Design the magnetic microencapsulated phase change materials with poly(MMA-MAA) @ n-octadecane modified by Fe3O4

Magnetic microencapsulated phase change materials (magnetic MicroPCMs) are hotly researched for their dual-functions with phase change and magnetic properties, which provided the new applications in fields of maneuverable phase change materials and infrared electromagnetic dual shield. A series of magnetic MicroPCMs samples are synthesized by polymerization and coprecipitation method and the chemical composition contained poly(MMA-MAA) @ n-octadecane modified by Fe3O4. In addition, the characterizations exhibit the excellent magnetic and phase change properties. The magnetic MicroPCMs samples present 20 emu·g−1 saturation magnetization with still high enthalpy of 132 J·g−1, which fully illustrates that the magnetic MicroPCMs fulfill both application on thermal energy storage and magnetic control.

idea and encapsulated n-octadecane to poly(MMA-MAA) firstly, then Fe 3 O 4 was coprecipitated on the surface of MicroPCMs after SDS modified. At last, the magnetic MicroPCMs was obtained and characterized.

Synthesis. Synthesis of MicroPCMs.
A 250 mL three-neck round-bottomed flask was equipped with a mechanical stirrer, a reflux condenser and a nitrogen gas inlet tube, which was put in a water thermostat bath as a reaction vessel. 0.25 g SDS as a dispersant dissolve in 100 mL distilled water, constitute the water phase. Oil phase consist of 9 g n-octadecane, 4 g MMA, 2 g MAA, and 0.1 g BPO. The water phase was added to the 250 mL three-neck round-bottomed flask is ready, and then was heated to 55 °C with stirring to 500 rpm for 10 min. Afterwards, the oil phase was poured into the water phase to form an oiliness suspension with stirring to 1200 rpm for 15 min. And then, the suspension was heated to 85 °C for 3.5 h with rotational rate was turned down 800 rpm, the nitrogen gas inlet tube was opened at the same time. 0.1 g K 2 S 2 O 8 was dissolved into 1.4 g of water to form an aqueous solution which was added into the reaction suspension and continued for 1. Characterization. The surface morphology and elemental compositions of the magnetic MicroPCMs were presented using a Hitachi S-4800 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) spectroscopy. Tecnai F20 was chosen to characterize diameter and microstructure of fabricated Fe 3 O 4 as the HRTEM. The chemical structures of the samples were investigated using a Nicolet 6700 FTIR spectrophotometer, and the simples were measured at a wave number of 4000-400 cm −1 using a KBr sampling sheet. A Germany Bruker D8 Advance X-ray diffractometer with Cu-Ka radiation (λ = 0.154 nm) gave the Powder X-ray diffraction (XRD) data at a scan rate of 10°/min in the 2θ range of 5°-90°. The phase-change behaviors of the samples were investigated using a PerkinElmer Diamond Dynamic differential scanning calorimetry (DSC) scans, coupled with a thermal analysis data station. All of the measurements were carried out under a nitrogen atmosphere at a heating or cooling rate of 10 °C/min in the range of 0-60 °C, and the mass for each sample was about 7-8 mg. Thermogravimetric analysis (TGA) of the magnetic microcapsules was carried out using a TA Instruments SDT Q600 thermal gravimetric analyzer under a nitrogen atmosphere. The sample with a mass of about 7-8 mg was placed in an aluminum crucible and then was ramped from room temperature up to about 600 °C at a heating rate of 10 °C/min. A Physical Property Measurement System (PPMS-9, Quantum Design) performed the magnetic properties of the samples in applied fields up to 20,000 Oe at room temperature (~298 K). The distribution of the particle diameter of magnetic MicroPCMs was measured on the Microtrac S3500 SI particle size analyzer, with the specimens were dispersed in ethanol.

