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Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes
Energy xxx (2014) 1e9
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Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes Wenhong Li a, b, Guolin Song a, Shuhua Li c, Youwei Yao a, b, Guoyi Tang a, b, * a b c
Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China Key Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Haidian District, Beijing 100084, China Shenzhen Entry-Exit Inspection and Quarantine Bureau, Shenzhen 518054, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2013 Received in revised form 26 March 2014 Accepted 30 March 2014 Available online xxx
MicroPCMs (microencapsulated phase-change materials) were successfully synthesized using MPS (3(trimethoxysilyl) propyl methacrylate) and VTMS (vinyltrimethoxysilane) as raw materials for hybrid shells, and n-octadecane as core materials. DSC (differential scanning calorimeter) results show that two types of crystals form in core materials of all MicroPCMs during crystallization. The weight ratio of MPS eVTMS, and pH values play significant roles on the performance of final product: n-Octadecane content of MicroPCMs increases from 58.7 wt.% to 76.0 wt.% and their thermal degradation temperature (2 wt.% weight loss) increases from 182.6 C to 188.9 C with the weight ratio of MPSeVTMS decreasing from 8:0 to 2:6 in neutral synthesis systems. However, the encapsulation of n-octadecane failed when only using VTMS as hybrid shell precursor; When the weight ratio of MPSeVTMS is fixed to 2:6, the encapsulation efficiency decreases in acidic or basic synthesis systems. The optimized final product, i.e., MicroPCMs obtained with the weight ratio of MPSeVTMS equaling to 2:6 in neutral conditions, display a best thermal properties with highest melting and crystallization latent heat of 166.74 J g1 and 169.35 J g1, and the n-octadecane content decreases only by 7.0 wt.% and 10.8 wt.% after thermal treatment and thermal cycling test, respectively. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: PCM (phase-change material) Microencapsulation Hybrid shells n-Octadecane Alkoxy silanes
1. Introduction With the increasing worldwide population growth, there is a heavy burden on conventional energy sources. In recent years, thermal energy storage technologies have attracted increasing attention to relieve the energy crisis [1]. Among different storage technologies, PCMs (phase-change materials) are able to store or release heat energy, while undergoing a phase change with a very small temperature variation. There are many aspects of studies concerning the application of PCMs, such as PCM integration in freezers or refrigerators to improve the system operating performance [2], or PCM tank storing cooling with refrigeration cycle with cheap electricity price during summer night time and releasing cooling during peak time for air conditioning application in summer daytime [3], or PCM integration with a solar air heater
* Corresponding author. Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. Tel./fax: þ86 75526036752. E-mail address: [email protected] (G. Tang).
collector inside a greenhouse [4], or PCM combination with construction materials in building walls to relieve indoor air temperature fluctuation [5], or active coolingeheating method with PCMs for batteries of electric vehicles in extreme conditions [6]. Organic PCMs are popular because of their excellent phase-change performance, chemical and thermal stability, and compatibility with various materials or fibers [7]. Among organic PCMs, n-octadecane is desirable for its high latent heat (appropriately 214 J g1) and thermal regulation in an appropriate phase-change temperature range (21e27 C), comfortable for the residential building applications and human body [8]. However, organic PCMs have the leakage issue when undergoing solideliquid phase-change transition, and they also suffer from low thermal conductivity which strongly suppresses the energy chargingedischarging rates. This is the reason why more attentions have been paid to encapsulation of PCMs in recent years [9,10]. MicroPCMs (microencapsulated phase-change materials) are PCMs encapsulated in micro-size polymer, inorganic or hybrid shells. MicroPCMs with polymer shells are widely prepared and studied, for examples, styrene-methyl methacrylate copolymer
http://dx.doi.org/10.1016/j.energy.2014.03.125 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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encapsulated paraffin wax [11], polyurea encapsulated n-octadecane [12], urea-formaldehyde resin encapsulated caprylic acid [13]. Qiu et al. [14,15] focused on the encapsulation of n-octadecane with methylmethacrylate-based polymer shells. Ma et al. [16,17] studied the encapsulation and properties of binary core. The inorganic materials used to encapsulate PCMs are mainly focused on silica, which has a much higher thermal conductivity and chemical stability than polymer. However, encapsulation efficiency or morphology of MicroPCMs with silica shell is too sensitive to the pH value of reaction solution: Zhang et al. [18] concluded that the synthesis of MicroPCMs using TEOS (tetraethoxysilane) failed at pH < 2.0 or >3.0, and the encapsulation efficiency varied significantly between 57.7% and 86.4% at pH 2e3. Similar phenomenon has been observed when it comes to encapsulation of paraffin using TEOS as precursor [19]. Latibari et al. [20] prepared nanocapsules with palmitic acid as core using TEOS precursor, and the mean diameter size of nanocapsules increased from 183.7 nm to 722.5 nm with pH value increasing from 11 to 12. He et al. [21] confirmed that microcapsule walls could be coarse or loose if pH value wasn’t elaborately controlled at 2.95e3.05 when encapsulating n-octadecane using sodium silicate. Up to now, two typical MicroPCMs respectively with poly(methyl methacrylate) network-silica hybrid shell [22] and covalently bonded polymereSiO2 hybrid shell [23] are reported, which possess high encapsulation efficiency or durability. This proves the feasibility of improving performance of MicroPCMs by adopting hybrid shell. Considering the requirements on MicroPCMs in various conditions, organosilicone shell can be an optimistic choice for the fact that organic component offers structural flexibility, whereas inorganic component is well-known for good chemical and thermal stability. In this work, MicroPCMs with hybrid shells are successfully prepared by employing two kinds of alkoxy silanes, MPS (3-(trimethoxysilyl) propyl methacrylate) and VTMS (vinyltrimethoxysilane) as shell precursors. The structure of MPS and VTMS is respectively characteristic of methacrylate ester and vinyl moiety, which indicating a potential stereo-chemical effect, chemical reactivity, and organic compatibility (as shown in Fig. 1). Self-cross-linkage of MPS and/or VTMS for hybrid shell may be realized based on hydrolysis, condensation and addition reaction [24e27]. To our best knowledge, preparing shell of MicroPCMs based on MPS and VTMS precursors has not been reported. In this study, using MPS and VTMS as precursors, a series of MicroPCMs with compact and smooth surface, high encapsulation efficiency, high thermal treatment and thermal cycling stability was developed.
