  The local glass transition temperature Tg of pyrene-labeled polystyrene (PS)
chains intermixed with end-tethered PS chains grafted to a neutral silica
substrate was measured by fluorescence spectroscopy. To isolate the impact of
the grafted chains, the films were capped with bulk neat PS layers eliminating
competing effects of the free surface. Results demonstrate that end-grafted
chains strongly increase the local Tg of matrix chains by $\approx$45 K
relative to bulk Tg, independent of grafted chain molecular weight from Mn =
8.6 to 212 kg/mol and chemical end-group, over a wide range of grafting
densities $\sigma$ = 0.003 to 0.33 chains/nm$^2$ spanning the mushroom-to-brush
transition regime. The tens-of-degree increase in local Tg resulting from
immobilization of the chain ends by covalent bonding in this athermal system
suggests a mechanism that substantially increases the local activation energy
required for cooperative rearrangements.
