The Mechanical Properties of Poly (Urea-Formaldehyde) Incorporated with Nano-SiO(2) 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-SiO(2) 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-SiO(2) models were analyzed at an atomic level based on molecular dynamics simulation. The interfacial interaction model of PUF on the SiO(2) surface was also constructed to further investigate the interaction mechanisms. The results showed that the incorporation of nano-SiO(2) 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-SiO(2) concentration. This trend was also confirmed by experimental tests. By analyzing the internal mechanism of the PUF–SiO(2) interfacial interaction, it was found that hydrogen bonds play a major role in the interaction between PUF and nano-SiO(2). 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 SiO(2). Hydrogen bonds interactions are involved in adsorption of PUF chains on the SiO(2) surface, reducing the distance between PUF chains and making the system denser, thus enhancing the mechanical properties of PUF materials.