Dramatic role of fragility in determining the magnitude of T$_{\mathrm{g}}$ perturbations to ultrathin film layers and near-infinitely dilute blend components

Using fluorescence, we measure the glass transition temperatures (T$_{\mathrm{g}})$ of ultrathin (11-14 nm) polystyrene (PS, bulk T$_{\mathrm{g}} =$ 103 $^{\circ}$C) layers which can be tuned over $\sim$ 80 $^{\circ}$C when sandwiched between two bulk neighboring layers of poly(4-vinyl pyridine) (P4VP), polycarbonate, poly(vinyl chloride) (PVC) or poly(tert-butyl acrylate). Between P4VP, an ultrathin PS layer has its dynamics slaved and reports the T$_{\mathrm{g}}$ of bulk P4VP. In contrast, an ultrathin PS layer is weakly perturbed (T$_{\mathrm{g}} =$ 97 $^{\circ}$C) when placed between PVC. These perturbations to the PS T$_{\mathrm{g}}$ become evident even for layers 10s of nanometers in thickness. Additionally, binary blends were prepared with 0.1 wt{\%} PS components surrounded by the same neighboring polymers as in the trilayers. The T$_{\mathrm{g}}$ reported by an ultrathin PS layer and a 0.1 wt{\%} PS blend component are the same for a given polymer pair indicating that the T$_{\mathrm{g}}$ perturbations in these two systems arise from a common physical origin. The strength of perturbations to PS correlate with the fragility of the neighboring domain in both blends and multilayers indicating that it is a key variable in determining the strength of T$_{\mathrm{g}}$-confinement effects. Fragility also tracks with the magnitude of T$_{\mathrm{g}}$-confinement effects observed in single layer polymer films supported on silicon wafers.