The viscoelastic behavior of polymer grafted nanocomposites (PGNs) with significantly different glass transition temperatures (Tgs) 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. This unique thermal stiffening behavior was attributed to the dynamic coupling of the high-Tg adsorbed chains and low-Tg matrix chains. The PGN studied in the current work consists of a nanoparticle grafted with high-Tg chains embedded in a low-Tg polymer matrix. The effect of the dynamic coupling of the grafted and matrix polymer chains is studied by molecular dynamics simulations. The influence of the graft brush height and matrix density on viscoelastic properties is investigated to identify the mechanism of the observed stiffening in these types of PGNs. This material is based upon work supported by the National Science Foundation under Grant No. CMMI-1825254.