Local Glass Transition Temperature Gradient in Polystyrene Matrices Caused by End-Grafted Chains Exhibits Inverse Molecular Weight Dependence of Grafted Chains

Grafted polymer chains are used extensively in composite materials and elsewhere for various purposes, but their impact on local material properties is not well understood. Here, we map the local matrix glass transition temperature Tg(z) in polystyrene (PS) as a function of distance from a silica substrate with PS end-grafted chains of different chain lengths. This is a system for which the local Tg is not perturbed in the absence of the grafted constraint of the chain end. With a localized pyrene fluorescence method, we find that the distance of ∼100 nm over which Tg(z) returns to bulk from the +45 K elevated Tg(z = 0) value at the grafted interface spans far beyond the extent of the grafted chains, from 6 to 16 times 2Rg. Interestingly, the range of the Tg(z) gradient depends on the grafted chain length with an inverse molecular weight scaling that is similar to Tg(M) in bulk polymers. We discuss the implications of this similarity and the possible mechanisms by which grafted chain connectivity could alter glassy dynamics. The breadth of the observed transitions in fluorescence intensity versus temperature is also characterized.