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Nanoencapsulated phase change materials with polymer-SiO2 hybrid shell materials: Compositions, morphologies, and properties
Energy Conversion and Management 164 (2018) 83–92
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Nanoencapsulated phase change materials with polymer-SiO2 hybrid shell materials: Compositions, morphologies, and properties ⁎
T
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Yalin Zhua,b, Yaosong Qina,b, Chengsha Weib, Shuen Liangb, , Xuan Luoa, , Jianhua Wangb, Lin Zhanga a b
Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase change materials Nanoencapsulation Hybrid shell Leakage proof Mechanical property
Organic-inorganic hybrid materials are promising for encapsulation of phase change materials (PCMs) to achieve exceptional capsule properties. In this work, novel polymer-SiO2 hybrid shelled nanoencapsulated PCMs (NanoPCMs) were fabricated in one-pot, through sequentially executed interfacial hydrolysis-polycondensation of alkoxy silanes and radical polymerization of vinyl monomers. The morphologies, chemical compositions, and crystal structures of the NanoPCMs were characterized by SEM, TEM, FT-IR, and XRD methods. The thermal energy storage capability, thermal reliability, and thermal conductivity were tested by DSC, accelerated thermal cycling test, and heat flux method, respectively. The leakage proof property and mechanical property were evaluated by seepage test and nanoindentation test, respectively. Compared with NanoPCMs with SiO2 shell, the NanoPCMs with polystyrene (PS)-SiO2 shell possess smaller size and bowl like shape, while NanoPCMs with poly (hydroxylethyl methacrylate) (PHEMA)-SiO2 shell possess larger size and perfect spherical shape. The polymer types have great impact on the supercooling behavior of the NanoPCMs. The polymer-SiO2 hybrid shell materials endow the NanoPCMs with improved thermal reliability, thermal conductivity, and leakage proof property. More importantly, the compressive load at yield increases remarkably from 14.7 μN for nanocapsules with SiO2 shell, to > 34.6 μN for that with PS-SiO2 shell, and 65 μN for that with PHEMA-SiO2 shell.
1. Introduction Phase change materials (PCMs) are substances that can undergo phase transitions at defined temperature range, and store/release large quantity of latent heats during the process [1]. Due to their high heat storage capacity, PCMs are good choice to overcome the uneven energy distribution in space and time, and the mismatch between energy supply and demand [2]. Moreover, based on their near isothermal characteristics during the thermal energy storage/release process, PCMs can also provide thermal comfort for buildings in an energy efficient and environmentally friendly way [3]. Therefore, PCMs can greatly contribute to development of renewable and sustainable energy sources, less consumption of fossil fuels, and less emission of green house gas. Most practically valuable PCMs, including paraffin, fatty acids, alcohols, and salt hydrates, perform thermal energy storage through solid–liquid phase transition, and suffer from leakage in liquid state and low thermal conductivity. In last decades, micro (1 μm < size < 1000 μm)/nano (size < 1 μm) encapsulation of PCMs
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with protective shell materials have been widely explored. Those capsules can effectively prevent the leakage of PCMs, improve the thermal conductivity and heat storage/release efficiency, and control the volume change during phase change process [4,5]. Through encapsulation, the application fields of PCMs have been greatly extended, such as latent functionally thermal fluids [6,7] and smart textiles [8]. Encapsulated PCMs with various organic and inorganic shell materials were prepared by using different methods, such as in-situ polymerization, suspension polymerization, interfacial polymerization, sol-gel method, and self-assembly method, etc [4]. Generally, the well developed organic shell materials, such as melamine-formaldehyde (M−F) resin, polyurea [9], polystyrene (PS) [10,11], polymethylmethacrylate (PMMA), can withstand the volume change during phase change process efficiently, but possess low thermal conductivity. Inorganic shell materials, mainly including SiO2 [12,13], TiO2 [14,15], and CaCO3 [16], have attracted intensive research interests during the latest ten years, due to their higher thermal conductivity and thermal stability than the organic counterparts. However, inorganic shell materials like SiO2 and TiO2 are generally obtained through sol-gel or interfacial
Corresponding authors. E-mail addresses: [email protected] (S. Liang), [email protected] (X. Luo).
https://doi.org/10.1016/j.enconman.2018.02.075 Received 3 January 2018; Received in revised form 20 February 2018; Accepted 20 February 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.
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much faster thermal energy storage/release [30]. When applied in latent functionally thermal fluid, NanoPCMs do not fracture easily in the course of flow [5]. However, it is obvious that the shell of NanoPCMs is generally much thinner than that of MicroPCMs, so its compactness and toughness are especially important to achieve desirable sealing performance and durability. Therefore, it is very valuable for practical application to develop novel NanoPCMs with organic-inorganic hybrid shell materials. In this work, we conduct modification of SiO2 shell material with PS and poly(hydroxylethyl methacrylate) (PHEMA), to obtain novel NanoPCMs with organic-inorganic hybrid shell materials. SiO2 is a classic type of inorganic shell material for micro/nano encapsulated PCMs, because it can be facilely and controllably synthesized through sol-gel (i.e. interfacial hydrolysis-polycondensation) process with inexpensive raw materials like tetraethyl orthosilicate (TEOS) [31,32] and sodium silicate [33]. PS and PHEMA [34] were selected as representatives of hydrophobic and hydrophilic polymers, respectively. The prices of the monomers for both polymers are also very cheap. The polymer-SiO2 hybrid shelled NanoPCMs were fabricated in one-pot, through sequentially executed interfacial hydrolysis-polycondensation of alkoxy silanes and free radical polymerization of vinyl monomers. The low costs of raw materials and simplicity of preparation can endow the target NanoPCMs with good potential for mass production and practical use. Through this preparation strategy, excellent morphologies can be obtained, and the polymer content can be facilely tuned. The influences of the two polymers with different contents on the morphologies, thermal energy storage properties, thermal conductivities, leakage proof properties, and mechanical properties of the NanoPCMs were comprehensively investigated. On the basis of the results of this research, the overall properties of the NanoPCMs can be optimized with respect to their compositions.
hydrolysis-polycondensation processes. They often exhibit mesoporous structure [17] and are mechanically weak. In recent few years, many research efforts have been devoted to organic-inorganic hybrid shell materials, which can combine the advantages of organic and inorganic materials, to achieve outstandingly high performances [18]. Consequently, several novel types of micro/nano encapsulated PCMs with organic-inorganic hybrid shell materials have been invented. Some inorganic materials like SiO2, TiO2, Al2O3, Si3N4, graphene oxide (GO), and graphene, were introduced into organic shell materials of microencapsulated PCMs (MicroPCMs) as fillers during the preparation process. These highly thermal conductive fillers were dispersed either by direct mixing or serving as Pickering stabilizers. Yin et al. [19] prepared MicroPCMs with PS-SiO2 hybrid shell material via Pickering emulsion polymerization method, in which organicallymodified SiO2 particles were used as stabilizer. The prepared MicroPCMs present good durability and thermal reliability. Zhao et al. [20] fabricated MicroPCMs with PMMA-TiO2 hybrid shell for thermal energy storage and UV-shielding. Sun et al. [21] synthesized a novel kind of MicroPCMs with PMMA/BN/TiO2 ternary hybrid shell material by Pickering emulsion method, in which nano-BN/TiO2 particles were used as stabilizer. Jiang et al. [22] synthesized new MicroPCMs based on paraffin wax core and poly(methyl methacrylate-co-methyl acrylate)-Al2O3 hybrid shell material. Yang et al. [23] reported MicroPCMs with PMMA-Si3N4 hybrid shell material. Chen et al. [24] prepared novel MicroPCMs with M−F resin-GO hybrid shell material by in situ polymerization, and the products possessed high latent heat and improved thermal conductivity. Zhang et al. [25] prepared MicroPCMs with PS-GO hybrid shell material by Pickering emulsion method, and achieved high encapsulation ratio and good thermal stability. Su et al. [26] synthesized MicroPCMs with methanol-modified M−F resin-graphene hybrid shell material through in situ polymerization, to improve the mechanical properties and thermal conductivities. In addition to the above mentioned method, a different method for preparation of MicroPCMs with hybrid shell material was reported by Li et al. [27]. Specifically, vinyl functionalized alkoxy silanes were employed as raw materials, which were subjected to simultaneous sol-gel reaction and free radical polymerization and resulted in polymer-SiO2 hybrid shell material to encapsulate the PCMs. The products possess high encapsulation ratios and low leakage rate. In summary, through these approaches, the obtained hybrid shell materials brought lots of merits for micro/nano encapsulated PCMs: enhanced encapsulation ratio [28], improved thermal conductivity [24,26], and better leakage proof properties [29]. It is observed that most of the previous works were focused on modification of organic shell materials with some inorganic substances, but the modification of inorganic shell materials with organic polymers was rare. In addition, from the view point of capsule size, most previously reported encapsulated PCMs with organic-inorganic hybrid shell materials were limited to MicroPCMs, while nanoencapsulated PCMs (NanoPCMs) were almost ignored. Compared with MicroPCMs, NanoPCMs have much larger specific surface area, which can result in
2. Experimental 2.1. Materials n-Octadecane (n-OD, 90 wt%), γ-methacryloxypropyl trimethoxy silane (MPS), and polyvinylpyrrolidone (PVP, M.W. 58,000 g/mol) were purchased from Alfa Aesar. Tetraethyl orthosilicate (TEOS), anhydrous ethanol, NH3·H2O (25 wt%), styrene (St), and 2,2′-azobisisobutyronitrile (AIBN) were purchased from Sinopharm Chemical Reagents. Hydroxyethyl methacrylate (HEMA) was purchased from Aladdin. St and HEMA were purified by passing through a neutral alumina column, to remove retardants. AIBN was recrystallized from 95% ethanol before use. Cetyltrimethylammonium bromide (CTAB) was commercially supplied by Tianjin Kermal Chemical Reagents. All chemicals were of reagent quality and used without further purification, unless stated otherwise.
