Self-healing that allows for autonomous recovery from microcracks would greatly improve the reliability and durability of materials . So far,the healing methods for polymers fall into two categories [2, 3, 4, 5, 6, 7]: extrinsic and intrinsic self-healing. The former operates by taking advantage of an intentionally pre-embedded healing agent,while the latter depends on interactions of the polymers themselves without the need of an additional healing agent. Compared to intrinsic self-healing,extrinsic self-healing can be more easily coupled with the target polymers because their molecular structures have no need to be changed.
To prepare polymers capable of extrinsically self-healing,the healing agent should be stored in fragile vessels,like microcapsules,in advance,which are then incorporated into the materials to be healed [8, 9, 10]. Upon cracking of the materials, the microcapsules also break,and the healing agent is released to the damaged portions by the capillary effect. Owing to the chemical reaction and/or physical interaction of the healing agent occurring in the cracks,the latter are re-bonded.
It is worth noting that the reported self-healing speed is not fast in most cases. Usually hours or dozens of minutes have to elapse to reach the steady state or maximum healing efficiency [11, 12, 13, 14, 15, 16], regardless of extrinsic or intrinsic healing strategies. As a result, practicality of the self-healing technique has to be improved.
To solve the problem,a healing agent based on dual capsules of epoxy monomer and its hardener SbF5was recently developed in our lab . Epoxy composite containing the microencapsulated healing agent was able to heal cracks within seconds,because of the rapid cure of epoxy monomer catalyzed by SbF5 following cationic chain polymerization mechanism. SbF5is a strong Lewis acid and reacts with almost all the known chemical compounds. Although it becomes milder after forming a complex with ethanol (i.e. SbF5·HOC2H5) so as to be handled,the resultant is still quite active and cannot be encapsulated by conventional methods like in situ polymerization in an oil-in-water [18, 19]. Otherwise,it would be easily deactivated in water and common organic solvents except ethanol.
Here in this work we show a novel route to prepare microencapsulated ethanol solution of SbF5·HOC2H5 (i.e. SbF5·HOC2H5/HOC2H5 ). Firstly,hollow silica microcapsules with nano-porous shell structure were preparedviasol-gel technique employing tetraethyl orthosilicate (TEOS) as precursor and (poly(styrene-methacrylate),PSMA) particles as templates. Secondly,silica walled microcapsules containing highly active SbF5·HOC2H5/HOC2H5 were produced by vacuum aided infiltration  of the core chemical into the hollow silica spheres. Effect of synthesis and infiltration conditions as well as reactivity of the encapsulated SbF5·HOC2H5/HOC2H5 were characterized and discussed. 2. Experimental 2.1. Materials
Epoxy,diglycidyl ether of bisphenol A (trade name: EPON 828), was purchased from Shell Co. Ethanol and ammonia (38 wt%) were purchased form Guangzhou Chemical Reagent Factory,China. Polyvinylpyrrolidone (PVP) K-90,TEOS,styrene,and methyl acrylate were all supplied by Alfa Aesar GmbH,Germany. Prior to use,styrene and methyl acrylate were washed with sodium hydroxide aqueous solution (5 wt%) and water,dried over magnesium sulphate,and evaporated to dryness in vacuum. Azobisisobutyronitrile (AIBN) was obtained from Sigma-Aldrich, and purified by recrystallization. 2.2. Synthesis of SbF5·HOC2H5/HOC2H5
Both absolute ethanol and the ice bag were cooled to -80°C in advance. In a glove box filled with pure nitrogen gas,the ethanol (20 g) was poured into a polytetrafluoroethylene bottle in the ice bag,and then SbF5 (5 g) was slowly added under stirring. The solution temperature was kept below 5°C by controlling the dropping. Afterwards,the solution was continuously stirred until it was warmed to room temperature,offering SbF5·HOC2H5/HOC2H5 , in which the fraction of SbF5is 20 wt%. 2.3. Preparation of PSMA particles
Monodispersed PSMA particles were prepared by dispersion polymerization . Typically,deionized water (28 g),ethanol (140 g),and PVP (3 g) were stirred at 200 rpm in a 500 mL threenecked flask for 60 min at 75°C under argon purge. The solution of AIBN (2.3 g),styrene (74 g),and methyl acrylate (6 g) was then added into the flask. After 12 h,PSMA particles were centrifuged, washed with water for three times,and then dried at 40°C. 2.4. Preparation of hollow silica microcapsules
