Control of molecular orientation, thermal stability, and photochemical stability in vapor-deposited organic glasses

Mark Ediger

University of Wisconsin-Madison

  We have used physical vapor deposition and the mobility of glassy surfaces to prepare what are likely the most stable glasses on the planet.(1) These materials have the properties expected for “million-year-old” glasses, including high density and high mechanical moduli.(2) Surprisingly, these glasses “melt” like crystals, with a constant velocity transformation front.(3) In addition, they resist photochemical degradation better than liquid-cooled glasses. Molecular orientation in vapordeposited glasses can be highly anisotropic.(4) These properties all depend systematically on the substrate temperature during deposition and can be measured efficiently with high throughput spectroscopic ellipsometry.(5)

  The interesting properties of vapor-deposited glasses arise from the high mobility of glass surfaces. During deposition, molecules near the free surface have the opportunity to sample many different packing arrangements. This mechanism allows the molecular orientation in vapor-deposited glasses to be predicted from molecular dynamics computer simulations(4, 6) and could be useful for optimizing the performance of organic light emitting diodes (OLEDs).

1. Swallen SF, et al. (2007) Organic glasses with exceptional thermodynamic and kinetic stability. Science 315(5810):353-356.

2. Kearns KL, Still T, Fytas G, & Ediger MD (2010) High-Modulus Organic Glasses Prepared by Physical Vapor Deposition. Adv. Mater. 22(1):39-+.

3. Swallen SF, Traynor K, McMahon RJ, Ediger MD, & Mates TE (2009) Stable Glass Transformation to Supercooled Liquid via Surface-Initiated Growth Front. Phys. Rev. Lett. 102(6):065503.

4. Dalal SS, Walters DM, Lyubimov I, de Pablo JJ, & Ediger MD (2015) Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors. P Natl Acad Sci USA 112(14):4227-4232.

5. Dalal SS, Fakhraai Z, & Ediger MD (2013) High-Throughput Ellipsometric Characterization of Vapor-Deposited Indomethacin Glasses. J. Phys. Chem. B 117(49):15415-15425.

6. Singh S, Ediger MD, & de Pablo JJ (2013) Ultrastable glasses from in silico vapour deposition. Nature Materials 12(2):139-144.