Jin Tian

To improve the mechanical properties of polyaspartate polyurea (PAEP), functionalized multi-walled carbon nanotubes reinforced polyurea composites (HDIT-MWCNTs/PAEP) were prepared by covalently functionalizing multi-walled carbon nanotubes (MWCNTs) with hexamethylene diisocyanate (HDI) trimer. The dispersibility, wettability and interfacial properties of HDI trimer functionalized MWCNTs (HDIT-MWCNTs) in PAEP were analyzed. The tensile properties of HDIT-MWCNTs/PAEP composites were tested and the reinforcement mechanism revealed. The results showed that compared with the pure multi-walled carbon nanotubes (P-MWCNTs), HDIT-MWCNTs were uniformly dispersed in the matrix and the wettability and interfacial properties were greatly improved. The contact angle of PAEP with HDIT-MWCNTs was 32.92°, a reduction of 46.3% compared with the P-MWCNTs. The interfacial energy and adhesion work between PAEP and HDIT-MWCNTs (25.14 mJ m⁻², 95.47 mJ m⁻²) were 46.7% and 24.2% higher than those of P-MWCNTs. When the content of HDIT-MWCNTs was 0.5 phr, the tensile strength and Young’s modulus of the composites reached a maximum of 17.78 MPa and 170.31 MPa; an increase of 28.4% and 90.8% respectively, compared with pure PAEP. According to the cross-section of HDIT-MWCNTs/PAEP, its strengthening mechanism was mainly manifested by the deflection and bifurcation of cracks and the plastic deformation of PAEP matrix due to HDIT-MWCNTs pulling out from the matrix.

In order to improve the mechanical and tribological properties of epoxy composites, the hyperbranched polyesters with terminal carboxyl (HBP) were used for covalent functionalization of multi-walled carbon nanotubes (MWCNTs). The dispersibility and wettability of functionalized MWCNTs (HBP-NH₂-MWCNTs) in epoxy resin (EP) were analyzed. The HBP-NH₂-MWCNTs reinforced EP composites (HBP-NH₂-MWCNTs/EP) were prepared. The microhardness and fracture toughness of HBP-NH₂-MWCNTs/EP were tested, and the tribological behavior and mechanism under dry friction conditions were investigated. The results show that the hyperbranched polyesters coated on the surface of MWCNTs forms a physical barrier layer, blocking the van der Waals force between MWCNTs, effectively reducing the agglomeration phenomenon and significantly improving its dispersibility and wettability. Meanwhile, The flexible hyperbranched polyester on the surface of MWCNTs alleviates the stress concentration of the interface and improves the load transfer efficiency, which greatly improves the microhardness and fracture toughness. Under dry friction conditions, the friction coefficient and wear loss of HBP-NH₂-MWCNTs/EP composites showed a trend of decreasing first and then increasing as the content of HBP-NH₂-MWCNTs increased from 0.3 phr to 1.2 phr. When the content of HBP-NH₂-MWCNT is 0.6 phr, the composites has the lowest friction coefficient and the wear loss, and the wear mechanism is mainly characterized by slight fatigue wear.

In order to improve the interfacial properties of graphene oxide (GO) and epoxy resin (EP), hyperbranched polyesters with terminal carboxyl (HBP) non-covalently functionalized graphene oxide (HBP-GO) was achieved by strong π-π coupling between hyperbranched polyesters and GO nanosheets. The effects of non-covalent functionalization of GO on the dispersibility, wettability and interfacial properties were analyzed. The mechanical properties and enhancement mechanism of HBP-GO/EP composites were investigated. The results show that the hyperbranched polyesters is embedded in the GO layer due to its highly branched structure, which forms the steric hindrance effect between the GO nanosheets, effectively prevents the agglomeration of GO nanosheets, and significantly improved the dispersibility of GO. Simultaneously, the contact angle of HBP-GO with EP is reduced, the surface energy, interfacial energy and adhesion work are increased, then the wetting property of HBP-GO is significantly improved. The main toughening mechanism of HBP-GO is microcrack deflection induced by HBP-GO and plastic deformation of the EP matrix. In the microcrack propagation zones, HBP-GO may produce the pinning effect near the microcrack tips and change their stress state, resulting in microcrack deflection and bifurcation. So, the microcrack propagation path is more tortuous, which will consume much more fracture energy. Therefore, the mechanical properties of the HBP-GO/EP composites are greatly improved.