Firstly,PSMA particles (5 g) were dispersed in ethanol (70 g) under ultrasonic agitation. Then,deionized water (10 g),ammonia (4 mL),and a certain amount of TEOS were added to the system. The mixture was kept for 24 h at ambient temperature and then filtered to obtain silica-coating particles. After removing the PSMA template through high temperature calcination,hollow silica microcapsules (~6mm) were obtained. Sizes of the microcapsules were determined by a Malvern MasterSizer 2000 particle size analyzer. Morphological observation of hollow capsules was conducted on a HITACHI model S-4800 field emission scanning electron microscope (SEM). 2.5. Preparation of silica-walled microcapsules containing
SbF5·HOC2H5/HOC2H5 The gas inside the hollow microcapsules (0.3 g) was evacuated for 1 h under vacuum circumstance of 0.001-0.002 mbar in a 25 mL one-necked flask. Then,SbF5·HOC2H5/HOC2H5 was vented into the system at -20°C to submerge the microcapsules for~50 h until the infiltration reached equilibrium. The low operation temperature (i.e. -20°C) reduced evaporation of the ethanol. The obtained SbF5·HOC2H5/HOC2H5 -loaded capsules (core content: 39 wt%) were filtered and purged with ethanol. Core content of the SbF5·HOC2H5/HOC2H5 -loaded microcapsules was determined by the following procedures: Firstly,a certain amount of capsules was mixed with ethanol,which was then completely ground in an agate mortar to release the encapsulated SbF5·HOC2H5/HOC2H5 . The mixture was filtrated and washed three times by ethanol to isolate the shells. The remaining liquid was added into the mixture of thick HNO3 and H2SO4(1/5 by volume) and treated in a 750 W microwave oven for 2 min to dispel the organic matter. After 24 h, the solution was examined by a TJA IRIS (HR) inductively coupled plasma-atomic emission spectrometer to determine the concentration of Sb,β. The core content,γ,was calculated from: γ=ΓβMSbF5/(0.2aMSb),where α is the capsules weight,Gthe volume of the acid solution containing the separated SbF5·HOC2H5/ HOC2H5,MSbF5 and MSb molecular weights of SbF5 and Sb, respectively. 2.6. Reactivity of encapsulated SbF5·HOC2H5/HOC2H5
To determine whether the activity of SbF5·HOC2H5/HOC2H5 was changed after encapsulation,epoxy monomer (2 g) and SbF5·HOC2H5/HOC2H5-loaded microcapsules (0.1 g) were mixed and ground. In the meantime,the drastic increase of temperature of the system due to rapid exothermic curing was measured by an infrared camera ImageIR® 8300 (InfraTec GmbH,Germany). For comparison,SbF5·HOC2H5/HOC2H5 (15mL) was directly injected to epoxy monomer under stirring,and the temperature rise was also recorded. 3. Results and discussion
As briefly mentioned in the introduction,hollow silica microcapsules are created through sol-gel route employing TEOS as the precursor. The latter builds up a gelatinous network (gel) in the form of spheres by taking the shape of the template (PSMA particles). After removing the template at elevated temperature, porous hollow silica microcapsules are obtained. In this context, the amount of TEOS involved in the reaction is the key factor,which decides whether the hollow silica microcapsules can be used for the subsequent infiltration of SbF5·HOC2H5/HOC2H5 .
Fig. 1a shows the micrograph of the hollow silica microcapsules made by using 1:1 mass ratio of TEOS to PSMA. Evidently,the TEOS precursor is insufficient to fully wrap the PSMA particles forming the compact gel coating. Once the templates are eliminated,the network has to collapse leading to incomplete hollow capsules, which cannot include any fluidic chemicals.
|Fig. 1. Hollow silica microcapsules created with different mass ratios of TEOS to PSMA: (a) 1:1, (b, c) 1.5:1, (d, e, f) 2:1, and (g) 2.5:1. (a, b, d, g) Full view,(c, e) close-up view, (f) squashed capsules.|
When the mass ratio of TEOS to PSMA is raised to 1.5:1 (Fig. 1b and c),complete hollow silica microcapsules are yielded,but the surface pores are too large (~14 nm). The low packing density would result in backflow of the infiltrated liquidviathe pores instead of holding it inside the capsules. It not only lowers longterm stability of the encapsulated healing agent,but also hinders the preparation of the self-healing epoxy composite. If the chemical leaks out of the SbF5·HOC2H5/HOC2H5 -loaded capsules, it would rapidly initiate polymerization with the surrounding epoxy monomer so that the compounding might no longer proceed due to the drastic increase of the system viscosity.
