Self-healing Materials: ROMP system
A well-reported microencapsulated system is a “ROMP” system: the microcapsules containing norborene-based monomers as healing agents and the Grubbs’ catalyst which can trigger “ROMP”.
“ROMP” stands for Ring-Opening Metathesis Polymerization. The Grubbs’ catalysts (1st, 2nd…generations) can initiate the polymerization and have advantages, such as high activity, tolerant to environment, moisture, oxygen and lots of other functionalities. But, they are very expensive.
What are suitable healing agents? 1. Capability of ROMP at room temperature or even lower, no extra heat needed; 2. Forming polymers with good mechanical properties: recover damaged matrix, thermoset preferred; 3. Low concentration of catalyst required; 4. Low viscosity: easy to encapsulated and filtrate to micro-cracks.
Commonly used norborene-based monomers, dicyclopentadiene (DCPD) and Ethylidene norbornene (ENB), both have very low viscosity and high ROMP activity, and meet the requirements.
To further tailor the properties of healing agents, the simplest way is mixing DCPD and ENB together. ENB has lower viscosity and higher reactivity but polymerizes into a linear polymer with a lower Tg than DCPD, which polymerizes into a cross-linked network. The modification of healing agents by mixing was reported by our collaborators.
Another modification method is to introduce extra cross-linkers. My project done in the first year of Ph.D. study is synthesis of cross-linkers and characterization of healing agents, such as cure kinetics, viscosity, and thermal-mechanical properties. More details were described in the published papers.