Effect of nanoparticles on the micromechanical and surface properties of poly(urea-formaldehyde) composite microcapsules

Microcapsules containing self-healing agents have been used to repair microcracks in polymeric matrices. These microcapsules must possess special properties, such as appropriate strength and stability in the surrounding matrix. Herein, poly(urea-formaldehyde) (PUF) microcapsules containing dicyclopentadiene (DCPD) were prepared by in situ polymerization. The elastic modulus and hardness of the microcapsules with and without a nanocomposite shell wall reinforced with carbon nanotubes and nanoalumina were examined using the nanoindentation method. The surface morphology, topography and roughness were investigated with scanning electron microscopy (SEM), optical microscope (OM), as well as atomic force microscopy (AFM). The results demonstrated significant increases in the elastic modulus and hardness due to the presence of reinforcement nanoparticles. In addition, it has been founded that the microcapsules with nanoalumina in the shell wall were stiffer and harder than the other microcapsules. The surface roughness parameters obtained from the AFM images showed that the nanoalumina nanoparticles resulted in a smoother surface of the microcapsules. In addition, the absence of nanoparticles in the shell wall resulted in the formation of microcapsules with rougher surfaces. Finally, the calculated plasticity index for the microcapsules increased with the addition of the nanoparticles. The results indicate that the PUF shell behaves as a viscoelastic-plastic material.