Confining polymerization at emulsion interface by surface-initiated atom transfer radical polymerization on reactive Pickering stabilizer

Confining polymerization at emulsion interface by surface-initiated atom transfer radical polymerization on reactive Pickering stabilizer

G Model JIEC 2593 1–7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx Contents lists available at ScienceDirect Journal of Indus...

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G Model

JIEC 2593 1–7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6

Confining polymerization at emulsion interface by surface-initiated atom transfer radical polymerization on reactive Pickering stabilizer Q1 Dezhong a b

Yin a,*, Wangchang Geng a, Qiuyu Zhang b,*, Baoliang Zhang b

School of Science, Northwestern Polytechnical University, Xi’an 710072, China Key Laboratory of Space Applied Physics and Chemistry of Ministry of Education, Northwestern Polytechnical University, Xi’an 710072, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 May 2015 Received in revised form 15 July 2015 Accepted 15 July 2015 Available online xxx

In this paper, surface-initiated atom transfer radical polymerization (SI-ATRP) on surface of Pickering stabilizer for microencapsulation was investigated. By SI-ATRP on initiator-immobilized SiO2 particles, emulsion droplets were involved into microcapsules. The density of PMMA chains on SiO2 surface reached 1.31 chain nm2 and the initiator efficiency was estimated to be 28%. The results confirmed that polymerization was confined successfully at the interface of emulsion and 80% of polymer chains were formed via SI-ATRP on Pickering stabilizer. This covalent between SiO2 and polymer endowed the prepared microcapsules with good durability and thermal reliability in phase change materials application. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Surface-initiated atom transfer radical polymerization Pickering emulsion Microencapsulated phase change materials Polymerization mechanism

7 8

Introduction

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Pickering emulsion, also referred as particles-stabilized emulsion, has been proven to be a feasible route for synthesizing particulates with unique microstructure [1–3]. Due to high adsorption energy of particle at oil–water interface, Pickering emulsion possesses inherent higher stability than traditional surfactant-stabilized emulsion. Furthermore, Pickering emulsion contains three phase (oil, water and solid particles), which allows various types of reactions to be practiced for products with different microstructure. A wide variety of particulates, such as solid microspheres [4], hollowed microspheres [5], porous material [6] and Janus particles [7], were successfully fabricated from Pickering droplet. The remits mentioned above make Pickering emulsion a versatile template for microencapsulation. Microcapsules are of great importance in applications of pesticide delivery, phase change material and self-healing materials [8,9]. Typically, the shell of microcapsule was formed by depositing polymer onto the droplet surface through suspension-like polymerization [10,11], solvent evaporation [12] or complex coacervation [13]. In these processes, restrictive matching between core liquids and polymer

Q2 * Corresponding authors. Tel.: +086 029 88430250; fax: +086 88430250. E-mail addresses: [email protected] (D. Yin), [email protected] (Q. Zhang).

is required, otherwise the polymeric chains deposit in the interior of particles rather than on the interface of droplet [14]. To overcome this problem, a second strategy, named in-situ polymerization at the interface of droplet, is advantageous over traditional deposition method. In in-situ formation of polymeric shell, special strategy is necessary to confine the polymerization at the interface. Reactive surfactant is commonly employed to introduce initiator onto the interface. Amphiphilic macroinitiators of atom transfer radical polymerization (ATRP) [15,16] or reversible addition-fragmentation transfer [17] were successfully reported to confine the polymerization at the interface. In Pickering emulsion, surface of Pickering stabilizer is an exclusive location to introduce initiator onto the interface. By this way, polymeric chains propagate on the surface of Pickering stabilizer, by which the radical polymerizations can be localized at the interface of Pickering droplets. Surface-initiated ATRP (SI-ATRP), as a ‘‘grafting from’’ technique, has become an indispensable tool for surface modification of solid substrate [18,19]. By introducing ATRP initiator onto solid substrate, polymer chains propagate from the surface of solid substrate, which can lead to the formation of long polymer chains and high grafting density brushes on the surfaces. Typically, SiO2 particles were functionalized by 3-aminopropyltriethoxy silane, followed by amination with 2-bromoisobutyryl bromide (BiBB) to archor ATRP initiator onto SiO2 particles [20,21]. In Pickering emulsion, Zhao [22] and Yang [23] carried out SIATRP on two hemispheres of silica particles to prepare Janus

http://dx.doi.org/10.1016/j.jiec.2015.07.010 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

