Molecular Dynamics Study on the Rheology of Polymer-Nanoparticles Mixture: Role of Inter-particle Interactions

We are motivated by experimental findings that mechanical reinforcement depends strongly on particle morphology in polymer nanocomposites (PNC). The results show that percolated particle morphologies offer a higher mechanical reinforcement than others. To understand this, we perform coarse-grained MD simulations on PNC by varying the polymer-particle interactions. A key finding is that while samples where the NPs are well-dispersed and clusters correspond to equilibrium states, others, such as percolated NP, depend on the preparation method. We explore the linear response of these samples using the Green-Kubo method and find that particle ordering strongly affects the temporal dependence of the stress-stress autocorrelation, with the percolated sample showing a solid-like response. Since their stress autocorrelation does not decay to zero, we perform steady and oscillatory shear simulations to obtain their storage ($G^{prime}$) and loss modulus $G^{primeprime}$). The percolated and dispersed samples show shear thinning over the range of accessible shear rates ($dot{gamma}$), and the ($G^{prime}$) and $G^{primeprime}$) of networked structures over an accessible range of angular frequency ($omega$) show a plateau characteristic of long relaxation time. Our results unequivocally verify the experimental conjectures that percolated networks provide maximum mechanical reinforcement in PNC systems.