Self-healing materials with ROMP healing agent system are well developed. However, the cost of ROMP system is very high. Assume that the matrix is made of epoxy, which is around $0.10/gram. The healing agent, DCPD, is a by-product of refinery, and it is $0.07/g even cheaper than epoxy matrix. The catalyst which triggers the ROMP, is really expensive, $71.80/g. Considering the composition of self-healing material, the overall cost of per gram self-healing material would be more than $1.89/g if take the processing cost in account. Does anyone want a self-healing materials cost so much? No, we need develop a brand new low-cost system with similar or better self-healing functionalities.
The azide/alkyne ‘click’ reaction is a recent re-discovery, and has many advantages: 1) high yielding; 2) reactive in ambient conditions; 3) no or benign solvents; 4) fast reaction kinetics; 5) minimal or inoffensive byproducts. All of these meets the healing agent requirements for fabricating a successful self-healing materials.

The most common Click reaction is called CuAAC, Cu-catalyzed Azide-Alkyne Cycloaddition, as shown as in the figure. Several azides and alkynes are designed and successfully synthesized. The overall cost of the CuAAC system we are working on is around $0.27/g, much cheaper than the self-healing materials based on 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.

Microcapsules are one of the most crucial factors in fabricating self-healing materials, and must be carefully engineered.

Making these microcapsules is a complex problem with many requirements: the encapsulation procedure must be compatible with the reactive healing agent, the liquid healing agent must not diffuse out of the capsule wall during its potentially long shelf-life, and the microcapsule walls must be strong and tough enough to handle the processing conditions of the host composite, while maintaining excellent adhesion with the cured polymer matrix to ensure that the capsules rupture upon composite fracture .