of materials in preparation of different samples have been summarized in Table 1.
3.1. Characterization of magnetic PCM microcapsules
2.5. Characterization
FTIR spectroscopy was used to characterize chemical structure and composition of the magnetic PCM microcapsules (Fig. 2). Modified magnetite with oleic acid (m-Fe3O4) was washed with ethanol to remove non-adsorbed oleic acid thoroughly. FTIR spectrum of m-Fe3O4 in Fig. 2 represents strong absorption bands at 577 and 449 cm−1 relating to Fe–O vibrations as the characteristic peaks of Fe3O4. The absorption band at 1718 cm−1 is relevant to the stretching vibration of carbonyl groups in the adsorbed oleic acid through hydrogen bonding [30]. The characteristic peaks of magnetite (577 and 449 cm−1) have been appeared in Mag-PCM 4 spectrum too. The comparison between FTIR spectra of HD and PMMA-N and peaks in Mag-PCM 4 shows the presence of HD and PMMA in the prepared magnetic PCM microcapsules. The crystalline structure of m-Fe3O4 NPs and magnetic PCM microcapsules were investigated by XRD and the corresponding patterns have been given in Fig. 3. Since HD is in molten state in the analysis condition, there would be no interference between crystalline structure of HD and magnetite particles in the XRD analysis. XRD pattern of mFe3O4 NPs displays a set of diffraction peaks at 2θ of 30.2°, 35.6°, 43.4°, 53.8°, 57.2°, 63.1°, and 74.2°, which are well assigned to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) planes, respectively [26,29]. This depicts that the magnetite nanoparticles have properly been crystallized and the position and relative intensity of the diffraction peaks match well with the standard XRD data for the bulk magnetite (JCPDS File No. 19-0629) crystalline structure with inverse spinal. Also, Mag-PCM 2 and Mag-PCM4 XRD patterns including mFe3O4 NPs revealed similar crystalline behavior, owing to all set of diffraction peaks for Fe3O4 NPs. On the other hand, no characteristic diffraction peak regarding to PMMA could be found in their XRD patterns, implying the amorphous PMMA matrix.
Fourier transform infrared (FTIR) spectra were recorded using Perkin-Elmer Spectrum RX1 spectrophotometer (USA) over the range of 400 to 4000 cm−1 using KBr pellet. Powder X-ray diffraction (XRD) measurement was performed to confirm the crystalline structure of magnetic particles on a Siemens D5000 (Germany) X-ray diffractometer with Cu-Kα radiation (k = 0.154 nm) and was operated at 40 kV and 40 mA with a scan rate of 1.2°/min in the range of 5–70°. Particle size and the morphology of the prepared magnetite and microcapsules were assessed by means of a scanning electron microscope (SEM, FEI Quanta 200 FEG, Eindhoven, Netherlands) equipped with a secondary electron detector. SEM micrographs were collected at operating in low vacuum mode and using an acceleration voltage of 30 kV. Thermal behavior of the microcapsules was determined by differential scanning calorimetry (DSC, Q20, TA instruments, New castle, USA). Samples of about 5 mg in aluminum pans were cooled and heated in the temperature range of −30 to 60 °C at the ramp rate of 10 °C/min under N2 atmosphere. Thermal gravimetric analysis (TGA, Q5000, TA Table 1 The recipe for preparation of magnetic PCM microcapsules. Sample
PMMA-N Mag-PCM 0 Mag-PCM0.5 Mag-PCM1 Mag-PCM2 Mag-PCM3 Mag-PCM4 a
HD (g)
0 8.60 8.6 8.6 8.6 8.6 8.6
m-Fe3O4 NPs g
wt%a
0 0 0.215 0.43 0.86 1.29 1.72
0 0 0.5 1 2 3 4
3.2. Morphological studies Microcapsules morphology has an important role for obtaining the
Relative to the total amount of monomers and HD.
