Stimuli-responsive microcapsules for self-protecting and self-healing material systems

Stimuli-responsive microcapsules are an attractive route to achieve smart functionality in material systems. Microcapsules isolate the active material from the surrounding environment while allowing programmable on-demand delivery in response to a variety of environmental triggers. These added functionalities are embedded in a material without large scale modification of the virgin material components. Despite significant progress in production of stimuli-responsive microcapsules, the ability to incorporate them in industrial environments remains a challenge due to intricate fabrication techniques, scalability, cost-effectiveness and material limitations. The research described here expands the fabrication techniques for production of stimuli-responsive microcapsules and subsequently, demonstrates a variety of smart microcapsules to develop self-protecting and self-healing material systems. Monodisperse, stimuli-responsive microcapsules are required for applications involving precise and controlled delivery of chemical payloads. However, such microcapsules are difficult to fabricate with high throughput and control over capsule geometry and shell wall properties, especially in the presence of organic solvents. In the first section of this research, an emulsification technique is adapted based on the interfacial tension of immiscible phases for the fabrication of monodisperse emulsion templates and microcapsules. In this technique, either one (single emulsion) or two (double emulsion) dispersed phases are simultaneously delivered while reciprocating across the interface of a stationary immiscible continuous phase. The interfacial tension of the continuous phase results in the separation of a monodisperse droplet in every cycle. Monodisperse single emulsion-templated microcapsules with cyclic poly(phthalaldehyde) (cPPA) and polymethacrylate (Eudragit E100) shell walls are formed with hydrophobic cores. The acid-triggered release of Eudragit and cPPA microcapsules containing an oil core is demonstrated. Tunable, monodisperse double emulsion templates with an aqueous core are formed with sizes ranging from 295 µm to 1200 µm. The double emulsion templates are converted to monodisperse, responsive microcapsules with a hydrophilic core through photocuring or selective solvent evaporation to form the polymer shell wall. Microcapsules with polymeric shell walls based on photocurable polyisocyanurate, cPPA and polylactide are fabricated. The acid-triggered release of cPPA microcapsules containing an aqueous core with a slower degradation rate is also demonstrated. Excellent control is achieved over the emulsion templates and microcapsules, with polydispersity less than 2% and the ability to predict the size reliably based on process parameters. Photo-degradable microcapsules are reported with cPPA shell wall containing an oil (single emulsion-templated) or aqueous (double emulsion-templated) core. Photo-responsive properties are achieved by incorporating a photoacid generator (PAG) additive to the polymeric shell wall. Localized generation of acid leads to acid-catalyzed depolymerization of cPPA. The capsules rapidly release their core material in under a minute with only 30 s of UV exposure. The photo-triggered response is further tuned by varying the PAG concentration or exposure period of UV light. The strategy is potentially generalizable to other acid degradable polymers. Subsequently, two applications of stimuli-responsive microcapsule-based material systems are demonstrated. Encapsulated anticorrosion agents provide a suitable alternative to dispersion of metal-based compounds in protective polymeric coatings on metal substrates. Stimuli-responsive microcapsules enhance protection abilities by autonomously responding to corrosion-induced environmental changes. pH-responsive microcapsules are reported with triggered release over a wide range of acidic pH values (pH < 6) and robust enough to be incorporated in commercial solvent-based epoxy coatings. The pH responsiveness is achieved by integrating acid labile ketals that undergo rapid hydrolysis within the cross-linked polyamide shell and readily release acetylenic diol or jojoba oil as the anticorrosive agent. The microcapsules are stable up to a temperature of 150 °C and provide long-term room temperature stability up to 3 months. Degradation and release kinetics of the microcapsules are quantified at various pH levels (1 ≤ pH ≤ 9) using NMR and gas chromatography, respectively. Microcapsules exhibit complete release in under 5 min at pH 1 to approximately 2 hours at pH 5. Coating performance is evaluated by electrochemical corrosion tests conducted in 5 wt% salt solutions with varying pH and concentration of the microcapsules in the coating. Inhibition efficiencies up to 70% are achieved in acidic saltwater solutions. Microcapsule-based strategies are attractive to achieve self-healing in composites as they are autonomous, scalable and avoid significant modification of the material components. Capsule-based self-healing has been widely shown in bulk polymers, and to some extent in thermoset composites, but autonomous healing in fiber-reinforced thermoplastic composites remains a challenge. The healing strategy described in this work utilizes multi-shell walled dual microcapsule system coated with polydopamine and containing a methacrylate-alkyl borane-based chemistry. The monomeric healing agents will undergo polymerization upon release from ruptured microcapsules within a damage zone. The core content of the microcapsules is quantified using NMR, and thermogravimetric analysis reveals prolonged thermal stability at temperatures of at least up to 180 °C. Subsequently, the microcapsules are functionalized on glass fibers to recover the interfacial bond strength at room temperature following complete glass fiber/polyethylene debonding. The healing efficiency increases with higher capsule coverage and healing time, with almost full recovery of interfacial shear strength obtained at 77% capsule coverage in 48 h. In addition to room-temperature autonomous healing, this healing strategy is also applicable to a variety of thermoplastic matrices.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste