Microcapsules derived from Pickering emulsions as thermal latent curing accelerator for epoxy resins

Journal Pre-proof Microcapsules derived from Pickering emulsions as thermal latent curing accelerator for epoxy resins Bowen Zhang, Yingying Zhao, Xin Sun, Xiaoma Fei, Wei Wei, Xiaojie Li, Xiaoya Liu

PII:

S1226-086X(19)30627-6

DOI:

https://doi.org/10.1016/j.jiec.2019.11.032

Reference:

JIEC 4875

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

19 September 2019

Revised Date:

19 November 2019

Accepted Date:

24 November 2019

Please cite this article as: Zhang B, Zhao Y, Sun X, Fei X, Wei W, Li X, Liu X, Microcapsules derived from Pickering emulsions as thermal latent curing accelerator for epoxy resins, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.032

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Microcapsules derived from Pickering emulsions as thermal latent curing accelerator for epoxy resins Bowen Zhang,1 Yingying Zhao,1 Xin Sun,1 Xiaoma Fei,2 Wei Wei,1,* Xiaojie Li,1 Xiaoya Liu1

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Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School

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of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, P. R. China

Wuxi Chuangda Advanced Materials Co., Ltd., Wuxi, Jiangsu 214028, P. R. China

*

Corresponding Author: Wei Wei (E-mail: [email protected])

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Graphical abstract

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Highlights

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1. Organic-inorganic hybrid microcapsules.

2. Long pot life for one-component curing composition at room temperature.

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3. Good resistance to shearing forces.

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4. Regulatory for latency and curing behavior.

Abstract

Organic-inorganic hybrid microcapsules encapsulating 2-phenylimidazole (2-PhIm) were prepared by Pickering emulsion polymerization of styrene and divinyl benzene using methacryloyloxy silane-modified silica particles as the stabilizer, and served as a

thermal latent curing accelerator for diglycidylether of bisphenol A epoxy/anhydride system. The one-component curing composition containing 5 wt% of the microcapsules exhibited an excellent storage stability. The pot life at room temperature was as long as 30 days, while which of 2-PhIm curing system was only 2 days. At elevated temperature, the encapsulated 2-PhIm could be released from microcapsules effectively, and instantly accelerated the curing reaction. Meanwhile, the addition of microcapsules did

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not cause an adverse effect on the thermomechanical performance of the epoxy

thermosets. In addition, the latency and curing behavior could be facilely regulated at a

wide range of temperature by adjusting monomer ratios of the polymer matrix, to match

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different process temperatures. The microcapsules with robust cross-linked polystyrene

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matrix structure also showed good resistance to shearing forces, indicating their feasibility for high-speed mixing or kneading process in practical applications. It is

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suggested that the novel latent curing accelerator is potential for high-performance one-

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pot epoxy formulations, particularly recommended for application in electronic

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packaging fields.

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Keywords: hybrid microcapsule, curing accelerator,epoxy resin

Introduction Owing to the outstanding thermal properties, remarkable chemical resistance, small curing shrinkage, and good adhesion and dimensional stability, epoxy resins have been widely applied in varied fields including adhesives[1], coatings[2], automobile[3],

electronics[4], architecture[5], and aerospace[6]. For use in electronic packaging fields, acid anhydride is one of the most common curing agents to initiate the polymerization of epoxy compounds[7]. Nevertheless, the curing temperature of those epoxy compound systems is very high due to their high curing activation energy. To reduce the curing temperature and increase processability, accelerators are usually added in epoxy system[8-10]. One of the most commonly used

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accelerators is imidazole compounds[11]. As epoxy curing accelerator, it can conspicuously reduce curing temperature and enhance the glass transition temperature (Tg) of epoxy thermosets, which is significant to the epoxy product in microelectronic

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packaging field[12]. However, imidazoles are usually not latent accelerator for one-pot

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epoxy resin systems[13]. The intrinsic high reactivity of imidazoles makes it easy for the liquid mixtures to increasingly transform into insoluble and infusible solids with cross-

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linked networks, which even occurs at room temperature. It would distinctly damage

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the storage stability and is disadvantageous for the curing molding of one-pot epoxy compound at high temperature[14-16]. Hence, developing effective methods to modify

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imidazole or imidazole derivatives for elevating the pot life and simultaneously maintaining the curing activity at high temperature has drawn much more attention in

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recent years to solve this conundrum[9, 17]. There are many approaches reported to modify imidazoles for reducing the activity,

such as preparation of metal-imidazole complexes[18] and imidazolium salts[19], formation of carboxylic acid esters via Michael addition[20], and encapsulation of imidazole compounds by polymer microcapsules[21]. Among these present methods to

modify imidazoles, microencapsulation is a feasible and enforceable strategy. After active materials being encapsulated into polymer microcapsules, a long shelf life could be achieved, and the active materials would be released from microcapsules through the polymer segment movement above Tg[22, 23]. Shin et al.[24] prepared encapsulated imidazole derivatives in polycaprolactone (PCL) microcapsules by either solvent evaporation method or spray-drying method. Li et al. [25] proposed a facile synthesis of

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imidazole-containing microcapsules via thiol-click chemistry for preparing a thermal latent curing agent for epoxy resins.