Scheme 1. Raw materials used for fabrication of polymer-SiO2 hybrid shell materials.
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Scheme 2. Schematic representation of the processes for preparation of nanoencapsulated n-octadecane with polymer-SiO2 hybrid shell materials.
2.2.2. Preparation of NanoPCMs with PHEMA-SiO2 hybrid shell materials The procedure is similar to that for NanoPCMs with PS-SiO2 hybrid shell materials, except that HEMA is selected as the vinyl monomer and dissolved in water phase. By changing the usage of HEMA (5 and 10 mL), two NanoPCMs samples were prepared, denoted as n-OD@ PHEMA-SiO2 (including n-OD@PHEMA-SiO2-1 and n-OD@PS-HEMA2). NanoPCMs with SiO2 shell materials was also prepared through the same procedure, except that no vinyl monomer (St or HEMA) was added, denoted as n-OD@SiO2. This sample was used as a control without polymer modification.
2.2. Preparation of NanoPCMs with polymer-SiO2 hybrid shell materials The molecular structures of raw materials for synthesis of organicinorganic hybrid shell materials were shown in Scheme 1. A one-pot procedure in miniemulsion was adopted to prepare the NanoPCMs with PS-SiO2 and PHEMA-SiO2 hybrid shell materials, as illustrated in Scheme 2. Initially, an O/W miniemulsion was produced by mixing the oil phase and water phase under high shearing (FA25 high shearing homogenizer, Fluke) and ultrasound (VCX 750 ultrasonic processor, Sonics & Materials). The oil phase is composed of n-OD, alkoxy silanes, hydrophobic monomer, and AIBN, while the water phase is composed of H2O, ethanol, PVP, and hydrophilic monomer, respectively. Two sequentially executed chemical reactions were involved in the preparation of the NanoPCMs. In step I, alkoxy silanes (TEOS and MPS) hydrolyzed and condensed at the water/oil interface catalyzed by NH3·H2O, and SiO2 shell with many methacryloxypropyl groups was formed, referring to our previous works [35]. In step II, the temperature was raised to 80 °C to initiate the free radical polymerization of the vinyl monomer (St or HEMA) and the methacryloxypropyl groups on the already formed SiO2 shell. Surfactant (CTAB), dispersing agent (PVP), free PHEMA polymer chains not attached to the SiO2 shell, and other impurities were removed by repeated washing with water and freeze drying.
2.3. Characterization Scanning electron microscopy (SEM) characterization of the asprepared NanoPCMs was conducted on a Zeiss Ultra 55 field emission scanning electron microscope. The specimen was sputter coated with a thin layer of Au. Micrographs were taken in high vacuum mode with 10 kV acceleration voltage and a medium spot size. Transmission electron microscopy (TEM) characterization was performed on a Hitachi H-800 transmission electron microscope operated at an accelerating voltage of 100 kV. Specimen was dispersed in ethanol under sonication, and a drop of the mixture was transferred to the carboncoated copper grid for observation. Based on the obtained SEM micrographs, sizes of the NanoPCMs were determined by using Nano Measurer 1.2 software. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet 6700 IR-spectrophotometer on KBr sampling sheet, with a scanning number of 32. X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance diffractometer (40 kV, 50 mA) with Cu Kα radiation (λ = 0.154 nm), at 2θ range of 5–90° and scanning rate of 10°/min. Thermogravimetric (TG) analysis was performed on a TA Instruments Q500 thermogravimetric analyzer under N2 atmosphere. The specimen with a mass of ca. 5 mg was placed in an aluminum crucible and then ramped from room temperature up to 550 °C, at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) analysis was conducted on a TA Instruments Q2000 apparatus in N2 atmosphere, the heating or cooling rate was 10 °C/min, and the sample weight was about 2 mg. For each sample, the measurement was repeated thrice. Accelerated thermal cycling tests were performed using Binder MK240 high-low temperature chamber, operated between 5 and 40 °C. Totally 500 melting/solidifying thermal cycles were conducted. Thermal
2.2.1. Preparation of NanoPCMs with PS-SiO2 hybrid shell materials Typically, n-OD (10.0 g), TEOS (12 mL), MPS (3.0 mL), styrene (5 mL) and AIBN (0.125 g) were mixed in a beaker (500 mL), to form a clear solution. Then, CTAB (1.64 g, 20 mM), PVP (2.135 g, 1 wt%), deionized water (142.5 mL), and anhydrous ethanol (71 mL) were added to the beaker in turn. The mixture was homogenized at the rate of 13,000 rpm for 10 min, followed by sonication for 10 min, to form a stable miniemulsion. The mixture was transferred into a three necked flask (500 mL) equipped with a magnetic stirrer and a reflux condensing tube, and thermo stated in an oil bath (35 °C). Aqueous ammonia (2.6 mL) was added to the flask to initiate the reaction of alkoxy silanes and the reaction proceeded with gentle stirring (300 rpm) for 2 h. Then, after bubbled with Ar for 40 min, the temperature was raised to 80 °C, and the free radical polymerization was continued for 4 h. The mixture was cooled down, vacuum filtered, repeatedly washed with deionized water, and freeze dried. Finally, white powder-like products were collected. By changing the usage of St (2.5, 5, and 10 mL), three NanoPCMs samples were prepared, denoted as n-OD@PS-SiO2 (including n-OD@PS-SiO2-1, n-OD@PS-SiO2-2, and n-OD@PS-SiO2-3). 85
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The final morphologies of all the prepared nanocapsules were shown in Fig. 2. n-OD@SiO2 possesses near spherical shape and its average capsule size is 335 nm. A small portion of capsules bear holes on their shells, which are formed during the preparation process because the SiO2 shell is amorphous and brittle. n-OD@PS-SiO2 possess bowl-like shape, and their average capsule sizes are smaller than that of n-OD@SiO2. With increasing St usage, the nanocapsules become more cupped, and their surface looks rougher. On the contrary, n-OD@ PHEMA-SiO2 possess perfectly spherical shape, and their capsule sizes and shell thicknesses are much larger than that of n-OD@SiO2 and nOD@PS-SiO2. In a word, the morphologies of the nanocapsules vary dramatically with their composition, which might be ascribed to the change of interfacial tension in the mini-emulsion systems, with addition of different vinyl monomers in varied usage. As is known that St is hydrophobic and HEMA is hydrophilic, respectively. For both n-OD@ PS-SiO2 and n-OD@PHEMA-SiO2, their capsule shells are intact, nearly without any holes on them, which implies that the polymer-SiO2 shells are more tougher than the SiO2 shell. Moreover, as shown in Fig. 2, homogeneous mono layered shell (polymer and SiO2 components are interpenetrated) instead of double layered shell (one polymer layer plus one SiO2 layer was formed obviously for all the nanocapsules. To explain this point, it is supposed that the vinyl monomers swell the mesoporous SiO2 shell and polymerize therein. The chemical compositions of the nanocapsules were confirmed by FT-IR characterization, as shown in Fig. 3a. All three kinds of the nanocapsules (n-OD@SiO2, n-OD@PS-SiO2, and n-OD@PHEMA-SiO2) show characteristic infrared absorption peaks of n-OD (2924, 2855, 1468, 1377, and 721 cm−1) and SiO2 (3438, 1070, and 459 cm−1) in common [36]. In addition, characteristic infrared absorption peaks of PS (νPheH = 3026 cm−1; νC]C = 1600, 1493 cm−1; σPheH = 758, 698 cm−1) and PHEMA (νC]O = 1728 cm−1) ascribed to n-OD@PSSiO2 and n-OD@PHEMA-SiO2 respectively were observed. Therefore, the successful formation of organic-inorganic hybrid shell materials was clearly demonstrated. Additionally, by comparing the XRD patterns of pure n-OD and the nanocapsules (Fig. 3b), it can be found that n-OD was successfully encapsulated in these nanocapsules and displayed the same triclinic crystal structure [33]. The composition of the nanocapsules can be further determined by