Fig. 1. Shell-forming reactions of MicroPCMs.
octadecane, MPS, VTMS and AIBN was mixed as the discontinuous phase. Then the discontinuous phase was added into the continuous phase to obtain W/O emulsion under vigorous agitation (1000 rpm) for 30 min. The polymerization process was followed with stirring (540 rpm) at 85 C for 6 h. The MicroPCMs were separated by centrifugation at 4000 rpm for 5 min twice, washed with n-hexane before filtrated, and dried in an oven at 45 C for 24 h. In order to study the effects of reaction conditions on the shell materials of MicroPCMs, solid polymer particles without n-octadecane in the conditions identical to those of MicroPCMs were prepared. The reaction conditions and recipes are listed in Table 1. 2.3. Thermal treatment of MicroPCMs Some samples were treated under 50 C for 7 days in an electro thermostatic blast oven. In order to absorb the core materials that
2. Experimental 2.1. Materials n-Octadecane (99 wt.%, Alfa) was used as a core material. MPS (>98.0%) and VTMS were employed as precursors, AIBN (2,2azobisisobutyronitrile) (98 wt.%) was used as a initiator, sodium salt of SMA (styrene-maleic anhydride) copolymer was selected as a dispersant, Hydrochloric acid (AR, 36e38%) and Ammonium hydroxide (AR) were used to adjust the pH values. n-Hexane (AR, 95.0%) was used as an ancillary detergent. All chemicals were used as received. 2.2. Preparation of MicroPCMs The preparation was carried out in a 250 ml three-neck round bottomed flask equipped with a mechanical agitator and immersed in a water bath. 5.5 g SMA was dissolved in 100 ml distilled water to form the continuous phase. An organic solution (Table 1) of n-
Table 1 Recipes for different weight ratios of MPSeVTMS and thermal stabilities regarded. Sample
MPS (g)
VTMS (g)
n-octadecane (g)
Ts ( C)
Char yield at 550 C (wt.%)
MV-1 MV-2 MV-3 MV-4 MV-5 S-1 S-2 S-3 S-4 S-5 n-octadecane
8 6 4 2 e 8 6 4 2 0 e
e 2 4 6 8 e 2 4 6 8 e
e e e e e 8 8 8 8 8 e
196.5 230.1 252.7 275.9 318.2 182.6 188.3 188.3 188.9 e 177.4
12.6 38.8 55.6 78.5 93.6 7.3 12.3 16.8 15.7 e 0.9
Note: MV-n (n ¼ 1e5) denotes the final products (solid polymer particles observed using SEM) from synthetic system without core materials; S-m (m ¼ 1e5) denotes the final products (microcapsules observed using SEM) from synthetic system with the addition of core materials; Ts denotes temperature corresponding to 2 wt.% weight loss.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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possibly leaked out of the microcapsules, each sample was enclosed in filter paper. 2.4. Thermal cycling test of MicroPCMs The thermal cycling tests up to 500 times were performed using heatingecooling cyclic oven. Each sample was enclosed in filter paper as in thermal treatment. The thermal cycling tests were performed at a heating or cooling rate of 1 C min1 in the temperature range of 15e50 C and kept for 5 min at 15 C and 50 C. 2.5. Characterizations of MicroPCMs The chemical structures of n-octadecane, MicroPCMs and polymer particles were obtained using a FT-IR (Fourier transformed infrared spectroscopy) at room temperature. The spectrum was collected by a scanning number of 32 at a resolution of 4 cm1 in the wave number range of 350e4000 cm1. The morphology, shell thickness and particle sizes of asprepared microcapsules were investigated using a FE-SEM (field emission scanning electron microscope). Samples were coated with a layer of gold before observation, and observed under secondary electron mode with an accelerating voltage of 5.0 kV. Thermal properties of n-octadecane and MicroPCMs were measured by DSC (differential scanning calorimeter) in the range of 0e50 C at a heating or cooling rate of 5 C min1 in an argon atmosphere with the flow rate of 60 ml min1. The content of noctadecane within the MicroPCMs was calculated by Eq. (1)
n octadecane content ¼ DHm;MicroPCMs þ DHc;MicroPCMs = DHm;PCMs þ DHc;PCMs
(1)
where DHm,MicroPCMs and DHc,MicroPCMs are melting enthalpy and crystallization enthalpy of MicroPCMs, respectively; DHm,PCMs and DHc,PCMs are melting enthalpy and crystallization enthalpy of PCM, respectively. Thermal stabilities were determined using a TGA (thermal gravimetric analyzer) in the range of 50e550 C at a heating rate of 10 C min1 in an argon atmosphere with the flow rate of 50 ml min1.
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S-m (m ¼ 1e4) [15]. The above observation confirms the encapsulation of n-octadecane in microcapsules. The peaks found in spectra of MV-n (n ¼ 1e4) and S-m (m ¼ 1e4) at around 1719 cm1 are attributed to CaO stretching vibration of the ester group of MPS. The most intense band in the spectra of S-m (m ¼ 1e4) at 1,091 cm1, assigned to SieOeSi asymmetric stretching vibration, indicates that the hydrolysis and condensation reactions of MPS and VTMS took place. The spectra at 1635 cm1 for MPS precursor and 1600 cm1 for VTMS precursor, attributed to the double bond (CaC), have disappeared in MV-n (n ¼ 1e4) and S-m (m ¼ 1e4), which gives evidence that MPS and VTMS have copolymerized [26]. The absorption bands between 3300 and 3700 cm1 in the spectra of MV-n (n ¼ 1e5) are assigned to SieOH stretching vibrations, which indicate that the condensation reactions were incomplete. Silanetriol in the shells were completely condensed since no SieOH absorption bands can be found in S-m (m ¼ 1e4). 3.2. Influence of the weight ratio of MPSeVTMS 3.2.1. Morphology of MicroPCMs SEM photographs in Fig. 3 show that all solid polymer particles (MV-n, n ¼ 1e5) and MicroPCMs (S-m, m ¼ 1e4) have sphericallike profiles. With the amount of VTMS increasing, the average particle sizes for MV-n (n ¼ 1e5) decrease, as shown in Fig. 3a, c, e, g and i. From Fig. 3b, d, f and h it can be observed that the asprepared S-m (m ¼ 1e4) have a similar trend. From Fig. 3j, a typical core/shell structure of S-4 with shell thickness about 138 nm is observed. S-m (m ¼ 1e4) prepared with different weight ratios of MPSeVTMS exhibit distinctly different structure and surface morphologies. S-1 displays a shrinkable sphere like structure with a relatively larger dimension compared with the rest samples, which is possibly caused by the flexible structure offered by the long carbon skeleton of MPS. It can be found that there are large amount of sub-micro scale particles and even finer fragments adhered on the surface of S-1, much less amount on S-2, whilst nearly disappear on S-3 and S-4. Namely, with the amount of VTMS increasing, the prepared S-m (m ¼ 1e4) spheres tend to be more regular, and their surface become smoother. This phenomenon is possibly due to the increase of SieOeSi linkage density in shell caused by increasing the weight ratio of VTMSeMPS. The increase of SieOeSi linkage is beneficial for enhancing structural strength of
3. Results and discussion Shell-forming reactions of MicroPCMs are shown in Fig. 1. When MPS and VTMS are mixed together in the emulsion, three main reactions are suggested to take place simultaneously: vinyl copolymerization between the organic moieties of MPS and VTMS; hydrolysis of the organoalkoxysilane into silanetriol molecules; and condensation of silanetriol molecules resulting in polysilsesquioxane networks. Optimizing three reactions is supposedly a key for the formation of the hybrid shell. While the vinyl copolymerization is found to be controllable by adjusting the weight ratio of MPSeVTMS, and the hydrolysis and condensation reactions are highly dependent on the pH value of the suspension medium [28], these two aspects are discussed next. 3.1. Chemical characterization FT-IR spectra of n-octadecane, MicroPCMs (S-m, m ¼ 1e4, described in Table 1) and solid polymer particles (MV-n, n ¼ 1e5, described in Table 1) are shown in Fig. 2. In the spectrum of noctadecane, the peaks at 1460 and 721 cm1 correspond to eCH2e bending vibration and the peak appears at 1373 cm1 arising from eCH3 bending vibration, which can also be found in the spectra of
Fig. 2. FT-IR spectra of n-octadecane, MicroPCMs (S-m, m ¼ 1e4) and solid polymer particles (MV-n, n ¼ 1e5).