Only in the case of 2:1 mass ratio of TEOS to PSMA qualified hollow silica microcapsules fabricated. Fig. 1d shows the appearance of the capsules is similar to that shown in Fig. 1b,but the magnified view in Fig. 1e reveals that the surface pores size has been significantly reduced (~5 nm). The improved packing density is adequate to provide aisles for the infiltration of curing agent SbF5·HOC2H5/HOC2H5 and prevent backflow. As the infiltration is carried out with the aid of vacuum,negative pressure is produced inside the capsules at the beginning. With the infiltration of SbF5·HOC2H5/HOC2H5 ,the pressure difference between the capsules compartment and outside environment gradually decreases to equilibrium. Consequently,the core chemical would not flow out for some time. In fact,the microencapsulated healing agent is not stored alone for time but mixed with matrix epoxy to manufacture the self-healing composite. The cured epoxy matrix plays the role of the outmost shield and no leakage occurs.
Thickness of the capsules is estimated to be about 200 nm from the ruptured versions (Fig. 1f). The relatively thin shell wall gives enough space for holding the infiltrated curing agent. Moreover, the uneven surface of the microcapsules and their polarity facilitate strong interfacial interaction in the composite. Rupture of the capsules is thus favored upon cracking of the latter.
When the mass ratio of TEOS to PSMA is further increased to 2.5:1 (Fig. 1g),the exterior surface of the hollow silica microcapsules looks rather coarse,and a lot of silica particles are deposited on the capsules surface. This is because the precursor is excessive,and the gelatinous network is too dense to create pores. In the course of high-temperature calcination treatment,the gas generated by degradated PSMA can barely slip away and burst of the microcapsules occurs. The deposited silica particles are the remains of the exploded portions.
The relationship between infiltration time and core content of hollow silica capsules is plotted in Fig. 2. With increasing time,the speed of loading decreases,which follows the general rule of adsorption. The equilibrium of 39 wt% is reached after 50 h. The data demonstrate that SbF5·HOC2H5/HOC2H5 can be successfully loaded by the hollow silica microcapsules through vacuum aided infiltration.
|Fig. 2. Loading of SbF5·HOC2H5/HOC2H5 by hollow silica microcapsulesversus infiltration time. Mass ratio of TEOS to PSMA = 2:1.|
To understand whether the high reactivity of SbF5·HOC2H5/HOC2H5 toward epoxy is deteriorated after encapsulation,the capsules were ground together with epoxy. In the meantime, temperature change of the mixture was monitored by a thermal camera (see Section 2). The reason why differential scanning calorimeter is not applied here lies in the fact that the epoxy-SbF5 cure proceeds so fast that the reaction is completed before the two substances are well mixed. Fig. 3 shows that the temperature increases from room temperature to 200°C within a few seconds as a result of the exothermic of the polymerization. The curve profile resembles that of the control experiment with the mixture of unencapsulated SbF5·HOC2H5/HOC2H5 and epoxy,implying that the encapsulated SbF5·HOC2H5/HOC2H5 is as active as its unencapsulated version.
|Fig. 3. Time dependences of temperature of epoxy cured by SbF5·HOC2H5/HOC2H5 from ground capsules and manual injection.|
Hollow silica microcapsules with porous shell structure are successfully produced by polymeric sphere template coupled with sol-gel technique. The mass ratio of TEOS to PSMA acts as the key factor and the optimal value is found to be 2:1. It ensures proper size of the microchannels on the shell wall,and hence favors infiltration of SbF5·HOC2H5/HOC2H5 with the help of vacuum and pressure balance inside and outside the capsules after infiltration. Further,the encapsulated SbF5·HOC2H5/HOC2H5 maintains its high reactivity toward epoxy,which meets the requirement of healing agent of the self-healing material. The method is suitable for packaging of other highly active compounds and has universal significance. Further exploration of capsule fabrication technique vianovel chemical approaches  is worth being conducted to expand the family of extrinsic healing polymers.Acknowledgment
The authors thank the support of the National Natural Science Foundation of China (Nos. 51273214 and 51333008),Doctoral Fund of Ministry of Education of China (No. 20090171110026),the Science and Technology Program of Guangdong Province (Nos. 2010B010800021,2010A011300004,2011A091102001 and S2013020013029),and the Basic Scientific Research Foundation in Colleges and Universities of Ministry of Education of China (No. 12lgjc08).
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