conductivities of the NanoPCMs were measured on an EKO HC-074-200 apparatus by using heat flux method, and the test temperature was 25 °C. The leakage proof properties of the NanoPCMs were investigated by heating the specimen (initial weight = 1.0 g, placed in a culture dish and cover by filter paper) in an air dry oven at 60 °C, which is much higher than the serving temperature of these NanoPCMs to accelerate their leakage. The specimen was weighed periodically, and its phase change enthalpies after heated for 504 h were tested by DSC. The mechanical properties of the NanoPCMs were tested on a nanoindenter (Hysitron Triboindenter). Specimens were dispersed in ethanol and ultrasonicated, and their concentrations were fixed at 5 g/mL. A uniform monolayer of the nanocapsules were deposited on a silicon wafer, and subjected to the nanoindentation mechanical tests. 3. Results and discussion 3.1. Morphologies, chemical composition, and crystal structure The NanoPCMs with polymer-SiO2 hybrid shell materials were prepared in one-pot through two steps of chemical reaction. In step I, nanocapsules with mesoporous SiO2 shell were fabricated through interfacial hydrolysis-polycondensation of TEOS and MPS, as shown in Fig. 1a and b. When St was selected as vinyl monomer, the obtained intermediate nanocapsules exhibited bowl-like morphology. Contrarily, when HEMA was selected as vinyl monomer, perfectly spherical intermediate nanocapsules were obtained. Subsequently, in step II, nanocapsules with polymer-SiO2 hybrid shell were produced through free radical polymerization of vinyl monomers on the preexistent SiO2 shell, and their morphologies did not change much compared with that of the intermediates after step I, as shown in Fig. 1c and d. MPS is a dual functional raw material, because it participates in both hydrolysispolycondensation reaction and free radical polymerization. As a result, SiO2 and polymer components are covalently bonded together in the hybrid shell materials. Through this strategy, excellent morphologies for NanoPCMs with polymer-SiO2 hybrid shell materials can be obtained, and the polymer content can be facilely tuned by adding different volume of vinyl monomer.
Fig. 1. SEM images of the intermediate nanocapsules obtained after step I, and the produced NanoPCMs after step II. Vinyl monomers: (a, c) 5 mL of St; (b, d) 5 mL of HEMA.
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Fig. 2. (a–c, g–i) SEM and (d–f, j–k) TEM images of nanoencapsulated n-octadecane: (a, d) n-OD@SiO2, (b, e) n-OD@PS-SiO2-1, (c, f) n-OD@PS-SiO2-2, (g, j) n-OD@PS-SiO2-3, (h, k) nOD@PHEMA-SiO2-1, and (i) n-OD@PHEMA-SiO2-2.
Fig. 3. FT-IR spectra and XRD curves of n-OD, n-OD@SiO2, n-OD@PS-SiO2-2, and n-OD@PHEMA-SiO2-1.
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Fig. 4. TG curves of n-OD, n-OD@SiO2, n-OD@PS-SiO2, and n-OD@PHEMA-SiO2.
which implies mitigated supercooling. The results suggest that the polymer type in the hybrid shell materials has great impact on the supercooling behavior of the NanoPCMs. The phase change temperatures, phase change enthalpies, and encapsulation ratios (R) of n-OD and the NanoPCMs were listed in Table 2. It can be observed that with increasing usage of vinyl monomers, the phase change enthalpies of the NanoPCMs decrease gradually. In order to obtain better durability, tighter encapsulation, higher thermal conductivity, and tougher capsule shell for the NanoPCMs, moderate phase change enthalpies are acceptable.
Table 1 Detailed information obtained from the TG curves of n-OD and NanoPCMs with various shell materials. Samples
Weight loss in stage I (100–230 °C)
Weight loss in stage II (330–500 °C)
Char yield at 550 °C
n-octadecane n-OD@SiO2 n-OD@PS-SiO2-1 n-OD@PS-SiO2-2 n-OD@PS-SiO2-3 n-OD@PHEMASiO2-1 n-OD@PHEMASiO2-2
98.1% 59.5% 55.0% 49.9% 46.1% 53.2%
– 6.2% 14.4% 21.6% 26.6% 12.4%
1.9% 26.2% 24.3% 22.3% 21.1% 26.6%
43.5%
16.6%
30.2%
3.3. Thermal reliability Thermal cycling tests consisting of 500 melting-solidifying cycles were performed to evaluate the thermal reliability of the NanoPCMs. The DSC curves of n-OD@SiO2, n-OD@PS-SiO2-2, and n-OD@PHEMASiO2-1 during the thermal cycling processes were shown in Fig. 6. No obvious variation can be found on the DSC curves of each NanoPCMs. The phase change enthalpies of all the NanoPCMs after different thermal cycles were presented in Fig. 7. It can be seen that the melting and solidifying enthalpies of n-OD@SiO2 after 500 thermal cycles decreased by 9.0% and 8.7%, respectively, but the phase change enthalpies of the NanoPCMs with hybrid shell materials (n-OD@PS-SiO2 and n-OD@PHEMA-SiO2) showed much less reduction. The results demonstrated that the polymer-SiO2 hybrid shell materials are more preferable than SiO2, for NanoPCMs to repeatedly store/release thermal energy in practical applications. After the NanoPCMs subjected to additional 200 thermal cycles (i.e. 700th thermal cycle), gradual variation in phase change properties is observable, as shown in Fig. S1 and Table S1. The α and β peaks of nOD@PS-SiO2 and n-OD@PHEMA-SiO2 are slightly strengthened, meanwhile the γ peak becomes weaker. The phase change enthalpies also fluctuate slightly. We speculate that during the repetitious melting/solidifying process, wetting between n-OD core and polymer–SiO2 shell is enhanced and interaction between them is improved, which benefit the heterogeneous nucleation induced by the inner shell of the nanocapsules.
TG characterization, as shown in Fig. 4. It can be observed that pure nOD evaporates completely at the temperature range of 100–230 °C. All the nanocapsules lose weight in two stages: stage I (100–230 °C) is mainly ascribed to the evaporation of core material (n-OD); stage II (330–450 °C) mainly corresponds to the decomposition of the polymers (PS or PHEMA). The char yield at 550 °C roughly represents the SiO2 content in the nanocapsules. These data were tabulated in Table 1, demonstrating that the polymer content in the nanocapsules increased gradually, while the n-OD and SiO2 content in the nanocapsules decreased accordingly, with increasing usage of vinyl monomers. 3.2. Phase change properties The thermal energy storage/release capability of the as-prepared NanoPCMs was assessed by DSC method, and the DSC curves during heating/cooling processes were shown in Fig. 5. n-OD@SiO2 melts through one step at around 27.5 °C, and solidifies through 3 steps at around 22.1 (α peak), 15.5 (β peak), and 5.6 °C (γ peak), respectively. As clarified in [37,38], the α and β peaks with lower supercooling are based on heterogeneous nucleation, and the γ peak with larger supercooling is based on homogeneous nucleation. Supercooling behavior is often encountered for NanoPCMs, mainly due to geometric confinement and absence of nucleating agent [14,39]. The extent of supercooling is related to the interaction between core material and shell material [37]. n-OD@PS-SiO2 melt at similar temperatures with that of n-OD@SiO2, but their solidification almost completely rely on homogeneous nucleation and show more obvious supercooling. However, n-OD@ PHEMA-SiO2 possess very similar phase change properties with that of n-OD@SiO2 during both the melting and the solidifying processes, except that the γ crystallizing peak moves to slightly higher temperature,
3.4. Thermal conductivities Fast heat transfer is vital for utilization of PCMs in highly efficient thermal energy storage or thermoregulation. Both high thermal conductivity and large specific surface area are significant for improving the heat transfer rate. As shown in Fig. 8, all the as-prepared NanoPCMs are more thermal conductive than pure n-OD, and the polymer-SiO2 88
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Fig. 5. DSC curves of the NanoPCMs with various shell materials.
