Surface Self-Diffusion of an Organic Glass

Abstract

Surface self-diffusion has been measured for an organic glass for the first time. The flattening of 1000 nm surface gratings of liquid indomethacin occurs by viscous flow at 12 K or more above the glass transition temperature and by surface diffusion at lower temperatures. Surface diffusion is at least 10(6) times faster than bulk diffusion, indicating a highly mobile surface. Our data suggest that surface diffusion is the leading mechanism of surface evolution for organic glasses at micrometer to nanometer length scales.

Long-range correlated dynamics in ultra-thin molecular glass films

“…In ultra-thin films, the dynamics are enhanced in the entire film, showing little thickness dependence, while in films with thicknesses of 40 nm or more, the dynamics in the entire film is bulk-like except for perhaps a few liquid-like mono-layers near the free surface. It is important to note that the temperature dependence of diffusion coefficients measured on the surface of bulk films [6,7] are also larger than those measured in ultra-thin films in this study, further confirming that the dynamics at the surface of a bulk film are different than those measured in thin films. These observations suggest that the dynamics of the glassy material are correlated over large length scales and the dynamics of thin films are influenced both by the interfacial dynamics and the glassy dynamics in the layers closer to the center of the film.…”

“…However, if the properties at nanoscale deviate significantly from bulk properties, the resulting films can have reduced kinetic and thermal stability. Recent experiments suggest that diffusion at the free surface of organic glasses can be several orders of magnitude faster [6,7], with weaker temperature dependence compared to bulk diffusion. Enhanced, weakly temperature-dependent dynamics on the surface of polymeric glasses [8,9] have been shown to significantly affect the properties of ultra-thin polymer films [9][10][11][12][13][14][15][16][17].…”

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

“…The first factor might be investigated in greater detail with atomistic simulations and surface sensitive spectroscopy (33), whereas a number of new experimental methods are available to probe dynamics at the surface of glasses (25,34). Structure at liquid surfaces is an active area of study (35,36), and our results indicate that physical vapor deposition might be a useful tool in this endeavor.…”

“…Discovered using model glass formers and labeled "stable glasses," these glasses have lower enthalpies (20), higher densities (21), and resist structural reorganization to higher temperatures than is possible with any other preparation route (22)(23)(24). The properties of stable glasses are explained by the high mobility of the free surface during the vapor deposition process (20,25). Because of lowered constraints to motion (26), molecules near the free surface can adopt near-equilibrium packing arrangements during deposition even at temperatures where the bulk structural relaxation time is thousands of years (21,27).…”

“…At lower values of T substrate /T g , there is a thermodynamic driving force to form the equilibrium SCL at that temperature resulting in higher stability, higher density materials. The mobility of the surface enables molecules to find these tight packing arrangements (20,25,27), even though this would require a prohibitively long time for a bulk material. T onset /T g and Δρ are maximized when high surface mobility is paired with a large thermodynamic driving force.…”