Impact of Domain Size on the Length Scale of Local Glass Transition Perturbations Caused by an Immiscible Glassy–Rubbery Interface

The depth-dependent profile in local glass transition temperature Tg(z) was measured by pyrene fluorescence within 75 nm thick glassy polystyrene (PS) domains either capped by 600 nm thick poly(n-butyl methacrylate) (PnBMA) layers or exposed to the free surface. In both systems, the total PS domain size is constrained by a "neutral" nonperturbing silica substrate. Remarkably, for this constrained PnBMA/PS bilayer system, we find the perturbing influence of the 6-7 nm PnBMA/PS interface to be essentially equivalent to that imposed by the free surface, in stark contrast to the previously reported long-range Tg(z) perturbations of up to ≈250 nm for unconstrained glassy-rubbery interfaces between semi-infinite domains. For the 75 nm PS domains, both the PnBMA interface and free surface impart a local Tg(z) reduction of ≈30 K, spanning ≈30 nm before bulk Tg is recovered, demonstrating that the total domain size strongly alters both the magnitude and extent of the dynamical gradient even when bounded by a nonperturbing interface.