to compare the thermal charging rates of the NanoPCMs with different shell composition, and the results were shown in Fig. S2. It can be found that the thermal charging is completed within 167, 128, and 137 s for nOD@SiO2, n-OD@PS-SiO2-2, and n-OD@PHEMA-SiO2-1, respectively. This result is in agreement with the result of thermal conductivity test. Therefore, it is evidenced the NanoPCMs with polymer-SiO2 hybrid shell materials are more feasible for practical application from the viewpoint of thermal charging rate, compared with n-OD@SiO2.
Table 2 Phase change properties of n-OD and the NanoPCMs with various shell materials. Samples
Melting
n-OD n-OD@SiO2 n-OD@PS-SiO2-1 n-OD@PS-SiO2-2 n-OD@PS-SiO2-3 n-OD@PHEMA-SiO2-1 n-OD@PHEMA-SiO2-2
Solidifying
R (%)
Tm (°C)
ΔHm (J·g−1)
Tc,α (°C)
Tc,β (°C)
Tc,γ (°C)
ΔHc (J·g−1)
30.5 27.5 27.9 28.1 27.9 27.9 28.7
202.0 108.6 101.1 86.1 76.7 104.3 77.3
24.2 22.1 – – – 23.4 23.0
23.0 15.5 – – – 17.2 16.9
– 5.6 7.0 7.8 8.6 8.6 8.7
194.2 99.8 97.3 86.1 77.4 104.8 76.6
– 53.8 50.1 42.6 38.0 51.6 38.2
3.5. Leakage proof properties The leakage proof properties of the nanocapsules with different shell materials were tested by treating them at 60 °C in an air dry oven. The weight loss of the nanocapsules was plotted against the heating time in Fig. 9a. It is shown that the weight loss is rapid in the initial 72 h due to the evaporation of water absorbed by the nanocapsules, then it turns slow. n-OD@SiO2 show fastest weight loss among all the as-prepared NanoPCMs. For n-OD@PS-SiO2, with increasing usage of St from 0 to 5 mL, the weight loss rate slows down remarkably. However, further increase of the usage of St to 10 mL only results in negligible influence. For n-OD@PHEMA-SiO2, the weight loss rate is also lower than that of n-OD@SiO2, but higher than that of n-OD@PS-SiO2, which implies that PHEMA-SiO2 shell material is inferior in leakage proof properties compared with PS-SiO2. Because of trace water absorption in the nanocapsules, the weight loss may not represent the exact n-OD evaporation during the heating process. In order to further verify the
R = (ΔHm of NanoPCMs)/(ΔHm of n-OD).
hybrid shell materials endow the NanoPCMs higher thermal conductivity than SiO2 shell. When the usage of St or HEMA was 5 mL, maximal thermal conductivity was achieved, increasing by 15.4% or 14.7%, respectively, compared with that of n-OD@SiO2. These results are a little unexpected because inorganic materials are generally more thermal conductive than polymers. It is proposed that the unique interpenetrated structure of the hybrid shell materials, which is more compact than the mesoporous SiO2 shell material, is responsible for the enhanced thermal conductivity. We conducted a preliminary heating test on the as-prepared samples
Fig. 6. Thermal reliability of the NanoPCMs with various shell materials: (a) n-OD@SiO2, (b) n-OD@PS-SiO2-2, and (c) n-OD@PHEMA-SiO2-1.
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Fig. 7. Variation of phase change enthalpies of the NanoPCMs during thermal cycling tests: (a) melting enthalpies, and (b) solidifying enthalpies.
leakage proof properties of the nanocapsules, their melting enthalpies before and after heated for 504 h were measured and illustrated in Fig. 9b. The drop percentage of melting enthalpy follows such an order: n-OD@PS-SiO2-3 < n-OD@PS-SiO2-2 < n-OD@PS-SiO2-1 < n-OD@ PHEMA-SiO2-1 < n-OD@SiO2 < n-OD@PHEMA-SiO2-2. Obviously, the order of leakage proof property is just inverse to the above mentioned order. Therefore, it can be concluded that the polymer-SiO2 hybrid shell materials endow the NanoPCMs with improved leakage proof property. 3.6. Mechanical properties Mechanical property is another important factor for practical application of encapsulated PCMs. In [26,40], mechanical properties of MicroPCMs were measured by a nanoindenter and an atomic force microscope, respectively. For the first time, we attempted the characterization of mechanical properties of NanoPCMs by using a nanoindenter in this work. Because of the rather small size of NanoPCMs, it is impossible to observe and locate these nanocapsules under the optical microscope equipped with the nanoindenter. Through preparing a monolayer of the nanocapsules on a silicon wafer, measurements were successfully conducted and reliable load-displacement curves were obtained in Fig. 10. The yield load of n-OD@SiO2 can be determined to
Fig. 8. Thermal conductivities of n-OD and the NanoPCMs with various shell materials.
Fig. 9. Leakage proof properties of the NanoPCMs with various shell materials: (a) weight loss vs. heating time; (b) drop percentage of melting enthalpies after heated for 504 h.
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Fig. 10. Load-displacement curves of the NanoPCMs with various shell materials: (a) n-OD@SiO2, n-OD@PS-SiO2, and (b) n-OD@PHEMA-SiO2.
be ca. 14.7 μN. However, yield plateaus are not found on the loaddisplacement curves of n-OD@PS-SiO2, which may be caused by the bowl-like shape of these samples. The load increases gradually with the displacement, until the tip of the nanoindenter reach the stiff silicon wafer and the load increases sharply. It is very likely that n-OD@PSSiO2-1, n-OD@PS-SiO2-2, and n-OD@PS-SiO2-3 are not damaged at loads lower than 19.5, 34.6, and 39.9 μN, respectively (Fig. 10a). For nOD@PHEMA-SiO2-1 and n-OD@PHEMA-SiO2-2, the yield loads can be determined to be ca. 65 and 79 μN respectively, which are much higher than that of n-OD@SiO2. Therefore, it is proved that the polymer-SiO2 hybrid shell materials are much superior in mechanical properties than the SiO2 shell material.
