Large Tg Shift in Hybrid Bragg Stacks through Interfacial Slowdown

Abstract

Studies of glass transition under confinement frequently employ supported polymer thin films, which are known to exhibit different transition temperature Tg close to and far from the interface. Various techniques can selectively probe interfaces, however, often at the expense of sample designs very specific to a single experiment. Here, we show how to translate results on confined thin film Tg to a “nacre-mimetic” clay/polymer Bragg stack, where periodicity allows to limit and tune the number of polymer layers to either one or two. Exceptional lattice coherence multiplies signal manifold, allowing for interface studies with both standard Tg and broadband dynamic measurements. For the monolayer, we not only observe a dramatic increase in Tg (∼ 100 K) but also use X-ray photon correlation spectroscopy (XPCS) to probe platelet dynamics, originating from interfacial slowdown. This is confirmed from the bilayer, which comprises both “bulk-like” and clay/polymer interface contributions, as manifested in two distinct Tg processes. Because the platelet dynamics of monolayers and bilayers are similar, while the segmental dynamics of the latter are found to be much faster, we conclude that XPCS is sensitive to the clay/polymer interface. Thus, large Tg shifts can be engineered and studied once lattice spacing approaches interfacial layer dimensions.