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SYAM: Symposium Amorphous materials: structure, dynamics, properties

SYAM 1: Amorphous materials: structure, dynamics, properties

SYAM 1.2: Hauptvortrag

Dienstag, 28. September 2021, 14:00–14:30, Audimax 1

Liquid-Liquid Phase Transition in Thin Vapor-Deposited Glass Films — •Zahra Fakhraai — Department of Chemistry, University of Pennsylvania, Philadelphia, PA, 19104

Physical vapor deposition can produce glasses with near-equilibrium properties at low temperatures. This phenomenon is enabled by the enhanced mobility at the surface region, which enables access low-energy states in the energy landscape that are otherwise kinetically inaccessible. In thin films (For TPD molecule, h~20-50 nm), where surface mobility is further enhanced, we observe a liquid-liquid phase transition to a high-density supercooled liquid (HD-SCL) phase. The HD-SCL is formed when vapor deposition is performed below the phase transition temperature (T_LL). Above T_LL films of the same thickness follow the supercooled liquid (SCL) state. Films deposited in the HD-SCL state have densities that exceed even the crystal density. The kinetic stability of these films, measured using solvent vapor-annealing, also shows a sharp change at T_LL, further confirming the liquid-liquid phase transition phenomenon. The HD-SCL is only energetically favored in the thin film regime and rapidly transforms to the ordinary SCL or glass upon further deposition (h > 60 nm). This rapid transition is a sign that the specific boundary conditions of thin films can enable the observation of phases that are otherwise unstable in bulk glasses. We discuss how this phenomenon may be related to the observation of low Tg in liquid-quenched thin films of molecular and polymeric glasses and how it may more generally elucidate the nature of phase transitions in glasses.