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the recipe in Mag-PCM 0, the particles became wrinkled with size of about 180 μm. This could be attributed to the shell shrinkage during polymerization to minimize internal stresses between PMMA shell and HD as the core material. High interfacial tension between PMMA and HD could impose internal stress on polymer chains and by considering the polymerization temperature (70 °C, which is lower than the Tg of PMMA homopolymer), rigid PMMA chains are not flexible enough to tolerate such a stress and tend to relax to give a wrinkled surface (Fig. 5b). However, no more changes in the morphology of PCM microcapsules were observed with the addition of m-Fe3O4 NPs (Fig. 5c and d). This demonstrates the proper modification of Fe3O4 NPs in a way that the addition of m-Fe3O4 NPs did not affect on the interfacial tension between core (HD) and shell (PMMA) remarkably. So, the final morphologies of magnetic PCM microcapsules were similar to MagPCM 0. In addition, no coalescence in microcapsules was found in these samples. In order to investigate core–shell structure of the prepared magnetic PCM microcapsules, the samples were crushed to observe the fractured surfaces and SEM images were taken (Fig. 6). For PMMA-N, filled particles were observed, on the contrary to multi-nucleus or pomegranate-like morphology for Mag-PCM 0 and Mag-PCM 4. These are the consequence of some thermodynamic (i.e. interfacial tension between HD and PMMA) and kinetic (i.e. polymerization rate) features which control the morphology of magnetic PCM microcapsules and were discussed in our previous work [6]. It is believed that the microencapsulation process and formation of core/shell morphology were mainly influenced by the thermodynamic features, but the high polymerization rate of MMA did not permit the phase separation to proceed completely. So, new micron-sized spherical PMMA particles were produced beside the formation of main PCM microcapsules and inside them (Fig. 6b and c). This dual particle growth was not affected by the added m-Fe3O4 NPs and the magnetite nanoparticles did not involve in the nucleation and particle growth stages (Fig. 6d and e). These multinucleus or pomegranate-like morphologies are quit fascinating due to the better encapsulation and protection of the core content against undesirable leakage. Moreover, EDX mapping was utilized to investigate the dispersion of m-Fe3O4 NPs in the Mag-PCM series samples (Fig. 7). It is evident that m-Fe3O4 NPs had good dispersion inside the magnetic PCM microcapsules and no serious agglomeration was found by increasing mFe3O4 NPs content. This is also another indication of proper modification of m-Fe3O4 NPs for reaching to efficient dispersion in the droplets during microencapsulation.
Fig. 2. FTIR spectra of Mag-PCM 4, m-Fe3O4, m-Fe3O4, PMMA-N and HD.
3.3. Thermal properties Thermal stability of Mag-PCM microcapsules and the actual content of Fe3O4 NPs in the microcapsules were investigated by TGA and differential thermogravimetric analysis (DTG) (Fig. 8). Pure n-hexadecane and neat PMMA (PMMA-N) exhibited a typical one-step thermal degradation with almost no residue and maximum decomposition temperature (Tdmax) at 190 and 380 °C, respectively. As expected, Fe3O4 NPs represented no specific weight loss upon heating up to 700 °C because of the inorganic nature of these nanoparticles. m-Fe3O4 NPs showed weight loss through a two-step thermal degradation with two Tdmax at 250 and 385 °C. Total weight loss at 700 °C for Fe3O4 NPs and m-Fe3O4 NPs were found to be 4% and 26%, respectively. Therefore, the weight percent of adsorbed OA onto Fe3O4 NPs during modification process was about 22 wt%. It is reasonable to say that thermal degradation profiles for MagPCM series samples look a cumulative of those for HD, PMMA-N and mFe3O4 NPs (Fig. 8b and d). However, the maximum degradation temperatures shifted slightly to higher ones with the increase in magnetite content. Moreover, Td5 (temperature for 5% weight loss) of the samples were moved from 128 to 168 °C by the incorporation of magnetite into the microcapsules (Table 2). Furthermore, Td95 (temperature for 95%
Fig. 3. XRD patterns of (a) m-Fe3O4 NPs, (b) Mag-PCM 2 and (c) Mag-PCM 4.