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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Fig. 3. SEM photographs of solid polymer particles and MicroPCMs: a) MV-1; b) S-1; c) MV-2; d) S-2; e) MV-3; f) S-3; g) MV-4; h) S-4; i) MV-5; j) the broken shell of S-4.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
W. Li et al. / Energy xxx (2014) 1e9
Fig. 4. DSC curves of n-octadecane and MicroPCMs (S-m, m ¼ 1e4).
the microcapsules. However, after being washed with n-hexane and dried in an oven, the fine powders of S-5 became oily, caused by leak of n-octadecane. This phenomenon indicates that microcapsules cannot be obtained using VTMS alone. This may be due to the stress in shell resulting from the molecular stereo effect of VTMS, i.e., the short carbon group may not provide a conformation for full linkages of SieOeSi or e(CH2)4e. Thus balancing the weight ratio of MPSeVTMS is an important factor for adjusting the linkage density and stress in microcapsule’s shell. 3.2.2. Thermal properties and thermal stability of MicroPCMs Thermal properties of n-octadecane and MicroPCMs (S-m, m ¼ 1e4) were evaluated by DSC. The relevant results are shown in Fig. 4 and summarized in Table 2. DSC cooling thermogram of bulk n-octadecane reveals a well-defined exothermic peak at around 20.92 C, which is attributed to the homogeneously nucleated liquidecrystal transition of the b-form crystals of n-octadecane. The shoulder next to the peak b at the higher temperature range is attributed to the rotatorecrystal transition [29]. The endothermic peak temperatures (Tpm) of S-m (m ¼ 1e4) are very close to each other as well as to that of bulk n-octadecane. This may suggest that the encapsulation of n-octadecane in S-m (m ¼ 1e 4) has little influence on the melting process of core material [30]. During the cooling process, another crystallization peak a can be observed from S-m (m ¼ 1e4). The peak a is induced by the heterogeneously nucleated liquiderotator transition, and it shifts to a low temperature with various intensities. After n-octadecane is encapsulated within the shell material, the interphase between
Table 2 Thermal properties of MicroPCMs prepared with different weight ratios of precursors. Sample
S-1 S-2 S-3 S-4 n-octadecane
Tpc ( C)
a
b
7.40 9.94 10.81 8.08 e
17.92 18.23 17.84 17.42 20.92
Tpm ( C)
DHc
DHm
(J g1)
n-octadecane content (wt.%)
26.89 26.98 27.66 27.84 26.61
129.92 144.63 147.32 169.35 226.34
129.58 140.08 146.78 166.74 215.96
58.7 64.4 66.5 76.0 100
(J g1)
Note: Tpc, Peak temperature on DSC cooling curve; Tpm, Peak temperature on DSC heating curve; DHc, Enthalpy on DSC cooling curve; DHm, Enthalpy on DSC heating curve.
5
Fig. 5. TGA curves of n-octadecane, MicroPCMs (S-m, m ¼ 1e4) and solid polymer particles (MV-n, n ¼ 1e5).
shell and core acts as the nucleus in promoting the heterogeneously nucleated phase transition from melting to a triclinic phase, and thus enhances the a-form crystals, as well as reduces their crystallization temperatures [18]. From Table 2, it can be seen that the latent heats and the noctadecane contents of S-m (m ¼ 1e4) increase with the amount of VTMS increasing. S-4 has the highest latent heat of melting (166.74 J g1) and crystallization (169.35 J g1) as well as highest noctadecane content (76.0 wt.%). TGA thermograms of n-octadecane, MicroPCMs (S-m, m ¼ 1e4) and solid polymer particles (MV-n, n ¼ 1e5) are shown in Fig. 5. Temperature corresponding to 2 wt.% weight loss and char yield at 550 C are also summarized in Table 1. A little weight loss of about 2 wt.% before 170 C is mainly caused by the incomplete removal of water in the samples. It can be noted from Fig. 5 that MV-n (n ¼ 1e 4) degrades in multiple steps while MV-5 degrades in two steps. This phenomenon indicates the degradation of solid polymer particles involving MPS in reaction is complex, which applies to the shell materials’ decomposition of S-m (m ¼ 1e4). FT-IR results approve that SieOH condensation may occur in all solid polymer particles’ degradations. Fig. 5 depicts that the S-m (m ¼ 1e4) degrades in multi-steps, which corresponds to the degradation of the core materials and shell materials. n-Octadecane starts to lose weight at about 177 C, completes at approximately 287 C, and the curve is of steep. The starting degradation temperature (Ts) of S-m (m ¼ 1e4) is all below 287 C, suggesting that the hybrid shells start to degrade before noctadecane diffuses out completely. Ts of S-m (m ¼ 1e4) range from 182 C to 189 C, higher than 177 C of n-octadecane, indicating the protection of the as-prepared shell materials, specially for the shell of S-4.
Table 3 Recipes with influence of pH and thermal stabilities regarded. Sample
MPS (g)
VTMS (g)
n-octadecane (g)
pH
Ts ( C)
Char yield at 550 C (wt.%)
S-6 S-7 S-8 S-9 S-10
2 2 2 2 2
6 6 6 6 6
8 8 8 8 8
8.0 8.5 9.0 6.0 5.5
187.5 194.4 173.0 183.2 180.6
21.6 18.4 24.2 18.9 25.1
Note: S-m (m ¼ 6e10) denotes the final products (microcapsules observed using SEM) from synthetic system with the addition of core materials.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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Fig. 6. SEM photographs of MicroPCMs with different pH values. a) and b) for S-6; c) and d) for S-7; e) and f) for S-8; g) and h) for S-9; i) and j) for S-10.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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Table 5 Thermal properties of a-form crystals in MicroPCMs. Sample
4
6
7
8
9
10
DHa (J g1) DHa-50 (J g1) DHa-500 (J g1)
32.72 46.32 43.46
15.92 21.51 34.51
19.77 30.87 41.51
23.51 34.39 42.96
13.75 20.54 29.24
10.89 13.21 19.10
Note: DHa, DHa-50, DHa-500 are the crystallizing enthalpy of a-form crystals in original samples, samples treated under 50 C for 7 days or subjected for 500 times thermal cycling test, respectively.
pH values, corresponding to basic (pH ¼ 8.0, 8.5, 9.0) and acidic (pH ¼ 6.0, 5.5) conditions, using otherwise the same recipe as S-4 (shown in Table 3). 3.3.1. Morphology of MicroPCMs SEM photographs in Fig. 6 show that changing pH value has little effect on the particle size, but influences significantly the surface of MicroPCMs (S-m, m ¼ 6e10). Under basic conditions (Fig. 6aef), the surfaces of S-m, (m ¼ 6e8) are very smooth. While under acidic conditions (Fig. 6gej), the surfaces of S-m (m ¼ 9e10) turn coarse and microcapsules begin to aggregate with acidity increasing. This can be explained by the structure difference in the inorganic networks formed in acidic and basic conditions. While acid catalysis promotes the formation of weakly cross-linked polymeric structures, more highly cross-linked and even fully dense inorganic networks can be formed in alkaline solutions [31]. The “extended” fractal structure of the dangling groups, such as Sie OH formed at low pH could promote interactions of two adjacent microcapsules and cause the aggregation of microcapsules obtained in acidic conditions [27].
