How Does the Length of End-grafted Polystyrene Chains Alter the Spatial Gradient in Local Glass Transition Temperature Tg(z) Near Silica Interfaces?

A first-principles understanding of how grafted chains alter material properties in polymer nanocomposites is not yet solidified due to the experimental difficulty of spatially resolving changes to material properties, especially within composite materials. We address this by assembling multilayer samples that enable using a localized fluorescence method to spatially resolve the local glass transition temperature Tg(z) at different positions z from the substrate interface. Focusing on polystyrene (PS) matrix chains intermixed with grafted PS chains at the silica interface allows us to isolate the topological changes to the local dynamics caused by grafting, as bare PS/silica interfaces do not perturb local Tg. Our recent work showed large +45 K increases in local Tg(z=0) next to the grafted silica interface, independent of grafted chain length between Mn = 9 kg/mol and 200 kg/mol and grafting density across the mushroom-to-brush transition regime. Here, we expand on that work by measuring the spatial gradient in local Tg(z) as a function of distance from the interface, constructing Tg(z) profiles for varying grafted chain lengths. We observe long gradients in Tg(z), significantly longer than the extent of the grafted chains themselves, with a saturating molecular weight dependence reminiscent of Tg(Mn) in bulk.