Effects of Microstructure Formation on the Stability of Vapor Deposited Glasses

TL;DR

This study investigates how microstructure formation in vapor deposited glasses affects their stability, revealing that longer molecular tails lead to decreased stability due to surface segregation and reduced surface mobility.

Contribution

It introduces a coarse-grained model showing how microphase separation impacts the stability of vapor deposited glasses with varying molecular tail lengths.

Findings

Longer molecular tails decrease thermodynamic stability.

Surface segregation of tails slows relaxation near the surface.

Reduced surface diffusion impairs glass formation.

Abstract

Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those of glasses aged for thousands of years. Exerting control over the structure and properties of PVD glasses formed with different types of glass-forming molecules is now an emerging challenge. In this work, we study coarse grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase separated domains in the supercooled liquid influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased thermodynamic and kinetic stability of the molecules in PVD films. The reduced stability is further linked to the reduced ability of these molecules to equilibrate at the free surface during PVD. We find that as the tail length is increased, the relaxation time near the surface of the supercooled equilibrium liquid films of these molecules are slowed and become essentially bulk-like, due to the segregation of the fluorocarbon tails to the free surface. Surface diffusion is also markedly reduced due to clustering of the molecules at the surface. Based on these results, we propose a trapping mechanism where tails are unable to move between local phase separated domains on the relevant deposition time scales.