Ionic glass formers show an inverted relation between fragility and non-exponential alpha-relaxation.

Abstract

Supercooled liquids undergo a rapid change in dynamics as they are cooled to their glass transition temperature and turn from a flowing liquid into an amorphous solid. Depending on how steeply the viscosity changes with temperature around the glass transition, glass formers are classified as strong or fragile. An empirical relation exists between the fragility of the liquid and the non-exponentiality of its α-relaxation. However, the microscopic origins of this correlation remain unclear and its generality has been debated. Here, we demonstrate that this relationship is inverted in organic materials with ionic interactions. We introduce a class of materials consisting of highly charged hydrophobic polymers cross-linked via moderated ionic interactions, and show that these combine a strong glass transition with an unusually stretched mechanical relaxation spectrum. By surveying a large variety of ionic liquids, polymerized ionic liquids, and ionomers, we show that all these charged materials follow a trend between fragility and non-exponential relaxation that is opposite to that of non-charged materials. This finding suggests a special role of long-ranged ionic interactions in vitrification and opens up a route toward developing new materials that combine the processability of strong glass formers with the mechanical dissipation of polymers.