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Glasses denser than the supercooled liquid.

Yi Jin1, Aixi Zhang1, Sarah E Wolf1

  • 1Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104.

Proceedings of the National Academy of Sciences of the United States of America
|July 31, 2021
PubMed
Summary
This summary is machine-generated.

Researchers discovered a new high-density supercooled liquid (HD-SCL) phase in thin films of TPD, exceeding ordinary liquid and crystal densities. This finding challenges previous understanding of glass density limits.

Keywords:
liquid–liquid phase transitionmobile surface layerphysical vapor depositionstable glassthin-film mobility

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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.