Results and Discussions
Synthetic strategy. The synthetic method of the magnetic MicroPCMs with Fe 3 O 4 is shown in Fig. 1. The MicroPCMs was fabricated by n-octadecane as the core and poly(MMA-MAA) as the shell firstly in this work. After the MicroPCMs was manufactured, a little of SDS was chosen to modify the surface of MicroPCMs. Then the Fe 2+ and Fe 3+ were dissolved and attached to MicroPCMs due to the amphipathic character of SDS, ammonia was dropped in reaction systems as precipitating reagent. And then, the Fe 3 O 4 particles were produced adopted coprecipitation method and attached to the shell of the MicroPCMs owing to physical adsorption. The SDS was used as surface modifers to prevent the Fe 3 O 4 particles getting off the surface of MicroPCMs. The chemical equation for fabrication of Fe 3 O 4 is shown as below: Molar ratio of Fe 2+ and Fe 3+ is 1:2 in theory according to the equation. But the ratio is 2:1 or 3:1 in actual experiments because the Fe 2+ was easy to be oxidized 36  The measured enthalpy, melting peak temperatures and crystallization peak temperatures of the MicroPCMs and pure n-octadecane are listed in Table 1 and the DSC curves are shown in Fig. 2. Compared the average enthalpy to pure enthalpy, the MicroPCMs are lower universally due to the content decrease of the n-octadecane (as Table 1 indicated). Moreover, the melting peak temperature was shifted left and the crystalline temperature was shifted right compare with pure n-octadecane, which means the thermal conductivity of the MicroPCMs is higher than pure n-octadecane's. It is observed that pure n-octadecane exhibits single peak at 21.1 °C in its solidification process. However, there are two peaks at 22.5 °C and 11.7 °C in the crystallization process of the MicroPCMs due to the impurity induced heterogeneous nucleation in the solidification process 37 .
It's also clearly shown that there are dramatic effects of different dosage of SDS on the content n-octadecane in Fig. 2. The ΔH a of the MicroPCMs were 143.0, 171 and 143.9 J·g −1 when synthesized with the SDS mass of   The diameter ranges of the MicroPCMs with 0.5 g SDS additive as dispersant is the narrowest, but the average diameter reached the maximum when the mass of dispersant SDS is 0.25 g. The dosage of dispersant SDS influences on the sizes of droplet in the suspension, and then affects the diameter of the microcapsules after the polymerization. It has been reported that the amount of the dispersant would make a great influence on the particle diameter of the MicroPCMs. Because the thickness of shell is tiny, the size distribution of particles can be changed largely by the shell's structure 38 . The PMMA-MAA structure could be changed by the amounts of SDS, as different amounts of surfactant were added in the same amounts of reactive monomers, the structure of copolymers will be changed in polymerization. In addition, the amounts of SDS would influence on the size of floating droplet in the suspension polymerization. And the particle diameter curve appeared inverted U-shaped with the increasing of the amount of the dispersant. As the increasing of the particle diameter, the content of the n-octadecane will increase and lead to raise of the enthalpy 38,39 . In this paper, the magnetic MicroPCMs was fabricated from the MicroPCMs with the maximum enthalpy.
The morphology, crystallinity and composition of magnetic MicroPCMs. There are four samples of different mass ratio of Fe 3 O 4 /MicroPCMs as controlling the amount of Fe 2+ and Fe 3+ in Table 2. And the SEM micrographs of the surface morphology of MicroPCMs and the magnetic MicroPCMs were presented in Fig. 4. Figure 4(a) shows the microscopic morphology of the MicroPCMs, they are excellent coated by Fe 3 O 4 and the surface looks smooth and flat. The surface morphology of the magnetic MicroPCMs is different to MicroPCMs without magnetic particles, and it's clearly shown in Fig. 4(b-d) that there are lots of nanoparticles on the MicroPCMs shell. Moreover, the magnetic MicroPCMs destroyed with ultrasounds still exhibits nanoparticles coated shell in Fig. 4(e). Therefore, the nanoparticles Fe 3 O 4 could attach the surface of the MicroPCMs and the thin Fe 3 O 4 presented better surface appearance. In addition, the EDX spectra of the magnetic MicroPCMs samples show peaks clearly corresponding to C, O and Fe elements in Fig. 4(f), confirming that the magnetic MicroPCMs shell consists of Fe 3 O 4 . The diameter and microstructures of Fe 3 O 4 of the magnetic MicroPCMs obtained in this study were examined using HRTEM, and the resulting TEM micrographs were presented in Fig. 5. As is shown in Fig. 5(a,b), the border of the magnetic MicroPCMs with 20% Fe 3 O 4 is not to distinguish clearly because the organic shell's carbon     It has been proved that the magnetic nanoparticles are coated on the shell of MicroPCMs from the SEM micrographs, EDX spectra, FTIR absorption spectra, and XRD pattern as mentioned above. This paper discussed the influences on the performance of MicroPCMs with different amounts of Fe 3 O 4 below.  Fig. 4, and the corresponding sample number is shown in Table 2. It's clearly to be found that the surface of the sample 1 is coated by lots of aggregative nanoparticles in Fig. 4(b), even they are easy to drop out from the surface, so that there are many nanoparticles fall out shown in Fig. 4(b). It is presented in Fig. 4(c) that the sample 2 have a complete coverage of the surface. Meanwhile, there are little nanoparticles out of the surface, it means that it is the proper dosage of the Fe 3 O 4 on the sample 2. There are many magnetic MicroPCMs adhered to each other in the sample 3 was shown in Fig. 4(d). However, they are flatter on the surface than sample 2. Therefore, the little amounts of Fe 3 O 4 could cover the surface of the magnetic MicroPCMs and make the surface smooth like the MicroPCMs. In addition, the distributions of the particles diameter with different mass of Fe 3 O 4 were given in Fig. 3(d-f). And the average diameter of the magnetic MicroPCMs with 70% Fe 3 O 4 has a great increase from 11 to 13 μm. However, the average diameter of the sample 2 and 3 just have a little increase. These prove that the small amount of Phase change characteristics and thermal stability performance. The phase change behaviors were investigated by DCS and the resulting DSC thermograms are presented in Fig. 8. Table 3 Fig. 8, there are not great influence on melting peaks and the crystalline peaks, and they all have the melting peaks at about 30 °C and the crystalline peaks at about 24 °C. Meanwhile, the average enthalpy ΔH a decrease with increase of the Fe 3 O 4 amounts. It is clearly known that the ΔH a is related to the content of the n-octadecane, and the increased amounts of Fe 3 O 4 must lead to the decrease of the n-octadecane's content in the magnetic MicroPCMs. In addition, the ΔH a are pretty close to the ΔH ac of the sample 2 and sample 3, it means that the results is corresponding to the anticipation. However, the sample 1 presents the great difference between ΔH a and ΔH ac , the value ΔH a is greater than ΔH ac in Table 3. It indicates The thermal stability of the magnetic MicroPCMs was investigated by TGA and the resulting thermograms are presented in Fig. 9. The MicroPCMs starts to loss the weight at about 180 °C in Fig. 9(d), it is clearly shown that high temperature make the MicroPCMs swell, n-octadecane flows out of the MicroPCMs and begins to decompose. The second weight loss happens at nearly 300 °C, because the polymer shell starts to decompose with the molecular chain ruptured. The weight loss curves of magnetic MicroPCMs are presented in Fig. 9(a-c). There is the second weight loss at around 200 °C differs from MicroPCMs, it is obviously known that the Fe 3 O 4 nanoparticles coated on the surface lead to high temperature of the weight loss for some n-octadecane. The first and third weight loss of the magnetic MicroPCMs are the same temperature as the first and second weight loss of MicroPCMs. Hence, compare with MicroPCMs, the magnetic MicroPCMs have a certain degree of ascension. In    addition, because the pure n-octadecane decomposition temperature (T d ) is just 128 °C 40 , the thermal stability of magnetic MicroPCMs has a substantial increase.