Employing Pickering emulsion polymerization to fabricate microcapsules is an 27]

. Pickering emulsions are stabilized by solid particles,

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effective approach[26,

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belonging to surfactant-free emulsions by self-assembly adsorption of colloid particles at the liquid-liquid interface[28]. Due to these solid particles owning the high adsorption

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energy and constant adsorption at the liquid-liquid interface, the emulsions have been

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considered as ideal templates for the preparation of organic-inorganic hybrid microcapsules by the fixation of the solid particles at the interface[29-31]. By conducting

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the polymerization of particle-stabilized emulsion droplets, pre-formed by a high-shear emulsification step, hybrid core-shell latexes are usually fabricated, where a polymer

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core is surrounded by an armour of inorganic particles. Recently, a mechanistic understanding of the Pickering emulsion polymerization process contributed by Lotierzo and Bon reveals that, inorganic particles would not spontaneously adhere to latex particles. Instead, the Pickering stabilizer needs wetting by adsorption of a growing radical from the water phase to trigger adhesion onto the growing latex

particles[32]. Compared with either solvent evaporation method or spray-drying method, incorporation of the solid particles into the prepared hybrid microcapsules would not reduce significantly the mechanical and thermal properties of epoxy thermoset[30, 33]. In this study, we prepared a microcapsule-type latent accelerator for epoxy curing system based on Pickering emulsion. The oil-in-water type Pickering emulsion was first prepared by mixing oil phase (2-phenylimidazole, vinyl monomers, solvent, and

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initiator) and water phase with silane-modified silica as particle emulsifier. Then the

microcapsule encapsulating 2-phenylimidazole was obtained by thermal initiation of Pickering emulsion polymerization using 2, 2'-azobis(isobutyronitrile) (AIBN) as

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initiator. The resulting microcapsule was further used as a curing accelerator for

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epoxy/anhydride system. The storage stability of the one-pot compound was measured by the change of apparent liquidity and viscosity, and the curing kinetics was analyzed

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through non-isothermal DSC compared with original 2-phenylimidazole. The

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thermomechanical properties of epoxy thermosets were characterized by DMA. In addition, the microcapsules with different release temperatures (i.e. latency) were

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facilely prepared by adjusting monomer ratios, implying their wide feasibility to different processing temperatures, especially for application in epoxy molding

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compound.

Experimental Materials 3-Methacryloxypropyltrimethoxysilane

(KH570,

98%

purity),

tetraethyl

orthosilicate (TEOS, 99% purity), 2-phenylimidazole (2-PhIm, 98% purity), and AIBN

(99% purity) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Hydroquinone (98% purity), acetone (AnalaR grade), aqua ammonia (25%-28% concentration), ethanol (AnalaR grade), and ethyl acetate (Chemical Pure grade) were obtained from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Styrene (St, AnalaR grade) and divinylbenzene (DVB, AnalaR grade) were received from Aladdin Chemistry Co., Ltd. (Shanghai, China) and treated with 5 wt% NaOH

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aqueous solution to remove the polymerization inhibitor. Diglycidyl ether of bisphenol

A (DGEBA) type epoxy resin (E51) with an epoxy equivalent of 186 g eq-1 and curing agent methylhexahydrophthalic anhydride (MHHPA) were supplied by Wuxi

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Chuangda Advanced Materials Co., Ltd. (Wuxi, China).

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Preparation of silane-modified SiO2 nanoparticles

Firstly, SiO2 nanoparticles were prepared by hydrolysis of TEOS. TEOS (10 g, 0.048

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mol), aqua ammonia (8.3 g, 25%-28% concentration), deionized water (16 mL), and ethanol (250 mL) were added in a single-neck flask equipped with refluxing condenser

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and reacted with magnetic stirring at 65 ℃ for 12 h. The obtained SiO2 particles were

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washed three times by ethanol, followed by drying at 40 ℃ for 8 h. Then, the SiO2 particles were modified by KH570. A few SiO2 nanoparticles and aqua ammonia were

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added in a single-neck flask, which contained 160 mL of 90 wt% ethanol aqueous solution dissolving a definite amount of KH570. The reaction was conducted with magnetic stirring at 50 ℃ for 12 h, and the modified SiO2 nanoparticles were obtained after being washed three times by ethanol and dried at 40 ℃ for 8 h. The chemical route of preparation and modification of SiO2 nanoparticles was illustrated in Scheme 1a.

