Polymer-tethered nanoparticles provide a strategy to improve particle dispersion in polymer nanocomposites and as materials themselves can exhibit self-healing behavior and enhanced mechanical properties. The few studies that previously characterized the glass transition temperature (T-g) behavior of neat polymer-grafted nanoparticles in the absence of a polymer matrix largely focused on average T-g response. We synthesized polystyrene-grafted silica nanoparticles (Si-PS) via ARGET ATRP, achieving the densely grafted state. Using differential scanning calorimetry, we investigated the brush molecular weight (MW) dependence of T-g, T-g breadth, heat capacity jump (Delta C-p), and fragility from 12 to 98 kg/mol. Compared with free PS chains of the same MW, brush Tg increases by 1-2 degrees C, brush Tg breadth remains unchanged within error down to 36 kg/mol and increases by 3-4 degrees C at brush MWs of 12 and 13 kg/mol, and,rush Delta C-p and fragility remain unchanged within error down to 52 kg/mol and then decrease with decreasing MW. Evidence of a significant Tg gradient from near the nanoparticle graft interface to near the free chain end was obtained for the first time via fluorescence of a pyrenyl dye labeled at specific regions along the brush chain length. In relatively high MW brushes, T-g = similar to 116 degrees C near the graft interface and T-g = similar to 102 degrees C near the chain end. Comparisons are made to results recently reported for similar PS brushes densely grafted to a flat substrate, which indicate that a larger T-g gradient is evident in a grafting geometry involving a flat interface as compared with a spherical nanoparticle interface. Other comparisons are also made with glass transition and fragility behaviors reported in the flat substrate geometry. Results of this study and others will help to better understand nanocomposites and tailor them for optimal properties.