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Title: Orientational anisotropy in simulated vapor-deposited molecular glasses

Enhanced kinetic stability of vapor-deposited glasses has been established for a variety of glass organic formers. Several recent reports indicate that vapor-deposited glasses can be orientationally anisotropic. In this work, we present results of extensive molecular simulations that mimic a number of features of the experimental vapor deposition process. The simulations are performed on a generic coarse-grained model and an all-atom representation of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), a small organic molecule whose vapor-deposited glasses exhibit considerable orientational anisotropy. The coarse-grained model adopted here is found to reproduce several key aspects reported in experiments. In particular, the molecular orientation of vapor-deposited glasses is observed to depend on substrate temperature during deposition. For a fixed deposition rate, the molecular orientation in the glasses changes from isotropic, at the glass transition temperature, T{sub g}, to slightly normal to the substrate at temperatures just below T{sub g}. Well below T{sub g}, molecular orientation becomes predominantly parallel to the substrate. The all-atom model is used to confirm some of the equilibrium structural features of TPD interfaces that arise above the glass transition temperature. We discuss a mechanism based on distinct orientations observed at equilibrium near the surface of the film, which get trapped within the film duringmore » the non-equilibrium process of vapor deposition.« less
Authors:
;  [1] ; ;  [2] ;  [3] ;  [4] ;  [1] ;  [5]
  1. Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637 (United States)
  2. Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
  3. Science et Ingénierie des Matériaux et Procédés, Grenoble INP, CNRS/UJF, 38402 Saint Martin d’Hères (France)
  4. (France)
  5. (United States)
Publication Date:
OSTI Identifier:
22493602
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 143; Journal Issue: 9; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ANISOTROPY; ATOMS; BENZIDINE; COMPUTERIZED SIMULATION; DEPOSITION; EQUILIBRIUM; FILMS; GLASS; INTERFACES; MOLECULES; ORIENTATION; SUBSTRATES; SURFACES; TRANSITION TEMPERATURE; TRAPPING; VAPOR DEPOSITED COATINGS