Fig. 7. DSC curves of MicroPCMs (S-m, m ¼ 4, 6e10); the black lines represent original samples, the red lines represent samples that were treated under 50 C for 7 days and the blue lines represent samples that were subjected for 500 times thermal cycling test.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
With the amount of VTMS increasing, the char yields of MV-n (n ¼ 1e5) at 550 C increase from 12.6 wt.% to 93.6 wt.%, which is resulted from the inorganic components increasing and organic components decreasing in solid polymer particles. The char yields of S-m (m ¼ 1e4) at 550 C are 7.3 wt.%, 12.3 wt.%, 16.8 wt.% and 15.7 wt.%, which are determined not only by the shell composition, but also by n-octadecane content. 3.3. Influence of pH To investigate influence of pH on the shell materials, the polymerization reaction was performed in systems with five different
3.3.2. Thermal properties and thermal stability of MicroPCMs Thermal properties of original samples of S-m (m ¼ 4, 6e10), treated under 50 C for 7 days or subjected to 500 times thermal cycling test are evaluated by DSC, as shown in Fig. 7 and summarized in Table 4. Under basic conditions, n-octadecane contents of S-m (m ¼ 6e8) first increase and then decline with basicity increasing. From neutral to acidic conditions, n-octadecane contents of S-m (m ¼ 4, 9, 10) decline with acidity increasing. The noctadecane content reaches the highest in neutral condition, which stands for S-4. The crystallizing and melting enthalpies of S-m (m ¼ 4, 6e10) decline after thermal treatment and thermal cycling test, while n-octadecane contents of S-4 decrease by 7.0 wt.% and 10.8 wt.% respectively, the least decrease in the S-m series. The influence of pH, thermal treatment and thermal cycling test on the crystallizing enthalpy of a-form crystals in S-m (m ¼ 4, 6e 10) are shown in Fig. 7 and summarized in Table 5. From Table 5, it can be concluded that thermal treatment and thermal cycling test increase the crystallizing enthalpy of a-form crystals in S-m (m ¼ 4, 6e10). Under basic conditions, the crystallizing enthalpy of a-form
Table 4 Thermal properties of MicroPCMs prepared with influence of pH. Sample
Tpc ( C)
a
b
S-4 S-6 S-7 S-8 S-9 S-10
8.08 8.88 9.39 8.18 8.91 9.06
17.42 17.71 18.88 17.12 17.45 17.51
Tpm ( C)
DHc (J g1)
DHm (J g1)
DHc-50 (J g1)
DHm-50 (J g1)
DHc-500 (J g1)
DHm-500 (J g1)
n-octadecane content (wt.%)
27.84 27.04 26.63 25.85 25.83 26.77
169.35 156.86 159.28 138.04 155.86 146.21
166.74 151.02 158.32 134.64 152.35 142.59
154.10 131.47 143.11 118.40 133.10 125.97
151.09 128.19 140.41 117.40 131.43 123.88
145.52 107.00 120.17 102.15 105.02 107.42
142.74 107.01 117.12 100.71 105.35 106.93
76.0 69.6 71.8 61.7 69.7 65.3
Note: DHc-50, DHc-500, DHm-50, DHm-500 are the crystallizing and melting enthalpy of samples treated under 50 C for 7 days or subjected to 500 times thermal cycling test, respectively; representation of other parameters are the same as in Table 2.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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Acknowledgment The authors gratefully acknowledge the financial supports from International Collaboration Project (No. 2008DFA51210). References
Fig. 8. TGA curves of n-octadecane and MicroPCMs (S-m, m ¼ 4, 6e10).
crystals in S-m (m ¼ 6e8) increases with basicity increasing. From neutral to acidic conditions, the crystallizing enthalpy of a-form crystals in S-m (m ¼ 4, 9, 10) decreases with acidity increasing. The crystallizing enthalpy of a-form crystals in S-4 is the highest, even after thermal treatment and thermal cycling tests. Thermal stabilities of S-m (m ¼ 4, 6e10) are shown in Fig. 8. Temperature corresponding to 2 wt.% weight loss and char yield at 550 C are also summarized in Table 3. The weight loss trend of S-m (m ¼ 4, 6e10) is similar, i.e., a two-step weight loss at about 173e290 C, 400e500 C. Under basic conditions, Ts of S-m (m ¼ 6e8) first increases and then declines with basicity increasing. From neutral to acidic conditions, Ts of S-m (m ¼ 4, 9, 10) declines with acidity increasing. Ts of S-7 is 194 C, the highest in the S-m (m ¼ 4, 6e10) series, however, encapsulation efficiency is lower than that of S-4. S-8 starts to degrade at 173 C, indicating the vulnerability of shell materials. The weight loss of S-m (m ¼ 4, 6e10) at first degradation increase with corresponding noctadecane content increasing, and S-4 shows the highest weight loss. 4. Conclusions MicroPCMs with n-octadecane core and hybrid shell from alkoxy silanes, MPS and VTMS precursors were prepared. All MicroPCMs have spherical-like profiles and core/shell structure. Two types of crystals were formed in core materials of all MicroPCMs during crystallization. The weight ratio of MPSeVTMS, and pH values play significant roles on the properties of MicroPCMs. With the weight ratio of MPSeVTMS decreasing from 8:0 to 2:6 in neutral synthesis systems, MicroPCMs tend to be more regular and their surface become smoother, and n-octadecane content increase from 58.7 wt.% to 76.0 wt.%. The encapsulation of n-octadecane failed when only using VTMS as hybrid shell precursor. When the weight ratio of MPSeVTMS is fixed to 2:6, the encapsulation efficiency decreases in acidic or basic synthesis systems, compared with that in neutral synthesis system. MicroPCMs obtained with the weight ratio of MPSeVTMS equaling to 2:6 in neutral conditions, display a best thermal performance with the highest melting and crystallization latent heat of 166.74 J g1 and 169.35 J g1, and the n-octadecane content decreases only by 7.0 wt.% and 10.8 wt.% after thermal treatment and thermal cycling test respectively, which show good potential as energy storage materials.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes Wenhong Li a, b, Guolin Song a, Shuhua Li c, Youwei Yao a, b, Guoyi Tang a, b, * a b c
Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China Key Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Haidian District, Beijing 100084, China Shenzhen Entry-Exit Inspection and Quarantine Bureau, Shenzhen 518054, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2013 Received in revised form 26 March 2014 Accepted 30 March 2014 Available online xxx