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particles. Wang [24] reported lightly cross-linked poly(2-hydroxyethyl methacrylate) brushes and subsequent capsule formation using Pickering emulsion SI-ATRP in the water phase. Sto¨ver [25] coated SiO2 nanoparticles electrostatically with an anionic polymeric ATRP initiator, followed by SI-ATRP of water-soluble monomers to encapsulate core solvents. These reports were carried out according to normal ATRP model where reaction mixtures must be rigorously deoxygenated by complex procedure. Low conversion of the monomer was observed because of the disproportionation of CuBr to Cu and CuBr2 and the oxidation of CuBr to CuBr2 during polymerization [25]. Recently, polymeric microcapsules through SI-ATRP polymerization based on activators regenerated by electron transfer (ARGET) in Pickering emulsion was reported [26]. Attention was focused on the characterization of products. In this paper, the polymerization mechanism and chain structure were investigated systematically through quantifying the initiator density on SiO2, percent of polymer formed by SIATRP, molecular weight of polymer chains and initiator efficiency for SI-ATRP. Our results confirmed that the polymerization was confined successfully at the interface of emulsion and majority of polymer was connected on Pickering stabilizer through surface initiated polymerization.

79

Materials and methods

80

Materials

81 82 83 84 85 86 87 88 89 90 91

Tris(2-pyridylmethyl)amine(TPMA, TCI Japan), fumed SiO2 particles (Wacker Silicones, Germany), 3-aminopropyl triethoxysilane (KH550, jkchemical, China), 2-bromoisobutyryl bromide (BiBB, jkchemical, China) were used as received. Methyl methacrylate (MMA) and ethyleneglycol dimethacrylate (EGDMA) (Sinopharm Chemical, China) were purified by alumina (neutral) packed column. CuBr2, azobisisobutyronitrile (AIBN), tetrahydrofuran (THF) and triethylamine were obtained from Beijing Chemical Reagent Co. Ltd (Beijing, China). Hexadecane and ascorbic acid (Vc) were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).

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Immobilization of ATRP initiators on SiO2 particles

93 94 95 96 97

ATRP initiators were immobilized on SiO2 particles according to method in literature [19] with some modification. The scheme was shown in Fig. 1. Anhydrous toluene (20 mL), KH550 (various amount) and triethylamine (1 g, 10 mmol) were introduced into a 100 mL three-neck flask. After cooling to 4 8C, 10 mL of anhydrous

toluene solution with BiBB (mole ratio of BiBB/KH550 = 1.2) was added dropwise. The reaction mixture was stirred for 2 h and then poured into a mortar containing 5 g SiO2 particles and milled the powder continuously. After evaporation of toluene, an ethanol/ ammonia mixture was introduced similarly to catalyze the hydrolysis and the condensation between Compound 3 and SiO2 particles. The modified SiO2 particles were washed with ethanol to remove the impurities.

98 99 100 101 102 103 104 105

Preparation of emulsion and ATRP for microencapsulation

106

SiO2 suspension, CuBr2 solution and TPMA were mixed as water phase. Separately, hexadecane and monomer were mixed as oil phase. Emulsification was carried out ultrasonically and the vessel was subsequently bubbled with N2 for 2 min to displace the air in the vessel. Then, Vc (reducing agent) was added into the vessel to reduce Cu(II) into Cu(I). The reaction mixture was sealed rapidly and placed in the roller reactor at 55 8C for 48 h. The microcapsules were filtrated and washed with water, and volatilized at ambient temperature. 2 g hexadecane was microencapsulated in each batch and mole ratio of Cu2+/TPMA/Vc was fixed to be 0.0384/0.101/ 0.292. Other experimental parameters were listed in Table 1. Cross-linked microcapsules were obtained in batch 8– 10 where EGDMA was added as cross-linker during Pickering emulsions preparation. The introduction of cross-linker has no any detectable effect on the formation of Pickering emulsion.

