A set of high grafting density mixed poly(tert-butyl acrylate)(PtBA)/polystyrene (PS) brushes with PtBA number-average molecular weight (M-n) fixed at 18.6 k Da and PS M-n ranging from 8.7 to 28.0 k Da was synthesized from 172 nm asymmetric difunctional initiator (Y-initiator)-functionalized silica particles by sequential surface-initiated atom transfer radical polymerization of tert-butyl acrylate and nitroxide-mediated radical polymerization (NMRP) of styrene. The Y-initiator-functionalized particles were prepared by the immobilization of a triethoxysilane-terminated Y-initiator onto the surface of bare silica particles via a hydrolysis/condensation process. The overall grafting densities of the obtained mixed brushes were 0.9 -1.2 chains/nm(2), which were significantly higher than those of mixed brushes prepared from silica particles that were surface functionalized by a monochlorosilane-terminated Y-initiator (0.6-0.7 chains/nm(2)). Differential scanning calorimetry analysis showed that all high density mixed brushes exhibited two distinct glass transitions, suggesting that the two grafted polymers were microphase separated in the brush layer. TEM showed that with the increase of PS M-a from 8.7 to 28.0 k Da the morphology of the mixed brushes changed from mostly isolated PS nanodomains buried in the PtBA matrix to a nearly cocontinuous nanostructure and two-layered nanostructures composed of a laterally microphase-separated bottom layer and a thin PS top layer. These morphologies were similar to the asymmetric mixed brushes with grafting densities of 0.6-0.7 chains/nm(2). However, the feature sizes of the patterns formed from the microphase separation were much smaller. The observed grafting density effect on the pattern feature size was further confirmed from the study of a high grafting density mixed brush sample with PtBA M-n of 23.7 kat and PS M-n of 25.7 k Da.