desired thermal storage properties. Here, the formation of Fe3O4 NPs was investigated by SEM analysis before and after stabilization with oleic acid (Fig. 4). It seems that Fe3O4 NPs are spherical-like with average size of 35–40 nm and addition of OA didn’t have any specific influence on particle size distribution. In other words, OA has just achieved to hydrophobize Fe3O4 NPs without affecting on their size distribution. Fig. 5 shows SEM images of PMMA-N and the prepared magnetic PCM microcapsules. Mag-PCM samples with size of 180–200 μm showed a wrinkled surface and opposite to PMMA-N with smooth surface. Based on our previous study, the interfacial tension between PMMA and HD plays a crucial role for obtaining core/shell structure with such a morphology [6]. PMMA-N sample contained spherical particles with average size of 140 μm. When HD was added to 21
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Fig. 4. SEM images of (a) Fe3O4 NPs and (b) m-Fe3O4 NPs.
Fig. 5. SEM images of (a) PMMA-N, (b) Mag PCM 0, (c) Mag PCM 2 and (d) Mag PCM 4.
samples were obtained according to the residue at 600 °C and upon complete degradation of the organic phases and they were calculated with respect to the residue for Mag-PCM 0 (Table 2). The results revealed a good agreement with the added m-Fe3O4 NPs in the recipes, depicting almost complete encapsulation of the primary used m-Fe3O4 NPs.
weight loss) in these samples were increased from 391 to 414 °C from Mag-PCM 0 to Mag-PCM 4. These elevations in Td5 and Td95 could be attributed to the capability of m-Fe3O4 NPs to absorb heat and improve thermal resistance of the obtained magnetic PCM microcapsules and retard HD leakage and shell degradation. However, the encapsulated magnetite content in Mag-PCM series
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Fig. 6. SEM images of the crushed (a) PMMA-N, (b, c) Mag-PCM 0, (c, d) Mag-PCM 4 samples.
Fig. 7. EDX mapping of (a) Mag-PCM 0.5, (b) Mag-PCM 2 and (c) Mag-PCM4 samples.
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Fig. 8. TGA (a) and DTG (b) of PMMA-N, Fe3O4 NPs, m-Fe3O4 NPs and HD; and TGA (c) and DTG (d) of Mag-PCM series samples.
agent in the crystallization process. This results in the appearance of two different Tcs in Mag-PCM 1 and resumes coherently until reaching to another single Tc in Mag-PCM 4. Observation of one Tc in the sample with highest level of m-Fe3O4 NPs returns to the dominant crystallization through nucleation process. Despite the crystallization peaks, all samples revealed a single melting temperature (Tm) between 18 and 20 °C, corresponding to Tm of pure HD (18 °C) (Fig. 9c). This illustrates that although two crystallization peaks have been emerged, the obtained HD crystalline structures are identical and m-Fe3O4 NPs can only influence on the crystallization process. In addition and due to the nucleating effect of m-Fe3O4 NPs, the number of HD crystal domains increases. These appear in more narrow and sharper melting peaks which is evident in Fig. 9c. Furthermore, the increase in thermal conductivity of the resulting microcapsules after incorporation of m-Fe3O4 NPs is another reason for this observation. Supercooling phenomenon is considered as a major obstacle to the practical application of PCM microcapsules [9]. Supercooling temperatures ((ΔTs) is determined from crystallization-melting hysteresis of a PCM (ΔTsi = Tm − Tci), where “i” refers to each crystallization temperature. As shown in Table 3, ΔTs2 for Mag-PCM samples is lower than ΔTs1 for Mag-PCM 0due to the incorporation of m-Fe3O4 NPs. This would be attributed to the increase in thermal conductivity by the presence of m-Fe3O4 NPs and the role of m-Fe3O4 NPs as a nucleating source for crystallization of HD [26]. The crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) are two important indexes for representing thermal storage-release capability of a PCM. It is understood (Table 3) that pure HD has considerably high phase-change enthalpy of 218 J/g, indicating an excellent phase change material. However, the encapsulation of HD into PMMA shell (Mag-PCM series) resulted in reduction of phase change enthalpies obviously. This reduction is quite expectable, as the heat changes in DSC are measured relative to the sample weight. and for Mag-PCM series samples, the quantity of HD as the responsible component for heat storage and release is lower with respect to pure HD.