107 108 109 110 111 112 113 114 115 116 117 118 119 120 121

Characterization

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Optical micrographs were collected with an optical microscope equipped with a digital camera (CAIKON, China). Diameter of Pickering droplets was evaluated by calculating 100 droplets in the optical micrograph image. Size distribution of SiO2 particles was evaluated by a Laser particle size analyzer. Contact angle of SiO2 particles was determined by a JY-82 contact angle goniometer (Chengde, China). Scanning electron micrographs (SEM) were collected with a JSM-6700F electron microscope (Japan). Thermal gravimetric analysis (TGA) of SiO2 particles and prepared microcapsules was carried out with a Q50 thermal gravimetric analyzer (TA Instruments, USA) with a heating rate of 10 8C min1. The conversion of MMA was tested by gas chromatography (GC) according to reported method [26]. Gel permeation chromatography (GPC) analysis was carried out with a Waters 515 pump and a Waters 2414 differential refractometer using SDV analytical columns (5 mm, PSt-DVB copolymer, PSS company, Germany) in THF as an eluent at 35 8C and at a flow rate of 1 mL min1. PMMA standard calibration (PSS ReadyCal) was used for calibration.

123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140

Fig. 1. Synthesis of modifier (Compound 3) and ATRP-active SiO2.

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JIEC 2593 1–7 D. Yin et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx Table 1 Experimental parameters for microencapsulation. Batch

SiO2-2 (g)

MMA (g)

EGDMA (g)

AIBN (mg)

C-Br/MMA mole ratio

1 2 3 4 5 6 7 8 9 10

0.2 0.3 0.3 0.3 0.4 0.3 (SiO2-1) 0.3 0.3 0.3 0.3

1.0 0.5 1.0 2.0 1.0 1.0 0.5 0.9 0.8 0.6

– – – – – – – 0.1 0.2 0.4

– – – – – – 5.0 – – –

0.069 0.207 0.1035 0.05175 0.138 0.0585 0.1035 – – –

141 142 143

Thermal properties of microcapsules were evaluated by differential scanning calorimeter (TA Instrument, DSC-2910) under a heating rate of 5 8C min1 and nitrogen atmosphere.

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Results and discussion

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ATRP initiator-functionalized silica nanoparticles

146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168

To obtain ATRP initiator-functionalized SiO2 nanoparticles, KH550 was amidated with BiBB to form Compound 3 (see Fig. 1), and then the obtained product was employed directly to modify SiO2. FT-IR of KH550, Compound 3 and modified SiO2 verifies the formation of amide group in Compound 3 and modified SiO2 (Supporting information 1). SiO2 was endowed with surface ATRP activity by C-Br group. Properties of prepared SiO2 were listed in Table 2. To probe the efficiency during modification, TGA analysis of modified SiO2 was carried out. The weight loss (Loss%) of modified SiO2 was calculated using unmodified SiO2 as reference. The utility efficiency of KH550 was calculated by dividing Loss% by theoretical weight loss. The theoretical weight loss was calculated from the stoichiometry on the hypothesis that all KH550 was covalentlybonded on SiO2. The result shows that the utility of KH550 reached 86% when 0.5 g KH550 was used to modify 5 g SiO2. In another paper [27], we modified SiO2 with KH570 (another silane couple reagent similar to KH550) through dispensing SiO2 in KH570 solution and the utility of KH570 was only 30%. This result shows high efficiency of dry modification process used in this research. From the Loss% of modified SiO2, we roughly estimated the ATRP initiator content and initiator density on SiO2 surface by the following equations: Initiator content ¼

170 169

172 171

Loss% Mlost

Number of initiator ¼

Surface area ¼

m  Loss%  NA M lost

mð1  Loss%Þ

Initiator density ¼

(2) 3mð1  Loss%Þ r pr p

(3)

r p  Loss%  NA Number ¼ Surface Area 3ð1  Loss%Þ  Mlost

(4)

r p  ð4=3Þpr 3p

174 173

(1)