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4. Conclusion NanoPCMs with polymer-SiO2 hybrid shell materials were prepared, and the polymer contents can be facilely tuned by adding different volume of vinyl monomers. n-OD@PS-SiO2 possess bowl-like shape, while n-OD@PHEMA-SiO2 possess perfectly spherical shape. Rather than double layered shell, homogeneous mono layered shell was formed for all the nanocapsules. The polymer type in the hybrid shell materials has great impact on the supercooling behavior of the NanoPCMs. The polymer-SiO2 hybrid shell materials endow the NanoPCMs with improved thermal reliability. Compared with n-OD@SiO2, the thermal conductivities of n-OD@PS-SiO2 and n-OD@PHEMA-SiO2 increase by 15.4% and 14.7%, respectively. The leakage proof property follows an order of: n-OD@PS-SiO2-3 > n-OD@PS-SiO2-2 > n-OD@PS-SiO21 > n-OD@PHEMA-SiO2-1 > n-OD@SiO2 > n-OD@PHEMA-SiO2-2. More importantly, the polymer-SiO2 hybrid shell materials are much superior in mechanical properties than the SiO2 shell material. Optimized overall properties of the NanoPCMs were obtained when the usage of vinyl monomers was 5 mL. The novel NanoPCMs with polymer-SiO2 hybrid shell materials may find broad applications in energy efficient buildings, smart textiles, and latent functionally thermal fluids. Acknowledgements The financial support from National Natural Science Foundation of China (No. 51703210) is gratefully acknowledged. We thank Prof. Jie Zeng, Dr. Wei Sang, and MD. Lingang Lan for their kind help on TEM and nanoindentation characterizations. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the 91
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Nanoencapsulated phase change materials with polymer-SiO2 hybrid shell materials: Compositions, morphologies, and properties ⁎
T
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Yalin Zhua,b, Yaosong Qina,b, Chengsha Weib, Shuen Liangb, , Xuan Luoa, , Jianhua Wangb, Lin Zhanga a b
Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase change materials Nanoencapsulation Hybrid shell Leakage proof Mechanical property
Organic-inorganic hybrid materials are promising for encapsulation of phase change materials (PCMs) to achieve exceptional capsule properties. In this work, novel polymer-SiO2 hybrid shelled nanoencapsulated PCMs (NanoPCMs) were fabricated in one-pot, through sequentially executed interfacial hydrolysis-polycondensation of alkoxy silanes and radical polymerization of vinyl monomers. The morphologies, chemical compositions, and crystal structures of the NanoPCMs were characterized by SEM, TEM, FT-IR, and XRD methods. The thermal energy storage capability, thermal reliability, and thermal conductivity were tested by DSC, accelerated thermal cycling test, and heat flux method, respectively. The leakage proof property and mechanical property were evaluated by seepage test and nanoindentation test, respectively. Compared with NanoPCMs with SiO2 shell, the NanoPCMs with polystyrene (PS)-SiO2 shell possess smaller size and bowl like shape, while NanoPCMs with poly (hydroxylethyl methacrylate) (PHEMA)-SiO2 shell possess larger size and perfect spherical shape. The polymer types have great impact on the supercooling behavior of the NanoPCMs. The polymer-SiO2 hybrid shell materials endow the NanoPCMs with improved thermal reliability, thermal conductivity, and leakage proof property. More importantly, the compressive load at yield increases remarkably from 14.7 μN for nanocapsules with SiO2 shell, to > 34.6 μN for that with PS-SiO2 shell, and 65 μN for that with PHEMA-SiO2 shell.
1. Introduction Phase change materials (PCMs) are substances that can undergo phase transitions at defined temperature range, and store/release large quantity of latent heats during the process [1]. Due to their high heat storage capacity, PCMs are good choice to overcome the uneven energy distribution in space and time, and the mismatch between energy supply and demand [2]. Moreover, based on their near isothermal characteristics during the thermal energy storage/release process, PCMs can also provide thermal comfort for buildings in an energy efficient and environmentally friendly way [3]. Therefore, PCMs can greatly contribute to development of renewable and sustainable energy sources, less consumption of fossil fuels, and less emission of green house gas. Most practically valuable PCMs, including paraffin, fatty acids, alcohols, and salt hydrates, perform thermal energy storage through solid–liquid phase transition, and suffer from leakage in liquid state and low thermal conductivity. In last decades, micro (1 μm < size < 1000 μm)/nano (size < 1 μm) encapsulation of PCMs
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with protective shell materials have been widely explored. Those capsules can effectively prevent the leakage of PCMs, improve the thermal conductivity and heat storage/release efficiency, and control the volume change during phase change process [4,5]. Through encapsulation, the application fields of PCMs have been greatly extended, such as latent functionally thermal fluids [6,7] and smart textiles [8]. Encapsulated PCMs with various organic and inorganic shell materials were prepared by using different methods, such as in-situ polymerization, suspension polymerization, interfacial polymerization, sol-gel method, and self-assembly method, etc [4]. Generally, the well developed organic shell materials, such as melamine-formaldehyde (M−F) resin, polyurea [9], polystyrene (PS) [10,11], polymethylmethacrylate (PMMA), can withstand the volume change during phase change process efficiently, but possess low thermal conductivity. Inorganic shell materials, mainly including SiO2 [12,13], TiO2 [14,15], and CaCO3 [16], have attracted intensive research interests during the latest ten years, due to their higher thermal conductivity and thermal stability than the organic counterparts. However, inorganic shell materials like SiO2 and TiO2 are generally obtained through sol-gel or interfacial
Corresponding authors. E-mail addresses: [email protected] (S. Liang), [email protected] (X. Luo).
https://doi.org/10.1016/j.enconman.2018.02.075 Received 3 January 2018; Received in revised form 20 February 2018; Accepted 20 February 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.
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much faster thermal energy storage/release [30]. When applied in latent functionally thermal fluid, NanoPCMs do not fracture easily in the course of flow [5]. However, it is obvious that the shell of NanoPCMs is generally much thinner than that of MicroPCMs, so its compactness and toughness are especially important to achieve desirable sealing performance and durability. Therefore, it is very valuable for practical application to develop novel NanoPCMs with organic-inorganic hybrid shell materials. In this work, we conduct modification of SiO2 shell material with PS and poly(hydroxylethyl methacrylate) (PHEMA), to obtain novel NanoPCMs with organic-inorganic hybrid shell materials. SiO2 is a classic type of inorganic shell material for micro/nano encapsulated PCMs, because it can be facilely and controllably synthesized through sol-gel (i.e. interfacial hydrolysis-polycondensation) process with inexpensive raw materials like tetraethyl orthosilicate (TEOS) [31,32] and sodium silicate [33]. PS and PHEMA [34] were selected as representatives of hydrophobic and hydrophilic polymers, respectively. The prices of the monomers for both polymers are also very cheap. The polymer-SiO2 hybrid shelled NanoPCMs were fabricated in one-pot, through sequentially executed interfacial hydrolysis-polycondensation of alkoxy silanes and free radical polymerization of vinyl monomers. The low costs of raw materials and simplicity of preparation can endow the target NanoPCMs with good potential for mass production and practical use. Through this preparation strategy, excellent morphologies can be obtained, and the polymer content can be facilely tuned. The influences of the two polymers with different contents on the morphologies, thermal energy storage properties, thermal conductivities, leakage proof properties, and mechanical properties of the NanoPCMs were comprehensively investigated. On the basis of the results of this research, the overall properties of the NanoPCMs can be optimized with respect to their compositions.
hydrolysis-polycondensation processes. They often exhibit mesoporous structure [17] and are mechanically weak. In recent few years, many research efforts have been devoted to organic-inorganic hybrid shell materials, which can combine the advantages of organic and inorganic materials, to achieve outstandingly high performances [18]. Consequently, several novel types of micro/nano encapsulated PCMs with organic-inorganic hybrid shell materials have been invented. Some inorganic materials like SiO2, TiO2, Al2O3, Si3N4, graphene oxide (GO), and graphene, were introduced into organic shell materials of microencapsulated PCMs (MicroPCMs) as fillers during the preparation process. These highly thermal conductive fillers were dispersed either by direct mixing or serving as Pickering stabilizers. Yin et al. [19] prepared MicroPCMs with PS-SiO2 hybrid shell material via Pickering emulsion polymerization method, in which organicallymodified SiO2 particles were used as stabilizer. The prepared MicroPCMs present good durability and thermal reliability. Zhao et al. [20] fabricated MicroPCMs with PMMA-TiO2 hybrid shell for thermal energy storage and UV-shielding. Sun et al. [21] synthesized a novel kind of MicroPCMs with PMMA/BN/TiO2 ternary hybrid shell material by Pickering emulsion method, in which nano-BN/TiO2 particles were used as stabilizer. Jiang et al. [22] synthesized new MicroPCMs based on paraffin wax core and poly(methyl methacrylate-co-methyl acrylate)-Al2O3 hybrid shell material. Yang et al. [23] reported MicroPCMs with PMMA-Si3N4 hybrid shell material. Chen et al. [24] prepared novel MicroPCMs with M−F resin-GO hybrid shell material by in situ polymerization, and the products possessed high latent heat and improved thermal conductivity. Zhang et al. [25] prepared MicroPCMs with PS-GO hybrid shell material by Pickering emulsion method, and achieved high encapsulation ratio and good thermal stability. Su et al. [26] synthesized MicroPCMs with methanol-modified M−F resin-graphene hybrid shell material through in situ polymerization, to improve the mechanical properties and thermal conductivities. In addition to the above mentioned method, a different method for preparation of MicroPCMs with hybrid shell material was reported by Li et al. [27]. Specifically, vinyl functionalized alkoxy silanes were employed as raw materials, which were subjected to simultaneous sol-gel reaction and free radical polymerization and resulted in polymer-SiO2 hybrid shell material to encapsulate the PCMs. The products possess high encapsulation ratios and low leakage rate. In summary, through these approaches, the obtained hybrid shell materials brought lots of merits for micro/nano encapsulated PCMs: enhanced encapsulation ratio [28], improved thermal conductivity [24,26], and better leakage proof properties [29]. It is observed that most of the previous works were focused on modification of organic shell materials with some inorganic substances, but the modification of inorganic shell materials with organic polymers was rare. In addition, from the view point of capsule size, most previously reported encapsulated PCMs with organic-inorganic hybrid shell materials were limited to MicroPCMs, while nanoencapsulated PCMs (NanoPCMs) were almost ignored. Compared with MicroPCMs, NanoPCMs have much larger specific surface area, which can result in
2. Experimental 2.1. Materials n-Octadecane (n-OD, 90 wt%), γ-methacryloxypropyl trimethoxy silane (MPS), and polyvinylpyrrolidone (PVP, M.W. 58,000 g/mol) were purchased from Alfa Aesar. Tetraethyl orthosilicate (TEOS), anhydrous ethanol, NH3·H2O (25 wt%), styrene (St), and 2,2′-azobisisobutyronitrile (AIBN) were purchased from Sinopharm Chemical Reagents. Hydroxyethyl methacrylate (HEMA) was purchased from Aladdin. St and HEMA were purified by passing through a neutral alumina column, to remove retardants. AIBN was recrystallized from 95% ethanol before use. Cetyltrimethylammonium bromide (CTAB) was commercially supplied by Tianjin Kermal Chemical Reagents. All chemicals were of reagent quality and used without further purification, unless stated otherwise.
