We report in this article the synthesis of a series of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes with PtBA number-average molecular weight (M-n) being fixed at 24.5 kDa and PS M-n ranging from 14.8 to 30.4 kDa on 160 nm silica particles and the study of their microphase separation behaviors using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The samples were synthesized by a two-step "grafting from" process from asymmetric difunctional initiator (Y-initiator)functionalized silica particles using two different "living"/controlled radical polymerization techniques. PtBA brushes were grown first from Y-initiator-functionalized particles by surface-initiated atom transfer radical polymerization of tert-butyl acrylate at 75 degrees C in the presence of a free initiator, followed by nitroxide-mediated radical polymerization (NMRP) of styrene at 120 degrees C. The "living" nature of NMRP allowed the synthesis of mixed PtBA/PS brushes with different PS molecular weights (14.8, 18.7, 24.9, and 30.4 kDa) in a one-pot polymerization. DSC studies showed that all thermally annealed mixed brush samples exhibited two glass transitions with the middle points located at similar to 47 and similar to 90 degrees C, suggesting that the two tethered polymers microphase separated in the brush layer. For TEM studies, the samples were dispersed in CHCl3, a good solvent for both PtBA and PS, drop-cast onto carbon films, thermally annealed in vacuum at 120 degrees C for 3 h, and then stained with RuO4 vapor. All samples showed clear microphase separation, consistent with the DSC results. With increasing PS M-n from 14.8, to 18.7, and 24.9 k Da, the morphology of mixed brushes evolved from isolated, nearly spherical PS microdomains buried inside the PtBA matrix, to short PS cylindrical microdomains in the PtBA matrix, and to nearly bicontinuous nanostructures. Further increasing the molecular weight of PS to 30.4 k Da resulted in the formation of isolated PtBA microdomains which were covered by PS chains.