MicroPCMs (microencapsulated phase-change materials) were successfully synthesized using MPS (3(trimethoxysilyl) propyl methacrylate) and VTMS (vinyltrimethoxysilane) as raw materials for hybrid shells, and n-octadecane as core materials. DSC (differential scanning calorimeter) results show that two types of crystals form in core materials of all MicroPCMs during crystallization. The weight ratio of MPS eVTMS, and pH values play significant roles on the performance of final product: n-Octadecane content of MicroPCMs increases from 58.7 wt.% to 76.0 wt.% and their thermal degradation temperature (2 wt.% weight loss) increases from 182.6 C to 188.9 C with the weight ratio of MPSeVTMS decreasing from 8:0 to 2:6 in neutral synthesis systems. However, the encapsulation of n-octadecane failed when only using VTMS as hybrid shell precursor; When the weight ratio of MPSeVTMS is fixed to 2:6, the encapsulation efficiency decreases in acidic or basic synthesis systems. The optimized final product, i.e., MicroPCMs obtained with the weight ratio of MPSeVTMS equaling to 2:6 in neutral conditions, display a best thermal properties with highest melting and crystallization latent heat of 166.74 J g1 and 169.35 J g1, and the n-octadecane content decreases only by 7.0 wt.% and 10.8 wt.% after thermal treatment and thermal cycling test, respectively. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: PCM (phase-change material) Microencapsulation Hybrid shells n-Octadecane Alkoxy silanes
1. Introduction With the increasing worldwide population growth, there is a heavy burden on conventional energy sources. In recent years, thermal energy storage technologies have attracted increasing attention to relieve the energy crisis [1]. Among different storage technologies, PCMs (phase-change materials) are able to store or release heat energy, while undergoing a phase change with a very small temperature variation. There are many aspects of studies concerning the application of PCMs, such as PCM integration in freezers or refrigerators to improve the system operating performance [2], or PCM tank storing cooling with refrigeration cycle with cheap electricity price during summer night time and releasing cooling during peak time for air conditioning application in summer daytime [3], or PCM integration with a solar air heater
* Corresponding author. Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. Tel./fax: þ86 75526036752. E-mail address: [email protected] (G. Tang).
collector inside a greenhouse [4], or PCM combination with construction materials in building walls to relieve indoor air temperature fluctuation [5], or active coolingeheating method with PCMs for batteries of electric vehicles in extreme conditions [6]. Organic PCMs are popular because of their excellent phase-change performance, chemical and thermal stability, and compatibility with various materials or fibers [7]. Among organic PCMs, n-octadecane is desirable for its high latent heat (appropriately 214 J g1) and thermal regulation in an appropriate phase-change temperature range (21e27 C), comfortable for the residential building applications and human body [8]. However, organic PCMs have the leakage issue when undergoing solideliquid phase-change transition, and they also suffer from low thermal conductivity which strongly suppresses the energy chargingedischarging rates. This is the reason why more attentions have been paid to encapsulation of PCMs in recent years [9,10]. MicroPCMs (microencapsulated phase-change materials) are PCMs encapsulated in micro-size polymer, inorganic or hybrid shells. MicroPCMs with polymer shells are widely prepared and studied, for examples, styrene-methyl methacrylate copolymer
http://dx.doi.org/10.1016/j.energy.2014.03.125 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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encapsulated paraffin wax [11], polyurea encapsulated n-octadecane [12], urea-formaldehyde resin encapsulated caprylic acid [13]. Qiu et al. [14,15] focused on the encapsulation of n-octadecane with methylmethacrylate-based polymer shells. Ma et al. [16,17] studied the encapsulation and properties of binary core. The inorganic materials used to encapsulate PCMs are mainly focused on silica, which has a much higher thermal conductivity and chemical stability than polymer. However, encapsulation efficiency or morphology of MicroPCMs with silica shell is too sensitive to the pH value of reaction solution: Zhang et al. [18] concluded that the synthesis of MicroPCMs using TEOS (tetraethoxysilane) failed at pH < 2.0 or >3.0, and the encapsulation efficiency varied significantly between 57.7% and 86.4% at pH 2e3. Similar phenomenon has been observed when it comes to encapsulation of paraffin using TEOS as precursor [19]. Latibari et al. [20] prepared nanocapsules with palmitic acid as core using TEOS precursor, and the mean diameter size of nanocapsules increased from 183.7 nm to 722.5 nm with pH value increasing from 11 to 12. He et al. [21] confirmed that microcapsule walls could be coarse or loose if pH value wasn’t elaborately controlled at 2.95e3.05 when encapsulating n-octadecane using sodium silicate. Up to now, two typical MicroPCMs respectively with poly(methyl methacrylate) network-silica hybrid shell [22] and covalently bonded polymereSiO2 hybrid shell [23] are reported, which possess high encapsulation efficiency or durability. This proves the feasibility of improving performance of MicroPCMs by adopting hybrid shell. Considering the requirements on MicroPCMs in various conditions, organosilicone shell can be an optimistic choice for the fact that organic component offers structural flexibility, whereas inorganic component is well-known for good chemical and thermal stability. In this work, MicroPCMs with hybrid shells are successfully prepared by employing two kinds of alkoxy silanes, MPS (3-(trimethoxysilyl) propyl methacrylate) and VTMS (vinyltrimethoxysilane) as shell precursors. The structure of MPS and VTMS is respectively characteristic of methacrylate ester and vinyl moiety, which indicating a potential stereo-chemical effect, chemical reactivity, and organic compatibility (as shown in Fig. 1). Self-cross-linkage of MPS and/or VTMS for hybrid shell may be realized based on hydrolysis, condensation and addition reaction [24e27]. To our best knowledge, preparing shell of MicroPCMs based on MPS and VTMS precursors has not been reported. In this study, using MPS and VTMS as precursors, a series of MicroPCMs with compact and smooth surface, high encapsulation efficiency, high thermal treatment and thermal cycling stability was developed.
