期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 116, 期 13, 页码 5937-5942出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1821761116
关键词
stable glass; surface dynamics; physical vapor deposition; surface diffusion; microstructure formation
资金
- National Science Foundation (NSF) Designing Materials to Revolutionize and Engineer our Future (DMREF) Grant [DMR-1628407]
- Materials Research Science and Engineering Center (MRSEC) Grant [DMR-1720530]
- Extreme Science and Engineering Discovery Environment (XSEDE) [TG-DMR150034]
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 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 times 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.
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