Microencapsulation of tris(dimethylaminomethyl)phenol using polystyrene shell for self-healing materials
Honglin Hu, Lu Zhang, Ying Zhang, Yunhua Yang, Ruilian Yu & Jinming Wang
Scientific Reports volume 10, Article number: 12315 (2020) Cite this article

【Abstract】
The self-healing function of the polymer material has been realized by the microencapsulation technology of the healing agent. A novel microcapsule contained tris(dimethylaminomethyl)phenol (DMP-30) with polystyrene as shell material was prepared via solvent evaporation technique in a W/O/W emulsion. Two key strategies were implemented to prepare the microcapsules successfully. First, a small amount of deionized water was added into DMP-30 to form a complex, and a stable W/O emulsion was successfully prepared. The second one is to form a stable W/O/W emulsion system with the high viscosity aqueous solution added with Arabia gum and surfactants as the third phase. In addition, the influencing factors of microcapsules preparation were investigated systematically. The chemical structure of DMP-30 microcapsule was investigated by Fourier transform infrared. The morphology and shell thickness of the microcapsules were observed by optical microscope and scanning electron microscope. The reactivity of the core material was studied by differential scanning calorimetry. The thermal properties of microcapsules were studied by thermogravimetric analysis. The environmental resistance of microcapsules was verified by the isothermal aging test. Results showed that DMP-30 was successfully coated by polystyrene and the microcapsule size was in the range of 2–40 μm. The synthesized microcapsules were thermally stable below 50 °C.

【Introduction】
Thermosetting polymeric structural composites are increasingly applied in the field of aeronautics and astronautics, automobile industry, machinery industry, sports equipment, etc., which is owing to their excellent performance such as high strength and stiffness, low weight, and environmental stability1. Inherent brittleness and faultiness of polymeric composites make them apt to form microcracks, propagating to failure2. Recently, smart materials3,4,5 inspired from the biological systems were developed to repair inside damage whenever and wherever it occurs during the lifetime of polymeric composites, which would provide a method to significantly extend the service life and reliability of polymeric structural composites6,7,8,9,10,11. To realize this purpose, a series of strategies have been exploited such as embedded healing microcapsule and its curing agent12,13, embedded dual-microcapsule including healing agent and hardener14,15,16, embedded vascular network containing healing agent17,18,19,20,21, and so on22,23,24. All of these methods aim to provide a carrier for the healing agent so as to maintain its reactivity and release while the microcrack occurs. For the key to achieving the self-healing function, the healing agent should be protected from the external environment.

It is a general approach that healing agent is microencapsulated by the polymeric shell material via interfacial polymerization25, situ polymerization26,27,28,29, or solvent evaporation30 in a stable emulsion system. The endo-dicyclopentadiene microcapsule was firstly reported for self-healing material, which was embedded together with the Grubby catalyst in an epoxy matrix. Compared with the original epoxy resin, the average fracture toughness recovery is 60%12. Subsequently, as a reactive monomer, epoxy resin can react with various curing agents such as amines and anhydrides at different temperatures. Therefore, the binary self-healing composite with epoxy and hardeners attracts wide attention. Microencapsulation of epoxy was successfully synthesized firstly by Yuan via situ polymerization26. The curing agent of epoxy resin mainly includes reactive curing agent and catalytic curing agent. Microencapsulation of reactive curing agents for the epoxy self-healing system has been attempted with only modest success31. Several strategies for the microencapsulating reactive hardeners had been reported. Typically, hollow microcapsule with poly(urea–formaldehyde) shell was put into a vacuum tank filling with diethylenetriamine. Diethylenetriamine-containing microcapsules were obtained after the vacuum filtration process22. Jin et al.14 reported a binary self-healing material consisted of polyoxypropylenetriamine capsules and epoxy capsules, which satisfied the rigorous requirements for structural polymer composites cured at rising temperatures. The polyoxypropylenetriamine microcapsules were obtained by the method of vacuum infiltration. Hollow microcapsule with poly(urea–formaldehyde) shell was immersed into polyoxypropylenetriamine. Diuron-containing capsule with polystyrene (PS) shell were prepared by the method of solvent evaporation in an oil-in-water emulsion30. The cationic catalytic curing agent (C2H5)2O·BF3 microcapsules for self-healing material were synthesized via infiltrating method32. Microencapsulation of an anionic catalyst curing agent of epoxy had been rarely reported.

Compared with the reactive curing agent, catalytic curing agent doesn’t participate curing reaction and isn’t consumed by epoxy functional groups. It needs only a small amount to cure the epoxy resin. Catalytic curing agent includes cationic catalyst via cation initiated ring-opening polymerization and anionic catalyst via anion initiated ring-opening polymerization of epoxy. Tris(dimethylaminomethyl)phenol (DMP-30) is an anionic catalyst and it can also act as an effective promoter of the epoxy curing system. Therefore, DMP-30 is an ideal embedment hardener or promoter for binary self-healing epoxy. In practice, however, microencapsulation of DMP-30 is quite difficult due to its solubility in water and many organic solvents. To the authors’ knowledge, encapsulated anionic catalyst DMP-30 for self-healing materials has not been reported yet. In this work, two key strategies were implemented to prepare the microcapsules successfully. The first one is that a small amount of deionized water is added to DMP-30 to form complex, and stable emulsion of water-in-oil is prepared successfully. The second one is that high viscosity water solution prepared by adding Arabic gum and surfactant is used as the third phase so as to form a stable W/O/W emulsion system.

In this study, DMP-30-containing microcapsules were prepared successfully by solvent evaporation in a W/O/W emulsion. The influence of preparation condition on the properties of microcapsules was systematically investigated by orthographic factorial design of three factors and three levels focused on the results of average core content, diameter, and shell thickness. The optimum preparation parameters of microcapsules were concluded finally. The chemical structure and core reactivity of microcapsules were confirmed and further proved that core material was microencapsulated successfully by shell material. To provide the performance of microcapsules for making binary self-healing composites in the subsequent works, the properties including surface morphology, shell thickness, size distribution and average diameter, thermal stabilization, isothermal aging, and interfacial properties were investigated. The performance parameters of microcapsules for fabricating self-healing composites are concluded finally.

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