Preparation of hybrid microcapsules The hybrid microcapsules were prepared using the following route (shown in Scheme 1b). To obtain stable Pickering emulsion, a definite amount of St, DVB, 2PhIm, ethyl acetate, and AIBN (2 wt% relative to total monomers) were mixed to get oil phase, which was then mixed with different amounts of aqueous SiO2 suspension. Hydroquinone (0.5 wt% relative to total monomers) was also added as an inhibitor to

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prevent polymerization in the aqueous phase. The concentration of SiO2 in the aqueous

suspension was varied from 3 wt% to 5 wt%. The volume ratio of St to DVB was 9:1,

7:1, 5:1, 3:1, and 1:1, respectively. The volume ratio of SiO2 suspension to oil phase

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was constantly 4:1. The stable Pickering emulsion was prepared by emulsification of

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oil in water using a high-shear homogenizer (Ultra-Turrax, IKA T10 basic) with 20,000 rpm for 2 min. Then the emulsion was heated to 70 ℃ to trigger the polymerization of

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St and DVB, and the reaction was carried out with a mechanical stirring for 8 h. After

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that, the resulting suspension was washed three times by water and acetone, respectively, to remove the free SiO2 nanoparticles and 2-PhIm. Finally, the hybrid microcapsules

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encapsulating 2-PhIm were obtained after being dried at 40 ℃ for 8 h. Table 1 sums the

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preparation formulations for the hybrid microcapsules. Preparation of epoxy thermosets Firstly, 45 g of MHHPA and a given amount of microcapsules (the content of 2-PhIm

in microcapsules is 2 wt% relative to epoxy resin) were added into a glass beaker and mixed by magnetic stirring for 1 h. Then 50 g of epoxy resin was added in the above glass beaker and stirred to organize a homogeneous mixture. To prepare the epoxy

thermosets, the mixture was degassed under vacuum for 1 h and poured into a mold. The mold was transferred to an oven and the mixture was cured at 130 ℃ for 2 h, 150 ℃ for 2 h, and 170 ℃ for 2 h. For comparison, 2-PhIm (2 wt% relative to epoxy resin) was also employed as the curing accelerator for E51/MHHPA. The mixing process was same as that of microcapsule system, and the corresponding epoxy thermosets were obtained according to the following curing procedure: 110 ℃ for 2 h, 125 ℃ for 2 h, and

non-isothermal DSC method, as shown in Fig. S1. Characterization

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135 ℃ for 2 h. The above curing procedures for the two systems were determined by

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The Fourier transform infrared (FT-IR) spectroscopy was obtained by Nicolet iS50

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FT-IR spectrometer (Thermo Fisher Scientific, USA) within 500 to 4000 cm-1. The Pickering emulsion droplets were observed with an optical microscope (DM-BA450,

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Motic, China). The water contact angle of SiO2 was characterized by a contact angle

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goniometer (OCA15EC, Dataphysics, Germany). The size and morphology of microcapsules were obtained using scanning electron microscope (SEM, S-4800,

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Hitachi, Japan). The thermal stability of the microcapsules and cured resins were tested by thermogravimetic analysis (TGA) from the TGA/1100SF apparatus (Mettler Toledo,

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Switzerland) with nitrogen atmosphere at the heating rate of 10 ℃ min-1 from 50 to 800 ℃. The curing behavior was performed by differential scanning calorimetry (DSC 204 F1, NETZSCH, Germany) at different heating rates (5, 10, 15, and 20 ℃ min-1). The release behavior of microcapsules was studied by UV spectrophotometer (UV-2550, Shimadzu, Japan). The viscosity of E51/MHHPA/microcapsule system was

investigated by rheometer (DHR-2, TA Instruments, USA) for evaluating storage stability of the system at room temperature. Dynamic mechanical experiment was measured using DMA Q800 instrument (TA Instruments, USA) with a double cantilever fixture. The cured specimens were tested from 30 to 200 ℃ with the heating rate of 3 ℃

Results and discussion Characterization of modified SiO2 nanoparticles

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min-1, and the test frequency was 1 Hz. The size of the specimens was 50 × 13 × 4 mm3.

As is known, the SiO2 particles which can adsorb on the droplet surface with high

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adsorption energy have been applied to stabilize the so-called Pickering emulsions via

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self-assembly at the oil/water interface[34]. The stability of emulsion depends on amphiphilicity of particles, particle size and concentration, interfacial tension, pH,

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salinity, etc[35]. Because the prepared silica had too many hydrophilic silyl hydroxyl

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groups on the surface, it could not well stabilize the oil-in-water Pickering emulsion. Hence, KH570 was applied to modify the surface of SiO2 in order to enhance the

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hydrophobicity of particles, and then strengthen the stability of the Pickering emulsion. In this study, 4 wt% of KH570 (relative to SiO2 mass) was determined as the optimal

