New Capsule Chemistries for Nanoscale Self-healing

Introduction Self-healing materials are being developed to improve the lifetime of materials used in various applications including coatings, adhesives, structural components, and even microelectronics. The approach of using microencapsulation for self-healing has been particularly promising with proven success in coatings and in bulk epoxy. However, the size of microcapsules limits their use to materials with larger features. In order to apply this approach to materials with smaller features, encapsulation of healing agent on a smaller size-scale must be achieved. The current scheme for healing using microcapsules consists of a twopart healing system: dicyclopentadiene (DCPD) monomer and Grubbs catalyst. The capsule shells consist of polyureaformaldehyde (PUF). DCPD is first emulsified into an aqueous solution of surfactant using a mixer blade to apply shear force. Urea and formaldehyde react in the aqueous phase to form oligomers. After depositing at the o/w interface, the oligomers further polymerize on the surface of the DCPD droplets. Using this method, capsules can be synthesized with diameters as small as 10 μm in diameter. This size can be reduced further using a higher shear force or by changing the chemical characteristics of the solution. In this research, we use sonication to reduce the average capsule diameter to less than 1.5 μm. Ultrahydrophobes have been shown to further reduce the average capsule diameter to 300 nm. However agglomeration is observed in these capsules, limiting their use in bulk material. A second goal of this work is to modify the surface of these capsules to eliminate agglomeration and improve dispersion in epoxy. It is expected that inorganic shells such as those formed by silica can improve the rigidity of the capsules and open up new opportunities for chemical functionalization. This in turn would limit the surface area of contact, reducing the Van der Waals interactions between the colloids. Together with functionalization, better dispersion of capsules in matrix is expected. Silica has been regrown on many surfaces such as polystyrene colloids and gold nanoparticles. Functionalization of the capsules can be accomplished using common sizings found in the composite industry. In this work, Glycidoxypropyltrimethoxysilane (GLYMO) has been used since it has two functionalities: a siloxane bridge to react with the silica surface and an epoxide functionality that matches the monomer functionality of the matrix. The epoxide functionality is expected to improve dispersion in the epoxy resin before curing. During the curing process, it can react with the matrix to improve the bond between the capsules and the matrix. In the future, additional functionalities can be investigated for different matrices.