Nanocapsules for self-healing materials

We report an in situ encapsulation method demonstrating over an order of magnitude size reduction for the preparation of urea–formaldehyde (UF) capsules filled with a healing agent, dicyclopentadiene (DCPD). Capsules with diameters as small as 220 nm are achieved using sonication techniques and an ultrahydrophobe to stabilize the DCPD droplets. The capsules possess a uniform UF shell wall (77 nm average thickness) and display good thermal stability. By controlling the ζ-potential, the capsules are uniformly dispersed in an epoxy matrix and shown to cleave rather than debond upon fracture of the matrix. Mechanical properties of the epoxy/capsule composite, including mode-I fracture toughness, elastic modulus, and ultimate tensile strength are measured and compared to previous data for larger capsules (ca. 180 μm).