(203c) Quantifying Nearly Isochoric Glass Formation and the Extent of Physical Aging toward Equilibrium in Confined Polymer Films

When confined to the nanoscale, the physical aging behavior of polymer films can deviate substantially from the bulk behavior. Using fluorescence intensity measurements, we have recently shown that as film thickness is decreased, poly(methyl methacrylate) (PMMA) films supported on silica, which result in hydrogen bonds between the hydroxyl groups on silica and the ester groups on PMMA, exhibit a reduction in physical aging rate. Through the use of a fluorescence / multilayer approach, in which physical aging was selectively measured at either the air-polymer or substrate-polymer interface, we showed that aging was reduced at both interfaces compared to the film interior.

In this study, a novel fluorescence approach in used to investigate both the glass transition temperature (Tg) and physical aging of PMMA films supported on silica. In particular, we compare quantitatively the extents of aging toward equilibrium in bulk and ultrathin polymer films and provide evidence of nearly isochoric (constant volume) glass formation in rapidly quenched, ultrathin films. When aged deep in the glass state, an ultrathin film ages more slowly than a bulk film. When aged over a 4000 min aging time deep in the glassy state, a 35-nm-thick film ages 1.6% of the extent needed to reach its equilibrium state while the bulk film ages 5.0% of the extent needed to reach its equilibrium state. Thus, the attractive polymer-substrate interactions (hydrogen bonds) that result in the ultrathin film undergoing nearly isochoric glass formation and having a greater departure from equilibrium also result in the film having reduced aging through restriction of segmental mobility.