Rheological mechanism of polymer nanocomposites filled with spherical nanoparticles: Insight from molecular dynamics simulation

Abstract

It is very crucial to understand the rheological behavior of polymer nanocomposites (PNCs) on the molecular level, which is very important for their processing and application. Thus, in this work the reverse nonequilibrium molecular dynamics simulation is employed to explore the rheological property of PNCs by tuning the volume fraction of nanoparticle (NP), the polymer-NP interaction and the NP size. The shear viscosity (η~γ˙-m) exhibits a power law with the shear rate where m varies from 0.42 to 0.53 at the high shear rates. By adopting the Carreau-Yasuda model, the calculated zero-shear viscosity gradually rises with increasing the volume fraction of NP, the polymer-NP interaction or reducing the NP size. This is attributed to the strong adsorption of chains by NPs and the formed network, which leads to the retarded dynamics. In addition, both the first and second normal stress differences also show the power laws on the shear rate. The chains are gradually extended as the increase of the shear rate, which is characterized by the mean-square end-to-end distance and the mean square radius of gyration. Especially, the evolution process of the NP network and the polymer-NP network is analyzed to deeply understand the shear thinning behavior. The number of the direct contact structure of NPs increases while the number of the polymer-NP bridged structure is reduced. This is further proved by the increase of the formation probability of the NP network and the decrease of the polymer-NP interaction energy. Finally, the chain dynamics is found to be enhanced due to the shear flow. In summary, this work provides a further understanding on the mechanism of the shear thinning of PNCs on the molecular level.