Fluctuation Electron Microscopy Study of Medium-Range Packing Order in Ultrastable Indomethacin Glass Thin Films
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Fluctuation Electron Microscopy Study of Medium-Range Packing Order in Ultrastable Indomethacin Glass Thin Films L. He1, A. Gujral2, M. D. Ediger2, P. M. Voyles1 1 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A. 2 Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A. ABSTRACT We have used fluctuation electron microscopy (FEM) to measure the medium range order in the molecular packing of 40 nm thick indomethacin glass films. Vapor deposition of indomethacin can create glasses with extraordinary kinetic stability and high density. We find peaks in the FEM variance at diffraction vector magnitudes between 0.03 and 0.09 Å-1, corresponding to intermolecular packing distances of 1-3 nm. FEM experiments were performed with a 13 nm diameter electron probe, so these data are sensitive to medium-range order in intermolecular packing. The FEM variance from an indomethacin glass with normal stability cooled from the liquid is significantly smaller than the variance from the ultrastable glass, suggesting that ultrastable glass is more structurally heterogeneous at a 13 nm length scale. A dose of ~7×105 e-/nm2 with a very low beam current of ~ 2.5 pA at 200 kV was used to minimize electron beam damage to the sample, and the average electron diffraction from the sample is unchanged at total electron doses fourteen times larger than required for a FEM experiment. These preliminary results on medium-range order in molecular glasses suggest that we may be able to provide insight into the structural differences between the remarkable ultrastable thin films and ordinary glasses. INTRODUCTION Molecular glasses are amorphous organic solids. Various molecular glasses are used for electrical and optical devices and as ion conductors, among other applications. Often, molecular glasses are formed by quenching a high temperature liquid. However, vapor-deposition of molecular glass thin films can result in extraordinarily high kinetic stability and high density in the glass state [1]. Indomethacin (IMC) films deposited onto a substrate held at 0.85Tg, the glass transition temperature, have higher Tg, lower fictive temperature, higher density, and lower enthalpy than liquid-cooled ordinary IMC glass [1]. Aging an ordinary glass to achieve the same stability would take at least decades, and maybe longer [1]. Large-area diffraction with x-rays, neutrons, or electrons, reveals the average short-range order of amorphous materials through a structure factor or pair distribution function. X-ray diffraction from ultrastable IMC thin films shows that there is tendency towards layering of the molecules parallel to the film surface [2]. Ordinary IMC glass thin films do not have the same diffraction feature. In the plane perpendicular to the film surface, x-ray diffraction shows random molecular packing in both ultrastable and normal IMC. Fluctuation electron microscopy (FEM) uses coherent electron nanodiffraction to probe medium-range order (MRO) in am
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