Table 2 Thermal degradation information of the samples from TGA. Sample
Td5 (°C)
Td95 (°C)
Residue at 600 °C (wt%)
HD Fe3O4 m-Fe3O4 Mag-PCM Mag-PCM Mag-PCM Mag-PCM Mag-PCM Mag-PCM
126 – 202 128 129 166 169 168 168
244 – – 391 410 398 407 410 414
0 96 74 ∼0 0.5 1 2 3 4
0 0.5 1 2 3 4
The phase-change behavior and thermal storage-release performance were investigated by DSC scans for the magnetic PCM microcapsules as well as pure HD as a control sample. The resulting DSC thermograms have been shown in Fig. 9 and meanwhile, the extracted phase change characteristic data from DSC thermograms have been summarized in Table 3. It is evident that the phase-change behavior of HD has been notably affected by the incorporation of m-Fe3O4 NPs. The presence of Fe3O4 in microcapsules causes a bimodal crystallization temperature from DSC thermograms [26]. Pure HD showed distinct and sharp melting and crystallization temperatures at 18 and 11 °C, respectively (Fig. 9a), but an interesting phenomenon was observed during crystallization process (Fig. 9b). For the samples with no (Mag-PCM 0) or low (Mag-PCM 0.5) or even higher (Mag-PCM 3 and 4) magnetite contents, the crystallization of HD occurred in a broad range with two different temperatures (Tc1 and Tc2) and the details have been reported in Table 3. With the increase in m-Fe3O4 NPs from 0.5 to 3 wt%, bimodality in crystallization temperature is quite obvious. This is a remarkable indicator of incorporation of m-Fe3O4 NPs into the prepared PCM microcapsules. In other words, magnetite nanoparticles were involved as a nucleating 24
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Fig. 9. DSC thermograms of (a) crystallization and melting of pure HD, (b) crystallization and (c) melting of the prepared Mag-PCM series samples.
Table 3 Thermal characteristics of pure HD and magnetic PCM microcapsules extracted from DSC thermograms. Tc1a (°C)
Tc2b (°C)
Tm (°C)
ΔTs1 (°C)
ΔTs2 (°C)
ΔHc (J/ g)
ΔH(Tc1) (J/g)
ΔH(Tc2) (J/g)
ΔHm (J/ g)
ΔH′ (J/ g)
C
0
11 7 8
– – –
18 19 18
7 12 10
– – –
218 53.1 52
53.1 52
– –
218 53.2 52.2
– 52.9 51.8
1 2 3 4
8 8 10 –
12 13 12 12
21 19 19 19
13 11 9 –
9 6 7 7
53 51.1 52.3 51.4
38.6 22.9 – –
14.1 28.4 52.3 51.4
53.1 51.3 52.5 51.5
52.7 51.0 52.3 51.2
Sample
HD Mag-PCM Mag-PCM 0.5 Mag-PCM Mag-PCM Mag-PCM Mag-PCM a b
E (%)
TCP (%)
Magnetite content from TGA (%)
– 24.4 23.9
– 97.6 95.6
– 99.4 99.2
– – 0.5
24.4 23.6 24.1 23.6
97.6 94.4 96.4 94.4
99.2 99.4 99.6 99.4
1 2 3 4
E
(%)
First crystallization peak at lower temperature. Second crystallization peak at higher temperature.
Experimental core content (CE, Eq. (1)) and encapsulation efficiency (E, Eq. (2)) of the prepared magnetic PCM microcapsules were used to evaluate their thermal storage properties (Table 3).
CE (%) =
E (%) =
ΔHm × 100 ΔHHD CE × 100 Ct
(1)
(2)
The encapsulation efficiencies were all above 94%, revealing successful encapsulation of HD by PMMA after polymerization. Moreover, no obvious change in E and CE of the microcapsules was observed by adding magnetite nanoparticles, depicting that the incorporation of mFe3O4 NPs did not sacrifice their thermal properties. Also, due to the presence of two crystallization temperatures, two crystallization enthalpies could be considered (ΔH (Tc1), ΔH (Tc2)). So, the crystallization curves were deconvoluted by “Origin V.9” software and the quantity of each deconvoluted peak has been given in Table 3. The enthalpies ratio
Fig. 10. ΔH (Tc1)/ΔHm and ΔH (Tc2)/ΔHm variations vs. m-Fe3O4 NPs content.