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dosage for the hydrophobic modification, depending on the emulsifying performance of the modified SiO2 particles (see Fig. S2). In the FT-IR spectrum of original SiO2 shown in Fig. 1a, the broad adsorption from 985 to 1299 cm-1, with the peak at 1106 cm-1, is attributed to the stretching vibration peak of Si-O-Si group. The peak observed at 470 cm-1 demonstrates the bending

vibration peak of Si-O group. In the FT-IR spectrum of modified SiO2, it can be obviously seen that the methylene vibration peak is presented at 2970 cm-1, compared with the spectrum of original SiO2. Meanwhile, the intensity of the broad adsorption peak of hydroxyl group decreases at 3500 cm-1, suggesting the successful modification of SiO2. Fig. 1b shows the TGA curves of SiO2 and silane-modified SiO2. The curve of the modified SiO2 displays a slight thermal decomposition at 330-580 ℃, which can be

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owed to the decomposition of the silane coupling agent introduced on the SiO2 surface[36]. The water contact angle (θ) of SiO2 and modified SiO2 are further presented

in Fig. 1c. The values of SiO2 and modified SiO2 are approximately 8.5ºand 62.2º,

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respectively. Thus, it can be speculated that the particles modified with a certain amount

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of KH570 would well stabilize Pickering emulsion. In addition, the SEM image of modified SiO2 is given in Fig. 1d. The particles have regular globular shape and uniform

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size. The particle size is approximately 150 nm.

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Characterization of Pickering emulsions and microcapsules Fig. 2a shows the optical microscope photos of the oil/water Pickering emulsions

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stabilized by the particle emulsifiers with different concentrations, and the insets are

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apparent digital photos of the emulsions. The statistical results of the droplet size of the corresponding Pickering emulsions from the optical microscopy images are shown in Fig. 2b. The concentration of modified SiO2 in aqueous phase was varied from 3 wt% to 5 wt%, and the volume ratio of SiO2 aqueous suspension to oil phase was constantly 4:1. The ingredient of oil phase is same as that of St-DVB51 (see Table 1). As described in the following picture, with the increase of the concentration of modified silica, the

size of emulsion droplets decreases gradually, but light flocculation is occurred in the emulsion with 5 wt% of modified SiO2 particles. The emulsion fluidity is declined which is not conducive to the synthesis of microcapsules. Thus 4 wt% of modified SiO2 particles was used to stabilize emulsions and further prepare microcapsules, and the average size of emulsion droplets is approximately 34 μm. The stable Pickering emulsions with different volume ratios of St to DVB (9:1, 7:1, 5:1, 3:1, and 1:1) are

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shown in Fig. S3. The droplet sizes of those emulsions are 30-40 μm.

The preparation process of microcapsules is shown as Scheme 1b. The microcapsules

were prepared by mixing oil phase (2-PhIm, St, DVB, ethyl acetate, and AIBN) and

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water phase (deionized water and hydroquinone) with silane-modified silica as particle

After

polymerization,

the

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emulsifier, followed by thermal initiation of polymerization of the Pickering emulsion. cross-linked

polystyrene/silica

(PS/SiO2)

hybrid

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microcapsule encapsulating 2-PhIm were formed. Actually, there should be some

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covalent bond links between polymer and SiO2, because of the presence of C=C groups on the surface of silane-modified silica[26]. The FT-IR spectra of the modified SiO2, 2-

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PhIm, PS, and microcapsules are presented in Fig. 3. In the spectrum of 2-PhIm, it can be seen that the characteristic peak at 1565 cm-1 belongs to the C=N stretching vibration,

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which probably coupled with the benzene ring vibrational mode. And the peak at 1460 cm-1 is assigned to the N–H in-plane deformation vibration. For the spectrum of PS, the C–H in-plane benzene ring bending vibration can be assigned at 702 cm-1. Meanwhile, by comparing the spectra of modified silica, 2-PhIm, PS, and microcapsules, it can be seen that the stretching vibration peak of Si-O-Si group at 1106 cm-1, C=N stretching

vibration at 1565 cm-1, and C–H in-plane benzene ring bending at 702 cm-1 are simultaneously appeared in the spectrum of the microcapsules. Hence, it is preliminarily indicated that the preparation of microcapsules is successful. The morphology of microcapsules characterized by SEM is shown in Fig. 4 at different magnifications. As can be seen in Fig. 4a, the size of the microcapsules is approximately 20-30 μm. The microcapsules are mainly regular globular shape, and

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some irregular ones may be caused by the contraction during synthesis[37]. The surface of microcapsules is relatively rough covered with several dense layers of SiO2 particles (see Fig. 4b and 4c). The surface of microcapsules with SiO2 particles can be ascribed

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to the followed effect: On the one hand, the silica is particle emulsifier for the Pickering

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emulsion, so it is located at the oil-water interface and exists on the surface of microcapsules after polymerization. On the other hand, the effect of covalent bond of

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C=C groups may enhance adhesion between SiO2 and the surface of microcapsules.

