Optimizing the Grafting Density of Tethered Chains to Alter the Local Glass Transition Temperature of Polystyrene near Silica Substrates: The Advantage of Mushrooms over Brushes

We measured the local glass transition temperature T-g(z) of polystyrene (PS) as a function of distance z from a silica substrate with end-grafted chains using fluorescence, where competing effects from the free surface have been avoided to focus only on the influence of the tethered interface. The local T-g(z) increase next to the chain grafted substrate is found to exhibit a maximum increase of 49 +/- 2 K relative to bulk at an optimum grafting density that corresponds to the mushroom-to-brush transition regime. This perturbation to the local T-g(z) dynamics of the matrix is observed to persist out to a distance z approximate to 100-125 nm for this optimum grafting density before bulk Tg is recovered, a distance comparable to that previously observed by Baglay and Roth [J. Chem. Phys. 2017, 146, 203307] for PS next to the higher-T-g polymer polysulfone.