Glass Formation near Covalently Grafted Interfaces: lonomers as a Model Case

In materials ranging from ionomers to semicrystalline polymers, the glass transition-the central phenomenon determining the dynamic and mechanical behavior of amorphous materials-can reflect profound alterations by covalent grafting to surfaces or inclusions. The understanding of these alterations in ionomers has long been underpinned by the view that these grafts serve as tethers, suppressing dynamics of attached chains out to a distance related to their molecular stiffness. Here we describe simulations of model ionomers suggesting the need for a fundamental reconsideration of this understanding. Specifically, as in nanocomposites and nanoconfined glass-formers, we find that near-interface mobility suppression is mediated by cooperative rearrangements intrinsic to glass-forming liquids rather than by a unique covalent tethering effect. While remaining consistent with many earlier predictions, these results bring dynamics near ionomeric aggregates and other grafted interfaces into agreement with a body of literature suggesting the universal presence of a cooperative dynamic length scale in glass-forming liquids that dominates their relaxation behavior and establishes their fundamental length scale of local dynamic gradients.