Physical vapor deposition technique reveals polyamorphism of an organic glass

Amorphous materials contain properties tunable by their degree of structural disorder and can provide advantages over crystalline materials, such as lower losses in optical signals for applications in fiber optics. Physical vapor deposition (PVD) is a preparation technique that can lead to enhanced efficiency and thermal stability of such materials. A new study examining the technique has revealed the existence of a liquid-liquid phase transition for 2-methyltetrahydrofuran (MTHF).

Riechers et al. report findings from dielectric spectroscopy on vapor-deposited MTHF. The group used PVD to create amorphous films with distinct signatures of polyamorphism, where two different liquid phases can simultaneously exist in a single material under certain conditions. The films can be triggered to inherit a low dielectric constant and a low loss contribution, both are hallmarks of materials well suited for transporting optical signals.

According to the authors, unlike the rapid quenching that occurs during conventional preparation of glasses, PVD is much slower and allows high mobility for the most recently deposited particles. This mobility leads to a more settled structure and allows for the establishment of preferred interactions between particles, but while also avoiding crystallization.

While the as-deposited films demonstrated reduced values of dielectric permittivity both in the glass state and the liquid state, a transition toward ordinary behavior of liquid-cooled MTHF was observed at elevated temperatures. The group’s findings also suggest that dipole-dipole interactions between particles are responsible for the differences in dielectric properties between the distinct amorphous states.

Author Birte Riechers hopes the group’s work will encourage discussions about driving forces for the emergence of polyamorphism and serve as a motivation for future studies exploring the formation of polyamorphous states involving PVD.

Source: “Ultrastable and polyamorphic states of vapor-deposited 2-methyltetrahydrofuran,” by Birte Riechers, A. Guiseppi-Elie, M. D. Ediger, and Ranko Richert, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5091796 .