One Micrometer Length Scale Controls Kinetic Stability of Low-Energy Glasses

Alternating current (AC) nanocalorimetry was used to measure the reversing heat capacity C-p of low-energy indomethacin glasses as they isothermally transform into the supercooled liquid. As the film thickness increases from 75 to 600 nm, the transformation time increases by more than an order of magnitude, consistent with a surface-initiated transformation mechanism. Eventually, the transformation time becomes constant for films between 1.4 and 30 mu m, indicating a distinct bulk transformation pathway. The observation of size-dependent transformation kinetics for glass samples approaching 1 mu m is unprecedented. We interpret the crossover in thickness dependence at 1 mu m to signify the average distance between transformation initiation sites in the bulk low-energy glass.