A new ionic glassy state of matter with counterintuitive properties

An international team of Dutch and Italian researchers has discovered a new class of ionic glassy materials that challenges one of the most established paradigms in the physics of the glassy state. The study, published in Nature Communications, shows for the first time that in these materials the relationship between glass “fragility” and the breadth of the relaxation spectrum is inverted compared to what has been observed in all conventional glasses known to date.

Alessio Zaccone, Professor of Mathematical Methods of Physics at the Department of Physics “A. Pontremoli” of the University of Milan, took part in this discovery. His work on mathematical modelling provided the conceptual framework needed to understand the new glassy state and the physical mechanism governing its dynamic properties.

A new paradigm for glasses

When a liquid is cooled until it becomes a glass, its dynamics slow down dramatically. Traditionally, supercooled liquids are classified as “strong” or “fragile” depending on how rapidly their viscosity increases as the glass transition temperature is approached. In fragile liquids, viscosity increases enormously—by about 13 orders of magnitude—over a very small temperature interval near the glass transition.

In classical glassy materials, higher fragility is associated with a broader, non-exponential relaxation spectrum, a hallmark of strongly cooperative dynamics.

The new study demonstrates that in organic ionic glasses—polymeric materials held together by long-ranged electrostatic interactions—this correlation is reversed: materials with low fragility (typical of “strong” glasses) display a surprisingly broad and strongly non-exponential relaxation spectrum.

The theoretical contribution: why the relationship is inverted

Professor Zaccone’s contribution was crucial in interpreting this anomalous behaviour. The theory developed in the study shows that long-ranged ionic interactions significantly increase the cohesive energy of the material and reduce its thermal expansion. This effect makes the glass less fragile from both a thermodynamic and mechanical standpoint.

At the same time, however, the presence of electric charges and covalent chemical bonds along the polymer chains introduces strong dynamic heterogeneities: different regions of the material relax on very different timescales. The result is an extremely broad relaxation spectrum, mathematically described by a very small stretching exponent (also known as the Kohlrausch relaxation exponent). In other words, the material is “strong” in the way it enters the glassy state, but “complex” and highly dissipative in its mechanical response.

This theoretical framework clarifies for the first time how fragility and relaxation spectrum width can be decoupled, demonstrating that the empirical relationship known for decades is not universal, but depends on the microscopic nature of the interactions.

Implications and applications

This discovery opens up new perspectives for the design of advanced materials. These new ionic glasses combine the processability of strong glasses, such as silica glass, with the energy-dissipation capability typical of polymers. This combination was previously thought to be unattainable, and it paves the way for applications ranging from resilient structural materials to vibration-damping devices.

“This work shows how a rigorous theoretical approach can reveal entirely new behaviours of disordered matter,” comments Professor Zaccone. “Understanding the role of ionic interactions in glasses allows us not only to explain unexpected experimental results, but also to guide the development of new materials with tailored properties.”

The study once again confirms the central role of the University of Milan in cutting-edge theoretical research on the physics of complex materials and the glassy state.