Scheme 1. Raw materials used for fabrication of polymer-SiO2 hybrid shell materials.
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Scheme 2. Schematic representation of the processes for preparation of nanoencapsulated n-octadecane with polymer-SiO2 hybrid shell materials.
2.2.2. Preparation of NanoPCMs with PHEMA-SiO2 hybrid shell materials The procedure is similar to that for NanoPCMs with PS-SiO2 hybrid shell materials, except that HEMA is selected as the vinyl monomer and dissolved in water phase. By changing the usage of HEMA (5 and 10 mL), two NanoPCMs samples were prepared, denoted as n-OD@ PHEMA-SiO2 (including n-OD@PHEMA-SiO2-1 and n-OD@PS-HEMA2). NanoPCMs with SiO2 shell materials was also prepared through the same procedure, except that no vinyl monomer (St or HEMA) was added, denoted as n-OD@SiO2. This sample was used as a control without polymer modification.
2.2. Preparation of NanoPCMs with polymer-SiO2 hybrid shell materials The molecular structures of raw materials for synthesis of organicinorganic hybrid shell materials were shown in Scheme 1. A one-pot procedure in miniemulsion was adopted to prepare the NanoPCMs with PS-SiO2 and PHEMA-SiO2 hybrid shell materials, as illustrated in Scheme 2. Initially, an O/W miniemulsion was produced by mixing the oil phase and water phase under high shearing (FA25 high shearing homogenizer, Fluke) and ultrasound (VCX 750 ultrasonic processor, Sonics & Materials). The oil phase is composed of n-OD, alkoxy silanes, hydrophobic monomer, and AIBN, while the water phase is composed of H2O, ethanol, PVP, and hydrophilic monomer, respectively. Two sequentially executed chemical reactions were involved in the preparation of the NanoPCMs. In step I, alkoxy silanes (TEOS and MPS) hydrolyzed and condensed at the water/oil interface catalyzed by NH3·H2O, and SiO2 shell with many methacryloxypropyl groups was formed, referring to our previous works [35]. In step II, the temperature was raised to 80 °C to initiate the free radical polymerization of the vinyl monomer (St or HEMA) and the methacryloxypropyl groups on the already formed SiO2 shell. Surfactant (CTAB), dispersing agent (PVP), free PHEMA polymer chains not attached to the SiO2 shell, and other impurities were removed by repeated washing with water and freeze drying.
2.3. Characterization Scanning electron microscopy (SEM) characterization of the asprepared NanoPCMs was conducted on a Zeiss Ultra 55 field emission scanning electron microscope. The specimen was sputter coated with a thin layer of Au. Micrographs were taken in high vacuum mode with 10 kV acceleration voltage and a medium spot size. Transmission electron microscopy (TEM) characterization was performed on a Hitachi H-800 transmission electron microscope operated at an accelerating voltage of 100 kV. Specimen was dispersed in ethanol under sonication, and a drop of the mixture was transferred to the carboncoated copper grid for observation. Based on the obtained SEM micrographs, sizes of the NanoPCMs were determined by using Nano Measurer 1.2 software. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet 6700 IR-spectrophotometer on KBr sampling sheet, with a scanning number of 32. X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance diffractometer (40 kV, 50 mA) with Cu Kα radiation (λ = 0.154 nm), at 2θ range of 5–90° and scanning rate of 10°/min. Thermogravimetric (TG) analysis was performed on a TA Instruments Q500 thermogravimetric analyzer under N2 atmosphere. The specimen with a mass of ca. 5 mg was placed in an aluminum crucible and then ramped from room temperature up to 550 °C, at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) analysis was conducted on a TA Instruments Q2000 apparatus in N2 atmosphere, the heating or cooling rate was 10 °C/min, and the sample weight was about 2 mg. For each sample, the measurement was repeated thrice. Accelerated thermal cycling tests were performed using Binder MK240 high-low temperature chamber, operated between 5 and 40 °C. Totally 500 melting/solidifying thermal cycles were conducted. Thermal
2.2.1. Preparation of NanoPCMs with PS-SiO2 hybrid shell materials Typically, n-OD (10.0 g), TEOS (12 mL), MPS (3.0 mL), styrene (5 mL) and AIBN (0.125 g) were mixed in a beaker (500 mL), to form a clear solution. Then, CTAB (1.64 g, 20 mM), PVP (2.135 g, 1 wt%), deionized water (142.5 mL), and anhydrous ethanol (71 mL) were added to the beaker in turn. The mixture was homogenized at the rate of 13,000 rpm for 10 min, followed by sonication for 10 min, to form a stable miniemulsion. The mixture was transferred into a three necked flask (500 mL) equipped with a magnetic stirrer and a reflux condensing tube, and thermo stated in an oil bath (35 °C). Aqueous ammonia (2.6 mL) was added to the flask to initiate the reaction of alkoxy silanes and the reaction proceeded with gentle stirring (300 rpm) for 2 h. Then, after bubbled with Ar for 40 min, the temperature was raised to 80 °C, and the free radical polymerization was continued for 4 h. The mixture was cooled down, vacuum filtered, repeatedly washed with deionized water, and freeze dried. Finally, white powder-like products were collected. By changing the usage of St (2.5, 5, and 10 mL), three NanoPCMs samples were prepared, denoted as n-OD@PS-SiO2 (including n-OD@PS-SiO2-1, n-OD@PS-SiO2-2, and n-OD@PS-SiO2-3). 85
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The final morphologies of all the prepared nanocapsules were shown in Fig. 2. n-OD@SiO2 possesses near spherical shape and its average capsule size is 335 nm. A small portion of capsules bear holes on their shells, which are formed during the preparation process because the SiO2 shell is amorphous and brittle. n-OD@PS-SiO2 possess bowl-like shape, and their average capsule sizes are smaller than that of n-OD@SiO2. With increasing St usage, the nanocapsules become more cupped, and their surface looks rougher. On the contrary, n-OD@ PHEMA-SiO2 possess perfectly spherical shape, and their capsule sizes and shell thicknesses are much larger than that of n-OD@SiO2 and nOD@PS-SiO2. In a word, the morphologies of the nanocapsules vary dramatically with their composition, which might be ascribed to the change of interfacial tension in the mini-emulsion systems, with addition of different vinyl monomers in varied usage. As is known that St is hydrophobic and HEMA is hydrophilic, respectively. For both n-OD@ PS-SiO2 and n-OD@PHEMA-SiO2, their capsule shells are intact, nearly without any holes on them, which implies that the polymer-SiO2 shells are more tougher than the SiO2 shell. Moreover, as shown in Fig. 2, homogeneous mono layered shell (polymer and SiO2 components are interpenetrated) instead of double layered shell (one polymer layer plus one SiO2 layer was formed obviously for all the nanocapsules. To explain this point, it is supposed that the vinyl monomers swell the mesoporous SiO2 shell and polymerize therein. The chemical compositions of the nanocapsules were confirmed by FT-IR characterization, as shown in Fig. 3a. All three kinds of the nanocapsules (n-OD@SiO2, n-OD@PS-SiO2, and n-OD@PHEMA-SiO2) show characteristic infrared absorption peaks of n-OD (2924, 2855, 1468, 1377, and 721 cm−1) and SiO2 (3438, 1070, and 459 cm−1) in common [36]. In addition, characteristic infrared absorption peaks of PS (νPheH = 3026 cm−1; νC]C = 1600, 1493 cm−1; σPheH = 758, 698 cm−1) and PHEMA (νC]O = 1728 cm−1) ascribed to n-OD@PSSiO2 and n-OD@PHEMA-SiO2 respectively were observed. Therefore, the successful formation of organic-inorganic hybrid shell materials was clearly demonstrated. Additionally, by comparing the XRD patterns of pure n-OD and the nanocapsules (Fig. 3b), it can be found that n-OD was successfully encapsulated in these nanocapsules and displayed the same triclinic crystal structure [33]. The composition of the nanocapsules can be further determined by