Fig. 1. Shell-forming reactions of MicroPCMs.
octadecane, MPS, VTMS and AIBN was mixed as the discontinuous phase. Then the discontinuous phase was added into the continuous phase to obtain W/O emulsion under vigorous agitation (1000 rpm) for 30 min. The polymerization process was followed with stirring (540 rpm) at 85 C for 6 h. The MicroPCMs were separated by centrifugation at 4000 rpm for 5 min twice, washed with n-hexane before filtrated, and dried in an oven at 45 C for 24 h. In order to study the effects of reaction conditions on the shell materials of MicroPCMs, solid polymer particles without n-octadecane in the conditions identical to those of MicroPCMs were prepared. The reaction conditions and recipes are listed in Table 1. 2.3. Thermal treatment of MicroPCMs Some samples were treated under 50 C for 7 days in an electro thermostatic blast oven. In order to absorb the core materials that
2. Experimental 2.1. Materials n-Octadecane (99 wt.%, Alfa) was used as a core material. MPS (>98.0%) and VTMS were employed as precursors, AIBN (2,2azobisisobutyronitrile) (98 wt.%) was used as a initiator, sodium salt of SMA (styrene-maleic anhydride) copolymer was selected as a dispersant, Hydrochloric acid (AR, 36e38%) and Ammonium hydroxide (AR) were used to adjust the pH values. n-Hexane (AR, 95.0%) was used as an ancillary detergent. All chemicals were used as received. 2.2. Preparation of MicroPCMs The preparation was carried out in a 250 ml three-neck round bottomed flask equipped with a mechanical agitator and immersed in a water bath. 5.5 g SMA was dissolved in 100 ml distilled water to form the continuous phase. An organic solution (Table 1) of n-
Table 1 Recipes for different weight ratios of MPSeVTMS and thermal stabilities regarded. Sample
MPS (g)
VTMS (g)
n-octadecane (g)
Ts ( C)
Char yield at 550 C (wt.%)
MV-1 MV-2 MV-3 MV-4 MV-5 S-1 S-2 S-3 S-4 S-5 n-octadecane
8 6 4 2 e 8 6 4 2 0 e
e 2 4 6 8 e 2 4 6 8 e
e e e e e 8 8 8 8 8 e
196.5 230.1 252.7 275.9 318.2 182.6 188.3 188.3 188.9 e 177.4
12.6 38.8 55.6 78.5 93.6 7.3 12.3 16.8 15.7 e 0.9
Note: MV-n (n ¼ 1e5) denotes the final products (solid polymer particles observed using SEM) from synthetic system without core materials; S-m (m ¼ 1e5) denotes the final products (microcapsules observed using SEM) from synthetic system with the addition of core materials; Ts denotes temperature corresponding to 2 wt.% weight loss.
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W. Li et al. / Energy xxx (2014) 1e9
possibly leaked out of the microcapsules, each sample was enclosed in filter paper. 2.4. Thermal cycling test of MicroPCMs The thermal cycling tests up to 500 times were performed using heatingecooling cyclic oven. Each sample was enclosed in filter paper as in thermal treatment. The thermal cycling tests were performed at a heating or cooling rate of 1 C min1 in the temperature range of 15e50 C and kept for 5 min at 15 C and 50 C. 2.5. Characterizations of MicroPCMs The chemical structures of n-octadecane, MicroPCMs and polymer particles were obtained using a FT-IR (Fourier transformed infrared spectroscopy) at room temperature. The spectrum was collected by a scanning number of 32 at a resolution of 4 cm1 in the wave number range of 350e4000 cm1. The morphology, shell thickness and particle sizes of asprepared microcapsules were investigated using a FE-SEM (field emission scanning electron microscope). Samples were coated with a layer of gold before observation, and observed under secondary electron mode with an accelerating voltage of 5.0 kV. Thermal properties of n-octadecane and MicroPCMs were measured by DSC (differential scanning calorimeter) in the range of 0e50 C at a heating or cooling rate of 5 C min1 in an argon atmosphere with the flow rate of 60 ml min1. The content of noctadecane within the MicroPCMs was calculated by Eq. (1)
n octadecane content ¼ DHm;MicroPCMs þ DHc;MicroPCMs = DHm;PCMs þ DHc;PCMs
(1)
where DHm,MicroPCMs and DHc,MicroPCMs are melting enthalpy and crystallization enthalpy of MicroPCMs, respectively; DHm,PCMs and DHc,PCMs are melting enthalpy and crystallization enthalpy of PCM, respectively. Thermal stabilities were determined using a TGA (thermal gravimetric analyzer) in the range of 50e550 C at a heating rate of 10 C min1 in an argon atmosphere with the flow rate of 50 ml min1.
3
S-m (m ¼ 1e4) [15]. The above observation confirms the encapsulation of n-octadecane in microcapsules. The peaks found in spectra of MV-n (n ¼ 1e4) and S-m (m ¼ 1e4) at around 1719 cm1 are attributed to CaO stretching vibration of the ester group of MPS. The most intense band in the spectra of S-m (m ¼ 1e4) at 1,091 cm1, assigned to SieOeSi asymmetric stretching vibration, indicates that the hydrolysis and condensation reactions of MPS and VTMS took place. The spectra at 1635 cm1 for MPS precursor and 1600 cm1 for VTMS precursor, attributed to the double bond (CaC), have disappeared in MV-n (n ¼ 1e4) and S-m (m ¼ 1e4), which gives evidence that MPS and VTMS have copolymerized [26]. The absorption bands between 3300 and 3700 cm1 in the spectra of MV-n (n ¼ 1e5) are assigned to SieOH stretching vibrations, which indicate that the condensation reactions were incomplete. Silanetriol in the shells were completely condensed since no SieOH absorption bands can be found in S-m (m ¼ 1e4). 3.2. Influence of the weight ratio of MPSeVTMS 3.2.1. Morphology of MicroPCMs SEM photographs in Fig. 3 show that all solid polymer particles (MV-n, n ¼ 1e5) and MicroPCMs (S-m, m ¼ 1e4) have sphericallike profiles. With the amount of VTMS increasing, the average particle sizes for MV-n (n ¼ 1e5) decrease, as shown in Fig. 3a, c, e, g and i. From Fig. 3b, d, f and h it can be observed that the asprepared S-m (m ¼ 1e4) have a similar trend. From Fig. 3j, a typical core/shell structure of S-4 with shell thickness about 138 nm is observed. S-m (m ¼ 1e4) prepared with different weight ratios of MPSeVTMS exhibit distinctly different structure and surface morphologies. S-1 displays a shrinkable sphere like structure with a relatively larger dimension compared with the rest samples, which is possibly caused by the flexible structure offered by the long carbon skeleton of MPS. It can be found that there are large amount of sub-micro scale particles and even finer fragments adhered on the surface of S-1, much less amount on S-2, whilst nearly disappear on S-3 and S-4. Namely, with the amount of VTMS increasing, the prepared S-m (m ¼ 1e4) spheres tend to be more regular, and their surface become smoother. This phenomenon is possibly due to the increase of SieOeSi linkage density in shell caused by increasing the weight ratio of VTMSeMPS. The increase of SieOeSi linkage is beneficial for enhancing structural strength of
3. Results and discussion Shell-forming reactions of MicroPCMs are shown in Fig. 1. When MPS and VTMS are mixed together in the emulsion, three main reactions are suggested to take place simultaneously: vinyl copolymerization between the organic moieties of MPS and VTMS; hydrolysis of the organoalkoxysilane into silanetriol molecules; and condensation of silanetriol molecules resulting in polysilsesquioxane networks. Optimizing three reactions is supposedly a key for the formation of the hybrid shell. While the vinyl copolymerization is found to be controllable by adjusting the weight ratio of MPSeVTMS, and the hydrolysis and condensation reactions are highly dependent on the pH value of the suspension medium [28], these two aspects are discussed next. 3.1. Chemical characterization FT-IR spectra of n-octadecane, MicroPCMs (S-m, m ¼ 1e4, described in Table 1) and solid polymer particles (MV-n, n ¼ 1e5, described in Table 1) are shown in Fig. 2. In the spectrum of noctadecane, the peaks at 1460 and 721 cm1 correspond to eCH2e bending vibration and the peak appears at 1373 cm1 arising from eCH3 bending vibration, which can also be found in the spectra of
Fig. 2. FT-IR spectra of n-octadecane, MicroPCMs (S-m, m ¼ 1e4) and solid polymer particles (MV-n, n ¼ 1e5).