 4pr 2p ¼

3

where rp is the density of the silica particles (2.07 g cm3, that of bulk silica), rp is the radium of SiO2 particles (Table 2), NA is Avogadro’s number, MLost is molecular weight of lost fragment in TGA(–CH2–CH2–CH2–NH–CO–CBr(CH3)2, M = 207 g mol1, as shown in Fig. 1). The initiator density is estimated to be 4.64 initiator nm2 when 0.5 g KH550 is used to modify 5 g SiO2. Other researches employed suspension process for immobilization of ATRP initiator on SiO2 and initiator density of 1.5 [24] and 2.86 [28] initiator nm2 was reported. The functionalization by ATRP initiator also changes the oil– water contact angle (uow) of SiO2. Here, SiO2 nanoparticles were pressed into a disk for sessile drop experiment to detect air-water contact angle (uaw) and air-hexadecane contact angle (uao), and the uow was calculated by Young’s equation, expressed in the following equation [29]: cosuow ¼

g aw g cosuaw  ao cosuao g ow g ow

(5)

where gaw is surface tension of water, 71.2 mN m1, gao is surface tension of hexadecane, 28.1 mN m1 [30], gow is hexadecanewater interfacial tension, 53.3 mN m1 [31]. The Determined uow was shown in Table 2. The result indicated that SiO2-1 and SiO2-2 are hydrophilic preferentially and SiO2-3 is hydrophobic preferentially.

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Pickering emulsion

198

Fig. 2 shows the appearance and optical micrograph of emulsion. Unmodified SiO2 was totally hydrophilic and no emulsion was obtained. Oil-in-water emulsions were obtained by SiO2-1 and SiO2-2. Cream of emulsion occurred because of large droplet size and density difference between oil and water. By SiO23, the emulsion was extremely unstable and released oil and water presented in the system. Because SiO2-3 is hydrophobic preferentially and the system is of low oil fraction, this match is improper for the formation of emulsion. Fig. 3 shows the diameter of emulsion prepared with different amount of SiO2-2 particle. The droplet diameter decreased as the amount of SiO2 particles increased, until a plateau at about 6 mm was reached. Higher amount of stabilizer do not lead to further reduction in droplet size, as reported by others [32,33]. The stabilization of Pickering emulsion depends on a particlesformed film on droplet surface. Herein, the particle coverage on droplet surface was calculated in the following equation [34]: pffiffiffi 3m p  Dd S (6) cov ¼ film ¼ Sinterf 2pr p  d p  V oil

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where mp is the mass of SiO2 particles, rp is the density of SiO2 particles (2.07 g cm3), Voil is the volume of oil (3 mL), Dd is the diameter of Pickering droplets and dp is the diameter of SiO2 particles. The droplet diameter and particle coverage under different amount of SiO2 were shown in Fig. 3. Densely particle monolayer on droplet surface is achieved when SiO2 amount is 0.2 and 0.3 g (coverage  1.0). SiO2 is arranged sparsely on droplet surface (coverage < 1.0) under low SiO2 amount, while free particles

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Table 2 Effect of KH550 on properties of modified SiO2 particles.

*

176 175 177 178 179 180 181 182 183 184 185 186 187 188 189 190

Sample

KH550 (g)

Weight loss by TGA (%)

Utility of KH550 (%)

Diameter (nm)

Initiator content (mmol g1)

Initiator density (nm2)

uow Calculated (8)

SiO2 SiO2-1 SiO2-2 SiO2-3

– 0.25 0.5 1.0

0.94 4.98 8.08 11.27

– 91.8 86.0 69.3

55.3 57.2 60.2 55.6*

– 0.195 0.345 0.499

– 2.41 4.64 6.42

13 44 64 95

Containing small particle formed by self-condensation of KH550. See Supporting information 2.

