Molecular Dynamics Simulations on the Tribological Properties of the Tunable Block Copolymers of Epoxy/Polyurethane in Different Temperatures

In order to investigate the tribological properties of copolymer at different temperatures, epoxy/polyurethane copolymer systems (2EP/PU and 2PU/EP) with molecular ratios of 2:1 and 1:2 were designed. Molecular dynamics (MD) simulations were conducted to simulate the physical performance and interfacial shear behavior of the sliding system. Firstly, the density, electrostatic interaction and van der Waals force of copolymers 2EP/PU and 2PU/EP at different temperatures before and after optimization were compared and analyzed. Moreover, the mechanical properties at 223 K, 298 K and 373 K were investigated. The results showed that the Young's modulus of 2PU/EP increased by 8.8%, 7.1% and 1.7% at 223 K, 298 K and 373 K, respectively, compared with that of 2EP/PU. It was mainly attributed to the linear structure of polyurethane, which increased the bulk density of molecular chains and stabilized the state of copolymer. At the same time, the sliding friction model between copolymer and iron layer was established, and the friction and wear properties of the materials were analyzed at atomic level. The lowest friction coefficient of 2EP/PU at 223 K was 0.067, which was nearly 40% lower than that at 298 K and 373 K. The friction coefficient of 2PU/EP was 0.105 at 373 K, and increased to 0.112 and 0.115 at 223 and 298 K, respectively. It was speculated that at 373 K, the electrostatic (Coulomb) interaction and van der Waals interaction between 2PU/EP molecules increased, which weakened the interaction between the Fe layer and the copolymer layer. In addition, the friction coefficient of 2PU/EP was higher than 2EP/PU, because the strong polarity of PU enhanced the interaction between the sliding interfaces and impeded the frictional shear process.

Comparing the total energy of 2EP/PU and 2PU/EP sliding systems, it was found that total energy of both copolymers was almost the same. Nevertheless, the temperature had a great influence on the molecular interaction energy. At 373 K, the total energy of 2EP/PU and 2PU/EP was about 4 250 kcal/mol. Decreasing the temperature, the total energy of both materials reduced. The energy of 2EP/PU and 2PU/EP were 3 500 kcal/mol and 2 500 kcal/mol at 298 K and 223 K, respectively, which was mainly related to the interaction energy between molecules. At high temperature, the molecular chain was relatively active, and the motion energy was large, while at low temperature, the molecules were almost frozen, and the motion energy was small. Combined with the radial distribution function result, it could be also confirmed that polymer chains were easier to aggregate at low temperature. With the increase of temperature, molecular chains gradually extended. Based on the relative concentration of atoms, it was confirmed that the two copolymer materials were mainly concentrated on the friction interface, so it was easier to form a transfer film. At 223 K, the relative concentrations of 2EP/PU and 2PU/EP were higher in the later stage of the movement, so wear may be more serious.