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Fig. 3. SEM photographs of solid polymer particles and MicroPCMs: a) MV-1; b) S-1; c) MV-2; d) S-2; e) MV-3; f) S-3; g) MV-4; h) S-4; i) MV-5; j) the broken shell of S-4.
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W. Li et al. / Energy xxx (2014) 1e9
Fig. 4. DSC curves of n-octadecane and MicroPCMs (S-m, m ¼ 1e4).
the microcapsules. However, after being washed with n-hexane and dried in an oven, the fine powders of S-5 became oily, caused by leak of n-octadecane. This phenomenon indicates that microcapsules cannot be obtained using VTMS alone. This may be due to the stress in shell resulting from the molecular stereo effect of VTMS, i.e., the short carbon group may not provide a conformation for full linkages of SieOeSi or e(CH2)4e. Thus balancing the weight ratio of MPSeVTMS is an important factor for adjusting the linkage density and stress in microcapsule’s shell. 3.2.2. Thermal properties and thermal stability of MicroPCMs Thermal properties of n-octadecane and MicroPCMs (S-m, m ¼ 1e4) were evaluated by DSC. The relevant results are shown in Fig. 4 and summarized in Table 2. DSC cooling thermogram of bulk n-octadecane reveals a well-defined exothermic peak at around 20.92 C, which is attributed to the homogeneously nucleated liquidecrystal transition of the b-form crystals of n-octadecane. The shoulder next to the peak b at the higher temperature range is attributed to the rotatorecrystal transition [29]. The endothermic peak temperatures (Tpm) of S-m (m ¼ 1e4) are very close to each other as well as to that of bulk n-octadecane. This may suggest that the encapsulation of n-octadecane in S-m (m ¼ 1e 4) has little influence on the melting process of core material [30]. During the cooling process, another crystallization peak a can be observed from S-m (m ¼ 1e4). The peak a is induced by the heterogeneously nucleated liquiderotator transition, and it shifts to a low temperature with various intensities. After n-octadecane is encapsulated within the shell material, the interphase between
Table 2 Thermal properties of MicroPCMs prepared with different weight ratios of precursors. Sample
S-1 S-2 S-3 S-4 n-octadecane
Tpc ( C)
a
b
7.40 9.94 10.81 8.08 e
17.92 18.23 17.84 17.42 20.92
Tpm ( C)
DHc
DHm
(J g1)
n-octadecane content (wt.%)
26.89 26.98 27.66 27.84 26.61
129.92 144.63 147.32 169.35 226.34
129.58 140.08 146.78 166.74 215.96
58.7 64.4 66.5 76.0 100
(J g1)
Note: Tpc, Peak temperature on DSC cooling curve; Tpm, Peak temperature on DSC heating curve; DHc, Enthalpy on DSC cooling curve; DHm, Enthalpy on DSC heating curve.
5
Fig. 5. TGA curves of n-octadecane, MicroPCMs (S-m, m ¼ 1e4) and solid polymer particles (MV-n, n ¼ 1e5).
shell and core acts as the nucleus in promoting the heterogeneously nucleated phase transition from melting to a triclinic phase, and thus enhances the a-form crystals, as well as reduces their crystallization temperatures [18]. From Table 2, it can be seen that the latent heats and the noctadecane contents of S-m (m ¼ 1e4) increase with the amount of VTMS increasing. S-4 has the highest latent heat of melting (166.74 J g1) and crystallization (169.35 J g1) as well as highest noctadecane content (76.0 wt.%). TGA thermograms of n-octadecane, MicroPCMs (S-m, m ¼ 1e4) and solid polymer particles (MV-n, n ¼ 1e5) are shown in Fig. 5. Temperature corresponding to 2 wt.% weight loss and char yield at 550 C are also summarized in Table 1. A little weight loss of about 2 wt.% before 170 C is mainly caused by the incomplete removal of water in the samples. It can be noted from Fig. 5 that MV-n (n ¼ 1e 4) degrades in multiple steps while MV-5 degrades in two steps. This phenomenon indicates the degradation of solid polymer particles involving MPS in reaction is complex, which applies to the shell materials’ decomposition of S-m (m ¼ 1e4). FT-IR results approve that SieOH condensation may occur in all solid polymer particles’ degradations. Fig. 5 depicts that the S-m (m ¼ 1e4) degrades in multi-steps, which corresponds to the degradation of the core materials and shell materials. n-Octadecane starts to lose weight at about 177 C, completes at approximately 287 C, and the curve is of steep. The starting degradation temperature (Ts) of S-m (m ¼ 1e4) is all below 287 C, suggesting that the hybrid shells start to degrade before noctadecane diffuses out completely. Ts of S-m (m ¼ 1e4) range from 182 C to 189 C, higher than 177 C of n-octadecane, indicating the protection of the as-prepared shell materials, specially for the shell of S-4.
Table 3 Recipes with influence of pH and thermal stabilities regarded. Sample
MPS (g)
VTMS (g)
n-octadecane (g)
pH
Ts ( C)
Char yield at 550 C (wt.%)
S-6 S-7 S-8 S-9 S-10
2 2 2 2 2
6 6 6 6 6
8 8 8 8 8
8.0 8.5 9.0 6.0 5.5
187.5 194.4 173.0 183.2 180.6
21.6 18.4 24.2 18.9 25.1
Note: S-m (m ¼ 6e10) denotes the final products (microcapsules observed using SEM) from synthetic system with the addition of core materials.
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Fig. 6. SEM photographs of MicroPCMs with different pH values. a) and b) for S-6; c) and d) for S-7; e) and f) for S-8; g) and h) for S-9; i) and j) for S-10.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
W. Li et al. / Energy xxx (2014) 1e9
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Table 5 Thermal properties of a-form crystals in MicroPCMs. Sample
4
6
7
8
9
10
DHa (J g1) DHa-50 (J g1) DHa-500 (J g1)
32.72 46.32 43.46
15.92 21.51 34.51
19.77 30.87 41.51
23.51 34.39 42.96
13.75 20.54 29.24
10.89 13.21 19.10
Note: DHa, DHa-50, DHa-500 are the crystallizing enthalpy of a-form crystals in original samples, samples treated under 50 C for 7 days or subjected for 500 times thermal cycling test, respectively.
pH values, corresponding to basic (pH ¼ 8.0, 8.5, 9.0) and acidic (pH ¼ 6.0, 5.5) conditions, using otherwise the same recipe as S-4 (shown in Table 3). 3.3.1. Morphology of MicroPCMs SEM photographs in Fig. 6 show that changing pH value has little effect on the particle size, but influences significantly the surface of MicroPCMs (S-m, m ¼ 6e10). Under basic conditions (Fig. 6aef), the surfaces of S-m, (m ¼ 6e8) are very smooth. While under acidic conditions (Fig. 6gej), the surfaces of S-m (m ¼ 9e10) turn coarse and microcapsules begin to aggregate with acidity increasing. This can be explained by the structure difference in the inorganic networks formed in acidic and basic conditions. While acid catalysis promotes the formation of weakly cross-linked polymeric structures, more highly cross-linked and even fully dense inorganic networks can be formed in alkaline solutions [31]. The “extended” fractal structure of the dangling groups, such as Sie OH formed at low pH could promote interactions of two adjacent microcapsules and cause the aggregation of microcapsules obtained in acidic conditions [27].
