Manipulating the properties of stable organic glasses using kinetic facilitation.

In contrast to ordinary glasses, when highly stable organic glasses are annealed at temperatures above T(g), they transform heterogeneously into the liquid state by a constant velocity propagating front that initiates at the free surface. The evolution of this growth front has been interpreted as kinetic facilitation, i.e.

Stable glasses of indomethacin and α,α,β-tris-naphthylbenzene transform into ordinary supercooled liquids.

Secondary ion mass spectrometry has been used to characterize translational motion in vapor-deposited glasses of indomethacin (IMC) and α,α,β-tris-naphthylbenzene (TNB). Vapor deposition onto substrates at ∼0.99 T(g) produced ordinary glasses that evolve according to fickian diffusion during annealing.

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.

Direct measurement of molecular motion in freestanding polystyrene thin films.

An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at T(g,DSC) - 25 K (T(g,DSC) is the glass transition temperature of bulk polystyrene).

Temperature-ramping measurement of dye reorientation to probe molecular motion in polymer glasses.

A temperature-ramping anisotropy measurement is introduced as an efficient way to study molecular motion in polymer glasses. For these experiments, fluorescent molecules were dispersed in the polymer glass and the reorientation of these dyes was used as a probe of segmental dynamics. For thick samples of polystyrene, poly (4-tert-butyl styrene), and poly(2-vinyl pyridine), temperature-ramping anisotropy measurements have a shape similar to differential scanning calorimetry measurements and nearly the same transition temperature.