Origin of two modes of non-isothermal crystallization of glasses produced by milling

To mechanistically explain the origin of two distinct non-isothermal crystallization modes, single-peak (unimodal) and two-peak (bimodal), of organic glasses.

Glasses of ten organic molecules were prepared by melt-quenching and cryogenic milling of crystals. Non-isothermal crystallization of glasses was monitored using differential scanning calorimetry and powder X-ray diffractometry.

The non-isothermal crystallization of glass, generated by milling, is either unimodal or bimodal, while that of melt-quenched glass without being milled is always unimodal. The mode of crystallization of amorphous phase depends on the relative position of the crystallization onset (T ( c )) with respect to glass transition temperature (T ( g )), and can be explained by a surface crystallization model. Bimodal crystallization event is observed when T ( c ) is below or near T ( g ), due to the fast crystallization onset at milled glass surfaces. Unimodal crystallization is observed when T ( c ) is well above T ( g ). We have verified this model by intentionally inducing flip between the two crystallization modes for several compounds through manipulating glass surface area and T ( c ).

The two modes of crystallization of organic glasses is a result of the combined effects of faster surface crystallization and variation in specific surface area by milling.