Fig. 7. DSC curves of MicroPCMs (S-m, m ¼ 4, 6e10); the black lines represent original samples, the red lines represent samples that were treated under 50 C for 7 days and the blue lines represent samples that were subjected for 500 times thermal cycling test.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
With the amount of VTMS increasing, the char yields of MV-n (n ¼ 1e5) at 550 C increase from 12.6 wt.% to 93.6 wt.%, which is resulted from the inorganic components increasing and organic components decreasing in solid polymer particles. The char yields of S-m (m ¼ 1e4) at 550 C are 7.3 wt.%, 12.3 wt.%, 16.8 wt.% and 15.7 wt.%, which are determined not only by the shell composition, but also by n-octadecane content. 3.3. Influence of pH To investigate influence of pH on the shell materials, the polymerization reaction was performed in systems with five different
3.3.2. Thermal properties and thermal stability of MicroPCMs Thermal properties of original samples of S-m (m ¼ 4, 6e10), treated under 50 C for 7 days or subjected to 500 times thermal cycling test are evaluated by DSC, as shown in Fig. 7 and summarized in Table 4. Under basic conditions, n-octadecane contents of S-m (m ¼ 6e8) first increase and then decline with basicity increasing. From neutral to acidic conditions, n-octadecane contents of S-m (m ¼ 4, 9, 10) decline with acidity increasing. The noctadecane content reaches the highest in neutral condition, which stands for S-4. The crystallizing and melting enthalpies of S-m (m ¼ 4, 6e10) decline after thermal treatment and thermal cycling test, while n-octadecane contents of S-4 decrease by 7.0 wt.% and 10.8 wt.% respectively, the least decrease in the S-m series. The influence of pH, thermal treatment and thermal cycling test on the crystallizing enthalpy of a-form crystals in S-m (m ¼ 4, 6e 10) are shown in Fig. 7 and summarized in Table 5. From Table 5, it can be concluded that thermal treatment and thermal cycling test increase the crystallizing enthalpy of a-form crystals in S-m (m ¼ 4, 6e10). Under basic conditions, the crystallizing enthalpy of a-form
Table 4 Thermal properties of MicroPCMs prepared with influence of pH. Sample
Tpc ( C)
a
b
S-4 S-6 S-7 S-8 S-9 S-10
8.08 8.88 9.39 8.18 8.91 9.06
17.42 17.71 18.88 17.12 17.45 17.51
Tpm ( C)
DHc (J g1)
DHm (J g1)
DHc-50 (J g1)
DHm-50 (J g1)
DHc-500 (J g1)
DHm-500 (J g1)
n-octadecane content (wt.%)
27.84 27.04 26.63 25.85 25.83 26.77
169.35 156.86 159.28 138.04 155.86 146.21
166.74 151.02 158.32 134.64 152.35 142.59
154.10 131.47 143.11 118.40 133.10 125.97
151.09 128.19 140.41 117.40 131.43 123.88
145.52 107.00 120.17 102.15 105.02 107.42
142.74 107.01 117.12 100.71 105.35 106.93
76.0 69.6 71.8 61.7 69.7 65.3
Note: DHc-50, DHc-500, DHm-50, DHm-500 are the crystallizing and melting enthalpy of samples treated under 50 C for 7 days or subjected to 500 times thermal cycling test, respectively; representation of other parameters are the same as in Table 2.
Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125
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Acknowledgment The authors gratefully acknowledge the financial supports from International Collaboration Project (No. 2008DFA51210). References
Fig. 8. TGA curves of n-octadecane and MicroPCMs (S-m, m ¼ 4, 6e10).
crystals in S-m (m ¼ 6e8) increases with basicity increasing. From neutral to acidic conditions, the crystallizing enthalpy of a-form crystals in S-m (m ¼ 4, 9, 10) decreases with acidity increasing. The crystallizing enthalpy of a-form crystals in S-4 is the highest, even after thermal treatment and thermal cycling tests. Thermal stabilities of S-m (m ¼ 4, 6e10) are shown in Fig. 8. Temperature corresponding to 2 wt.% weight loss and char yield at 550 C are also summarized in Table 3. The weight loss trend of S-m (m ¼ 4, 6e10) is similar, i.e., a two-step weight loss at about 173e290 C, 400e500 C. Under basic conditions, Ts of S-m (m ¼ 6e8) first increases and then declines with basicity increasing. From neutral to acidic conditions, Ts of S-m (m ¼ 4, 9, 10) declines with acidity increasing. Ts of S-7 is 194 C, the highest in the S-m (m ¼ 4, 6e10) series, however, encapsulation efficiency is lower than that of S-4. S-8 starts to degrade at 173 C, indicating the vulnerability of shell materials. The weight loss of S-m (m ¼ 4, 6e10) at first degradation increase with corresponding noctadecane content increasing, and S-4 shows the highest weight loss. 4. Conclusions MicroPCMs with n-octadecane core and hybrid shell from alkoxy silanes, MPS and VTMS precursors were prepared. All MicroPCMs have spherical-like profiles and core/shell structure. Two types of crystals were formed in core materials of all MicroPCMs during crystallization. The weight ratio of MPSeVTMS, and pH values play significant roles on the properties of MicroPCMs. With the weight ratio of MPSeVTMS decreasing from 8:0 to 2:6 in neutral synthesis systems, MicroPCMs tend to be more regular and their surface become smoother, and n-octadecane content increase from 58.7 wt.% to 76.0 wt.%. The encapsulation of n-octadecane failed when only using VTMS as hybrid shell precursor. When the weight ratio of MPSeVTMS is fixed to 2:6, the encapsulation efficiency decreases in acidic or basic synthesis systems, compared with that in neutral synthesis system. MicroPCMs obtained with the weight ratio of MPSeVTMS equaling to 2:6 in neutral conditions, display a best thermal performance with the highest melting and crystallization latent heat of 166.74 J g1 and 169.35 J g1, and the n-octadecane content decreases only by 7.0 wt.% and 10.8 wt.% after thermal treatment and thermal cycling test respectively, which show good potential as energy storage materials.
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Please cite this article in press as: Li W, et al., Preparation and characterization of novel MicroPCMs (microencapsulated phase-change materials) with hybrid shells via the polymerization of two alkoxy silanes, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.03.125