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The microcapsules do not have a hollow structure, as demonstrated by some broken morphology observed from SEM after the microcapsules were treated by liquid

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nitrogen and subjected to external mechanical force (see Fig. S4). Instead, they are hybrid core-shell PS spheres, where a solid cross-linked PS core is surrounded by an

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armour of SiO2 particles with 3 to 4 layers. Such matrix-type microcapsules usually exhibit an advantage of robust structure and good resistance to external force[38]. To further confirm the successful preparation of microcapsules, Fig. 5 gives the TGA curves of PS, 2-PhIm, and microcapsules. As is shown, the thermal decomposition curve of the microcapsules is divided into two parts. The first stage belongs to the

thermal decomposition of encapsulated 2-PhIm at 230-300 ℃, while the thermal decomposition temperature has increased by 30 ℃ compared with that of the pure 2PhIm. It is indicated that the microcapsules can provide good protection to 2-PhIm and its thermal stability is improved. The second stage is attributed to the thermal decomposition of cross-linked PS at 410-500 ℃. Compared with the pure PS, the microcapsules exhibit a slight increased thermal decomposition temperature, which is

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probably due to the coverage of inorganic SiO2 particles on the microcapsule surface.

According to the TGA results, the content of the encapsulated 2-PhIm in the microcapsules is accounted for about 19% of the microcapsule mass. In order to

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investigate the release mechanism of 2-PhIm from the microcapsules, the

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microcapsules were heated at 200 ℃ for 10 min, and then the SEM image of which was taken and shown as the inset of Fig. 5. It is clear that no obviously broken microcapsule

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was observed after the heat treatment, indicating that the cross-linked structure of PS

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matrix as well as inorganic SiO2 particles endow the microcapsules with the robustness to bear the heat treatment and maintain the integrity of morphology. Hence it is

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presumed that 2-PhIm should be released from the microcapsules by segment motion of PS and permeation at high temperature, rather than the rupture of microcapsules.

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Storage stability of one-pot epoxy compound containing microcapsules The storage stability of two epoxy resin compositions (E51/MHHPA/2-PhIm and

E51/MHHPA/microcapsule) was characterized from the changes in viscosity and liquidity at room temperature. The curves of viscosity versus storage time are given in Fig. 6. As is shown, the viscosity of 2-PhIm curing system rises suddenly after being

stored only for 36 h at 25 ℃, while the viscosity of microcapsule curing system rises slowly which still appears much lower fluidity viscosity after storage for 500 h. The tendency of apparent liquidity is also same as that of the viscosity for the two curing systems (Fig. 7). The microcapsule curing system presents good liquidity which can flow till after 30 days. But 2-PhIm curing system reveals poor liquidity which cannot flow only after 2 days. The improvement of storage stability for microcapsule

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curing system is attributed to the physical barrier of the polymer matrix on the active material. Curing kinetics and release analysis

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At elevated temperature, it is speculated that the encapsulated 2-PhIm should be

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released from the microcapsules by the segment movement above Tg, followed by accelerating the curing reaction of epoxy and anhydride (as illustrated in Scheme 2). In

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order to testify the catalytic ability of the latent curing accelerator and appraise the

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effect of adding microcapsule on the curing behavior of epoxy resins, non-isothermal DSC for the dynamic curing of epoxy compounds with 5 wt% of microcapsules and 1

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wt% of the control sample, at different heating rates of 5, 10, 15, and 20 ℃ min-1 were carried out to survey the curing kinetics of the curing systems. The degree of conversion

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(α) was calculated as the following expression: ∆H

α= ∆H T

tot

(1)

where ΔHtot is the total heat of curing reaction derived from the DSC thermogram, and ΔHT is determined by summating the calorimetric signal up to a temperature T, which is the heat liberated to this temperature.

Fig. 8 shows the DSC curves and plots of α against temperature at different heating rates of the epoxy curing systems including 2-PhIm and microcapsules as curing accelerator, respectively. With increasing heating rate, exothermic peak shifts to higher temperature regions for both the two curing systems. It is because that with a higher heating rate, the epoxy resin is not able to be cured completely until it reaches a higher temperature. For the microcapsule curing system, the exothermic peak is relatively

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broad compared with that of 2-PhIm curing system. This phenomenon is caused by that the release of 2-PhIm from microcapsules through the motion of chain segment of

polymer matrix needs a wide temperature range. In addition, the release is a slow and

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dynamic process. The initial curing temperature Ti of the microcapsule curing system

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shifts 15 degrees to the higher temperature compared to that of the 2-PhIm curing system (Table 2). The tendency is the same as that of the conversion. It is indicated that

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the activity of the microcapsule curing system is inhibited and the microcapsule has

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latency for epoxy resin as a curing accelerator.

