Understanding How Grafted Chains Create a Gradient in Local Glass Transition Tg(z) in the Athermal Polystyrene-Silica System

Surfaces with grafted polymer chains are of fundamental interest to the field of nanocomposites due to their widespread use in controlling dispersion of nanoparticles and likely perturbing local material properties of the matrix. Developing understanding in these materials is limited by the experimental difficulty of measuring how local properties are altered near interfaces. We focus on a model athermal system of polystyrene (PS) matrix chains intermixed with grafted PS chains at a silica interface in a planar film geometry. We investigate how the local glass transition temperature Tg(z) profile near the silica interface is altered by the presence of grafted chains of different lengths using a localized fluorescence method. Right at the silica interface, the Tg(z=0) of the pyrene-labeled PS matrix chains is found to be elevated by approximately 45 K relative to bulk Tg, regardless of grafted chain length and grafting density spanning the mushroom-to-brush transition regime. The independence of Tg(z=0) on these factors suggests that the mechanism for this Tg increase likely involves pinned chain ends inducing locally higher activation barriers for alpha relaxation, in a manner similar to ionomers, vitrimers, and other associating polymers.