Influence of Graft Density on Dynamically Coupled Polymer Grafted Nanocomposites*

The viscoelastic behavior of polymer grafted nanocomposites (PGNs) with significantly different glass transition temperatures (T_gs) between the graft and matrix polymers is investigated with molecular dynamics simulations. These types of PGNs have been shown to have reversible and repeatable stiffening behavior upon heating (Senses, E.; Isherwood, A.; Akcora, P. ACS Appl. Mater. Interfaces 2015, 7, 14682). This unique thermal stiffening behavior was attributed to the dynamic coupling of the high-T_g adsorbed chains and the low-T_g matrix chains. The PGN studied in the current work consists of a nanoparticle with grafted high-T_g polymer chains in a low T_g polymer matrix. The effect of the dynamic coupling of the grafted and matrix polymer chains is studied by molecular dynamics (MD) simulations. Results of MD simulations indicate that grafted chains drastically slow down matrix chain dynamics and also form a percolated network at high nanoparticle concentrations. The influence of the graft density on viscoelastic properties is investigated to identify the mechanism of the observed stiffening in these types of PGNs.