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Table 4 Magnetic parameters of the prepared samples. Sample
Ms (emu/ g)
Mr (emu/ g)
Hc (Oe)
Mr/Ms
Magnetization loss (%)a
Fe3O4 m-Fe3O4 Mag-PCM 0.5 Mag-PCM Mag-PCM Mag-PCM Mag-PCM
65 58 0.095
12 12 0.02
82 108 124.5
0.18 0.21 0.21
– 10.76 67.24
0.33 0.8 1.2 1.45
0.08 0.2 0.2 0.3
140 142 135 125
0.24 0.25 0.17 0.21
42.10 31.03 31.03 37.5
1 2 3 4
a Magnetization loss for m-Fe3O4 was calculated with respect to Fe3O4 NPs and for Mag-PCM series was calculated relative to m-Fe3O4 NPs.
65 emu/g with Mr/Ms. of 0.18 which demonstrates the achievement in preparation of almost superparamagnetic Fe3O4 NPs. Ms. for m-Fe3O4 NPs was 58 emu/g with 10.7% reduction relative to Fe3O4 NPs. This could be explained by two reasons: (i) 22 wt% surface modification of Fe3O4 NPs (from TGA results) in which the adsorbed OA on the surface of magnetite acts as an impurity and decreases the saturation magnetization[31], (ii) employment of ultrasonic irradiation in the procedure that causes partial oxidation of Fe3O4 and results in reducing magnetization per weight unit of magnetite [32]. Magnetization curve of Mag-PCMs have been demonstrated in Fig. 12 and the numerical data have been listed in Table 4. Low magnetization remanence and coercivity were observed, representing superparamagnetic behavior of the Mag-PCM samples. On the other hand, the observed Ms amounts from VSM analysis were lower than the expected ones. For instance, Mag-PCM 4 with 4 wt% magnetite content is expected to have Ms of 2.32 emu/g (relative to Ms of m-Fe3O4 NPs), but the observed Ms was 1.45 emu/g with 37.5% loss in magnetization. The amounts of magnetization loss for all the samples have been listed in Table 4 too. This would be attributed to the oxidation of magnetite during suspension polymerization. It is worth mentioning that the presence of peroxide initiators along with high shear rate of mixing during polymerization are responsible for promoting the above oxidation [32]. However, the increasing trend in magnetization from 0.095 to 1.45 emu/g for Mag-PCM 0.5 to Mag-PCM 4 is consistent with the aforementioned increase in magnetite content. Mr/Ms values imply that superparamagnetic properties of the obtained microcapsules have been preserved during this suspension polymerization. This means that these magnetic microcapsules can be attracted to by an external magnetic field and retain no residual magnetization when the magnetic field is removed at room temperature. Such a feature makes the magnetic microcapsules do not tend to aggregate together after removal of the magnetic field [33]. The prepared magnetic PCM microcapsules were readily dispersed in water to form an aqueous suspension by simple ultrasonication, and
Fig. 11. Thermal performance of Mag-PCM 1 and Mag-PCM 4 before (solid line) and after 300 heating-cooling cycles (dashed line).
of deconvoluted peaks to total melting peak showed a specific trend (Fig. 10). Both trends demonstrate a turning point in which, there will be no difference in crystallization behavior. Here, this point was appeared at 3 wt% of m-Fe3O4 NPs. Performance of the prepared magnetic PCM microcapsules was controlled by heating-cooling cycles for 300 times. The test gives useful information about efficiency of protecting HD by PMMA shell in the obtained PCM microcapsules. For this reason, DSC thermograms of the samples were recorded before and after thermal cycles. The observed thermal cycling performances (TCP) have been given in Table 3 and calculated from Eq. (3).
