Structure and Transport Properties of Polymer Grafted Nanoparticles

摘要

We perform molecular dynamics simulations on a bead-spring model of pure polymer graftednanoparticles (PGNs) and of a blend of PGNs with a polymer melt to investigate the correlationbetween PGN design parameters (such as particle core concentration, polymer grafting density, andpolymer length) and properties, such as microstructure, particle mobility, and viscous response. Constantstrain-rate simulations were carried out to calculate viscosities and a constant-stress ensemblewas used to calculate yield stresses. The PGN systems are found to have less structural order, lowerviscosity, and faster diffusivity with increasing length of the grafted chains for a given core concentrationor grafting density. Decreasing grafting density causes depletion effects associated with thechains leading to close contacts between some particle cores. All systems were found to shear thin,with the pure PGN systems shear thinning more than the blend; also, the pure systems exhibited aclear yielding behavior that was absent in the blend. Regarding the mechanism of shear thinning atthe high shear rates examined, it was found that the shear-induced decrease of Brownian stresses andincrease in chain alignment, both correlate with the reduction of viscosity in the system with the latterbeing more dominant. A coupling between Brownian stresses and chain alignment was also observedwherein the non-equilibrium particle distribution itself promotes chain alignment in the direction ofshear.