Molecular dynamics simulation of the viscoelasticity of polymer nanocomposites under oscillatory shear: effect of interfacial chemical coupling.

In this work by adopting coarse-grained molecular dynamics simulation, we focus our attention on investigating the effect of the chemical coupling between polymer and nanoparticles (NPs) on the viscoelastic properties of polymer nanocomposites (PNCs). Firstly we examine the effect of the interfacial chemical coupling on the non-linear behavior, such as the change of the storage moduli, the loss moduli and the loss factor as a function of the strain amplitude. Besides the reinforcing effect contributed by the interfacial chemical interaction, a much smaller loss factor is also observed attributed to less molecular friction and dissipation. Meanwhile, the effects of temperature, frequency, and the interfacial physical interaction between NPs and polymers on the viscoelastic properties are also probed. To uncover the structural and dynamic effect of the interfacial chemical coupling, we calculate the radial distribution function of polymer chains around NPs, the content of the polymer beads in the first layer of the interfacial region under quiescent and dynamic conditions, the incoherent intermediate dynamic structure factor of the polymer beads, which are chemically or physically tethered to the NPs, and all the polymer beads of the system, the quantitative comparison of the mean relaxation time for different interfacial chemical coupling, and the mean-square displacement of the polymer chains. Lastly we analyze the change of the interfacial energy such as the physical and chemical energies during oscillatory shear. Through these analyses, we conclude that with the increase of the interfacial chemical coupling, the change extent of the interfacial physical interaction the periodic strain decreases, attributed to a much smaller adsorption-desorption reversible process. This can rationalize the much weaker non-linear behavior or the "Payne effect". Based on these results, we anticipate that a better molecular-level understanding is provided on the effect of the interfacial coupling on the viscoelastic properties of PNCs.