Glasses denser than the supercooled liquid

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Glass density is typically limited by the metastable supercooled liquid (SCL) state, with exceptions requiring crystal nucleation or polyamorphic SCL states.
• Polymorphic liquid-liquid phase transitions are rare, observed mainly in network-forming systems, not simple liquids, even in aged or vapor-deposited glasses.

Purpose of the Study:

• To investigate if simple liquids can form high-density states exceeding SCL density.
• To explore the existence and characteristics of polyamorphic supercooled liquid states in vapor-deposited thin films.

Main Methods:

• Vapor deposition of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) films with controlled thicknesses.
• Density measurements of the deposited films to compare with SCL and crystalline states.
• Analysis of film thickness effects on supercooled liquid mobility and phase transitions.

Main Results:

• Thin TPD films (25-55 nm) exhibited densities up to 3.5% higher than their SCL state, and even higher than crystal density.
• A novel high-density supercooled liquid (HD-SCL) phase was identified with a liquid-liquid phase transition temperature approximately 35 K below the ordinary SCL's glass transition temperature.
• This HD-SCL state is favored in thin films due to enhanced surface mobility, but transforms to the ordinary SCL in thicker films (>60 nm).

Conclusions:

• Thin-film effects, specifically enhanced surface mobility, enable access to previously unobserved high-density supercooled liquid states in simple organic materials.
• The discovery of the HD-SCL phase in TPD expands the understanding of liquid-liquid phase transitions and glass polymorphism.
• This work suggests that kinetic barriers and surface effects play a crucial role in accessing thermodynamically distinct liquid states.