Surface Self-Diffusion of an Organic Glass

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.

Figure 1

Decay kinetics of 1000 nm IMC gratings. (a) Grating amplitude $h$ vs time at 303 K measured with AFM. Inset: AFM images at 0 and $5.9\times {10}^5\mathrm{s}$. (b) $h$ vs time at 333 K measured with diffraction. Inset: Diffraction patterns at 0 and 180 s. The side spots are from first-order diffraction. Contrast is enhanced in the 180 s pattern to make the diffraction spots more visible.

Figure 2

(a) Decay constants $K$ of 1000 nm gratings of IMC liquids and glasses. (b) $K$ vs grating wavelength at 295 K.

Figure 3

Surface and bulk diffusion coefficients of IMC liquid and glasses.