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Kissinger method and Crane method were adopted to calculate the average activation

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energy (Ea) and order of reaction (n) for the DGEBA cured with different curing accelerators, respectively[39, 40]. The Ea could be determined according to Kissinger approach using the following equation: β

AR

𝐸𝑎

Tp

a

R

ln [ 2 ] =ln [ E ] -

1

∙T

p

and n could be obtained through Crane method as follows:

(2)

d(lnβ) Ea =( +2Tp ) d(1⁄Tp ) nR

(3)

where β is the heating rate, Tp is the temperature of the maximum exothermic peak, A is the pre-exponential factor, and R is the gas constant (8.314 J mol-1 K-1). Ea could be obtained from the slope of the straight lines by plotting 1/ Tp versus ln(β/Tp2) at different heating rates, and n could subsequently be obtained from the value of first derivative of lnβ versus 1/Tp.

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Moreover, to calculate the value of Ea at different conversions, Starink method was used to analyze it[41]. The Ea could be determined according to Starink approach using the following equation: β

E

T1.92 f

) =1.0008 RTa +C

-p

-ln (

f

(4)

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where β, Ea, and R are the same as described above, Tf is the temperature corresponding to a certain conversion, and C is a constant independent of Tf. Ea could be obtained from

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the slope of the straight lines by plotting1/Tf versus -ln(β/Tf1.92) in a certain conversion.

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The Ea value and order of reaction obtained from Kissinger and Crane method for the 2-PhIm curing system are 71.521 kJ mol-1 and 1.03, respectively. While for the

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microcapsule curing system, the values are 85.868 kJ mol-1 and 1.09, respectively. It is indicated that the microencapsulated catalyst exhibited a blocked activity and enhanced

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the activation energy of curing reaction. Meanwhile, the reaction order of either curing system is approximately equal to 1, which is considered as first-order reaction. To further evaluate the activation energy at different extent of reaction, the Ea values calculated by Starink method are shown in Fig. 9. It can be seen that Ea values are obviously distinct at lower conversion rates (≤40%) for the two curing systems, but

they are relatively close at higher conversion rates. The tendency of gelation time for the two curing system is same as that of the Ea value calculated by Starink method (see Fig. S5). The gelation time for the two curing systems is obviously distinct at lower temperature while it is close at higher conversion temperature. In addition, when the conversion rate is greater than 60%, it is observed that Ea shows an upward trend for both the two curing systems, which is attributed to the increase of curing degree leading

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to the increase of viscosity of the curing system[42]. Combined with those conclusions,

the microcapsule curing system is regarded as the one owning both good curing activity at high temperature and good storage stability at room temperature.

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To survey the release behavior of microcapsules, DMSO was used as a medium to

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simulate the release of 2-PhIm from microcapsule at a certain temperature because of the good solubility for 2-PhIm. Quantitative release analysis of 2-PhIm encapsulated in

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microcapsules are presented in Fig. 10. The microcapsules were placed in DMSO and

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heated at 125 ℃ for different time to measure the release of 2-PhIm from the microcapsules by UV spectrophotometer. The UV absorption curve of 2-PhIm in

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DMSO with different heat treatment time is shown in Fig. 10a, and the curve of release amount calculated by standard curve versus time is shown in Fig. 10b. The release curve

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shows that the release amount increases gradually as increasing the time of heat treatment until 30 min later. The amount of 2-PhIm released from microcapsules is up to 82 wt% in 30 min. When the temperature is elevated, most of the 2-PhIm from microcapsules is released because of the segment movement of the cross-linked PS upon Tg. But the PS matrix is fairly thick so that the interior 2-PhIm cannot be released

from microcapsules completely. Moreover, the microcapsules were mainly composed of PS which had benzene ring structures. Therefore, the PS matrix might form π-π interaction with 2-PhIm, and thus affecting the release rate of 2-PhIm [43, 44]. The final release amount of which is about 82%. Generally, release of interior materials depends on the segment movement ability of polymer microcapsules above Tg. In this study, we facilely prepared a series of

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microcapsules with different release temperatures (i.e. latency) by adjusting monomer

ratios of St and DVB. The DSC curves of the epoxy curing systems containing different

microcapsules at a heating rate of 10 ℃ min-1 and Tg values of the corresponding

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microcapsules are presented in Fig. 11. With the increase of DVB moieties in

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microcapsules, exothermic peak shifts to higher temperature regions and becomes gradually broad. The tendency of the Tg of the corresponding microcapsules is

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consistent with that of the exothermic peak of DSC curves. It is demonstrated that the

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PS matrix which have different Tg lead to release the 2-PhIm at different temperatures and endow the microcapsules with different latency as curing accelerator. In practical

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applications, the microcapsules with different catalytic activity can be selected according to different processing temperatures, especially for application in epoxy

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molding compound.

