Effects of microstructure formation on the stability of vapor-deposited glasses

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.

Ultrastable glasses: new perspectives for an old problem

“…By measuring the relaxation dynamics of thin glassy TNB and α,α-A films, Samanta et al concluded that while the bulk of the glass is in a deeply quenched glassy state, the molecules within 10 nm from the surface can have a faster dynamics and are able to equilibrate before becoming part of the bulk [122]. Similar conclusions are obtained by More et al [145]. They used molecular dynamics simulations to study empirical coarsegrained models of organic molecules containing model fluorocarbon tails of increasing length: zero, one, four, and eight fluorocarbons, with a structure composed of a phenyl "body" and a fluorocarbon "tail", to simulate the formation of a vapor deposited glass.…”

Polyamorphism of vapor-deposited amorphous selenium in response to light

“…However, after the oligomers have combined to make larger chains (equilibrium chain length ∼200 atoms per chain) within 10 −6 to 10 −4 s (33), the barrier for relaxation is increased. The interaction between the oligomers can potentially slow the surface mobility and reduce the depth of the mobile region, limiting the packing efficiency during SME, analogous to what has been observed in molecular SGs with strong intermolecular interactions (5,16,35).…”

“…It is understood that SG formation is due to enhanced surface mobility (5,8,12,(14)(15)(16)(17). When T dep < Tg , at slow deposition rates (∼0.2 nm/s for most organic glasses), the surface region has sufficient mobility (18) to reach more energetically favored states before being buried into dynamically arrested states.…”

“…The stability of a PVD glass thus strongly depends on T dep (3,4,6,7,19) and the deposition rate (2,8,20). Surface-mediated equilibration (SME) also influences the structure and birefringence of PVD glasses, which are inherited from the equilibrium states at the glass surface (21,22) and the layers directly below the free surface (12,16).…”

Vapor deposition of a nonmesogen prepares highly structured organic glasses

“…The amphiphilic nature of posaconazole may be responsible for its vertical orientation at the free surface of the liquid. In recent simulations of vapor-deposited glasses of amphiphilic fluorinated molecules, the fluorinated tails segregate to the surface (49); it is plausible that posaconazole behaves similarly during deposition, and this provides a guide in selecting additional systems for study. Our investigation of posaconazole was motivated by its structural similarity with itraconazole, a system known to form smectic liquid crystals.…”