Universal link between polymer and glass physics inferred from double glass transitions in small–molecule liquids

At the glass transition, a supercooled liquid vitrifies into an amorphous solid and all viscous relaxation processes cease. The experimental observation of secondary or β relaxations below the glass transition temperature in molecular and metallic glasses suggests that this picture may be somewhat oversimplified but essentially correct. Polymers, on the other hand, are thought of as fundamentally different: on cooling, polymers typically show a cascade of transitions from a viscous liquid, through viscoelastic fluid and viscoelastic solid stages, and finally a brittle glass. Here we show that a family of homogeneous non-polymeric liquids—titanium tetraalkoxides—have double glass transitions giving rise to two distinct steps in the temperature-dependent heat capacities. The double glass transition causes the liquid to transition from viscous to viscoelastic to elastic, like a simple polymer liquid. This result allows us to link the physical behaviour of polymers and small-molecule glass formers in a single universal picture.