Glasses denser than the supercooled liquid

Abstract

When aged below the glass transition temperature, Tg, the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid–liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid–liquid phase transition temperature (TLL) ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.

Ultrastable glasses: new perspectives for an old problem

“…The properties of vapor-deposited glasses grown at low substrate temperatures, below 0.75 T g , have not been explored in detail and it is too early to draw conclusions about the mechanism of stabilization in this temperature regime, at least for organic glasses. Recent experiments from Fakhraai and coworkers [40] show that at temperatures around and below 0.85 T g it is possible to access to a different phase in TPD vapor deposited glasses but only for very thin films, below 70 nm. This is a clear example showing how complex can become the relaxation mechanisms that take place during the deposition process.…”

“…If N changes by two orders of magnitude the velocity of the liquid regions in the bulk would be one order of magnitude slower than the velocity of the surface propagation front. In this scenario, the difference in propagation velocity of the liquid between the front and the bulk could be due to a pressure increase arising from the difference in density of 1.7% between the glass and the liquid at the same temperature [40]. However, a study of the transformation of very thick films of an organic glass by means of dielectric spectroscopy showed that the relaxation time of the liquid surface propagation front and of the liquid formed in the bulk was not only the same, but equal to the alpha-relaxation time expected for an equilibrium supercooled liquid at that temperature [104].…”

Vapor-Deposited Thin Films: Studying Crystallization and α-relaxation Dynamics of the Molecular Drug Celecoxib

“… 40 A recent study by Jin et al demonstrated that the high density-supercooled liquid state is thermodynamically favored only in vapor-deposited glasses with the film thickness of 25 to 55 nm. 41 When it comes to OLED devices, the surface potential depends on the film thickness, especially for polar organic semiconductors. 42 It has also been demonstrated that interfacial molecular packing in vapor-deposited films of organic semiconductors as thin as ∼13 nm is more disordered than in bulk.…”

“…The confined glasses deposited at faster rates (∼15 nm s –1 ) were also found to possess an increased transformation rate . A recent study by Jin et al demonstrated that the high density-supercooled liquid state is thermodynamically favored only in vapor-deposited glasses with the film thickness of 25 to 55 nm . When it comes to OLED devices, the surface potential depends on the film thickness, especially for polar organic semiconductors .…”

Role of Molecular Layering in the Enhanced Mechanical Properties of Stable Glasses

“…Sample Preparation . For each molecule, 0.8–1 μm films were prepared by physical vapor deposition (PVD) from a thermal evaporation source, using a custom vacuum chamber with a base pressure of ∼10 –7 Torr (Figure B, details in our previous publications , ). The molecules were deposited onto silicon substrates (Virginia Semiconductor) bridged across two independently temperature-controlled copper sample holders and secured on each side using Apiezon PFPE 501 grease and a strong clamp to ensure good thermal contact.…”