conductivities of the NanoPCMs were measured on an EKO HC-074-200 apparatus by using heat flux method, and the test temperature was 25 °C. The leakage proof properties of the NanoPCMs were investigated by heating the specimen (initial weight = 1.0 g, placed in a culture dish and cover by filter paper) in an air dry oven at 60 °C, which is much higher than the serving temperature of these NanoPCMs to accelerate their leakage. The specimen was weighed periodically, and its phase change enthalpies after heated for 504 h were tested by DSC. The mechanical properties of the NanoPCMs were tested on a nanoindenter (Hysitron Triboindenter). Specimens were dispersed in ethanol and ultrasonicated, and their concentrations were fixed at 5 g/mL. A uniform monolayer of the nanocapsules were deposited on a silicon wafer, and subjected to the nanoindentation mechanical tests. 3. Results and discussion 3.1. Morphologies, chemical composition, and crystal structure The NanoPCMs with polymer-SiO2 hybrid shell materials were prepared in one-pot through two steps of chemical reaction. In step I, nanocapsules with mesoporous SiO2 shell were fabricated through interfacial hydrolysis-polycondensation of TEOS and MPS, as shown in Fig. 1a and b. When St was selected as vinyl monomer, the obtained intermediate nanocapsules exhibited bowl-like morphology. Contrarily, when HEMA was selected as vinyl monomer, perfectly spherical intermediate nanocapsules were obtained. Subsequently, in step II, nanocapsules with polymer-SiO2 hybrid shell were produced through free radical polymerization of vinyl monomers on the preexistent SiO2 shell, and their morphologies did not change much compared with that of the intermediates after step I, as shown in Fig. 1c and d. MPS is a dual functional raw material, because it participates in both hydrolysispolycondensation reaction and free radical polymerization. As a result, SiO2 and polymer components are covalently bonded together in the hybrid shell materials. Through this strategy, excellent morphologies for NanoPCMs with polymer-SiO2 hybrid shell materials can be obtained, and the polymer content can be facilely tuned by adding different volume of vinyl monomer.
Fig. 1. SEM images of the intermediate nanocapsules obtained after step I, and the produced NanoPCMs after step II. Vinyl monomers: (a, c) 5 mL of St; (b, d) 5 mL of HEMA.
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Fig. 2. (a–c, g–i) SEM and (d–f, j–k) TEM images of nanoencapsulated n-octadecane: (a, d) n-OD@SiO2, (b, e) n-OD@PS-SiO2-1, (c, f) n-OD@PS-SiO2-2, (g, j) n-OD@PS-SiO2-3, (h, k) nOD@PHEMA-SiO2-1, and (i) n-OD@PHEMA-SiO2-2.
Fig. 3. FT-IR spectra and XRD curves of n-OD, n-OD@SiO2, n-OD@PS-SiO2-2, and n-OD@PHEMA-SiO2-1.
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Fig. 4. TG curves of n-OD, n-OD@SiO2, n-OD@PS-SiO2, and n-OD@PHEMA-SiO2.
which implies mitigated supercooling. The results suggest that the polymer type in the hybrid shell materials has great impact on the supercooling behavior of the NanoPCMs. The phase change temperatures, phase change enthalpies, and encapsulation ratios (R) of n-OD and the NanoPCMs were listed in Table 2. It can be observed that with increasing usage of vinyl monomers, the phase change enthalpies of the NanoPCMs decrease gradually. In order to obtain better durability, tighter encapsulation, higher thermal conductivity, and tougher capsule shell for the NanoPCMs, moderate phase change enthalpies are acceptable.
Table 1 Detailed information obtained from the TG curves of n-OD and NanoPCMs with various shell materials. Samples
Weight loss in stage I (100–230 °C)
Weight loss in stage II (330–500 °C)
Char yield at 550 °C
n-octadecane n-OD@SiO2 n-OD@PS-SiO2-1 n-OD@PS-SiO2-2 n-OD@PS-SiO2-3 n-OD@PHEMASiO2-1 n-OD@PHEMASiO2-2
98.1% 59.5% 55.0% 49.9% 46.1% 53.2%
– 6.2% 14.4% 21.6% 26.6% 12.4%
1.9% 26.2% 24.3% 22.3% 21.1% 26.6%
43.5%
16.6%
30.2%
3.3. Thermal reliability Thermal cycling tests consisting of 500 melting-solidifying cycles were performed to evaluate the thermal reliability of the NanoPCMs. The DSC curves of n-OD@SiO2, n-OD@PS-SiO2-2, and n-OD@PHEMASiO2-1 during the thermal cycling processes were shown in Fig. 6. No obvious variation can be found on the DSC curves of each NanoPCMs. The phase change enthalpies of all the NanoPCMs after different thermal cycles were presented in Fig. 7. It can be seen that the melting and solidifying enthalpies of n-OD@SiO2 after 500 thermal cycles decreased by 9.0% and 8.7%, respectively, but the phase change enthalpies of the NanoPCMs with hybrid shell materials (n-OD@PS-SiO2 and n-OD@PHEMA-SiO2) showed much less reduction. The results demonstrated that the polymer-SiO2 hybrid shell materials are more preferable than SiO2, for NanoPCMs to repeatedly store/release thermal energy in practical applications. After the NanoPCMs subjected to additional 200 thermal cycles (i.e. 700th thermal cycle), gradual variation in phase change properties is observable, as shown in Fig. S1 and Table S1. The α and β peaks of nOD@PS-SiO2 and n-OD@PHEMA-SiO2 are slightly strengthened, meanwhile the γ peak becomes weaker. The phase change enthalpies also fluctuate slightly. We speculate that during the repetitious melting/solidifying process, wetting between n-OD core and polymer–SiO2 shell is enhanced and interaction between them is improved, which benefit the heterogeneous nucleation induced by the inner shell of the nanocapsules.
TG characterization, as shown in Fig. 4. It can be observed that pure nOD evaporates completely at the temperature range of 100–230 °C. All the nanocapsules lose weight in two stages: stage I (100–230 °C) is mainly ascribed to the evaporation of core material (n-OD); stage II (330–450 °C) mainly corresponds to the decomposition of the polymers (PS or PHEMA). The char yield at 550 °C roughly represents the SiO2 content in the nanocapsules. These data were tabulated in Table 1, demonstrating that the polymer content in the nanocapsules increased gradually, while the n-OD and SiO2 content in the nanocapsules decreased accordingly, with increasing usage of vinyl monomers. 3.2. Phase change properties The thermal energy storage/release capability of the as-prepared NanoPCMs was assessed by DSC method, and the DSC curves during heating/cooling processes were shown in Fig. 5. n-OD@SiO2 melts through one step at around 27.5 °C, and solidifies through 3 steps at around 22.1 (α peak), 15.5 (β peak), and 5.6 °C (γ peak), respectively. As clarified in [37,38], the α and β peaks with lower supercooling are based on heterogeneous nucleation, and the γ peak with larger supercooling is based on homogeneous nucleation. Supercooling behavior is often encountered for NanoPCMs, mainly due to geometric confinement and absence of nucleating agent [14,39]. The extent of supercooling is related to the interaction between core material and shell material [37]. n-OD@PS-SiO2 melt at similar temperatures with that of n-OD@SiO2, but their solidification almost completely rely on homogeneous nucleation and show more obvious supercooling. However, n-OD@ PHEMA-SiO2 possess very similar phase change properties with that of n-OD@SiO2 during both the melting and the solidifying processes, except that the γ crystallizing peak moves to slightly higher temperature,
3.4. Thermal conductivities Fast heat transfer is vital for utilization of PCMs in highly efficient thermal energy storage or thermoregulation. Both high thermal conductivity and large specific surface area are significant for improving the heat transfer rate. As shown in Fig. 8, all the as-prepared NanoPCMs are more thermal conductive than pure n-OD, and the polymer-SiO2 88
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Fig. 5. DSC curves of the NanoPCMs with various shell materials.
to compare the thermal charging rates of the NanoPCMs with different shell composition, and the results were shown in Fig. S2. It can be found that the thermal charging is completed within 167, 128, and 137 s for nOD@SiO2, n-OD@PS-SiO2-2, and n-OD@PHEMA-SiO2-1, respectively. This result is in agreement with the result of thermal conductivity test. Therefore, it is evidenced the NanoPCMs with polymer-SiO2 hybrid shell materials are more feasible for practical application from the viewpoint of thermal charging rate, compared with n-OD@SiO2.