TCP (%) =
ΔH ′ × 100 ΔHm
(3)
ΔH′ is the melt enthalpy of washed samples after thermal cycling for 300 times. Fig. 11 shows the performance of the two samples before and after heating-cooling cycles. No significant difference was observed for the enthalpies of Mag-PCMs after thermal cycling process, revealing the merit of these Mag-PCMs for versatile applications as dual-responsive phase change materials. 3.4. Studies on magnetic properties Magnetic properties of the prepared Fe3O4 NPs, m-Fe3O4 NPs and Mag-PCMs were evaluated by a vibrating sample magnetometer (VSM) at room temperature (25 °C) and the resulting magnetic hysteresis loops have been presented in Fig. 12. The superparamagnetic behavior is determined by high saturation magnetization (Ms), low remanence magnetization (Mr) and coercive force (Hc) values and also approaching of Mr/Ms to zero. The saturation magnetization of Fe3O4 NPs was
Fig. 12. Magnetic hysteresis loops of (a) Fe3O4 NPs and m-Fe3O4 NPs; and (b) Mag-PCM series microcapsules.
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Fig. 13. Thermal conductivity of the prepared Mag-PCM series samples (a) and its growth with increase in magnetite content (b).
they were shortly attracted by an external magnet. They were quickly redispersed with a slight shaking, once the magnetic field was removed. Such a good magnetic response may render the prepared magnetic PCM microcapsules in fast magnetic manipulation in actual applications.
and superparamagnetic characteristics were measured. Thermal conductivity of the Mag-PCM microcapsules increased linearly with the increase in Fe3O4 NPs content. It is believed that such Mag-PCM microcapsules could be considered in the future generation of PCMs that are called smart PCM materials. The introduced microcapsules with such dual functional features open a route toward application of these materials in manufacturing of microelectronic elements, electronic devices, textile and even special buildings such as hospitals with antijamming and thermal regulating effectiveness.
3.5. Thermal conductivity Although pure n-hexadecane has many desirable properties as a useful PCM, its organic nature with no free electron results in a low thermal conductivity [1]. This may delay its thermal response to the storage and release of the latent heat. Therefore, enhancement in thermal conductivity is absolutely essential when designing microencapsulated PCMs. It is expected that the encapsulation of n-hexadecane together with an inorganic material with higher thermal conductivity compensates this defect. Pure HD (in solid state) and PMMA display thermal conductivities as low as 0.15 [34] and 0.19 [35] W m−1 K−1, respectively. Thermal conductivities of the prepared magnetic PCM microcapsules were measured and their significant growth has been shown in Fig. 13. Interestingly, all of the prepared microcapsules showed thermal conductivities above than 0.19 W m−1 K−1. This observation is anticipated to the presence of Fe3O4 NPs in the microcapsules, while thermal conductivity of Fe3O4 NPs is 9.7 W m−1 K−1 [22,27]. These results confirm the encapsulation of m-Fe3O4 NPs in the presence of n-hexadecane and they actively impart more thermal conductivity to the resulting PCM microcapsules linearly. It is also noteworthy that the thermal conductivities for Mag-PCMs are associated with the quantity of included m-Fe3O4 NPs, and the higher thermal conductivity is attainable at higher magnetite contents. This depicts that m-Fe3O4 NPs play a critical role in enhancing heat transfer in Mag-PCMs with providing a continuous phase to the encapsulated HD. Such a continuous phase can be considered as a virtual heat transfer network and thus enhances the heat transfer rate over the whole microcapsule.
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4. Conclusion In this work, novel dual functional hexadecane-PMMA PCM microcapsules containing Fe3O4 nanoparticles were designed and prepared by suspension polymerization. Hydrophobic Fe3O4 NPs were prepared by co-precipitation method and their subsequent modification with oleic acid for better dispersion in the paraffinic core. A complete morphological study was carried out and the results showed a wrinkled surface for the magnetic PCM microcapsules. Extensive thermogravimetric and calorimetric observations were performed to elucidate thermal behavior of the microcapsules. The obtained Mag-PCM microcapsules were responsive to an external magnetic field due to the presence of Fe3O4 NPs and their magnetic parameters such as Ms, Mr, Hc 27
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