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JIEC 2593 1–7 D. Yin et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

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Fig. 2. Emulsions by different SiO2 (a) and micrograph of emulsion by SiO2-2 (b). Scale bar 20 mm.

the second step, SI-PMMA precipitates along with SiO2 in precipitation 2, while isolated PMMA dissolves in THF. The percent of SI-PMMA on SiO2 was calculated from PMMA/SiO2 ratio obtained by TGA analysis according to reported method [14,26], as expressed in the following equation: SI-PMMAð%Þ ¼

ðPolymer=SiO2 Þprecipitation 2 ðPolymer=SiO2 Þprecipitation 1

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(7)

226 227

suspend in aqueous phase when large amount of SiO2 is employed (coverage > 1.0).

We changed the amount of monomer and SiO2 to prepare microcapsule, and the conversion of monomer and percent of SIPMMA under different C-Br/MMA mol ratio were illustrated in Fig. 5. Amount of C-Br group is an important parameter to the conversion of monomer and the percent of SI-PMMA, verifying an ATRP mechanism rather than common radical polymerization. When AIBN was added in the system, common radical polymerization exists in the system, leading to a high conversion of 95% (Fig. 5a). However the percent of SI-PMMA dropped significantly to only 25.8% (Fig. 5b). On contrary, percent of SI-PMMA reached 80% in AIBN-free system. Increasing the C-Br/MMA is propitious to SIATRP because monomer molecules have more chance to react with initiator on SiO2.

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Mechanism of polymerization

GPC analysis of the cleaved PMMA and initiator efficiency

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With ATRP initiator on SiO2 surface, PMMA chains propagate from the SiO2 surface by surface initiated mechanism and the resultant PMMA chains (referred as SI-PMMA) were covalentlybonded on SiO2 surface. Because the SiO2 particles were archored on the droplet surface, polymerization of MMA was confined at the interface of Pickering droplets. To separate shell of microcapsules and SI-PMMA, procedure shown in Fig. 4 was employed. In the first step, unreacted monomer and core material were removed by ethanol. In precipitation 1 contains SI-PMMA and isolated PMMA (formed by mechanisms rather than surface-initiated polymerization). In

A typical characteristic of ATRP lies in the narrow distribution of molecular weight of obtained polymer. In a Teflon bottle, precipitation 2 was treated with hydrofluoric acid exhaustedly to cleave the grafted PMMA chains. After drying the cleaving solution, the residue was dissolved in THF for GPC analysis. Fig. 6 shows the molecular weight and polydispersity (PDI) of cleaved SIPMMA from uncross-linked microcapsule (by procedure in Fig. 4). The molecular weight distribution is relatively boarder than reported ARGET SI-ATRP in suspension system [35]. The complicated transport process of the monomer from droplets to SiO2 surface limits the level of control to the polymerization.

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Fig. 3. Average diameter (&) and particle coverage (*) of Pickering droplets stabilized by different amount of SiO2-2. The oil phase was composed of 1.0 g MMA + 2.0 g C16H34.

Fig. 4. Scheme for separating shell polymer and bonded PMMA.

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JIEC 2593 1–7 D. Yin et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

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Fig. 5. Conversion of monomer (a) and percent of SI-PMMA (b) under different C-Br/MMA ratio.

Fig. 6. Molecular weight and distribution of SI-PMMA in different batch.

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We roughly estimated the density of PMMA chain and the initiator efficiency for SI-ATRP. The grafting density was calculated by Eq. (4), where Loss% is obtained from TGA results of precipitation 2 and Mlost is the molecular weight of SI-PMMA. Fig. 7a shows the density of PMMA chain under different C-Br/ MMA ratio. The chain density range in 0.91–1.31 chain nm2, except under the circumstance of adding AIBN (common radical polymerization exist) or employing SiO2-1 (with low initiator density on it). The initiator efficiency was calculated by dividing chain density of PMMA by initiator density. The initiator efficiency was estimated to be 19–28%, depending on the C-Br/MMA ratio

(Fig. 7b). Also the introduction of AIBN has negatively effect on the initiator efficiency for SI-ATRP, because part of MMA polymerize by AIBN initiated radical polymerization.