Table 2 Phase change properties of n-OD and the NanoPCMs with various shell materials. Samples
Melting
n-OD n-OD@SiO2 n-OD@PS-SiO2-1 n-OD@PS-SiO2-2 n-OD@PS-SiO2-3 n-OD@PHEMA-SiO2-1 n-OD@PHEMA-SiO2-2
Solidifying
R (%)
Tm (°C)
ΔHm (J·g−1)
Tc,α (°C)
Tc,β (°C)
Tc,γ (°C)
ΔHc (J·g−1)
30.5 27.5 27.9 28.1 27.9 27.9 28.7
202.0 108.6 101.1 86.1 76.7 104.3 77.3
24.2 22.1 – – – 23.4 23.0
23.0 15.5 – – – 17.2 16.9
– 5.6 7.0 7.8 8.6 8.6 8.7
194.2 99.8 97.3 86.1 77.4 104.8 76.6
– 53.8 50.1 42.6 38.0 51.6 38.2
3.5. Leakage proof properties The leakage proof properties of the nanocapsules with different shell materials were tested by treating them at 60 °C in an air dry oven. The weight loss of the nanocapsules was plotted against the heating time in Fig. 9a. It is shown that the weight loss is rapid in the initial 72 h due to the evaporation of water absorbed by the nanocapsules, then it turns slow. n-OD@SiO2 show fastest weight loss among all the as-prepared NanoPCMs. For n-OD@PS-SiO2, with increasing usage of St from 0 to 5 mL, the weight loss rate slows down remarkably. However, further increase of the usage of St to 10 mL only results in negligible influence. For n-OD@PHEMA-SiO2, the weight loss rate is also lower than that of n-OD@SiO2, but higher than that of n-OD@PS-SiO2, which implies that PHEMA-SiO2 shell material is inferior in leakage proof properties compared with PS-SiO2. Because of trace water absorption in the nanocapsules, the weight loss may not represent the exact n-OD evaporation during the heating process. In order to further verify the
R = (ΔHm of NanoPCMs)/(ΔHm of n-OD).
hybrid shell materials endow the NanoPCMs higher thermal conductivity than SiO2 shell. When the usage of St or HEMA was 5 mL, maximal thermal conductivity was achieved, increasing by 15.4% or 14.7%, respectively, compared with that of n-OD@SiO2. These results are a little unexpected because inorganic materials are generally more thermal conductive than polymers. It is proposed that the unique interpenetrated structure of the hybrid shell materials, which is more compact than the mesoporous SiO2 shell material, is responsible for the enhanced thermal conductivity. We conducted a preliminary heating test on the as-prepared samples
Fig. 6. Thermal reliability of the NanoPCMs with various shell materials: (a) n-OD@SiO2, (b) n-OD@PS-SiO2-2, and (c) n-OD@PHEMA-SiO2-1.
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Fig. 7. Variation of phase change enthalpies of the NanoPCMs during thermal cycling tests: (a) melting enthalpies, and (b) solidifying enthalpies.
leakage proof properties of the nanocapsules, their melting enthalpies before and after heated for 504 h were measured and illustrated in Fig. 9b. The drop percentage of melting enthalpy follows such an order: n-OD@PS-SiO2-3 < n-OD@PS-SiO2-2 < n-OD@PS-SiO2-1 < n-OD@ PHEMA-SiO2-1 < n-OD@SiO2 < n-OD@PHEMA-SiO2-2. Obviously, the order of leakage proof property is just inverse to the above mentioned order. Therefore, it can be concluded that the polymer-SiO2 hybrid shell materials endow the NanoPCMs with improved leakage proof property. 3.6. Mechanical properties Mechanical property is another important factor for practical application of encapsulated PCMs. In [26,40], mechanical properties of MicroPCMs were measured by a nanoindenter and an atomic force microscope, respectively. For the first time, we attempted the characterization of mechanical properties of NanoPCMs by using a nanoindenter in this work. Because of the rather small size of NanoPCMs, it is impossible to observe and locate these nanocapsules under the optical microscope equipped with the nanoindenter. Through preparing a monolayer of the nanocapsules on a silicon wafer, measurements were successfully conducted and reliable load-displacement curves were obtained in Fig. 10. The yield load of n-OD@SiO2 can be determined to
Fig. 8. Thermal conductivities of n-OD and the NanoPCMs with various shell materials.
Fig. 9. Leakage proof properties of the NanoPCMs with various shell materials: (a) weight loss vs. heating time; (b) drop percentage of melting enthalpies after heated for 504 h.
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Fig. 10. Load-displacement curves of the NanoPCMs with various shell materials: (a) n-OD@SiO2, n-OD@PS-SiO2, and (b) n-OD@PHEMA-SiO2.
be ca. 14.7 μN. However, yield plateaus are not found on the loaddisplacement curves of n-OD@PS-SiO2, which may be caused by the bowl-like shape of these samples. The load increases gradually with the displacement, until the tip of the nanoindenter reach the stiff silicon wafer and the load increases sharply. It is very likely that n-OD@PSSiO2-1, n-OD@PS-SiO2-2, and n-OD@PS-SiO2-3 are not damaged at loads lower than 19.5, 34.6, and 39.9 μN, respectively (Fig. 10a). For nOD@PHEMA-SiO2-1 and n-OD@PHEMA-SiO2-2, the yield loads can be determined to be ca. 65 and 79 μN respectively, which are much higher than that of n-OD@SiO2. Therefore, it is proved that the polymer-SiO2 hybrid shell materials are much superior in mechanical properties than the SiO2 shell material.
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4. Conclusion NanoPCMs with polymer-SiO2 hybrid shell materials were prepared, and the polymer contents can be facilely tuned by adding different volume of vinyl monomers. n-OD@PS-SiO2 possess bowl-like shape, while n-OD@PHEMA-SiO2 possess perfectly spherical shape. Rather than double layered shell, homogeneous mono layered shell was formed for all the nanocapsules. The polymer type in the hybrid shell materials has great impact on the supercooling behavior of the NanoPCMs. The polymer-SiO2 hybrid shell materials endow the NanoPCMs with improved thermal reliability. Compared with n-OD@SiO2, the thermal conductivities of n-OD@PS-SiO2 and n-OD@PHEMA-SiO2 increase by 15.4% and 14.7%, respectively. The leakage proof property follows an order of: n-OD@PS-SiO2-3 > n-OD@PS-SiO2-2 > n-OD@PS-SiO21 > n-OD@PHEMA-SiO2-1 > n-OD@SiO2 > n-OD@PHEMA-SiO2-2. More importantly, the polymer-SiO2 hybrid shell materials are much superior in mechanical properties than the SiO2 shell material. Optimized overall properties of the NanoPCMs were obtained when the usage of vinyl monomers was 5 mL. The novel NanoPCMs with polymer-SiO2 hybrid shell materials may find broad applications in energy efficient buildings, smart textiles, and latent functionally thermal fluids. Acknowledgements The financial support from National Natural Science Foundation of China (No. 51703210) is gratefully acknowledged. We thank Prof. Jie Zeng, Dr. Wei Sang, and MD. Lingang Lan for their kind help on TEM and nanoindentation characterizations. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the 91
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