Ultrastable and polyamorphic states of vapor-deposited 2-methyltetrahydrofuran

This work reports results gained from dielectric spectroscopy on the organic molecular glass-former 2-methyltetrahydrofuran (MTHF), which was deposited onto an interdigitated electrode device by physical vapor deposition. By a suitable selection of preparation parameters (deposition temperature, deposition rate, and annealing conditions), various states of MTHF could be created: ultrastable glass, a liquid state with unusual dielectric properties, or the ordinary liquid state as obtained by supercooling. Observations on kinetic stability as well as on the suppression of dielectric loss in the ultrastable state resemble previous findings for other molecular glass-formers. Remarkably, after annealing just above T_g, all vapor-deposited films of MTHF display a static dielectric constant in the liquid state (ϵ_s) that is up to a factor of two below that of the ordinary bulk liquid. A structural transition to the ordinary liquid-cooled state of MTHF occurs at temperatures far above its conventional T_g, indicative of polyamorphism: the formation of an unusual structure that is achieved by physical vapor deposition and that differs from the ordinary liquid state obtained by supercooling. The present results also reveal that the dielectric constant of the as deposited glass (ϵ_∞) is reduced to practically the value of the squared refractive index, n².