In the fields of adhesives, coatings, and electronic packaging, microcapsule-type

latent accelerators are usually mechanically broken during a high-speed mixing or kneading process. For instance, the accelerators used in epoxy molding compound always suffer strong shear force when they pass through a screw extruder. Base on this,

we designed an experiment to investigate the anti-shearing ability of the as-prepared microcapsules. The microcapsules were subjected to different levels of shear force using high-speed disperser (DAC 150.1, HAUSCHILD, Germany) at 1000, 1500, and 2000 rpm. Then the treated microcapsules were added in E51/MHHPA system to test the curing behaviors. The anti-shearing performance of the microcapsules with crosslinked PS/SiO2 hybrid structure was characterized by DSC thermograms, and the

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morphology of microcapsules after being treated at different shearing rates was observed by SEM, as shown in Fig. S6. It can be seen that the curing behaviors revealed

by DSC curves of the three curing systems containing treated microcapsules show no

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obvious distinction compared to that of the original microcapsule curing system.

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Meanwhile, no fracture is observed by SEM for the microcapsules treated at different shearing rates. Therefore, it is demonstrated that the microcapsules own good anti-

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practical applications.

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shearing performance and feasibility for high-speed mixing or kneading process in

Thermomechanical and thermal properties of epoxy thermosets

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The thermomechanical properties obtained by DMA for the epoxy thermosets containing 2-PhIm and microcapsules are shown in Fig. 12a and 12b. Glass transition

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temperature and storage modulus determined at glassy state (E’[50℃]) and rubbery state (E’r) are summed up in Table 3. As is shown, the E’[50℃] of the epoxy thermosets containing microcapsules is slightly higher than that of the epoxy thermosets containing 2-PhIm. This is because the surface of microcapsules in epoxy thermosets has rigid silica particles, and the particles could enhance rigidity of epoxy thermosets. The

E51/MHHPA/microcapsule system exhibits a little decreased E’r compared to E51/MHHPA/2-PhIm system, indicating the slight decline of crosslink density of epoxy thermosets according to the rubbery elasticity theory[45]. It is probably caused by the addition

of

the

non-reactive

cross-linked

PS.

Nevertheless,

the

Tg

of

E51/MHHPA/microcapsule thermoset is comparable to that of E51/MHHPA/2-PhIm thermoset. In addition, an increase of initial decomposition temperature (T5%) and char

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yield at 800 ℃ of epoxy thermosets by adding the microcapsule-type curing accelerator

is also presented by TGA (see Fig. 12c and Table 3), which is profited from the organic-

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inorganic hybrid structure of microcapsules.

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Conclusions

In summary, an organic-inorganic hybrid microcapsule-type thermal latent curing

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accelerator for E51/MHHPA system was prepared by encapsulating 2-PhIm using

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Pickering emulsion polymerization of styrene and divinyl benzene stabilized by silanemodified silica particles. The prepared microcapsules had spherical shape covered with

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SiO2 particles, and the size was approximately 20-30 μm. The one-component epoxy curing system containing 5 wt% of the microcapsules had a pot life as long as 30 days

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at room temperature, while only 2 days was for 2-PhIm curing system. At elevated temperature, the encapsulated 2-PhIm could be released from microcapsules effectively, and instantly accelerated the curing reaction. The latency and curing behavior could be adjusted by changing the monomer ratios of the microcapsules, implying their wide feasibility to different processing temperatures. The microcapsules with robust cross-

linked PS/SiO2 hybrid structure also exhibited good resistance to shearing forces, indicating their feasibility for high-speed mixing or kneading process in practical applications. Moreover, the addition of microcapsules did not play an adverse effect on the thermomechanical performance of the epoxy thermosets, and the thermostability of which was even enhanced a little due to the organic-inorganic hybrid structure of microcapsules. Therefore, the microcapsule was proved to be an effective and

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promising latent curing accelerator for high-performance one-pot epoxy formulations.

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Declaration of interests We declared that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

We acknowledge the financial support from the Enterprise-University-Research

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Prospective Program, Jiangsu Province (BY2018041), Postgraduate Research &

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Practice Innovation Program of Jiangsu Provence (SJX18_0620), and MOE & SAFEA for the 111 Project (B13025). We are also grateful for the financial and material support

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from Wuxi Chuangda Advanced Materials Co., Ltd..

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[45] M. Kaji, K. Nakahara, T. Endo, J. Appl. Polym. Sci. 74 (1999) 690-698.

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SiO2. (d) SEM image of modified SiO2.

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Fig. 1 (a) FT-IR spectra, (b) TGA curves, and (c) images of water contact angle of SiO2 and modified

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Fig. 2 (a) Optical microscopy images and digital photographs (inset) of the oil/water Pickering emulsions stabilized by modified silica with different concentrations. (b) Statistical results of the

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droplet size of the Pickering emulsions based on the optical microscopy images.

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Fig. 3 FT-IR spectra of modified silica, 2-PhIm, PS, and microcapsules.

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Fig. 4 SEM micrographs of microcapsules.

