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

• Published in: The Journal of chemical physics 2017-05, Vol.146 (20), p.203307
• Main Authors: Baglay, Roman R., Roth, Connie B.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4975168

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 T g material to a higher T g material. Here we report local T g(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 T g of PS ( T g bulk = 101 °C) is recovered depends on whether PS forms the high-T g glassy component experiencing so-called soft confinement, z ≈ 225-250 nm for PS next to PiBMA ( T g bulk = 62 °C) and PnBMA ( T g bulk = 21 °C), or PS forms the low-T g rubbery component experiencing hard confinement, z ≈ 100-125 nm for PS next to PSF ( T g bulk = 186 °C) and PMMA ( T g bulk = 120 °C). The depth to which these T g(z) perturbations persist and the magnitude of the local T g perturbation at the interface are independent of the difference in T g bulk between the two polymers, the interaction parameter, and the chemical structure. We demonstrate that these broad, extended T g(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.