Origin of bulk transformation initiation sites

Determine whether the bulk transformation of low-energy indomethacin glasses into the supercooled liquid is initiated by heterogeneous nucleation at structural defects or by homogeneous nucleation arising from the mobility of glassy regions.

Background

The paper identifies an approximately 1 μm length scale in low-energy, vapor-deposited indomethacin glasses and interprets it as the average separation between sites where transformation into the supercooled liquid begins. The authors discuss two competing physical explanations for these sites. In the heterogeneous-nucleation picture, rare structural defects associated with inefficient amorphous packing become mobile above the glass-transition temperature and initiate local transformation. In the homogeneous-nucleation picture, any location in the glass can initiate transformation, but with an extremely small probability, so the observed length scale reflects the growth rate and the rate at which glassy regions become mobile.

The authors state that the available transformation kinetics cannot distinguish between these explanations. Although the Avrami exponent of approximately 4 is said to be consistent with sporadic nucleation in the homogeneous picture, this does not resolve whether the initiation sites are pre-existing structural defects or arise through homogeneous nucleation dynamics.