Local glass transition temperature Tg(z) of polystyrene next to different polymers: Hard vs. soft confinement

The depth to which the local glass transition temperature T-g and alpha-relaxations are perturbed near a boundary is believed to be related to the characteristic length scales associated with cooperative dynamics in dynamically heterogeneous glasses. Following our recent work [R. R. Baglay and C. R. Roth, J. Chem. Phys. 143, 111101 (2015)] that measured a very broad 350-400 nm local T-g(z) profile across a glassy-rubbery interface of polystyrene (PS)/poly(n-butyl methacrylate) (PnBMA), we compare here how the T-g(z) profile in PS varies when changing the neighboring polymer from a lower Tg material to a higher Tg material. Here we report local Tg(z) profiles for PS when in contact with polysulfone (PSF), poly(methyl methacrylate) (PMMA), and poly(isobutyl methacrylate) (PiBMA). We find that the distance from the interface before bulk Tg of PS (T-g(bulk) = 101 degrees C) is recovered depends on whether PS forms the high-Tg glassy component experiencing so-called soft confinement, z approximate to 225-250 nm for PS next to PiBMA (T-g (bulk) = 62 degrees C) and PnBMA (T-g (bulk) = 21 degrees C), or PS forms the low-Tg rubbery component experiencing hard confinement, z approximate to 100-125 nm for PS next to PSF (T-g (bulk) = 186 degrees C) and PMMA (T-g (bulk) = 120 degrees C). The depth to which these T-g(z) perturbations persist and the magnitude of the local Tg perturbation at the interface are independent of the difference in Tg bulk between the two polymers, the interaction parameter, and the chemical structure. We demonstrate that these broad, extended Tg(z) length scales appear to be universal across these different systems but show that the strong dynamical coupling across the dissimilar polymer-polymer interface only occurs when this interface has been annealed to equilibrium. We consider why dissimilar polymer-polymer interfaces exhibit continuous local dynamics across the interface in contrast to polymer-free surface, polymer-substrate, or polymer-liquid interfaces that show discontinuous local dynamics. Published by AIP Publishing.