The Mechanical Properties of Poly (Urea-Formaldehyde) Incorporated with Nano-SiO₂ by Molecular Dynamics Simulation

Self-healing materials can promote the sustainable reuse of resources. Poly (urea-formaldehyde) (PUF) microcapsules can be incorporated into dielectric materials for self-healing. However, the mechanical properties of PUF microcapsules need to be improved due to insufficient hardness. In this paper, PUF models incorporated with nano-SiO₂ of different filler concentrations (0, 2.6, 3.7, 5.3, 6.7, 7.9 wt.%) were designed. The density, the fractional free volume, and the mechanical properties of the PUF-SiO₂ models were analyzed at an atomic level based on molecular dynamics simulation. The interfacial interaction model of PUF on the SiO₂ surface was also constructed to further investigate the interaction mechanisms. The results showed that the incorporation of nano-SiO₂ had a significant effect on the mechanical properties of PUF. Density increased, fractional free volume decreased, and mechanical properties of the PUF materials were gradually enhanced with the increase of nano-SiO₂ concentration. This trend was also confirmed by experimental tests. By analyzing the internal mechanism of the PUF-SiO₂ interfacial interaction, it was found that hydrogen bonds play a major role in the interaction between PUF and nano-SiO₂. Moreover, hydrogen bonds can be formed between the polar atoms of the PUF chain and the hydroxyl groups (-OH) as well as O atoms on the surface of SiO₂. Hydrogen bonds interactions are involved in adsorption of PUF chains on the SiO₂ surface, reducing the distance between PUF chains and making the system denser, thus enhancing the mechanical properties of PUF materials.