The performance of MicroPCMs with different amounts of
Magnetic properties. The magnetic properties of the magnetic MicroPCMs were characterized by PPMS-9 at room temperature (~300 K), and the resulting magnetic hysteresis loops are shown in Fig. 10. It is obviously shown that three samples exhibit low residual intensity of magnetization and coercivity. It means that the magnetic MicroPCMs possess super paramagnetic behavior. Furthermore, the saturation magnetization of three magnetic MicroPCMs samples were measured to be 32.

Conclusions
In this work, we coated the Fe 3 O 4 nanoparticles on the surface of the MicroPCMs and fabricated the functional phase change materials with magnetic property. A series of products were prepared with differ amounts of Fe 3 O 4 and characterized by particle size analyzer for size distributions, SEM and TEM for microscopic morphology, FTIR for chemical structure, XRD for crystalline structure, DSC for phase change behaviors, TGA for thermal stability, and PPMS for magnetic properties. After these characterizations, the magnetic MicroPCMs exhibits excellent magnetic and phase change properties. For example, the 11 percent of content of Fe 3 O 4 in the magnetic MicroPCMs still present 20 emu·g −1 saturation magnetization with 132 J·g −1 enthalpy. This will provide new thinking on the applications of the phase change materials on electromagnetic protection clothing with thermostat for soldiers, gravida, or other special operations personnel, infrared-electromagnetic dual shield for stealth aircraft, and thermal manipulation in real production.