Fig. 5 TGA curves of PS, 2-PhIm, and microcapsule. The inset is the SEM image of the microcapsule heated at 200 ℃ for 10 min.

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Fig. 6 Curves of viscosity versus time for the E51/MHHPA/2-PhIm and E51/MHHPA/microcapsule

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systems at room temperature.

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after different days.

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Fig. 7 Liquidity performance of the E51/MHHPA/2-PhIm and E51/MHHPA/microcapsule systems

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Fig. 8 DSC curves for the curing process of the epoxy curing systems containing 1 wt% of 2-PhIm

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(a) and 5 wt% of microcapsules (b) at different heating rates (5, 10, 15, and 20 ℃ min-1). Plots of conversion degree (α) against temperature for the curing process of the epoxy curing systems

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and 20 ℃ min-1).

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containing 1 wt% of 2-PhIm (c) and 5 wt% of microcapsules (d) at different heating rates (5, 10, 15,

Fig. 9 Variation of Ea versus conversion α by Starink method.

Fig. 10 Release of 2-PhIm from microcapsules in DMSO after being heated at 125 ℃ for different

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time: (a) UV spectra and standard curve of 2-PhIm (inset). (b) Cumulative release amount of 2-

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PhIm from microcapsules versus time.

Fig. 11 (a) DSC curves of the epoxy curing systems containing a series of microcapsules papered by different ratios of St and DVB at a heating rate of 10 ℃ min-1. (b) Tg values obtained from DSC

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for the microcapsules papered by different ratios of St and DVB.

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Fig. 12 Curves of storage modulus versus temperature (a) and tan δ versus temperature (b) for the epoxy thermosets containing 2-PhIm and microcapsules obtained by DMA. (c) TGA curves of the

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epoxy thermosets containing 2-PhIm and microcapsules.

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Scheme 1 (a) Chemical illustration of preparation and modification of silica particles. (b) Schematic

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of fabrication of the hybrid microcapsules encapsulating 2-phenylimidazole by Pickering emulsion.

Scheme 2 Schematic of release of the encapsulated 2-PhIm from the microcapsules at elevated temperature and acceleration of the curing reaction of epoxy and anhydride.

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Table 1 Preparation formulations of microcapsules Water

St

DVB

(mL)

(mL)

(mL)

St-DVB91

32.0

6.5

0.7

St-DVB71

32.0

6.3

St-DVB51

32.0

St-DVB31 St-DVB11

2-PhIm

AIBN

(g)

(g)

1.0

1.00

0.9

1.0

6.0

1.2

32.0

5.4

32.0

3.6

acetate

Modified

a

Content of

Content of

SiO2

2-PhIm

SiO2

(g)

(wt%)

(wt%)

0.14

1.28

19

16

1.00

0.14

1.28

17

14

1.0

1.00

0.14

1.28

19

14

1.8

1.0

1.00

0.14

1.28

18

16

3.6

1.0

1.00

0.14

(mL)

1.28

17

content of 2-PhIm in the microcapsules was measured by TGA at air atmosphere.

bThe

content of SiO2 on the microcapsules was measured by TGA at air atmosphere.

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aThe

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Sample

Ethyl

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Table 2 Calorimetric

data

determined

for

the

curing

systems

of

E51/MHHPA/2-PhIm

and

E51/MHHPA/microcapsule E51/MHHPA/2-PhIm β (℃ min-1)

E51/MHHPA/Microcapsule

Tp (℃)

Tf (℃)

∆H (J g-1)

Ti (℃)

Tp (℃)

Tf (℃)

∆H (J g-1)

5

110.2

128.8

141.9

326.6

135.8

172.2

195.3

307.4

10

122.6

141.1

154.3

329.5

147.7

184.6

210.4

311.7

15

126.3

149.5

164.5

331.6

158.1

20

129.9

153.6

170.5

334.8

164.2

c

d

temperature in the curing process.

maximum temperature in the curing process.

cFinal

temperature in the curing process.

dTotal

heat in the curing reaction.

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bPeak

189.2

218.1

323.8

197.1

227.8

329.2

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aInitial

b

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Ti (℃)

a

Table 3 Thermomechanical

and

thermal

analysis

data

of

E51/MHHPA/2-PhIm

and

E51/MHHPA/microcapsule thermosets a

Tg

b

E’[50℃.]

c

E’r

d

T5%

e

Char Yield

Sample (MPa)

(MPa)

(℃ )

(%)

E51/MHHPA/2-PhIm

162.3

1945.2

26.6

368

5.5

E51/MHHPA/microcapsule

162.8

2032.6

23.2

379

7.2

of maximum of the tan δ.

bStorage

modulus determined at 50 ℃.

cStorage

modulus determined at tan  peak + 30 ℃

yield at 800 ℃

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eChar

decomposition temperature at 5% weight loss from TGA.

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dInitial

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aTemperature

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(℃ )