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Cross-linked microcapsule

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Cross-linked microcapsules were fabricated by adding EGDMA into the droplet and polymerizing accordingly (Batch 8–10 in Table 1). By EGDMA, PMMA chains on SiO2 surface were connected together, fixing the SiO2 particles and polymer into a whole shell. Typical SEM images of cross-linked microcapsules were shown in Fig. 8. The images showed that the microcapsule keep a spherical morphology of Pickering droplets. To verify the microcapsule structure, the microcapsules were soaked with ethanol or broken by pressing, followed by SEM observation. After removal of core material by ethanol, the microcapsules collapse but reserve the particulate shape (Fig. 8b). The SEM of broken microcapsules shows clearly a crust structure (Fig. 8c). Durability and thermal reliability of microcapsule are determinant to microencapsulated phase change materials (MePCM). Typically, MePCMs were practiced as fluid for thermal carrier [36] or solid material for temperature control [37,38]. Here, durability in MePCM suspension and thermal reliability in thermal cycling were practiced to investigate the performance of MePCMs. The protocol was described in our reported literature [10] and the percentage of leached PCM was summarized in Table 3. The result indicates that the crosslinking of shell have great effect on the performance of MePCMs. About 8.5% of core material leached from MePCMs without cross-linker, while less than 4% of core material

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Fig. 7. Grafting density (a) and initiator efficiency for SI-ATRP (b) under different C-Br/MMA ratio.

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Fig. 8. SEM images of (a) intact cross-linked microcapsules, (b) microcapsules after removal of core material and (c) broken microcapsules (showing a layer of crush).

Table 3 Percent of leached PCM from MePCMs in durability test and thermal reliability test. Batch

PCM/polymer

3 8 9 10

1.964 1.931 1.906 1.990

* #

Durability test*

Thermal reliability#

PCM/polymer

Leached PCM (%)

PCM/polymer

Leached PCM (%)

1.796 1.860 1.873 1.934

8.5 3.7 1.7 2.8

1.796 1.858 1.854 1.953

8.6 3.8 2.7 1.9

After suspended in water (35 8C) for 10 days. After 2000 thermal cycles.

310 311 312 313 314 315 316 317 318 319 320 321 322 323 324

lost when the shell of MePCMs was cross-linked. The cross-linking degree is an important parameter to the performance of MePCMs. MePCMs with 20% cross-linker (batch 9) present best performance, with only 1.7% and 2.7% of core material leached in the tests. Furthermore, we reported traditional suspension-like polymerization to prepare MePCMs in which polymer chain in the shell is not covalently-bonded on the SiO2 particles [10]. 2.2% to 4.2% and 5.8% to 7.7% of core material leached in the durability test and thermal reliability test, respectively. This result shows that MePCMs with covalently-bonded hybrid shell present much better performance. Because the PMMA chains are cross-linked together, the obtained microcapsules have better structural stability, which is important in the industrial application of microcapsule. For example, the microcapsule with poor structural stability is easy to be broken during the cycling of heat carrier fluid of MePCMs.

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Conclusions

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In a conclusion, polymerization of MMA was successfully confined at the oil/water interface by SI-ATRP on surface of Pickering stabilizer. A silane coupling agent containing ATRP initiating group was prepared and used to modify SiO2 particles. This modification endows the SiO2 particles with favorite wettability for Pickering stabilization and makes the SiO2 particles surface ATRP-active. In Pickering emulsion, this ATRP-active SiO2 acted as macroinitiator to initiate SI-ATRP. PMMA bonded on SiO2 particles in the shell was separated and used to calculate the conversion of monomer, percent of SI-PMMA, chain density of PMMA and initiator efficiency. The results show that 80% of MMA was polymerized through surface initiated mechanism. The chain density of PMMA on surface of SiO2 reached 1.31 chain nm2 and the initiator efficiency was estimated to be 28%. By EGDMA, PMMA chains on SiO2 surface were connected together, fixing the SiO2 particle and polymer into a whole shell. Only 1.7% of core material leached after suspended in water for 10 days and 2.7% of core material leached after 2000 thermal cycles. Our study shows that the covalent between SiO2 and polymer was important to the durability and thermal reliability of prepared MePCMs.

Acknowledgment

346

The supports from National Natural Science Foundation of China (51173147), the Natural Science Foundation of Shannxi Q4 Province (2014JM2038) are highly appreciated.

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Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2015.07.010.

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